Ultrafast polyene reaction and relaxation dynamics in solution: 1,3,5-hexatriene, 1,3-cyclohexadiene, and 7-dehydrocholesterol.

The reaction and relaxation dynamics of short polyenes in solution are important topics with both physical and biological relevance. This work presents ultrafast transient absorption studies of short polyenes in solution. The photochemical ring-opening reaction of 1,3-cyclohexadiene (CHD) to form 1,3,5-cis-hexatriene (Z-HT) is presented as a model system. In addition, the photophysics following excitation of 1,3,5-hexatriene are explored. In order to make the connection to a biologically relevant system, A study of the analogous photochemical ring opening of 7-dehydrocholesterol (DHC) to form previtamin D is presented. The lifetime of the initially prepared 1B (S2) state is found to be -1 of excess vibrational energy. In the model systems, CHD and cis-hexatriene, the cooling is composed of concurrent vibrational relaxation through energy dissipation to the solvent and single bond rotational isomerization. Although the initial rotamer population distribution is different for these two systems, the relaxation is quite similar in both, and largely independent of solvent viscosity. The ground state cooling dynamics following DHC excitation differ from the model systems. In the case of DHC, the relaxation and rotational isomerization processes are separable. The cooling occurs much faster in DHC because it is dominated by intramolecular vibrational redistribution rather than energy dissipation to the solvent. As a result, energy is removed from the reactive modes before rotational isomerization can occur.Ph.D.Physical chemistryPure SciencesUniversity of Michigan, Horace H. Rackham School of Graduate Studieshttp://deepblue.lib.umich.edu/bitstream/2027.42/132298/2/9963736.pd