Acylhydrazone Subunits as a Proton Cargo Delivery System in 7‐Hydroxyquinoline

The reimagined concept of long‐range tautomeric proton transfer using crane subunits is shown by designing and synthesising two new acylhydrazones containing a 7‐hydroxyquinoline (7‐OHQ) platform. The acylhydrazone subunits attached to the 7‐OHQ at the 8th position act as crane arms for delivering proton cargo to the quinoline nitrogen. Light‐induced tautomerization to their keto forms leads to Z/E isomerization of the C=C axle bond, followed by proton delivery to the quinoline nitrogen by the formation of covalent or hydrogen bonds. The axle‘s being either an imine or ketimine bond is the structural difference between the studied compounds. The −CH3 group in the latter provides steric strain, resulting in different proton transport pathways. Both compounds show long thermal stability in the switched state, which creates a tuneable action of bidirectional proton cargo transport by using different wavelengths of irradiation. Upon the addition of acid, the quinoline nitrogen is protonated; this results in E/Z configuration switching of the acylhydrazone subunits. This was proven by single‐crystal X‐ray structure analysis and NMR spectroscopy. Dropping your H′s: Two structurally different acylhydrazones containing imine (1 a) and ketimine (2 a) functions have been evaluated as proton cargo delivery subunits in the 7‐hydroxyquinoline platform. The tautomeric OH proton was delivered to the quinoline nitrogen by covalent (1 a) and hydrogen (2 a) bonding. The system is characterized by long stability in the switched state, thus enabling control of the bidirectional proton motion under different light energy.