PHOTOCHEMICAL RING-OPENING REACTIONS ARE COMPLETE IN PICOSECONDS - A TIME-RESOLVED UV RESONANCE RAMAN-STUDY OF 1,3-CYCLOHEXADIENE

Photoproduct formation, vibrational, and conformational relaxation kinetics in the photochemical ring-openings of 1,3-cyclohexadiene (CHD) and alpha-phellandrene (alpha-PHE) are determined by picosecond, time-resolved UV Stokes and anti-Stokes resonance Raman spectroscopy. The frequency-doubled output from an amplified, synchronously pumped dye laser system was used to perform transient, two-color resonance Raman Stokes experiments on the photoconversion of CHD to cis-hexatriene(c-HT) and of alpha-PHE to 3,7-dimethyl-1,3,5-octatriene(OT). The appearance time of the Stokes scattering from ground-state c-HT is 6 +/- 1 ps, while that of OT is 11 +/- 2 ps. In both reactions, the photoproduct anti-Stokes ethylenic intensity appears with a time constant of 8 +/- 2 ps and decays in 9 +/- 2 ps. This similarity demonstrates that the photoproduct appearance and intermolecular vibrational relaxation kinetics are not dramatically affected by the presence of alkyl substituents. Analysis of the photoproduct spectral evolution in the Stokes and anti-Stokes data as well as the observation of Raman lines characteristic of the all-cis conformer in the anti-Stokes data demonstrates that all-cis-HT first appears on the ground-state surface and then undergoes conformational relaxation to produce mono-s-cis-HT with a time constant of 7 ps. The photoproduct anti-Stokes ethylenic and single-bond stretch intensities further demonstrate that the initial photoproduct temperature at 4 ps is 1500 +/- 500 K and that the cooling time is 15 ps. This is the first complete analysis of the photoproduct formation, vibrational, and conformational relaxation dynamics characteristic of photochemical pericyclic ring-opening reactions.