Abstract:

Steady-state fluorescence anisotropy was measured for a series of carbocyanines in normal primary aliphatic alcohols as a function of temperature. From these values, rotational correlation times (φr) were obtained. They were compared with the ones calculated by modeling each molecule by a prolate rotor or an asymmetric rotor, under stick or slip boundary conditions for rotation. The two types of rotor render very similar results. The smaller carbocyanines show a stick rotation regime at low viscosity (η) and a contribution of dielectric friction to the total friction. One dicarbocyanine has a substick behavior at low η and reaches the stick limit at high η. The other dicarbocyanines follow the stick regime in the whole viscosity range. The dependence of φr on η is compared with literature values of the isomerization rate constant's (kiso) dependence on η. Both processes involve large movements of molecular fragments of similar sizes, which are influenced by the solvent viscosity. However, while φr is proportional to η, kiso shows a different behavior. The analysis of the data obtained for different carbocyanines in primary alcohols allows us to conclude that the different viscosity dependencies of kiso and of φr arise in the different time scales in which the molecule experiences the solvent friction opposite to the motion. © 1995 American Chemical Society.