ENERGY-DISPERSIVE EXCITATION TRANSPORT IN POLYPHENYLENE-VINYLENE PROBED BY FEMTOSECOND LUMINESCENCE-UP-CONVERSION

Excitonic relaxation in polyphenylene-vinylene. PPV, has been probed b y fs-luminescence optical Eating. We report on ultrafast luminescence patterns which have been interrogated for different energetic position s of the luminescence window. The data have been analyzed on the basis of a microscopic hopping model which takes into account the static si te-energy disorder, Forster site-to-site interaction and a density of localized singlet states (BOS) of molecular site excitations. A topic of great concern has been the analysis of the fs-data in terms of the luminescence lifetime space which directly reveals the spectrum of hop ping modes and the distribution of waiting times for energy-relaxing e xcitations traversing through the DOS. Wt: show that the absence of si gnificant rise terms in the S-0(v=0) <-- S-1(v=0) luminescence transit ion is caused by the ultrafast S-0(v=1) <-- S-1(v=0) decay from higher -lying DOS-states leading to the cancellation of amplitudes. The combi nation of the experimental and computational techniques thus favors th e molecular picture of exciton relaxation in PPV in which strongly cor related electron-hole pairs undergo dispersive excitation transport am ong the energetically disordered sites which can be related to segment al units caused by statistical interruption of the pi-bond conjugation length.