New arylhydrazones of β-diketones and their optical and thermal properties

New arylhydrazones of β-diketones, 2-(2-(4,4-dimethyl-2,6-dioxocyclohexylidene)hydrazinyl) benzoic acid (1) and (Z)-ethyl-2-(2-(2-nitrophenyl)hydrazono)-3-oxobutanoate (2), have been synthesized by reaction of activated β-diketones with the corresponding aryldiazonium salts, and characterized by IR, 1H and 13C NMR spectroscopies and elemental analysis. In DMSO solution, 1 is stable exclusively in the hydrazone form, while 2 exists in DMSO and H2O solutions as a mixture of enol–azo and hydrazone tautomeric forms. In the latter case, an increase in the solvent polarity shifts the tautomeric balance to the enol–azo form. Solvatochromic, thermal and photoluminescent properties of 1, 2 and the known related 2-(2-(2,4-dioxopentan-3-ylidene)hydrazinyl)benzoic acid (3), (2-(2-(2,4-dioxopentan-3-ylidene)hydrazinyl)phenyl)arsonic acid (4), (Z)-2-(2-(1,3-dioxo-1-phenylbutan-2-ylidene)hydrazinyl)benzoic acid (5) and 5-(2-(2,4-dioxopentan-3-ylidene)hydrazinyl)-2,3-dihydrophthalazine-1,4-dione (6) were studied. The obtained UV–vis absorption spectra confirm that εmах depends on the solvent and on the nature of AHBD, but no clear correlation was found. Upon heating, 1–3, 5 and 6 are stable up to 474 K and decompose in two steps whereas 4 decomposes in three steps starting from 343 K. The kinetic parameters of the thermal decomposition were estimated suggesting a low rate of the thermal decomposition. Compounds 2 and 3 exhibit luminescence with intense emission bands with maxima at λem of 466.0 (λex = 318 nm) and 532.5 nm (λex = 315 nm), respectively, while compounds 4 and 6 show low-intensity bands at λem of 501 (λex = 396 nm) and 503 nm (λex = 389 nm). Compounds 1 and 5 display no photoluminescence.