Abstract
Structures of the S1 and T1 states of benzil are examined based on the experimental results from nanosecond time-resolved infrared spectroscopy and picosecond time-resolved Raman spectroscopy. Nanosecond time-resolved infrared spectra of the T1 state of benzil as well as its three isotopically substituted analogues were measured in carbon tetrachloride. The observed infrared bands of T1 benzil were assigned based on the frequency shifts on isotopic (18O, and deuteration) substitutions. The infrared band at 1312 cm−1 is assigned to the CO anti-symmetric stretch vibration. An infrared band that has large contribution from the central C–C stretch is not observed. Picosecond time-resolved resonance Raman spectra of the S1 state of benzil were also measured. It has been reported that after the photoexcitation, the benzil molecule shows an ultrafast conformational change in the S1 state. The observed resonance Raman bands are attributable to the vibrations of the relaxed form of the S1 state. By comparing the Raman and infrared spectra of the S0, S1, and T1 states of benzil, the structures of benzil in the excited states are discussed. Upon going from the S0 state to the S1 or T1 state, the bond order of the CO bond decreases while that of the central C–C bond increases. Although several ground-state bands appear in both the infrared and Raman spectra, there is no band observed simultaneously in the infrared and Raman spectra of the T1 state, except for bands attributable to the phenyl ring vibrations. We conclude that T1 benzil has the inversion center that arises from the trans-planar structure. The spectral pattern of the resonance Raman scattering of the relaxed S1 state is very similar to that of the T1 state. This implies that the molecular structure of the relaxed S1 state is similar to that of the T1 state. The structure of the relaxed form of the S1 state is also considered to be trans-planar.
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