Time-Resolved Resonance Raman and Density Functional Studies on the Ground State and Short-Lived Intermediates of Tetrabromo-p-benzoquinone

Abstract

Time-resolved resonance Raman (TR3) and density functional theoretical (DFT) studies on the photogenerated transient intermediates of tetrabromo-p-benzoquinone (bromanil) are reported. The lowest triplet excited state, radical anion, and semiquinone radical have been observed. Computed spectra and normal-coordinate analysis have been used to make assignments of the observed bands. The lowest triplet state is computed to be a $\pi-\pi^*$ excited state of $^3B_g$ symmetry. The effect of electronic excitation on the triplet state structure is found to be more pronounced in bromanil as compared to that in p-benzoquinone. The changes in structure in the bromanil radical anion have been explained from the nodal structure of the LUMO of the ground state. The computed structure of the semiquinone radical shows that it is essentially a pentadienyl radical. Consequences of these structural changes on the observed vibrational spectra have been discussed.