Picosecond Solvation Dynamics in Nanoconfinement: Role of Water and Host-Guest Complexation

The dynamics of solvation of an excited chromophore, 5-(4-dimethylaminophenyl)-2-(4'-sulfophenyl)oxazole, sodium salt (DMO), has been explored in confined nanoscopic environments of beta-cydodextrin (beta CD) and heptalris(2,6-di-O-methyl)-beta-cyclodextrin (DIMEB). Solvation occurs on a distinctly slower time scale (tau(3)(s) similar to 47 ps, tau(4)(s) similar to 517 ps)) in the host cavity of DIMEB than in that of beta CD (tau(3)(s) similar to 20 ps, tau(4)(s) similar to 174 ps). The calculated equilibrium solvation response of DMO was characterized by four relaxation components (tau(1)(s) similar to 0.46-0.48 ps, tau(2)(s) similar to 3.2-3.4 ps, tau(3)(s) similar to 32.3-37.7 ps, and tau(4)(s) similar to 232-485 ps), of which the longer ones (tau(3)(s), iota(4)(s)) are well-consistent with experiments, whereas the ultrafast components (iota(1)(s), iota(2)(s)) are unresolved. The observed time constant (tau(3)(s)) within the similar to 20-47 ps range arises from slow water molecules in the primary hydration layers of the host CDs and is slower for DIMEB than for beta CD presumably due to longer-lived and stronger hydrogen bonds that water forms with the former host relative to the latter. Decomposition of the calculated solvation response of DMO has revealed that conformational fluctuations of the macrocyclic hosts give rise to the observed long-time relaxation component (tau(4)(s)), which is much slower for the inclusion complexes with DIMEB than for those with beta CD because of slower conformational dynamics of the former host than that of the latter.