Dramatic Pressure-Dependent Quenching Effects in Supercritical CO₂ Assessed by the Fluorescence of 4‘-Dimethylamino-3-hydroxyflavone. Thermodynamic versus Kinetics Control of Excited-State Intramolecular Proton Transfer

Steady-state fluorescence of 4‘-dimethylamino-3-hydroxyflavone (DMA3HF) was observed in supercritical carbon dioxide (scCO₂). Excited-state intramolecular proton transfer (ESIPT) occurs resulting in two well-separated emission bands corresponding to the normal and tautomer forms. As the scCO₂ density exceeds 0.7 g/mL, the relative intensity of the two bands tends to a constant value, comparable to that observed for organic solvents with E_T(30) = 33.0 ± 0.5 kcal/mol, such as toluene and di-n-butyl ether. At lower densities, the substantial decrease of the total fluorescence intensity (a 600-fold decrease as the pressure decreases from 100 to 80 bar) is accompanied by an even more accentuated decrease of the tautomer fluorescence. This can be explained by a shift in the equilibrium between normal and tautomer forms, concomitant with a more efficient quenching of the less solvated fluorophore, that may change the thermodynamic control of the relative population of the two emissive species to a kinetic control.