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.