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.

Resonance and high-pressure Raman studies on carbon peapods

“…Most of the high-frequency C 70 modes are well seen in the spectra with the modes at 1060, 1255, 1465, and 1476 cm Ϫ1 being anomalously enhanced relative to the most intense C 70 line at 1446 cm Ϫ1 . Additionally, the lines at 1465 and 1476 cm Ϫ1 are strongly shifted from their positions found in pristine-solid and solution spectra 19,21 (1458-1460 cm Ϫ1 and 1468-1471 cm Ϫ1 , respectively͒.…”

“…So far reported excitation profiles of the A g ͑2͒ line for a C 60 peapod reveal one maximum at 2.50 eV. 10,11 The excitation profiles of the C 70 peapod lines are generally downshifted by 0.1-0.15 eV relative to corresponding C 70 curves in solution as read out from the comprehensive study of Gallagher et al 21 , the only exception being the 1476 cm Ϫ1 line, whose dependence on excitation energy fits well into the REP of the C 70 1469 cm Ϫ1 line in benzene solution. This implies that the excited electronic states experience slight shifts upon insertion of the peas in the pods.…”

Assignment of the Internal Vibrational Modes of C70 by Inelastic Neutron Scattering Spectroscopy and Periodic‐DFT

“…In order to assign the modes as to whether they are symmetric or antisymmetric with respect to the improper rotation 2 S 5 , (subscript 1 or 2, respectively), we note that eight infrared modes are observed in the gas phase and 24 Raman modes in solution . In both cases, D 5 h symmetry is present.…”

Structural and Vibrational Properties of C60 and C70 Fullerenes Encapsulating Carbon Nanotubes

“…Our calculations based on DFT have clarified that the shift in the RBM frequencies of nanotubes containing fullerenes strongly depends on the diameters of the nanotubes. DFT calculations have also clarified that the shift in the RBM frequencies of nanotubes containing fullerenes strongly depends on the diameters of the nanotubes [51,[53][54][55][56][57].…”

“…Ref [53] Ref [54] Ref [55] Ref [56] Ref [57] Ref [21] low-and high-frequency component associated to the vibrations along the circumferential direction (G−) and the G direction of the nanotube axis (G+), respectively. In both metallic and semiconducting nanotubes, the former component is found to be dependent on the diameter of a nanotube, while the latter does not exhibit this dependence [40].…”