Time-resolved resonance Raman and density functional studies on the groundstate and short-lived intermediates of tetrabromo-p-benzoquinone

Time-resolved resonance Raman (TR3) and density functional theoretical (DFT ) studies on the photogenerated transient intermediates of tetrabromo-p-ben zoquinone (bromanil) are reported. The lowest triplet excited state, radica l anion, and semiquinone radical have been observed. Computed spectra and n ormal-coordinate analysis have been used to make assignments of the observe d bands. The lowest triplet state is computed to be pi-pi* excited state of B-3(8) 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 hav e been explained from the nodal structure of the LUMO of the ground state. The computed structure of the semiquinone radical shows that it is essentia lly a pentadienyl radical. Consequences of these structural changes on the observed vibrational spectra have been discussed.