Cooperative binding plays essential roles in many biochemical processes and has important implications for the development of sensory materials. Metal–organic frameworks (MOFs) can be designed to possess multiple binding sites on pore surfaces, but binding cooperativity within the confined space is rarely recognized. Here, we present a systematic study on cooperative protonation in MOFs in order to gain structural insights into the phenomenon. Three microporous Zr(IV) MOFs were studied for comparison, two furnished with sterically hindered but proton-accessible sites (Zr-PTB and Zr-PPTB) and one free of pyridyl sites (Zr-BTB, isostructural to Zr-PTB). Zr-PTB and Zr-PPTB show two opposite and simultaneous fluorescence transitions in narrow pH changes, which has the appeal for ultrasensitive pH probing. The dual-emission response is ascribable to pyridyl protonation, which turns off the (n, π*) emission and, meanwhile, turns on the (π, π*) emission. The abrupt fluorescence transitions arise from positive cooperativity of multisite protonation. Zr-PPTB shows stronger cooperativity (Hill coefficient h = 1.6) than Zr-PTB (h = 1.2). Structural inspection suggests that the arrangement of the pyridyl sites in Zr-PPTB is conducive to the interplay between pyridyl sites. Moreover, we demonstrate abrupt enhancement (up to 500 times) in proton conduction for Zr-PTB below the pH of cooperative protonation, with a maximum conductivity of 1.2 × 10^–2 S cm^–1 at 347 K and 98% RH. Zr-PPTB also shows abrupt conductivity enhancement upon cooperative protonation, but the enhancement is smaller. We ascribe the difference to the biased allocation of pyridyl sites between different channels of Zr-PPTB.

中文翻译：

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协同质子化是金属-有机框架酸响应的基础

协同结合在许多生化过程中发挥着重要作用，对感官材料的发展具有重要意义。金属有机框架 (MOF) 可以设计为在孔表面具有多个结合位点，但在有限空间内的结合协同性很少被认可。在这里，我们对 MOF 中的协同质子化进行了系统研究，以获得对该现象的结构性见解。研究了三种微孔 Zr(IV) MOF 以进行比较，其中两种具有空间位阻但质子可接近的位点（Zr-PTB 和 Zr-PPTB），另一种不含吡啶基位点（Zr-BTB，与 Zr-PTB 同构）。Zr-PTB 和 Zr-PPTB 在狭窄的 pH 变化中显示出两个相反且同时的荧光跃迁，这对超灵敏的 pH 探测具有吸引力。n , π*) 发射，同时打开 (π, π*) 发射。突然的荧光跃迁源于多位点质子化的正协同作用。Zr-PPTB 表现出比 Zr-PTB ( h = 1.2)更强的协同性 (Hill 系数h = 1.6)。结构检查表明，Zr-PPTB中吡啶基位点的排列有利于吡啶基位点之间的相互作用。此外，我们证明了在协同质子化的 pH 值以下，Zr-PTB 的质子传导突然增强（高达 500 倍），最大电导率为 1.2 × 10 ^–2 S cm ^–1在 347 K 和 98% RH 下。Zr-PPTB 在协同质子化时也显示出突然的电导率增强，但增强较小。我们将差异归因于 Zr-PPTB 不同通道之间吡啶基位点的偏差分配。