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Design of molecular structures showing fast ion conductive/transport pathways in the solid state has been a significant challenge. The amorphous or glassy phase in organic polymers works well for fast ion conductivity because of their dynamic and random structure. However, the main issue with these polymers has been the difficulty in elucidating the mechanisms of ion conduction and thus low designability. Furthermore, the amorphous or glassy state of ion conductive polymers often confronts the problems of structural/mechanical stabilities. Covalent organic frameworks (COFs) are an emerging class of crystalline organic polymers with periodic structure and tunable functionality, which exhibit potential as a unique ion conductor/transporter. Here, we describe the use of a COF as a medium for all-solid-state Li+ conductivity. A bottom-up self-assembly approach was applied to covalently reticulate the flexible, bulky, and glassy poly­(ethylene oxide) (PEO) moieties that can solvate Li+ for fast transport by their segmental motion in the rigid two-dimensional COF architectures. Temperature-dependent powder X-ray diffraction and thermogravimetric analysis showed that the periodic structures are intact even above 300 °C, and differential scanning calorimetry and solid-state NMR revealed that the accumulated PEO chains are highly dynamic and exhibit a glassy state. Li+ conductivity was found to depend on the dynamics and length of PEO chains in the crystalline states, and solid-state Li+ conductivity of 1.33 × 10–3 S cm–1 was achieved at 200 °C after LiTFSI doping. The high conductivity at the specified temperature remains intact for extended periods of time as a result of the structure’s robustness. Furthermore, we demonstrated the first application of a COF electrolyte in an all-solid-state Li battery at 100 °C.

The development of anhydrous proton-conducting materials is critical for the fabrication of high-temperature (>100 °C) polymer electrolyte membrane fuel cells (HT-PEMFCs) and remains a significant challenge. Covalent organic frameworks (COFs) are an emerging class of porous crystalline materials with tailor-made nanochannels and hold great potential for ion and molecule transport, but their poor chemical stability poses great challenges in this respect. In this contribution, we present a bottom-up self-assembly strategy to construct perfluoroalkyl-functionalized hydrazone-linked 2D COFs and systematically investigate the effect of different lengths of fluorine chains on their acid stability and proton conductivity. Compared with their nonfluorous parent COFs, fluorinated COFs possess structural ultrastability toward strong acids as a result of enhanced hydrophobicity (water contact angle of 144°). Furthermore, the superhydrophobic 1D nanochannels can serve as robust hosts to accommodate large amounts of phosphonic acid for fast and long-term proton conduction under anhydrous conditions and a wide temperature range. The anhydrous proton conductivity of fluorinated COFs is 4.2 × 10–2 S cm–1 at 140 °C after H3PO4 doping, which is 4 orders of magnitude higher than their nonfluorous counterparts and among the highest values of doped porous organic frameworks so far. Solid-state NMR studies revealed that H3PO4 forms hydrogen-boding networks with the frameworks and perfluoroalkyl chains of COFs, and most of the H3PO4 molecules are highly dynamic and mobile while the frameworks are rigid, which affords rapid proton transport. This work paves the way for the realization of the target properties of COFs through predesign and functionalization of the pore surface and highlights the great potential of COF nanochannels as a rigid platform for fast ion transportation.

We describe the encapsulation of mobile proton carriers into defect sites in nonporous coordination polymers (CPs). The proton carriers were encapsulated with high mobility and provided high proton conductivity at 150 °C under anhydrous conditions. The high proton conductivity and nonporous nature of the CP allowed its application as an electrolyte in a fuel cell. The defects and mobile proton carriers were investigated using solid-state NMR, XAFS, XRD, and ICP-AES/EA. On the basis of these analyses, we concluded that the defect sites provide space for mobile uncoordinated H3PO4, H2PO4(-), and H2O. These mobile carriers play a key role in expanding the proton-hopping path and promoting the mobility of protons in the coordination framework, leading to high proton conductivity and fuel cell power generation.

