Characterization of the short-range couplings in excitation energy transfer

The electronic coupling in excitation energy transfer (EET) is composed of a Coulomb coupling (which can be approximated as the Forster's dipole coupling), a Dexter's exchange coupling, and a term arising from the orbital overlap of the donor and acceptor fragments. We have developed a new scheme to account for the EET coupling by generalizing the fragment charge difference scheme [Voityuk, A. A.; Rosch, N. J. Chem. Phys. 2002, 117, 5607] for electron-transfer coupling. As a result, the EET coupling can be calculated in a general class of systems irrespective of the molecular symmetry. The short-range coupling, defined as the contribution from Dexter's exchange coupling and the overlap effect, was obtained as the difference of the EET coupling from this new scheme and a precise account of the Coulomb coupling. For a pair of stacked naphthalenes, the short-range coupling is very similar to the triplet-triplet energy-transfer coupling in both magnitudes and the distance dependences. We have observed cases in which the Coulomb coupling decays either faster or slower than the expected dipole-dipole R-3 distance dependence even at distances of 10-20 angstrom, due higher multipole interaction. For a well-studied series of rigidly linked naphthalene dimers, the role of through-bond interaction was studied with the new computational methods. Similar to previous reports, our results show that the through-bond component contains a Coulomb coupling, and in some cases, contributions from the short-range couplings can be seen.