On the unexpected mechanism of isomerization in tautomerizable azo photoswitches

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.