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

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(1H)‐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 (1H 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‐d6) and, if soluble in deuterated chloroform (CDCl3) and compared. The results reveal that a decrease in solvent polarity (upon changing the solvent from DMSO‐d6 to CDCl3) 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 CDCl3 and DMSO‐d6 indicate that the dipole moments of the Z configurations are significantly higher than those of the E structures.