Time-resolved site-selective spectroscopy of $\mathrm{poly}(p$-phenylene vinylene)

We report the dynamics of emission from the conjugated polymer $\mathrm{poly}(p$-phenylene vinylene) after ultrafast optical excitation with a range of photon energies. Subpicosecond temporal resolution of the emission allows us to distinguish between photoluminescence and intense resonant scattering that decays within a few picoseconds but dominates the time-integrated spectra. As the excitation energy is decreased the redshift of the photoluminescence over time is reduced, indicating a decreasing mobility of the excitons. The ratio between the intensities of the two highest-energy peaks in the spectrum increases for lower excitation energies and with increasing times after excitation. We deduce that the configurational energy change between ground and excited electronic states increases for excitons located on chain segments with shorter conjugation lengths. A Stokes shift of 20 meV between the excitation energy and the highest peak in emission is observed even when predominantly immobile excitons are generated. We attribute this shift to the preferential excitation into the higher levels of low-energy vibrational modes of states with electronic energy such that they are not in resonance with the excitation. This is supported by calculations that reproduce the experimental results only if these low-energy modes are considered. We show that when the low-energy phonon modes are important, site-selective spectroscopy excites a distribution of states that is broader than the spectral width of the excitation source.