Proton Conduction in Chiral Molecular Assemblies of Azolium-Camphorsulfonate Salts.

Chiral molecular assemblies have attracted considerable attention because of their interesting physical properties, such as spin-selective electron transport. Cation-anion salts of three azolium cations, imidazolium (HIm+), triazolium (HTrz+), and thiazolium (HThz+), in combination with a chiral camphorsulfonate (1S-CS-) and their racemic compounds (rac-CS-) were prepared and compared in terms of phase transitions, crystal structures, dynamics of constituent molecules, dielectric responses, and proton conductivities. The cation-anion crystals containing HIm+ showed no significant difference in proton conductivity between the homochiral and racemic crystals, whereas the HTrz+-containing crystals showed higher proton conductivity and lower activation energy in the homochiral form than in the racemic form. A two-dimensional hydrogen-bonding network consisting of HTrz+ and -SO3- groups and similar in-plane rotational motion was observed in both crystals; however, the HTrz+ cation in the homochiral crystal exhibited the rotational motion modulated with translational motion, whereas the HTrz+ cation in the racemic crystal exhibited almost steady in-plane rotational motion. The different motional degrees of freedom were confirmed by crystal structure analyses and temperature- and frequency-dependent dielectric constants. In contrast, steady in-plane rotational motion with the thermally activated fluctuating motion of CS- was observed both in homochiral and racemic crystals containing HIm+, which averaged the motional space of protons resulting in similar dielectric responses and proton conductivities. The control of motional degrees of freedom in homochiral crystals affects the proton conductivity and is useful for the design of molecular proton conductors.

Beyond the Conventional Limitation of Photocycloaddition Reaction in the Roomy Nanospace of a Metal-Organic Framework.

Topochemical reactions provide selective products based on the molecular position; however, they generally require molecules to be placed in strictly limited orientations and distances, making them less versatile. In this study, we found that by confining trans-4-styrylpyridine (4-spy) as a reactive substrate in a flexible metal-organic framework (MOF) nanospace, [2+2] cycloadducts can be selectively obtained, even when the distance between two C═C bonds of 4-spy in the crystal is 5.9 Å, which is much larger than the conventionally observed upper limit (4.2 Å). Such an unusual cyclization reaction is suggested to occur due to the transient proximity of the 4-spy due to the "swing" motion in the nanospace. The MOF nanospace, with its high degree of molecular structural freedom, can be applied to different platforms that do not require the fine constraints of reactive distances for solid-phase reactions.

A Steric-Repulsion-Driven Clutch Stack of Triaryltriazines: Correlated Molecular Rotations and a Thermoresponsive Gearshift in the Crystalline Solid.

We report that a newly developed type of triaryltriazine rotor, which bears bulky silyl moieties on the para position of its peripheral phenylene groups, forms a columnar stacked clutch structure in the crystalline phase. The phenylene units of the crystalline rotors display two different and interconvertible correlated molecular motions. It is possible to switch between these intermolecular geared rotational motions via a thermally induced crystal-to-crystal phase transition. Variable-temperature solid-state 2H NMR measurements and X-ray diffraction studies revealed that the crystalline rotor is characterized by a vertically stacked columnar structure upon introducing a bulky Si moiety with bent geometry as the stator. The structure exhibits correlated flapping motions via a combination of 85° and ca. 95° rotations between 295 and 348 K, concurrent with a negative entropy change (ΔS‡ = -23 ± 0.3 cal mol-1 K-1). Interestingly, heating the crystal beyond 348 K induces an anisotropic expansion of the column and lowers the steric congestion between the adjacent rotators, thus altering the correlated motions from a flapping motion to a correlated 2-fold 180° rotation with a lower entropic penalty (ΔS‡ = -14 ± 0.5 cal mol-1 K-1). The obtained results of our study suggest that the intermolecular stacking of the C3-symmetric rotator driven by the steric repulsion of the bulky stator represents a promising strategy for producing various correlated molecular motions in the crystalline phase. Moreover, direct and reversible modulation of the intermolecularly correlated rotation is achieved via a thermally induced crystal-to-crystal phase transition, which operates as a gearshift function at the molecular level.

