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.

设计可光切换的异双位点受体（heteroditopic receptor）极具挑战性，因其需同时对两个独立的结合位点进行激活与失活。在此，我们报道首例光控异双位点离子对受体（heteroditopic ion-pair receptor），其阳离子与阴离子结合位点均可通过单个光开关（photoswitch）实现同步且可逆的启闭切换。本受体结构简洁、分子量偏低，且可由商用前驱体简便合成。单晶X射线（single-crystal X-ray）衍射结构与核磁共振（NMR）光谱滴定实验证实，该受体在固态与溶液相中均能结合离子对，且阴阳离子结合过程存在显著的正协同效应（positive cooperativity）。该受体可通过紫外光触发酰腙（acylhydrazone）C=N双键的光异构化（photoisomerization）完全失活，且因分子内氢键（intramolecular hydrogen bond）的存在，失活状态下的受体具有动力学稳定性。其重新激活可通过光辐照实现，或更高效地通过酸催化反向异构化（acid-catalyzed back-isomerization）完成。这款简易光控离子对受体，可为新一代可切换受体、转运体、软材料及自组装体系的设计提供蓝图——此类体系中引入兼具功能的异双位点光控开关此前一直极具挑战性。