AN ISOMERIZATION REACTION OF A CYANINE DYE IN NORMAL-ALCOHOLS - MICROSCOPIC FRICTION AND AN EXCITED-STATE BARRIER CROSSING

The isomerization reaction of a cyanine dye, N,N'-diethyl-3,3'-tetramethylindocarbocyanine iodide (DiIC(2)), in eight n-alcohols at several temperatures was studied by using fluorescence lifetime measurements. In solutions from methanol to hexanol, hydrodynamic viscosity and a constant barrier seems to control the reaction. In long-chain solvents the constant-viscosity plots (n > 4) show nonlinear behavior, which becomes more apparent at high temperatures and low viscosities. The results suggest that when the size of the surrounding solvent molecule approaches the size of the isomerizing group the concept of hydrodynamic friction breaks down. Rotational molecular friction model for the reaction shows linear and solvent dependent eta/T dependencies of the nonradiative lifetimes. Correcting for the relative volumes of the solvent and the solute allows fitting of all the observed reduced rates by using a constant barrier of isomerization. The diffusion limit of Kramer's theory and a nonlinear boundary condition was used to account for the rates observed at high values of friction. It seems that non-Kramers' behavior at high viscosities is related to frequency-dependent frictional effects, which become important when solvent motion as compared to the reaction rate becomes slow.