engleski

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 probes of preferential solvation and ion clustering in a variety of binary 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 electrolytes has been extended to non-polar solvents. Whereas in tetrahydrofuran (THF) the association process is dominated by ion pairs of the electrolyte, in non-polar solvents the salt is present in the form of larger clusters consisting of several ions. The degree of aggregation strongly depends on the identity of the ions. These results find support in early cryoscopic and conductometric studies. Solvent mixtures consisting of a non-polar component and a strongly polar cosolvent were investigated over a range of concentrations and temperatures. A non-linear dependence of the free energy of solvation on the concentration of the polar component was observed in all instances, indicating that polar enrichment in the vicinity of the probe had occurred, The non-linearity cannot be fully accounted for by the simple dipole-dipole interaction model, marking the importance of more specific interactions between the probe and molecules of the polar component. The transient spectra of the probes exhibit thermochromic behavior in all the solvent mixtures studied, whereas analogous spectra in neat solvents are virtually independent of temperature. Thermochromic shifts approaching 50 cm(-1) K-1 were measured in toluene-dimethylsulfoxide mixtures.

Binary solvent mixtures; Non-polar solutions of electrolyres; Photoinduced charge trnsfer triplet states

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