Aromatic hydrazones derived from nicotinic acid hydrazide as fluorimetric pH sensing molecules: Structural analysis by computational and spectroscopic methods in solid phase and in solution

Abstract Structural analyses of aroylhydrazones were performed by computational and spectroscopic methods (solid state NMR, 1 and 2D NMR spectroscopy, FT-IR (ATR) spectroscopy, Raman spectroscopy, UV–Vis spectrometry and spectrofluorimetry) in solid state and in solution. The studied compounds were N ′-(2,3-dihydroxyphenylmethylidene)-3-pyridinecarbohydrazide ( 1 ), N ′-(2,5-dihydroxyphenylmethylidene)-3-pyridinecarbohydrazide ( 2 ), N ′-(3-chloro-2-hydroxy-phenylmethylidene)-3-pyridinecarbohydrazide ( 3 ), and N ′-(2-hydroxy-4-methoxyphenyl-methylidene)-3-pyridinecarbohydrazide ( 4 ). Both in solid state and in solution, all compounds were in ketoamine form (form I, CO NH N C ), stabilized by intramolecular H-bond between hydroxyl proton and nitrogen atom of the C N group. In solid state, the C O group of 1 – 4 were involved in additional intermolecular H-bond between closely packed molecules. Among hydrazones studied, the chloro- and methoxy-derivatives have shown pH dependent and reversible fluorescence emission connected to deprotonation/protonation of salicylidene part of the molecules. All findings acquired by experimental methods (NMR, IR, Raman, and UV–Vis spectra) were in excellent agreement with those obtained by computational methods.