Effect of substituents on the solvatochromism of stilbazolium merocyanines

Abstract

The visible spectra of a series of merocyanines, derivatives of 4-[(4-oxocyclohexa-2,5-dienylidene)ethylidene]-1,4-dihydropyridine, have been investigated in a number of solvents and solvent mixtures. The effect of substituents on the solvatochromic behaviour of these compounds is described. All investigated merocyanines exhibited a strong negative solvatochromism. Inverse solvatochromism was observed in solvents of very low polarity for polar dyes, and in polar solvents for dyes containing hydrophobic substituents. This inversion of the solvatochromic behaviour is attributed to aggregation phenomena. Keywords: merocyanines, solvatochromism, betaine, aggregation.