Long Chiang Larry Dalton, National Taiwan Univ Univ of Southern California
Alex Jen John Reynolds, Northeastern Univ Univ of Florida
Michael Rubner, MIT
National Taiwan University
- Taiwan Four Pillars
- Taiwan President Enterprises
- United Microelectronics Corporation
Proceedings published as Volume 488
of the Materials Research Society
Symposium Proceedings Series.
* Invited paper
SESSION J1: LIGHT EMITTING ORGANICS
Chair: Kenneth J. Wynne
Monday Morning, December 1, 1997
Salon G (M)
8:30 AM *J1.1
HIGH EXCITATION DENSITIES IN OPTICALLY AND ELECTRICALLY EXCITED CONJUGATED POLYMER STRUCTURES. Nick Harrison, Nir Tessler, Graham Denton, Mark Stevens, Seamus Burns, Richard Friend, Cavendish Laboratory, Cambridge, UNITED KINGDOM.
Conjugated polymers such poly(p-phenylenevinylene), PPV, show efficient luminescence, either optically or electically stimulated (the latter in electroluminescent diodes). There has been rapid progress in the development of optically-pumped structures which show stimulated emission and associated line-narrowing. We present measurements on a range of devices which show sufficient feedback to demonstrate lasing, and we discuss criteria for reducing the lasing threshold. We also present measurements of high peak brighness polymer LEDs, which can show current densities of up to 1000 A/cm2, and peak brightnesses up to 5 Mcd/m2. These devices show clear evidence for increased carrier mobilities at high charge injection densities.
9:00 AM J1.2
TRANSIENT ELECTROLUMINESCENCE UNDER STRONG VOLTAGE PULSES: POSSIBLE EVIDENCE FOR DIRECT INTER-BAND RADIATIVE TRANSITION. H. Chayet and D. Davidov, Racah Institute of Physics, The Hebrew University of Jerusalem, ISRAEL; R. Pogreb, Racah Institute of Physics, The Hebrew University of Jerusalem and the College of Judea and Samaria, Ariel; R. Neumann, Casali Institute of Applied Chemistry; and Y. Avny, Department of Organic Chemistry, The Hebrew University of Jerusalem, ISRAEL.
We present pulsed electroluminescence (EL) measurements on light emitting diodes (LED) based on thin films of poly(phenylenevinylene) (PPV) sandwiched between Al and ITO electrodes. We use high voltage pulser with voltages up to 800V and pulse width of 250 nsec. We show that the device can be operated at transient current densities as high as 150A/cm2 without appreciable degradation of the device. We observe two regimes in the operation of the LED depending on the pulsed current density. Up to 60A/cm2 (corresponding to pulsed voltage height of 250V) the EL pulse, which is strongly enhanced, follows its DC characteristics with green emission centered at 530 nm. Above this current density we observe additional UV-violet emission (centered at 390 nm, 3.17 eV) which increases exponentially with the height of the voltage pulses. At current density of 140 A/cm2 the UV-Violet emission is comparable to the green emission. For this current density the overall brightness is 105 cd/m2. The intensity of the UV-violet emission is significant higher for higher current densities. We provide evidence that the UV-Violet emission originates mostly near the Al electrode. We propose a possible explanation based ``hot'' carriers in strong electric fields which partially suppress the formation of singlet exitons and enhance the probability for direct LUMO-HOMO inter-band radiative transition. Impact excitations involving hot electrons and the Al electrode is another possibility. However, the emission wavelength at 390 nm agrees with the PPV LUMO-HOMO band-gap and supports the former mechanism. We shall discuss the prospects for lasing stimulated by electric fields.
9:15 AM J1.3
LIGHT-EMITTING AND HOLE-TRANSPORTING POLYMERS FOR LEDS. Toshihiro Ohnishi, Shuji Doi, Masato Ueda, Fumi Yamaguchi, Ta kanobu Noguchi, Sumitomo Chem. Co.Ltd., Tsukuba Research Laboratory, Ibaraki , JAPAN.
Many polymer LEDs consist of poly(phenylene vinylenes), PPVs as a light -emitting polymer because of their strong fluorescence. Various types of PPV have been synthesized and some of them showed strong fluorescence. We have already reported that copolymerization of various arylene vinylenes are effective to improve the fluorescent properties of PPV and that the random copolymerization of the monomers having different energy gaps or conjugation degrees can generate the irregularity in poly(arylene vinylenes) and enhance thefluorescence of the polymer. Among the copolymers synthesized, the m/p-phenylene vinylene copolymer showed fairly strong fluorescence and gave the most efficient polymer light-emitting diode(P-LED) device because of the good balance of the exciton confinement and the charge transporting property . The optimized P-LED device showed a maximum luminance of 70,000cd/m2. Polysilanes, such as poly(phenylmethylsilane), are known as a hole transporting polymer. There has been many attempts to increase the hole mobility so far. However, the mobility has not exceeded 0.0001cm2/Vs yet. We have successfully prepared polysilane having a triphenylamine group as a side chain, TPA-PS, and showing hole mobility as high as 0.001cm2/Vs. The high hole mobility is attributable to the intermolecular hopping process facilitated by the interaction between the polysilane backbone and the triph enyl amine group. The TPA-PS is soluble in organic solvents like toluene and chloroform and its homogeneous film is easily obtained by spin-casting. The thin film is effectively used as a hole transporting layer in LED devices. The bilayer LED device consisting of TPA-PS and tris(8-hydroxyquinoline) Aluminum, Alq3, shows high luminance (2000cd/m2) and high efficiency (4cd/A).
9:30 AM J1.4
WHITE LIGHT EMITTING ELECTROLUMINESCENCE DEVICES WITH ORGANIC OLIGOMERS. F. Meghdadia, S. Tascha, W. Graupnera, S.E. Döttingerb, M. Hanackb, G. Leisinga, aInstitute für Festkörperphysik, Technische Universität Graz, Graz, AUSTRIA; bInstitut für Organische Chemie, Eberhard Karls Universität Tubingen, Morgenstelle, GERMANY.
We report the properties of whit light emitting electroluminescence (EL) devices fabricated by the combination of blue and orange light emitting materials using parahexaphenyl (PHP) and a low concentration of isothianaphthene-S-oxide (ITSO) oligomers. The El spectrum covers the range between 400 to 650 nm. The El devices have an external El quantum efficiency of 1.5% photons/electron and CIE coordinates of X=0.33 and Y=0.338. The mechanism for orange emission is dominated by excitation energy transfer (EET) from PHP into the energetically lower lying energy states of ITSO. The results of photoinduced absorption, which is a technique sensitive to charged species and triplet states of these guest host systems, are discussed.
10:15 AM *J1.5
LASING AND CAVITY EFFECTS IN VACUUM DEPOSITED ORGANIC THIN FILMS. S. R. Forrest, V. Kozlov, V. Bulovic, P.E. Burrows, V. Khalfin, Center for Photonics and Optoelectronic Materials (POEM), Princeton Materials Institute, Department of Electrical Engineering, Princeton University, Princeton, NJ.
We demonstrate clear evidence for optically pumped lasing in vacuum deposited mixtures of the laser dye, DCM in Aluminum tris-(8 hyrdoxyquinoline). The thin films were deposited on InP substrates precoated with silicon dioxide, and then subsequently cleaved to form reflecting facets. The nitrogen laser-pumped structure lases at approximately 655nm. The several characteristics of lasing were all observed: linewidth narrowing to <1 Angstrom, optical modes consistant with the calculated free spectral range of the laser cavity, highly TE polarized output, a well-formed output beam, and a clear threshold with a high output differential efficiency. The threshold pump energy was 1uJ/cm2, which we estimate to correspond to an electrically pumped threshold current density of 10-100 A/cm2. A peak output power >50W was also observed. Using these materials, the prospects for electrically pumped organic lasers are greatly enhanced. Furthermore, we have also demonstrated lasing action for similar films deposited on thin polymer substrates. In this talk, we will discuss the dependence of threshold on several thin film and device parameters, and in particular will discuss measurements and analysis of cavity modes which influence the properties of organic LEDs and lasers.
10:45 AM J1.6
HIGH EFFICIENCY ORGANIC LIGHT-EMITTING DIODES. S. Joshua Jacobs, Francis G. Celii, Tae S. Kim, Texas Instruments, Components and Materials Research Center, Dallas, TX.
The insertion of two dielectric layers, one between the hole transporting layer and indium tin oxide anode and one between the electron transporting layer and cathode, in vapor deposited organic LEDs leads to dramatic Increases in quantum efficiency. For undoped Alq3 emission, quantum efficiencies approach 9 cd/A (linear regression fit to L-I data between 10 and 50 mA/cm2), while for quinacridone (QAC) doped Alq3, quantum efficiencies of 15 cd/A are realized. Here we describe the effects of varying the thickness and composition of the dielectric layers on the I-V and L-V data as well as the optimization of organic layer thickness for these devices, and we also discuss the mechanisms of charge injection and transport in these modified OLEDs. Our analysis indicates that further structure improvements targeting even higher efficiency devices are possible.
11:00 AM J1.7
PLASTIC MULTILAYERED MOLECULAR ORGANIC LIGHT EMITTING DIODES. George M. Daly, Hideyuki Murata, Charles D. Merritt and Zakya H. Kafafi, Optical Sciences Division, Naval Research Laboratory, Washington, DC; Yasuhiko Shirota, Department of Applied Chemistry, Osaka University, JAPAN.
Enhanced performance has been observed for plastic molecular organic light emitting diodes (MOLEDs) consisting of two to four organic layers sequentially vacuum vapor deposited onto patterned indium-tin oxide (ITO) on polyester films. For all device structures studied, the performance of plastic diodes is comparable to or better than their analogs on glass substrates. At 300 Cd/m2, a luminous power efficiency of 4.4 lm/W and external quantum yield of 2.7 is observed for a device structure consisting of two hole transport layers, a doped emitting layer and a electron transport layer on a ITO/polyester substrate. The same device made on a ITO/silica substrate has a luminous power efficiency of 3.5 lm/W and external quantum yield of 2.3. The inherent performance of the plastic devices compared to glass devices is considerable when transmission losses through the substrate are considered. At 550 nm the ITO/glass substrates show a percent transmittance of 90 while the ITO/plastic substrates, 71. Electrical and optical performance for comparative device structures has been characterized by current-voltage-luminance measurements and electroluminescence spectra, and thin film growth by scanning electron microscopy.
11:15 AM *J1.8
DYE DOPED ORGANIC LIGHT EMITTING DIODES FOR ACHIEVING SATURATED RED, GREEN AND BLUE OLEDS. Yujian You, Andre Shoustikov, Nicos Petasis, Mark E. Thompson, Department of Chemistry, University of Southern California, Los Angeles, CA; Paul E. Burrows, Stephen R. Forrest, Center for Photonic and Optoelectronic Materials and the Department of Electrical Engineering, Princeton University, Princeton, NJ.
Vacuum-deposited, single-heterojunction OLEDs have been prepared and studied, with a variety fluorescent dyes doped into the electron and hole transporting layers. The electron transporting layer consisted of range of different compounds, including aluminum tris(8-hydroxyquinoline), aluminum bis(8-hydroxyquinaldine)phenoxide and aluminum tris(5-hydroxyquinoxaline), while the hole transporting layer was 4,4'-phenyl-1-napthylaminobiphenyl (alpha-NPD). The dyes were doped at 0.2- 2 mol %, leading to saturated red, green and blue electroluminescence. The dyes examined included both conventional dye stuffs, such as indigo and squarilium dyes, as well as a novel family of emissive materials which can be tuned to emit from the blue to the red by suitable substitution on the dye framework. Current-voltage characteristics, UV-visible, photoluminescence and electroluminescence spectra have been measured and will be discussed. Correlations observed in the energy transfer efficiency between the host and guest dye, and the degree of overlap between the emission spectrum of the host and the absorption spectrum of the guest, suggest Förster energy transfer is the dominant pathway.
11:45 AM J1.9
ENERGY OFFSETS OF MOLECULAR LEVELS AT ORGANIC HETEROJUNCTIONS. Aparna Rajagopai and A. Kahn, Dept. of Electrical Engineering, Princeton University, Princeton, NJ.
The development of microelectronics and optoelectronics based on small molecule organic semiconductors requires a detailed understanding and control of the interface properties of these materials. In particular, the electrical behavior of organic-organic heterojunctions are of great importance for the efficiency of increasingly complex, multi layer devices. We present here a direct determination of energy offsets between molecular levels at interfaces between three small molecule organics, i.e. the bole-transporter 3,4,9,10 perylenetetracarboxylic dianilydride (PTCDA) and N,N'-diphenyl-N,N'-bis-(1-nappthyl)-l,1'biphenyl-4,4' diamine (-NPD, and the electron-transporter and light emitter tris(8-hydroxyquinoline)aluminum (Alq3). The offsets are measured with ultra-violet photoemission spectroscopy on layers incrementally deposited by thermal evaporation in ultra-high vacuum. We investigate the PTCDA-Alq3 and Alq3--NPD pairs. In each case, we measure two heterojunctions obtained by inverting the deposition sequence (A/B and B/A) and find no difference in the interface¢ offset within experimental resolution. For PTCDA-Alq3, we find the highest occupied molecular orbital (HOMO) of Alq3 0.4 eV above that of PTCDA. For -NPD-Alq3, the Alq3 HOMO is approximately 0.3 eV below that of -NPD. Using Me optical gap of each materials as a lower bound for the transport gap (2.2 eV, 2.7 eV and 3.1 eV for PTCDA, Alq3 and -NPD, respectively), these measurements indicate that the Alq3 lowest occupied molecular orbital (LUMO) is above that of PTCDA by about 0.8 eV, whereas, it is lower than that of -NPD. Thus electrons (holes) injected on the A1q3 (PTCDA) side of the heterojunction can cross into the PTCDA (Alq3). On the other hand, electrons are confined in Alq3 at the -NPD interface, a situation which may contribute to the higher efficiency of Alq3 electroluminescence in devices using the Alq3--NPD pair.
SESSION J2: NONLINEAR OPTICAL/PHOTOREFRACTIVE MATERIALS-I
Chair: Alex K.-Y. Jen
Monday Afternoon, December 1, 1997
Salon G (M)
1:30 PM *J2.1
STRUCTURE-PROPERTY RELATIONSHIPS FOR TWO-PHOTON ABSORBING MATERIALS. S. R. Mardera,b, S. Barlowa, B.H. Cumpstonb, J. Ehrlichb, A. A. Heikala, D. McCord-Maughona, J.W. Perrya,b, H. Roeckela, S. Thayumanvana, X.-L. Wua. (a) Beckman Institute, California Institute of Technology, Pasadena, CA; (b) Jet Propulsion Laboratory, Pasadena, CA.
Results from studies aimed at providing a fundamental understanding of structure/property relationships which will yield design guidelines for the rational design of molecules with large two-photon cross sections will be presented. In addition some initial results on using these molecules for a variety of applications will be discussed.
2:00 PM *J2.2
PHOTOFABRICATION OF SURFACE RELIEF GRATINGS USING POST FUNCTIONALIZED AZO POLYMERS. Sukant K. Tripathy1, Jayant Kumar2, Dong Yu Kim1, Xinli Jiang2, Xiaogong Wang1, Mongkol Sukwattanasinitt1 and Daniel J. Sandman1, Departments of Chemistry1 and Physics2, University of Massachusetts, Lowell, MA; Lian Li, Molecular Technologies Inc., Westford, MA.
We present novel observation and detailed investigation of a new photodynamic polymer transport process; optically induced surface relief grating (SRG) formation on azo polymer films. SRGs were photofabricated on various polymer films containing photoisomerizable azobenzene groups from post functionalization. These gratings were directly produced with modest intensities. Recording and erasing of the gratings are strongly dependent on polarization. Large surface modulation and high diffraction efficiency could be obtained under optimal recording conditions. Complex patterns have been inscribed on the films. These patterns are due to simple superposition of all the interfering recording waves. The SRG pattern mimics the light pattern and the gradient forces acting on the oriented azo dipoles are responsible for the large scale displacement of the polymers to which the chromophores are tethered to. These forces are, however, very small. Although long exposure times are used for significant deformation, such large deformation in a glassy medium is surprising. The SRG is the result of layer by layer plastic deformation of the polymer surface. Chromophore molecules in the viscous plasticized (by trans-cis-trans isomerization) surface layers are dragged by ``optical tweezer'' effect of the electric field gradient. Based on these investigations, possible mechanism for the formation of the SRGs is proposed.
2:30 PM J2.3
HIGH E-O COEFFICIENT POLYMERS BASED ON A CHROMOPHORE CONTAINING ISOPHORONE MOIETY FOR SECOND-ORDER NONLINEAR OPTICS. Jinghong Chen, Jingsong Zhu, Mingqian He, Larry R. Dalton, Department of Chemistry, University of Southern California, Los Angeles, CA; Sean Garner, Antao Chen, William H. Steier, Department of Electrical Engineering-Electrophysics, University of Southern California, Los Angeles, CA.
A high- chromophore APII utilizing isophorone as Pi-conjugation bridge was processed into both PMMA guest host and crosslinked(XL) polyurethane network with various loading densities. High electro-optic coefficients, r33 = 30 pm/V in PMMA and r33 = 27 pm/V at 1.06 mm were obtained. Alignment temporal stability ranged from 90 to 120C for XL polymer network. There is virtually no intrinsic absorption loss at 1.3 m (solution measurement). Also noteworthy is that high optimum loading densities of this chromophore are attainable in both cases without detectable chromophore aggregation due to intermolecular electrostatic interactions.
3:15 PM *J2.4
ELECTRO-OPTIC POLYMERS FOR DEVICE APPLICATIONS. Tony Kowalczyk, Andrew Zeng, Bill Bischel, Hilary Lackritz; Gemfire, Inc., Palo Alto, CA.
Polymers are seeing increasing use in passive and active waveguiding applications for integrated optics technology. Polymeric devices have advantages because of the ease of manufacture, low cost, and compatibility with other processing needs and components. This talk will concentrate on how to use polymers in IO applications to take advantage of materials compatibility issues. Low-cost laser diodes are enabling components of a number of inexpensive commercial products.
Unfortunately most diode lasers output TE-polarized light whereas many integrated optics applications require TM-polarized light. Therefore, an efficient compact, and integrated polarization rotator is necessary to permit coupling of TE-polarized laser diode array into a waveguide array and convert TE-polarized light to TM-polarized light. We have established the feasibility and proof-of principles underlying passive polymer polarization rotators by demonstrating extraordinarily high polarization conversion efficiency (97%) in a very compact device length. Polymer processing, materials characterization, and device design will be discussed.
3:45 PM J2.5
DESIGN OF SECOND ORDER NLO CHROMOPHORES FOR ELECTRO-OPTIC APPLICATIONS. S. Thayumanavan,a K. Staub,a J. Mendez,a A.K.Y. Jen,b L.R. Dalton,c S. Ermer,d B. Kippelen,e N. Peyghambarian,e S. R. Marder a,f aThe Beckman Institute, California Institute of Technology, Pasadena, CA; bDepartment of Chemistry, Northeastern University, Boston, MA; cLoker Hydrocarbon Institute, University of Southern California, Los Angeles, CA; d Lockheed Martin Research Lab, Palo Alto, CA; eOptical Sciences Center, University of Arizona, Tucson, AZ; fJet Propulsion Laboratory, California Institute of Technology, Pasadena, CA.
The main focus of our current work is to design and synthesize chromophores with high optical nonlinearities. The current design strategy should result in chromophores that are thermally and photochemically stable, soluble for procesibility, non-aggregating to reduce scattering loss, and appropriately functionalized for attachment to high performance polymers. We introduced bond length alternation (BLA) as a molecular parameter which correlates to the hyperpolarizability of a molecule. In addition to possessing increased ground state charge separation for an optimized BLA, the chromophores should be thermally and photochemically stable, and non-transparent. Therefore there is a need for the optimal design of the chromophores. With dialkylamino and diarylamino donors, cyanovinyls and phenylisoxazolone acceptors, and with thiophene or conformationally locked double bonds as the pi-system bridge, different chromophores are designed. The substitutions on the diphenylamino part, and the bridge system is designed to serve the purposes of increasing the donor strength, and solubility, while reducing aggregation. The chromophores will also bear appropriate functional groups for covalent attachment to a polymer back bone.
4:00 PM J2.6
MOLECULAR SECOND-ORDER OPTICAL NONLINEARITY OF PUSH-PULL BISDITHIOLENE NICKEL COMPLEXES. Chin-Ti Chen, Sish-Yuan Liao, Kuan-Jiuh Lin, Long-Li Lai, Academia Sinica, Institute of Chemistry, Taipei, TAIWAN.
We have investigated a series of novel square planar nickel complexes for molecular second-order optical nonlinearities. With donor and acceptor substituted bisdithiolene ligands, such complexes are analogs to organic chromophores Although with similar coordination structure, there are two types of the complexes in term of changes of dipole as well as the molecular first hyperpolarizabilities ('s). One type of the complexes shows a weak positive solvatochromic effect and small positive -value. The other, however, shows a strong negative solvatochromic effect and moderate to strong negative -value. Due to the broad and long wavelength (> 800 nm) absorption, -values of these push-pull nickel complexes were determined by solvatochromic method. The relationships among the bonding structure, change of dipole moment, and the sign of -values of the complexes will be discussed.
4:15 PM J2.7
EXCITON BANDWIDTH AND COUPLING TO INTRAMOLECULAR PHONONS IN PTCDA. M.H. Hennessy and Z.G. Soos, Princeton University, Dept. of Chemistry, Princeton, NJ; V. Bulovic and S.R. Forrest, Princeton University, Dept. of Electrical Engineering, Princeton, NJ.
PTCDA films have molecular stacks in closer than van der Waals contact and feature interesting absorption, emission, and photoconduction, including a CT band 0.2 eV below the Frenkel exciton's 0-0 line and a 0.6 eV red-shifted fluorescence relative to solution. Close PTCDA stacking implies a substantial exciton bandwidth, with k = 0 at the top, that has not been invoked in current assignments Resolved solution spectra indicate coupling to a 0.18 eV mode corresponding to out-of-phase C-C and C=C stretches; the same ag mode dominates polyene and polyacetylene spectra. Linear exciton-phonon coupling accounts for increased Franck-Condon displacement along ag in absorption (k = 0), decreased displacement in emission (k = ). The large in-plane PTCDA transition dipole yields a dipole derivative that enables phonon-assisted processes at k = . A one-dimensional band and coupling to the intramolecular ag mode provides consistent new PTCDA assignments for solution, films, and multiple quantum wells. The CT intensity is borrowed from the k = 0 exciton, the narrow CT band is within the exciton band, and CT mixing with k = excitons is weak due to similar transfer integrals for holes and electrons. Molecular PTCDA excitations reflect the D2h conjugation of perylene and are closely related to octatetraene spectra. The same connection emerges from a second -electron approximation for longer rylenes and pyrenes.
4:30 PM *J2.8
ADVANTAGES OF MODAL DISPERSION PHASE MATCHING AND MATERIAL REQUIREMENTS FOR DEVICES USING EFFICIENT SHG AT TELECOMMUNICATIONS WAVELENGTHS. Matthias Jager, Michael Canva, George Stegeman, Center for Research and Education in Optics and Lasers, University of Central Florida, Orlando, FL.
