Influence of confinement on the solvation and rotational dynamics of coumarin 153 in ethanol

The solvation and rotational dynamics of the chromophore coumarin 153 in ethanol confined in sol-gel glasses with average pore sizes of 50 and 25 Angstrom have been investigated and compared with the dynamics of the respective bulk solution. The measurements show that no adsorption of the chromophore at the inner pore surface occurs. Nevertheless, the amplitude and the dynamics of the Stokes shift as well as the time-resolved anisotropy are drastically changed upon confinement. The solvation dynamics in the confined solutions show nonexponential behavior comparable to that in 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. However, an increase of the mean rotational diffusion time constant sets in until the pore diameter decreases to less than 50 Angstrom. Two phenomenological models are put forward to rationalize this behavior. The model of a surface solvent layer with changed physical properties such as the viscosity and dielectric constant focuses on the interaction of the liquid with the inner pore surface reducing the molecular mobility and resulting in longer relaxation times. The modified polarization field model takes into account the confinement and predicts a variation of the solvent's reaction field induced by a preferential alignment along the pore walls. Probably both effects are of relevance in the confined solutions investigated. Confinement affects the steady-state spectra and leads to a red shift of the absorption and a blue shift of the fluorescence.