Proton‐conducting materials in the solid state have received immense attention for their role as electrolytes in proton‐exchange membrane fuel cells. Recently, crystalline materials—metal–organic frameworks (MOFs), hydrogen‐bonded organic frameworks (HOFs), covalent organic frameworks (COFs), polyoxometalates (POMs), and porous organic crystals—have become an exciting research topic in the field of proton‐conducting materials. For a better electrolyte, a high proton conductivity on the order of 10−2 S cm−1 or higher is preferred as efficient proton transport between the electrodes is ultimately necessary. With an emphasis on design principles, this Concept will focus on MOFs and other crystalline solid‐based proton‐conducting platforms that exhibit “ultrahigh superprotonic” conductivities with values in excess of 10−2 S cm−1. While only a handful of MOFs exhibit such an ultrahigh conductivity, this quality in other systems is even rarer. In addition to interpreting the structural–functional correlation by taking advantage of their crystalline nature, we address the challenges and promising directions for future research.

A microporous MOF {[Zn(SDB)(L)]·S} (IITKGP-5) with a polar pore surface has been constructed by the combination of a V-shaped -SO functionalized organic linker (HSDB = 4,4'-sulfonyldibenzoic acid) with an N-rich spacer (L = 2,5-bis(3-pyridyl)-3,4-diaza-2,4-hexadiene), forming a network with sql(2,6L1) topology. IITKGP-5 is characterized by TGA, PXRD and single crystal X-ray diffraction. The framework exhibits lozenge-shaped channels of an approximate size of 4.2 × 5.6 Å along the crystallographic b axis with a potential solvent accessible volume of 26%. The activated IITKGP-5a revealed a CO uptake capacity of 56.4 and 49 cm g at 273 K/1 atm and 295 K/1 atm, respectively. On the contrary, it takes up a much smaller amount of CH (17 cm g at 273 K and 13.6 cm g at 295 K) and N (5.5 cm g at 273 K; 4 cm g at 295 K) under 1 atm pressure exhibiting its potential for a highly selective adsorption of CO from flue gas as well as a landfill gas mixture. Based on the ideal adsorbed solution theory (IAST), a CO/N selectivity of 435.5 and a CO/CH selectivity of 151.6 have been realized at 273 K/100 kPa. The values at 295 K are 147.8 for CO/N and 23.8 for CO/CH gas mixtures under 100 kPa. In addition, this MOF nearly approaches the target values proposed for PSA and TSA processes for practical utility exhibiting its prospect for flue gas separation with a CO loading capacity of 2.04 mmol g.

A superprotonic conducting coordination polymer PCM-1 was synthesized. Joint experimental/modeling study revealed water-assisted proton dynamics through the formation of a long-range H-bonded network mediated by the Teixeira jump-diffusion model.

Design, synthesis, and applications of metal–organic frameworks (MOFs) are among the most salient fields of research in modern inorganic and materials chemistry. As the structure and physical properties of MOFs are mostly dependent on the organic linkers or ligands, the choice of ligand system is of utmost importance in the design of MOFs. One such crucial organic linker/ligand is terpyridine (tpy), which can adopt various coordination modes to generate an enormous number of metal–organic frameworks. These frameworks generally carry physicochemical characteristics induced by the π‐electron‐rich (basically N‐electron‐rich moiety) terpyridines. In this minireview, the construction of 3D MOFs associated with symmetrical terpyridines is discussed. These ligands can be easily derivatized at the lateral phenyl (4′‐phenyl) position and incorporate additional organic functionalities. These functionalities lead to some different binding modes and form higher dimensional (3D) frameworks. Therefore, these 3D MOFs can carry multiple features along with the characteristics of terpyridines. Some properties of these MOFs, like photophysical, chemical selectivity, photocatalytic degradation, proton conductivity, and magnetism, etc. have also been discussed and correlated with their frameworks.

Studies of proton conductivity in crystalline porous materials (CPMs), mainly metal–organic frameworks (MOFs) and coordination polymers (CPs), have received enormous attention due to their potential application in fuel cell membranes. These materials have well-defined structural features, easy synthetic routes, and functionalizable channels. These factors provide an added advantage of their targeted synthesis and control of framework–carrier interactions that eventually determine the orderly arrangement, mobility, and density of the proton carriers. The nature of framework–carrier interactions depends on a few characteristic features such as the choice of metal ions to build the framework, the nature of the ligands, the flexibility of the framework, and the polarity of the guest molecules. This Perspective focuses on understanding the fundamental principles of proton conduction, implicates various design strategies, and discusses the role of host–guest interactions in proton conductivity, a factor largely overlooked so far.