Meetings & Events

Spring 1999 logo1999 MRS Spring Meeting & Exhibit

April 5-9, 1999 | San Francisco
Meeting Chairs: Katayun Barmak, James S. Speck, Raymond T. Tung, Paul D. Calvert

Symposium F—Organic Nonlinear Optical Materials and Devices



Richard Claus
Dept of Elec & Computer Engr
Virginia Tech
Blacksburg, VA 24061-0356

Bernard Kippelen
Optical Sciences Center
Univ of Arizona
Tucson, AZ 85721

Hiliary Lackritz
Materials Dept
Gemfire Corp
Palo Alto, CA 94303

Symposium Support

*AlliedSignal, Inc.
*Lightwave Microsystems Corporation
*Lockheed Martin
*Office of Naval Research 
Proceedings published as Volume 561
of the Materials Research Society
Symposium Proceedings Series.
* Invited paper
Chair: Bernard Kippelen
Tuesday Morning, April 6, 1999
Franciscan II (A)
9:00 AM *F1.1

Recently we found that conjugated organic molecules with symmetrical charge transfer from the ends of the molecule to the middle have large two-photon cross sections. Based on this notion we designed and synthesized a variety of molecules with unprecedented two-photon cross sections. Here we will discuss the design strategy in detail as well as applications of these compounds in the field of optical limiting, two-photon microscopy, and two-photon induced polymerization.

9:30 AM F1.2
COMPARISON OF FEMTOSECOND AND NANOSECOND TWO-PHOTON CROSS SECTIONS MESUREMENTS OF AFX CHROMOPHORES IN VARIOUS SOLVENT ENVIRONMENTS. Max D. Alexander, Jr. , Lisa R. Denny, Jeffrey W. Baur, Richard A. Vaia, Paul A. Fleitz, Bruce A. Reinhardt, Materials and Manufacturing Directorate, Air Force Research Laboratory, Wright-Patterson AFB, OH; Sean M. Kirkpatrick, SAIC, Dayton, OH; Eric Pooler, SOCHE, Dayton, OH.

Accurate and reproducible determination of the intrinsic two-photon cross section remains one of the most difficult tasks of organic two-photon active materials. The measured response may represent a superposition of the fundamental two-photon and thermal perturbations as well as excited state absorption, small linear absorption contributions, and other multi-photon processes. The goal of this work is to separate the intrinsic two-photon cross section from these other contributions. The AFX family of chromophores, and specifically AF50, developed in the Air Force Research Laboratory have exhibited extremely large effective cross section values when measured with nanosecond pulses. These previously reported cross sections are seen to vary greatly with their local molecular environment and solvent system. Here we present intrinsic cross section data measured by nonlinear fluorescence and z-scan using femtosecond pulses. The effects local molecular environment has been examined in a variety of solvent systems to elucidate the intrinsic two-photon cross section. A Ti-sapphire pump-probe experiment has also been employed to examine the excited state contribution to the apparent two-photon cross section . The use of this technique allows for determination of the solvent relaxation kinetics and any excited state absorption, which can be separated as a function of the local molecular environment.

9:45 AM F1.3
MULTIFUNCTIONAL FULLERENE FILMS FOR PHOTONICS. Yanjing Liu 1, You-Xiong Wang2, Hongxia Lu2 and Richard O. Claus2, 1NanoSonic Inc., 2Virginia Tech.

Multilayer fullerene thin films up to more than one micron in thickness have been synthesized using a novel electrostatic self-assembly process, and their nonlinear optical properties characterized. The fullerenes, a C60 and C70 mixture (fullerite, 9:1), were made water soluble using a procedure detailed in prior publications. Consecutive monolayers of these molecules and water soluble polymers were then adsorbed from solutions by dipping. The regular linear buildup of the the thickness of the stable fullerene films was characterized by UV-vis spectroscopy and ellipsometry. Atomic force microscopy reveals ultrauniform structure of the outermost layers of the films, and uniform particle size. Electro-optic and pulsed laser measurements indicate both second order optical nonlinearity and optical limiting behavior. The relationship between nonlinear behavior and variations in the way the fullerene molecules are incorporated into the films is discussed.

10:00 AM F1.4
RESIN PHOTOCURE VIA TWO-PHOTON UP-CONVERTED FLUORESCENCE. Lisa R. Denny , Bruce A. Reinhardt, Max D. Alexander, Jr., Jeffery W. Baur, Paul A. Fleitz, Richard A. Vaia, Materials Directorate, Air Force Research Laboratory, Wright-Patterson AFB, OH; Sean A. Kirkpatrick, SAIC, Dayton, OH; Stephen J. Clarson, University of Cincinnati, College of Engineering, Cincinnati, OH.

The use of organic chromophores to produce two-photon up-converted emission as an initiation source for photocuring of resins represents a promising new processing technique for the fabrication of bulk structures. Resin photocure is a well-established polymerization technique which has been used commercially for decades. Traditional single-photon photoinitiated cures use light sources that, due to strong linear absorption of the material, are unable to penetrate effectively. Thus, limiting fabrication to thin configurations such as films and coatings. In contrast, use of internally generated uv-vis radiation from an infrared two-photon initiated fluorescence presents a tremendous advantage in terms of the ability of that light to penetrate deeper into the resin system. As a result thicker polymeric structures can be fabricated. The two-photon fluorescence photocure process involves an organic chromophore exhibiting two-photon up-converted fluorescence and a photoinitiator blended into the resin system. Once the chromophore fluoresces the photoinitiator is activated and polymerization can occur. The polymeric structure is fabricated in the portion of the resin where sufficient two-photon fluorescence is produced to activate the photoinitiator. Chromophores developed in the Air Force Research Laboratory exhibit large effective two-photon cross-section values and thus provide the efficient fluorescing behavior required to facilitate the photocure. In general, the most critical factors associated with this process are large two-photon cross-sections, efficiency of conversion, wavelenght of emission, power threshold, as well as resultant polymer initiator chemistry. Discussions from initial studies will include the observation of a strong correlation between the molecular environment surrounding the chromophore and the range in which fluorescence occurs. In these studies, using a femtosecond pulsed laser light source, the cure of optical epoxy and acrylic polymeric structures of greater than one centimeter in thickness have been demonstrated.

10:45 AM F1.5
LANGMUIR-BLODGETT FILMS OF SQUARYLIUM DYE J-AGGREGATES EXHIBITING FEMTOSECOND OPTICAL RESPONSES. Makoto Furuki , The Femtosecond Research Association, Tsukuba, JAPAN; Hitoshi Kawashima, Toshiro Tani, Electrotechnical Laboratory, Tsukuba, JAPAN; Lyong S. Pu, Corporate Research Lab., Fuji Xerox Co. Ltd., Kanagawa, JAPAN.

J-aggregates of squarylium dyes formed in their Langmuir-films have been found to exhibit efficient and ultra-fast non-linear optical properties, even in the form of a mono-molecular layer at an air-water interface. This time, we established a method to make a Langmuir-Blodgett (LB) film of J-aggregates with a single, intense (O. D. = 0.2 / monolayer) absorption band at 784 nm, almost that in its Langmuir-film. Deposition of the LB-film was carried out by withdrawing a glass substrate from an area slightly less than monolayer occupation during compression at a constant rate of 40$\%$ excess to the deposited area. Over-coating the surface of the LB-film with a glassy poly-perfluorocarbon film after deposition, enhanced and stabilized the formation of J-aggregates on the glass substrate. Characterizations by a near-field scanning optical microscope clarified a highly ordered structure of the LB-film, which filled with 2-dimensional domains of J-aggregates of sub-$\mu$m diameter and mono-molecular thickness, in which the dye molecules orient in the same directions. We also observed femtosecond nonlinear-optical responses from this LB-film. An ultra-fast decay of absorption change (200 fs) with a quite low saturation energy (3.4 mJ/cm2$\cdot$pulse) was observed in femtosecond pump-probe measurements. These results suggest a high delocalization of the excited states in the J-aggregates of the LB-film within ordered molecular arrangements, which maintaining 2-dimensional mono-molecular layer structures, initially formed in the Langmuir film at the air-water interface. This work was performed under the management of a technological research association: the Femtosecond Technology Research Association (FESTA), supported by New Energy and Industrial Technology Development Organization (NEDO).

11:00 AM *F1.6 RECENT ADVANCES IR POLYMERS FOR INTEGRATED OPTICS. William H. Steier , S.S. Lee, S. Garner, A. Chen, H. Zhang, Univ. Southern Calif., Dept. Electrical Eng., CA; Larry R. Dalton, Univ. Southern Calif., Dept. Chemistry, CA; Harold R. Fetterman, A. Udupa, D. Bhattaachaya, UCLA, Dept. Electrical Eng., CA.

The recent advances in passive and active polymer materials and their application to integrated optics will be reviewed. The latest results on high speed traveling wave electro-optic modulators, rf phase shifters, and recent advances in achieving three dimensional integrated optics using polymers will be reviewed.

Tuesday Afternoon, April 6, 1999
Franciscan II (A)
1:30 PM *F2.1
ENHANCEMENT OF NONLINEAR OPTICAL PROPERTIES THROUGH SUPRAMOLECULAR CHIRALITY. Thierry Verbiest, Sven Van Elshocht, Martti Kauranen, Louis Hellemans, Johan Snauwaert, André Persoons , University of Leuven, BELGIUM; Colin Nuckolls, Thomas Katz, Columbia University, New York, NY.

Nonlinear optical (NLO) effects like second-harmonic generation and the electro-optic effect require materials that are non-centrosymmetric on the molecular and macroscopic level. They are usually designed by incorporating noncentrosymmetric chromophores into a noncentrosymmetric macroscopic structure such as poled polymer films, Langmuir-Blodgett films, self-assembled films or crystals. Improvements to the nonlinearity can be made by optimization of the molecular nonlinear response or by improving the alignment of the chromophores in the macroscopic structure. Here we report the nonlinear optical properties of Langmuir-Blodgett (LB) films of chiral helicene derivatives that self-organize in LB-films to form supramolecular structures with extremely strong chiral properties. We show that the second-harmonic response of nonracemic (chiral) LB-films of the helicene is several orders of magnitude higher than the response of racemic (achiral) LB-films of the same material. The magnitude of the NLO response of the nonracemic films was estimated to be a respectable 50 pm/V, although the helicene structure lacks features that are commonly associated with high nonlinearity. We present evidence that it is the supramolecular organization of the helicene molecules that enhances the molecular NLO response, while the presence of chirality in the nonracemic films effectively breaks the macroscopic symmetry. Therefore, the NLO response of the nonracemic films is much higher.

2:00 PM *F2.2
IMPORTANCE OF MATERIAL AND OPERATIONAL PARAMETERS LIMITING THE PHOTOSTABILITY OF ELECTRO-OPTICS CHROMOPHORE DOPED POLYMERS. Adriana Galvan-Gonzales1, Michael Canva 1,2, George Stegeman1, Robert Twieg3, Seth Marder4; 1CREOL, School of Optics, University of Central Florida, Orlando, FL; 2IOTA, Laboratoire Charles Fabry du CNRS, Université d'Orsay, FRANCE; 3Liquid Crystal Institute, Kent State University; 4JPL and Beckmann Institute, CalTech, Pasadena, CA.

Electro-optics chromophore doped polymer materials have been studied for many years. They have been constantly improved and many efficient potential applications have now been demonstrated. Compared to other materials, they usually present the advantages of better efficiency, ease of integrability and low cost. Their Achille heel now remains in their chemical stability. The orientational stability issue, necessary to retain a chi(2) nonlinearity, has been addressed by many means, including covalently grafting the chromophores to the matrix, increasing the Tg of this latter and in parallel increasing the thermo-stability of the chromophore in order to resist the high temperature inherent to the electric poling process. Photostability of the chromophores, i.e. degradation reaction induced by absorption of a photon, has remained far less studied. However in an optical device which aim is to operate with photons, this issue will ultimately limit the component lifetime. We have investigated the photodegradation behavior of a number of chromophore doped systems, guest/host and side-chains, as a function of excitation wavelength. Systematic behaviours have been unravelled and allow us now to forecast material properties as a function of photon energy using only very few experimental data. We have also quantified the impact of operational parameter such as temperature, atmospheric composition,Ö Currently, we are interested in assessing the impact of chromophore structure and chromophore interactions with its host. Photostability figures of merit have been defined that easily allow to calculate device lifetimes in any operational configuration. Necessary material improvements are thus quantified.

2:30 PM F2.3
ENHANCED ELECTRO-OPTIC COEFFICIENT (R33) OF DOPED POLYMER FILMS THROUGH OPTIMIZATION OF CHROMOPHORE ENVIRONMENT. Richard A. Vaia , Stephen Caracci, Bruce Reinhardt, Materials and Manufacturing Directorate, Air Force Research Laboratory, WPAFB, Dayton OH; Michael Banach, Department of Materials Science and Engineering, University of Cincinnati, Cincinnati, OH.

The influence of chromophore-matrix and chromophore-chromophore interactions on processing and final electro-optical (EO) properties of guest-host polymer systems are discussed. Molecular interactions between chromophore and polymer define the local chromophore environment and dictate the alignment (poling) efficiency and subsequent thermal stability of the non-linear polymer. In addition, specific intermolecular interactions will retard cooperative relaxations (enhancing stability) and promote molecular level miscibility (increasing effective chromophore loading and EO coefficient (r33)). The molecular environment of Disperse Red 1 (DR1) and 4-(dicyanomethylene)-2-methyl-6-(4-dimethylaminostryl)-4H-pyran (DCM) doped in poly(styrene) (PS), poly(2-vinyl pyridine) (P2VP), poly(methyl methacyrlate) (PMMA), and styrene-methyl methacyrlate copolymers was examined with UV-VIS absorption, in-situ thermal ellipsometry and FTIR spectroscopy. These observations are correlated to processing parameters (poling field, temperature) and EO coefficient. The final EO coefficient varied as much an order of magnitude depending on the host polymer. Dispersion and local field effects due to the dielectric constant of the polymer do not account for these large variations. Specific chromophore-host interactions such as H-bonding increase chromophore solubility by preventing aggregate formation through dipole-dipole interactions. Additionally, a field-responsive host facilitates chromophore alignment. Combining these effects, such as in DR1-P2VP system, leads to an enhancement of r33 by greater than 40% with respect to conventional DR1-PMMA systems.

2:45 PM F2.4
BRIDGED POLYSILSESQUIOXANES WITH IMPROVED SECOND ORDER NONLINEAR OPTICAL PROPERTIES AND STABILITY. Stephen T. Hobson , US Army Inst of Chem Defense, Adv. Drug Assessment, Aberdeen Proving Ground, MD; Kenneth J. Shea, Univ of California-Irvine, Dept of Chemistry, Irvine, CA; Jarek Zieba, Paras N. Prasad, Dept of Chemistry, SUNY University at Buffalo, Buffalo, NY.

We report the synthesis and sol-gel polymerization of 4-nitro-N,N-bis[(3-triethoxysilyl)propyl]aniline. An efficient synthesis of the monomer was developed by the hydrosilylation of N, N-diallyl-4-nitroaniline. Optical quality thin films were synthesized by spin coating a n-butanol solution of 4-nitro-N,N-bis[(3-triethoxysilyl) propyl]aniline using formic acid both as catalyst and source of water. We improved the temporal stability of the NLO signal from films prepared from 4-nitro-N,N-bis[(3-triethoxysilyl)propyl]aniline by increasing the intensity of the poling field and extending the heating period during the poling/curing stage. By Maker fringe analysis, a $\chi$(2) value of 9 $\times$ 10-8 esu was measured for these polysilsesquioxanes. If one assumes that the major component of the NLO effect is along the z axis, the $\chi$(2) value corresponds to a d33 coefficient of 18.9 pm/V and a r33 value of 4.7 pm/V.

