Structures and Supramolecular Properties of Inclusion Complexes of Anthracene-Triptycene Nanocages with Fullerene Guests and Their Dynamic Motion as Molecular Gyroscopes

Three derivatives of macrocyclic cage compounds consisting of diarylanthracene and triptycene units were synthesized. These nanocages formed host-guest complexes with C₆₀ and other fullerene guests as confirmed by ