ASSESSMENT OF DIELECTRIC ENRICHMENT AROUND 2 FLUOROPHORES IN BINARY SOLVENTS

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)naphthalen (PROD AN), undergoes a large change in dipole moment in the excited stare. W e found that its emission energy varies linearly with the dielectric p arameter f(D) [f(D) = 2(D - 1)/(2D + 1)] in acetone/benzene and aceton e/cyclohexane mixtures. The emission of the second probe, N-acetyl try ptophanamide (NATA), also varies linearly with f(D) in ethylether/acet onitrile solvents. This lack of preferential solvation was further inv estigated by studies of NATA in acetonitrile!methanol mixtures where h ydrogen bonding is possible. We found that NATA-methanol interactions are not energetic enough to give rise to significant preferential solv ation. Since preferential solvation does not occur even for weakly hyd rogen bonding solvents it is thus highly unlikely that dielectric enri chment, which takes place in the absence of specific interactions, occ urs with any frequency. We postulate that previous reports of dielectr ic enrichment were due to a lack of consideration of changes in f(D) o f the solvent.