The positive halochromism of phenolate dyes in hydroxylic solutions of tetraalkylammonium cations

By contrast with the negative halochromic behaviour shown by phenolate betaines in the presence of alkaline and alkaline-earth cations, the addition of tetraalkylammonium salts to hydroxylic solutions of these dyes generate bathochromic shifts of their charge-transfer band. This positive halochromic behaviour by organic cations was examined systematically and its origin rationalized by nonspecific changes of the medium permittivity, and by specific dye–cation interactions in solution.

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► The halochromic behaviour of tetraalkylammonium salts was investigated in hydroxylic solvents.
► The observed positive halochromism depended on the solvent, and on the dye and cation structures.
► The effect was rationalized by invoking non-specific and/or specific solute–solvent interactions.
► Non-specific effects originate from a decrease of the medium permittivity by addition of the organic salt.
► Specific effects originate from dye–cation interactions, and were simulated with theoretical calculations.