The ultrafast ground and excited state dynamics of cis-hexatriene in cyclohexane

One-and two-color kinetics have been combined with broadband ultraviolet transient absorption spectroscopy in the 265-300 nm region to elucidate the photophysics of cis-hexatriene in cyclohexane solvent. The lowest 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 vibrationally hot. The excess vibrational energy permits ultrafast isomerization around the C-C single bonds in hexatriene. This results in a dynamic equilibrium of the three cis-hexatriene rotamers, which then relaxes 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%. Vibrational cooling results in trapping of a small amount, similar to 8%, of cZt-HT that relaxes on a much longer time scale as the barrier to isomerization becomes important. An estimate of the absorption spectrum of cZt-HT is deduced from analysis of the spectral data at 50 ps. (C) 1997 American Institute of Physics.