Yoichi Murakami

We report herein the first (i) covalent organic frameworks (COFs) with sln topology, (ii) drastic control of the shape of COF crystals with the same topology and chemical composition, (iii) insight that these different shapes are accompanied by conformational isomerism, and (iv) installation of dipolar rotors into three-dimensional (3D) COFs. We used a new building block, hexaarylbenzene with three dipolar 1,2-difluorophenyls. Depending on the solution composition, we generated two types of COF crystals with different shapes: hexagonal prism (TK-COF-P) and membrane (TK-COF-M). Although they exhibit distinctly different powder X-ray diffraction patterns, they are chemically identical. The structural determinations revealed that they have a low-symmetric, low-density sln topology that is yet to be reported for COFs. The two distinct shapes─shape dimorphism─is found to accompany conformational isomerism. Temperature-dependent dielectric and ¹⁹F NMR relaxation-time measurements reveal that the rotor motion is suppressed at room temperature, but the rotors respond to an external electric field at elevated temperatures owing to the high activation energy for rotation (∼20 kcal mol^-1), which is desired for room-temperature applications including molecular memories. These outcomes have not only expanded the diversity of COFs but have also provided a foundation for installing external-field-responsive functions into COFs that have high thermal stability, which is expected to invoke broad applications.

Covalently bonded crystalline substances with micropores have broad applications. Covalent organic frameworks (COFs) are representative of such substances. They have so far been classified into two-dimensional (2D) and three-dimensional (3D) COFs. 2D-COFs have planar shapes useful for broad purposes, but obtaining good crystals of 2D-COFs with sizes larger than 10 μm is significantly challenging, whereas yielding 3D-COFs with high crystallinity and larger sizes is easier. Here, we show COFs with 2.5-dimensional (2.5D) skeletons, which are microscopically constructed with 3D bonds but have macroscopically 2D planar shapes. The 2.5D-COFs shown herein achieve large single-crystal sizes above 0.1 mm and ultrahigh-density primary amines regularly allocated on and pointing perpendicular to the covalently-bonded network plane. Owing to the latter nature, the COFs are promising as CO₂ adsorbents that can simultaneously achieve high CO₂/N₂ selectivity and low heat of adsorption, which are usually in a mutually exclusive relationship. 2.5D-COFs are expected to broaden the frontier and application of covalently bonded microporous crystalline systems.

Assembly of coordination networks from Cd( ii ) and a multi-interactive hexaazaphenalene-based ligand was successfully modulated using magnetic fields and thermodynamic control.

Isomerism in covalent organic frameworks (COFs) has scarcely been known. Here, for the first time we show 3D COFs with three framework isomers or polymorphs constructed from the same building blocks. All isomers were obtained as large (>10 μm) crystals; although their crystal shapes were distinctly different, they showed identical FT-IR and solid-state NMR spectra. Our structural analyses revealed unprecedented triple isomerism in 3D COFs (noninterpenetrated dia, qtz, and 3-fold interpenetrated dia-c3 nets). Furthermore, this Communication reports the first known COF with qtz topology for which the structure determination was based on Rietveld analysis. We achieved triple framework isomerism by reticulating a tetrahedral building block with a flexible junction and a linear building block with PEO side chains and by varying solution compositions. Our energy calculations, along with the discovery of interisomer transition, revealed that the isomer with qtz topology was a kinetic isomer. Thus, this simple yet little-explored concept of reticulating only flexible building blocks is an effective pathway to significantly broaden the diversity of 3D COFs, which have been proposed for a myriad of applications.

Organic ionic plastic crystals (OIPCs) are a ductile soft material where the composing ions are in isotropic free rotation, while their positions are aligned in order. The rotational motion in its plastic phase promotes ion conduction by decreasing the activation energy. Here, we report novel OIPCs comprised of tetracyanoborate ([TCB]^-) and various organic cations. In particular, the OIPC composed of [TCB]^- and spiro-(1,1')-bipyrrolidinium ([spiropyr]⁺) cations can transform into its plastic phase at ultralow temperature (T_p = -55 °C) while maintaining a high melting point (T_m = 242 °C). Replacement of the cation with either tetraalkylammonium or phosphonium and comparing their phase behavior, the high T_m was attributed to the relatively small interionic distance between [spiropyr]⁺ and [TCB]^-. At the same time, the low T_p was realized by the restricted vibrational mode of the spirostructure, allowing the initiation of isotropic rotational motion with less thermal energy input.

