Chiral Honeycomb Lattices of Nonplanar π-Conjugated Supramolecules with Protected Dirac and Flat Bands.

The honeycomb lattice is a fundamental two-dimensional (2D) network that gives rise to surprisingly rich electronic properties. While its expansion to 2D supramolecular assembly is conceptually appealing, its realization is not straightforward because of weak intermolecular coupling and the strong influence of a supporting substrate. Here, we show that the application of a triptycene derivative with phenazine moieties, Trip-Phz, solves this problem due to its strong intermolecular π-π pancake bonding and nonplanar geometry. Our scanning tunneling microscopy (STM) measurements demonstrate that Trip-Phz molecules self-assemble on a Ag(111) surface to form chiral and commensurate honeycomb lattices. Electronically, the network can be viewed as a hybrid of honeycomb and kagome lattices. The Dirac and flat bands predicted by a simple tight-binding model are reproduced by total density functional theory (DFT) calculations, highlighting the protection of the molecular bands from the Ag(111) substrate. The present work offers a rational route for creating chiral 2D supramolecules that can simultaneously accommodate pristine Dirac and flat bands.

Optoelectronic conversion and polarization hysteresis in organic MISM and MISIM devices with DA-type single-component molecules.

Organic electronic devices offer various advantages, such as low cost and tunability. However, the organic semiconductors used in these devices have significant drawbacks, including instability in air and low carrier mobility. To address these challenges, we recently introduced organic MISM and MISIM (M = metal, I = insulator, S = semiconductor) devices, which effectively generate photo-induced displacement current and exhibit ferroelectric behavior. In previous studies, the S layer consisted of an organic donor-acceptor (DA) bilayer. In the present research, we fabricated MISM and MISIM devices using DA-type single-component molecules as the S layer and examined their photocurrent and polarization hysteresis. While the performance of these devices does not surpass that of DA bilayer devices, we discovered that DA-type single-component molecules can be utilized for photoelectric conversion and polarization trapping.

X-ray crystallographic analysis of the antiferromagnetic low-temperature phase of galvinoxyl: investigating magnetic duality in organic radicals.

Galvinoxyl, as one of the most extensively studied organic stable free radicals, exhibits a notable phase transition from a high-temperature (HT) phase with a ferromagnetic (FM) intermolecular interaction to a low-temperature (LT) phase with an antiferromagnetic (AFM) coupling at 85 K. Despite significant research efforts, the crystal structure of the AFM LT phase has remained elusive. This study successfully elucidates the crystal structure of the LT phase, which belongs to the P1̄ space group. The crystal structure of the LT phase is found to consist of a distorted dimer, wherein the distortion arises from the formation of short intermolecular distances between anti-node carbons in the singly-occupied molecular orbital (SOMO). Starting from the structure of the LT phase, wave function calculations show that the AFM coupling 2J/kB varies significantly from -1069 K to -54 K due to a parallel shift of the molecular planes within the dimer.

Enhanced Circularly Polarized Luminescence by a Homochiral Guest-Host Interaction in Gyroidal MOFs, [Ru(bpy)3] [M2(ox)3] (bpy = 2,2'-Bipyridyl, ox = Oxalate, M = Zn, Mn).

We report the circularly polarized luminescence (CPL) for [Ru(bpy)3]I2 (1) and [Ru(bpy)3][M2(ox)3] (M = Zn (2), Mn (3)). Whereas compound 1 is a simple salt of [Ru(bpy)3]2+, 2 and 3 are MOFs in which the chiral [Ru(bpy)3]2+ ions are encapsulated in a homochiral gyroidal skeleton of [M2(ox)3]2-. Whereas the solution of 1 exhibited weak CPL with a luminescence dissymmetry factor of |glum| ∼ 10-4, the CPL was significantly enhanced in solid-state 1-3 with |glum| = 2 × 10-2 for 1, 4 × 10-2 for 2, and 1 × 10-1 for 3. The enhanced CPL in 3 was attributable to an energy transfer between the homochiral guest and host in 3.

Ideal trigonal prismatic coordination geometry of Co(II) in a honeycomb MOF with a triptycene-based ligand.

