Investigation of Photochemical Upconversion Based on Triplet-Triplet Annihilation.

摘要

The potential benefits for societal adoption of solar energy are high in comparison to the adverse effects of climate change from using fossil fuels. However, the Shockley-Queisser limit sets a solar power conversion efficiency limit on all single bandgap silicon-based photovoltaic devices. Triplet-triplet annihilation upconversion (TTA-UC), which can convert the sub-bandgap photons to above-bandgap photons, has been proposed as a way to increase the solar cell efficiency. TTA-UC can be achieved with low power-density (<100 mW/cm 2), and non-coherent light as the excitation sources such as sunlight. Understanding the mechanism and increasing the efficiency of TTA-UC have an important technological implication in energy conversion, solar energy and biotechnology.;The TTA-UC process is a sequence of events: a sensitizer becomes excited and reaches a triplet state via intersystem crossing (ISC); triplet-triplet energy transfer (TTET) between sensitizer and emitter; triplet-triplet annihilation (TTA) between two emitters leading to a singlet excited state of one of them; and the final upconverted emission from the singlet excited acceptor. The studies on TTA-UC have been made a significant progressive, with varying degrees of successes. Still, there is ongoing need to better understanding the TTA-UC processes under different conditions and explore ways to enhance the efficiency of TTA-UC. This dissertation is to investigate the TTA-UC processes in different systems such as single-sensitizer/multi-acceptor systems, polymeric emitter systems, sensitizer attached on gold nanoparticles systems and TTA-UC included paramagnetic complex systems under external magnetic field. Based on these investigations, we try to figure out different ways to increase the efficiency of TTA-UC. For this purpose, several thorough investigations were carried out based on TTA-UC topics. Firstly, we constructed single-sensitizer/multi-acceptor systems, consisting of Platinum(II) octaethylporphyrin (PtOEP) as sensitizer and 9, 10-diphenylanthracene (DPA), 1,3-diphenylisobenzofuran (DPBF) and anthracene (AN) as acceptors, which display very high upconversion quantum yields. A hetero-TTA process between triplet acceptors of different types is believed to account for the synergistic effect leading to the high upconversion efficiency. Secondly, an investigation of TTA-UC based on polymeric emitters with tunable inter-chromophore distances was explored. Poly[(9-anthrylmethyl methacrylate)- co-(methyl methacrylate)] (Poly(AnMMA-co-MMA)) with different percentages of AnMMA were synthesized using reversible addition-fragmentation chain transfer (RAFT) polymerization, and used as emitters in association with platinum octaethylporphyrin as sensitizer to form TTA-UC systems. It is observed that TTA-UC intensity first increases with increasing AnMMA percentage in the polymers then decreases, and ultimately disappears, upon further increasing the AnMMA percentage. Thirdly, surface plasmon induced enhancement of homogeneous and heterogeneous TTA by gold nanoparticles (AuNPs) was observed. Excitation rate and intersystem crossing efficiency of the sensitizer, and efficiency of energy transfer between sensitizer and acceptor are believed to be enhanced by the surface plasmon of AuNPs, leading to the enhancement of overall TTA efficiency.;To sum up, these results shed light on the key factors affecting TTA-UC processes in different systems, provide ways to enhance TTA-UC efficiency, and have implication to the applications of TTA-UC in display, bioassay, bioimaging and solar energy, with promise for more.