Photon upconversion (UC) enhances the utilization efficiency of solar energy. However, UC materials generally exhibit a narrow optical absorption bandwidth of the sensitizing molecules that create excited triplet states, including the case of solid-state UC materials. Here, we demonstrate the concept of simultaneous dissolution of multiple sensitizers into crystals of fluorescent molecules to broaden the absorption bandwidth, by using an example of combining two sensitizing porphyrins. Consequently, we appreciably enhanced the UC emission intensity under simulated sunlight. In the air, the developed solid-solution crystals exhibited excellent photostability over 50 h and a low excitation threshold of <1-Sun intensity.

We impregnated the sugar alcohols (SAs) erythritol and mannitol (Man) in >0.1 mm single crystals of a covalent organic framework, COF-300, to propose new solid-state heat storing materials. The fusion-freezing cycles of the Man–COF composite occur in a narrow range of 130–155 °C without large or random supercooling, which has been a crucial problem of SAs, indicating the significance of this materials concept. The Man–COF composite displayed two endothermic and two exothermic features in the heating/cooling cycles. We measured the heat transmission properties in detail, based on which we discuss and suggest a postulate on the phase change behaviour of Man in the pores of the COF.

Photon upconversion (UC) is a technology that converts lower-energy photons (longer wavelength light) into higher-energy photons (shorter wavelength light), the opposite of fluorescence. Thus, UC is expected to open a vast domain of photonic applications that are not otherwise possible. Recently, UC by triplet−triplet annihilation (TTA) between organic molecules has been studied because of its applicability to low-intensity light, although the majority of such studies have focused on liquid samples in the form of organic solvent solutions. To broaden the range of applications, solid-state UC materials have been an active area of research. We recently developed air-stable, high-performance molecular UC crystals that utilize a stable solid-solution phase of bicomponent organic crystals. This article begins with a brief overview of previous challenges in developing and improving solid-state TTA–UC materials. Then, we briefly review and explain the concept as well as advantages of our molecular solid-solution UC crystals. We applied these organic crystals for the first time to a water environment. We observed blue UC emission upon photoexcitation at 542 nm (green–yellow light) and then measured the excitation intensity dependence as well as the temporal stability of the UC emission in air-saturated water. In nondegassed water, these organic crystals were stable, functioned with a low excitation threshold intensity of a few milliwatts per square centimeter, and exhibited high photo-irradiation durability at least over 40 h; indicating that the developed organic crystals are also viable for aqueous conditions. Therefore, the organic crystals presented in this report are expected to extend the domain of UC-based photonic applications in practical water systems including in vivo diagnostic, clinical, and therapeutic applications.

Organic films formed by temperature-gradient solidification up-convert visible light of subsolar intensity into UV light in air with record-long photostability.

The Seebeck coefficient (Se) and the viscosity of a redox electrolyte are the key characteristics of thermoelectrochemical cells that generate electric power from waste thermal energy. However, the recent upsurge of research in this field is seriously disconnected from the knowledge of solution chemistry explored in the previous century. Herein, we systematically investigate five redox couples of cobalt complexes containing different aromatic ligands and anions in γ-butyrolactone solvent to demonstrate how the Einstein relation of hydrodynamic theory and the Jones–Dole B coefficient obtained from viscosity measurements can be used to account for such electrolyte properties. In essence, we reveal that the outer-shell (solvent reorganization) and inner-shell (metal–ligand reorganization) contributions to the redox reaction entropy ΔS_rc (∝Se) can be quantified by the analyses using the B-coefficients and quantum-chemical simulations, respectively, while the distinct regimes found in the viscosity and conductivity are well accounted for by the Einstein relation, despite its classical hydrodynamic origin.