Azo‐hydrazone molecular switches: Synthesis and NMR conformational investigation

A series of five intramolecularly hydrogen‐bonded arylhydrazone (aryl = phenol, p‐nitrophenol, anisole, quinoline) derived molecular switches have been synthesized and characterized by NMR and HRMS techniques. It was found that the compounds exist as different isomers in solution. An investigation of both conformational and/or configurational changes of the azo‐hydrazone compounds was carried out by 1D 1H‐ and 13C‐ spectra, 2D NOESY, COSY, HSQC, and HMBC techniques. It was found that these stimuli‐responsive molecular switches exist mainly in the E form by intramolecularly hydrogen bonded between NH and the pyridine nitrogen at equilibrium. Deprotonation of the neutral E form yields the E′ deprotonated isomer. Prediction of 13C‐NMR chemical shifts was achieved by DFT quantum mechanical calculations. Anions have traditionally been difficult to calculate correctly, so calculations of the anion using different functionals, basis sets, and solvent effects are also included. Deuterium isotope effects on the 13C‐NMR chemical shifts were employed in the assignments and furthermore utilized as indicators of intramolecular hydrogen bonding. Studies in various organic solvents including CDCl3, CD3CN, and DMSO‐d6 were also performed aiming to monitor dynamic changes over several days. The effect of the hydrogen bonded solvents leads to Z forms. Current series of intramolecularly hydrogen bonded arylhydrazone molecular switches exists as different isomers in solution. They are found mainly in the E form by intra‐molecular hydrogen bond between the NH and the pyridine nitrogen at equilibrium.