PICOSECOND TIME-RESOLVED UV RESONANCE RAMAN-SPECTROSCOPY OF THE PHOTOCHEMICAL RING-OPENING OF 1,3,5-CYCLOOCTATRIENE AND ALPHA-PHELLANDRENE

The kinetics of the photochemical ring opening of 1,3,5-cyclooctatriene and α-phellandrene were determined by picosecond, time-resolved UV resonance Raman spectroscopy. An amplified, synchronously pumped dye laser provided a 275-nm pump pulse and probe pulses at either 284 or 298 nm with 2.6-ps time resolution. Transient Raman spectra were obtained from 0 ps to ∼10 ns after photolysis. The time evolution of the ethylenic intensity demonstrated that the octatetraene photoproduct of 1,3,5-cyclooctatriene appeared in 12 ± 2 ps. The 3,7-dimethyl-1,3,5-octatriene photoproduct produced from the photolysis of α-phellandrene appeared with a time constant of 11 ± 2 ps. The similarity of these times to the 8-ps production of cis-hexatriene from 1,3-cyclohexadiene (Reid, P.; Doig, S.; Mathies, R. Chem. Phys. Lett. 1989, 156, 163) indicates that the ∼10-ps photoproduct appearance time is a general feature of photochemical electrocyclic rearrangements. © 1990 American Chemical Society.