ENERGY-DEPENDENT BRANCHING BETWEEN FLUORESCENCE AND SINGLET EXCITON DISSOCIATION IN SEXITHIENYL THIN-FILMS

The fluorescence yield of thin films of sexithienyl drops rapidly above the S-1 <-- S-0 absorption edge while the yield of photocarrier generation increases simultaneously. This unusual behavior of a molecular solid is interpreted in terms of an energy-dependent branching between fluorescence and dissociation of a singlet excitation into a weakly bound electron-hole pair. This is shown to be a characteristic feature of a disordered system in which the energy levels of both neutral and charged excitations are subject to inhomogeneous broadening. In T6 the latter arises from torsional displacement of the thienylene moities.