Excited-state structure and photochemical ring-opening dynamics of 1,3,5-cyclo-octatriene from absolute resonance Raman intensities

Absolute resonance Raman cross sections are measured for 1,3,5-cyclo-octatriene (COT) in cyclohexane with excitation from 325 to 200 nm. These intensities and the absorption spectrum are modeled using a fully thermalized time-correlator theory to quantitate the excited-state equilibrium geometry displacements along 19 Raman-active normal modes. The resonance Raman spectra show significant intensity in low-frequency modes corresponding to planarization of the eight-membered ring. The 140 cm−1 twist-boat planarization (Δ=4.6) and the 339 cm−1 ring deformation (Δ=1.6) are particularly strong. However, no intensity is observed in modes which project onto the predicted disrotatory ring-opening motion, such as the nontotally symmetric CH2 twist fundamental or its overtone. Analysis of the fluorescence quantum yield (φF=2×10−6) gives an excited state lifetime on the order of ∼30 fs. These results show that ring planarization is the first step in the disrotatory ring opening of COT followed by rapid depopulation of the initially prepared state to a lower-lying excited electronic state upon which the actual ring opening occurs. Comparison of these results with the excited-state dynamics of other pericyclic systems suggests that pericyclic rearrangements occur only once a planar structure is established and that the bond rearrangement occurs predominantly on a low-lying, optically forbidden excited state.