The ultrafast photochemical ring-opening reaction of 1,3-cyclohexadiene in cyclohexane

The ring-opening reaction of 1,3-cyclohexadiene in cyclohexane solution and the subsequent photoproduct cooling dynamics have been investigated by using two-color transient absorption kinetic measurements and novel 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. Spectra deduced for time delays very close to zero, as well as calculated Rice-Ramsperger-Kassel-Marcus unimolecular reaction rates, provide strong evidence that the quantum yield for the reaction is determined before any relaxation occurs on the ground state. Upon formation, the vibrationally excited hexatriene photoproduct is able to isomerize around C-C single bonds freely. As a result, the evolution observed in the transient absorption measurements represents a combination of rotamer population dynamics and thermalization due to energy transfer to the solvent. Three distinct time scales for relaxation are observed, These time 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 isomerization of trapped cZt-HT to tZt-HT (much greater than 100 ps). (C) 1998 American Institute of Physics.