Katritzky salt fluorophores: Facile synthesis, bright solid-state emission, and mechanochromic luminescence

Katritzky Salts, readily synthesized from condensation between pyrylium salts and amines, were found to be bright solid-state fluorescent emitters. All these ionic fluorophores exhibited remarkable emission enhancement in the DMSO-H2O mixture with raising water fraction, which was identical to classical aggregation induced emission luminogens (AIEgens). After detailed investigation of their photoluminescence behavior in different solvents, the emission enhancement was attributed to polarity changes of solvent mixtures at low volume fraction of water, and aggregate formation at high volume fraction of water. Single crystal structures of TPP3 had directly indicated that the intermolecular π-π stacking interactions between fluorophores were severely suppressed as a result of their highly twisted structures, which had consequently contributed to the high emission quantum yield in the solid state. Furthermore, owing to intramolecular free rotations within these propeller-shaped ionic luminogens, obvious mechanochromic luminescence behavior was also observed with red-shifted emission after grinding and blue-shifted emission after solvent annealing. [Display omitted]