Energy relaxation of exciton states in molecular aggregates

Energy relaxation of exciton states was recently investigated in columnar molecular aggregates of ionic compounds. Quantum chemistry calculations revealed very strong interactions (1000 cm(-1) or more) among the chromophores. Such strong interactions are expected to induce large cooperation effects, even at room temperature. A model to describe the intraband scattering among the eigenmodes of the aggregate, in the strong coupling limit, is presented. Energy relaxation is analysed as an incoherent energy transfer process induced by the stochastic coupling of the exciton states to a thermal bath characterized by the amplitude and the correlation time of the site-energy fluctuations. Transfer rates are then calculated and used in a Pauli master equation to describe the time evolution of the eigenstates populations and the optical line shapes during the scattering process. The theoretical results are discussed for various configurations of linear aggregates as a function of the exciton-bath correlation time and are compared to time-resolved fluorescence experimental data. (C) 1998 Elsevier Science B.V. All rights reserved.