Deuterium isotope effect on femtosecond solvation dynamics in methyl beta-cyclodextrins.

Deuterium isotope effect on the solvation dynamics and fluorescence anisotropy decay of coumarin 153 (C153) bound to dimethyl beta-cyclodextrin (DMB) and trimethyl beta-cyclodextrin (TMB) is studied using femtosecond upconversion. In D(2)O, there is a marked increase in the steady state emission quantum yield and fluorescence lifetime of C153 bound to DMB and TMB. This suggests strong coupling between C153 and D(2)O inside the cyclodextrin cavity. In D(2)O, average solvation time of C153 in DMB is about 1.7 times slower compared to that in water. For TMB in D(2)O, solvation is 1.5 times slower. The deuterium isotope effect on solvation dynamics at long time arises mainly from the longer excited state lifetime. The longest components of solvation dynamics are ascribed to self-diffusion of C153 out of the cyclodextrin cavity. The nearly 1.5 times slower anisotropy decay of C153 bound to DMB and TMB in D(2)O (compared to H(2)O) is attributed to higher viscosity of D(2)O.