Photogenerated polarons in poly(paraphenylene vinylene) Original Research Article

Photo-induced spin species in poly(paraphenylene vinylene) (PPV) and its derivative have been investigated by means of the light-induced ESR (LESR) technique using variable excitation energy. LESR spectra at the X and K bands for stretch-oriented PPV samples exhibit clear anisotropy, which is well explained in terms of the anisotropic g-value and hyperfine coupling of unpaired π electron on the conjugated chain. The X band spectra were well reproduced by theoretical ESR spectra using a polaron spin density obtained from the Pariser-Parr-Pople model. The LESR signal increases significantly above around 3 eV. This threshold is interpreted as the interband gap above which free electron-hole pairs are created efficiently. From these results, we conclude that the observed light-induced spin species are polarons.