Absorption and Fluorescence Studies of Acridine in Subcritical and Supercritical Water

The isobaric acid/base equilibrium between acridine and the acridinium cation was measured from ambient temperature to 380 °C (above the critical temperature of water, T c = 374 °C) using absorption spectroscopy. At 3500 psia, the isobaric protonation of acridine is shown to be exothermic up to approximately 315 °C. Above 315 °C, protonation becomes endothermic due to changes in the dielectric constant of water with temperature, which has a profound influence on the solvation of ions. The results are interpreted using a modified Born equation to account for the temperature-dependent changes in acridinium cation and proton solvation. The absorption and fluorescence spectra and the fluorescence lifetime of acridine are sensitive to changes in solvent−solute hydrogen bonding. Hydrogen bonding between acridine and water is observed to decrease from ambient temperature to the critical temperature. A relatively rapid change in hydrogen bonding occurs between 100 and 200 °C.