Semi-empirical design and synthesis of novel 4-hydroxy-6-sulphonaphthyl azohydroxynaphthalenes with predominant hydrazone form

Due to their enhanced photochromic effects, hydrazone forms of tautomeric azo compounds are commercially preferred as photoreceptors and binary switches. While ortho-hydroxyazo compounds readily exhibit an intramolecular hydrazone re-arrangement, the predominance of hydrazone tautomers of para-hydroxyazo compounds is less frequently observed. The objective of this study was to design and synthesize sulphonated 4-naphthylazo naphth-1-ols with preferential hydrazone shift.PM6 calculations were used to quantify contributions of selected structural features to hydrazone stability in a set of model compounds. Synthesis of five computationally-optimized compounds (3a-e) was subsequently carried out via diazo coupling of 5‑hydroxy-7-sulphonaphthalene-2-diazonium ion with naphthol derivatives. The azo-hydrazone equilibria of 3a-e were further characterized using spectroscopic techniques and density functional theory (DFT).Computational design revealed that sulphonic acid substitution on the coupling components of the dyes and, solvation effect of the substituents were the highest contributors to hydrazone shift of the compounds. The ring position and electron withdrawing resonance effect of substituents were less contributory. Increasing substitution with sulphonic acid or another hydrophilic group (amino) also resulted in higher hydrazone stability. Mass spectral, IR and NMR data confirmed computational results with mono sulphonated 3a and poly substituted 3c-e existing predominantly as azo and hydrazone forms respectively. UV–Visible spectral analysis revealed that azo and hydrazone forms of 3a predominated in hydrogen bond acceptor and donor solvents respectively. In contrast, absorption spectra of 3b-e showed lone hydrazone bands in all solvents investigated. The DFT/M06-2X/6-31G(dp) characterization of the azo-hydrazone equilibrium of the compounds confirmed experimental results and showed that hydrazone tautomers of 3b, 3c and 3d were more stable than corresponding azo forms by 1.536, 0.289 and 1.359 kcal/mol respectively. DFT also confirmed involvement of sulphonic acid and imino groups in the transition of the dyes’ structures to the keto form.Three novel p-hydroxyazo dyes with increased hydrazone stability have been synthesized