Solvation dynamics of Nile Blue in ethanol confined in porous sol-gel glasses

We report on solvation dynamics measurements of the chromophore nile blue in ethanol confined to sol-gel glasses with 50 Angstrom and 75 Angstrom average pore size and compare them with the dynamics of the respective bulk solution. Both the amplitude of the dynamic Stokes shift as well as the dynamics of the solvation process are drastically changed upon confinement. In both confined solutions the dynamic Stokes shift is reduced by a factor of about 2. As the large majority of the chromophores is adsorbed at the inner pore surfaces the solute molecules interact with only a "half-space" of solvent molecules. In a first approximation this decreases the stabilization energy by a factor of 2. The solvation dynamics in the confined solutions show nonexponential behavior comparable to the bulk. However, the whole solvation process slows down and the single decay time constants characterizing it increase with decreasing pore size of the sol-gel glass. We have introduced two phenomenological models to rationalize this behavior. The enhanced polarization field model takes into account the confinement and predicts a strengthening of the solvent's polarization field in an extended solvation cage induced by the electrical field of the chromophore. The steric hindrance model focuses on the interaction of the liquid with the surface reducing molecular mobility resulting in longer relaxation times. Probably both effects are of relevance in the confined solutions investigated. Confinement affects the steady-state spectra as well leading to a redshift of the absorption and a blueshift of the fluorescence. Additional data on the reorientational dynamics of the chromophore are consistent with the fact that the molecules are predominantly adsorbed at the pore surfaces.