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

Abstract

Effect of temperature on the fluorescence anisotropy decay and the ultraslow component of solvation dynamics of coumarin 153 (C153) in a γ -cyclodextrin (γ -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 µ M C153 and 40 mM γ -CD are found to be quite large. This indicates formation of large linear nanotube aggregates of γ -CD linked by C153. It is estimated that >53 γ -CD units are present in each aggregate. In these aggregates with rise in temperature, the average solvation time (τ_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 (τ_{s corr}=f_d(τ_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.