Abstract
The isomerization of 1,1′-diethyl-2,2′-cyanine iodide in n-alcohols is studied by femtosecond pump-probe spectroscopy. Using a broad range of probe wavelengths, the dynamics of ground-state bleach recovery, excited-state absorption and stimulation emission are monitored. We attributed the observed spectral evolution to the excited-state torsional motion, which occurs in a barrierless fashion in all solvents investigated. Excitation initiates a fast downhill bond-twisting motion on steep part of the S 1 potential. The rate-limiting step of the isomerization is the stochastic bond twisting on a relatively flat part of the potential at the bottom of S 1. The dependences of the bond-twisting rates on solvent viscosity are different for the steep and flat parts of the S 1 potential. To the best of out knowledge, this work constitutes the first experimental observation of a process for which the viscosity dependence of the reaction rate changes along the reaction coordinate.
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