The femtosecond-resolved evolution of the emission spectrum of the important conjugated polymer poly(3-hexylthiophene) (P3HT) is presented. Detailed fluorescence up-conversion spectroscopy was performed on P3HT solid-state films and on P3HT in chlorobenzene solution. Two excitation wavelengths and several emission wavelengths, covering the entire fluorescence spectrum, were used. The data were complemented by polarization-sensitive measurements. Our global analysis allowed a reconstruction of the time-resolved emission spectra with 200 fs temporal resolution, so that spectral changes due to the early relaxation processes following π–π* interband absorption in the pristine polymer could be comprehensively characterized. Absorption occurs in isolated polymer chains in solution and in the solid state (including interchain interactions) for the film. In both cases, we find evidence of delocalization of the electrons and holes formed in the energy bands directly after photoexcitation with excess energy. This is followed by ultrafast (~100 fs) self-localization of the primary photoexcitation and by relatively slow exciton formation (~1 ps). Further relaxation occurs with time constants ranging from hundreds of femtoseconds to tens of picoseconds, due to exciton hopping to sites with lower energy and to a slow conformational planarization of the polymer backbone. Depolarization, a spectral red shift, and important changes in the vibronic structure are observed as a consequence of this relaxation. Finally, relaxed intrachain and interchain singlet excitons are formed in solution and film, respectively, on a 100–200 ps time scale. They decay with a ~500 ps time constant, by intersystem crossing in solution and by nonradiative recombination in the film. Our results are consistent with and strongly support the conclusions we obtained from a similar time-resolved fluorescence study of the polymer PCDTBT (J. Am. Chem. Soc.2010, 132, 17459): ultrafast charge separation in polymer:fullerene blends seems to occur before localization of the primary excitation to form a bound exciton.

The nature and time evolution of the primary excitations in the pristine conjugated polymer, PCDTBT, are investigated by femtosecond-resolved fluorescence up-conversion spectroscopy. The extensive study includes data from PCDTBT thin film and from PCDTBT in chlorobenzene solution, compares the fluorescence dynamics for several excitation and emission wavelengths, and is complemented by polarization-sensitive measurements. The results are consistent with the photogeneration of mobile electrons and holes by interband π-π* transitions, which then self-localize within about 100 fs and evolve to a bound singlet exciton state in less than 1 ps. The excitons subsequently undergo successive migrations to lower energy localized states, which exist as a result of disorder. In parallel, there is also slow conformational relaxation of the polymer backbone. While the initial self-localization occurs faster than the time resolution of our experiment, the exciton formation, exciton migration, and conformational changes lead to a progressive relaxation of the inhomogeneously broadened emission spectrum with time constants ranging from about 500 fs to tens of picoseconds. The time scales found here for the relaxation processes in pristine PCDTBT are compared to the time scale (<0.2 ps) previously reported for photoinduced charge transfer in phase- separated PCDTBT:fullerene blends (Phys. Rev. B 2010, 81, 125210). We point out that exciton formation and migration in PCDTBT occur at times much longer than the ultrafast photoinduced electron transfer time in PCDTBT:fullerene blends. This disparity in time scales is not consistent with the commonly proposed idea that photoinduced charge separation occurs after diffusion of the polymer exciton to a fullerene interface. We therefore discuss alternative mechanisms that are consistent with ultrafast charge separation before localization of the primary excitation to form a bound exciton.