A metal-organic framework (MOF) comprised of cobalt ions and triptycene-based 3-fold symmetric bridging ligands 9,10-[1,2]benzenoanthracene-2,3,6,7,14,15(9H,10H)-hexaone (o-TT) was prepared. Single-crystal structure analysis revealed a 2D honeycomb network structure and the ideal trigonal prismatic geometry of the Co(II) ion. The magnetic anisotropy of the Co(II) ion in the trigonal prism coordination geometry was analyzed via magnetic measurements and model calculations.

Magnetometric Characterization of Intermediates in the Solid-State Electrochemistry of Redox-Active Metal-Organic Frameworks.

Electrochemical energy storage has been a widely discussed application of redox-active metal-organic frameworks (MOFs) in the past 5 years. Although MOFs show outstanding performance in terms of gravimetric or areal capacitance and cyclic stability, unfortunately their electrochemical mechanisms are not well understood in most cases. Traditional spectroscopic techniques, such as X-ray photoelectron spectroscopy (XPS) and X-ray absorption fine structure (XAFS), have only provided vague and qualitative information about valence changes of certain elements, and the mechanisms proposed based on such information are often highly disputable. In this article, we report a series of standardized methods, including the fabrication of solid-state electrochemical cells, electrochemistry measurements, the disassembly of cells, the collection of MOF electrochemical intermediates, and physical measurements of the intermediates under the protection of inert gases. By using these methods for quantitatively clarifying the electronic and spin state evolution within a single electrochemical step of redox-active MOFs, one can provide clear insight into the nature of electrochemical energy storage mechanisms not only for MOFs, but also for all other materials with strongly correlated electronic structures.

Synthesis of Isomeric Tb3+ -Phthalocyanine Double-Decker Complexes Depending on the Difference in the Direction of Coordination Plane and Their Magnetic Properties.

Invited for the cover of this issue are Kentaro Tanaka at Nagoya University and co-workers. The image depicts three isomers of a terbium(III) phthalocyanine double-decker complex made from C4h symmetrically substituted phthalocyanines and their magnetic properties. Read the full text of the article at 10.1002/chem.202203272.

Graphite-like Charge Storage Mechanism in a 2D π-d Conjugated Metal-Organic Framework Revealed by Stepwise Magnetic Monitoring.

Quasi-two-dimensional (2D) fully π-d conjugated metal-organic frameworks (MOFs) have been widely employed as active materials of secondary batteries; however, the origin of their high charge storage capacity is still unknown. Some reports have proposed a mechanism by assuming the formation of multiple radicals on one organic ligand, although there is no firm evidence for such a mechanism, which would run counter to the resonance theory. In this work, we utilized various magnetometric techniques to monitor the formation and concentration of paramagnetic species during the electrochemical process of 2D π-d conjugated Cu-THQ MOF (THQ = tetrahydroxy-1,4-benzoquinone). The spin concentration of the fully reduced (discharged 1.5 V) electrode was estimated to be around only 0.1 spin-1/2 per CuO4 unit, which is much lower than that of the expected "diradical" form. More interestingly, a significant elevation of the temperature-independent paramagnetic term was simultaneously observed, which indicates the presence of delocalized π electrons in this discharged state. Such results were corroborated by first-principles density functional theory calculations and the electrochemically active density of states, which reveal the microscopic mechanism of the charge storage in the Cu-THQ MOF. Hence, a graphite-like charge storage mechanism, where the π-electron band accepts/donates electrons during the charge/discharge process, was suggested to explain the excessive charge storage of Cu-THQ. This graphite-like charge storage mechanism revealed by magnetic studies can be readily generalized to other π-d conjugated MOFs.

Synthesis of Isomeric Tb3+ -Phthalocyanine Double-Decker Complexes Depending on the Difference in the Direction of Coordination Plane and Their Magnetic Properties.

