XVIth International Workshop on
Quantum Systems in
Chemistry and Physics
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Theories and applications for electronic coupling in triplet energy transfer
Chao-Ping Hsu
Institute of Chemistry, Academia Sinica
The transport of charges and excitation energy are two processes of fundamental importance in diverse areas of research. Characterizations of electron transfer (ET) and excitation energy transfer (EET) rates are essential for a full understanding of many biological systems and opto-electronic devices. The electronic coupling factor is an off-diagonal Hamiltonian matrix element between the initial and final diabatic states in the transport processes. ET coupling is essentially the interaction of the two molecular orbitals (MOs) where the electron occupancy is changed. Singlet excitation energy transfer (SET) contains a Förster dipole–dipole coupling term as its most important constituent. Triplet excitation energy transfer (TET) involves an exchange of two electrons of different spin and energy; thus, it is like an overlap interaction of two pairs of MOs.
In the past, we have developed or improved some of the strategies for calculating ET, SET, and TET couplings. In the presentation, I plan to report our recent progresses with a focus on the TET and its application.
With a newly developed computational scheme, the Fragment Spin Difference (FSD), we can calculate the TET coupling over a general class of systems. It is therefore possible to calculate TET coupling values for systems that were previously hard to obtain for a number of reasons. Our recent results on photosynthetic light-harvesting complexes will also be discussed. In particular, TET the bacterial light-harvesting complex II (LH2) of Rhodospirillum molischianum and Rhodopseudomonas acidophila, and the peridinin-chlorophyll a protein (PCP) from Amphidinium carterae. The TET rates were estimated based on the couplings obtained. For all light-harvesting complexes studied, there exist nanosecond TET from the chlorophylls to the carotenoids. Our result supports a direct triplet quenching mechanism for the photoprotection function of carotenoids. The TET rates are similar for a broad range of carotenoid triplet state energy, which implies a general and robust TET quenching role for carotenoids in photosynthesis. This result is also consistent with the weak dependence of TET kinetics on the type or the number of π-conjugation lengths in the carotenoids and their analogs reported in the literature. Our results provide theoretical limits to the possible photophysics in the light-harvesting complexes.



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