Multiple mode exciton-phonon coupling: Applications to photoluminescence in oligothiophene thin films

The impact of exciton-phonon (EP) coupling including two or more vibrational modes on emission in herringbone aggregates with N molecules is investigated theoretically. When the total nuclear relaxation due to all modes is much smaller than the exciton band curvature at the band minimum, the effects of several modes are additive, with each mode contributing a single sideband to the emission spectrum in the limit of large N. Outside the weak coupling regime synergistic intermode effects arise. In particular, the presence of a strongly coupled mode enhances the sideband intensity of a weakly coupled mode. Applications are made to photoluminescence spectra of oligothiophene thin films.