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.
Universität Würzburg, Institut für Physikalische und Theoretische Chemie, Am Hubland, Campus Nord, Emil-Fischer-Str. 42, 97074 Würzburg, Germany. volker.engel@uni-wuerzburg.de.
Kilian
K
Hader
Institut für Physik, Humboldt Universität, Newtonstr. 15, D-12489 Berlin, Germany.
Volkhard
V
May
Universität Würzburg, Institut für Organische Chemie, Am Hubland, 97074 Würzburg, Germany.
Christoph
C
Lambert
Universität Würzburg, Institut für Physikalische und Theoretische Chemie, Am Hubland, Campus Nord, Emil-Fischer-Str. 42, 97074 Würzburg, Germany. volker.engel@uni-wuerzburg.de.
Volker
V
Engel