Polarization dependence of the ultrafast photoluminescence of oriented poly(p-phenylenevinylene)

The ultrafast dynamics of excitons in stretch-oriented poly(p-phenylenevinylene) (PPV) are studied by femtosecond time-resolved photoluminescence spectroscopy. It is shown that the total luminescence intensity at 1 ps after excitation is the same for excitation either parallel or perpendicular to the stretch direction. Thus the number of intrachain excitons initially generated is independent of the polarization of the excitation light. This indicates that direct interchain photogeneration processes are not important in PPV. A small polarization ratio is measured at very early times after excitation, and is consistent with the localized excitons having appreciable off-axis transition dipole moments. The confinement of the excitons places constraints on the maximum possible anisotropy of the luminescence. Significant differences between the luminescence rise and decay times are observed for different excitation and detection polarizations. These results are explained by a combination of nonradiative decay at quenching sites, whose strength depends upon the polarization of the excitation pulses, and exciton migration processes.