It would be of great importance to possess materials allowing strong Second Harmonic Generation (SHG) at telecommunication wavelengths. It is not an end in itself, but it is a prerequisite for numerous applications including devices based on frequency shifting for Wavelength Division Multiplexing (WDM), cascading (of (2) nonlinearities) for all optical signal processing, tunable frequency generation via optical parametric generation and amplification. Up to now, the best materials for this purpose were ferroelectric crystals such as LiNbO3, using Quasi-Phase Matching (QPM) techniques. We shall show that the latest results obtained with DANS or DRl doped poled polymers using Modal Dispersion Phase Matching (MDPM) techniques are now competitive in terms of figure of merit (0, in %/W cm2). The advantages of MDPM over QPM in the case of poled polymer devices will be discussed. To take full advantage of the polymer based device potential, it is now needed to decrease the propagation losses in the waveguide, especially at the second harmonic wavelength. This will allow us to achieve equal or better net SHG efficiency (, in %/W). Another way to improve the SHG is to increase the nonlinearity, d(2)eff,. These issues can be addressed both by using better chromophores and by improving the processing conditions, especially poling. The device designs may also be optimized with respect to the actual material possibilities.
SESSION J3: POSTER SESSION
Chair: Michael F. Rubner
Monday Evening, December 1, 1997
America Ballroom (W)
THE OXYGEN CHARGE TRANSFER COMPLEX AND ITS EFFECT ON THE ELECTRICAL, OPTICAL, AND LUMINESCENT PROPERTIES OF POLY(3-ALKYLTHIOPHENES). Mohamed S.A. Abdou, Francesco P. Orfino, Yongkeun Son, Steven Holdcroft, Simon Fraser University, Department of Chemistry, British Columbia, CANADA.
Molecular oxygen forms a reversible charge transfer complex (CTC) with poly(3-alkylthiophenes). The complex is weakly bound but possesses a distinct absorption band in the visible region. The CTC manifests itself as a fluorescence quencher of mobile polaronic excitons and it explains why oxygen quenches luminescence with much greater efficiency than predicted by a collisional quenching model. The electronic properties of the neutral polymer are modulated by the CTC. The extent of modulation of these properties is proportional to oxygen pressure, and is fully reversible. Analysis of poly(3-hexylthiophene) field effect transistors under increasing pressures of oxygen show that the carrier concentration increases, conductivity increases, and the charge carrier mobility is lowered by the formation of the charge transfer complex. Implications of the charge transfer complex on the photochemistry of poly(3-alkylthiophenes), photosensitization of singlet oxygen, and photobleaching are discussed.
POLY(PHENYLENEVINYLENE) GRAFTED WITH POLYIMINES AND POLYOLS CONTAINING COORDINATED TRANSITION METALS IN BLUE-GREEN LEDS WITH EXTENDED LIFETIMES. Iris Benjamin, Yair Avny, Department of Organic Chemistry, Dan Davidov, Racah Institute of Physics, Ronny Neumann, Casali Institute of Applied Chemistry, The Hebrew University of Jerusalem, Jerusalem, ISRAEL.
Grafted polymers of poly(phenylenevinylene) (PPV) were prepared by added polyimines and polyols to the PPV precursor polymer prior to formation of the conjugated polymer as outlined in the scheme.
PPV-PEI, X = NH
PPV-PEG, X = O
The photo- and electroluminescence spectra which were of similar intensity compared to PPV were blue shifted with a max between 475 nm for PPV-PEI and 490 nm for PPV-PEG. Further addition of metal salts, such as CuBr2, EuCl3 and TbCl3 to PPV-PEI at the polymer precursor stage yielded grafted PPV with transition metals. These polymers had identical luminescence spectra compared to the non-metal containing polymers, however, the electroluminescence intensity was increased twofold and the lifetime of a LED in a single layer configuration (ITO/PPV-PEI with complexed transition metal/Al) was increased seven fold. Incorporation of a transition metal without coordination yielded unchanged or inferior LEDs.
METAL COMPLEX POLYMERS FOR ELECTROLUMINESCENT APPLICATIONS. Xu-Tang Tao, Ya-Dong Zhang, Tatsuo Wada, Hiroyuki Sasabe, Biopolymer Physics Lab, Riken, JAPAN; Hironori Suzuki, Toshiuki Watanabe, Seizo Miyata, BASE, Tokyo Univ of Agriculture and Technology, JAPAN.
We report the synthesis and characterization of a set of soluble metal complex polymer for electroluminescent (EL) applications.ÝThese polymers were either prepared by a preformed metal complex or by the reaction of metal atom-donor atom coordination. One of such polymers was a polyurethane prepared by the reaction of a zinc Schiff base with 4,4'-diphenylmethane-diisocyanate. The polymer is amorphous and with glass transition temperature of 156ÝƒC and are soluble in common organic solvents such as chloroform, tetrahydrofuran (THF), and N-methylpyrrolidinone (NMP). The onset decomposition temperature of the polymer was above 300 ƒC. High optical quality films were obtained by spin-coating the polymer solution of THF. The zinc Schiff base, and the polyurethane show strong photoluminescence under a UV-lamp illumination. Single layer and bilayer devices containing a new polycarbazole as hole transfer layer have been fabricated and characterized. The results indicated that the complex polymer could act as both electron transport and emissive layers for EL devices. Complex polymers are of great interest in order to obtain material with high resistance to heat and oxidation or with new combinations of properties that are not encountered in the organic materials.
INVESTIGATIONS ON THE ALUMINUM/PARA-HEXAPHENYL INTERFACE IN LIGHT EMITTING DEVICES. Norbert Koch, Guenther Leising, Univ of Technology, Inst of Solid State Physics, Graz, AUSTRIA; Li-Ming Yu, Jean-Louis Guyaux, Jean-Jacques Pireaux, Univ Namur, LISE, Namur, BELGIUM; Yvon Morciaux, Guy Demortier, Univ Namur, LARN, Namur, BELGIUM.
Blue light emitting devices (LED) with para-hexaphenyl (PHP) as the active material and aluminum as cathode possess very high quantum efficiencies. To further optimize device performance it is crucial to understand the physical properties of the involved interfaces. We have performed Rutherford-backscattering experiments on actual devices to show the importance of oxygen in the interface formation at the cathode as this leads to the formation of a layer of aluminum-oxide between PHP and aluminum. In devices, where the organic film is exposed to air before the metal electrode is evaporated, an insulating layer on the metal-side therefore is inherent, but the thickness of the dielectric remains to be tuned to an optimum. It has been shown that the introduction of an intermediate layer between active material and electrodes results in a higher quantum efficiency of the LED, the most common concepts being charge-transport-layers, or insulators on the other hand. Our results underline the need for a better control of the LED processing. Ultraviolet- and X-ray photoelectron spectroscopy in situ growth studies of thin aluminum films on PHP have been made to reveal the change in the electronic structure of the active medium in a LED in the absence of oxygen. Also the direct interaction of oxygen with this organic material is investigated by photoelectron spectroscopy.
LUMINESCENCE IN POLYPARAPHENYLENE SUBJECTED TO DIFFERENT HEAT TREATMENT TEMPERATURES. S.D.M. Browna, M.J. Matthewsa, M.S. Dresselhausa,b, M. Endoc, T. Hiraokac. aDepartment of Physics, Massachusetts Institute of Technology, Cambridge, MA; bDepartment of Electrical Engineering and Computer Science, Massachusetts Institute of Technology, Cambridge, MA. cSchool of Engineering, Shinshu University, Nagano, JAPAN.
We investigated the structural and electronic properties of samples of polyparaphenylene (PPP) that have been subjected to different heat-treatment temperatures (=500C, 600C, 650C, 675C and 700C) as well as the polymer prior to heat-treatment (=0C). The samples of PPP were derived from two synthesis methods; the Kovacic method and the Yamamoto method. The photoluminescence (PL) spectrum of heat-treated PPP from the two methods indicates differences in electronic structure. The PL spectra from the =0C sample of PPP-Kovacic showed peaks in the blue-green located at 426nm, 453nm, 480nm and 515nm along with a large red luminescence band located at 668nm with a shoulder at 726nm. The intensity of the PL decreased for successively higher heat-treatment temperatures () but can still be observed at =675C. The PL spectra from the =0C sample of PPP-Yamamoto showed contributions in the blue-green located at 430nm, 455nm and 480nm along with a large contribution at 515nm. PL intensity from PPP-Yamamoto material can only be observed for up to 500C. The PL disappears at much lower in the heat-treated Yamamoto polymer than from the Kovacic material. Raman spectra taken of the heat-treated polymer indicated the presence of the phonon modes for PPP in samples at up to 650C, from both preparation methods. This indicates that delocalization of the -bond structure occurs at much lower in PPP-Yamamoto material due to the heat induced carbonization. The Yamamoto method causes PPP to polymerize in a less defective structure than for the Kovacic method. The broad red luminescence band from PPP-Kovacic has been attributed to defects in the polymer, such as macro-cycles, arising from the Kovacic preparation method. The present results are consistent with and reflect the high Li storage behavior characteristically observed in PPP-Kovacic based carbons heat-treated to 700C.
THERMAL CONVERSION OF POLY(P-PHENYLENE VINYLENE) PREPARED BY CHEMICAL VAPOR DEPOSITION. Ratna Shekhar, Kathleen M. Vaeth, Klavs F. Jensen, MIT Dept of Chemical Engineering, Cambridge, MA.
The fabrication of the conjugated polymer poly(p-phenylene vinylene) (PPV) based light emitting diodes has recently been realized. However, the optoelectronic qualities and the stability of conjugated polymers used in these devices have been shown to depend on the thermal conversion process of the respective prepolymers. A novel experimental set up has been designed to carry out the prepolymer conversion in a controlled UHV environment. The thermal conversion process of halogenated poly(1,4-xylylene), a precursor polymer deposited on a metal film via chemical vapor deposition technique, to the conjugated polymer poly(p-phenylene vinylene) (PPV) was studied under UHV conditions using in situ infra-red reflection absorption spectroscopy (IRRAS) and temperature programmed desorption (TPD) techniques. Windows of operation for thermal conversion of the prepolymer to PPV were identified by simultaneous monitoring of the volatile products with a quadrupole mass spectromete! r and the characteristic IR peak f or conjugation in the IRRA spectra. The chemical species produced during thermal conversion process between 90-190 !C was identified as HCl. Results obtained for different heating rates and the prepolymer film thickness provide insights into reaction-limited and mass-transfer limited regimes during the thermal conversion process. A theoretical model has been developed to explain the experimental observation under different conditions. The influence of process conditions on photoluminescence from the PPV film was characterized as well. In future studies, experimental results for thermal conversion process of the PPV prepolymer film obtained via other methods, such as spin-coating and self-assembly techniques, will be obtained.
INJECTION AND TRANSPORT OF CHARGE IN POLYMERIC ZINC BISQUINOLINE LIGHT EMITTING DIODES. Keith A. Higginson, D. Laurence Thomsen III, Fotios Papadimitrakopoulos, University of Connecticut, Storrs, CT.
Charge injection in single layer, self-assembled, polymeric zinc-bisquinoline light emitting diodes (LEDs) was studied by changing the height of the metal/semiconductor energy barrier. Electrons were determined to be the majority carriers, and light emission governed by the injection of holes. Cyclical current-voltage sweeps show reproducible doping-dedoping which result in a dramatic increase of current density flowing through these films. The nature of these trapped dopant (carriers) is presently under intense investigation. These carriers may be intrinsic (self doping) or extrinsic (doping impurities during self-assembly deposition) in nature. Cyclic voltammetry suggests a similar type of behavior indicating a strong stabilization of electrons on polymeric bisquinoline films.
FABRICATION OF POLYMER LIGHT EMITTING DIODES BY LAYER-BY-LAYER COMPLEXATION TECHNIQUE. Jaehyun Kim, Kethinni G. Chittibabu, Jayant Kumar and Sukant K. Tripathy, Center for Advanced Materials, Departments of Physics and Chemistry, University of Massachusetts, Lowell, MA.
Polymer Light Emitting Diodes (PLED's) are potential candidates in display applications. The efficiency of light emission can be improved by incorporating highly luminescent lanthanide metal ion complexes in PLED devices. Development of multilayers by depositing alternately rare earth metal ions and conjugated polymer derivatives using metal-ligand coordination bonding were examined in this investigation. We have fabricated multilayers of Europium (III) ions and thiophene based conjugated polymer, poly[2-(3thienyl)ethanol hydroxycarbonyl methyl urethane], via metal-organic ligands coordination bonding. Various factors, which affect the film thickness and uniformity are investigated. Growth of multilayers was monitored by UV/Visible spectrophotometer and spectrofluorometer. Current-voltage and luminescence-voltage characteristics of the fabricated devices will be reported.
LIGHT-EMITTING HETEROJUNCTION DEVICES OF CONJUGATED POLYMERS AND TRIS-CHELATED RU(II) COMPLEXES. Erika Abbas, Michael Durstock, Aiping Wu, Michael Rubner, MIT, Dept. of Materials Science and Engineering, Cambridge, MA; Jin-Kyu Lee, Erik Handy, MIT, Dept. of Chemistry, Cambridge, MA.
In previous work we have shown that device performance can be greatly enhanced through the formation of a heterojunction between layers containing poly(p-phenylene vinylene) and layers containing a phenanthroline-based ruthenium complex. The formation of this internal interface caused a ten-fold increase in device efficiency at luminances of approximately 100 cd/m2. In subsequent work we have examined other hole-transporting layers, such as poly(para-phenylene) and polyaniline, and other electrochemically active components, including a polyester and a polyurethane containing tris-bipyridyl Ru(II) segments. Devices using these materials show external quantum efficiencies greater than 2% and luminances of more than 150 cd/m2. Through choice of materials and control over the microstructure of these layers, deposited by spin-coating or by the sequential adsorption of polyelectrolytes, we have seen dramatic improvement in device performance. Details on the mechanisms of charge injection and transport in these films will be discussed.
LIGHT-EMITTING SEGMENTED POLYURETHANES BASED ON TRIS(BIPYRIDYL) RUTHENIUM(II), E.S. Handy, E.D. Abbas, M.F. Rubner, MIT, Cambridge, MA.
We report on the synthesis and electroluminescence (EL) of a new segmented polyurethane with hard segments derived from a tris(bipyridyl) ruthenium(II) species and hexamethylene diisocyanate. A poly(ethylene oxide) soft segment was chosen to facilitate ion transport during device operation. Submicron thin films of this material were spin-cast from pyridine solutions onto patterned ITO substrates, and an aluminum cathode thermally-evaporated on top. EL in excess of 100 cd/m2 was realized at 125-250 mA/cm2 and a 0.13% external quantum efficiency. These devices exhibited a time-dependent Œ¹charging¹¹ effect, suggesting that operation is electrochemical in origin. It remains to determine whether the delay in response time is electronic or ionic in nature. Structural modification of this material is readily performed to elucidate this mechanism. Relationships between hard and soft segment length, the effect of added salt, charging time, and emission characteristics will be discussed.
PHOTOVOLTAIC HETEROSTRUCTURE DEVICES MADE OF SEQUENTIALLY ADSORBED PPV AND C60. H. Mattoussia, X.L. Jiangb, J. Kumarb, S.K. Tripathyb, and M.F. Rubnera, aDepartment of Materials Science and Engineering, MIT, Cambridge, MA; bCenter for Advanced Materials, University of Massachusetts Lowell, Lowell, MA.
We present characterization of rectifying heterojunctions made of PPV and sulfonated C60. Both photocurrent and photovoltage were characterized as a function of the light power input, the incident wavelength, and the applied DC voltage. The heterojunctions were built from solution using the technique of layer-by-layer sequential adsorption; PPV was assembled from its charged soluble precursor form (a polycation) in conjunction with poly(acrylic acid), PAA, while sulfonated C60 was built layer-by-layer in conjunction with poly(allyl amine), PAH. The above technique permits one to control the heterojunction at the molecular scale. Upon illumination with laser light, the device shows a large photo-response that results from a photo-induced electron transfer across the interface between the PPV multilayers (donor) and the C60 multilayers (acceptor). The photocurrent was found to increase with the laser power and with the photon energy of the incident radiation. The photovoltage response, however, was found to saturate at about 700-800mV.
LIGHT EMITTING DEVICES BASED ON SEQUENTIALLY ADSORBED LAYERS OF POLY (P-PHENYLENE VINYLENE) (PPV) AND POLY (ACRYLIC ACID) (PAA). Michael F. Durstock, Michael F. Rubner, MIT, Department of Materials Science and Engineering, Cambridge, MA.
We have recently reported on the fabrication of organic light emitting devices based on sequentially adsorbed layers of a polycationic poly(p phenylene vinylene) (PPV) precursor and poly(methacrylic acid) (PMA). Here we have fabricated devices with PPV precursor and poly(acrylic acid) (PAA) in an effort to further improve device performance by controlling the nature of the polyanion with which the PPV precursor is assembled. We have seen dramatic differences in device performance by systematically varying the bilayer composition and the total film thickness by controlling the solution parameters and the total number of bilayers deposited. In addition, the conversion temperature has also been shown to strongly influence device characteristics. The current, best performing device has been for a system in which the bilayer thickness is around 60 , approximately half of which is due to the PPV. From this system we have been able to achieve luminance levels near 1000 cd/m2 using an aluminum cathode and an ITO anode. Such high brightness levels from a PPV single slab device with an aluminum top electrode are quite unusual.
OPTICAL PROPERTIES OF DISTYRYLBENZENE CHROMOPHORES AND THEIR SEGMENTED COPOLYMERS. Nicholas Benfaremo*, Daniel J. Sandman*, Sukant Tripathy*, Jayant Kumar, Ke Yang, Michael Rubner, Cormac Lyons, Center for Advanced Materials, U. Massachusetts Lowell, Lowell, MA; *Dept. of Chemistry, Dept. of Physics, Department of Materials Science, MIT, Cambridge, MA.
Electroluminescent devices based on poly(phenylene vinylene) (OPPV) have shown exceptional promise. The addition of poly(ethylene glycol) (PEG) to the PPV layer in light emitting electrochemical cells (LEC) markedly improves ionic conductivity and device efficiency, however, phase separation of the PPV and PEG materials contributes to overall device failure. In order to overcome this drawback, we have prepared segmented copolymers consisting of PPV and PEG subunits. End functionalized distyryl benzene chromophores were condensed with poly(ethylene glycol) either via a novel polymer analogous Mitsunobu reaction of by polycondensation with a PEG bis mesylate. The polymers were characterized by 1H and 13C NMR, IR, GPC and elemental analysis. The polymers have a very high photoluminescent quantum efficiency and when pumped with a 337 nm nitrogen laser (3 nsec pulse), a solution of the polymer exhibited laser action due to the amplified spontaneous emission. A thin films of the polymer also showed very strong fluorescence and experiments to establish evidence of stimulated emission when pumped in the UV are in progress.
POLYMER/POLYMER HETEROJUNCTION LIGHT-EMITTING DIODES WITH VOLTAGE-SWITCHABLE COLORS. Xuejun Zhang, Samson A. Jenekhe, Departments of Chemical Engineering and Chemistry, University of Rochester, Rochester, NY.
Light-emitting diodes fabricated from semiconducting polymer heterojunctions consisting of an electron-transport (n-type) polymer (polybenzobisthiazoles or polyquinolines) layer and a hole-transport (p-type) polymer (poly(p-phenylene vinylene)) layer are shown to exhibit reversible electroluminescence (EL) color changes with applied voltage. This class of heterojunction LEDs is exemplified by the device Al/poly(benzobisthiazole-1,4-phenylenebisvinylene)/PPV/ITO which switches reversibly from green at low voltage (4-17 V) to orange/red at higher voltage (>17 V). Because the voltage-switching of EL colors is only observed when the heterojunction layer thicknesses are less than 60-100 nm, it is a finite size effect. The turn-on voltage was as low as 4-5 V and efficiency and luminance were enhanced by about 20 times compared to single-layer devices. The observed finite size effects in these nanoscale heterojunction light sources are related to electric field-dependent charge transport and trapping processes in the materials. The results demonstrate that the electron-hole recombination and emission zone within polymer heterojunctions, and hence colors from the light-emitting diodes, can be regulated by controlling the relative thickness of the constituent p-type and n-type layers. In this way, heterojunction LEDs can be designed to exhibit one-color emission from either the p-type layer or the n-type layer, or switchable or multicolor emission from both layers. Light sources with voltage switchable colors offered by polymer heterojunctions are promising for a range of applications including multicolor pixels for full-color displays.
TOWARD ELECTRICALLY PUMPED ORGANIC DIODE LASERS: ELECTROLUMINESCENCE OF PROTON TRANSFER POLYMERS. Xuejun Zhang, Samson A. Jenekhe, Richard M. Tarkka, Departments of Chemical Engineering and Chemistry, University of Rochester, Rochester, NY; Joseph B. Schlenoff, Department of Chemistry, The Florida State University, Tallahassee, FL.
Organic molecules with intramolecular hydrogen-bonding which exhibit excited state intramolecular proton transfer (ESIPT) have previously been demonstrated as efficient laser dyes for four-level optically pumped lasers. We demonstrate here that such materials can also give rise to proton transfer electroluminescence which is a promising approach to electrically pumped organic diode lasers. Light-emitting diodes were fabricated using indium-tin-oxide as the hole injecting electrode, poly(vinyl carbazole) as the hole-transport layer, proton transfer polymers as the emissive layer, and aluminum as the electron injecting electrode. A series of new proton transfer copolymers which contain ESIPT moieties in the main chain, such as poly(1,4-(2-hydroxy)phenylene benzobisthiazole-co-decamethylene benzobisthiazole) (HPBT-co-PBTC10) or on the side chain, such as poly(styrene-co-3-hydroxy-4-ethenylflavone) (PS-co-V3HF), and homopolymers in which conjugation was disrupted by meta linkage or sp3 carbon, such as poly(2,5-benzoxazolediyl(2,2,2,-trifluoro-1-(trifluoromethyl)ethylidene)-5,2-benzoxazolediyl-4-hydroxy-1,3-phenylene) (mH6FPBO) were evaluated. The turn-on voltages of these devices were 10-12 V, and resulting in green color electroluminescence with peaks at 505-540 nm.
8:15 AM *J4.1
ELECTRONIC STRUCTURE AND OPTICAL RESPONSE OF ELECTRO-LUMINESCENT CONJUGATED POLYMERS. J.L. Breadas, J. Cornil, D. Beljonne, D.A. dos Santos, and Z. Shuai, Service de Chimie des Matériaux Nouveaux, Centre de Recherche en Electronique et Photonique Moléculaires, Université de Mons-Hainaut, Mons, BELGIUM.
We first discuss simulations of the absorption and emission spectra of long conjugated PPV oligomers, carried out by means of highly correlated quantum-chemical calculations. A detailed analysis of nature of the excited states associated to the optical excitations reveal their different origins, arising from combinations of localized and/or delocalized SymbolpTimes-molecular orbitals of the backbone. Comparison between the experimental measurements (performed by the Cambridge group) and the theoretical data indicates that there is a one-to-one correspondance between the absorption and photocurrent spectra: large amplitudes in photocurrent are correlated to a charge-transfer character of the corresponding excited states. These results have general implications: they provide a conceptual bridge between the descriptions of electronic excitations in molecular solids and semiconductors (where they are traditionally depicted in terms of electron-hole pairs (excitons) and free carriers, respectively); a proper description of conjugated polymers requires to rely on a proper combination of both the molecular picture and the band picture. Several experimental studies probing the photoluminescence properties of conjugated chains in condensed media have isolated various manifestations of intermolecular interactions. In a second part, we report on correlated quantum-chemical calculations on interacting conjugated systems. The results allow us to gain insight into the way interchain couplings affect the electronic and optical properties with respect to single chains, and hence the absorption and emission characteristics. This work is partly supported by the European Commission (TMR Network SELOA), the Ministère de la Région Wallonne, and the Belgian Federal Government PAI Program.
