AMINE QUENCHING OF AROMATIC FLUORESCENCE AND FLUORESCENT EXCIPLEXES

Quenching of the fluorescence of naphthalene, a number of naphthalene derivatives, and fluorene by tertiary amines has been studied. In most cases fluorescent exciplexes are produced, although 1,4-diazabicyclo[2.2.2]octane (Dabco) quenches both naphthalene and fluorene very efficiently without producing any exciplex emission. Stern-Volmer constants for quenching, quantum yields and lifetimes for exciplex fluorescence, and shifts of exciplex emission with structural change and solvent properties have been measured. Most of the results can be rationalized qualitatively in terms of structures of the exciplexes involving extensive charge transfer from the amines to the aromatic fluors. Substituent effects on exciplex emission frequencies have been analyzed semiquantitatively as have solvent effects on emission frequencies. Both approaches indicate that the ionic character of the exciplexes is high, but less than 100%. Shifts in emission frequency are well correlated as a function of dielectric constant and polarizability for most solvents, although unexpectedly large red shifts are observed in 1,4-dioxane, benzene, and toluene. In the case of dioxane the anomaly is attributed to submolecular dipole-dipole interaction, and, in the case of the aromatic solvent, to the fact that internal pressures are higher than for aliphatic solvents. The data for mixed solvents consisting of cyclohexane with dioxane or tetrahydrofuran indicate preferential solvation of the exciplexes by the ethers. © 1978, American Chemical Society. All rights reserved.