Solvent-side observation on vibrational energy transfer by transient grating spectroscopy: Bridged azulene-anthracene

Abstract: Transient grating acoustic spectroscopy has been applied to studies on the vibrational energy relaxation process of the electronic ground state of azulene, two 1-alkylazulenes, and five bridged azulene-anthracenes in three different solvents: 1,1,1-trichloro-1,2,2-trifluoroethane, acetonitrile, and xenon. The solute molecule was vibrationally excited by the photo-excitation of the auzlenyl group to the S1 state through the fast internal conversion, and the rate of solvent thermalization due to the vibrational energy relaxation was determined. The thermalization rates for 1-alkylazulenes and bridged azulene-anthracenes were faster than that of azulene. Based on the results of the thermalization rates of 1-alkylazulenes, we concluded that the acceleration of the energy dissipation from the azulenyl group induced the faster energy dissipation from the solute to the solvent. The vibrational normal mode analysis suggests that the density of the vibrational modes and anharmonic coupling between the vibrational modes induce the faster intramolecular energy redistribution. In xenon, the solvent thermalization rates were close to the energy dissipation rates of the solute molecule reported using transient absorption spectroscopy. On the other hand, in the chlorofluorocarbon, the ratio of the thermalization rate to the dissipation rate was a dependent of the molecular species, and especially faster solvent thermalization rate of 9-(6-(azulen-1-yl)hexyl)anthracene was predicted. The vibrational characteristics of this solvent are discussed in the relation to the vibrational modes and structures of the bridged compounds.