Vibrational structure of organic semiconductors: the role of conjugation length

Site-selective luminescence spectroscopy (SSL) was employed to resolve the vibracional modes in self-assembled poly(p-phenylene vinylene), PPV. Using Franck-Condon analysis, the PPV spectra can be well described by three effective vibrational modes at 330, 1160 and 1550 cm-1. As the temperature increases from 4 to 180 K the electron-phonon coupling (S factor) remains constant for the 1550 cm-1 mode while for other lower energy modes the coupling increases monotonically. This trend is consistent with the decrease of the conjugation length due to the increase of thermal disorder. We find that the temperature dependence of vibrational progression in PPV films is determined mainly by low frequency torsional modes.