Effect of solvent density and species on static and dynamic fluorescence Stokes shifts of coumarin 153

Solvent density dependence of the absorption and fluorescence spectra of coumarin 153 in argon (Ar), ethane (C2H6), carbon dioxide (CO2), and trifluoromethane (CF3H) has been studied at 323.2 K over a wide density region up to rho(r) similar to 3, where rho(r) is the reduced density by the critical density (rho(c)) of the solvent (rho(r) equivalent to rho/rho(c)). By analyzing the line shape functions of the absorption and fluorescence spectra, the first moment and the full width of the half maximum (FWHM) have been determined. Both the free energy difference (Delta G) between the ground and the S-1 states and the fluorescence Stokes shift (Delta nu), evaluated by the first moments of the absorption and fluorescence line shape functions, show larger shift to the values in vapor in the order of Ar, C2H6, CO2, and CF3H if compared at the same rho(r). There are three characteristic density regions on the density dependence of Delta G and Delta nu; these values show larger dependence in the lower-density region (rho(r) < 0.5) and the higher-density region (rho(r) > 2), than that in the medium-density region. The solvent density and species dependence of the FWHM of the absorption spectrum is quite similar to that of Delta nu. The density dependence of Delta nu and FWHM in Ar and C2H6 indicates that the intramolecular reorganization energy decreases with increasing the solvent density, while the effect is small. We have estimated the solvent reorganization energy (lambda(S)) in CO2 and CF3H by assuming no contribution of lambda(S) in C2H6 and Ar. We have demonstrated that the dielectric continuum model makes a good prediction on the relation between Delta G and lambda(S), while it is poor on their density dependence. The invariance of Delta G and lambda(S) in the medium-density region can be interpreted as the density saturation around the solute molecule. In the lower-density region (rho(r) < similar to 0.5), the way of the solvation is suggested to be different from those in the medium- and high-density regions. The lifetime of S-1 shows a large dependence on the solvent density and species in the very low-density region (rho(r) similar to 0.05). We have also found that slow solvation dynamics exists in the lower-density region of CF3H. The origin of the dynamics is discussed in relation with the static Stokes shift. (C) 1999 American Institute of Physics. [S0021-9606(99)01036-3].