Collective optical response of molecular aggregates

Relative fluorescence quantum yield, fluorescence lifetime, optical dephasing, and absorption line shift data are presented for both excitonic transitions of the pseudoisocyanine (PIC) J‐aggregate. The data indicate activation of the optical dephasing by exciton‐acoustic phonon scattering at low temperatures. Above approximately 80 K the optical dephasing, line shift as well as the fluorescence lifetime lengthening call for activation by high frequency phonons. Moreover, resonance Raman data indicate the delocalization of low frequency vibrational modes in the aggregate.