Syntheses and Excited‐State Intramolecular Proton Transfer of 3‐Hydroxythioflavone and Its Sulfone Analogue

Abstract 3‐Hydroxythioflavone ( 3‐HTF ) was synthesized upon oxidation with selenium dioxide, in contrast to 3‐hydroxyflavone, a prototype molecule undergoing excited‐state intramolecular proton transfer (ESIPT) reaction. Also achieved were the syntheses of the 3‐HTF sulfone analogue and an interesting sulfonyl selane intermediate. Their structures were fully characterized. 3‐HTF showed an ultrafast ESIPT property, as evidenced by >150 fs −1 reaction rate constant and it solely exhibited a proton‐transfer tautomer emission (590 nm) in cyclohexane at room temperature and in a 77 K solid matrix. The lower electronegativity of the sulfur atom in 3‐HTF leads to a significant redshift of the tautomer emission compared with that of 3‐hydroxyflavone. The sulfone analogue showed a lack of ESIPT, mainly owing to the decrease in carbonyl basicity.