Cooperative Protonation Underlying the Acid Response of Metal–Organic Frameworks

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.