8:45 AM *J4.2
ELECTROACTIVE POLYMERS: APPLICATION IN SENSORS FOR VOLATILE ORGANIC COMPOUNDS AND IN NOVEL LED's. A.G. MacDiarmid, W.J. Zhang and F. Huang, Univ. of Pennsylvania, Dept. of Chemistry, Philadelphia, PA.
Interdigitated gold arrays coated with a thin film of soluble polypyrrole doped with dodecylbenzene sulfonic acid, ``DBSA'' or octaaniline doped with DBSA serve as excellent reversible sensors for the detection of selected hydrocarbons and their derivatives. Sensitivity was measured as the % change in resistance, (R) defined as R=[(R Ro)/Rox100]% where Ro is the initial resistance of the sensor and R is the resistance in the presence of analyte vapor. The polypyrrole sensor gave a R 250% when placed in static air saturated with toluene vapor at room temperature, for 2 minutes. The corresponding value of R for an octaaniline sensor was 3,000%. A similar response was obtained after four weeks intermittent testing and storage in air. Modifications have resulted in R values > 30,000% under the same experimental conditions. The performance of polymer LEDs having the configuration Al/MEHPPV/ITO (where MEH -PPV=poly(2-methoxy-5-(2'-ethyl-hexoxy)-1,4-phenylenevinylene) have been improved by light iodine doping of the emissive MEH-PPV polymer layer whereby the turn-on voltage is reduced and the external quantum efficiency is increased by an order of magnitude. It differs from non doped MEH-PPV LEDs in that light emission is observed in both forward and reverse bias modes. LEDs having the configuration Al/MEH PPV/EB/ITO where EB=emeraldine base, (the non-doped form of polyaniline), exhibited a slow increase in intensity of emitted light at an applied forward bias of 32 volts during 30 minutes. This is believed to possibly result from an electrochemical process.
9:15 AM *J4.3
TRANSITION METAL-CONDUCTING POLYMER HYBRID MATERIALS. Timothy M. Swager*, S. Sherry Zhu, Richard P. Kingsborough, Department of Chemistry, Massachusetts Institute of Technology, Cambridge, MA.
Conducting polymers with active transition metal functionality offer new opportunities for the formation of sensors, catalysts, and novel conductors. This potential is largely due to the ability of transition metals to exhibit multiple stable oxidation states under ambient conditions. To harness this potential we have designed materials that intimately incorporate redox active transition metals into the conjugated polymerís structure. By tailoring the redox potential of the organic polymers to match that of the transition metal, we have produced materials with high conductivity. The metalí s contribution to the conductivity imparts special characteristics to the polymers. In particular interactions of the metal centers with small molecules and ions creates a chemoresistive response that enables the formation of sensory devices. The rapid transport of electrons in these materials also opens up possibilities for efficient catalysis of multiple electron redox processes.
9:45 AM J4.4
LOW BAND GAP CONJUGATED POLYMERS AS HIGH CONTRAST ELECTROCHROMIC MATERIALS. John R. Reynolds, David J. Irvin, Anil Kumar, Mark C. Morvant, Jerry L. Reddinger, Gregory A. Sotzing, Christopher A. Thomas, and Dean M. Welsh, Department of Chemistry, Center for Macromolecular Science and Engineering, University of Florida, Gainesville, FL.
Fully conjugated polymers of designed electron density and overlap, with electronic band gaps (Eg) that range from 1.15 eV to 3.0 eV (1100 to 410 nm), have been synthesized. Low gap polymers are of special interest as they are cathodically coloring electrochromic materials and can often be both p- and n-type doped. During oxidative doping, the electronic absorption is transferred from the visible region to the near infrared making the materials highly transmissive. Alkyl and aryl substituted poly(3,4-ethylenedioxythiophene) [poly(EDOT)] derivatives with Eg values of 1.7 eV show repression of the 1.2 eV intermediate bipolaron absorption, enhancing the electrochromic contrast relative to the parent poly(EDOT). These substituted polymers show no distinct electronic absorption processes between 1.25 eV (1000 nm) and 4.0 eV (310 nm). Multi-aromatic ring and vinylene linked monomers allow for band gap control throughout the entire visible, and a portion of the near-lR, region. Cyanovinylene linked polymers have been synthesized for band gap reduction. For example, the EDOT/thiophene derivative, poly[l-cyano-1-(2-thienyl)-2-(2 (3,4-EDOT))vinylene], exhibits an Eg at 1.25 eV while the bis-EDOT derivative, poly[1-cyano-1,2-bis(2-(3,4EDOT))vinylene], has its Eg lowered further to 1.15 eV. Band gap reduction can also be effected in fluorinated derivatives as exemplified by comparison of poly[1,4-bis(2 (3,4-EDOT))benzene] (Eg = 1.8 eV) and poly[1,4-bis(2-(3,4EDOT)) 2,3,5,6-tetrafluorobenzene] (Eg = 1.5 eV). The combination of electron rich (donor) and electron poor (acceptor) units adjacent to one another in a conjugated chain is further being studied as a means for band gap reduction and stabilization of n-doped states.
10:15 AM *J4.5
AN EMULSION POLYMERIZATION PROCESS FOR ORGANICALLY SOLUBLE AND ELECTRICALLY CONDUCTING POLYANILINE. Patrick J. Kinlen, Yiwei Ding, Charles R. Graham, Jingyue Liu and Edward E. Remsen, Monsanto Company, St. Louis, MO.
A new emulsion process has been discovered for the direct synthesis of the emeraldine salt of polyaniline (PANI) that is soluble in organic solvents. The process entails forming an emulsion composed of water, a water soluble organic solvent (e.g., 2-butoxyethanol), a water insoluble organic acid (e.g., dinonylnaphthalene sulfonic acid) and aniline. Aniline is protonated by the organic acid to form a salt which partitions into the organic phase. As oxidant (ammonium peroxydisulfate) is added, PANI salt forms in the organic phase and remains soluble. As the reaction proceeds, the reaction mixture changes from an emulsion to a two phase system, the soluble PANI remaining in the organic phase. With dinonylnaphthalene sulfonic acid (DNNSA) as the organic acid, the resulting product is truly soluble in organic solvents such as xylene and toluene(not a dispersion), of high molecular weight (Mw >22,000), film forming and miscible with many polymers such as polyurethanes, epoxies and phenoxy resins. As cast, the polyaniline film is only moderately conductive (10(-5) S/cm), however treatment of the film with surfactants such as benzyltriethylammonium chloride (BTEAC) or low molecular weight alcohols and ketones such as methanol and acetone increases the conductivity 3-4 orders of magnitude.
10:45 AM J4.6
DEVELOPMENT OF NOVEL PHOTOCHROMIC AND AFFINITYCHROMIC POLYTHIOPHENE DERIVATIVES. Mario Leclerc, Isabelle Levesque, Karim Faid, University of Montreal, Department of Chemistry, Montreal, PQ, Canada.
Conjugated polymers are now well-known as highly conducting materials in the doped (oxidized, reduced or protonated) state. The delocalized electronic structure of these polymers is, in part, responsible for the stabilization of the different charge carriers created upon doping. In most cases, the conjugated electronic structure involves also a strong absorption in the UV-visible range. The coupling between the UV-visible absorption spectrum of neutral conjugated polymers and the conformation of the backbone has recently led to the development of thermochromic and solvatochromic materials. In particular, regioregular poly(3-alkoxy-4-methylthiophene)s have shown interesting and reversible violet-to-yellow color changes (associated to a modification of the conformation of the main chain)which seems driven by order-disorder transition within the side chains. Consequently, it is believed that various external stimuli can generate a conformational transition within the side chains. In this respect, we have recently prepared different polythiophene derivatives bearing photo-isomerizable or binding moieties which have led to novel photochromic or affinitychromic polymers. This work has also revealed an amplification of the detection mechanism which is particularly promising for the development of biosensors. Different examples of applications will also be discussed.
11:00 AM J4.7
MODULATION OF THE ENERGY GAP IN POLYDITHIENOTHIOPHENE MATERIALS. Marinella Catellani, Silvia Luzzati, Ist. Chimica delle Macromolecole-MITER, CNR, Milano, ITALY; Catia Arbizzani, Dip. Chimica, Univ. Bologna, ITALY; Marina Mastragostino, Dip. Chimica Fisica, Univ. Palermo, ITALY.
Organic conjugated materials with narrow band gap has been receiving a considerable attention stimulated by the capacity of both n- and p- doping, along with a suitable transparency in the doped state. This opens the possibility of application in electrochemical devices such as electrochromics and symmetrical redox supercapacitors. Using fused heteroaromatic rings as monomers we have prepared a series of polydithienothiophenes with band gaps of 1.1 - 1.2 eV, stable in both n- and p- doped state. The co-polymerisation of two dithienothiophene isomers in well defined ratios leads to materials with different electronic properties depending from the chemical composition of the chain. It is possible to modulate continuously the band gap of the copolymer from 1.8 eV to 1.1 eV by electrochemical copolymerisation. The synthesis of both dithienothiophene polymers and copolymers will be presented along with its optical and electrochemical characterisation.
11:15 AM J4.8
DESIGN AND SYNTHESIS OF NEW CONJUGATED PORPHYRIN COPOLYMERS FOR OPTICAL-ELECTRONIC APPLICATIONS. Biwang Jiang, Wayne E. Jones Jr., Department of Chemistry and Institute of Material Research, State University of New York at Binghamton, Binghamton, NY.
A series of structure-defined conjugated porphyrin polymers have been prepared in which the porphyrin units are connected with a variety of diethynylaryl bridges. The photophysical and electronic properties can be tuned through the structural modification of the synthetically accessible aryl bridge units due to the strong electronic communication between the porphyrin and bridge units. These materials have electronic absorptions ill near IR region (780 nm) and are expected to have large third-order optical nonlinearity Fluorescence dynamics in solution and the solid state also provide insight into energy transfer and electronic coupling. The solubility, thermal stability, and tunable electronic properties of these novel conjugated porphyrin copolymers make them especially useful for optical-electronic devices, solar energy conversion materials, and sensors.
11:30 AM J4.9
STRUCTURAL APPROACH TO IMPROVE THE RESPONSE CHARACTERISTICS OF COPPER PHTHALOCYANINE THIN FILM-BASED NO2 GAS SENSOR. Tadashi Nagasawa, Graduate School of Electronic Science and Technology; Kenji Murakami and Kenzo Watanabe, Research Institute of Electronics, Shizuoka University, Shizuoka, JAPAN.
Copper phthalocyanine (CuPc) is a p-type organic semiconductor with high thermal and chemical stability. It has been applied to gas sensors because its electrical conductivity is very sensitive to the adsorption of oxidizing gases. The conductivity of the CuPc film, being sensitive to a variation in the mobility of -electrons as a charge carrier, can be influenced by the structure of CuPc molecules in the film. We have already reported that the gas sensitivity and the electrical conductivity of the -form CuPc thin films sublimed on the glass substrate in vacuum are strongly influenced by the film microstructure and morphology. This paper focused on a novel attempt to improve the response characteristics of the -CuPc film-based NO2 gas sensor through a modification of the film microstructure. Firstly, the gas sensitivity is remarkably increased by a insertion of higher-sensitive layer (vanadil Pc film) between the film and the substrate in a low gas concentration range. Secondly, a reversibility in cycles of gas doping and dedoping is completely improved by the film deposition on the hydrofluoric acid treated substrate. The detailed microstructure is discussed based on the investigations with an AFM.
11:45 AM J4.10
UNIMOLECULAR RECTIFICATION IN HEXADECYLQUINOLINIUM TRICYANOQUINODIMETHANIDE. Robert M. Metzger, Bo Chen, Dominique Vuillaume, Ulf Hopfner, Jeffrey W. Baldwin, M. V. Lakshmikantham, and M. P. Cava, Dept. of Chemistry, University of Alabama, Tuscaloosa, AL.
Hexadecylquinolinium tricyanoquinodimethanide, HDQ-3CNQ, was first studied by Geoffrey J. Ashwell and J. Roy Sambles, who saw rectification in Ag | Mg | Langmuir-Blodgett (LB) multilayer | Ag sandwiches. We have improved its synthesis, measured its dipole moment (43 Debyes), and found that even a monolayer of HDQ-3CNQ rectifies, either as a macroscopic Al | monolayer of HDQ-3CNQ| Al sandwich, or by scanning tunneling spectroscopy on an HOPG surface. This may be the world's smallest electronic device.
12:00 NOON J4.11
SOLVATO-CONTROLLED DOPING OF CONDUCTING POLYMERS. Mikhail Y. Lebedev, Michael V. Lauritzen, Steven Holdcroft, Department of Chemistry, Simon Fraser University, British Columbia, CANADA.
Silver (I) is employed to dope poly(3-alkylthiophenes) (P3ATs) and other conjugated polymers. Solutions containing silver (I) salts and P3ATs, which usually undergo spontaneous reaction, doping, and precipitation, were stabilized using coordinating ligands which modify the reduction potential of the Ag(I) species. Upon removal of the coordinating ligand, spontaneous oxidative doping of the polymer occurrs. Uniform electronically conducting films were thus obtained by simultaneous evaporation of both solvent and Ag(I)-coordinating ligand - a process termed ìsolvato-controlled dopingî. Doping was observed with a variety of silver (I) salts consisting of relatively electronegative counter ions, whereas a series of poly(3-alkylthiophenes) possessing increasing head-to-head dyad content showed a decreasing tendency to undergo Ag(I)-doping due to the increasingly positive oxidation potential of the polymer. Solvato-controlled doping of polyaniline and poly(3-methoxythiophene) with Ag(I) is also described, thus demonstrating the applicability of this approach to other conducting polymer systems.
SESSION J5: SUPRAMOLECULAR ENGINEERING
Chair: Michael F. Rubner
Tuesday Afternoon, December 2, 1997
Salon G (M)
1:30 PM *J5.1
MOLECULAR SELF-ASSEMBLY APPROACHES TO ORGANIC LED STRUCTURES. T.J. Marks, H. Chou, W. Ma, Dept of Chem and Mats Res Center, Northwestern Univ, Evanston, IL; N. Peyghambarian, B. Kippelen, S. Shaheen, N. Armstrong, Optical Sciences Center, U. of Arizona, Tucson, AZ.
Molecular self-assembly offers the prospect of constructing optically-functional organic multilayers composed of tailored building blocks deposited with self-sealing conformal coverage and Angstrom-level precision. We report here an approach to organic LED structures in which the hole transport, emissive, and electron transport layers are assembled into robust thin films using siloxane-based molecular self-assembly techniques. The film microstructures are characterized by a battery of spectroscopic and microscopic techniques. Characteristics of the resulting self-assembled devices include ultra-thin layers, high brightnesses, and low driving voltages.
2:00 PM *J5.2
SUPRAMOLECULAR MATERIALS AND THEIR NOVEL PROPERTIES THROUGH SELF ORGANIZING MOLECULES. Samuel I. Stupp, Gregory N. Tew, Martin U. Pralle, Departments of Materials Science and Engineering and Chemistry, Beckman Institute for AdvancedScience and Technology, Materials Research Laboratory, University of Illinois at Urbana-Champaign, Urbana, IL.
The ability to create regularly shaped nanoscale objects which serve as the building blocks of supramolecular materials is an extremely important goal in materials science. We report on a general class of supramolecular materials created by self organizing triblock molecules that form nanostructures lacking a center of invasion. The hierarchical organization of these objects could offer materials with defined nanopores, chemically and topographically defined surfaces, as well as with other interesting properties such as second order nonlinear optical response, piezoelectricity, pyroelectricity, and strong luminescence. The supramolecular materials formed by these molecules have been characterized by a number of techniques including; transmission electron microscopy, second harmonic generation, small angle x-ray scattering, and photoluminescence. These supramolecular unites organize spontaneously into films thousands of layers thick. Interestingly, second harmonic generation measurements demonstrate the stacking of nanostructures in the films is polar for several chemical structures.
2:30 PM J5.3
MOLECULAR SELF ASSEMBLY AS A ROUTE TO STRUCTURED POLYION LAYER-BY-LAYER FILMS. Sarah L. Clark, David DeWitt, and Paula T. Hammond*, Massachusetts Institute of Technology, Dept. of Chemical Engineering, Cambridge, MA.
In previous studies, we have demonstrated that self assembled monolayers (SAMs) patterned onto a surface can act as a template for the selective adsorption of polyions. Micron sized features can be formed using this in-situ approach to patterning ultrathin films. Surface selectivity of the polymer adsorption can be critically altered by varying adsorption conditions such as pH or ionic content. Further variations in the chemical nature of the patterned SAM may also be used as a means of controlling the amount of deposition of a specific polymer onto the surface region. We will discuss these variables and the viability of forming more complex microstructures by manipulating process conditions. The ability to mechanize this patterning process, and the ultimate limit of selective adsorption in terms of number of layers and resolution will also be addressed. Finally, new investigations on the adsorption of side chain liquid crystalline systems and other rigid and semi-rigid organic systems have been initiated. The control of order within the polyion layers will be addressed for these systems, and the adsorption of rigid molecules onto various SAMs surfaces will be presented.
3:15 PM J5.4
GLUING MOLECULES TO SEMICONDUCTORS; A ROUTE TO MOLECULE-CONTROLLED DEVICES. David Cahen, Stephane Bastide, Rami Cohen, Ayelet Vilan, Dept of Materials and Interfaces, Weizmann Institute of Science; Konstantin Gartsman, Dept of Chemical Services, Weizmann Institute of Science; Abraham Shanzer, Jacqueline Libman, Dept of Organic Chemistry, Weizmann Institute of Science; Ron Naaman, Rachel Ussishkin, Alex Kadyshevitch, Dept of Chem. Physics, Weizmann Institute of Science, Rehovot ISRAEL.
The surface properties of a semiconductor dictate many of the electronic characteristics of optoelectronic devices. Indeed, surface treatments form the basis for much of today's electronics. Because control over the semiconductor surface permits fine-tuning of the resulting materials properties, controlled adsorption of organic compounds on semiconductors presents an attractive hybrid approach to molecule-based devices. The structural and functional versatility and flexibility of organics allows incorporation of one or more properties, that can be varied systematically. Ideally electronic control requires control over the electron affinity, band bending and surface recombination characteristics. Optical control comes about by coupling the optical absorption and/or emission properties of the molecules to the semiconductor. Recent work by us and others covers several families of organic compounds and types of semiconductors (II-VI, III-V and IV). We show that the semiconductor work function may be tuned by adsorbing just a monolayer of molecules that possess a surface binding group as well as auxiliary groups that change the molecules' dipole moment (to tune electron affinity) and/or LUMO energy (to tune band bending). Modifying band bending depends on the donor/acceptor character of the auxiliary group (LUMO levels) relative to that of the semiconductor (Fermi level). Use of multifunctional molecules allows to add additional properties, such as hydrophobicity (for molecular protection). Use of chromophores adds optical adressability and recognition. Interactions of the molecules with the semiconductor surface (coverage and mode of binding) are analyzed by FT-IR. Surface modifications of the work function are measured by Kelvin probe technique and by adsorption on field effect transistors (FET's). The relevance to devices is seen in results on CuIn(Ga)Se2/CdS heterojunctions and on GaAs FET's and High Electron Mobility Transistors. This hybrid approach transforms the latter in versatile sensing devices.
3:30 PM *J5.5
QUANTUM-WELL HETEROSTRUCTURES IN SEMICONDUCTING POLYMERS. Samson A. Jenekhe, X. Linda Chen, Departments of Chemical Engineering and Chemistry, University of Rochester, Rochester, New York.
Semiconducting polymer quantum-well heterostructures such as multiple quantum wells, quantum wires, quantum boxes, and superlattices are of fundamental interest for exploring spatial confinement effects on electronic, optoelectronic, and optical phenomena. Such low dimensional organic semiconductors are also expected to exhibit enhanced or novel electronic and optical properties that could significantly improve thin film transistors, light emitting diodes, photodetectors, and lasers made from semiconducting polymers in ways analogous to low dimensional inorganic semiconductors. We have successfully prepared semiconducting polymer heterostructures, which exhibit strong two- and three-dimensional quantum confinement of excitons, through self-organization of binary blends of a block copolymer and its parent homopolymer. A huge number (ca. 1014 to 1019/cm3) of organic quantum wires and boxes with extremely uniform sizes and ultrasmall confinement volumes (1-2 nm3) are obtained. Among the quantum size effects observed in thin films at room temperature include: new optical transitions; absorption and emission spectral narrowing; enhanced luminescence efficiency; enhanced exciton lifetime; exciton stability in high electric fields; efficient energy transfer; efficient stimulated emission. Other aspects of electronic localization phenomena in semiconducting polymers, including charge confinement and cooperative intramolecular interactions are also being explored through various block conjugated copolymer architectures. The implication of semiconducting polymer quantum-well nanostructures for light emitting diodes, lasers, information storage, and molecular electronic devices will also be discussed.
4:00 PM J5.6
SELF-ASSEMBLED HETEROCYCLIC CHROMOPHORE MULTILAYERS AS SECOND-ORDER NONLINEAR OPTICAL MATERIALS. Wenbin Lin, Joshua Malinsky, George K. Wong, Tobin J. Marks, Department of Chemistry and the Materials Research Center, Northwestern University, Evanston, IL.
Organic materials capable of manipulating photonic signals have been actively sought because of their applications in optical communications, optical computing, and data storage. In particular, there is great current interest in the synthesis of organic second-order nonlinear optical (NLO) materials owing to their advantages over traditional inorganic crystals. This group has successfully developed a self-assembly approach to the synthesis of intrinsically acentric, high-hyperpolarizability chromophoric multilayers via sequential deposition of a silane coupling layer, a chromophoric layer, and a siloxane capping layer. In this contribution we will present our recent results on the synthesis of high-hyperpolarizability heterocyclic chromophores based on benzothiazole- and thienyl-linkages, and their incorporation into self-assembled multilayers. The theoretical and experimental molecular hyperpolarizabilities (determined by harmonic light scattering) of the chromophores will be correlated with the second-order NLO susceptibilities of the self-assembled multilayers. Also presented will be the microstrutural characterization of these self-assembled chromophoric multilayers using UV-Vis spectroscopy, ellipsometry, second harmonic generation, X-ray photoelectron spectroscopy, and X-ray reflectivity measurements.
4:15 PM J5.7
SELF-ASSEMBLED HETEROSTRUCTURES BASED ON CONJUGATED POLYMERS: STRUCTURAL AND OPTICAL STUDIES. D. Davidov, M. Tarabia, H. Hong, Racah Institute of Physics, R. Neumann, Casali Institute of Applied Chemistry and Y. Avny, Department of Organic Chemistry, The Hebrew University of Jerusalem, ISRAEL; S. Kirstein, Max-Planck-Institute for Colloids and Interfaces, Berlin, GERMANY; and R. Steitz, Berlin Neutron Scattering Center, Hahn-Meitner-Institut, GERMANY.
We describe studies of modulated heterostructures consisting of alternate layers of conjugated and non-conjugates polymers with emphasis on neutron and X-ray reflectivity. The heterostructures were prepared by the layer-by-layer self-assembly technique using the precursor of poly(phenylenevinylene) (D-PPV), and other polyelectrolyte spacers. Heat treatment after the layer assembly converted the pre-D-PPV to a conjugated semiconducting polymer. We are able to observe quasi-Bragg reflections (up to the third order) due to the formation of ordered modulated structures. The fitting procedure yields the buried inter-layer roughness at the D-PPV/spacer interface. This roughness parameter, of the order of 123, was found to be smaller than the thickness of the D-PPV and the spacer layers, suggesting that the length over which interdigitation between neighboring polymer layers occurs is significantly smaller than the spacer. The fabrication of high quality modulated structures with controlled layer thickness and relatively small interfacial roughness may be a first step towards polymer-based multi quantum-wells analogous to such devices in inorganic heterostructures. We demonstrate the fabrication of efficient light emitting diodes (LEDs) based on these heterostructures with emission in the blue due to the confinement of the active polymer layer.
