VIBRONIC STRUCTURE IN THE MULTIPLE STATE FLUORESCENCE-SPECTRUM OF C-70 - A THEORETICAL INVESTIGATION

The vibronic structure of the fluorescence spectrum of C-70 is analyze d on the basis of semi-empirical quantum-chemical calculations followe d by modeling of the spectra. Excitation energies of the lowest electr onic states of C-70 and transition dipole moments are computed with th e semi-empirical complete neglect of differential overlap/spectroscopi c parametrization (CNDO/S) Hamiltonian combined with configuration int eraction calculations which include single and double excitations. Vib ronic interactions required to model the structure of the spectra are computed at the same level of theory and the emission spectra of the l owest dipole-forbidden and dipole-allowed excited states of C-70 are s imulated on the basis of a perturbative expansion of vibronic wavefunc tions. The comparison between simulated and observed luminescence spec tra indicates that the lowest state responsible for the observed emiss ion is a dipole-fosbidden A(2)' state which borrows intensity mainly f rom the lowest two dipole-allowed states of E-1' symmetry. The weakly allowed 1 E-1' state, lying slightly above S-1, whose simulated emissi on shows almost negligible vibronic activity, is assigned to the secon d emitting state which contributes to the multiple state emission obse rved for C70 (C) 1998 American Institute of Physics.