Mechanical and Thermal ON-OFF Switching of the Vapochromic Behavior of a Luminescent Polymorphic Pt(II) Complex.

Vapochromic materials that exhibit color/luminescence changes induced by vapor exposure have attracted considerable attention. Herein, we report the grinding- and heating-induced ON-OFF switching of the vapochromic behavior of [Pt(ppyCl2)(Clacac)] (1; ppyCl2 = 2-(3-chlorophenyl)-4-chloropyridinato, Clacac = 3-chloroacetylacetonato). 1 formed yellow and orange polymorphs (1-Y and 1-O), and 1-Y could be converted to 1-Og, which showed a very similar crystal structure but with a broadened X-ray diffraction pattern compared with that of 1-O. Moreover, 1-Og can be reversibly transformed into 1-O via heating and grinding. Notably, 1-Og underwent a N,N-dimethylacetamide vapor-induced transformation to 1-Y, whereas 1-O did not undergo such a transformation. These results indicate the ON-OFF switching of vapochromic behavior induced via grinding and heating. This finding will be beneficial for developing intelligent molecular devices.

Proton Conduction at High Temperature in High-Symmetry Hydrogen-Bonded Molecular Crystals of RuIII Complexes with Six Imidazole-Imidazolate Ligands.

A new H-bonded crystal [RuIII (Him)3 (Im)3 ] with three imidazole (Him) and three imidazolate (Im- ) groups was prepared to obtain a higher-temperature proton conductor than a Nafion membrane with water driving. The crystal is constructed by complementary N-H⋅⋅⋅N H-bonds between the RuIII complexes and has a rare Icy-c* cubic network topology with a twofold interpenetration without crystal anisotropy. The crystals show a proton conductivity of 3.08×10-5  S cm-1 at 450 K and a faster conductivity than those formed by only HIms. The high proton conductivity is attributed to not only molecular rotations and hopping motions of HIm frameworks that are activated at ∼113 K, but also isotropic whole-molecule rotation of [RuIII (Him)3 (Im)3 ] at temperatures greater than 420 K. The latter rotation was confirmed by solid-state 2 H NMR spectroscopy; probable proton conduction routes were predicted and theoretically considered.

Proton Conduction at High Temperature in High-Symmetry Hydrogen-Bonded Molecular Crystals of RuIII Complexes with Six Imidazole-Imidazolate Ligands.

Invited for the cover of this issue is mainly the group of Makoto Tadokoro and co-workers at Tokyo University of Science. Other co-workers are Masaki Itoh, Ryota Nishimura, Kensuke Sekiguchi (TUS students), Dr. Norihisa Hoshino (Tohoku Univ.), Dr. Hajime Kamebuchi (Nihon Univ.), Dr. Jun Miyazaki (Tokyo Denki Univ.), Prof. Motohiro Mizuno (Kanazawa Univ.) and Prof. Tomoyuki Akutagawa (Tohoku Univ.). The image depicts on two mechanisms of proton transport rotations of the proton-conductive starburst molecule [RuIII (HIm)3 (Im)3 ]. Read the full text of the article at 10.1002/chem.202201397.

Three-Dimensional Metal-Organic Network Glasses from Bridging MF62- Anions and Their Dynamic Insights by Solid-State NMR.

Glassy-state coordination polymers (CPs) are a new class of network-forming glasses. In this work, we constructed glass-forming CPs composed of both anionic and neutral ligands as network formers. With the use of hexafluoro anions (MF62-) and 1,3-bis(4-pyridyl)propane (bpp), two isostructural CP crystals, [Zn(SiF6)(bpp)2] (ZnSi) and [Zn(TiF6)(bpp)2] (ZnTi), were synthesized. Solid-state 19F NMR revealed rotational motion of MF62- with dissociation and re-formation of the Zn-F coordination bonds in both CP crystals, which reflects the thermodynamic parameters related to the glass formability. The mobility of SiF62- is larger than that of TiF62-, suggesting a higher glass formability of ZnSi. When mechanical ball milling was conducted, ZnSi completely changed into a glassy state, whereas ZnTi showed incomplete glass formation. Examination of the amorphous structures elucidated retention and partial destruction of the Zn-F coordination bonds in ball-milled ZnSi and ZnTi, respectively. These results provide the relationship between the ligand dynamics and glass formability of CPs.

