ASSESSMENT OF DIELECTRIC ENRICHMENT AROUND TWO FLUOROPHORES IN BINARY SOLVENTS

Abstract

Abstract We investigated dielectric enrichment, or the preferential interaction between a probe and a polar solvent, using two different fluorophores in binary solvents that will not form strong hydrogen bonds with the probe. The first probe, 6‐propionyl(N,N‐dimethylamino)naphthalene (PRODAN), undergoes a large change in dipole moment in the excited state. We found that its emission energy varies linearly with the dielectric parameter f(D) [f(D) = 2(D ‐ 1)/(2D + 1)] in acetone/benzene and acetonekyclohexane mixtures. The emission of the second probe, N‐acetyl tryptophanamide (NATA), also varies linearly with f (D) in ethylether/acetonitrile solvents. This lack of preferential solvation was further investigated by studies of NATA in acetonitrile/methanol mixtures where hydrogen bonding is possible. We found that NATA‐methanol interactions are not energetic enough to give rise to significant preferential solvation. Since preferential solvation does not occur even for weakly hydrogen bonding solvents it is thus highly unlikely that dielectric enrichment, which takes place in the absence of specific interactions, occurs with any frequency. We postulate that previous reports of dielectric enrichment were due to a lack of consideration of changes in f (D) of the solvent.