THE ULTRAFAST GROUND AND EXCITED-STATE DYNAMICS OF CIS-HEXATRIENE IN CYCLOHEXANE

One-and two-color kinetics have been combined with broadband ultraviol et transient absorption spectroscopy in the 265-300 nm region to eluci date the photophysics of cis-hexatriene in cyclohexane solvent. The lo west singlet excited state, the 2 (1)A(1) state, is observed to have a lifetime of 200+/-50 fs. The ground-state hexatriene is produced vibr ationally hot. The excess vibrational energy permits ultrafast isomeri zation around the C-C single bonds in hexatriene. This results in a dy namic equilibrium of the three cis-hexatriene rotamers, which then rel axes multiexponentially to the room-temperature distribution in which the di-s-trans-Z-hexatriene form predominates. The peak of the mono-s- trans (cZt-HT) population is estimated to be similar to 50%. Vibration al cooling results in trapping of a small amount, similar to 8%, of cZ t-HT that relaxes on a much longer time scale as the barrier to isomer ization becomes important. An estimate of the absorption spectrum of c Zt-HT is deduced from analysis of the spectral data at 50 ps. (C) 1997 American Institute of Physics.