Excitation energy transfer in condensed media

Abstract

We derive an expression for resonance energy transfer between a pair of chromophores embedded in a condensed medium by considering the energy splitting of the chromophores from their resonant excited states. We employ time-dependent density functional response theory in our derivation. The linear response theory treatment is rigorous within the framework of time-dependent density functional theory, while in obtaining the energy transfer coupling, the standard first-order approximation is used. The density response function for the medium, which can be replaced by the macroscopic dielectric susceptibility, enables the inclusion of the medium influence on the energy transfer coupling between the donor and acceptor. We consider the Coulomb coupling, and determine that our result is isomorphic to the Coulomb interaction between two charge densities inside a dielectric medium. The isomorphism we found not only provides a general and useful expression for applications, but additionally offers a basis for the extension of the dielectric response model to energy transfer coupling, which has been implicitly used earlier. An illustrative model shows that for two separated molecules, the medium adds a dielectric screening effect to the Coulomb coupling of their transitions. However, if the two molecules are so closely spaced that they effectively reside in a single cavity, the medium can enhance or reduce the strength of the coupling depending on the orientation and the alignment of the two chromophores.