Absorption and emission spectra of monomers and dimers of [tetramethoxytetrakis(octyloxy)phthalocyaninato]silicon have been measured as a function of solvent and concentration. In contrast to the monomer, the fluorescence of the dimer is shifted to lower energy by about 4000 cm -1 , and the emission quantum yield is reduced by a factor of 10 -3 . The spectroscopic data are analyzed in the strong coupling limit of dimer models and lead to an excitation exchange interaction term of 1770 cm -1 and an evaluation of the excited state lifetime of ca. 24 ps. This fast decay is dominated by nonradiative processes. The admixture of ion pair configurations is suggested to provide an efficient coupling to the environment and channel of decay of the exited state energy. Small contributions of such configurations to the ground state are also invoked to explain the marked changes in the absorption spectra as a function of environment (solvent) and concentration (aggregation).

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.