Isotropic Anhydrous Superprotonic Conductivity Cooperated with Installed Imidazolium Molecular Motions in a 3D Hydrogen-Bonded Phosphate Network.

Utilizing molecular motion is essential for the use of anhydrous superprotonic molecular proton conductors (σ beyond 10-4  S cm-1 ) as electrolytes in hydrogen fuel cells. However, molecular motion contributing to the improvement of intrinsic proton conduction has been limited and little clarified in relation to the proton conduction mechanism, limiting the development of material design guidelines. Here, a salt with a three-dimensional (3D) hydrogen-bonded (H-bonded) phosphate network with imidazolium cations installed inside was studied, whose components are known to exhibit molecular motions that contribute to proton conduction. Despite its anisotropic H-bonded network, the salt exhibits isotropic anhydrous superprotonic conductivity exceeding 10-3  S cm-1 at ≈351 K, which is the first example for organic molecular crystal. Variable-temperature X-ray structural analysis and solid-state 2 H NMR measurements revealed significant 3D molecular motion of imidazolium cations, which accelerate proton conduction via the 3D H-bonded phosphate network.

Room-Temperature Ring-Opening Polymerization of δ-Valerolactone and ϵ-Caprolactone Caused by Uptake into Porous Pillar[5]arene Crystals.

Confined space provides a reaction platform with altered reaction rate and selectivity compared with a homogeneous solution. In this work, porous phenolic pillar[5]arene crystals were used as a reaction space to promote and perturb equilibrium between lactones and their corresponding polyesters. Immersion of porous pillar[5]arene crystals in liquid lactones induced ring-opening polymerization of δ-valerolactone and ϵ-caprolactone at room temperature because the phenolic hydroxy groups have catalytic activity via hydrogen bonds and the pillar[5]arene cavities prefer linear guests. After the reaction, pillar[5]arene and polyesters formed pseudo-polyrotaxanes.

Thermally Responsive Poly(ethylene oxide)-Based Polyrotaxanes Bearing Hydrogen-Bonding Pillar[5]arene Rings*.

Poly(ethylene oxide)s (PEOs) are useful polymers with good water solubility, biological compatibility, and commercial availability. PEOs with various end groups were threaded into pillar[5]arene rings in a mixture of water and methanol to afford pseudopolyrotaxanes. Corresponding polyrotaxanes were also constructed by capping COOH-terminated pseudopolyrotaxanes with bulky amines, in which multiple hydrogen bonds involving the pillar[5]arene OH groups were critically important to prevent dethreading. The number of threaded ring components could be rationally controlled in these materials, providing a simple and versatile method to tune the mechanical and thermal properties. Specifically, a polyrotaxane with a high-molecular-weight axle became elastic upon heating above the melting point of PEOs and exhibited temperature-dependent shape memory property because of the topological confinement and crosslinked hydrogen bonds.

Thermally Responsive Poly(ethylene oxide)-Based Polyrotaxanes Bearing Hydrogen-Bonding Pillar[5]arene Rings.

Invited for the cover of this issue are Tomoki Ogoshi and co-workers at Kyoto University, Kanazawa University and Tokyo University of Agriculture and Technology. The image depicts musical notation to represent hydrogen bond networks and poly(ethylene oxide) chains. Read the full text of the article at 10.1002/chem.202005099.

Guest Vapor-Induced State Change of Structural Liquid Pillar[6]arene.

State change is a key phenomenon in materials science. We report the first observation of vapor-responsive reversible structural liquid-to-solid and solid-to-structural liquid state changes. We observed that a macrocyclic compound, a pillar[6]arene derivative bearing 12 n-hexyl substituents, is a room temperature structural liquid with unique properties. Formation of a host-guest complex between the pillar[6]arene cavity and the n-hexyl substituent results in a structural liquid with nanoscale structural heterogeneities. The structural liquid solidifies when exposed to competitive cyclohexane guest vapor, whereupon cyclohexane replaces the n-hexyl substituents in the pillar[6]arene cavity and the n-hexyl substituents located outside of the cavity crystallize into distinct nanolayer assemblies. The solid reverts back to the structural liquid when the cyclohexane guest is removed through heating under reduced pressure because of rethreading of the n-hexyl substituents into the cavity. The structural liquid-to-solid and solid-to-structural liquid changes are reversible through the uptake and release of cyclohexane guest vapor.

