M. Levitus et al., ROTATIONAL RELAXATION OF CARBOCYANINES - COMPARATIVE-STUDY WITH THE ISOMERIZATION DYNAMICS, Journal of physical chemistry, 99(39), 1995, pp. 14231-14239
Steady-state fluorescence anisotropy was measured for a series of carb
ocyanines in normal primary aliphatic alcohols as a function of temper
ature. From these values, rotational correlation times (phi(r)) were o
btained. They were compared with the ones calculated by modeling each
molecule by a prolate rotor or an asymmetric rotor, under stick or sli
p boundary conditions for rotation. The two types of rotor render very
similar results. The smaller carbocyanines show a stick rotation regi
me 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 dicarbocyanine
s follow the stick regime in the whole viscosity range. The dependence
of phi(r) on eta is compared with literature values of the isomerizat
ion rate constant's (k(iso)) dependence on eta. Both processes involve
large movements of molecular fragments of similar sizes, which are in
fluenced by the solvent viscosity. However, while phi(r) is proportion
al 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 p
hi(r) arise in the different time scales in which the molecule experie
nces the solvent friction opposite to the motion.