Electronic spectral shifts, reorganization energies, and local density augmentation of Coumarin 153 in supercritical solvents

Solvent reorganization energies and local densities of Coumarin 153 (C153) in supercritical C2H6, CO2, and CHF3 (Tc=5 K) are measured using fluorescence spectroscopy. Reorganization energies are 320±70 and 770±70 cm−1 in CO2 and CHF3, respectively – nearly independent of density (0.3≤ρ/ρc≤2.1). The spectral shifts imply similar effective local densities in all three solvents. These effective densities can exceed 3–5 times the bulk density. In CO2 and CHF3, the maximum density augmentation calculated from emission shifts is ∼20% greater than that calculated from excitation shifts. No such difference is found in C2H6.