Ultrafast polyene dynamics in solution:: The conformational relaxation and thermalization of highly excited cis-1,3,5-hexatriene as a function of initial conformation and solvent

Femtosecond transient absorption spectroscopy has been used to investigate the ground and excited-state dynamics of cis-1,3,5-hexatriene (Z-HT) and 1,3-cyclohexadiene following excitation to the strongly allowed 1(1)B state in cyclohexane and n-hexadecane solvents. The internal conversion from the excited state manifold to produce vibrationally excited ground electronic state Z-HT occurs with rates of ca. 2-4 ps(-1). The relaxation of highly excited Z-HT is used to probe the solvent-solute interaction and the influence of solvent on the competition between thermalization and single-bond isomerization dynamics. The data are also used to compare and contrast the hexatriene relaxation as a function of solute initial conditions (cZc-NT vs tZt-HT). A formalism for transforming the observed spectral data into time-dependent photoproduct yields is developed. These data are then compared with time-dependent quantum yields obtained from a transition state, rate equation approach to modeling the anticipated population dynamics. The detailed analysis of the results presented here are compared with the analysis of Lochbrunner et al. [Lochbrunner, S.; Fuss, W.; Schmid, W. E.; Kompa, K.-L. J. Phys. Chem. A 1998, 102, 9334. In press.] for the conformational relaxation of highly excited ZHT in ethanol. Although the conformational evolution observed in hexadecane and cyclohexane are similar, the decays of cZc-HT and cZt-HT appear to be substantially faster in ethanol. These results are discussed in the context of Kramers theory for condensed phase reaction dynamics.