Study of transient state structures of quinones from Resonance Raman Spectroscopy

Abstract

The time-resolved resonance Raman spectra of the triplet excited state, radical anion, and semiquinone radical of bromanil have been recorded. Observed vibrational modes have been assigned for all three transient species by comparison with the ground-state frequencies, spectra computed using DFT calculations, and with other tetrahalogen-substituted quinones. Depolarization ratios have been measured for all the observed bands in the triplet excited state. Normal coordinate analysis has been carried out to understand the changes that occur on halogen substitution in the ground and excited states. These data have been used along with the computed frontier molecular orbitals of the ground state to understand the structures of the triplet state, radical anion, and the semiquinone radical. Our calculations have shown that the participation of the lone pairs on bromine in the frontier molecular orbital is significant. The effect of halogen substitution is observed to be larger in the triplet excited state than in the ground state. Electronic excitation distorts the molecule, making the ring narrower and the C–C bonds shorter, as well as distorting the molecule out-of-plane to a lower C?h symmetry compared to the ground state. Vibrational frequencies observed in the triplet state were predicted with larger errors than observed for the ground state. The radical anion geometry is also affected similarly. The changes due to reduction on the structure of bromanil are very similar to the changes due to electronic excitation. However, the C–Br bond lengths show larger changes on reduction than on electronic excitation. These structural changes imply that the radical anion and triplet state of bromanil would be more reactive than the ground state.