Evaluation of Substituent Effect in Z-Isomer Stability of Arylazo-1H-3,5-dimethylpyrazoles: Interplay of Steric, Electronic Effects and Hydrogen Bonding

Abstract

The electronic and steric effects of aryl substituents and the influence of hydrogen bonding in Z-isomer stability of phenylazopyrazole derivatives have been investigated. In this regard, 38 substituted phenylazopyrazole derivatives and 6 N-methyl phenylazopyrazoles (with meta substitutions) have been synthesized. Their photoswitching behavior, photostationary states (PSS), and kinetics of thermal reverse isomerization were evaluated experimentally using UV-vis and NMR spectroscopic techniques. Furthermore, density functional theory (DFT) computations have been performed for more detailed insights. Despite the presence of substantial substituent effects inferred through Taft and Hammett relationships, the concentration dependency in controlling the isomerization rates has also been observed. Kinetics studies at different concentrations, solvent effects, and computations have confirmed the decisive role of hydrogen bonding and solvent-assisted tautomerism in this regard. Through this study, a complex interplay of steric, electronic effects and hydrogen bonding as factors in dictating the stability of Z-isomers in arylazo-1 H-3,5-dimethylpyrazoles has been demonstrated.

Arylazo‐3,5‐dimethylisoxazoles: Azoheteroarene Photoswitches Exhibiting High Z‐Isomer Stability, Solid‐State Photochromism, and Reversible Light‐Induced Phase Transition

“…Even at a very high concentration (1.6 m m ), the E – Z isomerization was found to show 40 % isomerization (Supporting Information, Figure S1). This clearly supported our understanding, based on the earlier studies on azopyrazoles, that the photoisomerization step can be limited by hydrogen bonding . Surprisingly, the 2‐hydroxy‐ and 2‐amino‐substituted derivatives ( 12 d and 32 d , respectively) were also found to undergo photoswitching.…”

“…In spite of significant changes in those parameters, the computed structural data did not show any trend that can account for the electronic effects of the substituents. In fact, this observation was also made in the case of arylazopyrazoles with meta substituents . Likewise, in our earlier studies on arylazopyrazoles, the inspection of molecular orbitals corresponding to HOMO−1, HOMO, and LUMO of the E‐ isomers of phenylazoisoxazole revealed n, π, and π* orbitals, respectively.…”

“…Commercially available aniline derivatives were used as starting materials. Both the generation of hydrazone derivatives 1 – 37 c and the consecutive cyclization step with hydroxylamine hydrochloride to access the target isoxazole based derivatives 1 – 37 d adopted the literature procedure . However, the presence of a base provided good to excellent yields in the cyclization step.…”

“…To understand the photoisomerization efficiency, the isomerization conversion was estimated at their respective PSSs for the forward E – Z and reverse Z – E steps in the solid state and in the solution phase. The PSS compositions were estimated by literature methods and the results tabulated (Table and Supporting Information, Table S2 for solid‐state PSS data).…”

“…Inversion and rotational transition states were reported to be the major mechanistic channels for thermal Z – E isomerization in many azoheteroarenes. Depending on the heterocycles, the pathway can vary, and this necessitates investigation of the mechanistic aspects . Hence, we computed both the mechanistic channels in the Z – E isomerization pathways of arylazo‐3,5‐dimethylisoxazole.…”