β-Diketones bearing electron-donating chromophores and a novel β-triketone: synthesis and reversible fluorescence behavior

Saturated and unsaturated β-diketones bearing 4-N,N-dimethylaminophenyl substituent and a novel β-triketone were synthesized. These β-diketones exist in both cis-enol and keto forms in solution, and their relative contents were determined by ¹H NMR measurements. In contrast, for the β-triketone, only enol form was observed in a solution. A strong fluorescence quenching was observed for unsaturated diketones as compared with the saturated ones. These β-diketones displayed reversible photoinduced ketonization as revealed by UV-vis spectroscopy. The fluorescence emission of the saturated β-diketones faded gradually during UV irradiation. The fluorescence emission of these saturated β-diketones can thus be reversibly switched between the enol form ‘ON’ and the keto form ‘OFF’ based on keto-enol tautomerization.