On the rate of radiationless intermolecular energy transfer

The effects of higher multipole interactions (up to electric quadrupole - quadrupole) and small exchange contributions on the rate of intermolecular energy transfer are examined. Second-order Coulombic interactions are described within a molecular quantum electrodynamical framework. A correction due to a small first-order exchange mechanism is then proposed. It is concluded that use of the multipole expansion in the interaction Hamiltonian is not always a good approximation at interchromophore separations of less than about 10 Å. This is attributed to a combination of large molecular dimensions compared to intermolecular separation, and wave function overlap effects. For larger separations, the interaction is described well by the usual dipolar coupling formalism. The inclusion of small exchange effects in a simplistic model at small to intermediate separations demonstrates the likelihood of a substantially greater rate of energy transfer than that predicted by either a Förster-type (dipole-dipole) or a purely exchange-type mechanism. The relevance to the photophysics of multichromophoric systems, such as aromatic polymers, is discussed.