POLARIZATION DEPENDENCE OF THE ULTRAFAST PHOTOLUMINESCENCE OF ORIENTED POLY(P-PHENYLENEVINYLENE)

The ultrafast dynamics of excitons in stretch-oriented poly(p-phenylen evinylene) (PPV) are studied by femtosecond time-resolved photolumines cence 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 th e excitation light. This indicates that direct interchain photogenerat ion 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 m oments. The confinement of the excitons places constraints on the maxi mum possible anisotropy of the luminescence. Significant differences b etween the luminescence rise and decay times are observed for differen t excitation and detection polarizations. These results are explained by a combination of nonradiative decay at quenching sites, whose stren gth depends upon the polarization of the excitation pulses, and excito n migration processes.