Dynamics of Frenkel excitons in disordered molecular aggregates

This article reports on the optical dynamics in aggregates of pseudoisocyanine-bromide and iodide. For PIC-Br in an ethylene glycol/water glass, the results of resonance light scattering (RLS), time-resolved emission, and photon echo decay measurements are discussed. Band structure calculations based on a linear-chain model for the J aggregate have also been performed. The results show that the J band can be described as a disordered Frenkel exciton band in which superradiant states exist that extend over about 100 molecules. Numerical simulation studies of the J band, based on Anderson's Hamiltonian with uncorrelated diagonal site energies, show that the ratio kappa of the disorder parameter D over the nearest-neighbor coupling parameter J12 is about 0.11. Using the frequency dependence of the ratio between the yields of vibrational fluorescence and Raman scattering as a probe, the dephasing process and derived parameters for the bath correlation function at three different temperatures have also been examined. It is shown that at all temperatures the exciton dephasing process occurs in the fast modulation limit. For PIC-I in a Langmuir-Blodgett film the optical excitation can be described also in terms of a band transition but the disorder is much larger than in a glass. For this system, a low-temperature fluorescence lifetime of about 10 ps is measured, which suggests that the excitation is much more delocalized than in the case of self-assembled aggregates in a glass. Resonance Rayleigh scattering experiments on PIC in a bilayer show that the exciton-dephasing rate increases dramatically at energies above the renormalized band edge.