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 (Delta = 4.6) and the 339 cm(-1) ring deformation (Delta = 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 (phi(F) = 2 x 10(-6)) gives an excited state lifetime on the order of similar to 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 .