Identification of effective exciton–exciton annihilation in squaraine–squaraine copolymers
Ultrafast time-resolved transient absorption spectroscopy is able to monitor the fate of the excited state population in molecular aggregates or polymers. Due to many competing decay processes, the identification of exciton–exciton annihilation (EEA) is difficult. Here, we use a microscopic model to describe exciton annihilation processes in squaraine–squaraine copolymers. Transient absorption time traces measured at different laser powers exhibit an unusual time-dependence. The analysis points towards dynamics taking place on three time-scales. Immediately after laser-excitation a localization of excitons takes place within the femtosecond time-regime. This is followed by exciton–exciton annihilation which is responsible for a fast decay of the exciton population. At later times, excitations being localized on units which are not directly connected remain so that diffusion dominates the dynamics and leads to a slower decay. We thus provide evidence for EEA tracked by time-resolved spectroscopy which has not been reported that clearly before.