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A theoretical approach is described t o rationalize and predict the solvatochromic behaviour of vinylogous y-pyridones. The theoretical curves obtained are shown t o reproduce the experimental UV-VIS spectroscopic behaviour of dyes 1, 2, 3 and 5. The model reconciles previous conflicting reports on the solvatochromism of these and related dyes.Solvatochromic dyes generally exhibit steady bathochromic (positive solvatochromism) or hypsochromic shifts (negative solvatochromism) in solution, as polarity of the medium is increased. Some dyes may change their behaviour in solution with a change of solvent, and revert the sign of their solvatochromic shifts at a given polarity. Compound 1 is generally Paper 4/05332J

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