Intraband relaxation and dephasing of Frenkel exciton states in one-dimensional J-aggregates

A theoretical model is proposed to analyse the relaxation and the dephasing mechanisms of Frenkel excitons in one-dimensional J-aggregates. At low temperature, the exciton dynamics originates from the coupling of excitons to phonons which creates temperature dependent local fluctuations in the site energies of the molecules. A microscopic model of exciton–phonon coupling is used assuming a Bose–Einstein distribution for the phonon occupation density. Intraband scattering rates among the exciton states are calculated and used in a master equation to obtain the time evolution of the exciton populations after initial excitation of the optically allowed state. At higher temperatures, the dephasing of the exciton states originates from the coupling of excitons to the vibrational modes. Both processes (intraband scattering and dephasing) contribute to the lengthening of the radiative exciton lifetime with increasing temperature. Using a generalized participation ratio taking into account the homogeneous width allows to define, at each temperature, the number of coherently coupled molecules and to interpret the anomalous observed temperature dependence of the radiative lifetime of excitons in PIC aggregates at low, intermediate and high temperature.