Competitive Effect of Surface Hydrophobicity–Hydrophilicity and the Number of Accessible Protons on Water-Assisted Proton Conductivity in Metal–Organic Frameworks: Dielectric σ-Relaxation Triggered by Disordered Guest Ionic Migration and Its Mechanism

The rapid upsurge of metal–organic frameworks (MOFs) has sparked profound interest in their potential as proton conductors for proton exchange membrane fuel cells. However, proton-conducting behaviors of hydrophobic MOFs remain poorly understood compared with their hydrophilic counterparts, largely due to the absence of a microscopic phase separation structure akin to that found in Nafion membranes. Herein, we demonstrate a strategy for regulating the structures and proton conductivities of MOFs by separately incorporating hydrophobic −C(CF3)2– group alongside hydrophilic −O– and −SO2– groups into organic ligands as linkers. Three analogous MOFs, {[Zr6(obba)4(μ3-O)4(μ3–OH)4(OH)3(H2O)3(HCOO)]·3.5DMF·3CH3COCH3}n (Zr-obba, H2obba = 4,4′-oxybis(benzoic acid)), {[Zr6(sdba)4(μ3-O)4(μ3–OH)4(OH)2(H2O)2(HCOO)2]·2(CH3)2NH2+·2HCOO–·7(CH3)2NH·6.5H2O}n (Zr-sdba, H2sdba = 4,4′-sulfonyldibenzoic acid), and {[Zr6(hfipbba)4(μ3-O)4(μ3–OH)4(OH)4(H2O)4]·7.5(CH3)2NH2+·7.5HCOO–·3.5(CH3)2NH}n (Zr-hfipbba, H2hfipbba = 4,4′-(hexafluoroisopropylidene)bis(benzoic acid)), were prepared and characterized. Interestingly, we observed a trend in proton conductivity: Zr-obba > Zr-sdba > Zr-hfipbba, which can be attributed to the competitive effect of surface hydrophobicity–hydrophilicity and the availability of accessible protons. More importantly, we identified dielectric σ-relaxation in Zr-sdba and Zr-hfipbba, which is linked to the thermally activated migration of disordered guest ions within the MOFs, highlighting the significant relationship between proton conduction and dielectric relaxation. This finding contributes to a deeper understanding of the critical roles that hydrophobicity–hydrophilicity play in proton conduction and the dielectric σ-relaxation mechanism in crystalline MOFs.