Vibronic spectra of perylene bisimide oligomers: effects of intermolecular charge-transfer excitation and conformational flexibility.

We have recently presented a theoretical study on the temperature-dependent absorption and photoluminescence spectroscopy of rubrene multichromophores by combining the time-dependent long-range-corrected density functional theory with the Frenkel exciton model (Gao; et al. J. Phys. Chem. A2009, 113, 12847). The spectra of rubrene multichromophores up to heptamers have been calculated and the effects of exciton-phonon coupling and temperature on the photophysical properties of both H- and J-aggregated oligomers were addressed. However, in that work the contribution of intermolecular charge-transfer excitons (CTEs) to vibronic spectra was not addressed. Here we take into account the effect of CTEs for the absorption and emission spectra of the aggregated perylene bisimide (PBI) oligomers in order to have a quantitative explanation to the experimental absorption and emission spectra of the PBI dyes. The role of intermolecular CTEs is discussed for different intermolecular orientations and distances. The simulations demonstrate that the contribution of CTEs becomes significant when the intermolecular distance is less than 4.5 Å for the π-π stacked PBI aggregates, and the mixed exciton model is prerequisite to explain the experimentally observed red-shift of the absorption spectra in this case. The large Stokes shift of the emission spectra can be reproduced by our model, and it is induced by the asymmetric nature of the lowest excitonic state of the H-aggregated oligomers. The experimentally observed broad emission bands come from two species with different conformations. As for J-aggregated PBI oligomers, the interactions of FEs induce the red-shift and the increase of the relative intensity of 0-0 peak of the absorption spectra with more aggregated units.