8:00 PM - EN10.03.13
Photon Upconversion from Near Infrared to Blue Light with TIPS-Anthracene as an Efficient Triplet-Triplet Annihilator
Naoyuki Nishimura1,2,Victor Gray1,3,Zhilong Zhang1,Akshay Rao1
University of Cambridge1,Asahi Kasei Co2,Uppsala University3
Photon upconversion (PUC) via triplet-triplet annihilation (TTA) is an attractive means for solar energy concentration, bio-imaging, and photochemical reaction, such as photoinduced drug delivery. In particular, generation of blue light (< 500 nm) via upconversion is important, because blue light is required for most photochemical reactions1,2, as well as photocatalytic water splitting to produce hydrogen, since most materials with the efficient quantum yield for photocatalytic water splitting can harvest only photons with energy above 500 nm (e.g., GaN:ZnO, BiVO4 - SrTiO3:Rh(,La))3,4. On the other hand, utilizing near infrared (NIR) photons is crucial for applications such as bio-imaging and drug delivery, as the window for high transparency of biological tissue lies in the NIR region (700 - 900 nm).1,4-6 Thus, PUC from NIR to blue would be beneficial to many emerging applications.
As the difference in energy between blue and NIR photons is large (e.g., 470 nm: 2.64 eV and 800 nm: 1.55 eV), a reduction of energy loss in PUC process, leading to larger anti-Stokes shift (: (PUC emission energy) - (excitation energy)), is also important. However, there has been no report, to the best of our knowledge, of a material with an anti-Stokes shift greater than 1.0 eV that also has a linear excitation intensity dependency (slope 1 for the PUC process).7 One of the keys to reduce loss in energy for PUC is to minimize the driving force of TTA process (difference in energy between 2T1 and S1 of TTA material). Reports of annihilators fluorophores which can accept low-energy triplets (< 1.55 eV) and generate blue light have so far mainly focused on 9,10-Bis(phenylethynyl)anthracene (BPEA)8-10 and perylene derivatives.1,11,12 However, these materials function with low efficiency of TTA (e.g., BPEA)8-10 and/or require large driving force for TTA (e.g., perylene derivative: 2 T1- S1 ≈ 0.38 eV) 1,11,12. Therefore, there is a clear need for PUC materials with a large anti-Stokes shift as well as efficient PUC from NIR to blue.
Here, we demonstrate efficient PUC converting from NIR energy to blue photons using the commercially available material 9,10-Bis[(triisopropylsilyl)ethynyl]anthracene (TIPS-Ac) as an efficient TTA material despite a low driving energy for the TTA process (< 0.32eV). When TIPS-Ac was sensitized with Pd(II) meso-Tetraphenyl Tetrabenzoporphine (PdTPBP), a generates triplets via intersystem crossing, the PUC photoluminescence quantum yield with excitation at 635 nm was 27 ± 0.5 % with a TTA efficiency of 77 ± 3 %. Combining TIPS-Ac with Pt(II) meso-Tetraphenyl Tetrabenzoporphine (PtTPBP), where we show directly generates triplet via NIR excitation at 785nm, the PUC achieved an anti-Stokes shift of 1.03 eV with a linear excitation intensity dependency.
Our results highlight the use of direct triplet generation via NIR excitation as a useful path to achieving large anti-Stokes shift and also show that very high TTA efficiencies can be achieved even in the absence of large driving energies for the TTA process.
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