ESR studies of photo-generated polarons in undoped poly (p-phenylene vinylene)/PPV

Photo-generated spin species in undopedpoly (p-phenylene vinylene)/PPV and its derivative, CH3O-PPV have been studied by means of the light-induced ESR (LESR) technique using variable excitation energy. The LESR signal increases with the incident photon-energy and it becomes significant above the threshold energy of 3.1 eV for PPV and 2.7 eV for CH3O-PPV. Similar increase has been reported for the excitation spectra of photocarriers in MEH-PPV, suggesting that the light-induced spins are charged species. Furthermore, a clear anisotropy of a π-electron on the conjugated chain have been confirmed in the LESR spectra in stretch oriented PPV samples and they were well reproduced by the ESR spectrum simulation using a polaron spin density obtained from Pariser-Parr-Pople (PPP) model. From these results, we conclude that the observed light-induced spins are photo-generated polarons.