Electronic Structure of Solvent-Polarity Indicators. Solvatochromism and Tautomeric Epuilibria of Styryl-Merocyanines

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Abstract
Solvatochromic and halochromic behaviour of two unsymmetrical merocyanine-type dyes, namely 2-[2′- and 4′-hydroxystyryl]-pyridinium methiodide, are studied. In alcoholic and hydrogen-bond-acceptor (HBA) solvents their visible absorption spectra exhibit an additional long-wavelength band suggesting the existence of tautomeric equilibria in their solutions. The tautomeric equilibrium constants in triethanolamine-acetone mixed solvents are determined. Quantum mechanical MO method PPP-π-SCF-CI is used to calculate the electronic spectra and the underlying keto-enol-tautomeric stability. Deprotonation of the OH group is shown to provide destabilization in binding energy in excess of 5 eV in agreement with the experimental results indicating that these indicators exist mainly as protonated species (enol form) in their solutions. Furthermore, solvent-induced changes in the ground-state electronic structures are examined by means of 1H-NMR spectroscopy and the PPP calculations.

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