Rotational relaxation of carbocyanines. Comparative study with the isomerization dynamics

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 (φ_r) 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 φ_r on η is compared with literature values of the isomerization rate constant's (k_iso) dependence on η. Both processes involve large movements of molecular fragments of similar sizes, which are influenced by the solvent viscosity. However, while φ_r is proportional to η, k_iso 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 k_iso and of φ_r arise in the different time scales in which the molecule experiences the solvent friction opposite to the motion.