Temperature dependence of anisotropy decay and solvation dynamics of coumarin 153 in γ-cyclodextrin aggregates

Effect of temperature on the fluorescence anisotropy decay and the ultraslow component of solvation dynamics of coumarin 153 (C153) in a gamma-cyclodextrin (gamma-CD) nanocavity are studied using a picosecond set up. The steady-state anisotropy (0.13 +/- 0.01) and residual anisotropy (0.14 +/- 0.01) in fluorescence anisotropy decay in an aqueous solution containing 7 mu M C153 and 40 mM gamma-CD are found to be quite large. This indicates formation of large linear nanotube aggregates of gamma-CD linked by C153. It is estimated that > 53 gamma-CD units are present in each aggregate. In these aggregates with rise in temperature, the average solvation time (<tau(s)>(obs)) decreases markedly from 680 ps at 278 K to 160 ps at 318 K. The dynamic Stokes shift is found to decrease from 800 cm(-1) at 278 K to 250 cm(-1) at 318 K. The fraction of dynamic Stokes shift (f(d)) detected in a picosecond set up is calculated using the Fee-Maroncelli procedure. The corrected solvation time (<tau(s)>(corr) = f(d)(<tau(s)>(obs)) displays an Arrhenius type temperature dependence. From the temperature variation, the activation energy and entropy of the solvation process are determined to be 12.5 kcal M-1 and 28 cal M-1 K-1, respectively. The ultraslow component and its temperature dependence are ascribed to a dynamic exchange between bound and free water molecules.