A C4h symmetrically substituted phthalocyanine, 1,8,15,22-tertrakis(2,4-dimethylpent-3-oxy)phthalocyanine (H2 TdMPPc), was used to synthesize Tb3+ -phthalocyanine double-decker complexes ([Tb(TdMPPc)2 ]s). Because H2 TdMPPc has C4h symmetry, S,S, R,R, and meso isomers of [Tb(TdMPPc)2 ] were obtained depending on the difference in the direction of the coordination plane of two C4h -type phthalocyanines with respect to a central Tb3+ ion. We investigated the physical properties of these [Tb(TdMPPc)2 ] isomers, including their single-ion magnetic properties, and found that the spin-reversal energy barrier (Ueff ) of the meso isomer was apparently higher than that of the enantiomers. Detailed crystal structural analyses indicated that the meso isomer has a more symmetrical structure than do the enantiomers, thereby suggesting that the higher Ueff of the meso isomer originated from the more highly symmetrical structure.

N-doped nonalternant aromatic belt via a six-fold annulative double N-arylation.

The design and synthesis of nitrogen (N)-doped molecular nanocarbons are of importance since N-doped nanocarbons have received significant attention in materials science. Herein, we report the synthesis and X-ray crystal structure of a nitrogen-inserted nonalternant aromatic belt. The palladium-catalyzed six-fold annulative double N-arylation provided an aromatic belt bearing six nitrogen atoms in one step from cyclo[6]paraphenylene-Z-ethenylene, the precursor of the (6,6)carbon nanobelt. The C 3i-symmetric structure of the aromatic belt in the solid state was revealed using X-ray crystallography. The multistep (electro)chemical oxidation behavior of the belt, which was facilitated by the six p-methoxyaniline moieties, was studied, and a stable dication species was successfully identified by X-ray crystallography. The present study not only shows the unique structure and properties of the N-doped nonalternant aromatic belt but also expands the scope of accessibility of synthetically difficult belt molecules by the conventional intramolecular contraction pathway.

Improvement in Cycle Life of Organic Lithium-Ion Batteries by In-Cell Polymerization of Tetrathiafulvalene-Based Electrode Materials.

Redox-active organic molecules are promising candidates for next-generation electrode materials. Nevertheless, finding low-molecular-weight organic materials with a long cycle life remains a crucial challenge. Herein, we demonstrate the application of tetrathiafulvalene and its vinyl analogue bearing triphenylamines as long-cycle-life electrodes for lithium-ion batteries (LIBs). These molecules were successfully synthesized using palladium-catalyzed C-H arylation. Electrochemical analysis revealed that a polymer formed on the electrode. LIBs comprising these molecules exhibited noteworthy charge-discharge properties with a long cycle life (the capacity after 100 cycles was greater than 90% of the discharge capacity in the third cycle) and a high utilization ratio (approximately 100%). "In-cell" polymerization during the first charge process is considered to contribute to the effect. This study indicates new avenues for the creation of organic materials for rechargeable batteries.

Exfoliation of Al-Residual Multilayer MXene Using Tetramethylammonium Bases for Conductive Film Applications.

This study describes the concise exfoliation of multilayer Ti3C2T x MXene containing residual aluminum atoms. Treatment with tetramethylammonium base in a co-solvent of tetrahydrofuran and H2O produced single-layer Ti3C2T x , which was confirmed via atomic force microscopy observations, with an electrical conductivity 100+ times that of Ti3C2T x prepared under previously reported conditions. The scanning electron microscopy and X-ray diffraction measurements showed that the exfoliated single-layer Ti3C2T x MXenes were reconstructed to assembled large-domain layered films, enabling excellent macroscale electric conductivity. X-ray photoelectron spectroscopy confirmed the complete removal of residual Al atoms and the replacement of surface fluorine atoms with hydroxy groups. Using the exfoliated dispersion, a flexible transparent conductive film was formed and demonstrated in an electrical application.

Stabilization of Interfacial Polarization and Induction of Polarization Hysteresis in Organic MISIM Devices.