3:30 PM *F2.5
HIGH SPEED LAYER-BY-LAYER SELF-ASSEMBLY OF POLARIZED CHROMOPHORIC SUPERLATTICES. Yanjing Liu , NanoSonic, Inc., Richard O. Claus, Dept of Electrical and Computer Engineering, Dept of Material Science and Engineering, Virginia Tech, VA; James R. Heflin, Daniela Marciu, and Charley Figura, Dept of Physics, Virginia Tech, VA.

A new method for high speed build-up of the macroscopic non-centrosymmetric NLO films at room temperature has been developed, using layer-by-Layer electrostatic self-assembly (ESAM) technique. It has been demonstrated that this molecular-assembly technique can provide not only highly homogeneous, micron-thick films, but also relatively high $\chi$(2) values. The ESAM films have exhibited no measurable decay of $\chi$(2) at room temperature over a period of more than eighteen months in contrast to electric field poling films. It will also be demonstrated that $\chi$(2) values can be increased significantly by applying a dc electric field during the self-assembly process.

4:00 PM F2.6
SECOND ORDER NONLINEAR OPTICAL FILMS BY ALTERNATING POLYELECTROLYTE DEPOSITION ON HYDROPHOBIC SUBSTRATES. M. Joseph Roberts , Geoff A. Lindsay, Naval Air Warfare Center, Chemistry and Materials Division, China Lake, CA; Warren N. Herman, Naval Air Warfare Center Aircraft Division, Electro-Optic Sensors Branch, Patuxent River, MD.

Alternating polyelectrolyte aqueous solution deposition technique (APD) may be used to process nonlinear optical polymers (NLOP) into noncentrosymmetric ordered films near room temperature. In this study, second-order NLOP APD films of a stilbazolium-substituted polyepichlorohydrin and poly(sodium 4-styrenesulfonate) were prepared by alternately dipping a substrate into aqueous solutions of each polymer. Evidence for uniform layer to layer deposition includes a linear increase of UV-Visible absorbance and quadratic increase of second harmonic generated light intensity as a function of film thickness. Films have been uniformly deposited up to 116 bilayers. The Maker fringes exhibit additional minima due to interference of the second harmonic generated at the two sides of the substrate that supports films on both sides. The fact that these minima go to zero indicates high quality films. Azimuthal scans indicate that the polar order is isotropic in the plane of the film. Thermal stability experiments show that these films can maintain 90$\%$ of the polar order up to 150$^{\circ}$C during a slow temperature ramp of 1$^{\circ}$C/min. Films have been further characterized by contact angle measurements, profilometry, and polarized light microscopy. Work is in progress to deposit thicker films of the same quality and to quantify NLO figures of merit.

4:15 PM F2.7
STRUCTURE-PROPERTY RELATIONSHIPS IN ORGANIC NONLINEAR OPTICAL MATERIALS. Eric M. Breitung and Robert J. McMahon, University of Wisconsin, Department of Chemistry and Materials Science Program, Madison, WI.

Tuning the degree of bond-length alternation in organic nonlinear optical materials is a powerful paradigm for the design of organic materials with large molecular hyperpolarizabilities ($\beta$). Several research groups employed this paradigm in the design and synthesis of NLO materials incorporating donor-acceptor polyenes. Increased bond-length alternation in polyenes leads to decreased barriers to rotation about C=C bonds and, hence, increased conformational flexibility. Since the degree of bondlength alternation is solvent dependent, so is the degree of conformational flexibility. In an effort to probe the influence of conformational flexibility on NLO response, we synthesized a series of simple donor-acceptor polyenes that are either conformationally flexible (eg. dimethylaminopentadienal) or rigid (eg. 7-dimethylamino-4,4a,5,6-tetrahydro-2(3H)-naphthalenone). For each pair of molecules ZINDO sum-over-states calculations predict a larger value of $\beta$$_\mu$, for the conformationally flexible isomer, but EFISH measurements (CHCl3) reveal a larger value of$\beta$$_\mu$, for the conformationally rigid system. Various explanations for this behavior will be considered.

4:30 PM F2.8
CARBAZOLE CYCLIC OLIGOMERS AND CHROMOGENIC CALIX[4]ARENES FOR NONLINEAR OPTICS. Tatsuo Wada 1,2, Atsushi Gunji1,2, Yadong Zhang1, Sumio Maruyama2, Stephan Houbrechts2, Hiroyuki Sasabe1,2, 1Core Research for Evolutional Science and Technology (CREST), JST, Frontier Research Program, The Institute of Physical and Chemical Research (RIKEN), Wako, JAPAN; Yuji Kubo, Saitama University, Dept. of Applied Chemistry, Urawa, JAPAN.

We have developed cyclic oligomers as a new class of multifunctional molecules. We investigated second-order nonlinear optical responses on two kinds of molecular systems such as carbazole cyclic oligomers and chromogenic calix[4]arenes. By Knoevenagel condensation between bisformyl carbazole and bis(cyanoacetate), we can control the ring size of carbazole cyclic oligomers with a high yield. Efficient energy transfer was observed from carbazole moieties to acceptor-substituted ones in the tetramer. This carbazole cyclic oligomer can be used as an electron-transport and luminescent materials in electroluminescent devices. Acceptor-substituted carbazole and indoaniline derivatives with a different number of moieties were introduced into the calix[4]arenes platform. The topological shapes of indoaniline-derived calix[4]arenes were studied by hyper-Rayleigh scattering. The two indoaniline moieties in the chromogenic calix[4]arenes were pre-aligned and net molecular hyperpolarizability was enhanced.

4:45 PM F2.9
POLING AND RELAXATION DYNAMICS OF 2ND ORDER NLO-ACTIVE NEMATIC LIQUID CRYSTALLINE MAIN CHAIN POLYMERS. A.T.H. Koch , M. Warner, J.J.M. Halls, S.V. Fridrikh, Cavendish Laboratory, Cambridge, UNITED KINGDOM; E. Toussaere, Ecole Normale Supíerieure de Cachan, FRANCE; C.E. Schwarzwîalder, S.C. Moratti, Melville Lab for Polymer Synthesis, Cambridge, UNITED KINGDOM; R.H. Friend, Cavendish Laboratory, UNITED KINGDOM.

We report the characterisation of the second order nonlinear optical properties of a semiflexible main chain random copolymer with a head to tail structure synthesized from 4-carboxy-4$^\prime$-[N-(11-hydroxyundecyl)-N-methylamino]-azobenzene and hydroxybenzoic acid. This polymer exhibits a nematic liquid crystal mesophase with a nematic-isotropic transition temperature of $180^{\rm o}$ C. We have carried out measurements of the temperature and field dependent poling behaviour by means of a time-resolved ellipsometric set-up after Teng and Man. We measure values of $\chi^{(2)}_{333}$ of up to 90 pm/V. As a function of temperature, the plateau values of the poling response to an applied electric dc field vary stepwise with increased mobility of the various segments of the polymer chain. These changes in mobility correspond to conformational changes of the polymer which we have studied by differential scanning calorimetry (DSC), thermally stimulated discharge current (TSDC) and powder diffractometry x-ray analysis. In the time domain, we find that above the glass transition temperature Tg of $35^{\rm o}$ C up to a temperature of $110^{\rm o}$ C a fast and a slow poling response can be distinguished. In that temperature range is not possible to anneal out the nematic defects even with applied field strengths up to $3{\rm x}10^8$ V/m. Between $110^{\rm o}$ C and the nematic-isotropic transition only the fast response can be observed. We show that the $\chi^{(3)}$ contribution to the measured NLO-response is less than 5% for the particular material studied even at the highest applied fields. We model the dependence of the poling on the applied field strength with a modified self-consistent Maier-Saupe approach. We have used electroabsorption to measure the imaginary part of $\chi^{(2)}$ over a wavelength range from 350 to 1000 nm and to determine the degree of polar order in the polymer films. Furthermore, we find sub-band gap features in the electroabsorption spectra which we assign to charge transfer states resulting from charge injection into the polymer films.

Tuesday Evening, April 6, 1999
8:00 P.M.
Metropolitan Ballroom (A)
APPLICATION OF THE FÖRSTER MODEL TO A STUDY OF REVERSE SATURABLE ABSORPTION IN FULLERENE-CONTAINING POLYIMIDE. N.V. Kamanina , L.N. Kaporskii, Vavilov State Optical Inst, St. Petersburg, RUSSIA; V.I. Berendyaev, B.V. Kotov, Karpov Research Physical-Chemical Inst, Moscow, RUSSIA.

Effect of reverse saturable absorption (RSA) is the characteristics property of polymers doped with fullerenes. In the present work RSA has been investigated in the structures based on polyimide. The structures were a 1$\mu$m thick film of the aromatic polyimide solution in 1,1,2,2-tetrachloroethane doped with the fullerene mixture (C60:C70=87:13) and/or the malachite green dye. The RSA investigations were carried out under an irradiation of a pulsed Nd-YAG laser at wavelength of 532 nm.
RSA was detected in both types of the structures with a level of optical limiting depended on the concentrations of the fullerene mixture and of the dye. The most optical limiting was observed in the polyimide structure doped with the fullerene mixture simultaneously with the dye.
Since the absorption spectrum of the fullerene-polyimide system is overlapped with the fluorescence spectrum of the malachite green dye at the wavelength of the excitation radiation (532 nm), resonance conditions are fulfilled in the polyimide-dye-fullerene structure. This fact allows the RSA peculiarities of that structure to be explained in the framework of the Förster model [1]. In this case overlapping the electron shells of the dye and fullerene molecules provides the favourable conditions for the formation of the charge transfer complexes as the result of the free electron exchange between donor (dye) and acceptor (fullerene).
[1] T. Förster, Disc. Farad. Soc., 27 (1959) 7.


The polymerization of acetylene on a catalytic system, composed of binuclear rhenium complexes and reducing agent, in solution of saturated polymers leads to soluble compositions containing nanoparticles of polyacetylene with low content of conformational and chemical defects. Studying the structure of polyacetylene obtained in different polymeric matrices indicates that the compositions of polyacetylene with poly(vinyl butiral) are of greatest interest for theory and practice. In this report, fundamental regularities of Raman scattering by the trans-nanopolyacetylene (trans-NPA) compositions are reported. The results make it possible to formulate four basic features that distinguish the Raman scattering by the trans-NPA compositions from the standard trans-PA modifications. (1) The weak frequency dispersion of polyene C=C and C-C stretching bands, which is evidence for the narrow length distribution of conjugation chains and the narrow energy distribution of the lowest-lying excited states 1Bu and 2Ag in trans-NPA. (2) The occurrence of long sequences of overtone and combination bands in room-temperature resonance Raman spectra and the occurrence of at least two overtones and combination band in room-temperature off-resonance Raman spectra (785- and 1064 nm excitations) of trans-NPA compositions. (3) High intensities of the anti-Stokes lines of the fundumentals C=C and C-C stretching vibrations of the polyene chain. (4) Extremely high intensities of the fundamentals C=C and C-C stretching vibrations of the polyene chain. Compositions of trans-NPA could be used for elaboration of new types of Raman lasers and security of objects and documents against forgery.


NLO properties of push-pull molecules are usually modeled in terms of the DA dimer model, a simple two-state model. This model proved useful in understanding the physical behavior of these molecules, as well as in providing guidelines for the synthesis of molecules with large NLO responses. Recently I have shown that this model can be easily extended to account for the coupling of electrons with molecular vibrations: In push-pull molecules vibrations originate a large amplification of static NLO responses, and strongly affect the responses even at optical frequencies. In the same spirit, I show that the coupling of the molecular dipole moment to the polarization of the surrounding solvent can be modeled in terms of an effective DA dimer. In fact, the fast component of the solvent polarization simply renormalizes the parameters of the DA model. Moreover, in polar solvents, the slow orientational polarization adds to vibrations in amplifying the static NLO responses. Solvent effects on spectroscopic properties of push-pull molecules are highly non-trivial and cannot be caught, even at a qualitative level, without properly accounting for the non-linearity of the solvent-solute interaction. In particular, the implications of this non-linearity are discussed in connection with solvatochromic effects (for both absorption and emission bands) and electroabsorption measurements in solution. The ground and excited state dipole moments of push-pull molecules are usually extracted from these experiments using standard theories accounting for linearized solvent-solute interactions: the corresponding estimates are proved to be unreliable for molecules characterized by large non-linear responses and alternative interpretative schemes, based on truly non-linear interactions, are presented.


In this paper, we describe result of measurement on a group velocity dispersion for an optical wave guide utilizing siloxane polymer on ceramic substrate. The single-mode optical waveguide utilizing siloxane polymer on ceramic substrate has been developed for opto-electronic multichip modules to operate femto seconds light wave pluses and THz electrical signals. Propagation loss of the single-mode optical waveguide formed on ceramic substrate is 0.14 dB/cm at 1.3 micron meter wavelength. However, siloxane polymer has an absorption spectrum band at about 1.53 micron meter which is assumed by combination band of C-H vibration modes. Then propagation loss of the waveguide is 2.0 dB/cm at 1.55 micron meter wavelength. The group velocity dispersion of the optical waveguide is measured by a following method. At first, an optical signal was modulated by a tunable laser source and passed through the optical waveguide. Second, that optical signal passed through the optical waveguide was detected by an optical receiver and compared against its modulation electrical signal as reference. Third, the difference of phase between the two signals was measured as a group delay. A group velocity dispersion is calculated from the value. The result showed that the group velocity dispersion of the optical waveguide was significantly related to the absorption spectrum of siloxane polymer. The group velocity dispersion of the optical 50mm long waveguide is about 2 fsec/nm at 1.55 micron meter wavelength. For example, when a 500 femto seconds optical pulse which has 16 nm for spectrum width passed through the optical wave guide, an expansion of the spectrum width is estimated as about 32 nm. Then, this expansion of spectrum is well to distinguish femto seconds optical signals. Thus, the optical waveguide utilizing siloxane polymer is well capable for femto seconds optical pluses transmission.

TWO PHOTON SPECTROSCOPY OF DITHIENYL POLYENES. Thomas M. Cooper , Paul A. Fleitz, Laura Sowards, Air Force Research Lab, Wright-Patterson Air Force Base, OH; Lalgudi V. Natarajan, Sean M. Kirkpatrick, SAIC, Dayton, OH; Charles W. Spangler, Dept. of Chemistry, Montana State University, Bozeman, MT.

To develop novel nonlinear absorbers for optical limiting applications, we have been investigating the properties of dithienyl polyenes. Spectroscopic measurements include UV/Vis, fluorescence, fluorescence lifetime and two photon absorption and fluorescence as a function of the polyene's conjugation length. Structure-property trends will be used to explore the feasibility of using these compounds for optical limiter applications.

COMBINED MAIN- AND SIDE-CHAIN AZOBENZENE POLYESTERS: A POTENTIAL FOR PHOTOINDUCED NONLINEAR WAVEGUIDES. Frederik Sahlén, Tommy Geisler, Svren Hvilsted, Condensed Matter Physics and Chemistry Department, Ris National Laboratory, Roskilde, DENMARK; N.C.R. Holme, P.S. Ramanujam, Optics and Fluid Dynamics Department, Ris National Laboratory, Roskilde, DENMARK; Jan C. Petersen , Danish Institute of Fundamental Metrology, Lyngby, DENMARK.

