Femtosecond Study of Exciton Transport Dynamics in (Phthalocyaninato)polysiloxane

Abstract

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 photoinduced bleaching and transient absorption at lower excitation densities is made by including, in addition to the monomolecular, exponential decay with lifetime τ, a time-dependent bimolecular exciton−exciton annihilation rate constant γt-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.