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 (phi(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 (eta) and a contribution of dielectric friction to the total friction. One dicarbocyanine has a substick behavior at low eta and reaches the stick limit at high eta. The other dicarbocyanines follow the stick regime in the whole viscosity range. The dependence of phi(r) on eta is compared with literature values of the isomerization rate constant's (k(iso)) dependence on eta. Both processes involve large movements of molecular fragments of similar sizes, which are influenced by the solvent viscosity. However, while phi(r) is proportional to eta, 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 phi(r) arise in the different time scales in which the molecule experiences the solvent friction opposite to the motion.