Evidences for charge‐transfer complex formation in the benzene adsorption on sulfated TiO2–a resonance Raman spectroscopy investigation

Benzene adsorbed on highly acidic sulfated TiO₂ (S‐TiO₂) shows an intriguing resonance Raman (RR) effect, with excitation in the blue‐violet region. There are very interesting spectral features: the preferential enhancement of the e_2g mode (1595 cm⁻¹) in relation to the a_1g mode (ring‐breathing mode at 995 cm⁻¹) and the appearance of bands at 1565 and 1514 cm⁻¹. The band at 1565 cm⁻¹ is probably one of the components of the e_2g split band, originally a doubly degenerate mode (8a, 8b) in neat benzene, and the band at 1514 cm⁻¹ is assigned to the 19a mode, an inactive mode in neat benzene. These facts indicate a lowering of symmetry in adsorbed benzene, which may be caused by a strong interaction between S‐TiO₂ and the benzene molecule with formation of a benzene to Ti (IV) charge transfer (CT) complex or by the formation of a benzene radical cation species. However, the RR spectra of the adsorbed benzene cannot be assigned to the benzene radical cation because the observed wavenumber of the ring‐breathing mode does not have the value expected for this species. Moreover, it was found by ESR measurements that the amount of radicals was very low, and so it was concluded that a CT complex is the species that originates the RR spectra. The most favorable intensification of the band at 1595 cm⁻¹ in the RR spectra of benzene/S‐TiO₂ at higher excitation energy corroborates this hypothesis, as an absorption band in this energy range, assigned to a CT transition, is observed. Copyright © 2008 John Wiley & Sons, Ltd.