4:30 PM J5.8
SELF-ASSEMBLED MULTILAYERS AND PHOTOLUMINESCENCE PROPERTIES OF A NEW WATER SOLUBLE POLY(PARA-PHENYLENE). Xiaobo Shi, DeQuan Li, Los Alamos National Laboratory, Los Alamos, NM.
A new water-soluble poly(para-phenylene) (PPP) was synthesized through the sulfonation reaction of poly(p-quarterphenylene-2,2'-dicarboxylic acid). The incorporation of sulfonate groups has dramatically improved PPP's solubility in water at a wide pH range, whereas previous PPP is only slightly soluble in basic solutions. Dilute aqueous solutions of this polymer with acidic, neutral or basic pH emit brilliant blue light while irradiated with UV light. The sulfonated PPP emits from 350 nm to 455 nm with maximum intensity at 380 nm. Self-assembled multilayers of this sulfonated PPP were constructed with a positively charged polymer (poly(diallyl dimethyl ammonium chloride) and characterized with various surface analyses. Both conductive (RuO2) and non-conductive (SiO2) substrates were used in the preparation of self-assembled multilayers. Electrical and optical properties of these novel self-assembled thin films will be discussed.
4:45 PM J5.9
MICRODOMAIN SURFACE ANALYSIS BY SCANNING PROBE MICROSCOPIES OF MULTIBLOCK COPOLYMERS INCORPORATING A CONJUGATED POLYMER SEGMENT. Ph. Leclere, R. Lazzaroni, J.L. Bredas, Service de Chimie des Materiaux Nouveaux, Universite de Mons-Hainaut, Mons, BELGIUM; B. Francois, C.N.R.S., FUPPA-FLRMP, Pau, FRANCE.
A number of strategies for nanostructure fabrication are currently being developed on the basis of self-assembly of organic materials, block copolymers, for optoelectronics devices and nanotechnologies. It has been widely recognized that, in such compounds, there exists a close relationship between the physical properties and morphology. Moreover, the block copolymer specific chemical, electrical, optical, or mechanical properties can be controlled via the choice of the constituent polymers. In this work, we have applied Scanning Probe Microscopies (AFM with Phase Detection Imaging and Electrostatic Force Microscopy) to study the topography as well as the viscoelastic and electrical properties of thin film surfaces of di- and triblock copolymers. The blocks are made from thermoplastic polymer segments (poly(styrene), poly(methylmethacrylate)) and from a doped conjugated polymer segment (poly(paraphenylene)). Periodically ordered nanometer-size domain structures are observed and the corresponding morphology is analyzed by adapted image processing methods. This work is partly supported by the European Commission (TMR Network SELOA), the Ministere de la Region Wallonne, and the Belgian Federal Governmemt PAI Program.
SESSION J6: ORGANIC METALS/MAGNETIC MATERIALS
Chair: Ronald L. Elsenbaumer
Tuesday Evening, December 2, 1997
Salon H/I (M)
8:00 PM *J6.1
LANGMUIR-BLODGETT ANALOGS OF LAYERED TRANSITION METAL PHOSPHONATES: MAGNETIC MONLAYERS AND "DUAL-NETWORK" ASSEMBLIES. Daniel R. Talham, Gail E. Fanucci, Melissa A. Petruska, Candace T. Seip, Department of Chemistry , University of Florida, Gainesville, FL; Garrett E. Granroth, Mark W. Meisel, Department of Physics, University of Florida, Gainesville, FL.
The Langmuir-Blodgett (LB) method is perhaps the oldest technique to afford molecular level control over organic solid-state assemblies. However, many potential applications have not been realized because of the metastable nature of the layered organic films. In particular, the demonstration of physical properties that require long-range structural order, such as superconductivity or magnetic order, has been elusive. We have recently developed a method for introducing long-range two-dimensional structural order into LB films. By taking advantage of the inorganic lattice energy of known solid-state layered structures, we have incorporated inorganic extended solid structures into the hydrophilic portion of LB assemblies. In addition to adding structural order to the film, this approach provides the opportunity to introduce physical properties normally associated with the inorganic solid-state. As a demonstration of this concept, we recently observed weak ferromagnetism in an LB film of manganese octadecylphosphonate, the first example of cooperative magnetic ordering in an LB film. In this presentation a series of Langmuir-Blodgett film analogs of solid-state transition metal organophosphonates will be described. Systems that include functionalized organic molecules in the metal phosphonate LB films will be also presented. The objective is to prepare ``dual network'' assemblies, where both the organic and inorganic components add function to the LB films. Phosphonic acids containing phenyl, biphenyl, azobenzene and tetrathiafulvalene groups have been synthesized, and metal phosphonate LB films with both divalent and tetravalent metals have been prepared and characterized using FTIR, XPS, X-ray diffraction, AFM and optical spectroscopy.
8:30 PM J6.2
AC SUSCEPTIBILITY STUDIES OF NEW MAGNETIC MOLECULAR SOLIDS. Brenda C. Korte, Scott P. Sellers, Roger D. Sommer, Mahesh K. Mahanthappa, Gordon T. Yee, Department of Chemistry and Biochemistry, University of Colorado, Boulder, CO; and William S. Durfee, Department of Chemistry, Buffalo State College, Buffalo, NY.
The magnetic properties of molecular solids are typically determined by dc magnetic susceptibility measurements. If ac measurements are made at all, they are typically performed at one frequency and a peak in the in-phase component, together with a non-zero out-of-phase component is used to show the occurrence of a phase transition. While important, such measurements do not allow conclusive assignment of the magnetic state. By way of example, we will show that it is necessary to examine the frequency dependence of this phase transition to determine its true nature. Only a lack of frequency dependence in the low audio range (1-1000 Hz) indicates ferromagnetism. The presence of frequency dependence points to superparamagnetism or a spin-glass-like state. Unlike most examples of ``molecule-based'' magnetism, square-planar, neutral tetraazaporphyrin complexes are true molecular solids in which the molecules are held together only by van der Waals attraction. We have synthesized iron(II) octaethyletraazaporphyrin and determined it to be a new canted ferromagnet. Interestingly, the manganese(II) analog of this compound, also expected to be a ferromagnet, is isomorphous and shows significant hysteresis, but evidence for a spin-glass-like state below Tm as well. Possible explanations for these observations will be discussed.
8:45 PM J6.3
MAGNETIC PROPERTIES OF ORGANIC FERROMAGNET TDAE-C60 - SINGLE CRYSTALS AND THIN FILMS. Madoka Tokumoto, Y. Tsubaki, K. Pokhodnya, Y. Sakakibara, Electrotechnical Laboratory, Ibaraki, JAPAN; A. Omerzu, D. Mihailovic, Jozef Stefan Institute, Ljubljana, SLOVENIA; T. Uchida, Science University of Tokyo, Chiba, JAPAN.
Recent progress in the study on magnetic properties of molecular soft ferromagnet TDAE C60 (where TDAE is tetrakis(dimethylamino) ethylene) is reported. Both crystalline and thin film samples are prepared by the diffusion method and the temperature and field dependence of magnetization are studied by SQUlD magnetometer. The magnetic behaviors of single crystals were found to vary from sample to sample. In best single crystals, the saturation magnetization as high as 0.9 B/C60 has been observed so far. And an extremely sharp phase transition around 16 K as well as the remanence or the irreversibility, i.e. difference between zero-filed-cooled(ZFC) and field-cooled (FC) measurements, were observed in the temperature dependence of magnetization at very low magnetic fields (<10G). The ferromagnetic phase transition was also observed in TDAE C60 thin films.
9:00 PM J6.4
NEW -ELECTRON DONORS AND THEIR RADICAL CATION SALTS: SYNTHESIS, STRUCTURAL AND ELECTRICAL PROPERTIES. Martin R. Bryce, Adrian J. Moore, Alexanderr K. Lay, Andrei S. Batsanov, Judith A.K. Howard, Department of Chemistry, University of Durham, Durham, UNITED KINGDOM.
We have a continuing interest in the study of new crystalline organic metals1. We will discuss the synthesis, X-ray crystal structures, conductivity data and other solid-state properties of new -electron donor molecules and their radical ion salts. Representative TTF derivatives include the 4-(N-methylthiocarbamoyl) and 4-iodo derivatives, respectively, where the substituents on the TTF framework engage in close interchain contacts in salts, thereby increasing the dimensionality of the system. We will also discuss progress on polycyclic donors containing the 1,4-dithiin heterocycle, which have yielded many crystalline radical cation salts for which X-ray crystal structures have been obtained.
9:15 PM J6.5
ELECTRONIC STRUCTURE OF THE ORGANIC METALS -ET2Cu(SCN)2 and -ET2Cu[N(CN)2]BR MEASURED BY SOFT X-RAY EMISSION AND SOFT X-RAY ABSORPTION. Cristian B. Stagarescu, L.-C. Duda and Kevin E. Smith, Department of Physics, Boston University, Boston, MA; J. H. Guo and J. Nordgren, Department of Physics, Uppsala University, Uppsala, SWEDEN; D.-K. Seo and M.-H. Whangbo, Department of Chemistry, North Carolina State University, Raleigh, NC; R. Haddon, Bell Laboratories, Lucent Technologies, Murray Hill, NJ; J.S. Brooks, Department of Physics, Florida State University, Tallahassee, FL; D. Jeromé, Laboratoire de Physique des Solides (associé au CNRS), Université de Paris-Sud, Orsay, FRANCE.
The electronic structure of the two similar organic, layered metals $kappa-ET_2Cu(SCN)_2-ET2Cu[N(CN)2]Br have been studied using a combination of soft X-ray emission and soft X-ray absorption spectroscopy. These techniques are powerful probes of the site and orbital angular-momentum resolved partial density of states (PDOS) for both occupied and unoccupied states. Therefore these spectroscopies are particularly suited for an analysis of the density of states of multi-atomic, complex materials as the ET-based organic conductors, allowing site-specific electronic structure to be measured. Furthermore, for certain materials, the electronic structure of specific layers can be either measured or inferred. We present a detailed picture of the electronic structure of $kappa-ET_2Cu(SCN)_2-ET2Cu[N(CN)2]Br as measured by soft X-ray emission and soft X-ray absorption performed at the C-1s, S-2p and N-1s core levels. In particular we show that the measured contributions of the C-2p orbitals to the occupied and unoccupied densities of states are very similar for the two studied compounds. The measured C-2p occupied PDOS compares well with results of a recent tight binding calculation. The small contribution to the PDOS from C atoms residing in the inorganic, non-conductive layers is identified through an analysis of the observed changes of the X-ray emission spectra with the excitation energy. This allows us to identify a C-2p, inorganic layer specific character of the unoccupied states existing in the immediate proximity of the Fermi level. Work supported in part by NSF CAREER award DMR-95-01174 and by NSF INT-9515370.
9:30 PM J6.6
PREPARATION OF C60 CHARGE TRANSFER COMPLEXES WITH ORGANIC DONOR MOLECULES AND ALKALI DOPING TO THEM. Akihiro Otsuka, Gunzi Saito, Seiji Hirate, Sang-Soo Park, Takeshi Ishida, Div of Chemistry, Graduate School of Sci, Kyoto Univ, Kyoto, JAPAN; Anvar A. Zakhidov, Kyuya Yakushi, Inst for Molecular Sci, Okazaki, JAPAN.
Superconductivity so far observed in C60 compound is mainly restricted to the -3 charged C60 substances generated by alkali, alkaline ammonia, or alkaline earth doping. In such compounds, the crystal and electronic structures have been also restricted to three-dimensional ones because of the cubic-like arrangement of C60 molecules in the lattice. An extensive study on development of new C60 compounds is required. In order to explore new superconductors based on C60 which possess different characters in dimensionality of crystal and electronic structures, electronic state of C60, we have been investigating the alkali doping to charge transfer (CT) complexes of C60 with organic donor molecules. For example, after potassium or rubidium doping under a mild condition to the 1:1:1 CT complex of OMTTFC60benzene, it showed superconductivity at 17-18.8 K or 23-26 K, respectively, with its pristine crystal structure constructed by two-dimensional arrangement of C60 preserved. As the starting CT complexes for alkali doping, new C60 complexes as well as the known ones have been prepared. The first redox potential of 1, 3, 6, 8-tetrakis(dimethylamino)pyrene (TDAP) is +0.02 V vs. SCE and it is a stronger electron donor than OMTTF (+0.29 V) or TMPD (+0.15 V). Its CT complex with C60 was obtained from benzene solution as a 1:2:1 composition including benzene, TDAP(C60)2benzene. From absorption spectra, this TDAP complex showed the CT band at around 12 103 cm-1 and proved to be in neutral ground state. The results of the alkali doping to C60 complexes of TDAP, HMTTeF, hydroquinone, ferrocene, etc. will be presented.
9:45 PM J6.7
STRUCTURAL, ELECTRONIC, AND MAGNETIC PROPERTIES OF SELF-ASSEMBLED SUPRAMOLECULAR GRID STRUCTURES. Joerg Hassmann, Cathérine Y. Hahn, Oliver Waldmann, Hans-Joerg Schleemilch, Nikolaus Hallschmid, Eduard Volz, Paul Mueller, Physikalisches Institut III, Universitaet Erlangen-Nuernberg, Erlangen, GERMANY; Garry S. Hanan, Dirk Volkmer, Ulrich S. Schubert, Jean-Marie Lehn, Laboratoire de Chimie Supramoléculaire, Université Louis Pasteur, Institut Le Bel, Strasbourg, FRANCE.
Supramolecular Chemistry has become an important tool for the design and synthesis of self-assembled molecular architectures. One of the major goals is the ordered arrangement of metal ions. We present structural, electronic and magnetic properties of tetranuclear -grid complexes containing four metal ions such as cobalt(II) or cadmium(II) and four 4,6-bis(6-(2,2'-bipyridyl))-pyrimidine ligands. The ligands can be derivatized with various end groups. For the preparation of thin films, the Langmuir-Blodgett technique and electrochemical adsorption were used. By electrochemical under potential deposition, ordered monolayers of the grids could be transferred on gold(111) surfaces. The electronic structure of the thin films was studied by UPS and XPS spectroscopy. The grid films exhibit an insulating gab of ca. 2.5 eV indicating semiconducting behaviour. Additionally, conductivity measurements of thin films of the grids and their derivatives were performed. By doping the grid films e.g. with Cd(CF3SO3)2, their conductivity can be increased by several orders of magnitude. Measurements of the magnetization of the Co--grid revealed intramolecular antiferromagnetic coupling of the four Co spins.
10:00 PM J6.8
ANISOTROPIC ELECTRICAL CONDUCTIVITY AT THIN FILMS OF TTF-TCNQ AND SOME TTF-TCNQ DERIVATIVES. Waltraud Vollmann, H.-Ulrich Sonntag, Bettina Siegmund, Heike Kluge, Technical University Chemnitz, Chemnitz, GERMANY.
Correlations between morphology and electrical conduction at vacuum deposited thin films of TTF-TCNQ and some of its derivatives have been investigated. All thin films are polycrystalline, but the crystallite sizes, shape and orientation differ considerably. Because of the grain boundaries acting as electron barriers the electrical conduction mostly is thermal activated. Moreover it has been found to be very anisotropic. We observed three different types of anisotropic behaviour: 1. In TTF-TCNQ and HMTTF-TCNQ the in-plane room temperature conductivities range from 2..70 -1 cm-1, but the sandwich values are 5..7 orders smaller. Metallic conduction only could observed at one HMTTF-TCNQ in-plane measurement. Explanations could been given by SEM investigations. The thin films consist of flattened bar-shaped (TTF-TCNQ) or long needle-like (HMTTF-TCNQ) crystallites, whereby the needle axis, which is also the high conductive b-axis, is oriented within the film plane. 2. The OMTTF-TCNQ thin film conductivities are in good agreement with reported values of monocrystals (10-5 -1 cm-1). The in-plane values differ only slowly from the sandwich values. Here the crystallites are block-shaped without a preferred orientation. 3. In DPTTF-TCNQ thin films the sandwich value is 3..5 orders higher than the in-plane value. The crystallites grow on cauliflower-like with a preferred direction out of the film plane.
SESSION J7: NONLINEAR OPTICAL/PHOTOREFRACTIVE MATERIALS-II
Chair: Larry R. Dalton
Wednesday Morning, December 3, 1997
Salon G (M)
8:30 AM *J7.1
NEW PORPHYRIN-BASED CHROMOPHORES AND MATERIALS WITH EXCEPTIONAL NONLINEAR OPTICAL PROPERTIES. Steven M. LeCours, Michael J. Therien, University of Pennsylvania, Department of Chemistry, Philadelphia, PA; Alex K.-Y. Jen, Northeastern Technology, Monmouth Junction, NJ.
A new class of chromophores has been fabricated that features both electron-releasing and electron-withdrawing groups fused via an intervening ethynyl moiety to the carbon framework of a (porphinato)metal complex. These species possess a number of unusual optical properties that include molecular first-order hyperpolarizabilities () of exceptional magnitude. [5-(4'-dimethylaminophenylethynyl)-15-(4''-nitrophenylethynyl)-10,20-diphenylporphinato]copper(lI) (I) and [5-(4'-dimethylaminophenylethynyl)-15-(4''-nitrophenylethynyl)- 10,20(diphenylporphinato]zinc(II)() represent two archetypal members of this new class of exceptional nonlinear chromophores; for example, electric field induced second harmonic generation experiments (EFISH) carried out on II show that exceeds 1000 x 1030 esu. This work suggests that this structural motif may find exceptional utility in the development of electrooptic devices as well as materials for efficient frequency doubling of incident irradiation (second harmonic generation); this is underscored by further work showing that exhibits remarkable thermal stability, exhibiting no perceptible decomposition at 250C in thin polyimide films, a matrix thought to be ideal for electrooptical applications.
9:00 AM J7.2
RECENT DEVELOPMENT OF HIGHLY EFFICIENT CHROMOPHORES AND POLYMERS FOR ELECTRO-OPTIC DEVICE APPLICATIONS. Alex K-Y. Jen, Qing Yang, Xijun Wang and Guy Beardlsey, Department of Chemistry, Northeastern University, Boston, MA.
A new series of highly efficient ( as high as 7,000 x lO-48 esu), chemically and thermally stable (275C) nonlinear optical (NLO) chromophores have been developed by using 1-perfluoro-2 dicyanovinyl as electron-accepting and diphenylamino as electron donating groups for a series of thiophene stilbenes. Excellent tradeoffs among absorption, molecular nonlinearity and thermal stability were achieved. Electro-optic polymers based on guest/host or covalently attachment of chromophores onto polyimide and polyquinoline backbones demonstrate high E-O activities and good optical, electrical and mechanical properties. Combination of these properties with simple trench and fill waveguide process will enable the design and integration of compact E-O switches into high speed digital optical modulators.
9:15 AM *J7.3
CHARACTERIZING AND CIRCUMVENTING ELECTROSTATIC INTERMOLECULAR INTERACTIONS IN HIGHLY ELECTRO-OPTIC POLYMERS. Aaron W. Harper, Jingsong Zhu, Mingqian He, Larry R. Dalton, Department of Chemistry, University of Southern California, Los Angeles, CA; Sean M. Garner, William H. Steier, Department of Electrical Engineering, University of Southern California, Los Angeles, CA.
Polymers possessing non-resonant electro-optic activity exceeding 20 pm/V require chromophores with strong electron withdrawing groups (cyanovinyls, carbon acid moieties, etc.) as well as highly polarizable bridges. Although much progress has been made on designing and preparing materials with molecular ``electro-optic'' activities, their incorporation into polymers to show comparably large bulk electro-optic activity has been met with little success. We report here the nature of the difficulty of the translation of microscopic to macroscopic electro-optic activity. The optimization of molecular activity increases intermolecular electrostatic interactions between chromophores, and these interactions impede induction of polar acentric order in the polymers. Theoretical analysis of the problem is presented, as well as one example of a material that is designed to circumvent these interactions. The resulting material possesses electro-optic coefficients of 2530 pm/V.
9:45 AM J7.4
ELECTRONIC PROPERTIES OF A NOVEL CLASS OF CONJUGATED SYSTEMS: TRANSITION METAL SUBSTITUTED OLIGOTHIOPHENES. Riccardo Tubino, Luca Lancellotti, Dipartimento di Scienza dei Materiali e INFM Universita di Milano, ITALY; Silvia Luzzati, Istituto Chimica delle Macromolecole e MITER, CNR, Milano, ITALY; Antonio Papagni, Emanuela Licandro, Stefano Maiorana, Dipartimento di Chimica Organica e Industriale, Universita di Milano, ITALY.
The incorporation of a transition metal in a conjugated structure is particularly interesting because enhanced nonlinear activity is expected due to the the induced asymmetry in the charge distribution. In this paper we report a spectroscopical study of a novel class of push-pull molecules containing a Chromium or Tungsten atom connected to an oligothiophene through a carbenic bond. The electronic coupling between the electrons of the conjugated system and the d electrons of the transition metal has been monitored through absorption, emission and Raman spectra in and off- resonance. This interaction between d and electrons leads to a red shift of the thienylene absorption band and to the appearance of a new strong metal to ligand charge transfer band at lower energies. It is established that in this latter transition the electrical dipole reverses its direction upon photoexcitation from the ground to the first excited state, thus accounting for the enhanced non linear optical response of these molecules. Solvatochromic effect has been used to estimate the second order molecular hyperpolarizability.
10:30 AM *J7.5
NEW PHOTOPOLYMERS BASED ON TWO-PHOTON ABSORBING CHROMOPHORES AND APPLICATION TO THREE DIMENSIONAL MICROFABRICATION AND OPTICAL STORAGE. B. Cumpston1, J. Ehrlich1, A. Heikal1, Z.-Y. Hu2, I.-Y.S. Lee1, S.R. Marder1,2, J.W. Perry1,2, H. Rockel2, and X.L. Wu1, 1Jet Propulsion Laboratory, California Institute of Technology, Pasadena, CA; 2Beckman Institute, California Institute of Technology, Pasadena, CA.
Molecules exhibiting strong two-photon absorption hold great potential for a wide range of applications including: two-photon fluorescence imaging, three-dimensional (3D) optical data storage, 3D microfabrication, photorefraction and optical limiting. We have observed large two-photon absorptivities in bis-donor diphenylpolyene derivatives, that is correlated to simultaneous charge transfer from the end groups to the polyene bridge in the molecule. These molecules are also excellent photoexcitable electron donors that can initiate charge-transfer reactions. In initial applications of these materials we have demonstrated their use in two-photon initiation of polymerization and optical limiting.
Two-photon initiated polymerization holds tremendus promise for ultrahigh density optical memory and 3D microfabrication. One can selectively polymerize very small volumes of polymer near the focus of a laser beam, thus creating a change in refractive index and permitting data storage densities of 1012 bits/cm3. Bis-donor diphenylpolyenes are excellent electron donors and we have shown that upon excitation they are able to initiate polymerization of acrylate monomers and multifunctional acrylates. These materials have allowed us to develop a new class or two-photon writable photopolymers, that have high writing speed as a result of their large two-photon absorptivity. We will present our recent results on direct two-photon writing of 2- and 3-D microscale features.
