A Photoswitchable Heteroditopic Ion-Pair Receptor

Designing
light-switchable hetero­ditopic receptors is challenging
because it necessitates simultaneous (de)­activation of two separate
binding sites. Herein, we present the first photo­switchable
hetero­ditopic ion-pair receptor in which both cation and anion
binding sites are simultaneously and reversibly switched OFF and ON
by a single photo­switch. Our receptor is simple, low molecular
weight, and readily synthesized from commercially available precursors.
Single-crystal X-ray structures and NMR spectroscopic titrations support
ion-pair binding to the receptor both in the solid state and in solution,
with strong positive cooperativity between the cation and anion binding.
The receptor can be completely switched OFF by UV light-triggered
photo­isomerization of an acyl­hydrazone CN double
bond and remains kinetically stable in the deactivated form due to
an intra­molecular hydrogen bond. Its re-activation could be
achieved by light irradiation or, more effectively, by fast acid-catalyzed
back-isomerization. Our simple photo­switchable ion-pair receptor
may serve as a blueprint for the design of new generations of switchable
receptors, transporters, soft materials, and self-assembled systems,
where incorporation of a functional hetero­ditopic ON/OFF photo­switch
has been challenging up to now.