Hexatrienes. Investigations of Ground State Vibrational Spectroscopy, Triplet Sensitized Photo-Isomerization, and Structure, Kinetics, and Dynamics of the Lowest Excited Triplet State

摘要

Liquid phase Raman, and gas or liquid phase FTIR spectra are obtained for E-1,3,5-hexatriene, Z-hexatriene, E,E-heptatriene, E,E,E-octatriene, and E-2,5-dimenthyl-hexatriene, and assigned on the basis of symmetry consideration, Raman depolarization ratios and IR band shapes, previous literature, and through a comparison among the hexatrienes. Previous literature on the photochemistry of hexatrienes is surveyed emphasizing the principle of Non Equilibration of Excited Rotamers (NEER) and the triplet state photochemistry. The photochemical mechanisms that are regarded to be operative in the present study are presented, and the photochemical conversion observed for each of the compounds investigated is described. Time-resolved differential absorption spectroscopy of hexatrienes excited to their lowest triplet state by pulse radiolysis, and time-resolved resonance Raman spectroscopy of hexatrienes excited to their lowest triplet state by sensitized laser flash photolysis, are described as well as time resolved Raman spectroscopy of the lowest triplet states of 2,5-dimethyl-hexatriene and hexatriene, respectively. The experimental procedures and the results are discussed. The discussion brings out information on structure, kinetics, and dynamics in the lowest excited triplet state and on potential energy surfaces in the triplet as well as in the ground state. The results suggest that the NEER principle can be extended from excited singlet to triplet states. A comparison of the triplet spectra of the various hexatrienes forms the basis of an assignment of the transient spectra, and so increases the knowledge of the structure of the triplet state of hexatrienes. This is probably an important step in extending the usefulness of time-resolved resonance Raman spectroscopy in the study of excited states. (EG). 42 refs. (ERA citation 12:045944)