Solvatochromic, computational chemical and E/Z geometrical isomerism studies on some aryl‐hydrazones synthesised by reacting of aryldiazonium chlorides with 6‐butyl‐4‐hydroxyquinolin‐2(1H)‐one

Abstract A series of 3‐(2‐aryl‐hydrazone)‐quinolin‐2,4‐diones were synthesised by the reaction of aryldiazonium chlorides with 6‐butyl‐4‐hydroxyquinolin‐2(1 H )‐one dissolved in water containing equimolar amount of sodium hydroxide. All the reactions were carried out at 0–5°C and the corresponding hydrazones were obtained in satisfactory yields and purified easily by recrystallisation from hot dimethylformamide. According to proton nuclear magnetic resonance ( 1 H NMR) and ultraviolet‐visible (UV‐visible) results, there are two kinds of intramolecular hydrogen bonds in these synthesised aryl‐hydrazones which enables the compounds to be rotated about the hydrazone C=N bond and through that leads to a reversible isomerisation between their E and Z configurations. The isomers populations and the rotation phenomenon can be controlled by changing the polarity of the solvents. The E/Z ratio of each synthesised compound was studied in deuterated dimethyl sulfoxide (DMSO‐ d 6 ) and, if soluble in deuterated chloroform (CDCl 3 ) and compared. The results reveal that a decrease in solvent polarity (upon changing the solvent from DMSO‐ d 6 to CDCl 3 ) shifts the isomeric balance to Z ‐geometrical form and substantially increase the E/Z ratio. A density functional theory (DFT) analysis regarding highest occupied molecular orbital (HOMO), HOMO −1, lowest unoccupied molecular orbital (LUMO), and LUMO +1 has been conducted and discussed. The results obtained from DFT calculations at B3LYP/6‐31G(d) level of theory, demonstrated that the hydrazone tautomeric forms are more stable with the lowest total energy and showed a good agreement with the experimental findings in both gas and solution states. As it is expectable, DFT calculation in CDCl 3 and DMSO‐ d 6 indicate that the dipole moments of the Z configurations are significantly higher than those of the E structures.