Resolving the turnover of temperature dependence of the reaction rate in barrierless isomerization

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 similar to 17 cP; the range of viscosities studied was from similar to 2 to similar 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.