This paper reports on the solvatochromism of a new typical merocyanine dye of stilbazolium betaine. Solvatochromic absorption and fluorescence energy shifts of 2-(4-hydroxystyryl)-l -methylpyridini u m betai ne { 4-[2-( 1methyl-2-pyridinio)vinyl]phenolate), M, in 10 polar solvents are 15.0 and 6.0 kcal mol-' (1 cal = 4.184), respectively.Absorption and fluorescence energies vary rectilinearly with Dimroth-Reichardt's empirical solvent polarity

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The correlation between solvatochromy and the solvent-polarity-dependent rate constant k, of thermal cis-trans isomerization (solvatokinetic behavior) has been investigated for a stilbazolium-type merocyanine and an amphiphilic analogue in a polarity series of eight neat solvents (protic and aprotic). At room temperature k,, varies over 7 orders of magnitude, decreasing with increasing solvent polarity. It has been demonstrated that the first transition energy AE-and the free activation enthalpy AG*, of thermal cis-trans isomerization are linearly correlated with each other and with Dimroth's solvent polarity parameter ET. These observations and the solvent-polarity-dependent increase of absorption band width are explained in terms of a model emphasizing the strong coupling between molecular conformation, electronic charge distribution, and solvent polarization.

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