Synthesis, X-ray crystal structure and fluorescent spectra of novel pyrazolo[1,5-a]pyrazin-4(5H)-one derivatives

A series of fluorescent compounds, containing pyrazolo[1,5-a]pyrazin-4(5H)-one moiety, were designed and synthesized from ethyl 1-(2-oxo-2-phenylethyl)-3-phenyl-1H-pyrazole-5-carboxylates. The structures of the compounds have been confirmed by IR, 1H NMR, HRMS and X-ray crystal diffraction. The optical properties of the compounds were investigated by UV–vis absorption and fluorescence spectroscopy. The effect of pH on the UV–vis absorption of compound 2a in methanol–H2O solutions was studied and interpreted by theory calculation. The pKa value of compound 2a was determined by the absorption spectra.

Graphical abstractA series of fluorescent compounds, containing pyrazolo[1,5-a]pyrazin-4(5H)-one moiety, were designed and synthesized from ethyl 1-(2-oxo-2-phenylethyl)-3-phenyl-1H-pyrazole-5-carboxylates. The structures of the compounds have been confirmed by IR, 1H NMR, HRMS and X-ray crystal diffraction. The optical properties of the compounds were investigated by UV–vis absorption and fluorescence spectroscopy. The effect of pH on the UV–vis absorption of compound 2a in methanol–H2O solutions was studied and interpreted by theory calculation. The pKa value of compound 2a was determined by the absorption spectra.Figure optionsDownload full-size imageDownload as PowerPoint slideHighlights• Novel pyrazolo[1,5-a]pyrazin-4(5H)-ones were prepared and fully characterized. • X-ray crystal structure of compound 2c was reported. • UV–vis absorption and fluorescence spectroscopy of all compounds were measured. • Influence of pH on the structure of compound 2a in solution was examined. • Ionization constant of compound 2a was determined by spectrophotometric method.