Radiative decay and energy transfer in molecular aggregates: The role of intermolecular dephasing

Abstract

A microscopic theory for radiative lifetimes and energy transfer rates in molecular aggregates is developed. Using an effective Hamiltonian, we show that the radiative decay rate of isolated small aggregates with N molecules is Nγ, where γ is the radiative rate of a single molecule. This linear dependence on N saturates when the aggregate size becomes comparable with the wavelength of light associated with the optical transition, and for large N the rate attains a limiting value independent of N. Intermolecular dephasing processes are incorporated by introducing a thermal bath and solving for the time evolution of the density matrix with use of an effective Liouville operator. Intermolecular dephasing destroys the coherent interaction of the aggregate with the radiation field and the rate changes from Nγ to γ as the dephasing rate increases. A coherence length which depends on the strength of intermolecular interactions and on the dephasing rate is defined and calculated.