11:00 AM *J7.6
CONJUGATED POLYMER BASED NANOCOMPOSITES FOR PHOTONICS APPLICATIONS. P.N. Prasad, N. Deepak Kumar, Manjari Lal, Mukesh P. Joshi, Photonics Research Laboratory, Department of Chemistry, State University of New York at Buffalo, Buffalo, NY.
Nanoscale synthesis and processing provides a novel approach for making a new generation of nanocomposite materials with novel optical and electrical properties for the development of new technologies. This presentation will be focused on the preparation of nanocomposites of Poly (para-phenylene vinylene) (PPV) with other polymeric systems and inorganic glasses and semiconductors. A new approach using nanoscale polymerization for making processable monodispersed oligomeric species of PPV using the base catalyzed polymerization reaction of PPV monomer within the cavity of a reverse micells nanoreactor will be presented. Application of this approach for fabricating novel materials for a variety of applications in photonics will be discussed. Fabrication of bulk nanocomposites of PPV and silica by in-situ polymerization of monomer within a porous glass and their lasing properties will be presented.
11:30 AM J7.7
GUEST-HOST OPTICAL LIMITERS WITH HIGH LASER DAMAGE THRESHOLD EPOXY ELASTOMER MATRICES. Michael E. De Rosa, Weijie Su, Doug Krein, Materials Directorate, Wright Laboratory, WL/MLPJ, Wright-Patterson Air Force Base, OH; Mark C. Brant, Daniel G. McLean, Science Applications International Corp., Dayton, OH.
Conventional guest-host optical limiters typically consist of a nonlinear absorbing chromophore dissolved in solution. Recently there have been efforts to incorporate these types of NLO dyes into solid polymeric matrices. Though solid-state organic limiters have been demonstrated, their performance suffers due to the low laser damage threshold of the polymer matrix. We report the results of optical limiting in an optical grade thermosetting epoxy elastomer matrix doped with silicon naphthalocyanine and zinc octabromotetraphenyl porphyrin. These elastomeric limiters demonstrate high bulk laser damage threshold (>100 J/cm2) due to their viscoelastic properties. The damage thresholds observed are more than one oreder of magnitude higher than brittle thermosets and thermoplastics. We show how thermomechanical properties of the matrix affect laser damage threshold in undoped and doped matrices. Results of recent efforts in processing and characterizing limiters with dye concentration gradients will also be presented.
11:45 AM J7.8
ULTRAFAST AND NONLINEAR OPTICAL CHARACTERIZATION OF OPTICAL LIMITING PROCESSES IN DERIVATIZED FULLERENES. R. Kohlman, V. Klimov, X. Shi, M. Grigorova, B. R. Mattes, and D. McBranch, Los Alamos National Laboratory, Los Alamos, NM; H. Wang and F. Wudl, University of California, Los Angeles, CA; B. Moreshead and J. L. Nogues, GELTECH, Orlando, FL.
We review the results of spectral studies of the ultrafast excited-state absorption in fullerenes and derivatized fullerenes, which allow determination of both the spectral response of reverse saturable absorption (RSA) nonlinearities in these compounds, as well as the dynamical response for different morphologies. We have investigated the effect of various sol-gel glass environments on the optical limiting of derivatized fullerenes. We find that by appropriate pretreatment, the optical limiting response of derivatized fullerene sol-gels can be enhanced to approach that of the same molecules in solution while significantly enhancing the damage threshold. The ultrafast absorption and optical limiting of these derivatived fullerenes will be compared with that of various porphyrin and phthalocyanine molecules as well as new broadband limiting materials which consist of phthalocyanines derivatized directly onto the fullerene molecule. The experimental data are compared with modeling using a five-level model.
SESSION J8: LIGHT-EMITTING POLYMERS
Chair: Richard H. Friend
Wednesday Afternoon, December 3, 1997
Salon G (M)
1:30 PM *J8.1
LIGHT EMISSION. A. Heeger.
Abstract not received
2:00 PM *J8.2
ADVANCED PHOTOREFRACTIVE AND LIGHT-EMITTING ORGANIC MATERIALS. Nasser Peyghambarian and Bernard Kippelen, University of Arizona, Optical Sciences Center, Tucson, AZ.
By following novel design strategies for the active dopant dye molecules, we have obtained photorefractive polymers with an unprecedented performance level. We have optimized the orientational effects occurring in these materials by designing dye molecules with a high dipole moment and a high linear polarizability anisotropy. Complete diffraction is observed at 830 nm in 105 um-thick films with sub-second response time. In the visible, these new polymers outperform the best previous photorefractive polymers by a factor of four in refractive index modulation amplitude and accelerated aging studies indicate that their shelf lifetime should be improved by two orders of magnitude. By developing photorefractive polymers doped with isomeric mixtures of chromophores we have developed highly efficient materials with an estimated shelf lifetime of several years at room temperature. With these new materials we demonstrated imaging through scattering media using an all-optical holographic time-gate technique. In the area of organic light-emitting materials for display and laser applications, we have fabricated light emitting devices with an Al cathode that exhibits unprecedented levels of performance. A thin LiF insulating layer between the light-emitting layer and the cathode enhances the electron injection process and is responsible for the high performance. We will present the results of an extensive study of the emission properties of optically pumped conjugated polymers and optically pumped lasers.
2:30 PM J8.3
ORGANIC AND POLYMERIC MATERIALS FOR LOW THRESHOLD PLASTIC LASERS. Zhentan Bao, Magnus Berggren, Ananth Dodabalapur, Richard E. Slusher, E.A. Chandross, Bell Laboratories, Lucent Technologies, Murray Hill, NJ.
A variety of organic materials have previously been reported as emitters in lasers. In most cases, these emitters consist of organic dyes which had been dissolved either in nonabsorbing and non-emitting solvents or ill solid matrices. More recently, optically stimulated emission has been reported for neat conjugated polymer films. However, very high pumping power is needed in order to observe spectrum narrowing. It is very important to reduce the absorption coefficient at emission wavelength. We present results in achieving very low threshold stimulated emission using a Forster transfer mechanism. The emitter materials we employed contain a strongly absorbing waveguiding host organic material that is doped with organic dye molecules (0.1-1 wt%) or conjugated polymers (about 10 wt%). The excited host can act as the excitation source for the emitters through Forster energy transfer, and hence the observed stimulated emission wavelength can be substantially shifted from the absorption edge of host material by several hundred nanometers. Consequently, low pumping threshold is achieved due to a more efficient population inversion. By using the above approach, we are able to obtain stimulated emission from the ultraviolet (392 nm) to the near infrared (805 nm) by using 2-(4-biphenylyl)-5-(4-tert-butylphenyl) 1,3,4-oxadiazole (PBD) as the host and appropriate dyes as the emitters with the lowest threshold powers of 1-2 kW/cm2. We observed threshold power as low as 200 W/cm2 using PBD as host and a conjugated polymer as emitter. The polymeric emitters include alkyl and alkoxy substituted poly(phenylene vinylene) and poly(phenylene thiophene) derivatives.
3:15 PM J8.4
SYNTHESIS AND ELECTROLUMINESCENCE STUDIES OF A NOVEL COPOLYMER: POLY (PHENYLENE VINYLENE-CO-QUINOLINE VINYLENE). Ronald L. Elsenbaumer*, Ramesh K. Kasim and Martin Pomerantz*,a, *Materials Science & Engineering Program, aDepartment of Chemistry & Biochemistry, The University of Texas at Arlington, Arlington, TX.
Highly efficient polymer LEDs are generally fabricated using multi layered structures with separate carrier transport and emission layers. We recently reported on the synthesis and electroluminescent characteristics of poly(2,6-quinoline vinylene) (PQV) and its use as electron transport layer in poly(phenylene vinylene) (PPV) based LEDs. To take advantage of high emission efficiency of PPV and electron accepting ability of PQV, a copolymer of PPV and PQV (PPVQV) was synthesized via a precursor route and converted to the conjugated form by thermal elimination. When used as the active emission layer in LEDs with ITO and aluminum as positive and negative electrodes, PPVQV showed yellow emission. Higher emission efficiencies were achieved when blends of the copolymer with PPV were used in conjunction with PPV in a multi-layered structure. Along with chemical characterization data of the copolymers, comparison of results from EL studies on single and multi-layered devices will be presented and discussed. We will also report on a simple and cost effective device fabrication technique involving chemical deposition of metal electrode and compare EL characteristics of devices with vacuum deposited and chemically deposited metal electrodes.
3:30 PM J8.5
LECs MADE OF mLPPP. F.P. Wenzl1, S. Tasch1, J. Gao3, U. Scherf2, G. Leising1, A.J. Heeger3, 1Institut für Festkörperphysik, Technische Universität Graz, Graz, AUSTRIA; 2Max-Planck-Institut für Polymerforschung, Mainz, GERMANY; 3Institute for Polymers and Organic Solids, University of California, Santa Barabara, CA.
We report the application of the blue emitting conjugated polymer mLPPP (methyl substituted laddertype polyparpphenylene) in light emitting electrochemical cells. The cative layer of the LEC consists of a blend of mLPPP with the ionically conductive polymer PEO and LiCF3SO3 as Salt. Photoluminescene (PL) and Electroluminesscence (EL) spectra and I/U- characteristics were taken for diffferent concentrations of PEO and salt. The I/U characteristics show low turn on voltages both for current and electroluminescence which are equal to the energy gap divided by the elementary charge. The initial electroluminescence spectra are quite the same as those for LEDs made of mLPPP but turn into green after some time of operation. By optimising the concentrations of the compositions in the blend we are able to realise LECs with response times <30s.
3:45 PM J8.6
TUNING POLYMER DESIGN FOR HIGH PERFORMANCE ELECTROLUMINESCENT DISPLAYS. M.J. Watson, R.H. Friend, Cavendish Laboratory, Cambridge University, Cambridge, UNITED KINGDOM; I. Grizzi, C.R. Towns, Cambridge Display Technology, Cambridge, UNITED KINGDOM.
There is currently a great deal of interest being shown in the use of poly(p-phenylene vinylene) (PPV) as the active element for electroluminescent (EL) display devices. PPV is completely insoluble and intractable and is therefore fabricated from a solution-based precursor polymers namely poly(p-xylene sulphonium bromide). Copolymer precursors were investigated, where a known proportion of sulphonium bromide units were substituted with acetate groups. The acetate containing precursors were thermally converted to generate PPV and appear to include a proportion of acetate substituted entities which segment the conjugation of the polymer. Compared to the PPV obtained from homopolymer precursors, these acetate-containing PPV films exhibit blue-shifted UV-vis and photoluminescence (PL) spectra as well as showing increased PL efficiency. The thermal conversion is influenced in the presence of an underlying indium tin oxide (ITO) layer, which is a common device anode material. The lTO leads to the elimination of a larger proportion of the precursor acetate groups during thermal treatment. PPV obtained from a precursor containing 40% of acetate substituted moieties is therefore very similar to PPV generated from a homopolymer precursor. However, precursors with higher acetate levels lead to a blue-shifted. higher efficiency, green emitting PPV, even on ITO.
4:00 PM J8.7
LIGHT EMITTING DIODES FROM SELF-ASSEMBLED HETEROSTRUCTURES OF A POLYMERIC RUTHENIUM (II) COMPLEX AND POLY (ACRYLIC ACID). Aiping Wu, MIT, Dept. of Materials Science and Engineering; Jinkyu Lee, MIT, Dept. of Chemistry; Michael F. Rubner, MIT, Dept. of Materials Science and Engineering, Cambridge, MA.
We have explored a new light emitting material, Ru(bpy)3+2 polyester, and employed it to fabricate solid state light emitting diodes using the layer-by-layer self-assembly processing technique. By controlling the thin film processing conditions such as solution pH, we can systematically alter the composition of the polycation/polyanion building block. This in turn has made it possible to dramatically alter the device performance. Using this approach, we have obtained up to 3% external quantum efficiency from the self-assembled multilayers of the Ru(bpy)3+2 polyester and poly(acrylic acid) and a light output of 50 cd/m2. Furthermore, by building compositionally graded heterostructures or placing insulating blocks at the electrode/polymer interfaces, we can basically control the light emission which can be symmetric in both biases or asymmetric in the forward and the reverse bias. We also demonstrated that the self-assembled Ru(bpy)3+2 polyester/PAA device can be operated under the AC mode.
4:15 PM J8.8
ELECTROLUMINESCENCE FROM HETEROSTRUCTURES OF SELF-ASSEMBLED PPV AND INORGANIC CdSe NANOCRYSTALS. H. Mattoussi, L.R. Radzilowski, B.O. Dabbousi, M.G. Bawendi, M.F. Rubner, and E.L. Thomas, MIT, Center for Materials Science and Engineering, Cambridge, MA.
Electroluminescence (EL) and photoluminescence (PL) from heterostructure thin films made of poly(phenylene vinylene), PPV, and inorganic semiconductor CdSe nanocrystals were studied. In these devices, the PPV was built next to the ITO anode, using the technique of molecular layer-by-layer sequential adsorption, and served primarily as the hole transport layer. The inorganic layer, adjacent to the electrode, was made of spin cast CdSe nanocrystals, passivated with either organic groups or a few monolayers of ZnS (a wider band gap semiconductor, e.g., ZnS). In these devices, we found that the EL signal is almost exclusively generated within the inorganic layer. The performance of these heterostructure devices was influenced by the thickness of the dot layer. Lifetime tests revealed long term stability, as samples with continuous operation of 50 l00 hours were made. Values of the external quantum efficiency (QE) as high as 0. l% can be reached. In addition, the QE was not necessarily enhanced by the presence of ZnS overcoating, as opposed to the observed increase in the PL quantum yield. This reflects a difference in the efficiency of charge injection into the dots due to the ZnS overlayer.
4:30 PM J8.9
IMPROVED ELECTRON INJECTION IN ORGANIC LIGHT EMITTING DEVICES BY APPLYING THIN INSULATING LAYERS. R. Pairleitner1, C. Hochfilzer1, S. Tasch1, G. Leising1, U. Scherf2, K. Müllen2; 1Institut für Festkörperphysik, TU Graz, Graz, AUSTRIA; 2Max-Planck-Institut für Polymerforschung, Mainz, GERMANY.
The organic light emitting devices are constructed with an ITO-anode and an aluminum cathode. For the active layer we use either ladder type poly para-phenylene (mLPPP) or parahexaphenyl (PHP). A thin film of insulating material is applied between the active layer and the cathode, in order to achieve a better tunnel injection due to a higher electric field at the interface. We compared different insulating materials with various thickness. The best results are obtained by using a 10 LiF-layer. Thereby the onset voltage decreases and the current density in the device increases. The influence of additional electron transport layers (ETL) in these devices is also presented.
4:45 PM J8.10
CHEMICAL VAPOR DEPOSITION OF PPV FOR FABRICATION OF LIGHT EMITTING DIODES WITH LOW TURNON VOLTAGES. Kathleen M. Vaeth, Klavs F. Jensen, MIT, Dept of Chemical Engineering, Cambridge, MA.
Light emitting polymers such as poly(p-phenylene vinylene) (PPV) are an attractive alternative to inorganic materials used as the active layer in light emitting diodes (LEDs) and lasers. Thin films of PPV are usually incorporated into LEDs by solution processing, which can have problems of side reactions with solvent and residual oxygen. In this research, chemical vapor deposition (CVD) of PPV is explored as an alternative to solution processing methods. The advantages of the CVD process include good control of film uniformity, elimination of side reactions with solvents and oxygen, and the potential to control the film architecture and composition on the molecular level. There have been several reports of CVD of PPV in the literature, but none have demonstrated single layer LEDs with reasonable turnon voltages. We have shown reproducible fabrication of unoxidized PPV films with the CVD method. The quantum efficiency of the polymer is 28 percent, which matches the best reported values for pristine solution processed PPV. Ex-situ fluorescence microscopy reveals several different growth morphologies of the polymer below the critical deposition temperature, which has important implications for LED fabrication. Electrical and optical characteristics of single layer CVD LEDs with low turnon voltages will presented. Comparison of devices made from chlorine and bromine based precursors reveals very different electrical and optical behavior, with the chlorine route demonstrating significantly lower turnon voltages. The best devices have turnon voltages as low as 4 volts, and light output easily detected with the naked eye in a well lit room. Polarized light emission from the PPV films has also been achieved by rub-aligning the CVD precursor polymer before thermal conversion. These ! films have been incorporated into single layer LEDs that exhibit parallel/perpendicular EL intensity ratios of 2:1, and turnon voltages of 6 volts.
SESSION J9: POSTER SESSION
Chair: Alex K.-Y. Jen
Wednesday Evening, December 3, 1997
America Ballroom (W)
SYNTHESIS AND INVESTIGATION OF THE OPTICAL STORAGE CHARACTER OF A SERIES OF AZO COPOLYMERS. Yanqing Tian, Yingying Zhao, Department of Chemistry, Jilin University, Changchun, CHINA; Yanqiang Yang, Zhengqian Wei, Department of Physics, Jilin University, Changchun, CHINA.
Over the past few years, the search for ideal organic/polymer reversible optical storage materials has caused scientists' attention. Actually azo polymer is a kind of ideal material for optical storage because of their potential uses in various photonic applications. Until now some papers have reported the character of data/high-density-data storage by using azo polymers, but the reports of image storage especially the real time/long time multiple image storage which is very important for the practical application and a method for increasing the density of storage are relatively few. In this paper we prepared a series of copolymers--poly2-(4-nitrophenylazophenyl methyl amino) ethyl methacrylate-co-6-[4-(4-methyloxy-benzoyloxy)phenyloxy]hexyl methacrylate, investigated their photoinduced birefringence and studied the real-time and long-time of multiple image storage of the copolymer with 50% of azo unit by using He-Ne laser light at 632.8nm. The results showed that the maximum photoinduced birefringence at 632.8nm increased with increasing of azo unit in these polymers, and the maximum induced birefringence of the polymer with 100% azo unit under the irradiation at 632.8nm is 0.019. This value is much lower compared to the values reported in references due to the use of different laser light, the reason will be discussed. The results also showed that the copolymer with 50% azo unit could realize excellent triple real-time and long-time optical spot storage by using the method of degenerate four wave mixing (DFWM) and double image storage with satisfactory resolution. At room temperature the optical and image storage have been proved to be stable with lifetime exceeding 7 days. The writing, reading and erasing cycles are over 1000.
THIRD ORDER OPTICAL NONLINEARITY IN COFACIAL PHTHALOCYANINE DIMERS AND TRIMERS. Eric S. Manas and Frank C. Spano, Department of Chemistry, Temple University, Philadelphia, PA; Lin X. Chen, Chemistry Division, Argonne National Laboratory, Argonne, IL.
The influence of inter-macrocycle interactions on the second hyperpolarizabilities (-;, -, ) of cofacial phthalocyanine dimers and trimers is studied theoretically. The analysis presented is based on the Frenkel exciton model for a chain of three level molecules. Using a simplified treatment in the static and near-resonant regimes we identify two mechanisms which lead to enhancements in the dimer or trimer value of (-;, -, ) over that of the monomer. The first mechanism is a disruption of the balance between type I and type II terms in the sum over states expression for the second hyperpolarizability tensor kjih(-;, -, ), caused by weak inter-macrocycle interactions. The second is a near-resonance enhancement of the type II terms due to an inter-macrocycle interaction induced shift in the monomer derived two-photon allowed states towards twice the laser photon energy. This analysis is in good agreement with recent degenerate four wave mixing experiments [SPIE Proc., 2527, 61 (1995)] which showed a strong enhancement of (-;, -, ) for SiPcO oligomers as a function of the number of macrocycles. Our calculations suggest that the first mechanism is responsible for the 25-fold monomer to dimer enhancement measured in this system, and that the additional four-fold enhancement found in going from the dimer to the trimer is primarily the result of the second mechanism.
CHARACTERIZATION OF A CROSS-LINKED METHACRYLATE-DYE BASED POLYMER WITH NONLINEAR OPTICS PROPERTIES. Paulo António Ribeiro, Universidade Nova de Lisboa, Faculdade de Ciências e Tecnologia, Departamento de Física, Monte Caparica, PORTUGAL; Débora Terezia Balogh, José Alberto Giacometti, Universidade de São Paulo Instituto de Física de São Carlos, São Carlos, BRAZIL.
A cross-linked polymeric material was obtained from three main components: the homopolymer backbone poly(2-hydroxyethyl-methacrylate), the azo dye 4-[N-ethyl-N-(2-hydroxyethyl] amino-4'-nitroazo-benzene (DR1) and a tri-isocyanate. Films are easily produced dissolving, at room temperature, the three components in N,N dimetilformamide and spinning the solution onto glass substrates. The DR1 molecules were chemically bounded to the urethane isocyanate groups or to the polymer backbone in a thermally activated reaction. High temperature promotes the cross-linking reaction as well facilitates the dipole orientation of the polar group in the polymer matrix. To obtain the second order nonlinear response the polar groups were oriented by submitting films to corona discharge in a corona triode. During corona poling the temperature was increased at constant rate up to a final temperature. Orientational order of the polar groups was monitored by measuring the electrochromic changes in the UV-VIS absorption spectra before and after the poling process. A systematic study was carried out varying the temperature increase rate, final temperature, poling time, poling voltage and chemical composition of the films. Results showed that the orientational order in the polymer matrix depends mainly on the final poling temperature and on the chemical composition of the films, namely on the ratio of isocyanate to hydroxyl and on the DR1 dye content. The efficiency of dipole orientation is improved when films are poled at the final temperatures in the range of 150 C to 160 C.
FEMTOSECOND OPTICAL NONLINEARITY IN THE SQUARYLIUM DYE J-AGGREGATES. M. Furuki, FESTA Laboratories, Tsukuba, Ibaraki, JAPAN; L.S. Pu, Fuji Xerox Co. Ltd., Foundation Res. Lab., Nakai-machi, Kanagawa, JAPAN; F. Sasaki, S. Kobayashi, T. Tani, Electrotechnical Laboratory, Tsukuba, Ibaraki, JAPAN.
We report on the clear observation of strong and ultrafast nonlinear optical responses at room temperature from a mono-molecular layer of the squaryliun dye J-aggregates. The dynamics of Frenkel-excitons in J-aggregates has so far been studied on dispersed cyanine dyes, high optical nonlinearity and fast decay of their excited states have been shown. For practical applications, however, a highly continuous and uniform 2-dimensional J-aggregate is desired.
Through our study on squarylium dye Langmuir films, we have found that a molecular structure with two propyl groups substituted on both ends of the chromophore form J-aggregates. A linear-absorption measurement for a 0.8 nm thick Langmuir film of bis[4-(N-propylhexylamino)phenyl]squarylium showed a J-band at 777 nm under 5 C with an extremely high absorbance of 0.5 (O.D.) (corresponding to 107 cm-1), suggesting the highly continuous property of the film. A transient femtosecond pump-probe absorption measurement using a pump at 780 nm (2.5 200 Jcm2) exhibited breached absorption on the long-wavelength side, and induced absorption on the opposite side of the J-band. This dispersion type absorption change is characteristic to J-aggregates, and indicates saturation of the transition from the ground state to the 1-exciton state as well as the induction of transitions between higher excited states. The decay rates of these absorption changes were found to have two time constants of 250350 fs and 1020 ps the faster response was dominant under a lower pump power.
From the magnitude of absorption bleaching, the saturation intensity was determined to be 7.5106 Wcm2, which is extremely low for an organic monolayer,suggesting exciton delocalization enabling the excitation of several tens molecules by one photon. The observed large nonlinearity as well as the fast response at room temperature confirms high density and high uniformity of this J-aggregate film. This work was supported by NEDO within the framework of the Femtosecond Technology Project.
