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<p>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 <i>cis</i>-<i>trans</i> 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.</p><p></p>

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On the unexpected mechanism of isomerization in tautomerizable azo photoswitches

  • 期刊论文

  • 出版模式：

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  • 发表日期：

    2026年2月24日

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  • Coral Hillel,
  • Christopher J. Barrett,
  • William J. Pietro,
  • Ozzy Mermut

摘要

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.