Energy-dispersive excitation transport in polyphenylene-vinylene probed by femtosecond luminescence-up-conversion

Abstract

Excitonic relaxation in polyphenylene-vinylene, PPV, has been probed by fs-luminescence optical gating. We report on ultrafast luminescence patterns which have been interrogated for different energetic positions of the luminescence window. The data have been analyzed on the basis of a microscopic hopping model which takes into account the static site-energy disorder, Förster site-to-site interaction and a density of localized singlet states (DOS) 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 hopping modes and the distribution of waiting times for energy-relaxing excitations traversing through the DOS. We show that the absence of significant rise terms in the S 0 v = 0 ← S 1 v = 0 luminescence transition 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 combination of the experimental and computational techniques thus favors the molecular picture of exciton relaxation in PPV in which strongly correlated electron-hole pairs undergo dispersive excitation transport among the energetically disordered sites which can be related to segmental units caused by statistical interruption of the π-bond conjugation length.