LARGE AND FAST THIRD-ORDER NONLINEARITY OF ONE-DIMENSIONAL FOUR-NUCLEAR ORGANOMETALLIC COMPLEX STUDIED BY FEMTOSECOND OPTICAL KERR EFFECT AND Z-SCAN METHODS. Kenji Kamada, Toru Sakaguchi, Koji Ohta, Osaka National Research Institute, Dept of Optical Materials, Ikeda, Osaka, JAPAN; Kazushi Mashima, Kazuhide Tani, Osaka University, Dept of Chemistry, Faculty of Engineering Science, Toyonaka, Osaka, JAPAN.
Organometallic complexes have attracted considerable attention in the community of organic nonlinear optics due to variety of metal species and self-organization ability which could increase the third-order susceptibility. Recently we have synthesized novel one-dimensional tetranuclear organometallic complexes containing four metal atoms forming one-dimensional d-d conjugation system. We investigated the third-order nonlinearity of the tetranuclear complexes by picosecond degenerate four-wave mixing experiments and found one of the complexes, Mo2Pd2Cl2(pyphos)4 (pyphos = 6-diphenylphos-phino-2-pyridonate), has large third-order nonlinearity ( = 1.5 10-30 esu at 532 nm). In this paper, we report the results of ultrafast response of the third-order nonlinearity of Mo2Pd2Cl2(pyphos)4 measured by femtosecond Optical Kerr Effect (OHD-OKE) experiment, and of wavelength dispersion measured by femtosecond Z-scan experiment. The third-order nonlinear response measured by OHD-OKE experiment for dichloromethane solution followed incident laser pulse (pulsewidth: 100 fs), indicating purely electronic contribution dominates the response. Z-scan experiment for the same solution showed that the magnitude of the hyperpolarizability in the wavelength region of 760-820 nm is in the order of 10-31-10-32 esu and the absolute value of the hyperpolarizability decreases with longer observation wavelength. The sign of the polarizability was determined to be negative by both the experiments. The lowest one-photon electronic transition band of Mo2Pd2Cl2(pyphos)4 is centered at 640 nm and tails up to 800 nm. Hence, the wavelength dispersion is considered to be due to the one-photon transition. The previous values of the hyperpolarizability at 532 nm is larger by one or two orders of magnitude than those by the present experiment. The difference can be explained in terms of resonance enhancement.
FEMTOSECOND DYNAMICS OF EXCITED STATES OF PERYLENE DIIMIDE PIGMENTS. A. J. Makinen, S. Xu, S. Diol, Z. Zhang, A. R. Melnyk, D. A. Mantell, Y. Gao, G. M. Mason, A. A. Muenter, Center for Photoinduced Charge Transfer, University of Rochester, Rochester, NY.
For the first time we have studied the lifetimes of the excited electron states of different perylene diimide pigments with femtosecond photoemission spectroscopy. Perylene diimides are organic compounds similar to photoreceptor materials widely used in many imaging applications. In our measurements, we have found extremely short lifetimes for the excited states. They range from 20 fs to 40 fs at 3.0 eV and 2.2 eV above the valance band maximum respectively. Such short lifetimes are several orders of magnitude less than the fluorescence lifetimes previously observed in these materials. The lowest excited state measured, 2.2 eV above the valance band maximum, is still approximately 0.4 eV above the lowest lying fluorescent singlet state of the perylene molecule and hence we explain the fast lifetimes by the almost instantaneous relaxation of hot electrons to low lying excited states. We attribute the rapid relaxation of the carriers to strong carrier-phonon coupling. Fast initial relaxation of photoexcited electrons is an underlying assumption in both the Onsager Model and in the Internal Conversion Theory for carrier generation by photoexcitation, and our results support the validity of this assumption.
FEMTOSECOND PUMP AND PROBE SPECTROSCOPY ON POLY(PARA-PHENYLENES). W. Graupner, G. Kranzelbinder, G. Leising, Institut fuer Festkoerperphysik, TU Graz, AUSTRIA; G. Lanzani, Istituto di Matematica e Fisica, Universita' di Sassari, ITALY; M. Nisoli, S. De Silvestri, Dipartimento di Fisica, Politecnico de Milano, ITALY; U. Scherf, K. Muellen, Max-Planck-Institut fuer Polymerforschung, GERMANY.
The conjugated ladder-type poly(para-phenylene) (LPPP) shows blue photoluminescence and electroluminescence emission with high quantum yields. A high degree of intrachain order and low concentration of electronic defects allows to produce excellent optoelectronic devices from these materials. We present results of transient pump and probe spectroscopy on guest host systems, where the host is the blue emitting LPPP and the guest is either a charge acceptingspecies as C60 or an efficient long wavelength emitter, accepting the electronic excitation energy from the LPPP. In addition these experiments were done underdifferent high static external electric fields. We acknowledge the support of the bilateral Italo/Austrian 1997 Research Program.
MAGNETIC FIELD EFFECT IN HIGHLY PURE, HIGHLY FLUORESCENT CONJUGATED POLYMERS. W. Graupner, C. Zenz, G. Leising, Institut fuer Festkoerperphysik, TU Graz, AUSTRIA; M. Sacher, G. Grampp, Institut fuer Physikalische und Theoretische Chemie, TU Graz, AUSTRIA; U. Scherf, K. Muellen, Max-Plank-Institut fuer Polymerforschung, GERMANY.
The conjugated ladder-type poly(para-phenylene) (LPPP) shows blue photoluminescence (PL) and electroluminescence emission with high quantum yields. A high degree of intrachain order and low concentration of electronic defects allows to produce excellent optoelectronic devices from these materials. We present measurements of magentic field effect on the PL of LPPP. The magnetic measurements are done on both highly pure samples and after controlled degradation. Based on PL emission and excitation spectra as well as transient PL data and infrared spectra we propose that the magnetic field effect is no intrinsic effect in pure conjugated polymers and thererfore a tool to estimate the defect concentration in the samples.
PUSH-PULL OLIGOTHIOPHENE AND OLIGOTHIENOQUINONOID DERIVATIVES FOR NONLINEAR OPTICS. Tatsuo Wada, Keiko Tanaka, Takashi Isoshima, Hiroyuki Higuchi Juro Ojima, Shinobu InoueS, Yoshio AsoS, Tetsuo OtsuboS and Hiroyuki Sasabe*, *Core Reserach for Evolutional Science and Technology (CREST), JST, Frontier Research Program, The Institute of Physical and Chemical Research (RIKEN), Saitama, JAPAN; Department of Chemistry, Toyama University, Toyama, JAPAN; SDepartment of Applied Chemistry, Hiroshima Unversity, Hiroshima, JAPAN.
Conjugated systems containing a thiophene moiety have received much more attention than benzene systems from the view point of their electroactive and optical applications. The thiophene system has a lower aromatization energy which is advantageous for extended conjugation. In order to examine the influence of the donor and acceptor substituent groups on the nonlinear optical property of the conjugation path, we studied the , -oligothiophene and oligothienoquinonoid derivatives. In this paper, we describe linear and nonlinear optical properties of push-pull oligothiophenes and oligothienoquinonoid derivatives. Third-harmonic generation, hyper-Rayleigh scattering and electroabsorption measurements were performed. The head-to-tail arrangement in 3-alkyl oligothiophenes has a red-shifted absorption, and enhanced second- and third-order susceptibilities. Enhancements of second- and third-order susceptibilities were also obtained in the extended push-pull oligothienoquinonoid system. These oligothienoquinonoid systems are a new class of efficient nonlinear optical chromophores.
DESIGN, METHODOLOGY AND PREPARATION OF NOVEL POLYMERS FOR NON-LINEAR OPTICS. Xiaogong Wang1, Mongkol Sukwattanasinitt1, Srinivasan Balasubramanian1, Ke Yang2, Jayant Kumar2, Daniel J. Sandman1 and Sukant K. Tripathy1, Departments of Chemistry1 and Physics2, University of Massachusetts, Lowell, MA; Lian Li and K.G. Chittibabu, Molecular Technologies Inc., Westford, MA.
Novel nonlinear optical (NLO) polymers have been developed based on the post functionalization reactions of a number of precursor polymers. The precursor polymers include epoxy based polymers, polydiacetlenes (with large (3)), semi-rigid tethered polyimides, among others. The precursor polymers were post-functionalized by azo coupling reaction and tricyanovinylation. The NLO polymers contain chromophores with different or conjugation system and electron acceptors. The correlation between chromophore structures and polymer properties was investigated. The properties of polymers containing the same chromophore and different types of polymer main-chains were compared. Depending on both chromophore and polymer structures, the NL0 chromophores can be organized into noncentrosyrmmetric alignment by electric field poling or self-assembly, leading to large second order optical nonlinearities. For example, d33 value of 80 pm/V at 1.55 m was determined for a post functionalized polymer containing 4-(5-tricyanovinylthienyl-2 )aniline chromophores.
CHARACTERIZING THE NLO CHROMOPHORE ORIENTATION OF POLYMERIC FILM BY ELECTROABSORPTION SPECTROSCOPY. Ke Yang1, Xiaogong Wang 2, Woohong Kim2, Aloke Jain1, Lian Li1, Jayant Kumar1, and Sukant Tripathy2, Department of Physics1 and Department of Chemistry2, Center for Advanced Materials, University of Massachusetts, Lowell, Lowell, MA.
The dispersion of third order nonlinear coefficients (3)1133 and (3)3333 of three different NLO polymer films were determined by electroabsorption spectroscopy. The first material investigated is an epoxy based polymer BP-2A-NT, with azobenzene NLO chromophore 4-[((4-Nitrophenyl)(azo)phenyl)azo]aniline in its side chain. The other materials are two polydiacetylenes poly(BPOD) and poly(4-BCMU), in which the delocalized polymer chains contribute to the third order nonlinearity. The complex spectrum of (3)3333 of each material is very similar in shape to the corresponding (3)1133 spectrum. The ratio of (3)3333 to (3)1133 is 3.2 for BP-2A-NT, 1.5 for both poly(BPOD) and poly(4 BCMU). These ratios indicate that the distribution of the side chain NLO chromophores of BP-2A-NT is very close to three dimensional isotropic distribution, and the distribution of the main chain chromophores of poly(BPOD) and poly(4-BCMU) is concentrated on the film plane.
SECOND-ORDER NONLINEAR OPTICAL POLYMERS DERIVED FROM POLYETHERSULFONE. Harold E. McCarron, Kwang S. Kang and Braja K. Madal, Department of Chemistry, Illinois Institute of Technology, Chicago, IL.
Abstract not available.
8:30 AM J10.1
MULTILAYER SELF-ASSEMBLIES AS ELECTRONIC AND OPTICAL MATERIALS. DeQuan Li, Xiaobo Shi, Yongwoo Lee, Michael Luett, Michael Fitzsimmons, and Duncan McBranch, Los Alamos National Laboratory, Chemical Science and Technology Div and Los Alamos Neutron Scattering Center, Los Alamos, NM.
This paper will discuss a broad spectrum of multilayer self-assemblies based on strong and weak interactions. Multiple functionalities were built in these molecular self-assemblies in order to emphasize advanced materials applications in these systems. Multulayer thin films ( 100 nm thick) consisting photoluminescence polymers and electron acceptors are constructed for electrical and optical measurements. Potential applications are using their electronic and optical properties in optoelectronic devices such as charge carriers, light emitting layers, or photorefractive materials. While incorporating molecular recognition receptors, these systems can be tailored to function as smart skins or chemical or bio-sensors. Indeed, we have demonstrated that cyclodextrins can be used as surface-bound receptors for chemical detections due to their well-defined cavities.
8:45 AM J10.2
IMPROVED PERFORMANCES OF SEXITHIOPHENE BASED THIN-FILM TRANSISTOR USING A SILICON OXIDE SURFACE FUNCTIONALIZATION BY SELF-ASSEMBLED MONOLAYERS. O. Tharaud, J. Collet, C. Legrand, D. Vuillaume, A. Chapoton, IEMN-CNRS, Villeneuve dí Ascq, FRANCE.
Thin-film transistors (TFT) made of conducting oligomers with good electrical performances are obtained for organic films well ordered at a molecular level. Highly ordered films are obtained provided that oligomers have a sufficient mobility on the substrate surface. One possible way to fulfill such a condition is to evaporate oligomers on heated substrate. In this work, high surface mobility is obtained by a chemical functionalization of the silicon dioxide surface, and the corresponding improvements of the TFT performances are evidenced. A self-assembled monolayer of octadecyltrichlorosilane (OTS) was deposited on the SiO2 (250 nm thermally grown oxide on a heavily doped silicon) by chemisorption from solution. This OTS monolayer reduces the surface tension from 78 mN/m (untreated SiO2) to 20.5-21 mN/m (contact angle measurements). Then, a 50 nm thick layer of sexithiophene was deposited by thermal evaporation onto the treated substrate that was held at 80 C. Devices were completed by evaporating an 80 nm thick gold layer (optical lithography and lift-off) to form source and drain contacts (L=5m, W=100m). For comparison, similar devices without the OTS monolayers were made simultaneously. Room temperature current-voltage measurements indicate that the presence of the OTS monolayer improves TFT performances : threshold voltage decreases from 22 V to 10 V; subthreshold swing decreases from 14 V/decade to 8 V/decade; finally a current ratio Ion/Ioff of 1E6 is obtained for a gate voltage excursion of 75 V instead of 115V. We surmise that these improved performances are due to a better molecular order in the sexithiophene layer. A low surface tension of the OTS treated SiO2 substrate tends to increase the surface mobility of oligomers during the evaporation of the film.
9:00 AM *J10.3
DETERMINATION OF THE INTERNAL ELECTRIC FIELD IN THE ELECTROOPTIC ACTIVE LAYER OF MULTI LAYER POLYMER STACKS. S. Grossmann, S. Saal, T. Weyrauch, and W. Haase, Institute of Physical Chemistry, Darmstadt University of Technology, Darmstadt, GERMANY.
Electrical field poling of polymer multi layer structures is of basic interest for fabricating a variety of integrated optic polymer devices, e.g. waveguide devices with electively located passive and active electrooptic (EO) polymers. Due to different resistivities of the involved layers one expect a field distribution according to the Ohm¥s law. The effect of internal surfaces, charges, and polarisations are difficult to estimate. In order to achieve efficient poling of these polymer structures a large part of the applied voltage must be applied to the active layer. Up to now no direct method to investigate the effective electric field in each layer of a polymer stack is known. In this paper we present a method, which allows to measure the field strength selective in the EO active polymer layer, using the Stark effect of the EO chromphores as sensor. The investigated polymer structures are made up of two layers. In order to measure the electric field strength in the active layer an electric AC field is applied to the sample in addition to the DC poling field. Because of the different spectral behaviour of the quadratic and the linear electroabsorption (Stark effect) we can distinguish between effects caused by the internal electric field and the polar order of the chromophores, respectively. Thus the value and the time dependency of the internal field in the active layer is detectable. The influences of different layer conductivities and interface effects on the internal effective field can be studied. We observed both, an increase and a decrease of the internal DC field, with time, depending on the polymers in the stacks. Information about time course and magnitude of the polar order of the dissolved EO chromophores is also available.
9:30 AM J10.4
CONTROLLING MATERIALS ARCHITECTURE ON THE NANOMETER SCALE: PPV NANOCOMPOSITES USING POLYMERIZABLE LYOTROPIC LIQUID CRYSTALS. Ryan C. Smith, Walter M. Fischer, Hai Deng, and Douglas L. Gin, University of California, Department of Chemistry, Berkeley, CA.
Nanometer-scale architectural control is a common feature of biological materials, which is primarily responsible for their impressive properties. Due to the limitations of conventional processing techniques, control over this realm has typically been inaccessible for the construction of synthetic materials. One of the questions that we wanted to address is whether engineered nanoarchitecture, combined with modern engineering materials, would result in new manmade materials with superior or unique bulk properties. We have developed a general synthesis strategy for the construction of ordered nanocomposites, using polymerizable Iyotropic (i.e., amphiphilic) liquid crystals (LCs). In this strategy, self-organizing Iyotropic LC monomers are used to form an ordered template matrix in the presence of a reactive hydrophilic solution. Subsequent photopolymerization to lock-in the matrix architecture, followed by initiation of chemistry within the ordered hydrophilic domains to afford solid-state fillers, yields the anisotropic nanocomposites. Composites have been synthesized that have a regular hexagonal arrangement of extended poly (-phenylenevinylene) (PPV) domains, with an interchannel spacing of 4 nm. The photoluminescence of these materials is significantly altered from that of bulk PPV. The nanometer-scale dimensions of these composites can be tuned by varying the size of the hydrophobic tails and/or the nature of the counterion associated with the hydrophilic headgroup of the monomers.
9:45 AM J10.5
HIGH MOBILITY AND LIQUID PHASE PROCESSABLE TFT SEMICONDUCTORS. H.E. Katz, J. Laquindanum, and A.J. Lovinger, Bell Laboratories-Lucent Technologies, Murray Hill, NJ.
Mobilities ranging from 0.02-1 cm2/Vs are obtainable from a wide variety of hole-transporting compounds in thin film transistors (TFTs). Comparable mobilities from n-channel materials and liquid phase deposition processes are desirable for technologically attractive all-organic electronics. We have synthesized new compounds with various conjugated frameworks aimed at meeting those goals. For example, oligomeric and fused aromatic compounds that combine hole mobilities up to 0.2 cm2/Vs with solubilizing side chains have been discovered. Some of these show extraordinary thin film crystallinity and the most definitive chemical evidence so far of two-dimensional transport. In addition to the above p-type compounds, we have continued our investigation of complementary n-type materials based on the naphthalene framework, developing new methods for introducing the electron withdrawing and solubilizing groups. Compatibilization of these compounds with liquid-phase patterning and deposition will be discussed.
10:30 AM *J10.6
PHOTOINDUCED STIMULATED EMISSION IN SEXITHIOPHENE SINGLE CRYSTALS. Francis Garnier, Gilles Horowiz, Pierre Valat, Fayçal Kouki and Véronique Wintgens, CNRS, Laboratoire des Matériaux Moléculaires, Thiais, FRANCE.
Stimulated emission in organic conjugated polymers has attracted much attention recently, aiming at the development of organic based laser diodes. In the present communication, we show evidence for stimulated emission in sexithiophene (6T) single crystals. At room temperature, the PL spectrum presents a first set of three peaks, consisting of a 0-0 transition at 16860 cm1 and its vibronic replica. When decreasing the temperature, tow other sets of peaks appear successively, in which the 0-0 transitions are located at higher energies, 17510 and 17980 cm-1 respectively. The low temperature emission level of these latter high energy 0-0 peaks is much lower than that of their replica (0-1 and 0-2 peaks), which suggests that these two sets of emission peaks originate from excited state(s) with Ag symmetry, associated with forbidden transitions, made partially allowed by vibronic coupling. On the other hand, the low temperature excitation spectrum presents a fine structure, with a very narrow 0-0 peak at 18350 cm-1, which is very close to the Ag state estimated from two photon excitation. At low excitation level, the transient PL presents a single exponential decay with a time constant of 1.9 ns. At higher excitation levels, a shorter and more intense emission superimposed to the low excitation signal. Which we attribute to stimulated emission. This is confirmed by the emission spectrum, which consists, at 10 K, of a very narrow line (FWHM 13-1) centered at 16870 cm-1, close to the origin of the room temperature steady state emission.. We therefore attribute the latter transition to the bottom of the Bk exciton band.
11:00 AM J10.7
ELECTRICAL CHARACTERIZATION OF MESOSCOPIC SINGLE CRYSTALS OF SEXITHIOPHENE. C. Daniel Frisbie, Eric L. Granstrom, Michael J. Loiacono, Department of Chemical Engineering and Materials Science, University of Minnesota, Minneapolis, MN.
Single crystals of the molecular semiconductor alpha-sexithiophene (6T) are grown by vacuum sublimation onto conducting (eg., Au) or insulating (eg., SiO2) substrates. The crystals range from 50-800 nm in diameter and from 1-6 monolayers (2.5-15 nm) in thickness. The lattice parameters of these crystals are established by atomic force microscopy which confirms that the molecules are oriented with their long axes approximately perpendicular to the substrate. The electrical conductivity of the 6T crystals are probed using two methods. In one approach, individual 6T grains are grown in a small gap between individually addressable gold electrodes fabricated on SiO2 substrates by electron beam lithography. A gate electrode lying underneath the SiO2 on which the 6T crystals are grown allows the lateral conductivity of the 6T crystal to be probed as a function of field induced carrier concentration. These transport measurements are made as a function of the number of monolayers in the 6T island and reveal that the first monolayer adjacent the SiO2 is at least 10 times less conductive than the subsequent layers. These transistor measurements also allow determination of the charge carrier mobility in individual 6T crystals. The second type of measurement utilizes a gold-coated tip in an atomic force microscope as a positionable, nanoscopic electrical contact. Conductive probes are used to measure both lateral and vertical conductivity through 6T crystallites doped with I2. In vertical measurements which probe conduction through the thickness of the 6T island and hence down the long axes of the 6T molecules, the gold-coated probe is positioned on top of the crystal with a predetermined load, and the I-V trace is recorded using the substrate as the second contact. Plots of conductance vs. number of 6T layers in the crystal display a sharp peak at 3 monolayers with conductance through 3 monolayers being 100 times greater than conductance through a single monolayer. Lateral conductivity measurements of 6T crystals using conducting probe AFM confirm findings from the transistor experiments but also are used to measure conductance across individual 6T grain boundaries.
11:15 AM J10.8
NOVEL LIQUID CRYSTAL DISPLAYS BASED ON HIGHLY POLARIZED PHOTOLUMINESCENT POLYMER FILMS. Ch. Sarwa, A. Montali, C. Bastiaansen, Ch. Weder, P. Smith, ETH Zürich, Dept of Materials, Zürich, SWITZERLAND.
Since the early 90's, much research has focused on the photoluminescence (PL) and electroluminescence (EL) properties of conjugated polymers, because of their potential application as emitting layer in EL devices. The introduction of uniaxial molecular orientation into films of luminescent polymers was naturally found to yield structures that emit polarized light. Rather surprisingly, the photoluminescence properties of oriented, conjugated polymers have attracted substantially less attention, especially from an application point of view. In this paper we report the fabrication of highly-polarized photoluminescent polymer films based on poly(2,5-dialkoxy-p phenyleneethynylene)s (PPE), and their use in a new family of liquid crystal displays (LCDs). As one relevant example, a back-lit twisted nematic configuration of an LCD was built, in which one of the absorbing polarizers was replaced by a polarized PL film, characterized by a dichroic ratio in excess of 70. Such devices can exhibit a substantial improvement in brightness, contrast and viewing angle, since the polarized photoluminescent films can combine two separate features, i.e. the functions of a polarizer and an efficient color filter.
11:30 AM J10.9
ELECTROOPTIC EFFECTS IN POLYMER DISPERSED LIQUID CRYSTAL SYSTEMS. Richard Blacker, Keith Lewis, Ian Mason, Ian Sage, Kate Webb, DERA Malvern, Worcs, ENGLAND.
Research into electro-optic effects in nanophase polymer dispersed liquid crystal (PDLC) materials has highlighted their potential as materials for a new class of tuneable filters. The structures, based on UV cured phase separated composites, contain liquid crystal both as discrete nano-scale droplets, and as material dissolved in the polymeric host. The essential difference between these materials and more conventional PDLC's is the scale of the refractive index inhomogeneity which is considerably smaller than the wavelength of visible light. Based upon effective medium approximations, the composite thus acts as a single isotropic medium, whose average refractive index is dependant on the level of applied electric held. Visible band Fabry-Perot etalon devices have been fabricated. These structures yield a series of electrically tuneable optical resonances. A summary of the factors that control the properties of such devices and how these relate to potential areas of application will be presented.
