Highly proton-conductive metal–organic coordination polymers (MOCPs) have attracted a great deal of attention because of their potential applications in proton exchange membrane fuel cells and electrochemical sensors. Currently, the precise control of proton conductivity is mainly achieved through the rational design of organic ligands and the effective modification of pore structures. In contrast, little attention has been paid to the influence of metal ions of MOCPs on their proton conductivities due to the absence of isostructural compounds for comparison and analysis. Herein, we designed three MOCPs {[(H₃O⁺)₂][Zn(pzdc)₂]}_n, {[(H₃O⁺)₂][Mn(pzdc)₂]}_n and [Cu(Hpzdc)₂·2H₂O]_n (H₂pzdc = 2,3-pyrazinedicarboxylic acid) with very similar chemical formulae and single-crystal structures as models to investigate how the coordination interactions/abilities of metal ions affect their proton conductivities and water stabilities. Remarkably, these three MOCPs show proton conductivities as high as 10⁻³ S cm⁻¹ at 323 K and ∼97% RH along with excellent stabilities to temperature, water, acid and base (over a broad pH range of 0–12). Density functional theory (DFT) calculations show that the binding energies of the O–H bonds of the carboxyl groups in the solvation models are a predominant factor in precisely tuning the proton conductivity at ∼97% RH. Furthermore, the extraordinary stabilities, especially in water, are more likely to be associated with the coordination interactions resulting in a significant decrease in the system energies of the generated coordination compounds. These results provide a novel perspective for the design and synthesis of MOCPs with high proton conductivity and good stability.

The interaction between cysteine with Li+ and LiF in the microcosmic water environment was investigated to elucidate how ions interact with amino acids and the cation–anion correlation effect involved. The structures of Cys·Li+(H2O)n and Cys·LiF­(H2O)n (n = 0–6) were characterized using ab initio calculations. Our studies show that the water preferentially interacts with Li+/LiF. In Cys·Li+(H2O)0–6, Li+ interacts with amino nitrogen, carbonyl oxygen, and hydrophobic sulfur of Cys to form a tridentate mode, whereas in Cys·LiF­(H2O)n, Li+ and F– work in cooperation and interact with carbonyl oxygen and hydroxyl hydrogen of Cys to form a bidentate type. The neutral and zwitterionic forms are essentially isoenergetic when the water number reaches three in the presence of Li+, whereas this occurs at four water molecules in the presence of LiF. Further research revealed that the interaction between Li+/LiF and Cys was mainly electrostatic, followed by dispersion, and the weakest interaction occurs at the transition from the neutral form to zwitterionic form. Natural population analysis charge analyses show that for Cys·Li+(H2O)n, the positive charge is mostly concentrated on Li+ except for the system containing three water molecules. For Cys·LiF­(H2O)n, the positive charge is centered on the LiF unit in the range n = 0–6, and at n = 5, electron transfer from Cys to water occurs. Our study shows that the contribution of anions in zwitterionic state stabilization should be addressed more generally along with cations.

The intrinsic negative Poisson’s ratio effect in 2-dimensional nanomaterials have attracted a lot of research interests due to its superior mechanical properties, and new mechanisms have emerged in the nanoscale. In this paper, we designed a novel graphyne-like two-dimensional carbon nanostructure with a “butterfly” shape (GL-2D-1) and its configuration isomer with a “herring-bone” form (GL-2D-2) by means of density functional theoretical calculation and predicted their in-plane negative Poisson’s ratio effect and other mechanical properties. Both GL-2D-1 and GL-2D-2 present a significant negative Poisson’s ratio effect under different specific strains conditions. By contrast, GL-2D-2 presents a much stronger negative Poisson’s ratio effect and mechanical stability than does GL-2D-1. It is hoped that this work could be a useful structural design strategy for the development of the 2D carbon nanostructure with the intrinsic negative Poisson’s ratio.