Exciton versus band description of the absorption and luminescence spectra in poly(p-phenylenevinylene)

Abstract

The fluorescence spectra of poly(p-phenylenevinylene) have been investigated at 6 K employing site-selective laser spectroscopy. They delineate the existence of a well-defined energy in the tail of the absorption edge below which the fluorescence emission is quasiresonant with an excitation featuring a Stokes shift of 100 ${\mathrm{cm}}^{\mathrm{-}1}$ only. Above this localization threshold ${\nu }_{\mathrm{loc}}$ spectral diffusion is observed with the emission independent of excitation yet carrying a significant polarization memory for excitation energies up to 1.9 eV in excess of ${\nu }_{\mathrm{loc}}$. This is incompatible with the band picture involving photogeneration of an uncorrelated electron-hole pair, yet consistent with the concept of random walks of neutral excitations through an inhomogeneously broadened density of states. The chromophores are associated with a distribution of segments of the polymer chains along which the excitation is delocalized. Published results on time-resolved fluorescence support the exciton picture.