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A new approximate model of the solvatochromism of merocyanines is presented. In solution merocyanines exist in the electronic ground state as a mixture of three different structures. Since only one longest wavelength band is observed in the UV-visible absorption spectra of such merocyanines, their lowest excited states are roughly assumed to be represented by a single structure. The molar fraction of each of the three structures of merocyanines in the ground state is a function of the solvent polarity. This (VBHB) model has been verified using merocyanines prepared from 5-(hydroxyaryl)-7,7-dimethyl-7H-indolo [ 1,2-a ] quinolinium perchlorates. The analysis of the experimental data suggests that the three ground-state structures of merocyanines are the vinylogous amide (V) and the betaine as the free (B) or hydrogen-bonded (HB) form.

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The fundamental energetic parameters (intercepts and slopes of straight lines approximating energetic levels) of solvatochromic betaines generated by deprotonation of 2‐(4‐hydroxystyryl)‐3H‐indolium salts were determined from solvatochromic data in one‐component solvents. It was demonstrated that the solvatochromism of the investigated dyes can be described by the VBHB (vinylogous amide–betaine–hydrogen‐bonded betaine) physical model. In comparison with similar dyes, the ability to form hydrogen‐bonded solvates is weaker owing to the higher acidity of the dye precursor used. The type of observed solvatochromism was determined. Copyright © 2004 John Wiley & Sons, Ltd.