Vibronic structure in the multiple state fluorescence spectrum of C70: A theoretical investigation

The vibronic structure of the fluorescence spectrum of C70 is analyzed on the basis of semi-empirical quantum-chemical calculations followed by modeling of the spectra. Excitation energies of the lowest electronic states of C70 and transition dipole moments are computed with the semi-empirical complete neglect of differential overlap/spectroscopic parametrization (CNDO/S) Hamiltonian combined with configuration interaction calculations which include single and double excitations. Vibronic interactions required to model the structure of the spectra are computed at the same level of theory and the emission spectra of the lowest dipole-forbidden and dipole-allowed excited states of C70 are simulated on the basis of a perturbative expansion of vibronic wavefunctions. The comparison between simulated and observed luminescence spectra indicates that the lowest state responsible for the observed emission is a dipole-forbidden A2′ state which borrows intensity mainly from the lowest two dipole-allowed states of E1′ symmetry. The weakly allowed 1 E1′ state, lying slightly above S1, whose simulated emission shows almost negligible vibronic activity, is assigned to the second emitting state which contributes to the multiple state emission observed for C70.