Connectivity-Rewired Construction of Hydrogen-Bonded Azo-Macrocycles Enables Photoswitchable Recognition of Lithium Ions.

Abstract

Photoresponsive hydrogen-bonded azo-macrocycles capable of selectively recognizing lithium cation were constructed by reversing the amide-azobenzene connectivity, which redistributes electron density and preorganizes four carbonyl oxygen donors into a smaller, more convergent cavity. Compared with a connectivity-isomeric reference macrocycle, the new receptor displays a pronounced preference for Li⁺, in which complexation with LiClO₄ shows a slow exchange on the ¹H NMR timescale and an association constant (<i>K_a</i>) exceeding 10⁴ M^-1, whereas the reference binds Li⁺ weakly (<5 M^-1). In contrast, both hosts exhibit only modest binding toward Na⁺ (10²~10³ M^-1) and fast exchange, consistent with size/geometry matching of the compressed cavity to Li⁺. The newly designed azo-macrocycles reveal a highly selective recognition of Li⁺ thanks to the more evenly arrayed four amide oxygens enclosing a cavity of small dimension. Notably, <i>E</i>/<i>Z</i> photoisomerization of macrocycle switches the binding regime, enabling reversibly light-triggered Li⁺ binding under UV irradiation and recapture under visible light. This work establishes a new photoresponsive receptor based on H-bonded azo-macrocycles for photoswitchable recognition of Li⁺.