Theoretical MO study on the electronic spectra of novel merocyanine dyes bearing maleimide ring system

Abstract

Theoretical MO study is presented on the electronic spectra of new polymethine dyes with a heterocyclic ring incorporated into the methine chain. 1-Alkyl-2 or 4-methylpyridinium salts and related compounds reacted at the exocyclic double bond with cyano- or methoxycarbonyl heterocycles bearing methylthio group in the presence of potassium carbonate in dimethyl sulfoxide at room temperature to give new polymethine dyes in good yields. This reaction occurred by an addition-elimination mechanism, involving nucleophilic attack of cyclic enamines at carbon deficient atoms on heterocycles followed by elimination of methylmercaptan. These polymethine dyes are red, violet, and blue in color and appeared at 450-615 nm in absorption spectra in ethanol. Some MO calculations including INDO/S, RPA, TDDFT etc. were performed to investigate their electronic spectra and fairly good agreements were obtained between experimental and calculated their absorption maxima.