PROBING PREFERENTIAL SOLVATION AND ION AGGREGATION WITH CHARGE-TRANSFER TRIPLET-STATES OF AROMATIC AMINO-NITRO COMPOUNDS

The charge transfer triplet states of p-aminonitroterphenyl (PANT), p- aminonitrobiphenyl (PANE) and aminonitrofluorene (ANF) were used as pr obes of preferential solvation and ion clustering in a variety of bina ry media. The excited states of the above molecules are characterized by very large static dipole moments (''giant dipoles''). As a result, the corresponding transient absorption spectra are highly sensitive to the local polarity of the environment and are very well suited to the study of solvation and association in mixed solvents and solutions of electrolytes. The spectroscopic study of the dynamics and energetics of association of charge-separated species with chemically inert elect rolytes has been extended to non-polar solvents. Whereas in tetrahydro furan (THF) the association process is dominated by ion pairs of the e lectrolyte, in non-polar solvents the salt is present in the form of l arger clusters consisting of several ions. The degree of aggregation s trongly depends on the identity of the ions. These results find suppor t in early cryoscopic and conductometric studies. Solvent mixtures con sisting of a non-polar component and a strongly polar cosolvent were i nvestigated over a range of concentrations and temperatures. A non-lin ear dependence of the free energy of solvation on the concentration of the polar component was observed in all instances, indicating that po lar enrichment in the vicinity of the probe had occurred, The non-line arity cannot be fully accounted for by the simple dipole-dipole intera ction model, marking the importance of more specific interactions betw een the probe and molecules of the polar component. The transient spec tra of the probes exhibit thermochromic behavior in all the solvent mi xtures studied, whereas analogous spectra in neat solvents are virtual ly independent of temperature. Thermochromic shifts approaching 50 cm( -1) K-1 were measured in toluene-dimethylsulfoxide mixtures. (C) 1997 Elsevier Science S.A.