Time-resolved site-selective spectroscopy of poly(p-phenylene vinylene) - art. no. 115206

We report the dynamics of emission from the conjugated polymer poly(p-pheny lene vinylene) after ultrafast optical excitation with a range of photon en ergies. Subpicosecond temporal resolution of the emission allows us to dist inguish between photoluminescence and intense resonant scattering that deca ys within a few pico-seconds but dominates the time-integrated spectra. As the excitation energy is decreased the redshift of the photoluminescence ov er time is reduced, indicating a decreasing mobility of the excitons. The r atio between the intensities of the two highest-energy peaks in the spectru m increases for lower excitation energies and with increasing times after e xcitation. We deduce that the configurational energy change between ground and excited electronic states increases for excitons located on chain segme nts with shorter conjugation lengths. A Stokes shift of 20 meV between the excitation energy and the highest peak in emission is observed even when pr edominantly immobile excitons are generated. We attribute this shift to the preferential excitation into the higher levels of low-energy vibrational m odes of states with electronic energy such that they are not in resonance w ith the excitation. This is supported by calculations that reproduce the ex perimental results only if these low-energy modes are considered. We show t hat when the low-energy phonon modes are important. site-selective spectros copy excites a distribution of states that is broader than the spectral wid th of the excitation source.