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ABSTRACT:We have previously suggested that crystalline Bombyx mori silk in silk 16 II form (the silk structure after spinning) is not a simple antiparallel β-sheet but is 17 intrinsically heterogeneous. Using the peptide (AG) 15 36 and dried under mild conditions) has been shown to possess a 37 repeated type II β-turn structure. 7−9 On the other hand, the 38 precise intermolecular packing in the Silk II form (representing 39 the core of the spun silk fiber) has not yet been determined. 40 Using X-ray fiber diffraction of the crystalline region, the 41 structure of Silk II was first characterized by Marsh, Corey, and 42 Pauling 10 as a regular array of antiparallel β-sheets: this 43 structure remains the classic image of β-sheet silk. We call this 44 cxs00 | ACSJCA | JCA10.0.1465/W Unicode | research.3f (R3.6.i5 HF05:4232 | 2.0 alpha 39) 2014/10/10 09:17:00 | PROD-JCA1 | rq_3109040 | 12/15/2014 13:54:33 | 9 | JCA-DEFAULT 65 using a small (Ala-Gly) 15 peptide as the model. The alternating 66 copolypeptide (Ala-Gly) n has been generally accepted as a good 67 model of the crystalline region, NMR spectra of (AG) n 68 correspond closely to those obtained using the crystalline 69 fraction of native silk II fibers, 7−16 and the torsion angles of the 70 straight backbone chains correspond to the typical angles of an 71 antiparallel β-sheet. 17 In previous 13 C solid state NMR studies 72 of (AG) n , the 13 Cβ signal of the Ala residues has been reported 73 to consist of three peaks. 15,16 The high-field peak was assigned 74 to a distorted β-turn/random coil, while the other two peaks 75 were assigned to antiparallel β-sheet structures with different 76 intermolecular arrangements. The key challenge lies in the ability to discern and resolve the 92 two kinds of antiparallel β-sheet chains with different 93 intermolecular packing arrangements, as detected here and in 94 the earlier 13 C CP/MAS NMR study. 15,16 We therefore carried 95 out a search of packing arrangements, guided by crystallo-96 graphic and NMR data; refined the resulting structures; and 97 tested them against experimental data. The peptide (AG) n 98 crystallizes in space group P2 1 , a rectangular unit cell with the 99 parameters a = 9.38 Å, b = 9.49 Å, and c = 6.98 Å. The Marsh 100 model places the molecular axis along b but is otherwise very 101 similar: a = 9.40 Å, b = 6.97 Å, and c = 9.20 Å. In order to 102 generate two kinds of β-sheet models with different 103 intermolecular arrangements, we had the idea to calculate 104 atomic coordinates for the chains, setting either c or b along the 105 molecular axis. For each of these two models, energy 106 optimization was performed. 9 1 H, 13 C, and 15 N chemical shifts 107 were then predicted for the two antiparallel β-sheet structures 108 using the GIPAW method. 23

Although magic angle spinning (MAS) solid-state NMR is a powerful technique to obtain atomic-resolution insights into the structure and dynamics of a variety of chemical and biological solids, poor sensitivity has severely limited its applications. In this study, we demonstrate an approach that suitably combines proton-detection, ultrafast-MAS and multiple frequency dimensions to overcome this limitation. With the utilization of proton-proton dipolar recoupling and double quantum (DQ) coherence excitation/reconversion radio-frequency pulses, very high-resolution proton-based 3D NMR spectra that correlate single-quantum (SQ), DQ and SQ coherences of biological solids have been obtained successfully for the first time. The proposed technique requires a very small amount of sample and does not need multiple radio-frequency (RF) channels. It also reveals information about the proximity between a spin and a certain other dipolar-coupled pair of spins in addition to regular SQ/DQ and SQ/SQ correlations. Although 1H spectral resolution is still limited for densely proton-coupled systems, the 3D technique is valuable to study dilute proton systems, such as zeolites, small molecules, or deuterated samples. We also believe that this new methodology will aid in the design of a plethora of multidimensional NMR techniques and enable high-throughput investigation of an exciting class of solids at atomic-level resolution.

Nuclear magnetic resonance (NMR) spectroscopy of protons in protonated solids is challenging. Fast magic angle spinning (MAS) and homonuclear decoupling schemes, in conjunction, with high magnetic fields have improved the proton resolution. However, experiments to quantitatively measure 1 H− 1 H distances still remain elusive due to the dense proton−proton dipolar coupling network. A novel MAS solidstate NMR pulse sequence is proposed to selectively recouple and measure interproton distances in protonated samples. The phase-modulated sequence combined with a judicious choice of transmitter frequency is used to measure quantitative 1 H− 1 H distances on the order of 3 Å in Lhistidine•HCl•H 2 O, despite the presence of other strongly coupled protons. This method provides a major boost to NMR crystallography approaches for structural determination of pharmaceutical molecules by directly measuring 1 H− 1 H distances. The band-selective nature of the sequence also enables observation of selective 1 H− 1 H correlations (e.g., H N −H N /H N −H α /Η Ν −Η Methyl ) in peptides and proteins, which should serve as useful restraints in structure determination.