THE ULTRAFAST PHOTOCHEMICAL RING-OPENING REACTION OF 1,3-CYCLOHEXADIENE IN CYCLOHEXANE

The ring-opening reaction of 1,3-cyclohexadiene in cyclohexane solutio n and the subsequent photoproduct cooling dynamics have been investiga ted by using two-color transient absorption kinetic measurements and n ovel time-resolved absorption spectroscopy in the 260-300 nm spectral region. The initial photoproduct in this reaction, s-cis,Z,s-cis-1,3,5 -hexatriene (cZc-HT) is formed on a similar to 250 fs time scale. Spec tra deduced for time delays very close to zero, as well as calculated Rice-Ramsperger-Kassel-Marcus unimolecular reaction rates, provide str ong evidence that the quantum yield for the reaction is determined bef ore any relaxation occurs on the ground state. Upon formation, the vib rationally excited hexatriene photoproduct is able to isomerize around C-C single bonds freely. As a result, the evolution observed in the t ransient absorption measurements represents a combination of rotamer p opulation dynamics and thermalization due to energy transfer to the so lvent. Three distinct time scales for relaxation are observed, These t ime scales correspond approximately to the development of an evolving equilibrium of Z-HT rotamers (1-5 ps), vibrational cooling and thermal equilibration with the surroundings (10-20 ps), and activated isomeri zation of trapped cZt-HT to tZt-HT (much greater than 100 ps). (C) 199 8 American Institute of Physics.