Smart Hydrogen Atoms in Heterocyclic Cations of 1,2,4-Triazolium-Type Poly(ionic liquid)s

[Image: see text] Discovering and constructing molecular functionality platforms for materials chemistry innovation has been a persistent target in the fields of chemistry, materials, and engineering. Around this task, basic scientific questions can be asked, novel functional materials can be synthesized, and efficient system functionality can be established. Poly(ionic liquid)s (PILs) have attracted growing interest far beyond polymer science and are now considered an interdisciplinary crossing point between multiple research areas due to their designable chemical structure, intriguing physicochemical properties, and broad and diverse applications. Recently, we discovered that 1,2,4-triazolium-type PILs show enhanced performance profiles, which are due to stronger and more abundant supramolecular interactions ranging from hydrogen bonding to metal coordination, when compared with structurally similar imidazolium counterparts. This phenomenon in our view can be related to the smart hydrogen atoms (SHAs), that is, any proton that binds to the carbon in the N-heterocyclic cations of 1,2,4-triazolium-type PILs. The replacement of one carbon by an electron-withdrawing nitrogen atom in the broadly studied heterocyclic imidazolium ring will further polarize the C–H bond (especially for C5–H) of the resultant 1,2,4-triazolium cation and establish new chemical tools for materials design. For instance, the H-bond-donating strength of the SHA, as well as its Bro̷nsted acidity, is increased. Furthermore, polycarbene complexes can be readily formed even in the presence of weak or medium bases, which is by contrast rather challenging for imidazolium-type PILs. The combination of SHAs with the intrinsic features of heterocyclic cation-functionalized PILs (e.g., N-coordination capability and polymeric multibinding effects) enables new phenomena and therefore innovative materials applications. In this Account, recent progress on SHAs is presented. SHA-related applications in several research branches are highlighted together with the corresponding materials design at size scales ranging from nano- to micro- and macroscopic levels. At a nanoscopic level, it is possible to manipulate the interior and outer shapes and surface properties of PIL nanocolloids by adjusting the hydrogen bonds (H-bonds) between SHAs and water. Owing to the interplay of polycarbene structure, N-coordination, and the polymer multidentate binding of 1,2,4-triazolium-type PILs, metal clusters with controllable size at sub-nanometer scale were successfully synthesized and stabilized, which exhibited record-high catalytic performance in H(2) generation via methanolysis of ammonia borane. At the microscopic level, SHAs are found to efficiently catalyze single crystal formation of structurally complex organics. Free protons in situ released from the SHAs serve as organocatalysts to activate formation of C–N bonds at room temperature in a series of imine-linked crystalline porous organics, such as organic cages, macrocycles and covalent organic frameworks; meanwhile the concurrent “salting-out” effect of PILs as polymers in solution accelerated the crystallization rate of product molecules by at least 1 order of magnitude. At the macroscopic scale, by finely regulating the supramolecular interactions of SHAs, a series of functional supramolecular porous polyelectrolyte membranes (SPPMs) with switchable pores and gradient cross-sectional structures were manufactured. These membranes demonstrate impressive figures of merit, ranging from chiral separation and proton recognition to switchable optical properties and real-time chemical reaction monitoring. Although the concept of SHAs is in the incipient stage of development, our successful examples of applications portend bright prospects for materials chemistry innovation.