Visualizing overdamped wavepacket motion: Excited-state isomerization of pseudocyanine in viscous solvents

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 S1 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 S1. The dependences of the bond-twisting rates on solvent viscosity are different for the steep and flat parts of the S1 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.