Cooperative radiative dynamics in molecular aggregates

We theoretically investigate the radiative dynamics of molecular aggregates with physical dimensions much smaller than an optical wavelength. The fluorescence decay rate of a one-dimensional aggregate consisting of N electronically coupled two-level molecules interacting with acoustic and optical lattice phonons is calculated. The linear dependence (superradiance) of the radiative decay rate on the aggregate size N is shown to be quenched by exciton–phonon coupling. An increase of aggregate size N eventually leads to a convergent, size independent decay rate, which is N* times faster than the monomer decay rate. The coherence size N* is generally a function of the exciton–phonon coupling strength, the phonon bandwidth, and the aggregate temperature. For low frequency phonons, a scaling law is obtained and an empirical relation for the temperature dependence N*∼T−1/3 is derived.