Abstract
6-Methoxyquinoline demonstrates proton transfer in the lowest excited singlet state in acetonitrile-water mixtures as well as in pure water. The photohydrolysis reaction of the directly excited neutral molecule is strongly solvent dependent and independent of solution alkalinity. Proton abstraction from the conjugate acid, however, depends predominantly on the alkalinity of the solution and only on the continuum properties of the solvent. The separability of the hydrolysis and proton abstraction reactions, using steady-state methodology, allows the proton abstraction to be treated independent of the hydrolysis. The linear relationship between the ratio of the relative fluorescence efficiencies of base and conjugate acid and hydroxide ion concentration is obtained only if proper Brønsted activity factors are included in the relationship. These factors can be calculated from classical electrostatics and are simple powers of the activity coefficients necessary to convert the formal hydroxide ion concentration to formal hydroxide ion activity. Using this approach it is possible to calculate pOH from hydroxide ion concentration in acetonitrile-water solutions containing a mole fraction of acetonitrile up to 0.13.
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