New combined main- and side-chain azobenzene polyesters, which exhibit an intensity dependent refractive index, have been prepared in order to optically fabricate nonlinear waveguides. The general design idea of our polymers comprises a $\chi$(3)-active chromophore in the main-chain, and a chromophore, which can be oriented by polarized light, in the side-chain. The photoinduced orientation of the side-chains results in anisotropy or birefringence, which provides the basis for the waveguide properties. Furthermore large surface-relief gratings have been found to arise in azobenzene polymers. This grating formation could facilitate the coupling of light into and out of the waveguides. Novel sulfonyl azobenzenes, a diester, (4-[[5-(ethoxycarbonyl)pentyl]sulfonyl]-4í- [[5-(ethoxycarbonyl)pentyl]methylamino] azobenzene, and a diol, 4-[[(8-hydroxy-7-methylhydroxy)-octyl]sulfonyl]- 4í-${\it N}$${\it N}$-dimethylamino azobenzene, have been used to prepare new polyesters by transesterification in the molten state. The polyesters have been characterized by 1H NMR, 13C NMR and UV-visible spectroscopy, differential scanning calorimetry (DSC), size exclusion chromatography (SEC), third harmonic generation (THG) and optical anisotropy measurements. The molar masses of the polyesters were in the range of 5000-19000 g mol-1, which was sufficient in order to spin coat thin films. Optical anisotropy measurements have been carried out in thin films of these polymers. Polarization holography was employed to write both an anisotropic and surface relief grating. A diffraction efficiency of approximately 12$\%$ was obtained from preliminary measurements. From atomic force microscopic investigations, it was found that a stable surface relief grating with a typical depth of 700 nm was also formed. The peaks of the surface relief grating were found to coincide with linear polarization perpendicular to the grating vector. From THG measurements the polymers are shown to possess an off-resonance electronic $\chi$(3) of the order 10-12 esu corresponding to a non-linear refractive index of 2.3$\times$10-14 cm2/W.

DESIGN AND SYNTHESIS OF DENDRIMERS WITH ENHANCED TWO-PHOTON ABSORPTION. El Hadj Elandaloussi , Charles Spangler, Montana State Univ, Dept of Chemistry and Biochemistry, Optical Technology Center, Bozeman, MT; Carl Dirk, Dept of Chemistry, Univ Texas, El Paso, TX; Martin Casstevens, Deepak Kumar, John Weibel, Ryszard Burzynski, Laser Photonics Technology, Inc, Amherst, NY.

There has been considerable recent interest in the design of new organic chromophores, oligomers and polymers with potentially large two-photon cross-sections. One particularly attractive system is based on poly[p-phenylene vinylene] (PPV) oligomers containing electron-donating substituents. We have recently designed and synthesized several PPV dimers, with bis-(diphenylamino) donor groups attached to the terminal phenyl rings, which have demonstrated large TPA cross-sections. We have now extended this design concept to dendrimer structures based on bis-(diphenylamino)stilbene repeat units. The efficacy of this approach will be discussed, as well as projected future design paradigms for even greater TPA enhancement.

PHOTONICS APPLICATIONS OF BIPOLARON FORMATION IN BIS-(DIPHENYLAMINO)DIPHENYLPOLYENES. Kimba Ashworth , Benjamin Reeves, Amy Frost, Charles Spangler, Montana State Univ, Dept of Chemistry and Biochemistry, Optical Technology Center, Bozeman, MT.

Bis-(dialkylamino) and bis-(diphenylamino) substituted diphenylpolyenes have recently been shown to have exceptionally large two-photon cross-sections, and have also been used in optical power limiting (OPL) applications. Another potential mechanism for OPL in these materials involves the photogeneration of highly absorbing bipolaronic dications which can then function as reverse saturable absorbers (RSAs). The synthesis of two series of these unique polyenes has been carried out, as well as studies of bipolaron formation in solution. These new chromophores have also been incorporated into pendant polymers, copolymers and dendrimer formulations, and the efficiency of photonic processes based on polymer type will be presented.

THE SYNTHESIS OF DIPHENYLPOLYENES AND PPV-OLIGOMERS INCORPORATING DIPHENYLPHOSPHINO DONOR GROUPS. Luis Madrigal , Charles Spangler, Dept of Chemistry and Biochemistry, Optical Technology Center, Montana State Univ, Bozeman, MT; Carl Dirk, Dept of Chemistry, Univ Texas, El Paso, TX.

Previous studies of the second and third order nonlinear optical properties of organic molecules have shown enhancement of the nonlinear optical response when second row elements replace first row elements in the structure (e.g. S for O in donor groups). Various substituted amino groups have been used extensively in the design of molecules for both second and third order NLO applications as strong donor groups. However, to date there have been no reported systematic studies of the effect of replacing P for N in various chromophore functionalities. In this presentation we will discuss the syntheses of several organic polyenes with diphenylphosphino substituents, and compare structure-property relationships to the equivalent diphenylamino series.

SYNTHESIS AND OPTICAL LIMITING CHARACTERIZATION OF A PORPHYRIN-BUCKMINSTERFULLERENE DYAD. Kenneth J. McEwan and Keith L. Lewis, Defence Evaluation and Research Agency, Malvern, UNITED KINGDOM; How-Ghee Ang, Zhi-Heng Loh , Leng-Leng Chng and Yiew-Wang Lee, DSO National Laboratories, SINGAPORE.

Buckminsterfullerene (C60) and porphyrins are well-known optical limiters in the visible region. Furthermore, the enhanced nonlinear absorption by the fullerene anion (C60-) relative to pristine C60 is expected, due to the increased electronic transition dipole moment of the unpaired electron in the excited state. We synthesized a porphyrin-C60 dyad in which the spherical C60 moiety is positioned above the porphyrin plane. Molecular modeling yielded a structure with a through-space inter-chromophore distance of $\sim$3.28 $\AA$. It is hoped that the close proximity of the two moieties would facilitate inter-chromophore electron transfer to yield the charge-separated species, which in turn, is expected to exhibit enhanced optical limiting performance relative to its components. However, electronic spectroscopy did not show any through-space interaction. This was attributed to the unfavorable disposition of electron-rich bonds in each chromophore towards one another. Optical limiting measurements using 3-ns laser pulses at 532 nm were performed on solutions of the porphyrin-C60 dyad, the reference porphyrin, and C60. The linear transmittances of the solutions was $\sim$40$\%$. Optical limiting was observed for all the samples. However, the transmitted saturation level exhibited by the porphyrin-C60 dyad was about 4 times higher than that of C60 and 1.3 times higher than that of the reference porphyrin. This could be due to the absence of through-space interaction as predicted from molecular modeling, as well as a shift in the excited state absorption spectra of the dyad relative to its individual components. The latter awaits further confirmation from time-resolved laser spectroscopy experiments. Nevertheless, the vast structural variation possible in such dyads, as presented by their ease of chemical modification, should enable the rational design of dyads with enhanced optical limiting performances relative to their respective components.

ELETRICAL FIELD EFFECTS ON CRYSTALLINE PERFECTION OF MBA-NP CRYSTALS BY MAPPING OF BRAGG-SURFACE DIFFRACTION. S.L. Morelhao , Sao Paulo Univ., Dept of Applied Physics, Sao Paulo, BRAZIL; L.H. Avanci, M.A. Hayashi, L.P. Cardoso, Univ. of Campinas, Inst. of Physics, Campinas, BRAZIL.

High quality non-centrosymmetric organic single crystals, such as 2-(alfa-ethylbenzylamino) - 5-nitropyridine (MBA-NP), have exceptionally large second order polarisabilities. The possibilities of exploiting this fundamental property are perceived to be extensive because of the almost endless variations in chemical structure that can be produced through modern organic synthesis. In addition, to exceptional nonlinearities the space groups are all of the type that also produces piezoelectricity. In terms of controlling material non-linear properties, characterization of the piezoelectric tensor has been important. X-ray diffraction techniques are often used to determine small changes in the crystal lattice under an applied electric field. Recently [Phys. Rev. Lett., 1998 (in-press)], a method based on the X-ray multiple beam diffraction phenomenon were developed for determining more than one piezoelectric coefficients from a single scan. A strong limitation for applying such method is the non-constant mosaicity of the MBA-NP crystals. In this article, we have essentially investigated effects of weak electric fields to the spatial misorientation of the large perfect diffracting regions (mosaic blocks) of the crystals. The misorientation are monitored by mapping of Bragg-surface diffraction (BSD) [Appl. Phys. Lett. 71(18), 2614 (1997)], which is the unique diffraction technique to allow the spatial view of the misorientation. The BSD is a special three-beam diffraction that involve one Bragg reflection and one surface reflection (diffracted beam in the surface parallel direction). Reciprocal space mapping are also used for comparison. The misorientation effects are correlated to the direction of molecular electrical dipole.

TWO-PHOTON ASSISTED NONLINEAR ABSORPTION OF DIFLUORENYLIDENETHENE. Anders Eriksson , Linkoping University, Dept of Physics and Measurement Technology, Linkoping, SWEDEN; Per Andersson, Bertil Eliasson, Umea University, Dept of Organic Chemistry, Umea, SWEDEN; Mikael Lindgren, Svren Svensson, Defence Research Establishment, Linkoping, SWEDEN.

A cumulene named difluorenylidenethene (DFE) has been examined with regard to its third order optical nonlinearities. The material was found to be stable with a strong absorption peak at 479 nm but with very weak absorption above 500 nm. The z-scan method and nonlinear absorption measurements were used to specify the origin of the optical nonlinearity. A Sunlite OPO laser allowing for wavelength tuning was used to characterize the wavelength dependence of the response of the material. Luminescence spectra were collected for a number of different exciting wavelengths and energies. From a preliminary analysis the maximum effective nonlinear absorption coefficient (beta) was found to be ca 44 cm/GW at 532 nm for 3.4 ns pulse-length, however, with a large spreading in experimental data depending on pulse energy. The nonlinear behaviour can possibly be explained in terms of two-photon assisted reverse saturable absorption.

SYNTHESIS AND CHARACTERIZATION OF SOME NEW METAL-CONTAINING POLYMERS FOR THIRD ORDER NONLINEAR OPTICAL APPLICATIONS. Mamoun M. Bader , Phuong-Truc T. Pham, Justin Moser, Leslie Albright, Kevin Myeres and Angela Molli, Department of Chemistry, Pennsylvania State University, Hazleton, PA.

Continuing with our earlier theoretical investigations of 8-hydroxyquinoline and its derivatives, we report here on the synthesis and characterization of a new family of metal-containing polymers incorporating 8-hydroxyquinoline, for third order nonlinear optical applications. These polymers have the following structural features: a metal center, a chromophore, and a flexible spacer group with variable lengths. These structural features allow us to have an appreciable degree of control over the various physical properties of these materials. We report on three different types of such polymers: polyethers, polyesters, and polyamides. Finally the chromophores in these polymers are either incorporated in the backbone and/or as a side chain.

OPTICAL NONLINEAR PROPERTIES OF SELF-ASSEMBLED J-AGGREGATES CYANINE MOLECULES DOPED IN SPIN-CASTED SILICATE FILMS. Itaru Honma , *T. Watanabe, H.S. Zhou, *K. Ishigure and *K. Asai, Energy Division, Electrotechnical Laboratory, AIST, Umezono, Tsukuba, Ibaraki, JAPAN; *Department of Quantum System Engineering, Faculty of Engineering, University of Tokyo, Hongo, Bunkyo-ku, Tokyo, JAPAN.

The functional opto-electrical materials with molecular-to-nano size structure has attracted much attention for devices with excellent properties. We present, in this paper, a new synthetic route of self-organization process to produce self-assembled J-aggregates cyanine molecules doped in sol-gel derived silicate films. The J-aggregates of cyanine molecules (1-methyl-1octadecylquino cyanine perchlorate) with sufficient optical density are spontaneously doped in the sol-gel derived silicate thin films through spin casting technique. The cyanine molecules are solvated in the solution of HCl/H2O mixture, ethanol and TEOS. The mixed solution was spin casted on the silica substrate at the rotational speed of 2000 rpm. The as-prepared film was red-colored where only isolated molecular absorption can be observed, however, as the hydrolysis and condensation of silica proceeds, the doped molecules start forming J-aggregates in the glassy matrix and optical absorption changes to that of the aggregates. The process provide simple and efficient method to prepare optical grade J-aggregates-doped silica thin films while the optical density is much larger than the that formed by LB technique. The third order nonlinear optical properties of the film was measured by Z-scan method in a vacuum at liquid nitrogen temperature. The measuring wavelength of 577 nm and light intensity of 20-80 MW/cm2 was used. The large nonlinear susceptibility of 5 x 10-7 esu was obtained for doped J-aggregated in the transparent silica film.

ULTRAFAST OPTICAL RESPONSES OF THE SQUARYLIUM DYE J-AGGREGATES FILMS. Satoshi Tatsuura , Min-quan Tian, Yasuhiro Sato, Lyong Sun Pu, Fuji Xerox Co., Ltd., Corporate Research Labs., Kanagawa, JAPAN; Makoto Furuki, FESTA Laboratories, Ibaraki, JAPAN.

Some organic molecules are attractive candidates for the ultrafast optical devices because of their fast recovery of the bleached and induced absorption. Recently they have been observed that the J-aggregates of pseudoisocyanine bromide have a fast decay component of 1.3 ps at 77K and that the monolayer of squarylium(SQ) dye J-aggregates on water surface has 280 fs decay time at room temperature(RT). But in consideration for utilizing them as optical devices, operation at RT and easiness of handling should be very important. We tried to fabricate the films consist of SQ J-aggregates on solid substrates and evaluate their decay times of the bleached absorption. We fabricated the SQ films on glass substrates by spincoating method. The SQ derivatives differed from each other markedly in their spectra and behavior under heating according to their alkyl side chain length. Progress of aggregation was observed for some SQ derivatives under heating and/or the existence of acid vapor. Especially we found it for the first time that the SQ derivatives with dipropylamino bases(SQ33) and dibutylamino bases(SQ44) showed a sharp absorption peak of J-aggregates in the spincoated films. Time-resolved difference absorption spectra were measured about the SQ33 and SQ44 spincoated films by femtosecond pump-probe spectroscopy. The fast decay time of SQ44 was determined to 220 fs at 50 MW/cm2 pump power and the max rate of an absorbance change was about 25 % at 220 MW/cm2 pump power. All measurements were performed in air at RT. We suppose that the SQ dye has great advantages both in ability and cost in the field of the ultrafast optical applications. This work was supported by New Energy and Industrial Technology Development Organization (NEDO) within the framework of the Femtosecond Technology Project.

INFLUENCES ON THE SPECTROSCOPIC PROPERTIES OF AFX SERIES TWO-PHOTON ABSORBING HETEROCYCLIC CHROMOPHORES. Jeffery W. Baur , Max D. Alexander, Jr., Lisa R. Denny, Bruce A. Reinhardt and Richard A. Vaia, Materials and Manufacturing Directorate, Air Force Research Laboratory, Wright-Patterson AFB, OH; Ramamurthi Kannan, Systran Corporation, Dayton, OH; Sean A. Kirkpatrick, SAIC, Dayton, OH.

