Electronic Relaxation and Ground-State Dynamics of 1,3-Cyclohexadiene andcis-Hexatriene in Ethanol

Abstract

The transient absorption induced by UV excitation of 1,3-cyclohexadiene and Z-hexatriene dissolved in ethanol was measured in the wavelength range 255−450 nm. Repopulation of the ground state takes 470 fs in tZt-hexatriene. The same process in 1,3-cyclohexadiene, accompanied by ring opening to Z-hexatriene, occurs in less than 300 fs. The analysis of the long-wavelength wings of the ground-state spectra reveals a cooling time constant of 7 ps for the hot product Z-hexatriene. The transient absorption in the UV furthermore reflects the single-bond isomerizations of this product to different conformers. The evolution of their concentrations was simulated by rate equations using temperature-dependent isomerization rates calculated with the Arrhenius law. In this model, the concentrations of the conformers reach thermal equilibrium in a few picoseconds; during the first 10−20 ps, the concentrations follow the cooling of the molecule by the solvent, staying near thermal equilibrium; and a small quantity of cZt-hexatriene is trapped in its potential well on a time scale of 100 ps at the final temperature (300 K). At this temperature, equilibration takes longer. This model well reproduces the time dependence of the integrated spectra in solution and recent experimental results in gas phase. In addition it allows a consistent interpretation of previous, time-resolved resonance−Raman experiments. The fact that single-bond isomerization in the hot product was not suppressed by contact with the cold solvent suggests that this could also be the case for photorhodopsin, the postulated primary conformer of the photoproduct of rhodopsin which always eluded trapping even at cryogenic temperatures.