Femtosecond study of exciton transport dynamics in (phthalocyaninato)polysiloxane

The transport and relaxation dynamics of singlet excitons of rodlike [tetramethoxytetrakis(octyloxy)-phthalocyaninato]polysiloxane (PcPSi) in tetrahydrofuran solution were investigated on the femtosecond scale by transient absorption spectroscopy. Singlet excitons are generated upon excitation into the ground-state absorption at 555 nm using laser pulses of 120 fs duration with different intensities. Transient absorption spectra were recorded at variable delays using a white-light continuum probe pulse. Both photoinduced bleaching and transient absorption show nonexponential decay dynamics with a pronounced dependence on the excitation energy density. The analysis of the decay dynamics of the photo-induced bleaching and transient absorption at lower excitation densities is made by including, in addition to the monomolecular, exponential decay with lifetime tau, a time-dependent bimolecular exciton-exciton annihilation rate constant gamma t(-1/2) characteristic of one-dimensional diffusive exciton transport. At high excitation densities, the observed decay strongly deviates from this behavior, as a new relaxation channel is opened, which is attributed to short-lived, charge-separated states.