Synthesis,Structure,and Solvatochromism of Merocyanine Dyes Based on Barbituric Acid

摘要

Di-,tetra-,and hexamethinemerocyanines were synthesized on the basis of barbituric and 1,3-dimethylbarbituric acids and heterocycles characterized by a strong (1-benzyl-1,2-dihydrobenzo[cd]indole),medium (1,3-dihydro-1,3,3-trimethyl-2H-indole and 3-ethyl-2,3-dihydro-1,3-benzothiazole),and strong electron-donor power (1,3-diphenyl-2,3-dihydro-1H-benzimidazole).Their solvatochromic properties were analyzed using both conventional parameters (absorption maxima and molar absorption coefficients) and centroids,oscillator strengths,widths,asymmetry and excess coefficients,and fine structure of the long-wave absorption bands,which were determined by mathematical processing (method of moments).A combination of these parameters unambiguously characterizes limiting electronic states of merocyanines (neutral polyene,polymethine,or charged polyene) responsible for the type of solvatochromism.Relations were found between solvatochromic properties,on the one hand,and donor-acceptor power of the terminal groups,length of the polymethine chain,and solvent nature,on the other.The difference in solvatochromism (up to reversal of sign) of merocyanines derived from barbituric acid and their analogs based on 1,3-dimethylbarbituric acid originates from both the difference in their electronic asymmetry and the ability of the NH groups in the former to participate in hydrogen bonding with proton-acceptor solvents.