Due to the recent development of organic chromophores with relatively large effective two-photon cross-section values, there has been intense interest in utilizing and optimizing nonlinear absorption processes in a variety of applications including optical limiters, nondestructive inspection of paint with underlying corrosion, and low energy photocuring. In addition to being thermally stable and processible, asymmetric chromophores consisting of $\pi$-electron donating and $\pi$-electron accepting heterocyclic moieties separated by a conjugated aromatic core have been shown to give some of the largest nonlinear responses. However, these nonlinear responses have also been shown to vary with local molecular environment and with the pulse width of the incident radiation. Here we consider the influence of molecular environment on the linear and nonlinear spectroscopic properties of a series of structurally similar chromophores. Because the two-photon fluorescence was nearly identical to that of the single-photon fluorescence for a given molecular environment, information about the fluorescing state obtained from a linearly excited molecule could be applied to that of a nonlinearly excited molecule. For the series of chromophores examined, the Stokeís shift was observed to increase linearly and substantially with solvent polarity. Such behavior was adequately described by existing solvent relaxation theory and indicates that the fluorescing species is highly sensitive to a local molecular environment which can be described in terms of the properties of the bulk solvent. An excited state absorption from this solvent sensitive state has been suggested as a possible cause for the solvent sensitivity of the nanosecond effective two-photon cross-section values and for the large discrepancy between values recorded with femtosecond and nanosecond laser pulses. Detail studies of intermolecular energy transfer interactions between chromophore substituents will further the understanding of this fluorescing excited state.

ELECTROSTATIC SELF-ASSEMBLY OF MULTILAYER NONCENTROSYMMETRIC THIN FILMS AND DEVICES. Kristie M. Lenahan , You-Xiong Wang, Wei Zhao, Richard O. Claus, Virginia Polytechnic Institute & State University, Fiber & Electro-Optics Research Center, Blacksburg, VA; Yanjing Liu, NanoSonic, Inc., Blacksburg, VA.

The electrostatic self-assembly of multilayer thin films by alternate adsorption from polyelectrolyte solutions spontaneously leads to the formation of noncentrosymmetric structures if the molecules themselves have net dipole moments. The observation of significant second-order nonlinear optical susceptibility in such films has been observed, using both commercially available chromophores and molecules specifically designed to yield an enhanced net dipole moment. The nature of the deposition process results in an alignment of the chromophores that is stable over time and to temperatures up to 150$^{\circ}$C, in contrast with poled polymers. ESA films offer the additional major advantages of excellent homogeneity and low optical loss, high thermal and chemical stability, and low cost.

Wednesday Morning, April 7, 1999
Franciscan II (A)
8:30 AM *F4.1
DESIGN AND OPTIMIZATION OF CHROMOPHORES FOR LIQUID CRYSTAL AND PHOTOREFRACTIVE APPLICATIONS. R.J. Twieg , M. He, L. Sukhomlinova, F. You, Department of Chemistry, Kent State University, Kent, OH; K.D. Singer, V. Ostroverkhov, Department of Physics, Case Western University, Cleveland, OH; W.E. Moerner, M.A. Diaz-Garcia, D. Wright, J.D. Casperson, B. Smith, E. Glazer, Department of Chemistry, UC San Diego, La Jolla, CA; R. Wortmann, C. Glania, P. Kramer, K. Lukaszuk, R. Matschiner, Institute of Physical Chemistry, University of Mainz, Mainz, GERMANY.

Organic chromophores are employed in a wide range of discrete optical or electronic functions as well as combined opto-electronic functions. Well-known and established applications include photosensitizers and charge transport agents for electrophotography and liquid crystals and dichroic dyes for displays. New and emerging applications for organic chromophores include those for electro-optic, electroluminescent and photorefractive devices. Fortunately, many common structure features can be shared amongst these chromophores and then synthetic, design and function properties are portable amongst them. Our own current efforts on organic chromophores reflect this diversity of structure and function but also attempt to utilize common structure concepts as often as possible. For both photorefractive and liquid crystal applications we are working with chromophores that contain the diphenylacetylene and methylenedihydropyridine functionalities. We are also attempting to extend these pyridine based systems to the analogous methylenetetrahydro- and methylenehexahydro- systems as well. Photorefractive materials are generally fabricated and their diffraction efficiency and speed are evaluated as composite systems in a charge transport material such as PVK. Chirality is another general feature of current interest as it applies to NLO materials with helical pi-systems for Kleinman-disallowed hyperpolarizability. Here the pseudotensor contributions to the nonlinearity are evaluated by hyper-Rayleigh light scattering experiments. Presently the important design features that will give rise to large nonlinearities via these contributions is not well understood. The chiral molecules we are presently evaluating almost invariably are derived from natural sources such as camphor or the steroid estrone. Chirality, of course, has already been of great relevance to liquid crystals including cholesteric and ferroelectric liquid crystals. The potential liquid crystal behavior of our chromophores is evaluated by thermal analysis and polarized optical microscopy. Many of the relevant electronic and optical properties of these photorefractive and liquid crystal systems are also conveniently obtained by electro-optical absorption measurements (EOAM). In all cases we are also attempting to design these molecules mindful of ultimate requirements for durabilty as well.

9:00 AM *F4.2
NEW INSIGHTS INTO TRAPPING AND COMPENSATION IN PHOTOREFRACTIVE POLYMERS. W.E. Moerner , Stanford Univ, Dept of Chemistry, Stanford, CA; D. Wright, M. Diaz-Garcia, A. Goonesekera, J. Casperson, B. Smith, M.S. DeClue, E. Glazer and J.S. Siegel, Univ of California San Diego, Dept of Chemistry, La Jolla, CA; R.J. Twieg, Kent State Univ, Dept of Chemistry, Kent, OH.

In the past few years, several photorefractive polymer materials have attained impressive performance, with two-beam coupling gain coefficients of 100's per centimeter. We have utilized these large gain coefficients coupled with multiple layer geometries and/or grating translation to achieve beam fanning, self-pumped phase conjugation and single-pass gains of a factor of 500. In spite of these large strides, the understanding of trapping is still in its infancy. We consider a class of photorefractive polymers based on the photoconducting host polymer poly(N-vinyl carbazole), doped with various nonlinear optical chromophores based on dicyanostyrenes, and with a liquid plasticizer and the sensitizer fullerene C60. Detailed measurement of the trapping dynamics as a function of time, intensity, and electric field allows us to propose a new trapping model. Using a combination of infrared spectroscopy, extensive photorefractive performance measurements, and electrochemical measurements, we show that the fullerene anion acts as the primary hole trap with compensation provided by the nonlinear optical chromophore. New insights into the optimal design of photorefractive polymers will be presented.

9:30 AM *F4.3
PHOTOREFRACTIVE POLYMERS WITH HIGH SPEED. N. Peyghambarian , K.B. Ferrio, J. Herlocker, E. Hendrickx, B.D. Guenther, Univ. of Arizona, Tucson, AZ; S. Mery, GMO/IPCMS, Stasbourg, FRANCE; Y. Zhang, B. Kippelen, Univ. of Arizona, Tucson, AZ.

We report on a photorefractive polymer with a 4-ms response time constant in transient four-wave mixing experiments at 0.5 W/cm2 writing irradiance, 95 V/$\mu$m applied electric field, and a grating period of 3.1 $\mu$m. Complementary transient ellipsometry, however, reveals orientational birefringence response two orders of magnitude faster, with a time constant of 30 $\mu$s. Orientation does not limit the dynamic formation of photorefractive gratings in this polymer, which suggests that even faster photorefractive responses are possible for polymer composites with improved charge generation and transport properties.

10:00 AM F4.4 PHOTOREFRACTIVE EFFECT IN A NEW COMPOSITE BASED ON BI-FUNCTIONAL HOST POLYMER. Yi-Wang Chen , Yuan-Kang He, Hui-Ying Chen, Institute of Polymer Science, College of Chemistry, Peking University, Beijing, P.R.CHINA; Feng Wang, Zhi-Jian Chen, Qi-Huang Gong, Department of Physics, Peking University, Beijing, P.R. CHINA.

The guest-host approach for photorefraction always leads to a polymeric system with low stability. Instead, bi-functional side chain polymeric materials, which have both the photoconductivity and the electro-optic effect intrinsically, exhibit higher stability of the EO effect. We have synthesized a new bi-functional side chain polymeric material obtained from poly(carbazolylpropyl methacrylate) (PCPMA) as starting material. A portion of the carbazole groups were reacted with p-nitroaniline to form 3-(p-nitrophenyl)azo carbazole (NPAC) groups having second-order NLO properties. The photorefractive effect was observed in composites based on such bi-functional side chain copolymer of N-methacryloxypropyl-3-(p-nitrophenyl)azo carbazole with N-methacryloxypropyl carbazole (PCPMA-NPAC). A two-beam coupling gain coefficient of 9.4 /cm and an electro-optic coefficient of 9.3 pm/V were measured at the applied electric field of 92.4V/mm by the typical two-beam coupling experiment and the compensation electro-optic measurement technique. The erase-write properties of the gratings were discussed as well.

Wednesday Afternoon, April 7, 1999
Franciscan II (A)
1:30 PM *F5.1
ORGANIC AND POLYMER TRANSISTORS: DEVICE PHYSICS AND APPLICATIONS. A. Dodabalapur , Z. Bao, R. Sarpeshkar, Y.Y. Lin, H.E. Katz, W.J. Li, A.J. Lovinger and V.R. Raju, Bell Laboratories, Lucent Technologies, Murray Hill, NJ.

This presentation will begin with a description of the basic physics of typical organic and polymer transistors and the factors which determine and influence the apparent mobility. The transient characteristics and modeling of organic transistors will be described. The integration of organic light emitting diodes and transistors is promising for emissive displays. We have developed designs for `smart' pixels in which an analog circuit consisting of 6-7 transistors drives each LED. The simulated and experimental characteristics of such pixels will be presented. The development of air-stable n-channel organic transistors led to our demonstrating the first organic complementary circuits. The design considerations and characteristics of organic complementary circuits with > 100 transistors will be described. Such circuits will be compared with circuits based upon only p-channel transistors.

2:00 PM F5.2
SYNTHESIS, MATERIAL PROPERTIES, AND TRANSISTOR PERFORMANCE OF OLIGOTHIOPHENES AND THEIR THIAZOLE ANALOGUES. Wenjie Li , Howard E. Katz, Andrew J. Lovinger, Karl R. Amundson, Bell Laboratories, Lucent Technologies, Murray Hill, NJ.

Beginning with the original reports of Garnier on the utility of vacuum-sublimed thiophene oligomers in field-effect transistors (FETs), this compound class has been an important vehicle for exploring the performance limits and applications of these devices. We have previously reported the synthesis and semiconducting properties of oligothiophenes with solubilizing end-substituents, especially dihexylsexithiophene (DH$\alpha$6T) and dihexylquinquethiophene (DH$\alpha$5T). In this paper, the liquid phase fabrication of large area FETs based on these compounds is investigated. Thin films cast from solutions of these compounds onto different dielectric surfaces showed high field-effect mobilities (> ca. 0.03 cm2Vs) and good film continuity over large areas (1-2 cm2). In addition, several new p-type semiconducting materials with lower electron-donating ability than the parent oligothiophenes were synthesized and their thermal, morphological, and FET properties were studied. The incorporation of thiazole rings into oligothiophenes was designed to make them less susceptible to p-doping. The thiazole-containing compounds generally have lower melting points and better solubility than the corresponding oligothiophenes. The off currents are also lower due to the reduced susceptibility. Relatively high on/off current ratios (> 103) can be routinely obtained from devices operating in air, eliminating the need for strict exclusion of oxygen. However, the trade-off is their lower field-effect mobility which is possibly caused by the larger charge injection barrier and poorer ordering in the film.

2:15 PM *F5.3
NANOSCALE LIGHT AMPLIFICATION AND CONVERSION IN ORGANIC CRYSTALS. Denis Fichou , Laboratoire des Materiaux Moleculaires, C.N.R.S., Thiais, FRANCE; Fabrice Charra, Alexandre Gusev, DRECAM, CEA Saclay, Gif sur Yvette, FRANCE; Jean-Michel Nunzi, Vincent Dumarcher, LETI, CEA Saclay, Gif sur Yvette, FRANCE.

We recently observed amplified stimulated emission (ASE) in plane-parallel single crystals of quaterthiophene (4T), sexithiophene (6T) and octithiophene (8T) under one-photon transverse pumping. Following a single-pass amplification, SE light is waveguided throughout the organic crystals on several millimeters and comes out essentially from the edges in a total internal reflection process. These lamellar crystals also generate two-photon fluorescence when they are pumped in a longitudinal geometry by a near-infrared light beam, i.e. when the beam of a Ti:sapphire laser is directed right in between the two lower and upper faces of the crystal. Importantly, these two phenomena are highly dependent on the structural morphology of the crystal samples and it becomes necessary to investigate the origin of light emission at the microscopic and molecular levels in order to determine its primary mechanism. In this context, we used the tip of a scanning tunneling microscope (STM) to image the morphology of the front face of 4T, 6T and 8T crystals. Although these bulk materials (typically a few microns thick) are resistive, their good photoconductivity allows to flow enough charges when they are deposited on top of a transparent conducting ITO substrate and irradiated by a continuous He-Cd laser light. The weak photocurrent which is produced then allows to realize the structural mapping of the crystal surface. But more importantly, laser-induced STM also provides a powerful tool to generate photovoltaic conversion of light into electricity at the microscopic level. The STM tip, the air gap and the crystal then form a metal-insulator-semiconductor (MIS) junction aimed at injecting charges into the photoconductive material under laser irradiation. Correlation between the structural and the photovoltaic images of the 4T, 6T and 8T crystals unveils some unexpected features with a spatial resolution of a few nanometers.

2:45 PM F5.4
TIME RESOLVED TRANSPORT OF ELECTRONS AND HOLES IN CONJUGATED POLYMERS. D.J. Pinner , N. Tessler, R.H. Friend, Cavendish Laboratory, University of Cambridge, UNITED KINGDOM.

We present a wide range of time resolved properties of various polymers in light emitting diode configurations. Using a combination of time resolved current, voltage, charge induced absorption, light emission, and light emission spectra we are able to provide direct evidence for the motion of both types of carriers. Using the charge induced absorption, the unknown properties of the contact interface are decoupled from the charge transport. We report reliable data of both electron and hole mobility in efficient LEDs and address the issue of pulsed versus CW properties. Knowing the above properties, efficient light emitting diodes are constructed which allow us to demonstrate light intensity in excess of 20 million cd/m2. We discuss the implications of this work for future electrically-pumped polymer-based lasers.

3:30 PM F5.5
ORGANIC LASERS BASED ON ONE- AND TWO-DIMENSIONAL PHOTONIC BANDGAP STRUCTURES. Martin Meier1 , Attila Mekis2, Ananth Dodabalapur1, Zhenan Bao1, John A. Rogers1, John D. Joannopoulos2, Richard E. Slusher1, 1Bell Laboratories, Lucent Technologies, Murray Hill, NJ; 2MIT, Cambridge, MA.

Organic gain media have been very useful in investigating he properties of several classes of laser resonators such as distributed feedback (DFB), distributed Bragg reflectors (DBR), planar microcavities and whispering-gallery mode. We describe the characteristics of two-dimensional distributed feedback lasers with organic thin-film gain media (Alq /DCM) deposited on lithographically patterned Si/SiO2 photonic band gap structures. Bragg reflections caused by the gratings diminsh the group velocity of photons along some directions of crystallographic symmetry to zero, and the resulting feedback gives rise to laser oscillations. Dispersion relations for photons were calculated analytically and are used to interpret the laser emission spectra. We will describe techniques which make possible the realization of a complete two-dimensional photonic bandgap in the visible. A promising and quite ease method to fabricate one- and two dimensional distributed feedback resonators for organic lasers are soft lithographic techniques. We demonstrate the successful application of microcontact printing and replica molding techniques to the fabrication of DFB, DBR and two-dimensional photonic bandgap structures with a feature size of  300nm. Using Alq:DCM as organic gain media either the precursor polymer PPV or a acrylate-based pre-polymer (NOA 72) can be used to form the resonator structures. Thus photoexcited plastic lasers have narrow emission profiles. The good performance of the lasers demonstrates the suitability of these techniques for fabrication certain types of functional optoelectronic devices that require submicron features size.

