Light-Triggered Crystallization of a Molecular Host−Guest Complex

The control of structural changes in supramolecular assemblies is a key point in the development of molecular machines. The reversible photoisomerization of organic compounds such as azobenzene using light as an external input is especially suited because no waste products are generated. Based on our previous studies on the quantitative encapsulation of suitably sized bis-sulfonate guests by a self-assembled, metal−organic cage consisting of four rigid, bent bis-monodentate pyridyl ligands and two Pd(II) ions, we show here how the light-switchable guest cis-4,4′-azobenzene bis-sulfonate can be expelled from its 1:1 host−guest complex triggered by its photoisomerization to the trans-isomer. Using a highly soluble, PEGylated cage derivative, the full reversibility of this light-driven encapsulation/release process is demonstrated. In contrast, a sample of the less soluble, unsubstituted cages including 1 equiv of the cis-guest was shown to result in immediate crystallization upon photoisomerization of the guest. X-ray structure analysis confirmed the guest molecules having left the cavity of the host and on the contrary joining the cages into a polymeric material by binding to their Pd(II) centers from outside.

超分子组装体（supramolecular assemblies）的结构变化调控是分子机器（molecular machines）研发的核心要点。以光作为外部输入的偶氮苯（azobenzene）等有机化合物的可逆光致异构化（reversible photoisomerization）尤为适用，因其不会产生任何副产物。基于我们此前针对由四个刚性弯折双单齿吡啶配体（rigid, bent bis-monodentate pyridyl ligands）与两个二价钯（Pd(II)）离子自组装得到的金属有机笼（self-assembled metal−organic cage）对尺寸匹配的双磺酸根客体（bis-sulfonate guests）实现定量包封的研究，本文展示了光可控客体（light-switchable guest）顺式-4,4′-偶氮苯双磺酸根（cis-4,4′-azobenzene bis-sulfonate）如何在光致异构化为反式异构体（trans-isomer）后，从其1:1主客体复合物中被释放。借助高溶解性的聚乙二醇化笼状衍生物（PEGylated cage derivative），我们证实了该光驱动包封/释放过程具备完全可逆性。与之形成对比的是，包含1当量顺式客体的低溶解性未取代笼状样品，在客体发生光致异构化后会立即结晶。X射线晶体结构分析（X-ray structure analysis）证实，客体分子已脱离主笼空腔，反而通过结合到笼结构外侧的二价钯中心，与笼骨架共同形成了聚合物材料（polymeric material）。