Fluorinated Phenylazopyrazoles for Monitoring the Photoisomerization in Complex Systems

Photoswitches have been impacting diverse areas of research, introducing means of dynamically regulating their environment through reversible photochemical isomerization. Among the different classes of photoswitches, phenylazopyrazoles (PAPs) stand out due to their facile synthesis, beneficial photochemical properties, and the long thermal stability of their metastable forms. Not surprisingly, they have been applied in different fields, from material sciences to pharmacology. However, once incorporated into complex systems, following the photoisomerization behavior of PAPs with routinely used analytical methods becomes challenging. In this work, we focused on synthesizing and studying the isomerization behavior of a series of differently substituted PAPs possessing two trifluoromethyl groups on the pyrazole moiety (F-PAPs). By studying their isomerization with steady-state and transient absorption spectroscopy, we found marked trends in the isomerization efficiency and photophysical properties of the different switches. Most importantly, these molecules can quantitatively isomerize between their stable and metastable forms, with relatively long lifetimes of thermal back relaxation. Leveraging these characteristics, we highlight in this work the potential application of F-PAPs as photoswitchable ¹⁹F-NMR probes to generate light-responsive vesicles.