Photoactivatable Hydrazone Photoswitches

Photoactivatable molecules enable on-demand generation of active species via external light stimulation, allowing for precise spatiotemporal control over their activation. Molecular photoswitches, which typically undergo reversible transitions between two thermodynamically stable states upon irradiation with distinct wavelengths, are highly valuable tools with broad utility in different research areas. Herein, we introduce a new class of photoactivatable photoswitches: the switchable molecules are designed to undergo initial photochemical activation from a stable precursor upon irradiation at λ₁, generating active photoswitches that exhibit reversible isomerization when irradiated with two other wavelengths (λ₂ and λ₃) of light_. Crucially, neither λ₂ nor λ₃ alone activates the photoswitch precursor, ensuring temporal and orthogonal control over both activation and switching processes. This photoactivatable system operates through a sequential photochemical pathway that involves a photoinduced tetrazole-hydrazone transformation, followed by E/Z photoisomerization of the resulting hydrazone motif. Upon irradiation at λ₁, the diaryl tetrazole precursor undergoes rapid photolysis with concomitant nitrogen extrusion, generating a reactive nitrile imine (NI) intermediate. This intermediate subsequently undergoes an intramolecular nucleophilic addition to furnish the hydrazone-based photoswitch in situ. Notably, strategic incorporation of heteroatoms within the molecular architecture significantly enhances the thermal stability of both E/Z isomers by facilitating intramolecular hydrogen-bonding interactions.