Positive Cooperative Protonation of a Metal–Organic Framework: pH-Responsive Fluorescence and Proton Conduction

Abstract

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.

Modulating fluorescence sensing properties of excited-state intramolecular proton transfer (ESIPT)-based metal organic frameworks (MOFs) by metal polarization

“…Metal-organic frameworks (MOFs) 5,6 are crystalline porous materials formed by self-assembly of appropriate metal ions/ clusters and organic ligands. Due to their large specific surface area and modifiable pore environment arising from diverse metal centers and versatile organic ligands showing rich connection modes, MOFs are widely used in the fields of gas storage and separation, 7,8 catalysis, [9][10][11] proton conduction 12,13 and fluorescence sensing. 14,15 In particular, luminescent metal-organic framework materials (LMOFs) 16,17 combine critical porosity 18 and luminescence, 19 leading to rich host-guest investigations 20 in a specific confined environment, and it thus has received extensive attention in sensing detection.…”

Switched Proton Conduction in Metal–Organic Frameworks

“…These changes are due to pH-changed cooperative protonation of the pyridyl sites. 89 Li’s group observed the proton-conducting response to formic acid vapor in two 3D isostructural Ln(III) MOFs, ZZU-1 and ZZU-2. 90 They can distinguish formic acid vapor from other organic small-molecule vapors such as methanol, ethanol, acetone, toluene, acetic acid, etc.…”

Designable Guest‐Molecule Encapsulation in Metal–Organic Frameworks for Proton Conductivity

“…The pH‐dependent proton conductivity in MOFs is demonstrated for the first time by Hong [9] in 2014 by soaking Ni 2 (dobdc) in sulfuric acid solutions at different pH values, affording new proton conducting frameworks, H + @Ni 2 (dobdc). Recently, a stable MOF, Zr 6 ( μ 3 ‐O) 4 ( μ 3 ‐OH) 4 (COO) 12 , was synthesized by Yang and co‐workers for the pH‐sensitive proton conductivity [10] . As shown in Figure 1, a 3D framework is formed, in which Zr 6 ( μ 3 ‐O) 4 ( μ 3 ‐OH) 4 (COO) 12 secondary building units (SBUs) are linked by TCPBP 4− ligands.…”

“…Deposition Numbers CCDC2039554 (for Zr-TCPBP [10] ), 1564881 (for BUT-8 [12] ), 1911731 (for Ni-MOF [Ni 2 (H 4 L)(H 2 O) 9 (C 2 H 7 SO)(C 2 H 7 NCO)] [13] ), 605510 (for MIL-101 [14,16,22] ), 2041165 (for PHOS-100 [15] ), 881244 (for [Cu-(bpdc)(H 2 O) 2 ] n [19] ), 881245 (for {H[Cu(Hbpdc)(H 2 O) 2 ] 2 [PMo 12 O 40 ]•nH 2 O} n [19] ), 881246 (for {H[Cu(Hbpdc)(H 2 O) 2 ] 2 [PW 12 O 40 ]•nH 2 O} n [19] ), 1562195 (for NENU-3 [21] ), 1993021 (for MOF-217 [26] ), 1908861 (for [Eu 2 (HBDPP) 2 (H 2 O) 2 (DMF) 2 ](H 2 O) 2 [29] ), 1002678 (for MOF-801 [30] ), 1585550 (for VNU-23 [31] ) and 1810933 (for His 8.2 �VNU-23 [31] ), 1405751 (for UiO-66-NH 2 [32] ), 1013322 (for PCN-777 [35] ), 902900 (for DUT-68 [36] ), 1885639 (for FJU-10 [41] ), 1013734 (for UPG-1 [42] ) contain the supplementary crystallographic data for this paper. These data are provided free of charge by the joint Cambridge Crystallographic Data Centre and Fachinformationszentrum Karlsruhe Access Structures service.…”

“…Recently, a stable MOF, Zr 6 (μ 3 -O) 4 (μ 3 -OH) 4 (COO) 12 , was synthesized by Yang and co-workers for the pH-sensitive proton conductivity. [10] As shown in Figure 1, a 3D framework is formed, in which Zr 6 (μ 3 -O) 4 (μ 3 -OH) 4 (COO) 12 secondary building units (SBUs) are linked by TCPBP 4À ligands. The ligand is highly disordered and shows two perpendicular orientations (Figure 1a), and its structure can be simplified to a (4,12)-connected ftw topology (Figure 1b).…”