Influence of Confinement on the Solvation and Rotational Dynamics of Coumarin 153 in Ethanol

Abstract

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 Å 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 Å. 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.

Simulations of time-dependent fluorescence in nano-confined solvents

“…16 For C153 ͑a neutral molecule͒ the absorption spectrum was unchanged and the fluorescence redshifted between 25 and 50 Å pores. 17 The results for the neutral C153 molecule are consistent with our simulations; spectral shifts for charged solutes such as nile blue can display distinctly different properties. 52 For both molecules they found S(t) exhibited a triexponential decay ͑they could not resolve any subpicosecond component to the dynamics͒ in both bulk and confined solutions; the fastest components were on the time scale of a few picoseconds while the slower ones were tens of picoseconds, the longest being ϳ100 ps.…”

“…16,17 They measured the steady-state spectra and time-dependent Stokes shift of nile blue and Coumarin 153 ͑C153͒ in ethanol inside the sol-gel pores. In the case of nile blue ͑a cationic dye with a negative counterion͒ they found a small blueshift in the steady-state absorption spectrum and a redshift in the fluorescence spectrum with increasing pore diameter ͑50 vs 75 Å͒.…”

“…46 Such measurements have been carried out by Levinger and co-workers in a wide variety of reverse micelles, [9][10][11][12][13][14][15] Bhattacharyya and co-workers in reverse micelles, vesicles, solgels, and zeolites, 4 -8 Baumann et al in sol-gels, 16,17 and a number of other groups. 18 -25 In paper I we found that a chromophore with a charge transfer transition in a hydrophobic spherical nanometer-scale cavity displays different trends in the steady-state absorption and fluorescence spectra with cavity size.…”

Structural transition in alcohol-water binary mixtures: A spectroscopic study

“…Also, the longest time constant (τ L ) of the multi-exponential decay of emission intensity collected at magic angle (54⋅7°) compares well with the values reported by these groups. 62,67,68 All these evidences support the conclusion that the observed bi-exponential decay of the anisotropy of C153 in these binary mixtures is rather generic in nature and might therefore not be associated with any impurity in the probe or misalignment in the measurement system. Also note that the shortest life time obtained from the fit of the magic angle decay for C153 in ethanol is ~70 ps (table 2) which is at least 4 times greater than the fastest component in the anisotropy decay (figure 5, left lower panel, filled circles).…”

Solute location in a nanoconfined liquid depends on charge distribution

“…The focus is on examining how a change in the charge distribution of a solute molecule affects its position. This system was the subject of a previous experimental study, 25 but information about the solute position was not obtained. The C153 molecule possesses the properties required to model a charge transfer process as discussed above.…”