Synthesis, properties and quantum chemical evaluation of solvatochromic pyridinium-phenyl-1,3-thiazol-4-olate betaine dyes

The syntheses and characterizations of four zwitterionic betaines are presented. These dyes possess an uncommon heterocyclic 1,3-thiazol-4-olate donor moiety. The natures of the HOMO/LUMO transitions and of the intramolecular charge-transfer state were assigned with the help of quantum chemical calculations. Multiple intermolecular solute/solvent interactions were discussed using linear solvation energy relationship (LSER) with Kamlet–Taft and Catalán parameters. The dyes show a pronounced negative solvatochromism ranging from λmax 392 nm in TFE to 820 nm in THF (Δλ=428 nm or 1.65 eV). The X-ray structures of one of the tetrafluoroborate salts and of one of the betaines are discussed.