Shpolski effect in optical spectra of frozen solutions of the organic C 60 fullerene derivative in toluene

The optical spectra of a solution of the C 60 ethyl acetate (C 60 -EA) in toluene are investigated. The absorption spectra measured at T = 80 K reveal a combination of an intense featureless background pedestal deriving from absorption of charge-transfer cluster complexes on which weakly structured molecular absorption is superposed. A decrease in the temperature from 80 to 2 K leads to the appearance of a multiplet of narrow lines (with a width of down to 5 cm −1 ) corresponding to the optical excitation of C 60 -EA molecules occupying different inequivalent positions in the matrix. A similar structure is observed in the luminescence spectrum of the C 60 -EA solution at T = 2 K. The structural luminescence spectrum is in mirror symmetry with the absorption bands. The frequencies of the corresponding transitions and the related molecular vibrations are determined. The allowed character of both mirror-symmetry spectra combined with the exact matching of their constituent frequencies suggests that the vibronic interaction in the molecule is determined by the Frank-Condon mechanism.