11:45 AM J10.10
MOBILITY AND OTHER ELECTRICAL PROPERTIES FROM BULK AND FET STUDIES ON 6T, 4T AND PENTACENE SINGLE CRYSTALS. H. Schona, Ch. Kloca, R.A. Laudise and B. Batlogg, Bell Laboratories, Lucent Technologies, Murray Hill, NJ; a University of Konstanz, Konstanz, GERMANY.
Single crystals of the organic semiconductors -hexathiophene, (6T), -quarterthiophene, (4T), and pentacene have been used to measure their electrical bulk properties and to fabricate single crystal field effect transistors, FET. From temperature dependent measurements of space-charge-limiting current, along directions parallel and perpendicular to the crystal surfaces, we have determined the acceptor concentration, hole mobility, and trap densities. Further studies focus on the dependence of those electrical properties on technological parameters, such as (1) transport gases used during crystal growth, and (2) post-growth annealing in oxidizing and reducing atmosphere. Optimized conditions yield trap densities and acceptors densities in 6T crystals as low as 5x1014 cm-3 and 7x1010 cm-3, respectively. FETs on 4T and 6T crystals give mobilities of 0.05 and 0.52 cm2/Vs, and on/off ratios of 5x103 and 105, respectively. These high mobilities are in close agreement with values derived independently from I-V measurements on the bulk crystals.
SESSION J11: CONJUGATED POLYMERS
Chair: Alan G. MacDiarmid
Thursday Afternoon, December 4, 1997
Salon G (M)
1:30 PM *J11.1
SYNTHESIS OF CONJUGATED POLYMERS BY VAPOR DEPOSITION POLYMERIZATION. Chain-Shu Hsu, Department of Applied Chemistry, National Chaio Tung University, Hsinchu, TAIWAN.
The synthesis of phenylated poly(p-phenylene vinylene) and poly(2,5-thienylene vinylene) by vapor deposition polymerization is presented.
2,3-Diphenyl-1,4-bis(chloromethyl)benzene and 2,3-diphenyl- bis(chloromethyl)thiophene are subjected to vapor deposition pyrolysis above 650 centigrade to yield corresponding reactive intermediates. The reactive intermediates are polymerized spontaneously in bulk to yield phenylated poly(p-phenylene vinylene) and poly(2,5-thienylene vinylene). The obtained polymeric flms are characterized by UV, IR, and solid state NMR spectroscopies. Their thermal properties are measured by DSC and TG analysis. The potential application of the obtained conjugated polymeric films in the field of organic LED is also discussed.
2:00 PM J11.2
ELECTROCONDUCTIVE HYDROGELS: ELECTRICAL, ELECTROCHEMICAL, AND IMPEDANCE PROPERTIES. Anthony Guiseppi-Elie, Andrew R. Sujdak, Ann M. Wilson, Research and Development Dept, ABTECH Scientific, Inc, Yardley, PA.
A novel class of polymers - the electroconductive hydrogels is introduced. These materials are formed by sequential photopolymerization of methacrylate-functionalized prepolymers and monomers followed by chemical or electrochemical polymerization of aniline or pyrrole. This yields an interpenetrating network of inherently conductive polyaniline or polypyrrole, within the water-swellable, freestanding or electrode-supported, hydrogel. The water content of these gels was measured by mass change following exhaustive drying and found to correlate with the conductive polymer content. Electroactivity was measured by multiple scan rate cyclic voltammetry. Ion transport characteristics were greatly enhanced in these gels relative to pure electroconductive polymers. Electrical conductivity was measured by 4-point technique on microfabricated, co-planar, interdigitated microsensor electrode arrays. Hydrogels were equally conductive but more stable than pure electroconductive polymers. In-plane and standard across-the-solution electrochemical impedance spectroscopy over the range 1 x 10E-4 Hz to 1 x 10E5 Hz showed the conductive polymer to principally influence the faradic component of a modified Randles equivalent circuit. Applications as field-responsive materials in immuno-biosensors and in controlled electrorelease of bioactive peptides will be discussed.
2:15 PM J11.3
DOUBLE-STRAND POLYANILINE AS MOLECULAR MEMORY TO CHEMICAL STIMULI. Gowri P. Kota, Linfeng Sun, Huaibing Liu, Sze C. Yang, Univ of Rhode Island, Dept of Chemistry, Kingston, RI.
Intramolecular cooperative interactions within a macromolecule may lead to bistability in molecular structures or conformations. If the structure and property of the macromolecule respond to an externally controllable stimulus in a measurable way, the response function will show a hysteresis loop. Because the hysteresis response, the bistable states of the molecule is a demonstration of a molecular memory. In this article we report the chemical bistability in the conductor-insulator transition (an emeraldine salt to emeraldine base transition) for soluble doulbe-strand polyaniline. The water-soluble polymeric complex of polyaniline (a molecular complex with poly(styrene sulfonic acid)) was verified to be dissolved as non-aggregated single molecules in water. These molecules show strong hysteresis behavior in the acid/base titration. Under suitable conditions, the two bistable states are stable in the same pH solutions at room temperature for 3 months without losing its chemical memory. It is postulated that the double-strand structure is responsible for the molecular bistability.
2:30 PM J11.4
EFFECT OF MORPHOLOGY ON THE ELECTRICAL TRANSPORT PROPERTIES OF POLYANILINE FILMS FOR ELECTRONIC APPLICATIONS. S.S. Hardaker, K. Eaiprasertsak, and R.V. Gregory, School of Textiles, Fiber, and Polymer Science, Clemson University, Clemson SC; G. X. Tessema, Department of Physics, Clemson University, Clemson, SC.
Recent studies in our laboratory have demonstrated and confirmed the effect of differing morphologies in polyaniline films on their electronic properties and electrical transport mechanisms. Films formed from a variety of different solvents are structurally different resulting in the observered changes in electrical properties. Studies determining and characterizing these structural differences in polyaniline films by X-ray, DMA, DSC, and optical spectroscopies are reported and contrasted with observed changes in charge transport, thermopower, and optical properties. Determination of the effect of initial oxidation state on the properties of the formed polymer and changes in the polymers electronic and electrical structure/property relationships due to changes in initial oxidation state of the polymer will also be discussed in detail.
3:15 PM J11.5
SYNTHETIC APPROACHES TOWARD DENDRITIC CONJUGTED POLYMERS BASED ON POLYFUNCTIONAL C60 PRECUSORS. Long Y. Chiang, Jayendra Bhonsle, and Lee Y. Wang, Center for Condensed Matter Sciences, National Taiwan University, Taipei, TAIWAN.
Extensive research on fullerene functionalization leads to various synthetic chemistry suitable for the preparation of new C60-based polymers. Highly functionalized fullerenes with attached polar hydroxy or amino groups can be utilized as a molecular core for anchoring dense polymer arms via directly bonding with hydroxy or amino functions. These polymer arms may branch out from the ball-shaped fullerene core and stretch in three dimensions, making the resulting macromolecule a truly star-shaped dendritic polymer. By the utilization of polymerizable precursors, such as aniline and thiophene, covalently attached on the fullerene core, it is possible to synthesize the corresponding dendritic conjugated polymers. Recently, we have developed a number of C60 functionalization chemistry for introducing multiple polar functions onto a fullerene cage. Detailed dendritic fullerene syntheses and materials characterization will be discussed.
3:30 PM J11.6
MELT PROCESSED ELECTRICALLY CONDUCTIVE BINARY AND TERNARY IMMISCIBLE POLYMER/POLYANILINE BLENDS. Meital Zilberman1, Arnon Siegmann2 and Moshe Narkis2, Technion-Israel Institute of Technology, Departments of 1Materials and 2Chemical Engineering, Haifa, ISRAEL.
There is an increasing interest in processible polymeric materials, using conventional melt processing techniques, whose electrical conductivity can be tailored for a given application. Intrinsically conductive polymers (ICPs) are expected to yield this attractive combination of properties. Polyaniline (PANI) is one of the most promising ICPs. The main disadvantage of PANI, like other ICPs, is its limited thermal processability. One of the methods to process PANI without altering the polymer's structure, is by blending with conventional polymers. These blends may combine the desired properties of the two components. i.e., electrical conductivity of PANI together with physical and mechanical properties of the matrix polymer. In the present study, conductive PANI was blended with several thermoplastic matrices, in a Brabender mixer. The PANI characterization included conductivity measurements, morphology, crystallography, and thermal behavior. The Binary blends' investigation focused on their morphology in light of the interaction between the components, and on the resulting electrical conductivity. Generally, the level of interaction between the PANI and the matrix polymer determines the blend morphology, and the resulting electrical conductivity. Similar solubility parameters of the two components lead to high level of PANI dispersion within the matrix, and to formation of conducting paths at low PANI contents. The morphology of a conducting network is described by a primary structure of small dispersed PANI particles, interconnected by a secondary short range fibrillar structure. Ternary blends, containing two immiscible thermoplastic polymers and PANI exhibit high electrical conductivities at low PANI contents, when the PANI is preferably located in one of the phases of a co-continuous blend.
3:45 PM J11.7
THERMOREVERSIBLE GELS OF POLYANILINE: VISCOELASTIC AND ELECTRICAL PROPERTIES. Terhi Vikki, Janne Ruokolainen, Olli Ikkala, Helsinki Univ of Technology, Materials Physics Lab, Espoo, FINLAND; Mika Torkkeli, Ritva Serimaa, Helsinki Univ, Dept of Physics, Helsinki, FINLAND; Heikki Isotalo, State Research Centre, Espoo, FINLAND; Pentti Passiniemi, Neste Oy, FINLAND.
We show that polyaniline (PANI) dissolved in dodecyl benzene sulphonic acid (DBSA) using formic acid medium allows thermoreversible gelation after formic acid is removed. Formic acid allows particle-free complexes in optical microscopy. Below the gelation temperature the materials are elastic in compression experiments, the storage and loss moduli do not essentially depend on frequency, and the samples are electronically conducting. Above the gelation temperature the moduli indicate flow-like behavior and substantial ionic conduction component is observed. These results suggest reversible i.e. fusible network formation. Other electrically conducting thermoreversible gels and their transitions are also discussed.
4:00 PM J11.8
THE CONDUCTIVITY OF BULK DOPED POLYPYRROLE. B. Chapman, R.G. Buckley, New Zealand Institute for Industrial Research, Lower Hutt, NEW ZEALAND; N.T. Kemp, A.B. Kaiser, D. Beaglehole, and H.J. Trodahl, School of Chemical and Physical Sciences, Victoria University of Wellington, Wellington, NEW ZEALAND.
We have performed temperature-dependent (1.5 - 300K) conductivity and thermoelectric power measurements on a wide range of doped polypyrrole samples prepared under various conditions and with various dopants, and spectral conductivity measurements (0.004 - 5 eV) on a subset of optical quality samples. The temperature-dependent conductivity decreases with decreasing temperature and for the samples with the highest conductivity we observe an increase at the lowest temperature (10K). The results emphasise that the conductivity in these materials is limited by charge transfer between metallic regions or chains. The optical conductivity shows a depression in the conductivity at energies below 200 meV. This may be due to localisation or inhomogeneity and can be shown to fit a two component model. The response below 100 meV is dominated by the presense of a low-frequency excitation and displays a positive dielectric constant. Conductivity measurements have also been made on material which has been exposed to various gasses in order to help identify the source of the gas sensitivity and selectivity, an effect which is being developed as the transducer in an ''electronic nose''. Again the results establish that it is the inter-chain transport which is affected by the absorbed gas.
4:15 PM J11.9
ELECTROCHEMICALLY DEPOSITED POLYPYRROLE ON GOLDYBa2Cu3O7 COMPOSITE THIN FILMS. Chris E. Jones, John T. McDevitt and Jianai Zhao, Department of Chemistry and Biochemistry, University of Texas at Austin, Austin, TX.
Gold/YBa2Cu3O7 composite films are evaluated as template layers for the study of conductive polymer/superconductor interactions. Scanning electron microscopy, 4-point resistivity and electrochemical techniques are utilized to characterize the electrodeposition of polypyrrole layers grown onto such composite thin film structures. These studies demonstrate that the weak-link rich gold/YBa2Cu3O7 composite films are receptive to polypyrrole growth by electrochemical means. Influence of the polymer charging/discharging characteristics on the superconductor properties are discussed.
4:30 PM J11.10
DYNAMIC HOLOGRAPHY OF PHOTOCHROMIC LIQUID HYDROGELS. Joseph D. Gresser, Charles M. Lyons, Edgardo J. Mantilla, Debra J. Trantolo, Donald L. Wise, Cambridge Scientific, Inc., Boston, MA.
Hydrogels are crosslinked macromolecular networks swollen in water or biological fluids. These moieties have become excellent carriers for release of drugs and bioactive macromolecules either in their equilibrium swollen state or a dynamically swelling system. The biocompatibility of hydrogels is attributed to their ability to simulate the natural tissue due to their high water content and their special surface properties. Their major disadvantage or their low mechanical strength can be overcome either by crosslinking, by formation of interpenetrating networks, or by crystallization which induces crystallic formation and drastic reinforcement of their structure. Another application of crosslinked hydrogels is in the development of optical materials with potential applications for real time holography. Cambridge Scientific, Inc. has collaborated with the Hardened Materials Branch, Materials Directorate at Wright-Patterson Air Force Base (Ohio) in the investigation of this property of hydrogels. The focus of this talk will be on the preparation and characterization of oriented-crosslinked biopolymers. Results of the modification of these hydrogels by using spiropyran and by changing the crosslinker will be discussed.
4:45 PM J11.11
ADVANCES IN PRINTED PLASTIC TRANSISTORS. Zhenan Bao, V. Reddy Raju, Ananth Dodabalapur, Andrew J. Lovinger, Bell Laboratories, Lucent Technologies, Murray Hill, NJ; Yi Feng, Goodyear Tire & Rubber Co., Akron, OH; Janelle Brown, Department of Chemistry, Massachusetts Institute of Technology, Cambridge, MA.
In this paper, we will present results on high performance materials for field-effect transistor (FET) applications with emphasis on the processability and stability of the organic and polymeric semiconducting materials. Various soluble and processable conjugated polymers have been investigated as the semiconducting layers in FETs. Regioregular poly(3-alkylthiophene)s were found to have the highest field-effect mobility and the largest on/off ratio due to their ordered structures in the solid state. This performance is the best reported so far for polymeric semiconducting materials and enables us to fabricate printed plastic transistors in which all components, a polymer dielectric layer, a semiconducting regioregular poly(3-alkylthiophene) layer, and two silver electrodes, have been printed on an ITO-coated plastic substrate. The performance of these transistors is comparable to those formed on Si substrate using SiO2 dielectric and lithographically defined Au electrodes. The field effect mobilities are on the order of 10-2 cm2/Vs. In addition, we have made air-stable n-channel transistors with very high mobility ca. 0.03 cm2/Vs
SESSION J12: NONLINEAR OPTICAL/PHOTOREFRACTIVE MATERIALS-III
Chair: Sukant K. Tripathy
Friday Morning, December 5, 1997
Salon G (M)
8:30 AM *J12.1
SYNTHESIS AND NONLINEARITY OF TRIENE CHROMOPHORES CONTAINING THE CYCLOHEXENE RING STRUCTURE. Susan Ermer, Steven M. Lovejoy, Doris S. Leung, and Hope Warren, Lockheed Martin Advanced Technology Center, Palo alto, CA; Christopher R. Moylan and Robert J. Twieg, IBM Almaden Research Center, San Jose, CA.
A series of conjugated donor-acceptor trienes in which the central double bond is incorporated into an unsaturated isophorone, verbenone or chromone ring has been synthesized. In each case, the donor group consists of an amine and an aromatic or heterocyclic ring system, and the acceptor is the dicyanomethylidene group. The nonlinear optical properties of each of the compounds has been measured and correlated with its structure. The dipole moments and molecular hyperpolarizabilities of these compounds, like those of other conjugated polyenes, are large enough to be used as the active components of electro-optic polymers. Unlike other donor-acceptor polyenes, however, these compounds exhibit the thermal stability required for such applications.
9:00 AM *J12.2
MULTIFUNCTIONAL HYPER-STRUCTURED MOLCULES. Tatsuo Wada , Yadong Zhang, Tetsuya Aoyama, Yuji Kubo* and Hiroyuki Sasabe, Core Research for Evolutional Science and Technology (CREST), JST; Frontier Research Program, The Institute of Physical and Chemical Research (RIKEN), Saitama, JAPAN; *Department of Applied Chemistry, Saitama University, Saltama, JAPAN.
For well-defined and topologically controlled molecular systems we studied hyperbranched oligomers, cyclic oligomers and chromogenic calixarenes. Systems exhibiting multifunctional properties such as photoconductivity, electro-optic activity, and photorefractive responses, were designed and investigated. We selected acceptor-substituted carbazole as a building block for carbazole oligomers. Molecular solids of carbazole oligomers exhibit a glass-rubber transition. Their thin films were prepared by spin-coating without a polymer matrix and can be poled above the glass transition temperature. These compounds were confirmed to be multifunctional and displayed both photoconductive and second-order nonlinear optical responses. Unlike multi-component photorefractive polymers, these multifunctional hyper-structured molecules can be applied to monolithic photorefractive materials. The topological shapes of indoaniline-derived calixarenes were studied by hyper-Rayleigh scattering. The two indoaniline moieties in calixarene derivatives were pre-aligned and net molecular hyperpolarizability was enhanced. Besides dendric oligomers, cyclic oligomers can be used as a molecular platform which allow molecular level tuning of shape, size and topology for superior optoelectronic functions.
9:30 AM J12.3
INFLUENCE OF HOST ENVIRONMENT ON THE POLING CHARACTERISTICS AND CHROMOPHORE-CHROMOPHORE INTERACTIONS IN ELECTRO-OPTIC GUEST-HOST POLYMER SYSTEMS. Max D. Alexander Jr, University of Dayton Research Institute, Dayton, OH; Richard A. Vaia, Michael Banach, Stephen Caracci, Bruce Reinhardt, Robert Spry, Aterials Directorate, Air Force Research Laboratory, Wright-Patterson AFB, OH.
Electro-optic (EO) systems are currently of intense interest for switching applications in high speed, ultra-large bandwidth communication devices. Nonlinear optical response originates from a non-centrosymmetric alignment of the chromophores. In a polymer matrix, the non-centrosymmetric alignment is conventionally achieved by aligning the dipole moment of the chromophores in a large dc electric field near the glass transition temperature of the polymer. Once the chromophores are sufficiently poled, the sample is cooled to ``freeze-in'' the dipole alignment and the field is removed. Whether the chromophore is solvated or present in associated structures before or during poling will depend critically on the processing conditions and the host matrix. Thus in addition to matrix and chromophore relaxation, the molecular environment and ``morphology'' of the chromophore will have a substantial effect on the poling characteristics of the EO polymer system.
We report on our results relating to N,N-diphenyl-7-[2-(4 pyndinyl)ethenyl]-9,9-di-n-decylfluoren-2-amine, known as AF-50, in a variety of host matrices of differing polarity. In contrast to many well known chromophores, such as disperse red one (DR1), AF-50 exhibits a strong photoluminescence under both single and two photon excitation. Photoluminescence and absorption measurements are used to quantify the influence of temperature and chromophore concentration on the chromophore-chromophore interactions in various host polymers. Additionally, the extent of molecular orientation prior to and after poling is determined by waveguide index measurements. These results are correlated to in-situ EO measurements of an equivalent r33. The host matrix and processing solvent are shown to influence vvhether the chromophores are preferentially solvated or occur as dimers and higher order associated structures in the EO polymer film The subsequent molecular environment and ``morphology'' of the chromophore has a profound influence on the ultimate macroscopic nonlinear response of the system.
10:15 AM J12.4
OXYGEN REACTIVE ION ETCHING(RIE) OF UV CURABLE ACRYLATE POLYMER FOR THE FABRICATION OF POLYMERIC ARRAYED WAVEGUIDE GRATING OPTICAL DEVICES. J.-H. Ahn, W.-Y. Hwang, M.-C. Oh, M.H. Lee, H.J. Lee, S.G. Han, Y.-H. Won, Photonics Switching Section, Electronics and Telecommunications Research Institute, Yusong , Taejon, KOREA.
The main features of reactive ion etching properties are smooth roughness and high anisotropy(vertical to lateral etch rate) for the arrayed waveguide grating devices because the optical performances are sensitive to waveguide width and roughness. This study demonstrates RIE conditions which affect the roughness, etch rate and anisotropy of UV curable acrylate polymer. The combinations of pressure(50-150mTorr), rf power(0.11-0.56W/cm2) and oxygen flow rate(30-150sccm) were chosen to investigate the etching properties. UV cured acrylate films coated on Si substrates with a si-based resist as the etch mask. The ratio of etch rate of the acrylate polymer/Si-based resist is more than 10. Higher rf power gives a higher etch rate when the pressure and oxygen flow rate are kept constant. With constant oxygen flow rate and rf power, there is a peak on the etch rate to pressure. The pressure value corresponding to the peak of etch rate shifts to the higher pressure at higher power. In our etching condition(0.11W/cm2 rf power, 50sccm oxygen flow rate), the peak of the pressure is 80mTorr. The roughness of etched side-wall and surface is improved with higher pressure and lower rf power. However, the anisotropy is inferior due to the dominant chemical etching with higher pressure. In 50sccm oxygen flow rate and 0.11W/cm2 rf power, the variances of the anisotropy vs. pressure(80-150mTorr) are measured between 7.15 to 3.54. The optimum RIE conditions for the smooth roughness and higher anisotropy are found to be 80-100mTorr pressure at 50sccm flow rate and 0.11W/cm2 rf power. The measured etch rate and the anisotropy are 90nm/min and 7.15, respectively. An arrayed waveguide grating wavelength router operating at 1550nm has been designed and fabricated using oxygen RIE techniques. The waveguide is 4x4um2 square buried channel structure. The performances were measured.
10:30 AM J12.5
INFLUENCE OF SOLVENT ENVIRONMENT ON THE DETERMINATION OF TWO-PHOTON CROSS SECTION AND FIRST ORDER HYPERPOLARIZABILITY OF NLO CHROMOPHORES. Richard A. Vaia, Michael Banach, Bruce Reinhardt, Robert Spry, Materials Directorate, Air Force Research Laboratory, Wright-Patterson AFB, OH; Max D. Alexander Jr, University of Dayton Research Institute, Dayton, OH; Paras N. Prasad and Lixiang Yaun, Photonics Research Laboratory, State University of New York at Buffalo, Buffalo, NY.
Organic molecules with large optical nonlinearities are currently of intense interest for diverse applications ranging from communication to optical limiting. Most potential uses of these molecules requires that they be incorporated in a host matrix, either liquid or solid. Additionally, the fundamental physical coefficients such as two-photon cross section () and first order hyperpolarizability ()are determined for solvated chromophores. The degree to which the chromophore is solvated or forms associated structures depends critically on the host environment. Thus the local environment of the chromophore should have a substantial effect on the determination of and . Here we report our results on the role of molecular environment on the second and third order NLO properties of N,N-diphenyl-7-[2-(4pyridinyl)ethenyl] -9,9-di-n-decyl-fluoren-2-amine, known as AF-50.