3:45 PM F5.6
MORPHOLOGY AND CHARGE TRANSFER IN INTERPENETRATING NANOPARTICLE/CONJUGATED POLYMER NETWORK PHOTOVOLTAICS. Alexi Arango , Sue Carter, UCSC, Dept of Physics, Santa Cruz, CA; Thomas Daubler, Dieter Neher, Max-Planck-Institut fur Polymerforschung, Mainz, GERMANY; Phil Brock, IBM Almaden Research Center, Almaden, CA.

We study the effect of nanoparticle morphology and dye-sensitization on quantum efficiency and photovoltage in conjugated polymer / nanoparticle blends and layered photovoltaic cells. Additionally, we examine the consequence of exposure to ambient atmosphere and the effect of a self-assembled nanoparticle layer at the ITO electrode. In ambient atmosphere, blended sandwich cell devices with ITO and Au electrodes, reveal large photocurrents of over 100 electrons per incident photon at low bias voltages of $\pm$ 1 Volt, yet produce no significant photovoltage or short circuit current. Layered devices have lower photocurrents under bias, yet generate open circuit voltages of 0.7 Volts and short circuit current quantum efficiencies approaching 5%. We utilized DC photocurrent action spectra and current-voltage characteristics, from which conclusions about photo-induced charge transfer at the nanoparticle interface, charge trapping and hole injection may be drawn.

Wednesday Afternoon, April 7, 1999
Franciscan I (A)

1:30 PM *F6.1/B13.1/E8.1
EXCIMER FORMATION IN COPOLYMERS OF 9,9-DIALKYL FLUORENES AND ANTHRACENE(S). R.D. Miller , G. Klaerner, F.G. Klaerner, M. Kamieth, M.H. Davey, J.C. Scott, J.P. Chen, IBM Almaden Research Center, San Jose, CA.

Poly(fluorene) derivatives comprise an interesting class of electroactive materials with potential application in polymeric light emitting diodes. The formation of excimers upon thermal annealing or the passage of current is a ubiquitous problem for these materials. The interaction of polymer chains with themselves and/or their environment depends on the chemical structure and shape of the chain and the nature of the functional groups. We have prepared a variety of novel linear, U- and S- shaped rigid rod polymers by the copolymerization of 2,7-dibromo-9,9-dihexylfluorene with 2,6-, 1,8, 1,5-, and 9,10-dihaloanthracenes. Polymer chain conformations have been studied by molecular modeling and predictions regarding excimer formation have been verified by spectroscopic studies.

2:00 PM F6.2/B13.2/E8.2
PHOTOLUMINESCENT POLYMER FILMS FOR DISPLAY APPLICATIONS. Anja R.A. Palmans , Andrea Montali, Christoph Weder, Paul Smith, ETH Zurich, Dept. of Materials, Zurich, SWITZERLAND.

In order to increase the efficiency of liquid crystal displays, our research has focused on the introduction of photoluminescent (PL) polarizers into traditional displays. These PL polarizers typically comprise rigid poly-p-phenyleneethynylene (PPE) polymers, embedded and uniaxially oriented in a polyethylene (PE) matrix. Since only a limited fraction of incident light can directly be used by these PL polarizers - because their absorption is also anisotropic - we recently proposed a new approach in which coumarin dyes have been isotropically mixed into such oriented PPE/PE films to act as sensitizer. In these ternary blends, the overall efficiency of transferring the unpolarized incident light to polarized colored light has been dramatically improved through a polarizing energy transfer from the sensitizer to the PPE. To explore the scope and limitations of this approach, a variety of sensitizers was mixed into the PPE/PE blends. However, we observed that upon mixing certain sensitizers into the PE/PPE polymer blends, problems related to phase separation and crystallization arise. As a result, we have introduced the concept of covalently attaching sensitizers to the conjugated polymer backbone. This approach allows us to introduce a well-defined distance between sensitizer and PPE, which ameliorates the control over the required proximity between the sensitizer and PPE. As such, energy transfer efficiency may be further improved. In addition, problems related to phase separation are circumvented. Finally, the polarizing nature of the energy transfer can be enhanced.

2:15 PM F6.3/B13.3/E8.3
LASING IN ORGANIC THIN FILMS USING CASCADE FORSTER ENERGY TRANSFER. Martin Meier , John A. Rogers, Ananth Dodabalapur, Zhenan Bao, Richard E. Slusher, Bell Laboratories, Lucent Technologies, Murray Hill, NJ; Attila Mekis, MIT, Dept. of Physics, Cambridge, MA.

Optically pumped lasing in thin films of an absorbing host 8-hydroxyquinolinato aluminum (Alq) or PBD doped with different laser dyes (C490, DCM II, LDS 821) is investigated. In our materials, an initial molecular excited state is generated in the host compound by absorption of light. This state is then resonantly and non-radiatively transferred through one or more steps down in energy. Suitably matched dye molecules dispersed in the host ensure that the absorption losses at the final emission wavelength are very small. This approach can lower the threshold power density required for light amplification and provide broad tunability of the emission wavelength. Furthermore the optical emission properties are decoupled from the transport properties which is important for the design of future electrically driven device structures. In order to provide the necessary feedback, we employed distributed feedback (DFB), distributed bragg reflectors (DBR) and two-dimensional photonic bandgap structures based on thermally oxidized Si wafer as resonators. The carefully chosen gain media together with the resonator structures which act as wavelength-selective mirrors allow us to fabricate optically pumped lasers in the whole visible spectral range (390nm -800nm). We also fabricated rib-waveguide based DFB and DBR lasers in which the gratings are defined by soft-lithographic procedures.

2:30 PM F6.4/B13.4/E8.4
ELECTRO-LUMINESCENCE AND PHOTOLUMINESCENCE OF POLYFLUORENE-PHK COPOLYMER LED. O. Stephan, J.C. Vial , F. Muller, Lab. de spectrometrie Physique, Univ. J. Fourier and CNRS, St. Martin d'Heres cedex, FRANCE.

Thin layers of polyfluorene-polyhexylcarbazole copolymer sandwiched between ITO and aluminum electrodes show stable photoluminescence and electro-luminescence at room temperature and ambient atmosphere. I(V) and Electro-luminescence versus voltage curves present a threshold at 7 Volts for layer thicknesses of the order of 0.1 $\mu$m. The emission degrades slowly when the layer is electrically polarized or UV exposed (The lifetime of the device is typically 12 hours for 25 Volts - 150 $\mu$A polarization conditions). Photoluminescence and electro-luminescence spectra show the same features and are similar to those obtained for low dilution of the polymer in chloroform. Time resolved photoluminescence spectra present a fast blue band (in the sub nanosecond range) centered at 450 nm and a slower yellow one (in the range of 5 ns) centered at 550 nm. The yellow band is absent for fresh samples but become dominant (specially for the electro-luminescence) for aged samples. As already proposed, the spectral behavior suggests that this band results from the re-crystallization of the polymer, here, we add other observations in favor of this explanation. In addition, We explain the slower decays as taking their origins in indirect type electron-hole recombination, enhanced in the crystalline phase.

2:45 PM F6.5/B13.5/E8.5
FLUORESCENT, HEXACATENAR LIQUID CRYSTALS. Ben Hoag, Douglas L. Gin , Univ of California, Dept of Chemistry, Berkeley, CA; Udo Theissl, Emil J.W. List, Egbert Zojer, Guenther Leising, Technische Univ Graz, Inst für Festköperphysik, Graz, AUSTRIA.

A novel family of phasmidic liquid crystals incorporating an oligo($\it{p}$-phenylenevinylene) core was produced. These compounds exhibit blue luminescence in both solution and the solid state. The emission profiles of these compounds are influenced by the degree of organization in the solid state. The synthesis and liquid-crystalline behavior of these compounds will be discussed. Their photophysical properties, as studied by c.w. photoluminescence excitation, emission spectroscopy, and site selective spectroscopy, as well as their optoelectronic properties will be presented.

3:15 PM *F6.6/B13.6/E8.6
MULTICOMPONENT DESIGNS FOR POLYMER LEDs. Frank E. Karasz and Bin Hu, Department of Polymer Science & Engineering, University of Massachusetts, Amherst, MA.

Multilayer architectures provide substantially enhanced efficiencies in polymer LEDs principally by virtue of their ability to provide a more balanced charge injection and good excitonic confinement within the chromophore layer. Further improvement may be gained by using the same charge transport materials in a multi-component design which relies on the immiscibility of the respective polymeric constituents to provide micro-domains (``quantum wells'') of the chromophore in a matrix of the other components. Using alternating hard-soft block copolymers in which the oligomeric hard block forms the chromophore while the soft block provides solubility and additional phase separation, we have designed systems which emit blue, green or red, as required, from a single layer device. Further advantages of the multicomponent design include more versatility in optimizing charge injection by adjusting the ratio of the electron and hole transporting materials, and reduced concentration quenching in the chromophore because of its dispersed state. By using a chromophores blend, one may also take advantage of energy transfer from the higher to the lower band gap material. Fabrication of a single layer device is simplified in comparison to a multi-layer architecture.
Furthermore the multi-component design has been developed to produce a white light emitting polymer LED. The chromophore domain is a blend of the blue, green and red emitters in appropriate ratio, while the charge transport components optimized by the usual criterion of balanced injection from the matrix. Because of the differing band gaps and ionization potentials of the chromophore components the emission spectrum is field dependent; a CIE chromaticity diagram analysis showed a shift in the spectrum as the bias voltage was increased, progressing from orange-red to white as the green and blue chromophores became activated.

3:45 PM F6.7/B13.7/E8.7
OPTICAL PROPERTIES AND CHARACTERIZATION OF Eu3+ ACTIVATED PMMA:PAAc COPOLYMER SAMPLES: Eu CONCENTRATION EFFECTS. Rebeca Sosa , Mario Flores, Antonio Muñoz and Rogelio Rodríguez, Dept. of Physics, Universidad Autónoma Metropolitana-Iztapalapa, Distrito Federal, MÉXICO.

Samples of Eu3+-activated PMMA:PAAc copolymers were prepared in order to investigate their spectroscopic properties. The densities, refractive indices. UV-Visible absorption and emission spectra and fluorescence decay times were obtained experimentally. These data were used in conjunction with Judd-Ofelt theory to calculate the spontaneous emission probabilities and branching ratios, as a function of the europium content. The range of the concentrations was 0.1 to 2.5$\%$ mol. Our results are combined with previous information on the optical properties of rare-earth activated polymers in order to get a better understanding of the role played by the Eu3+ ions on the polymerization process in our samples.

4:00 PM F6.8/B13.8/E8.8
PHOTOLUMINESCENCE OF POLY(P-PHENYLENE VINYLENE) ENCAPSULATED IN MESOPOROUS SILICA. Andras G. Pattantyus-Abraham, Michael O. Wolf , Dept of Chemistry, University of British Columbia, Vancouver, BC, CANADA.

We have prepared poly(p-phenylene vinylene) (PPV) encapsulated in a mesoporous silica, MCM-41. In situ polymerization of xylylene bis(tetrahydrothiophenium chloride) yields a highly luminescent, yellow powder with a fluorescence spectrum matching that of unencapsulated PPV. Nitrogen absorption isotherms show that the pore size of the PPV-loaded MCM-41 is significantly smaller than that of empty MCM-41 and a polymer loading of ca. 8% by weight is obtained by thermogravimetric analysis. The hybrid material has also been characterized by IR and solid-state NMR. Detailed photoluminescence data for the encapsulated PPV will be presented, and the use of these nanostructured materials in device applications will be discussed.

4:15 PM F6.9/B13.9/E8.9
FLUORESCENCE EMISSION FROM SINGLE STRETCH-ORIENTED CONJUGATED POLYMER MOLECULES. Paul J. Carson , Jessie A. DeAro, Steven K. Buratto, Department of Chemistry, University of California, Santa Barbara, CA; Cuiying Yang, IPOS, University of California, Santa Barbara, CA.

Fluorescence from blends of derivatized poly(p-phenylenevinylene) (PPV) in stretch-oriented polyethylene (PE) has been observed with single-molecule spatial isolation using confocal microscopy. The photoluminescence and electroluminescence, as well as mechanical properties and ease of processing, have made conjugated polymers desirable as potential active materials in device applications. Single-molecule fluorescence has the advantage over ensemble spectroscopies in that it allows the direct observation of the heterogeneity of spectral line shapes and fluorescence intermittency among individual emitters. These photophysical properties, as well as others, reveal the distribution of nanoenvironments and optically relevant conformational distributions of the fluorescent species in the active material. Results on single-molecule fluorescence intermittency and spectral line shapes will be presented for the derivatized-PPV/PE system. Polarization anisotropy in both stretch-oriented and unoriented polymer blends will be discussed. New techniques for characterizing the underlying rates of molecular state fluctuation will be presented.

4:30 PM F6.10/B13.10/E8.10
PROBING OF TRIPLET STATES IN POLYMER LEDS. V. Cleave , N. Tessler, R.H. Friend, University of Cambridge, Physics Department, Cambridge, UNITED KINGDOM; G. Yahioglu, Imperial College, Chemistry Department, London, UNITED KINGDOM; P.Le Barny, Thomson-CSF, Orsay, FRANCE.

We report a study of the triplet state in doped polymer light-emitting diodes (LEDs). Using a system consisting of a luminescent polymer doped with triplet-emitting molecules, we employ time resolved electroluminescence (EL) and photoluminescence (PL) as well as sub-gap (triplet) absorption, to give direct evidence for the transfer of triplet as well as singlet excitons from the polymer host to the molecular dopant. These transfer processes offer normally non-radiative triplet excitons, as well as singlet ones, access to a radiative decay channel, lifting the 25% efficiency limit imposed by the ratio of 3:1 triplet:singlet excitons created in conventional polymer LEDs. Device efficiency can be maximized by optimising the trapping of carriers, singlet excitons (Förster transfer), and triplet excitons (Dexter transfer) by a single guest molecule. The incorporation of triplet emitting molecules also enables us to probe the position of the triplet level in the polymer host via knowledge of the guest molecule, and build a more complete picture of the polymer energy levels. We show that the established procedure can be applied to a wide range of luminescent polymers to shed light on their basic photophysics.

4:45 PM F6.11/B13.11/E8.11
FROM A REACTION MECHANISM TO NEW PPV'S FOR HIGH PERFORMANCE POLYMER-LEDS. H. Spreitzer , H. Becker, W. Kreuder, E. Kluge, H. Schenk, Aventis Research & Technologies, Frankfurt, GERMANY.

In the recent years, organic light emitting materials made dramatic progress in comparison to classical semiconductor light-emitting diodes. Especially light emitting polymers provide an ideal combination of processability and performance to allow for an innovative low cost technology for LCD-backlighting, illumination, and display applications. Recent advances in materials and device preparation lead to high-performance polymer LEDs (PLEDs) with power efficiencies exceeding 10 lm/W.
Soluble Poly-para-phenylenes vinylenes (PPVs) have been proven to be highly efficient light emitting materials for different parts of the visible spectrum. Those materials can be prepared by dehydrohalogenation polymerization of 1,4-bischloromethyl benzenes. The mechanism of this polymerization is assumed to incorporate a p-quinodimethane species as a key intermediate. We could show, that this mechanism may lead to polymerization defects in the polymer which then will cause interruptions in conjugation. Due to the fact, that high charge carrier mobilities are required for a stable polymeric emitter, they have been closely investigated by 13C NMR measurements of 13C isotope labeled polymers. It has now been shown, that the expected breaks in the conjugation chain could indeed be identified. The amount of defects differs regarding the substitution pattern of the PPV. The detrimental effect of conjugation breaks can clearly be observed in lifetime tests of PLEDs.
By modifying the substitution pattern of the PPV, the occurrence of polymerization defects was reduced drastically. Those new materials now allow the preparation of very stable and highly efficient PLEDs. Efficiencies of up to 16 lm/W (@ 100 cd/m2) and lifetimes exceeding 25000 h (as determined by accelerated aging) are now routinely achieved. The new class of high performance polymers allows easy access to various colors.

