Solvatochromism and electrophilicity

► The electrophilicity index ω is a useful predictor of the solvatochromic behaviour of a dye. ► As polarity increases, ω decreases/increases for a negative/positive solvatochromism. ► For different media, the electrophilicity varies linearly with the dye solvation energy. ► Negative dyes behave as hard, zwitterionic, positive dyes as soft, non-polar electrophiles. ► Ten examples of phenolate betaine dyes illustrate the protocol.