Cooperative Protonation Underlying the Acid Response\nof Metal–Organic Frameworks

Abstract

Cooperative binding plays essential\nroles in many biochemical processes\nand has important implications for the development of sensory materials.\nMetal–organic frameworks (MOFs) can be designed to possess\nmultiple binding sites on pore surfaces, but binding cooperativity\nwithin the confined space is rarely recognized. Here, we present a\nsystematic study on cooperative protonation in MOFs in order to gain\nstructural insights into the phenomenon. Three microporous Zr­(IV)\nMOFs were studied for comparison, two furnished with sterically hindered\nbut proton-accessible sites (Zr-PTB and Zr-PPTB) and one free of pyridyl\nsites (Zr-BTB, isostructural to Zr-PTB). Zr-PTB and Zr-PPTB show two\nopposite and simultaneous fluorescence transitions in narrow pH changes,\nwhich has the appeal for ultrasensitive pH probing. The dual-emission\nresponse is ascribable to pyridyl protonation, which turns off the\n(<i>n</i>, π*) emission and, meanwhile, turns on the\n(π, π*) emission. The abrupt fluorescence transitions\narise from positive cooperativity of multisite protonation. Zr-PPTB\nshows stronger cooperativity (Hill coefficient <i>h</i> =\n1.6) than Zr-PTB (<i>h</i> = 1.2). Structural inspection\nsuggests that the arrangement of the pyridyl sites in Zr-PPTB is conducive\nto the interplay between pyridyl sites. Moreover, we demonstrate abrupt\nenhancement (up to 500 times) in proton conduction for Zr-PTB below\nthe pH of cooperative protonation, with a maximum conductivity of\n1.2 × 10^–2 S cm^–1 at 347\nK and 98% RH. Zr-PPTB also shows abrupt conductivity enhancement upon\ncooperative protonation, but the enhancement is smaller. We ascribe\nthe difference to the biased allocation of pyridyl sites between different\nchannels of Zr-PPTB.