Wednesday Evening, April 7, 1999
8:00 P.M.
Metropolitan Ballroom (A)
OBSERVATION OF LARGE BAND BENDING AT ORGANIC SEMICONDUCTOR HETEROINTERFACES. P.G. Schroeder 1, M.W. Nelson1, P.A. Lee2, K.W. Nebesny2, N.R. Armstrong2, R. Schlaf1 and B.A. Parkinson1, 1Colorado State University, Department of Chemistry, Fort Collins, CO; 2University of Arizona, Department of Chemistry, Tucson, AZ.

Band bending of up to 0.8eV has been observed at heterointerfaces involving organic semiconductor materials with different Fermi level energies. Our results demonstrate that this effect is very significant for many of the active materials being investigated for luminescent displays (OLEDís) and organic thin film transistors (OTFTís). The interfaces investigated were grown by vapor deposition in a multi step growth procedure. After each growth step the samples were characterized in situ using a combination of x-ray and ultraviolet photoemission spectroscopy (XPS, UPS). In these measurements UPS was used to measure the highest occupied molecular orbital (HOMO) positions and interface dipoles while XPS was used to independently determine the band bending at the interface. Our experiments show that, due to the large magnitudes of band bending observed, it is crucial to incorporate band bending into the determination of the energy level alignment. This procedure leads to a more precise determination of the electronic structure at heterocontacts in order to gain better understanding of the charge injection processes and rectification behavior of such interfaces.

TOWARDS THE SYNTHESIS OF MONO- AND BI-FUNCTIONAL AMPHIPHILIC OLIGOTHIOPHENES. Rigoberto C. Advincula , Seiji Inaoka and Dan Phillips, Department of Chemistry, University of Alabama at Birmingham, AL; Mi-Kyoung Park, Dong-Myung Shin, Department of Chemical Engineering, Hong-Ik University, Seoul, KOREA.

It is our objective to investigate the mono- and bi- functionalization of oligothiophene derivatives to achieve amphiphilicity and telechelic functionality. The telechelic functionality is attractive for further polymerization reactions and utility in copolymer architectures. In this report, we describe our initial synthesis and characterization of 5,5'''''-di((6-bromohexyl)-2,2':5', 2-terthiophene) 2,2':5',2:5,2''': 5''',2:5,2''''' - a sexithiophene derivative. The synthesis was done primarily by the symmetrical coupling of terthiophene derivatives. We also synthesized a thiophene pentamer, bromoalkyl-oligothiophene derivate 5-(6-bromohexyl)- 2,2';5',2;5,2';5',2- quinquethiophene and characterized its initial photoluminescence properties. N,N'-dimethyl-N,N'-bis(6-(a-quinquethienyl)hexyl)- 1,6-hexanediamine was formed from the reaction with N,N-dimethyl-1,6-hexanediamine. These materials can eventually be functionalized as an amphiphile or attached to an ionene polymer backbone as side groups.

DETERMINATION OF CRYSTALLITE SIZE AND LATTICE STRAIN IN HEXAPHENYL THIN FILMS. Hans-Jürgen Brandt a, Roland Resela, Jozef Keckesb, Brigitte Koppelhuber-Bitschnauc, Norbert Kocha and Günther Leisinga, aInstitute of Solid State Physics, Technical University Graz, Graz, AUSTRIA; bDepartment of Theoretical Chemistry, Slovak Academy of Sciences, Bratislava, SLOVAKIA; cInstitute of Physical and Theoretical Chemistry, Technical University Graz, Graz, AUSTRIA.

Hexaphenyl (6P) thin films are used as an organic material for efficient blue light emitting diodes. Well known structural properties are of special interest for achieving high efficiency electroluminescence. 6P thin films are prepared by thermal vacuum deposition on glass substrates, one substrate held at room-temperature (RT) and the other one heated to 170°C (HT). The thin films were studied by x-ray diffraction line profile analysis and by a atomic force microscope. We investigated the crystallite size and lattice strain of the RT-sample, using the HT-sample and highly pure silicon as standard. We determine the crystallite size and lattice strain on 3 different types of preferred grown crystallites. Multiple line analysis was performed on (00l)-reflections (l = 1 .. 13) and by single line analysis of the (11-1), (11-2) and the (20-3) reflections. We observe, that the size of the 6P crystallites in (00l)-direction are in the same order of magnitude as the film thickness. The line broadening is determined by the small size of the crystallites.

THE VALENCE ELECTRONIC STRUCTURE OF N-DOPED P-SEXIPHENYL. Udo Theissl , Emil J.W. List, Norbert Koch, Guenther Leising, Technical Univ Graz, Inst of Solid State Physics, Graz, AUSTRIA; Antje Vollmer, Free Univ Berlin, Inst of Physical Chemistry, Berlin, GERMANY; Sigurd Schrader, Univ Potsdam, Inst of Solid State Physics, Potsdam, GERMANY; Jean-Jacques Pireaux, Univ Namur, LISE, Namur, BELGIUM.

Thin films of p-sexiphenyl (6P) were doped with increasing amounts of potassium in situ, and the change in the valence electronic structure of 6P upon the alkali metal deposition was followed with ultraviolet photoelectron spectroscopy. We observe the evolution of new intra-gap emissions, which are attributed to the formation of bipolarons, even for very low doping concentrations. The low binding energy intra-gap emission exhibits a pronounced asymmetric lineshape, in contrast to the findings when cesium is used as dopant. In order to investigate whether this lineshape is due to different emissive electronic species in the bulk and on the surface of the 6P film, the take-off angle for the photoelectrons was varied. As no change in the lineshape is found when going from normal to near-grazing emission we can exclude that charged 6P molecules in the bulk and on the surface yield different valence electronic spectra. Therefore, the characteristic lineshape of the low binding energy emission is proposed to be related to the interaction of the doped organic molecule with the different counterions.

EL BEHAVIORS OF STYLYL COMPOUNDS WITH BENZOXAZOLE AND BENZOTHIAZOLE FOR ORGANIC LIGHT-EMITTING-DIODE. Koichi Yamashita , Tatsuo Mori, Teruyoshi Mizutani, Nagoya Univ, Dept. of Electrical Engineering, Nagoya, JAPAN.

The use of distylylbenzene derivatives in blue organic light-emitting-diodes (OLEDs) was first discovered in Idemitsu Kousan by Hosokawa et al. In our previous work, we have studied on the stylyl derivatives having triphenylamine and benzoxazole at side group as emitting layer. These devices emitted a blue-green light. However, these devices did not have an excellent EL efficiency (0.1 lm/W). For the current density versus applied voltage characteristics, the current densities of devices using these stylyl compounds as an emitting material were higher than that of device using Alq3 as an emitting materials at same applied voltage. From these results, we think that these stylyl compounds having a triphenylamine group can flow out holes from emitting layer to cathode metal. In addition, the poly-crystallized rates of these materials were faster than that of TPD at room temperature in air. From these reasons, we designed new stylyl compounds. In this presentation, we study on two series of new stylyl compounds as emitting and hole transport materials for the characteristics of OLEDs. We synthesized two series of new stylyl compounds. One have distylyl structure eliminated triphenylamine group which have benzoxazole or benzothiazole groups at both sides. Another have the structure of ditriphenylamine derivative (TPD) having benzoxazole or benzothiazole groups as side group. We studied emitting and hole transport characteristics using these new stylyl compounds for OLED.

ENHANCEMENT OF THE MORPHOLOGICAL STABILITY OF ALUMINUM(III) 8-HYDROXYQUINOLINE FILMS BY INCREASING ENTROPIC DISORDER. K.A. Higginson , B. Yang and F. Papadimitrakopoulos, Department of Chemistry, Polymer Science Program, Nanomaterials Optoelectronics Laboratory, Institute of Materials Science, University of Connecticut, Storrs, CT.

Aluminum(III) 8-hydroxyquinoline (Alq3), a premier material for the development of organic light-emitting diode (OLED) technology, has been shown to be a semicrystalline material which undergoes structural relaxations at temperatures well below its glass transition. This crystallization and/or aging affected the fluorescence and conductivity of Alq3 films. In an effort to develop OLED materials with greater stability, materials which form thermally stable amorphous glasses have been pursued. Aluminum(III) 5-methyl-8-hydroxyquinoline (AlMeq3) was used to retard crystallization in emitting films, both as a pure material and in coevaporated blends with Alq3. Crystallization and aging kinetics of Alq3, AlMeq3, and their blends were studied with x-ray diffraction and luminescence techniques. The blends of Alq3 and AlMeq3 were found to be resistant to crystallization in OLED configurations.

CURRENT-VOLTAGE CHARACTERISTICS OF SINGLE LAYER POLYMER LIGHT EMITTING DIODES UNDER HIGH ELECTRICAL EXCITATION. B. Ruhstaller , S.A. Carter, Univ. of California, Dept. of Physics, Santa Cruz, CA; P.J. Brock, IBM Almaden Research Center, San Jose, CA; U. Scherf, Max Planck Inst. fur Polymerforschung, Mainz, GERMANY.

We study the current-voltage characteristics of both double and single carrier polymer light emitting devices in the high intensity electrical excitation regime. Single layer devices are being investigated with either the orange emitting MEH-PPV or the blue emitting ladder polymer Me-LPPP as the active polymer. Performing very low duty ($\approx$0.001$\%$) pulsed electroluminescence measurements up to 80 volts we observe a crossover from a space charge limited to an ohmic regime with a weakly field dependent mobility and a saturating external quantum efficiency of 1%. We report current densities of 100 A/cm2 and brightnesses of 5$\times$105 cd/m2. The results are compared to DC measurements and the spectral features are monitored below the onset of degradation.

CONJUGATED POLY-P-PHENYLENE (PPP) FROM POLY(1,3-CYCLOHEXADIENE) (PCHD) HOMO- AND BLOCK COPOLYMERS: CONTROLLED PROCESSABILITY AND PROPERTIES. Kunlun Hong, Jimmy Mays, Yuming Wang, Rigoberto Advincula , University of Alabama at Birmingham, Department of Chemistry, Birmingham, AL.

Poly(p-phenylene) (PPP) and its derivatives are an interesting class of polymers, which exhibit conductivity upon doping. It also exhibits photo-or electro-luminescence upon excitation with light or application of a bias voltage in a diode configuration. Pure PPP is an insoluble and infusible material, stable at high temperatures. Its thermal stability is an attractive property, considering the requirements for future opto-electronic applications. In this report, we indicate our initial results on the synthesis of PCHD by living anionic polymerization, block copolymer configuration, and subsequent conversion to PPP derivatives. We have used a variety of initiators, solvent, and temperature conditions to determine the right parameters for obtaining narrow MWD polymers. We then proceeded with conversion to PPP derivatives using a dehydrogenation reaction with chloranil. Our results indicate a systematic conversion to a conjugated polymer with increased solubility and photoluminescence properties. The microstructure, MW and block copolymer composition affecting the processability and conversion properties dramatically.

HOLOGRAPHIC NANOPATTERNING OF POLY(P-PHENYLENE VINYLENE) AND THE INFLUENCE ON PL EFFICIENCY AND RADIATIVE DYNAMICS. G. Gigli, M. Lomascolo, R. Rinaldi, C. Turco, P. Visconti and R. Cingolani, Unita INFM, Dipartimento Scienza dei Materiali, Universite di Lecce Lecce, ITALY; F. Cacialli , Cavendish Laboratory, University of Cambridge, Cambridge, UNITED KINGDOM.

We report a flexible method for the patterning of organic semiconductors in the submicrometer range, which we have successfully applied to thin films of poly(p-phenylene vinylene) (PPV) prepared on a variety of substrates, such as quartz, indium-tin oxide coated glass, or inorganic dielectric mirrors. The method is based on the holographic lithography of a standard photo-resist layer, followed by Ar-ion etching of the PPV. The holography was carried out with the help of an Ar-ion laser beam (363.8 nm) which we used in combination with a corner cube interferometer of our own design and construction, in order to fabricate 2-dimensional periodic structures. The use of multiple exposures of the photo-resist allows effective control of the geometry of the repeat element which we exemplify for stripes and pillars of virtually arbitrary shape (squares, circles, diamonds, ellipses), with feature size ranging between 250 nm and 1 µm. We characterised the morphology of the structures by contact mode atomic force microscopy, and the impact of the technology on the material properties by monitoring the photoluminescence quantum yield at different stages of the process. We also report and discuss the temporal decay of the radiative photoexcited species with a view to the damage introduced by the patterning procedure.

Thursday Morning, April 8, 1999
Metropolitan I (A)
8:30 AM *F8.1/B15.1
APPROACHES TO COLOR GENERATION IN ORGANIC LIGHT EMITTING DEVICES. S.R. Forrest , M.A. Baldo, V. Bulovic, R. Desphande, D. O'Brien, Princeton University, Department of Electrical Engineering, Princeton, NJ; A. Shoustikov and M.E. Thompson, University of Southern California, Department of Chemistry, Los Angeles, CA.

We discuss the use of both phosphors and polarizable fluorescent dyes as means for generating highly efficient, saturated color emission from organic light emitting devices (OLEDs). We have recently discovered that electrophosphorescence coupled with Dexter energy transfer, can lead to red light emission in vacuum deposited OLEDs with external efficiencies approaching 6%. Using electrophosphorescence, we have accurately measured the spin statistics of exciton formation in such hosts as Alq to result in a singlet fraction of only 16+/-2%. This surprising result is of both fundamental and technological importance. While phosphorescence can be extremely efficient at low intensities, due to triplet-triplet annihilation and color center saturation, the efficiency tends to fall off at higher luminance levels. In other work, we have demonstrated that some fluorescent dyes are highly polarizable, leading to large shifting of the emission color as a function of dopant density. This effect, which we term polarization color shifting, has numerous uses in tuning the emission of OLEDs. In this talk, we will discuss energy transfer, electrophosphorescence, polarization color shifting and related effects observed in OLEDs based on vacuum deposited molecular organic materials.

9:00 AM F8.2/B15.2
HIGHLY TRANSPARENT POLYMER LIGHT-EMITTING DEVICE. Shun-Chi Chang, Yang Yang , Dept. of Materials Science and Engineering, UCLA, Los Angeles, CA.

Transparent light-emitting devices are desired for many unique applications, such as head-on displays. Usually, transparent organic LEDs requires the sputter deposition of the transparent ITO electrode on top of the EL organic materails, and this process is rather time consuming. We have developed a novel technique for the fabrication of highly transparent polymer LEDs without the sputter deposition of the top transparent ITO electrode. The transparency of the device is higher than 90% in the visible spectrum, when a blue polymer poly [9,9-bis(3,6-dioxaheptyl)-fluorene-2,7-diyl] (BDOH-PF) is used as the active material. When unbiased, the transparent LED is as transparent as glass. On biasing, the brightness of the LED can reach 100cd/m2 under10 volts. This device represents a new approach for the fabrication of a large area transparent display panel.

