Novel Nitroxide-Substituted Hydrazone Switch: Experimental and Theoretical Insights into Photoswitching Behavior

Hydrazones are a versatile class of molecular switches with dual responsiveness to both light and pH. To investigate their switching properties, we incorporated a nitroxide moiety, enabling analysis by not only conventional techniques such as ¹H NMR and UV-vis spectroscopy but also EPR spectroscopy, which provides valuable insights into structure and dynamics. A novel nitroxide-substituted hydrazone switch (2) was synthesized and fully characterized. However, initial experiments using ¹H NMR and UV-vis revealed restricted photoisomerization of 2. Theoretical studies employing DFT and TD-DFT methods revealed the presence of the D₁ excited state related to π → π* electron transfer of the nitroxide moiety, and D₂ excited state related to π → π* electron transfer within the hydrazone moiety. The latter excitation results in weakening of the CN bond and enables the rotation around the hydrazone bond; however, the internal conversion D₂ → D₁ process is most likely responsible for the quenching of photoisomerization in 2. Additionally, pH-induced switching was monitored using UV-vis and EPR spectroscopy, revealing that strong acids such as trifluoroacetic acid had no significant effect on the paramagnetic center.