Shanghai Key Laboratory of Green Chemistry and Chemical Processes, School of Chemistry and Molecular Engineering, East China Normal University, Shanghai 200062, China.
Engineering Research Centre for Nanophotonics and Advanced Instrument, School of Physics and Electronic Science, East China Normal University, Shanghai 200062, China.
J Am Chem Soc. 2021 Jun 16;143(23):8838-8848. doi: 10.1021/jacs.1c03432. Epub 2021 Jun 2.
Positive cooperative binding, a phenomenon prevalent in biological processes, holds great appeal for the design of highly sensitive responsive molecules and materials. It has been demonstrated that metal-organic frameworks (MOFs) can show positive cooperative adsorption to the benefit of gas separation, but potential binding cooperativity is largely ignored in the study of sensory MOFs. Here, we report the first demonstration of positive cooperative protonation of a MOF and the relevant pH response in fluorescence and proton conduction. The MOF is built of Zr-O clusters and bipyridyl-based tetracarboxylate linkers and has excellent hydrolytic stability. It shows a unique pH response that features two synchronous abrupt turn-off and turn-on fluorescent transitions. The abrupt transitions, which afford high sensitivity to small pH fluctuations, are due to cooperative protonation of the pyridyl sites with a Hill coefficient of 1.6. The synchronous dual-emission response, which leads to visual color change, is ascribable to proton-triggered switching between (n, π*) and (π, π*) emissions. The latter emission can be quenched by electron donating anion-dependent through photoinduced electron transfer and ground-state charge transfer. Associated with cooperative protonation, the proton conductivity of the MOF is abruptly enhanced at low pH by two orders, but overhigh acid concentration is adverse because excessive anions can interrupt the conducting networks. Our work shows new perspectives of binding cooperativity in MOFs and should shed new light on the development of responsive fluorescent MOFs and proton conductive materials.
正协同结合是一种普遍存在于生物过程中的现象，对于设计高灵敏度响应分子和材料具有很大的吸引力。已经证明，金属有机骨架（MOFs）可以对气体分离表现出正协同吸附作用，但在研究感应 MOFs 时，潜在的结合协同性在很大程度上被忽视了。在这里，我们首次报道了 MOF 的正协同质子化及其在荧光和质子传导中的相关 pH 响应。该 MOF 由 Zr-O 簇和基于联吡啶的四羧酸连接体构建，具有优异的水解稳定性。它表现出独特的 pH 响应，具有两个同步的猝灭和开启荧光转变。这些猝灭和开启的转变提供了对小 pH 波动的高灵敏度，这是由于吡啶基位点的协同质子化，其希尔系数为 1.6。同步双发射响应导致视觉颜色变化，归因于质子触发的（n，π*）和（π，π*）发射之间的开关。后者的发射可以通过光诱导电子转移和基态电荷转移被供电子阴离子依赖性猝灭。与协同质子化相关的是，MOF 的质子电导率在低 pH 值下突然增强了两个数量级，但过高的酸浓度是不利的，因为过多的阴离子会中断导电网络。我们的工作展示了 MOFs 中结合协同性的新视角，应该为响应性荧光 MOFs 和质子导电材料的发展提供新的思路。