Host-Guest Complexation Using Pillar[5]arene Crystals: Crystal-Structure Dependent Uptake, Release, and Molecular Dynamics of an Alkane Guest.

Host-guest complexation has been mainly investigated in solution, and it is unclear how guest molecules access the assembled structures of host and dynamics of guest molecules in the crystal state. In this study, we studied the uptake, release, and molecular dynamics of n-hexane vapor in the crystal state of pillar[5]arenes bearing different substituents. Pillar[5]arene bearing 10 ethyl groups yielded a crystal structure of herringbone-type 1:1 complexes with n-hexane, whereas pillar[5]arene with 10 allyl groups formed 1:1 complexes featuring a one-dimensional (1D) channel structure. For pillar[5]arene bearing 10 benzyl groups, one molecule of n-hexane was located in the cavity of pillar[5]arene, and another n-hexane molecule was located outside of the cavity between two pillar[5]arenes. The substituent-dependent differences in molecular arrangement influenced the uptake, release, and molecular dynamics of the n-hexane guest. The substituent effects were not observed in host-guest chemistry in solution, and these features are unique for the crystal state host-guest chemistry of pillar[5]arenes.

Solid-state 23Na NMR spectroscopy studies of ordered and disordered cellulose nanocrystal films.

Cellulose nanocrystal films with either disordered or chiral nematic structures of varying helical pitch were investigated using 23Na solid-state nuclear magnetic resonance (NMR) spectroscopy. Spin lattice relaxation of 1H correlated with 23Na analyzed by indirect observation using polarization transfer from 1H nuclei to 23Na nuclei showed that the Na+ cations are well hydrated in the cellulose nanocrystal films. Linewidth analysis in solid-state 23Na NMR showed that the Na+ cations move in confined spaces, and that the Na+ cations in the film having disordered structure are more dynamic than in the films having ordered structure. From lineshape analysis of the 23Na 2D nutation NMR spectra, we can distinguish the Na+ environments within the ordered and disordered films, and find trends in anisotropic interaction parameters between ordered samples with different pitches. These are the first detailed 23Na NMR spectroscopic studies of CNC-Na+ films, and they show that this technique may be a powerful probe for characterizing the extent of order in nanocellulose samples.

Local structure and hydrogen bond characteristics of imidazole molecules for proton conduction in acid and base proton-conducting composite materials.

Composite materials of acidic polymers and basic molecules have high proton-conductivity. Understanding the proton conduction mechanism of the composite materials, which depends on hydrogen bond characteristics, is an important task for developing materials with high proton-conductivity. This work is focused on poly(vinylphosphonic acid)-imidazole and alginic acid-imidazole as examples of composite materials of acidic polymers and basic molecules and examines the local structure and hydrogen bond characteristics of imidazole (Im) molecules in composite materials using density functional theory. The results show that Im molecules interact strongly with polymeric acids in these composite materials and that the interaction energy increases with the increase in the number of Im molecules. The rotational motion of Im molecules occurs in the segment where only Im molecules without excess protons are hydrogen-bonded to each other. The calculation results for the various segments, which depend on the hydrogen bonding environment, show that the proton conduction process in composite materials consists of the following steps: proton transfer in the segment where Im molecules interact with polymeric acids, proton transfer in the segment where Im molecules are affected by excess protons, and Grotthuss diffusion with reorientation of Im molecules in the segment where only Im molecules without excess protons are bonded to each other.

Investigation of the effects of sintering and indium-doping of zinc oxide using 67Zn magic angle spinning NMR analysis.

Indium-doped zinc oxide, a potential alternative material to indium tin oxide, was analyzed in powder form via 67Zn magic angle spinning nuclear magnetic resonance (MAS NMR). The 67Zn MAS NMR results show that the line shapes of zinc oxide were broadened by sintering, which was also observed for indium-doped zinc oxide, in which the broadening also depended on the sintering time. Furthermore, the line shapes of indium-doped zinc oxide were significantly broader than those of the corresponding zinc oxide, and were independent of the degree of indium doping. This indicates that indium atoms are associated into cluster-like structures in this compound.

