Dynamics of optical excitations in a ladder-type pi-conjugated polymer containing aggregate states

The dynamics of optically generated excitations in a poly-(para)-phenylene-type ladder polymer has been investigated by means of femtosecond pump-probe as well as femtosecond luminescence up-conversion spectroscopy. In addition to the bulk emission a low-energy emission band assigned to physical aggregates is observed. By comparing the photoluminescence (PL) decay of the bulk emission with the time-resolved PL traces of the aggregate emission we show that in the presence of aggregates the dynamics of the initially generated excitons are determined by transfer from bulk to aggregate states. Additionally stimulated emission (SE) as well as photoinduced absorption are observed for different spectral regions in the pump-probe experiments. The SE is ascribed to bulk singlet excitons as evidenced by the correspondence of the temporal behavior of SE and the decay of bulk emission, whereas the PA is attributed to spatially separated excitons.