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

Abstract

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 β-cyclodextrin (βCD) and heptakis(2,6-di- O-methyl)-β-cyclodextrin (DIMEB). Solvation occurs on a distinctly slower time scale (τS3 ∼ 47 ps, τS4 ∼ 517 ps) in the host cavity of DIMEB than in that of βCD (τS3 ∼ 20 ps, τS4 ∼ 174 ps). The calculated equilibrium solvation response of DMO was characterized by four relaxation components (τS1 ∼ 0.46-0.48 ps, τS2 ∼ 3.2-3.4 ps, τS3 ∼ 32.3-37.7 ps, and τS4 ∼ 232-485 ps), of which the longer ones (τS3, τS4) are well-consistent with experiments, whereas the ultrafast components (τS1, τS2) are unresolved. The observed time constant (τS3) within the ∼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 βCD 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 (τS4), which is much slower for the inclusion complexes with DIMEB than for those with βCD because of slower conformational dynamics of the former host than that of the latter.