Chiral triptycene-pyrene π-conjugated chromophores with circularly polarized luminescence.

A pair of optically pure triptycene derivatives ((R,R)- and (S,S)-3) containing fluorescent pyrene-based π-conjugated pendant groups attached through amide spacers were prepared via a resolution step using chiral high-performance liquid chromatography. Their absorption, circular dichroism, photoluminescence and circularly polarized luminescence (CPL) properties were investigated under various solution conditions. (R,R)- and (S,S)-3 exhibited clear solvent- and concentration-dependences of the optical and chiroptical properties as a result of the interconversion between molecularly dispersed and aggregate states. We also observed that (R,R)- and (S,S)-3 emitted left- and right-handed circularly polarized light, respectively, upon UV irradiation under aggregation conditions, and their dissymmetry factors were found to be greater than 1.0 × 10-3. Based on the contrasting result that almost no CPL signal appeared in the monomeric solution state, the resulting CPL was considered to arise from the supramolecular chirality induced in the hydrogen-bonded aggregate, wherein the pyrenyl pendants of 3 were most likely arranged in a preferred-handed twisting structure.

Potential energy construction in the diabatic picture for quantum mechanical rate constants of intermolecular proton transfer.

We propose a simple method for potential construction in the diabatic picture and the estimation of thermal rate constants for intermolecular proton transfer reactions using quantum dynamics simulations carried out on the constructed potentials. For symmetrical and asymmetrical proton transfer pairs, the obtained potentials and rate constants were in good agreement with the reference values. Furthermore, our method is used for the analysis of proton transfer in crystalline imidazolium succinate and discusses the proton conductivity in terms of intermolecular proton transfer. This approach can be used to estimate proton transfer rate constants for large molecular systems, even when the calculation of the transition state is impossible.

Conducting π Columns of Highly Symmetric Coronene, The Smallest Fragment of Graphene.

Coronene, which is the smallest D6h -symmetric polycyclic aromatic hydrocarbon, attracts particular attention as a basic component of electronic materials because it is the smallest fragment of graphene. However, carrier generation by physical methods, such as photo- or electric field-effect, has barely been studied, primarily because of the poor π-conduction pathway in pristine coronene solid. In this work we have developed unprecedented π-stacking columns of cationic coronene molecules by electrochemical hole-doping with polyoxometallate dianions. The face-to-face π-π interactions as well as the partially charged state lead to electrical conductivity at room temperature of up to 3 S cm(-1) , which is more than 10 orders of magnitude higher than that of pristine coronene solid. Additionally, the robust π-π interactions strongly suppress the in-plane rotation of the coronene molecules, which has allowed the first direct observation of the static Jahn-Teller distortion of cationic coronene molecules.

An NMR study on the mechanisms of freezing and melting of water confined in spherically mesoporous silicas SBA-16.

Thermodynamic and dynamic properties of water confined in mesoporous silica glass SBA-16 were investigated by DSC, and (1,2)H NMR spectroscopy and (2)H NMR spin-lattice relaxation time (T1) as a function of pore size. SBA-16 possesses the main spherical pores, interconnecting channels and micropores (corona). Water in the characteristic spherical pores of SBA-16 freezes at the homogeneous nucleation temperature of water. Between room and freezing temperatures, the correlation time of the isotropic rotation of water in the pores of SBA-16 followed the Vogel-Fulcher-Tammann (VFT) relation, which reflects the formation and growth of clusters of fragile water for changing to the strong water. The vitrification of water in micropores around 200 K was observed by (2)H NMR. Above 200 K, the correlation time of the rotation of water in micropores exhibited non-Arrhenius behavior, which is correlated with the gradual decrease in the mobility of water due to the growth of hydrogen bonding, forming low density water before vitrification. After vitrification, the activation energy of the rotation of water in micropores was 25-33 kJ mol(-1), which was similar to that in ice Ih for all samples. The freedom of cluster formation and water rotation increased with the increasing the pore size.