Electrochemical Gating of Host-Guest Charge-Transfer Interactions Enabled by Viologen-like Functionality Engineered Metal-Organic Frameworks.

Using electrochemically responsive metal-organic frameworks (MOFs) as host matrices to afford gating properties for functional guests is rather attractive but remains unexplored. Herein, a series of functionalized Zr-MOFs with viologen-like skeletons were created by engineering 2,2'-bipyridinium bay substitution with different alkyl chains. Of the series, benefiting from the enhanced rigidity, the one bearing N,N'-ethylene bridge, UiO-67-EE, exhibited the strongest electron deficiency due to the lowest LUMO level, thereby leading to efficient electron transfer and favorable redox activity, which further endowed it with outstanding electrochromic properties. More importantly, the highly electron-deficient framework of UiO-67-EE could allow the accommodation of electron-rich guest molecules through host-guest charge transfer (CT) interactions. By leveraging the electroresponsiveness of the viologen-like functionality, UiO-67-EE served as an adaptable platform for controlled guest release and capture through efficient control of dynamic CT interactions upon stimuli of alternate potentials. This smart electrochemical gating behavior of the host-guest systems was also monitored in real time by distinguishable optical changes of the guests. Besides, it was exploited to develop high-performance sensing platforms by integrating a molecular gate constructed from the target-aptamer complex.