Photoluminescence and absorption measurements were used to quantify the effect of host environment on chromophore-chromophore interactions. As the solvent polarity or chromophore concentration increases the photoluminescence undergoes a sizable Stoke's shift, with an insignificant change in the UV absorption spectra in nonpolar solvents the photoluminescence shows distinct vibronic structure. Molar absorptivity and total photolurninescence output are seen to increase as the concentration of chromophore is reduced over several orders of magnitude. This behavior is typical of excimer formation in concentrated solution or polar solvent. These observations coincide well with two-photon cross sections and molecular hyperpolarizability determined in like solvents. In general, the molecular environment has a profound effect on the molecule's nonlinearity. The ultimate nonlinear response may be optimized by carefully engincenng the chromophore environment through thc addition of optically inert pendants.
10:45 AM J12.6
SYNTHESIS AND OPTICAL LIMITING CAPABILITY OF A NEW CLASS OF POLYPHENYLATED PHTHALOCYANINES. Christopher J. Walsh, Kwang S. Kang and Braja K. Mandal*, Department of Chemistry, Illinois Institute of Technology, Chicago, IL.
We wish to report the synthesis and optical limiting capability of a new class of peripherally modified, polyphenylated phthalocyanines and metallophthalocyanines. These compounds were synthesized by cyclic tetramerization of functionalized pentaphenylbenzophthalonitriles. The phthalonitriles were made in several steps via Diels-Alder cycloaddition between phenylethynylphthalonitrile and substituted tetracyclones. The polyphenylated design leads to enhanced solubility and good film forming properties. Cyclodehydrogenation studies have been attempted which would lead to completely planar, highly conjugated systems and the results will be discussed. The optical limiting properties in both solutions and evaporated films have been studied and will be presented.
11:00 AM J12.7
STUDY ON THE RELATIONS OF STRUCTURE AND THE ULTRAFAST OPTICAL KERR EFFECT OF POLYNITRILES. Q. Wang, W.W. Pei, Y.K. He, C.Q. Luo, H.Y. Chen, Peking Univ., Inst. of Polymer Science, College of Chemistry, Beijing, CHINA; C.F. Wang, Y.H. Zon, Peking University, Dept. of Physics, Beijing, CHINA.
Much attention has been paid on conjugated polymers during recent years because of their potential in various high technology applications. We have reported a novel class of conjugated polyntriles with unique third order nonlinear optical properties. In this paper we report the further study on the synthesis of various polybenzonitrile derivatives, and the relationship between molecular structure and optical nonlinearity of polybenzonitrile.
Polybenzonitriles with different substituent groups, such as polybenzonitrile (PBN), polytonlenenitrile (PTN), polyaminobenzonitrile (PABN), polymethoxbenzonitrile (PMBN), polycyanopyridine (PCP), plynitrobenzonitrile (PNBN) and poly (e,4-dimethoxbenzonitrile) (PDMBN) were prepared by ratio-frequency plasma glow discharge techniques or chemical polymerization. Polymers were characterized by FT-IR, UV-Vis, VPO, etc., and their ultrafast Kerr effect were measured by femtosecond dye laser light equipment. The nonresonant third-order NLO susceptibility and the second-order hyperpolarizability () increased with the enhancement of conjugated systems, i.e. PCP=PMBNPABN
11:15 AM J12.8
NOVEL POLYMER STRUCTURES WITH IMPROVED STABILITY OF NLO PROPERTIES, C.D. Eisenbach, Inst. of Technical Chemistry II, Univ. of Stuttgart, Stuttgart, GERMANY; M. Biber, K. Fischer, R. Hagen, D. Haarer, Macromol. Chem. II and Exp. Physics IV, Univ. of Bayreuth, Bayreuth, GERMANY.
A general problem in the service properties of polymer materials with nonlinear (NLO) properties is the loss of NLO chromophor orientation with time. This is due to the relatively unrestricted mobility of both the chromophor and matrix (chain) elements in conventional guest-host and NLO side-chain polymers where the chromophor simply is dissolved in the polymer or separated from the main chain by a spacer, respectively. The scope of this work was to stabilize the optical anisocropy of the system by restriction of the mobility of the oriented chromophors. This was achieved by the incorporation of the N,N-dialkylaminonitrostilbene based NLO-chromophor partly (pseudo-main chain NLO polymers) or completely (main chain NLO polymers) into the polymer backbone of polyurethanes or polyesters. These polymers were obtained by polycondensation of stilbene diols with diacyl chlorides or diisocyanates, and stilbene hydroxy carboxylic acid derivatives. The variation of the building blocks allowed to systematically vary the glass transition temperatures Tg between 5C and 140C; the completely amorphous polymers exhibited thermostability of up to 250 C. The NLO properties of the materials were investigated with thin films obtained by spin coating or casting from trifluoro acetic acid solution; chromophor orientation was done by corona poling. The SHG-signal of the polymers did not change upon heating up to the Tg region; this was in distinct contrast to conventional side chain NLO polymers where a loss of NLO properties was observed already well below Tg (-relaxation) due to higher relaxation processes In both the main chain and pseudo main chain NLO polymers, the change of the SHG-signal when going through the glass-rubber transition was almost a mirror image of the -CP change in the DSC trace. The different aspects of the incorporation of the NLO-chromophor in the polymer backbone on the electrooptical properties with regard to the longtime stability will be discussed with particular examples.
11:30 AM J12.9
SPONTANEOUS EMISSION AND ABSORPTION IN MODEL AGGREGATES OF CONJUGATED OLIGOMERS. Eric S. Manas and Frank C. Spano, Department of Chemistry, Temple University, Philadelphia, PA.
Interchain interactions in polymer films lead to picosecond energy transfer and possibly to cooperative emission or superradiance. Observation of the latter phenomenon in poly(para-pheylenevinylene)(PPV) films is currently a matter of some controversy. Here, we present model calculations of spontaneous emission and absorption in an aggregate of oriented -conjugated oligomers. Each oligomer is taken as a ring of N carbon atoms with average bond length a, and is treated using the Su Schrieffer Heeger(SSH) Hamiltonian with alternation parameter supplemented with Coulomb interactions. Interactions between rings are treated to second order. For aggregates in which the oligomers are aligned to normal to the aggregate axis with nearest neighbor distance d, intermolecular interactions cause a blue-shift in the absorption spectrum due to first order interchain excitation transfer. There is also a much smaller red-shift due to second order interactions which is negligible for >0.2. The average absorption frequency increases with N, peaks, and ultimately tends to a small value when the ring diameter greatly exceeds d. At this point the entire spectrum for an aggregate is indistinguishable from the isolated ring spectrum. The emission from an aggregate of M aligned rings is superradiant with a rate that is M times faster than an isolated oligomer. When , the superradiant rate increases linearly with over most of the range 01. Near =1, however, there is a sharp transition from a one-dimensional superradiant rate () to a two-dimensional rate, with scaling as the product MN when =1.
11:45 AM J12.10
SYNTHESIS OF NEW MULTIFUNCTIONAL POLYMERS INCORPORATING BIS-(DIPHENYLAMINO)DIPHENYLPOLYENE MOIETIES. Charles W. Spangler, Tami Faircloth, El Hadj Elandaloussi, Benjamin Reeves, Department of Chemistry and Biochemistry, Montana State University, Bozeman, MT.
In recent years the diphenylamino electron-donating group has been utilized extensively in the design of new thermally stable chromophores for applications in nonlinear optics and in organic light-emitting diodes. Recently bis(diphenylamino)stilbene has also been shown to have a large two-photon absorption cross section, which points to the possibility that molecules of similar structure may have utility as optical limiters or as two-photon up-conversion lasers. In this presentation we will present a general synthetic scheme for the synthesis of a variety of chromophores incorporating these donor groups, and strategies for the design of both main-chain and pendant polymers incorporating these covalently attached chromophores. We will also discuss the formation of bipolaron-like dications in these systems by oxidative doping, and how these doping studies can be utilized as models for photogeneration of bipolarons which may function as reverse saturable absorbers (RSAs).
SESSION J13: LIGHT EMITTING MATERIALS
Chair: Arthur J. Epstein
Friday Afternoon, December 5, 1997
Salon G (M)
1:30 PM *J13.1
CONTROL OF LIGHT EMITTING POLYMER DEVICES USING POLYMER/POLYMER INTERFACES. A.J. Epstein, Y.Z. Wang, D.D. Gebler, The Ohio State University, Dept of Physics, Columbus, OH; D.K. Fu, T.M. Swager, Massachusetts Institute of Technology, Dept of Chemistry, Cambridge, MA.
The central role of polymer/polymer interfaces in controlling the operation of several types of devices is presented. Trilayer devices consisting of blends of pyridine-phenylene and thiophene-phenylene based copolymers sandwiched between the emeraldine base and sulfonated forms of polyaniline with ITO and Al as electrodes are shown to operate under either polarity of driving voltages. Different colors of light are emitted, red under forward bias and green under reverse bias. The key roles of the polymer/polymer interfaces and balancing hole and electron transport in the emitter layer in obtaining the color variable bipolar operation are discussed. Bilayer heterojunctions formed from poly(pyridyl vinylene phenylene vinylene) (PPyVPV) derivatives and poly(vinyl carbazole) (PVK) have bright photoluminescence (PL) and electroluminescence (EL) from exciplexes formed at the interface. Exciplex formation reduces the PL and EL quenching that typically occurs near metal/polymer contacts. The difusion rate of excitons to the interface is estimated as 0.001 cm2/s.
2:00 PM J13.2
NEW LUMINESCENT PPV DERIVATIVES FOR LED APPLICATIONS. Beng Sim Chuaha, Andrew B. Holmesa,b, Stephen C. Morattia, Neil Feederb, John E. Daviesb, Franco Caciallic, Richard H. Friendc, Elisabeth A. Marsegliac, Donizetti A. dos Santosd and Jean-Luc Brédasd, aMelville Laboratory for Polymer Synthesis, Department of Chemistry, University of Cambridge, Cambridge, UNITED KINDGOM; bUniversity Chemical Laboratory, Department of Chemistry, University of Cambridge, Cambridge, UNITED KINGDOM; cCavendish Laboratory, Department of Physics, University of Cambridge, Cambridge, UNITED KINGDOM; dCentre de Recherche en Electronique et Photonique Moléculaires, Université de Mons-Hainaut, Mons, BELGIUM.
Poly(1,4-phenylene vinylene) (PPV) derivatives are becoming increasingly important in their applications as electroluminescent polymers. The substitution opportunities on the phenylene ring, the vinylene linkage and the polymer backbone influence the electronic, optical and morphological properties of the polymers, and thus offer a wide range of tunability for various applications. For example, 2,5-dialkoxy-substitution as in MEH PPV, results in a significant red shift and a solution-processible polymer, compared with the green-emitting and intractable unsubstituted parent PPV. We present our recent results on a poly(phenylene vinylene) polymer 1 with dialkoxy substitution at the unconventional 2,3-positions on the benzene ring. This polymer exhibits a high solid state photoluminescence efficiency (ca. 40%) with a blue-shifted emission compared with MEH PPV. Double layer light-emitting devices (LEDs) have been fabricated and exhibit reasonable efficiencies. In order to understand better the origin of the interesting optoelectronic properties of 1 the model compounds 2 have been synthesized and characterized by single crystal X-ray analysis. Corresponding semi empirical model calculations have also been performed.
2:15 PM J13.3
ALTERNATING CURRENT LIGHT-EMITTING DIODES WITH LANGMUIR-BLODGETT FILMS OF THIOPHENES. R.Österbackaa, T. Österga, A. J. Pal a,b, K-M. Källmana, and H. Stubb a, aDepartment of Physics, Åbo Akademi University, Turku, FINLAND; bIndian Association for the Cultivation of Science, Calcutta, INDIA.
Alternating current (ac) light-emitting diode (LED) structures: ITO - insulating polymer - active polymer - insulating polymer - Al have been prepared using the Langmuir- Blodgett technique for the polymer layers. Using polyaniline (PANI) or polymethylmethacrylate (PMMA) and poly(3-hexylthiophene) (PHT) or quinquethiophene (QT) as the insulating and active materials, respectively, electroluminescence (EL) was observed for both polarities with varying degrees of assymmetry. By reducing the thickness of the active layer - an advantage of the LB technique- one can reach higher operation frequencies (beyond 10kHz) at a particular electric field. However for thicknesses of the active material below some 10 LB layers (30-50 nm) a reduced thickness decreases the operation frequency. The latter effect reflects the importance of the accumulation of charges at the interfaces in organic LEDs, which has been documented in various studies recently. The effect of interfaces is also reflected in other ways in LB-LEDs: Single layer ITO/PHT/Al devices show a delayed EL-response to a square voltage pulse. In addition to the contribution of the charge carrier transit time to the delay one also has to consider charge accumulation. The latter causes field redistribution and an enhancement of electron injection increasing the EL output. Multilayer ac-structures, however, show an immediate response. This response can be interpreted as as a result of charges initially present at the various interfaces, including the polymer/polymer ones.
2:30 PM J13.4
DYNAMICS OF POLARONS IN GUEST-HOST-SYSTEM POLYMER LIGHT EMITTING DEVICES. E.J.W. List, W. Graupner, S. Tasch, G. Leising, Inst. f. Festkörperphysik, Technische Universität Graz, Graz, AUSTRIA; J. Partee, J. Shina, Ames Laboratory, Iowa State University, IA; P. Schlichting, Y. Geerts, U. Scherf and K. Müllen Max-Planck-Institut für Polymerforschung, Mainz, GERMANY.
The conjugated ladder-type poly(paraphenylene) (LPPP) is an attractive material for blue polymer light emitting diodes (PLED). Blending the active layer with small amounts of a red emitting guest polymer, the emission shifts from blue to red with increasing guest concentration due to efficient excitation energy transfer. The results of electroluminescence detected magnetic resonance measurements and current detected magnetic resonance measurements on PLED based on 0.05 weight % - 2 weight % red emitting poly(perylene-co- diethynylbenzene) in the active layer of the PLED are presented and discussed.
3:15 PM *J13.5
POLYMERIC ZINC-BISQUINOLINE BASED SELF-ASSEMBLED LIGHT EMITTING DIODES. Fotios Papadimitrakopoulos, D. Laurence Thomsen III, Keith A. Higginson, Baocheng Yang, University of Connecticut, Storrs, CT.
Our group has introduced a novel self-assembly technique to grow thin films made of polymeric metal-chelate derivatives of 8-hydroxyquinoline. The driving force is to achieve insoluble and intractable, pin-hole films, suitable for LEDs and other semiconducting applications. The growth of a 1,200 ‰ film in about 2.7 hrs, with the potential for multilayer fabrication at high purity levels, offers an alternative to vacuum deposition and spin-coating. The orientation and order of the self-assembled growing polymer chain will be closely examined based on spectroscopic characterization. Electrons were determined to be the majority carriers by varying the energy barrier of the metal/semiconductor junction of single layer poly(zinc-bisquinoline) LEDs, with light emission governed by the injection of the minority carrier (holes). Cyclic current-voltage sweeps show reproducible doping-dedoping characteristics that result in a dramatic increase of current density flowing through these films. This is going to be discussed in light of the underlying order w hich appears to increase upon heat-treatment at elevated temperatures.
3:45 PM J13.6
POLYMER LAYER ORDERING OF POLYANILINE DERIVATIVES IN PLED DEVICES: SURFACE CHARACTERIZATION AND ADSORPTION. R.C. Advincula , University of Alabama at Birminigham, Dept. of Chemistry, Birmingham, AL; C.W. Frank , Stanford University, Dept. of Chem. Eng., Stanford, CA; W. Knoll ; Max Planck Institute for Polymer Research, Mainz, GERMANY; D. Roitman, J. Sheats, R. Moon, Hewlett-Packard Laboratories, Palo Alto, CA.
The use of self-assembled polyaniline derivatives for polymer light emitting diode (PLED) devices is investigated primarily as a hole-injecting layer on the anode surface (ITO coated glass). The layer ordering and self assembly process is facilitated using the alternate polyelectrolyte deposition process. This involves the use of counter polyelectrolytes that allow multilayer build-up depending on the solution and deposition conditions as initially reported by Decher and Rubner et.al.. Spectroscopy and microscopy is used to probe the layer order and integrity, foremost of which is surface plasmon spectroscopy and atomic force microscopy. Different solution and pairing conditions resulted in varrying degrees of order and properties for these layers. The objective of these iterations being to determine the polyaniline layer properties towards efficient PLED device performance. Using current-voltage (J-V) and luminance-voltage (L-V) measurements, the efficiency and lifetime is compared to previously spin-casted systems. The improvements are compared from systems using the same luminescent polymer (specifically MEH-PPV) and fabrication methodology. The effect of parameters such as layer number, pairing system, annealing, etc. are discussed as they relate to the device. Overall, the presence of these films of the order of 20 nm improves the device performance and stability. While these results are important towards the eventual objective of an efficient device, effort is made to understand the mechanism of this improvement and the importance of layer order at the nm scale. This will involve investigating the effects of polymer chain orientation and complexation with the counter polyelectrolytes on a layer by layer basis.
4:00 PM J13.7
ORGANIC LIGHT EMITTING DEVICES WITH A PULSED LASER DEPOSITED In2O3:Sn TOP-ELECTRODE. Y. Wu, C.H.M. Marée, R.F. Haglund, R.A.Weller, L.C.Feldman, Vanderbilt University, Dept of Phyics & Astronomy, Nashville, TN; H.W.H. Lee, Lawrence Livermore National Laboratory, Livermore, CA.
There is a considerable interest in using organic electroluminescent (EL) devices for display applications because of the low processing costs, the high efficiencies and the wide range of emission wavelengths achieved in organic semiconductor films. The common structure of organic LED devices consists of organic films deposited on transparent In2O3:Sn (ITO) and capped by a low work-function metal as a cathode. However, for useful integration in active matrix displays, a direct connection of the LED pixels on top of silicon field effect transistors is preferable. In this structure the transparent ITO will establish the capping electrode on top of the organic LED. ITO films are routinely deposited by RF sputtering. However, the presence of a plasma as well as the elevated temperatures necessary for optimized ITO properties can cause damage to the organic layers during the deposition. We present the growth of an ITO top contact by means of pulsed laser deposition (PLD). We investigated the influence of oxygen pressure and other growth conditions in order to optimize the transparency, oxygen content and conductivity of the films. A high quality, room temperature deposited film, with a conductivity of 310-4 cm has been obtained. Using PLD, ITO was deposited on heterojunction organic LED devices consisting of tris-(8-hydroxyquinoline) aluminum (Alq) and a hole transporting film. Results on current-voltage and electroluminescence measurements as well as ion beam analysis of the structures will be presented.
4:15 PM J13.8
INTERNAL FIELD DISTRIBUTION IN ORGANIC LIGHT EMITTING DIODES WITH DOUBLE LAYER STRUCTURE. C. Hochfilzer,T. Jost, W. Graupner, G. Leising, Institut fuer Festkoerperphysik, TU Graz, AUSTRIA; U. Scherf, K. Muellen, Max Planck Institut fuer Polymerforschung, Mainz, GERMANY; E. Forsythe, Y. Gao, Dept of Physics and Astronomy, University of Rochester, NY; C. W. Tang, Imaging Research and Advanced Development, Eastman Kodak Company, Rochester, NY.
Double layer organic light emitting devices (OLED) are constructed by evaporating 8-hydroxyquinoline aluminum (Alq3) on a spin cast thin film of ladder type poly-para-phenylene (mLPPP). A thick layer of Mg:Ag is used as the cathode material. These organic materials are very suitable for application in OLEDs both, as transporting materials as well as active layers. (Alq3) predominantly transports electrons while mLPPP is a conjugated polymer having higher hole mobilities. Due to these transport properties the formation and radiative recombination of the excitons in ITO/mLPPP/(Alq3)/Mg:Ag devices occur close to the mLPPP/(Alq3) interface although the energy levels of the HOMO and LUMO of (Alq3) and mLPPP are similar. We compare the device performance of OLEDs with varying (Alq3) layer thickness (0, 2, 5, 15, 30, 50 nm) and constant mLPPP layer thickness (90 nm). A difference in the onset voltage, electroluminescence (EL) quantum efficiency and shape of the EL-spectra as a function of the Alq3 layer thickness is observed. We analyze these results with respect to the internal electric field distribution of the double layer devices derived from electroabsorption measurements.
4:30 PM J13.9
FLUORENE-BASED THERMOPLASTIC AND THERMOSETTING COMPOSITIONS: A STUDY OF THEIR PHOTOLUMINESCENCE, MORPHOLOGY, BIREFRINGENCE AND OTHER PROPERTIES. Ed Woo, Michael Inbasekaran, Bill Shiang, Weishi Wu, Materials R&D Laboratory, Central & New Businesses R&D, The Dow Chemical Company, Midland, MI.
We have developed a number of fluorene-based thermoplastic polymers and crosslinkable oligomers for the fabrication of electroluminescent devices. This presentation will deal with the characterization of these novel emitting materials by photoluminescence (PL), differential scanning calorimetry, and optical birefringence. The effect of substitution at carbon-9 of the fluorene on the liquid crystallinity, semi-crystallinity, and the PL emission spectra of these blue emitters will be discussed. Also presented will be the relationship between crosslinking of the oligomers and the PL behavior of the cured resins.
4:45 PM J13.10
PHOTOEXCITATIONS OF OLIGOTHIOPHENES WITH REDUCED CONFORMATIONAL MOBILITY. L. Rossi, Istituto Nazionale per la Fisica della Materia, ITALY, and Dipartimento di Fisica ''A. Volta'', Università di Pavia, Pavia, ITALY; G. Bongiovanni, Istituto Nazionale per la Fisica della Materia, ITALY, and Dipartimento di Scienze Fisiche, Università di Cagliari, Cagliari, ITALY; C. Botta, Istituto di Chimica delle Macromolecole, CNR Milano, ITALY; G. Cerullo, CEQSE-CNR Dipartimento di Fisica, Politecnico di Milano, Milano, ITALY; G. Lanzani, Istituto di Matematica e Fisica, Università di Sassari, Sassari, ITALY; A. Mura, Istituto Nazionale per la Fisica della Materia, ITALY, and Dipartimento di Scienze Fisiche, Università di Cagliari, Cagliari, ITALY; F. Sannicoò, Dipartimento di Chimica Organica ed Industriale, Università di Milano, Milano, ITALY; R. Tubino, Istituto Nazionale per la Fisica della Materia, ITALY, and Dipartimento di Scienze dei Materiali, Università di Milano, Milano, ITALY.
We present a comprehensive study of the optical and electronical properties of a series oligothiophenes in which one or two inter-ring torsional angles have been blocked by chemical bridging. These give us the possibility to investigate the role of the conformational mobility and the electron-torsional coupling on the deactivation process of the singlet excited states. Studying the molecules in solution we demonstrate that a simple size-dependent model based on the free-electron molecular orbital approximation fail to explain the whole experimental behaviour pointing to the role of inter-ring torsions in the relaxation branching between radiative and non-radiative states. The effects of the reduced torsional mobility on the optical properties are analysed with several spectroscopic techniques (optical absorption, steady state and time resolved photoluminescence (PL), PL quantum yields) on solutions and solid state samples. In diluted solutions we observe a red shift of the optical absorption and an increase in the PL quantum efficiencies by reducing the conformational mobility of the oligothiophenes. A strong and sharp excitonic absorption band has been observed in cast films and evaporated films due to a highly ordered card pack arrangement attained by self assembly. Novel data on the decay kinetics of these excitonic states have been investigated in order to correlate their dynamic to the crystal structure in particular the intermolecular coupling.