New β-diketone-containing styrenic monomers and their polymers: Synthesis, keto-enol tautomerism and related fluorescence behavior

Three new β-diketone-containing styrenic monomers and their polymers were synthesized. The phenol and naphthol groups, and the electron-donating N,N-dimethylaniline groups were covalently attached to the β-diketone monomers at a designed position. The keto-enol tautomerism was characterized by 1H, 13C NMR and UV-vis absorption spectroscopy. It was found that the β-diketone monomers exist in three forms, i.e. two cis-enol forms and one keto form. Their relative contents were determined by NMR spectroscopy. The β-diketone monomers bearing the phenol (1) and naphthol (2) groups display photoinduced enolization during UV irradiation due to the formation of intramolecular hydrogen bonds between the phenolic or naphtholic hydroxyl groups and the carbonyl groups. For their polymers and copolymers, however, photoinduced ketonization occurred during UV irradiation. The β-diketone monomer bearing N,N-dimethylaniline group (3) is a novel charge transfer fluorophore, which can be potentially employed as a dual-purpose functional monomer.