Synthesis and electronic spectra of novel merocyanine dyes bearing a maleimide ring incorporated into the methine chains

The present article describes the synthesis of new polymethine dyes with a heterocyclic ring incorporated into the methine chain. 1-Alkyl-2 or 4-methylpyridinium salts and related compounds 1a-e reacted at the exocyclic double bond with cyano- or methoxycarbonyl heterocycles bearing a methylthio group 2a-d to give new polymethine dyes 3a-n in good yield. This reaction occurred by an addition-elimination mechanism, involving nucleophilic attack of cyclic enamines at carbons deficient atoms on the heterocycles followed by elimination of methylmercaptan. The polymethine dyes obtained are red, violet, and blue in color with the, absorption peaks at 524-614 nm. Semi-empirical as well as ab-initio quantum chemical calculations were used to theoretically characterize their pi-pi* absorption maxima in the visible region Which plays-a decisive role in their color appearances. (C) 2002 Elsevier Science Ltd. All rights reserved.