9:15 AM F8.3/B15.3

The fabrication of efficient and bright organic light-emitting devices (OLEDs) based on more environmentally stable cathodes, such as aluminum (Al), has already been made possible by incorporating an inorganic buffer layer such as lithium fluoride (LiF), cesium fluoride (CsF) etc., between the cathode and the organic layer. A systematic study of the effects of the buffer layer thickness on device performance shows that OLEDs with sub-monolayer thickness of LiF, for example, have the same external quantum efficiency and light output as devices with LiF thickness of 0.8 nm. While the mechanism behind the enhanced performance in not yet completely understood, the above experimental results eliminate few of several arguments currently used to explain the enhancement in OLEDs performance, mainly, tunneling-through-an-insulator and elimination of gap-states. Short time operational stability data of devices with LiF thickness ranging from 0 nm to 1.5 nm will be presented.

9:30 AM F8.4/B15.4
INVESTIGATION OF THE ELECTRODE/ORGANIC INTERFACE IN MOLECULAR ORGANIC LIGHT EMITTING DIODES USING COMBINED XPS AND UPS MEASUREMENTS. R. Schlaf , 1L.A. Crisafulli, 2P.G. Schroeder, 1M.W. Nelson, 1H. Murata, 2C.D. Merritt, 2B.A. Parkinson, 1Z.H. Kafafi, 21Colorado State University, Dept of Chemistry, Fort Collins, CO; 2US Naval Research Laboratory, Optical Sciences Division, Washington, DC.

The electronic properties of the electrode/organic interfaces in molecular organic light emitting diodes (MOLEDs) have considerable influence on the device performance. Device performance strongly depends on the charge injection barrier at both the anode/hole transport layer and the cathode/electron transport layer interfaces. We have determined the frontier orbital alignment between a number of electron transporters (ex. tris-(8-hydroxyquinolinato) gallium or indium (Gaq3, Inq3)) and commonly used cathode materials (ex. Ag, Mg, Al), and hole transporters (ex. 4,4í -bis(1-naphtylphenylamino) biphenyl (NPB)) and commonly used anode materials (ex. indium tin oxide (ITO)). The experiments were carried out in ultra high vacuum by growing well-defined organic thin films in multiple steps on metal or ITO coated glass substrates. After each growth step, the surface was characterized by combined XPS and UPS measurements. This method uses UPS to measure the highest occupied molecular orbital (HOMO) position and the work function while XPS is employed to detect the band bending developing across the interface during film growth. This procedure allows the distinction between interface dipole and band bending related spectral phenomena with high accuracy. Our results demonstrate that the metal/organic interfaces show strong interface dipoles and band bending.

9:45 AM F8.5/B15.5
POLYMER PHOTONIC DEVICES WITH FROZEN P-I-N JUNCTIONS. Jun Gao , Alan J. Heeger, Institute for Polymers and Organic Solids, University of California, Santa Barbara, CA; Gang Yu, Uniax Corp., Santa Barbara, CA.

Polymer light-emitting electrochemical cells (LECs) and polymer photovoltaic cells have been made from polymer blends.The operation of these solid-state devices involves both electronic and electrochemical processes, as a result of the introduction of polymer electrolyte in the polymer blend film. When a large enough bias is applied across the device, a p-i-n junction is induced due to electochemical doping on both sides of the film. The p-i-n junction which is created in situ is dynamic. When the external bias is removed, the junction will disappear by relaxation. We report the stabilization of the p-i-n junction in these devices by lowering the temperature. We found that at temperature below 200K, the ions were essentially immobile a nd the p-i-n junction was ``frozen''. Detailed study shows that polymer ``frozen-junction'' devices exhibit significant advantages over ``dynamic-junction'' devices.

10:30 AM *F8.6/B15.6
DOPED POLYMER LIGHT-EMITTING DEVICES. Alan G. MacDiarmid and Feng Huang, University of Pennsylvania, Dept of Chemistry, Philadelphia, PA.

Even unsuspected trace amounts of residual doping either in the light-emitting polymer layer (MEH-PPV) or in a polymer layer (polyaniline) adjacent to an ITO electrode in a light-emitting device, can greatly reduce electron and/or hole injection energies and mask the conventional work functions of the electrode materials. This effect can improve certain important parameters of a device including light emission in a reverse bias mode in addition to the conventional forward bias mode. Poly(ethylene oxide) can enhance this effect. The effects are consistent with the presence of (polaron/bipolaron)+ A- p-doped ionic species (A-: dopant anion) initially present in the polymer, the number of which does not change during operation. Potential-induced diffusion of A- ions controls the properties of the systems. Application of a pulsating DC or AC potential promotes operating lifetime of the devices. General conclusion : in the past, electronic polymers in the highly conducting (fully doped) metallic regime have attracted the most attention. In the future, it seems likely that the effect of unsuspected traces of impurities on certain properties of organic semiconductors will attract considerably more attention as the semiconducting regime becomes technologically more important.

11:00 AM *F8.7/B15.7
CONJUGATED POLYMERS AS MULTI-COLOR ELECTROCHROMIC AND ELECTROLUMINESCENT MATERIALS. John R. Reynolds , David J. Irvin, Michael B. Ramey, Christopher A. Thomas, Dean M. Welsh, Department of Chemistry, Center for Macromolecular Science and Engineering, University of Florida, Gainesville, FL; Wing-Keung Woo, Michael F. Rubner, Department of Materials Science and Engineering, MIT, Cambridge, MA.

A series of variable gap conjugated polymers have been synthesized as multi-color electrochromic and electroluminescent materials. Low bandgap (ca. 1.0 to 1.5 eV), cathodically coloring and high visible light contrast, electrochromic polymers are designed utilizing a broad selection of electron rich (e.g. 3,4-alkylenedioxythiophene) units, along with electron poor (e.g. pyridinyl and cyanovinylene) units. Electrochromic polymers with three distinct color states are prepared using polymers containing chromophores of discrete lengths. Using carbazole and biarylamine linked chromophores, yellow neutral, green cation-radical, and blue dication states are attained. Electropolymerization methodologies are shown to produce electrochromic materials with the most rapid and long-lived switching characteristics as compared to solution processed films. Relatively high bandgap (2.4 to 3.0 eV), luminescent, conjugated polyelectrolytes based on poly(p-phenylene), poly(2,5-thienylene-alt-p-phenylenes), and poly(p-phenylene ethynylenes) have been utilized in layer-by-layer electrostatic depositions to yield luminescent films with emission colors ranging from yellow to green to blue.

11:30 AM F8.8/B15.8
EFFECTS OF IONIZATION POTENTIAL OF THE HOLE TRANSPORT LAYER ON THE ELECTROLUMINESCENT CHARACTERISTICS OF ORGANIC LIGHT-EMITTING DEVICES. B. Kippelen , G.E. Jabbour, S.E. Shaheen, M.M. Morrell, J.D. Anderson, P. Lee, Univ. of Arizona, Tucson, AZ; E. Bellmann, S. Thayumanavan, S. Barlow, R.H. Grubbs, CalTech, Pasadena, CA; S.R. Marder, N.R. Armstrong, N. Peyghambarian, Univ. of Arizona, Tucson, AZ.

We report on organic electroluminescent devices with Al cathode with luminous efficiency of 20 lm/W and external quantum efficiency of 4.6 $\%$. When pulsed in air at room temperature and without any encapsulation, remarkably high peak brightness of 4.4 x 106 cd/m2 and high efficiency of 4.4 cd/A are obtained. In contrast to current belief, we demonstrate that device quantum efficiency can be increased by tuning the ionization potential of the hole transport moieties to reduce energy barrier at the hole transport/light-emitting interface. We also show that high efficiency and long stability can be combined by cascading hole transport layers with increasing ionization potential.

11:45 AM F8.9/B15.9
TEMPERATURE AND FIELD DEPENDENT MOBILITIES IN POLYMER LED'S. L.D. Bozano , S.A. Carter, Univ. of California, Dept. of Physics, Santa Cruz, CA; J.C. Scott, G.G. Malliaras, P.J. Brock, IBM Almaden Research Center, San Jose, CA.

Transport properties of electron and hole-dominated MEH-PPV devices are reported. The IV characteristics were measured in the range of temperature from 200 K to 300 K. Different anode/cathode combinations were used to obtain single and double polarity devices. The temperature dependent mobilities are derived from the space charge limited behavior at high electric fields. At 300 K the zero field mobility $\mu_{o}$ and the electric coefficient $\gamma$ for electrons and holes are compared. For holes we found $\mu_{o}$ = 1. x 10-7 cm2/Vs and $\gamma$ = 4.3 x 10-4 (m/V)1/2. For the electrons, we found a $\mu_{o}$ an order of magnitude lower than holes but a larger $\gamma$ of 7.0 x 10-4 (m/V)1/2. Due to the stronger electric field dependence of the electron mobility, electron and hole mobilities are comparable at working voltages in the trap-free limit.

Thursday Afternoon, April 8, 1999
Metropolitan I (A)
1:30 PM *F9.1/B16.1
MOLECULAR SELF-ASSEMBLY APPROACHES TO ORGANIC PHOTONIC ARRAYS. T.J. Marks , W. Li, Q. Wang, J. Cui, J. Malinsky, Chem Dept and the Materials Research Center, Northwestern U, Evanston IL; G. Jabbour, J. Anderson, B. Kippelen, N. Armstrong, N. Peyghambarian, Optical Sciences Center, U. of Arizona, Tucson, AZ.

Molecular self-assembly as embodied in siloxane-based self-limiting chemisorptive processes offers the prospect of constructing optically-functional organic superlattices composed of tailored chromophoric building blocks deposited with conformal, self-sealing/healing coverage and Angstrom-level precision. Reported here is a new approach to organic LED assemblies in which hole transport, emissive, and electron transport layers are assebled into robust, covalently interlocked thin films using siloxane condensation/ coupling processes. The resulting structures have been characterized by a battery of microstructural, electrochemical, spectroscopic, and photonic techniques. Properties of the resulting assemblies include high structural regularity, absence of pinholes, the possibility of ultra-thin layers, hight quantum efficiencies, and low driving voltages.

2:00 PM F9.2/B16.2
ORGANIC QUANTUM-CONFINED STRUCTURES VIA MOLECULAR LAYER EPITAXY. Vladimir Burtman and Shlomo Yitzchaik, The Hebrew University of Jerusalem, Department of Analytical and Inorganic Chemistry, Jerusalem, ISRAEL.

A new molecular assembly approach - molecular layer epitaxy (MLE) is introduced for the vapor-phase assembly of organic multilayers integrated in molecular electronic devices. The MLE approach uses carrier gas-assisted epitaxial deposition, covalent bonding and horizontal p-stacking, in a pulsed mode for layer-by-layer growth. The substrates in this process are firstly self-assembled with a template layer, n-propylamine, susceptible for chemisorption. The subsequent steps rely on self-limiting reactions that forms monolayers from the vapor phase and self-cleaning processes where the excess precursor is sublimed away. Repeating such a reactions cycles with 1,4,5,8-naphtalenetetracarboxylic-dianhydridde (NTCDA) and 1,6-Diamino-n-hexane (DAH) or/and 4,4- Diamino-diphenylmethan (DAPM) or/and 1,4-Phenylenediamine (PDA) building blocks leads to an imide linked layer-by-layer growth. Resulting organic superlattices were studied by a spectroscopic ellipsometry, absorption and emission spectroscopy and current voltage measurements. MLE-derived structures are highly regular, exhibiting exciton confinement effects and are thermo-chemically robust. An ultra-thin (ca. 4 nm) organic light-emitting devices (OLED) with low turn on voltage (0.5 V) were obtained via MLE where the emission is also size and voltage dependent. Structural manipulations in the frame of MLE method on the molecular level are highly flexible and therefore are very suitable for obtaining tunable quantum-confined layered structures.

2:15 PM F9.3/B16.3
CRYSTALLINE SELF-ASSEMBLIES FROM ZINC-BISQUINOLINE; STRUCTURE, MORPHOLOGY TOWARDS EPITAXIALLY GROWN OLEDS. D. Laurence Thomsen III and Fotios Papadimitrakopoulos , Department of Chemistry, Polymer Science Program, Nanomaterials Optoelectronics Laboratory, Institute of Materials Science, University of Connecticut, Storrs, CT.

The reactive self-assembly of 8,8'-dihydroxy-5,5'-biquinoline (bisquinoline) with diethyl zinc has shown to form a linear coordination polymer. The packing and film uniformity of these films has been instrumental for the fabrication of pinhole free films capable of electroluminescence. Spectroscopic ellipsometry indicates that these assemblies possess the highest refractive index among prior reported multilayer assemblies (n = 1.69 at 633 nm). The suspected long-range crystalline order is currently elucidated as a result of a detailed morphological and spectroscopic characterization. Optical microscopy and selective area electron diffraction demonstrates the presence of randomly-oriented poly(zinc-bisquinoline) single-crystals dispersed throughout the self-assembled film. Crystal structure characterization indicates a preferred growth orientation with the molecular axis perpendicular to the growth substrate, along with a strong lateral bisquinoline self-organization. The potential of this method to produce epitaxially grown organic semiconductors for light emitting diodes is presently discussed.

2:30 PM F9.4/B16.4
SYNTHESIS AND SELF-ASSEMBLING PROPERTIES OF POLY (PHENYLENE VINYLENE) WITH DENDRITIC SIDE-CHAINS. X. Linda Chen , Zhenan Bao, Karl R. Amundson and Andrew J. Lovinger, Bell Laboratories, Lucent Technologies, Murray Hill, NJ.

Novel types of poly(phenylene vinylene) with dendritic side-chains have been synthesized to investigate their self-assembling and device properties. These conjugated polymers, which have weight-average molecular weights 20 - 60 kDalton, were synthesized by Heck reactions. The introduction of the bulky side-chains improved the main-chain PPV separation and enhanced photoluminescence was observed. In solid state, the polymers self-organized into thermotropic liquid crystals with nematic textures, suggesting potential application as polarized light-emitting media. In addition, results of poly(phenylene vinylene)s with various structure modifications, their associated self-assembling properties as well as device applications will be presented.

2:45 PM F9.5/B16.5
SYNTHESIS AND CHARACTERIZATION OF NOVEL METAL COMPLEXES FOR ELECTROLUMINESCENCE APPLICATIONS. Xu-Tang Tao ,Tatsuo Wada, Hiroyuki Sasabe, Bio-Polymer Physics Laboratory, The Institute of Physical and Chemical Research (RIKEN), JAPAN; Hironori Suzuki, Toshiyuki Watanabe, Seizo Miyata, Graduate School of Bio-Applications and Systems, Tokyo University of Agriculture and Technology (TUAT), Tokyo, JAPAN.

We report the synthesis and characterization of a series of new metal complexs for electrolminescence applications.The complexes are prepared by the reaction of 2-[3-(2H-benzotriazol-2-yl)-4- hydroxyphenyl]ethyl methacrylate with various metal salts. These complexes can be polymerized and show strong photoluminescence under a UV-lamp illumination. High optical quality films were obtained by vacuum evaporation or by spin-coating methods. The electroluminescence of single and double layer devices have been characterized. The results indicated that these metal complexes could act as both electron transport and emissive layers for EL devices.

3:30 PM *F9.6/B16.6
POLYQUINOLINES: MULTFUNCTIONAL POLYMERS FOR ELECTRO-OPTIC AND LIGHT-EMITTING APPLICATION. Alex K.Y. Jen , Hong Ma, Xiaoming Wu, Jianyao Wu, Yunqi Liu, Michelle Liu, Sen Liu, Craig Urian, Department of Chemistry, Northeastern University, Boston, MA.

