In recent years, there has been no shortage of research achievements in light-responsive materials based on azobenzene photoswitches. Of growing interest is the ability to reversibly tune the competing “dark" thermal cis-trans back-isomerization through protonation effects. The hydroxy-substituted azobenzenes are well-known for their complex pH-dependent behavior, including azo-hydrazone tautomerism. Presently, experimental studies rationalize only qualitatively the marked acceleration in thermal switching upon acquiring the hydrazone tautomer, while the results of theoretical treatments have experienced a persistent cusp problem in calculated potential energy surfaces. Here, using density functional theory, spin-flip, and multireference wavefunction quantum chemical methods, we provide for the first time a comprehensive explanation of thermal switching in the hydrazone tautomer. We show that, through concerted torsion of two dihedral angles, the hydrazone tautomer unexpectedly acquires a maximally puckered transition state, enabling rapid rotation of the entire system. This study demonstrates the exploitative advantages of protonation for tuning thermal isomerization in azobenzene photoswitches. Azobenzene photoswitches have garnered significant attention for their potential in light-responsive materials, yet challenges remain in understanding their thermal cis-trans isomerization. Here, the authors employ advanced quantum chemical methods to elucidate the rapid thermal switching mechanism in the hydrazone tautomer, revealing puckering of the tautomeric nitrogen as a key factor for accelerating isomerization kinetics.
Hillel et al. (Tue,) studied this question.
Synapse has enriched 5 closely related papers on similar clinical questions. Consider them for comparative context: