Ultrafast non-linear optical properties and photoinduced relaxation dynamics of poly(2,5-thienylenevinylene)

The transient absorption spectra of a thin film of poly(2,5-thienylenevinylene) at photon energies between 1.15 and 2.55 eV are measured by femtosecond pump-probe spectroscopy, where the pump pulse (≈ 100 fs) is resonant with the π ∗ → π transition. Several ultrafast non-linear optical processes are observed during the temporal overlap of the pump and probe beams, including bleaching, Raman gain and induced phase modulation. The photoinduced absorbance change signals are found to shift spectrally to higher photon energies with an estimated time constant of 100–200 fs in the spectral region from 1.15 to 1.90 eV, indicating the geometrical relaxation to a self-trapped state. The transient decay curves are not described well by single exponential decays but can be fitted to a biexponential decay function. The fast and slow decay time constants are both found to increase from 0.16 to 0.86 ps and 4 to 14 ps, respectively, with higher probe photon energies from 1.44 to 1.82 eV. The fast decay component included both the relaxation of the hot self-trapped exciton to the adiabatic ground-state potential-energy surface and a phonon emission process. The slow decay component corresponds to both the decay of a thermalized self-trapped state via tunnelling to the ground state and a thermalization process. The observed spectral shift and decay kinetics are similar to photoexcitation dynamics previously observed in polydiacetylenes and polythiophenes.