On the nature of exciton–phonon coupling in crystalline benzene

Abstract

A new technique for determining the nature of the exciton−phonon coupling in molecular crystals is applied to the S1 (0,0) exciton band. The phonons associated with the origin line of the neat crystal fluorescence are found to derive their intensity by a vibronic mechanism. In addition, the recently proposed ’’localized exciton sideband method’’ is applied to the fluorescence and phosphorescence spectra of pure and isotopic mixed benzene crystals to further test its applicability to molecular crystals. The difficulties expected for the general application of the localized exciton sideband method are discussed in some detail. It is shown that the method can be applied to pure as well as mixed crystals, removing uncertainties about possible impurity induced perturbations of the phonon modes. The correspondence between observed and calculated features in the phonon spectrum is not as satisfactory as in the naphthalene crystal. The strength of the exciton−phonon coupling is found to be similar, but measurably different for the T1 and S1 states of benzene. Furthermore, it is shown that the exciton−phonon coupling for the S1 state is less in the pure than in the isotopic mixed crystal. Thus, even though the pure crystal excitation exchange integrals are as small as ∼2 cm−1, the delocalization of the excitation causes a ∼10% reduction in the phonon coupling.