Synthesis and characterization of a new class of unsymmetrical squaraine dyes

By starting from several types of nucleophilic compounds XH, such as electron-rich ethylenic, aromatic, or heteroaromatic compounds, and either sqaric acid dichloride QCl(2) or dialkyl squarates Q(OR)(2) as activated squaric acid derivatives, some new semisquaric acid derivatives QXOH have been prepared. These compounds condense with a further nucleophilic compound YH to yield unsymmetrically substituted squaraines XQY whose analytical and spectroscopic data are recorded. The squaraines XQY are deeply coloured, strongly solvatochromic compounds whose longest-wavelength absorption maxima are hypsochromic shifted in comparison to the ones of the corresponding symmetrically substituted squaraines XQX and YQY. The shift can be correlated with the donor strength of the nucleophilic compounds XH or YH used as educts and, hence, with their reactivity in the condensation reaction with squaric acid and its derivatives. (C) 2001 Elsevier Science Ltd. All rights reserved.