Site torsional motion and dispersive excitation hopping transfer in π-conjugated polymers

Photoexcitations coupled to intrachain, backbone modes of a flexible pi-conjugated polyarvlene polymer and their effect on the relaxational dynamics of migrative excitation transfer have been examined by using femtosecond and picosecond time resolved fluorescence techniques. In poly[4,4'-diphenylene-1,2-di(3,4-dimethoxyphenyl)vinylene], DMOP-PDPV, a vertical excitation into the biphenylic segmental unit is subject to a quasi-instantaneous torsional, energetic relaxation that gives rise to large bathochromic shift of the gated fluorescence spectrum on time scales < 200 fs. For times > w 200 fs the (new-equilibrium) excited-state population evolves in the subsequent, electronic excitation energy transfer process (EET), but the rates in the ongoing, dissipative energy relaxation (up to several tens of nanoseconds!) are relatively slow and significantly delayed as compared to those in the more rigid poly(phenylenevinylene), PPV. The results are interpreted in terms of factored time scales for nuclear motion and electronic hopping modes. The significant slowing of the EET is related to the reduced Forster spectroscopic overlap integral, as a result of the sudden (<200 fs), large Stokes shift between vertical and structurally relaxed excitations. The energy shift of fluorescence photons in the course of EET, comparatively smaller than that brought about by the nuclear relaxation process and, due to the deceleration of EET, even still detectable on a nanosecond scale, can be understood as the result of a dissipative relaxation along a site energy cascade. The latter energy-dispersive pathway stems from a density of states (DOS) of localized S-1-states whose incoherent electronic communication can be adequately described within the framework of a molecular, excitonic picture in the limit of incoherent, non-Markovian motion.