Polarons, localization, and excitonic coherence in superradiance of biological antenna complexes

A real-space formulation of time-resolved fluorescence of molecular aggregates is developed using the one-exciton density matrix rho(t) of the optically driven system. A direct relationship is established between the superradiance enhancement factor L-s, and the exciton coherence size L-rho associated with the off-diagonal density matrix elements in the molecular representation. Various factors which affect the latter, including finite temperature, energetic disorder, coupling with phonons, and polaron formation are explored. The theory is applied for the interpretation of recent measurements in the B850 system of the LH2 photosynthetic complexes. (C) 1997 American Institute of Physics.