Synthesis and characterization of a group of new medium responsive non-symmetric viologens. Chromotropism and structural effects

A group of new non-symmetric viologens was synthesized and characterized spectrally using various techniques. These viologens contain active methylene moieties being capable to act as hydrogen bond donors (HBD) and interact in solution with various solvents, the latter acting as hydrogen bond acceptors (HBA). These specific solute-solvent interactions result in intense coloration of the resulting solutions. The chromotropic behavior of these compounds is triggered by basic solvents, and no significant change in color is observed in poorer HBA-solvents, such as several alcohols. The dependence of the color of the solutions on the polarity parameters of solvents (solvatochromism) was investigated. Solvent polarity parameters expressing both dipolarity/polarizability and basicity of solvents have been used. Finally the correlation of the chromotropic behavior of these compounds with their structure is examined, using both experimental data and Hammett equation.

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► We report the synthesis and characterization of new non-symmetric viologen dyes.
► These viologens present a unique chromotropic behavior triggered by HBA-solvents.
► Solvatochromic species are produced by specific solute-solvent interactions.
► Their medium responsive behavior is quantified and dipole moments are determined.
► The structure–properties relation is studied using 13C-NMR and Vis spectral data.