Raman Excitation Profiles of C70 in Benzene Solution. Assignment of the Electronic Spectrum in the 380−510-nm Region

Abstract

The resonance Raman (RR) spectrum of C70 has been studied in benzene using 11 laser excitation energies across the main visible absorption band (MVAB) of C70 between 514.5 and 406.7 nm. Raman excitation profiles (REPs) were constructed for the 15 most intense RR bands of C70, and symmetry assignments have been made partly on the basis of polarization work. Contrast is made to work performed on thin films where problems have arisen from the symmetry-lowering effect of the surface and from neglect of resonance. Assignments for nine other less intense RR bands are suggested. Three electronic transitions under the MVAB are identified and assigned definitively: the HOMO − 4 (e2‘‘) → LUMO + 1 (e1‘‘) transition in the 514/501-nm excitation region, the HOMO − 5 (e1‘) → LUMO + 1 (e1‘‘) transition in the 476/472-nm excitation region, and the HOMO (a2‘‘) → LUMO + 2 (a1‘‘) in the 457/452-nm excitation region. The REPs reveal that these three electronic transitions are vibronically coupled to the strong electronic transition at 382 nm which is assigned to the HOMO − 2 (a2‘) → LUMO + 3 (e1‘) transition. RR B-term scattering mechanisms are the major source of intensity enhancement for bands of the totally-symmetric A1‘ and the non-totally-symmetric E1‘‘ and E2‘ Raman modes. The REPs of the 15 bands are grouped into four types that provide insight into the change in the electronic distribution upon excitation for each transition. Unlike C60, whose extraordinarily high symmetry makes it very sensitive to solvent-induced symmetry lowering and whose RR spectrum is rich in forbidden, overtone, and combination bands, C70 displays a restricted subset of RR scattering phenomena. The lower symmetry and more localized molecular orbitals of C70 make it a better model for the RR scattering mechanisms and vibronic coupling expected in the higher fullerenes.