Delocalization of electronic excitations in disordered molecular systems

Picosecond photon echo experiments are used to examme the nature of the electronic states of interacting molecules in the mixed molecular crystal system, pentacene in p-terphenyl at 1.5 K. The onset of delocalization of the pentacene first singlet electronic excited state with increasing pentacene concentration is manifest in very fast photon echo decays. The concentration dependence of the echo decay rates accords with theoretical estimates. 1.