Abstract
Excitation energy transfer (EET) refers to the nonradiative transfer of an electronic excitation from a donor molecule to an acceptor molecule (or fragment). The electronic coupling is one of the important factors in describing the transition rate. The electronic coupling for EET has been dissected with a perturbation theory, which gives rise to a dipole–dipole interaction (Förster theory) and an electron exchange integral (Dexter coupling) between electronic transitions of donor and acceptor and they are valid only for donor/acceptor separated at a large distance. To calculate the electronic coupling, we have developed the fragment-spin difference (FSD) and fragment-excitation difference (FED) schemes for triplet and singlet energy transfer, respectively. As eigenstate-based methods, FSD and FED allow us to extract the coupling from the full electronic Hamiltonian. The EET coupling obtained is not subject to a long-distance condition. The FSD and FED schemes are generally applicable to both intermolecular and intramolecular EET, regardless of their symmetry. The FSD scheme can be used to study triplet excitation energy transfer (TET). Numerical benchmark for the FSD were performed in two series of π-conjugation model systems. The FSD coupling exhibits weak dependence on molecular size and shows an exponential dependence on the intermolecular distance. The FSD results were also compared with those obtained from the conventional exchange integral, and we found that FSD yields more consistent results. The FSD was used to study TET in the light-harvesting complexes of purple bacteria and dinoflagellates. For all light-harvesting complexes studied, there exist nanosecond TET from the chlorophylls to the carotenoids. The 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 The 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 exhibits the robustness of carotenoids in photosynthesis systems. The FED scheme can be used to study singlet excitation energy transfer (SET). For a pair of stacked naphthalenes, the short-range coupling was found with similar value and distance dependence as TET coupling. The FED was employed to calculated SET coupling in special pairs of the reaction centers of purple bacteria. We found that the short-range coupling represents the dominant contribution to the total SET coupling. We have developed the FED and FSD schemes to calculate excitation energy transfer coupling. We found them useful for a large number of systems. The coupling values obtained are mostly consistent with those calculated with other methods, with FED and FSD results being more reliable, as judged by their distance dependent behavior. FED and FSD are useful in offering physical insights in applications. With their general applicability, we expect them to be useful in the future.