Enhanced Proton Conductivities in Cucurbit[6]uril-Based Metal−Organic Rotaxane Frameworks by Proton Donor−Acceptor

It is crucial to design and construct new crystalline solid-state proton conductors (SSPCs) for the advancement of clean energy applications. The pKa values of the functional groups in the ligand were calculated by using density functional theory (DFT) calculations. Three new cucurbit[6]uril (CB[6])-based metal−organic rotaxane frameworks (MORFs) (named Zn5(PR63)2(APDC)4(BDC)Cl4H2O·14H2O(CUST-813), [Zn2(PR63)(stp)2(H2O)2]·3H2O (CUST-814), and [Cd2(PR63)(APDC)2(HAPDC)2(H2O)2]·3H2O (CUST-815)) have been successfully designed and synthesized under solvent thermal conditions. The powder X-ray diffraction (PXRD) pattern under different temperatures and relative humidities (RHs) and thermogravimetric analysis (TGA) indicated that all three compounds possess excellent stabilities. The alternating current (AC) impedance tests showed the proton conductivity value of CUST-813 can reach 0.87 × 10−4 S cm−1, which is approximately 1.67 times that of CUST-814 and 11 times that of CUST-815 (75 °C, 98% RH), respectively. The results indicated that the simultaneous introduction of proton donors and proton acceptors into MOFs can effectively enhance their proton conductivities. This work provides new insights into the design and synthesis of CB[6]-based solid-state proton conductors.