Ester‐Aroyl‐S,N‐Ketene Acetals with Solid‐State Luminescence: AIEgens from Sequential Three‐Component Desymmetrization

Di(hetero)aroyl dichlorides are desymmetrized upon sequential reaction with alcohols and 2-methyl N-benzyl thiazolium salts within the course of a one-pot three-component reaction yielding ester-substituted aroyl-S,N-ketene acetals under mild conditions in good yields. A prerequisite for the concise one-pot process is the different nucleophilicity of the alcohols and in situ generated S,N-ketene acetals. The resulting compounds are merocyanines with dominant charge-transfer absorption bands which are fluorescent in the solid state, but not in solution. In water/ethanol solvent mixtures of increasing water content, the water-insoluble dyes display typical aggregation-induced emission (AIE) characteristics. The water fraction inducing AIE as well as the emission color, and fluorescence quantum yield (Φ_f) of the aggregated dyes can be controlled by the alcohol part of the ester moiety. Encapsulation into polystyrene nanoparticles can lead to a considerable increase of the fluorescence quantum yield Φ_f to 30% as shown for a representatively chosen dye revealing the highest Φ_f of 11% within the dye series in the water/ethanol mixtures and enabling the usage of these dyes as fluorescent reporters in aqueous environments.