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.

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.

Self-assembled honeycomb lattice in the monolayer of cyclic thiazyl diradical BDTDA (= 4,4'-bis(1,2,3,5-dithiadiazolyl)) on Cu(111) with a zero-bias tunneling spectra anomaly.

Scanning tunneling microscopy (STM) observation reveals that a cyclic thiazyl diradical, BDTDA (= 4,4'-bis(1,2,3,5-dithiadiazolyl)), forms a well-ordered monolayer honeycomb lattice consisting of paramagnetic corners with unpaired electrons on a clean Cu(111) surface. This BDTDA lattice is commensurate with the triangular lattice of Cu(111), with the former being 3 × 3 larger than the latter. The formation of the BDTDA monolayer structure, which is significantly different from its bulk form, is attributed to an interaction with the metal surface as well as the intermolecular assembling forces. STM spectroscopy measurements on the BDTDA molecules indicate the presence of a characteristic zero-bias anomaly centered at the Fermi energy. The origin of this zero-bias anomaly is discussed in terms of the Dirac cones inherent to the honeycomb structure.

Discovery of the K4 Structure Formed by a Triangular π Radical Anion.

The K4 structure was theoretically predicted for trivalent chemical species, such as sp(2) carbon. However, since attempts to synthesize the K4 carbon have not succeeded, this allotrope has been regarded as a crystal form that might not exist in nature. In the present work, we carried out electrochemical crystallization of the radical anion salts of a triangular molecule, naphthalene diimide (NDI)-Δ, using various electrolytes. X-ray crystal analysis of the obtained crystals revealed the K4 structure, which was formed by the unique intermolecular π overlap directed toward three directions from the triangular-shape NDI-Δ radical anions. Electron paramagnetic resonance and static magnetic measurements confirmed the radical anion state of NDI-Δ and indicated an antiferromagnetic intermolecular interaction with the Weiss constant of θ = -10 K. The band structure calculation suggested characteristic features of the present material, such as a metallic ground state, Dirac cones, and flat bands.

The key role of vibrational entropy in the phase transitions of dithiazolyl-based bistable magnetic materials.

The neutral radical 1,3,5-trithia-2,4,6-triazapentalenyl (TTTA) is a prototype of molecule-based bistable materials. TTTA crystals undergo a first-order phase transition between their low-temperature diamagnetic and high-temperature paramagnetic phases, with a large hysteresis loop that encompasses room temperature. Here, based on ab initio molecular dynamics simulations and new X-ray measurements, we uncover that the regular stacking motif of the high-temperature polymorph is the result of a fast intra-stack pair-exchange dynamics, whereby TTTA radicals continually exchange the adjacent TTTA neighbour (upper or lower) with which they form an eclipsed dimer. Such unique dynamics, observed in the paramagnetic phase within the whole hysteresis loop, is the origin of a significant vibrational entropic gain in the low-temperature to high-temperature transition and thereby it plays a key role in driving the phase transition. This finding provides a new key concept that needs to be explored for the rational design of novel molecule-based bistable magnetic materials.

Multidimensional network structures and versatile magnetic properties of intermolecular compounds of a radical-anion ligand, [1,2,5]thiadiazolo[3,4-f][1,10]phenanthroline 1,1-dioxide.

The crystal structures and magnetic properties of seven kinds of [1,2,5]thiadiazolo[3,4-f][1,10]phenanthroline 1,1-dioxide (tdapO2) radical-anion salts, namely, K·tdapO2, K·tdapO2·0.5MeCN, K·(tdapO2)2, Rb·(tdapO2)2, Cs7·(tdapO2)6·ClO4, (NH4)2·tdapO2·I, and Hppda·tdapO2·MeCN, were investigated. Single-crystal X-ray analyses of these radical-anion salts revealed formation of π-stacking columns and the presence of intercolumnar coordination bonding or hydrogen bonding. The intermolecular magnetic coupling constants in these salts range from strong antiferromagnetic (J/kB = -310 K) to ferromagnetic (J/kB = 24 K). Ab initio calculations performed on the nearest-neighbor radical pairs in the π-stacking columns suggested that the magnetic interactions are strongly governed by the overlap between the two anionic radical species and well explain the observed ferromagnetic and antiferromagnetic interactions. In addition, calculations of a hypothetical oxygen-less tdap analogue suggested that the presence of oxygen in tdapO2 significantly reduces the hopping integral and enhances the probability of ferromagnetic interaction.

Monovalent and mixed-valent potassium salts of [1,2,5]thiadiazolo[3,4-f][1,10]phenanthroline 1,1-dioxide: a radical anion for multidimensional network structures.

A novel phenanthlorine derivative, [1,2,5]thiadiazolo[3,4-f][1,10]phenanthroline 1,1-dioxide (tdapO(2)), was prepared to act as a radical-anion building block for coordination polymers. The crystal structures and magnetic properties of the monovalent and mixed-valent radical-anion salts K·tdapO(2) and K·(tdapO(2))(2) were elucidated and confirm the possibility of tdapO(2) to act as a bridging ligand and its capability to exhibit magnetic ordering at 15 K.

Electronic structure of disjoint diradical 4,4'-bis(1,2,3,5-dithiadiazolyl) thin films.

The electronic structure of the thiazyl diradical, 4,4'-bis(1,2,3,5-dithiadiazolyl) (BDTDA), has been investigated by ultraviolet photoemission spectroscopy. Stacked BDTDA dimers showed an energy band dispersion of about 0.3 eV for the highest occupied molecular orbital in the direction of the surface normal of the BDTDA solid film. The pi-orbital overlap between the stacked dimers therefore evolves into a quasi one-dimensional energy band along the dimer stacking direction.

Interactive radical dimers in photoconductive organic thin films.

Fully interactive: Overlap between extended unoccupied molecular orbitals leads to the high photoconductivity of interactive radical dimers. Sandwich-type cells (see picture; ITO = indium tin oxide) comprising highly oriented thin films of a disjoint diradical, 4,4'-bis(1,2,3,5-dithiadiazolyl) (BDTDA) exhibit a photocurrent with a high on/off ratio at reverse bias voltages and photovoltaic behavior at zero bias voltage.

Synthesis and properties of hetero-halogenated TTFs.

Novel hetero-halogenated TTFs containing both chlorine and iodine atoms were prepared and their unique characters based on the halogen atoms were examined.