Photochemical isomerization in the absence of a potential barrier

Abstract

Different time resolved spectroscopic techniques have been used to investigate the photophysics of the isomerization reaction of 1,1′-diethyl-4,4′-cyanin. The molecule is characterized by a very short excited state lifetime, linear viscosity dependence over a wide viscosity range and no or negative temperature dependence of the reaction rate. The wavelength dependence of the ground state recovery experiment reported earlier (Åkessonet al 1986,Chem. Phys. Lett. 126 385) has been shown to be the result of dependence mainly on the analyzing light. We believe that this molecule can be a representative of the barrierless reaction type (E₀ < 0) and that the probe wavelength dependence in the GSR experiment is due to the fact that different spectroscopic techniques may probe different physical events in the case of barrierless reactions, and suggest that it is a result of stimulated emission in combination with the resolution of the movement of the population on the excited state surface.