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In this work, the temperature dependence of the barrierless cis-trans isomerization reaction in the excited state of 1,1′-diethyl-4,4′-cyanine (1144-C) has been investigated. Transient absorption kinetics as well as transient absorption spectra were measured at different temperatures and viscosities, making it possible to obtain dynamic information regarding the isomerization process at constant viscosity. Our experimental results suggest a crossover of the reaction rate from a negative temperature dependence (decreasing relaxation rate with increasing temperature) at low viscosities to a positive temperature dependence at higher viscosities. This crossover of the relaxation rate was observed to occur at ∼17 cP; the range of viscosities studied was from ∼2 to ∼30 cP. We discuss the origin of this turnover behavior in terms of the BFO (Bagchi, Fleming, and Oxtoby) theory and show that the turnover can be explained by this model, as the competition between two processes following the excitation, namely transport of the population on the excited-state surface and internal conversion. We also discuss the validity of using the solvent shear viscosity as a measure of the friction experienced by the isomerizing group. Our results suggest that, at least for 1144-C in short-chain alcohols (methanol to hexanol), the shear viscosity can indeed be used as a reasonable measure of the solvent friction in the barrierless isomerization process.

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The barrierless isomerization of 1,1-diethyl-4,4-cyanine in alcohol solutions has been studied with 75-fs laser pulses in transient absorption experiments. The measured dynamics are compared with Monte-Carlo simulations based on the BFO-theory (Bagchi, B.; Fleming, G .R.; Oxtoby, D.W. J. Chem.Phys. 1983.78: 7375) for condensed-phase barrierless isomerizations, and the viscosity dependence of the long-time exponential decay is well reproduced with a coherent set of parameters. In the most viscous solvents, early-time (::: 1 ps) kinetics are observed which are characteristic of damped wavepacket dynamics in the reaction coordinate.