Photon-energy dependence of light-induced ESR for poly(para-phenylene vinylene) and its derivatives

To study the nature of photoexcitation in poly(para-phenylene vinylene) (PPV), we have performed a combined study of light-induced electron spin resonance (LESR) and photoluminescence (PL). The LESR signal increases with increasing incident photon energy and it becomes significant above 3.1 eV for PPV and 2.9 eV for poly(2,5-dimethoxy-para-phenylene vinylene) (CH3O-PPV), respectively. The observed absorption and PL peaks of CH3O-PPV are slightly lower than those of PPV, corresponding to a smaller bandgap. The results are consistent with the LESR action spectra. We also measured the polarized LESR for stretch-oriented PPV. The LESR signal being excited at 340 nm (3.7 eV) is larger for light polarized perpendicular to the chain direction than for the parallel case, suggesting that interchain charge separation is important for the LESR.