Molecule-based ferroelectrics has attracted much attention because of its advantages, such as flexibility, light weight, and low environmental load. In the present work, we examined an organic metal|insulator|semiconductor|insulator|metal (MISIM) device structure to stabilize the interfacial polarization in the S layer and to induce polarization hysteresis even without bulk ferroelectrics. The MISIM devices with I = parylene C and S = TMB (=3,3',5,5'-tetramethylbenzidine)-TCNQ (=tetracyanoquinodimethane) exhibited hysteresis loops in the polarization-voltage (P-V) curves not only at room temperature but also over a wide temperature range down to 80 K. The presence of polarization hysteresis for MISIM devices was theoretically confirmed by an electrostatic model, which also explained the observed thickness dependence of the I layers on the P-V curves. Polarization hysteresis curves were also obtained in MISIM devices using typical organic semiconductors (ZnPc, C60, and TCNQ) as the S layer, demonstrating the versatility of the interfacial polarization mechanism.

Enhanced proton conductivity in a flexible metal-organic framework promoted by single-crystal-to-single-crystal transformation.

MFM-722(Pb)-DMA undergoes a single-crystal-to-single-crystal (SCSC) transformation to give MFM-722(Pb)-H2O via ligand substitution upon exposure to water vapour. In situ single crystal impedance spectroscopy reveals an increase in proton conductivity due to this structural transition, with MFM-722(Pb)-H2O showing a proton conductivity of 6.61 × 10-4 S cm-1 at 50 °C and 98% RH. The low activation energy (Ea = 0.21 eV) indicates that the proton conduction follows a Grotthuss mechanism.

Quantum Spin Liquid State in a Two-Dimensional Semiconductive Metal-Organic Framework.

Two-dimensional metal-organic frameworks (2D MOFs) have attracted much attention, as they are the crystalline materials that exhibit both conductivity and microporosity. Numerous efforts have been made to advance their application as chemiresistive sensors or electrochemical capacitors. However, the intrinsic physical properties and spin states of these materials remain poorly understood. Most of these 2D MOFs possess a honeycomb lattice, with a Kagomé lattice arrangement of metal cations. These structural characteristics suggest that these MOFs would be candidates for geometrically frustrated spin systems with unprecedented magnetic phenomena. Herein, by performing magnetic susceptibility and specific heat measurements at an ultralow temperature down to 38mK on a 2D semiconductive MOF, Cu3(HHTP)2, a quantum spin liquid state that arises from the geometrical frustration was suggested. This result illustrates the potential of strongly correlated MOFs as systems with emergent phenomena induced by unusual structural topologies.

Chemical potentials of electric double layers at metal-electrolyte interfaces: dependence on electrolyte concentration and electrode materials, and application to field-effect transistors.

When a metal is soaked in an electrolyte solution, the metal and solution affect each other through the formation of electric double layers (EDLs) at their interfaces. The EDLs at metal-electrolyte interfaces can realize high-density charge-carrier injections and accumulations, and thus have recently attracted attention for their potential application to energy storage, and electronic and electrochemical devices. In such EDL-based devices, including field-effect transistors (FETs), the potential energy of surface electrons in the metal electrodes (EM) governs the transistor device performance. This is in clear contrast to redox-driven electrochemical devices such as dye-sensitized solar cells and electrochromic devices, whose performance is primarily governed by the potentials of the redox-active species. However, there has been no systematic research to bridge the distance between metal electrons and electrolyte ions. In the present study, we carefully examined the dependence of EM of ITO, Au and Pt electrodes on the concentration of the PEG solutions of LiCl and MgCl2, because it has been well established that the chemical potential of electrolyte solutions is dependent on the solution concentrations. Our results showed that, at the same electrolyte concentration, the values of EM increased in the order of ITO, Au and Pt; moreover, on the same electrode, EM showed linear decreases as a function of the logarithm of the electrolyte concentrations. To understand these behaviors, we developed a theoretical treatment of the EDLs based on the simple Gouy-Chapman model, and obtained the theoretical expressions of EM in terms of the concentration of electrolyte and the work function of the metal electrode (ΦM), which were found to successfully explain the dependences of EM on the electrolyte concentration and the electrode materials. We also examined the EDL-FETs of platinum phthalocyanine (PtPc), with various LiCl-PEG solutions of different concentrations as gate electrolytes. The threshold voltage eVT and EM exhibited a linear relation, which was well explained by the relation between EM and the valence band energy EVB of PtPc. The transfer characteristics at various gate voltage VG were found to be well normalized by a function of eVG + EM.