The effect of pressure and freezing on luminescence. I. 3-hydroxyflavone and indole

The effect of pressure and freezing on the radiative and nonradiative rates from the excited states of 3-hydroxyflavone and indole has been studied. A series of solvents which have a wide range of dielectric constants (1.96–38.3) and which freezes at 295 K in the available pressure range were used. The radiative rates (kr) correlate well with the low frequency dielectric constant of the media (ε). Clear evidence of this correlation is demonstrated at the phase transition where both kr and ε drop sharply in polar solvents while in nonpolar solvents kr and ε stay almost constant. The nonnradiative rates (knr) appear to depend on solute–solvent collisions. As the viscosity of the media increases, the number of these collisions decreases causing a drop in knr. A brief discussion on the tautomerization process of 3-hydroxyflavone is presented in the Appendix.