Polyquinolines (PQs) represent a class of high performance polymers with outstanding thermal stability (> 450$^{\circ}$C), low dielectric constants (2.5-3.0), low moisture absorption (0.1-0.15$\%$), very high Tg (250-400$^{\circ}$C), and compatibility with plasma or reactive ion etching. Our studies of their electro-optic (E-O) properties have shown very promising results for side-chain polyquinoline systems. They possess large E-O coefficients after poling (r33 as high as 40 pm/V at 1.3 $\mu$m), and good temporal alignment stability at 100$^{\circ}$C. Recently, polyquinolines have also been explored as good electron transporting materials for light-emitting diodes (LEDs). The results from electrical and optical characterization of these polymers showed that they possessed very good LED performance (low turn-on voltage, bright light emission and high rectification ratio). In addition, they possessed high thermal stability, good electrochemical reversibility and excellent film-forming property.

4:00 PM F9.7/B16.7
1.5 $\mu$M LUMINESCENCE FROM ERQ BASED OLED'S. Richard Curry , William Gillin, Dept. of Physics, Queen Mary and Westfield College, University of London, UNITED KINGDOM.

A 1.5 $\mu$m emission source which is compatible with silicon technology would have huge potential in optoelectronic communications. To this end there has been a considerable research effort over the last 10 years to study the incorporation of erbium into silicon. Erbium is of interest because it has a sharp intra-atomic transition at 1.5 $\mu$m. We have produced the chelate erbium (III) tris(8-hydroxyquinoline) (ErQ) and demonstrated bright, sharp luminescence lines at  1.5 $\mu$m which are due to the erbium intra-atomic transitions. We present photoluminescence data as a function of excitation energy and absorption data. These show that the erbium is being excited via absorption into the molecule rather than by direct absorption into the atom. Organic light emitting diodes (OLED's) have been fabricated on indiun tin oxide (ITO) using N, N'-bis(3-methylphenyl)-N, N'-diphenyl-[1, 1'-biphenyl]-4, 4'-diamine (TPD) as a hole transporting layer and ErQ as an electron transporting/emitting layer. As AlQ based OLED's have been demonstrated to operated using silicon as one electrode we propose that ErQ may be a potential candidate for a silicon compatible 1.5 $\mu$m emission source.

4:15 PM F9.8/B16.8
NARROW SPECTRUM LIGHT-EMISSIVE DEVICES MADE FROM BLENDS WHICH EXHIBIT ENERGY TRANSFER FROM CONJUGATED POLYMERS TO RARE EARTH COMPLEXES. Michael D. McGehee , Troy Bergstedt, Andrew P. Saab, Chi Zhang, Marie O'Regan, Alan J. Heeger, Guillermo Bazan and Vojislav Srdanov, Univ of California, Santa Barbara, CA.

Conjugated polymers have proven to be very useful as the emissive component in light-emitting diodes and more recently in lasers and photonic band gap materials. One of the limitations for some of these applications is the very large width of the emission spectrum (50-100 nm). To address this problem we have blended $\beta$-diketone chelated europium complexes into blue emitting polymers such as CN-PPP and PVK. In these blends energy can be transferred in a multistep process from the polymer to the $\beta$-diketone ligands and then to the europium atoms. The fluorescence spectrum from the europium is centered at 613 nm and has a linewidth of less than 10 nm. To achieve efficient energy transfer we have synthesized a whole family of europium complexes with different $\beta$-diketone ligands to tune the energy levels involved in the energy transfer process. We will present spectroscopic data which show the mechanisms of energy transfer and that energy transfer can be efficient if the energy levels of the europium chelates are chosen properly . We will also present quantum efficiency measurements and temperature dependent fluorescence lifetime measurements which show that highly efficient fluorescence can be achieved from the europium if it is shielded in an environment which prevents quenching. Furthermore, we will show the characteristics of light-emitting diodes made from the blends and discuss research which is presently being done to make lasers and photonic band-gap devices.

4:30 PM F9.9/B16.9

We have fabricated Schottky diodes using PPV-oligomer:C60 blends and measured their dark and illuminated I-V characteristics. We found that the current of the illuminated devices under reverse bias increases strongly with the applied voltage. This result is similar to that observed with Schottky diodes fabricated using other conducting polymers [1]. Both LED and solar cell structures show this behaviour. This behaviour is strikingly different from that of a Si Schottky diode where the current is independent of the reverse bias voltage. We suggest that this behaviour arises on two accounts, (1) the excitons dissociate under the action of the applied field [2] increasing the number of charge carriers and hence the current, and (2) the mobility of the carriers increases as the applied field increases [3]. We have modified the chemical rate equations to take into account the effect of the electric field on the rate of dissociation and derived an expression for the equilibrium fraction of dissociated excitons. Using this expression for the number of carriers and the expression for mobility given in [3], we have derived an expression for the current as a function of applied field. Exciton binding energy and field enhanced mobility occur as fitting parameters in this equation. We have derived the value of the binding energy and field enhanced mobility by fitting this equation with the experimental results for several different cases. We find that even for the same organic semiconductor, the binding energy comes out to be different for samples prepared in different laboratories. [1] X. Wei et al., Phys. Rev. B 49, 17480 (1994); J. Gao et al., Synth. Metals 84, 979 (1997). [2] R. Kersting et al., Phys. Rev. Lett. 73, 1440 (1994); V. I. Arkhipov et al., Phys. Rev. B 52, 4932 (1995). [3] S. Karg et al., Synth. Metals 67, 165 (1994).

4:45 PM F9.10/B16.10
DELAYED FLUORESCENCE AND TRIPLET-TRIPLET ANNIHILATION IN PI-CONJUGATED POLYMERS. J. Partee, E.L. Frankevich, B. Uhlhorn, J. Shinar , Ames Laboratory - USDOE* and Physics Department, Iowa State University (ISU), Ames, IA; Y. Ding and T.J. Barton, Ames Laboratory - USDOE and Chemistry Department, ISU, IA.

The delayed fluorescence (DF) of poly(para-phenylene vinylene) (PPV) and poly(para-phenylene ethynylene) (PPE) derivative solids and frozen solutions at 20 K was measured by modulating the photoexcitation power at a frequency f and monitoring the response of the photoluminescence at 2f. The behavior of the response vs f provides strong evidence for triplet-triplet annihilation to singlets excitons accounting for up to $\sim$3% of the total emission in PPV films and $\sim$1.5% in PPE powder. It also yields triplet lifetimes of 70 and 110 microsec in PPV films and frozen solutions, and $\sim$200 and $\sim$500 microsec in PPE powder and frozen solutions, respectively. However, while the response of the PPV films and solutions agreed with a single monomolecular triplet lifetime, the behavior of the PPE powder with such a single lifetime was only rough, and that of the PPE solution was poor. In the films, the DF was slightly red-shifted from the prompt fluorescence, but in the frozen solutions the two coincided. This behavior is consistent with the high energy of the lowest-energy singlet excitons in the shortest conjugated segments in the films, which appears to be more than twice that of the triplet excitons. *Ames Lab is operated by ISU for USDOE under Contract W-7405-Eng-82.

Friday Morning, April 9, 1999
Metropolitan I (A)
8:30 AM *F10.1/B17.1

Recent developments from the DARPA High Definition Systems program will be reviewed. A brief review will be presented showing how new display technologies are being incorporated into military systems, as well as some of the new technologies which are being supported by the DARPA HDS program.

9:00 AM *F10.2/B17.2
MULTICOLOR POLYMER ELECTROLUMINESCENT DEVICES. Junji Kido , Takayuki Suzuki, Satoshi Shirai, Graduate School of Engineering, Yamagata University, Yonezawa, Yamagata, JAPAN.

Fabrication of micropatterned multicolor polymer electroluminescent (EL) devices by the newly developed photobleaching method is described. In this approach, we take advantage of the instability of organic fluorescent molecules upon photooxidation; that is, organic molecules can be oxidized or bleached by exposure to light in the presence of oxygen. For color tuning and patterning, a polymer emitter layer containing fluorescent dyes is exposed to ultraviolet (UV) light after formed by spin coating so that a specific dye is photooxidized and becomes non-emissive. Due to this photobleaching process, the UV exposed area have different emission colors from the non-exposed area, and, using a fine-patterned photomask, micropatterned multicolor EL devices are fabricated. This photobleaching method is the simplest method for the fabrication of multicolor polymer EL devices.

9:30 AM F10.3/B17.3
R-G-B MULTI-COLOR ORGANIC LIGHT-EMITTING PIXELS PROCESSED BY HYBRID INKJET PRINTING. Shun-Chi Chang, Yang Yang , Dept. of Materials Science and Engineering, UCLA, Los Angeles, CA; Jun Onohara and Junji Kido, Graduate School of Engineering, Yamagata University, Yamagata, JAPAN.

Organic light-emitting diodes represent a novel approach for fabricating future high quality multicolor emissive displays. Traditionally, the deposition of organic semiconductors is carried out by the thermal sublimation process in an ultra-high vacuum environment. This is a rather time consuming process and the patterning of fine multicolor pixels is also quite complicated. In this presentation, we report the successful demonstration of controllable patterning of red-green-blue (R-G-B) multi-color organic light-emitting pixels using a hybrid inkjet printing technique. For this demonstration, the blue polymer, which serves as the buffer layer, was partially soluble in the solvent using for the red and green dopants. We observed an efficient charge transfer when small amount of dopants, DCM and Almq3, were introduced into the buffer layer. This demonstration represents a new approach to solving an old problem of patterning the multi-color organic LEDs for the future display application.

9:45 AM F10.4/B17.4
SURFACE-MODIFIED TRANSPARENT ELECTRODES FOR ORGANIC LIGHT-EMITTING DIODES. Peter K.H. Ho , Univ of Cambridge, Cavendish Laboratory, Cambridge, UNITED KINGDOM; Jeremy H. Burroughes, Cambridge Display Technology Ltd, Cambridge, UNITED KINGDOM; Richard H. Friend, Univ of Cambridge, Cavendish Laboratory, Cambridge, UNITED KINGDOM and Cambridge Display Technology Ltd, Cambridge, UNITED KINGDOM.

Super-efficient poly(p-phenylenevinylene) derivative-based single-layer polymer light-emitting diodes (LEDs) with external forward quantum efficiencies near to one-quarter of the internal photoluminescence efficiency of the emitter layers have been obtained on modified indium-tin oxide (ITO) anodes. The ITO surface is modified with an ultrathin functional polymer interlayer of the order of 100-$\AA$ thick in a two-step process comprising first a surface silylation and derivatisation reaction to anchor ionic charges on the oxide surface, followed by self-assembly (alternate polyelectrolyte deposition) of the desired polymer interlayer. Switching of the polymers during assembly allows composition-graded interlayers to be fabricated with charge-transport and other properties tunable with nanometer precision in the assembly direction. These interlayers can be designed to give a virtual anode interface with the emitter polymer layer that is conducive to hole injection but which presents a blocking barrier to electrons transiting in the opposite direction. The resultant devices exhibit a sharp turn-on soon after attaining the thermodynamic flat-band condition. This is a signature of facile bipolar carrier injection at the respective interfaces, together with efficient carrier confinement, capture and radiative recombination in the emitter polymer layer. For a polyfluorene derivative-based single-layer LED emitting in the green, where the eye phototopic response peaks, we have recorded a forward luminous efficiency of 25lm/W peaking at a drive voltage of 3.0V; and a luminance level of 1000cd/m2 at 5.6V.

10:30 AM *F10.5/B17.5
THE PHYSICS OF ORGANIC LIGHT-EMITTING DEVICES. J. Campbell Scott , IBM Research Division, Almaden Res. Ctr., San Jose, CA; George G. Malliaras, Dept. of Materials Science and Engineering, Cornell Univ., Ithaca, NY; Luisa Bozano and Sue A. Carter, Dept. of Physics, Univ. of California, Santa Cruz, CA.

Qualitatively, the basic principles behind the operation of OLEDs are well established. In order to optimize device parameters such as power efficiency and operating voltage, to explore the limits of performance and to understand changes in the electrical properties as diodes age, it is necessary to develop a quantitative understanding of each of the relevant processes: injection of electrons and holes at the cathode and anode, charge transport, recombination and exciton formation, and emission. In this talk, I will summarize our experimental, theoretical and numerical studies to address these issues.

11:00 AM *F10.6/B17.6
HIGHLY EFFICIENT POLYMER LEDS FOR FULL COLOUR DISPLAY APPLICATIONS. Jeremy Burroughes, David J. Lacey , Ian Millard, Craig Murphy, Carl Towns, Cambridge Display Technology Ltd, Cambridge, UNITED KINGDOM.

We report the results of our programme of work on a range of poly(fluorene)-based Polymer Light Emitting Diodes (PLEDs). These have been developed with a common device architecture for all emission colours, which achieves facile injection of both electrons and holes, and features polymers with common core chemistry. It is a primary concern to ensure that the emission colour of our PLEDs makes them suitable for use in full colour display applications. By variation of the chemistry of the emissive layer we have been able to tune the emission colour whilst retaining the general device architecture and with little compromise of the electrical performance. This approach has enabled rapid development of the different PLED colours and enabled demonstration of efficient, stable EL emission at various wavelengths. Synthesis of high molecular weight polymers based on the fluorene unit has been achieved using a Suzuki polymerisation route. Careful control of both the polymerisation reaction and the work-up has yielded polymers of high purity and with high photoluminescence (PL) efficiency. Variation of the chemistry of the monomers allows control of PL emission colour, variation of the HOMO and LUMO levels and independent variation of the chemical compatibility of the resultant polymer. We are now able to demonstrate significant device efficiencies for red, green and blue PLEDs using bilayer structures on Indium-Tin Oxide substrates with evaporated metal cathodes. For example, green light emitting structures with efficiencies in excess of 20 lm/W have been produced regularly. Similar green PLEDs driven with constant current show very low drive voltage drift over thousands of hours (less than 0.4 mV per Hour). The most recent efficiency and lifetime data on our Red and Blue structures will also be presented.

11:30 AM F10.7/B17.7
LONG-TERM DEGRADATION MECHANISM OF ORGANIC LIGHT EMITTING DEVICES BASED ON SMALL MOLECULES. Hany Aziz , McMaster University, Department of Materials Science and Engineering, Hamilton, Ontario, CANADA; Zoran D. Popovic, Nan-Xing Hu, Ah-Mee Hor, Xerox Research Centre of Canada, Mississauga, Ontario, CANADA; Gu Xu, McMaster University, Department of Materials Science and Engineering, Hamilton, Ontario, CANADA.

The intrinsic degradation of (8-hydroxyquinoline) aluminum-based organic light emitting devices, that leads to the long-term drop in the electroluminescence efficiency of the devices operated under constant current conditions, has been studied. The role of stabilizing agents, such as introducing a copper pthalocyannine buffer layer at the hole-injecting contact and using mixed layers of hole and emitting electron transport molecules, has been investigated. Based on the results, the major cause of the long-term degradation in the devices has been elucidated.

11:45 AM F10.8/B17.8
THREE-COLOR DOPING OF POLYMER OLEDS BY MASKED DYE DIFFUSION. F. Pschenitzka , J. C. Sturm, Princeton University, Dept. of Electrical Engineering, Princeton, NJ.

Achieving three-color OLEDs is problematic since the local deposition and/or local patterning of three different organic materials is difficult especially for polymer based devices. In this work, we demonstrate masked dye diffusion technique to locally change the emission spectrum of an OLED, and apply this method for three-color integration. The key novel feature of the work is the use of a large-area dopant source plate, as opposed to a conventional evaporation point source. At elevated temperatures, the dye will transfer from the source and diffuse into the organic layer of the OLED which is in close proximity. The pattern of the transferred dopant may be controlled by either a conventional shadow mask or a printed diffusion barrier. We can create the source plates and transfer the dopant without vacuum processing. This method allows us to integrate RGB-OLED into a single polymer film with resolution. The device was made with a PVK and PBD as the matrix and bimane (blue), C6 (green) and Nile Red (red) as emissive dyes. The method can be easily applied to other polymers and small molecule devices.

System Administrator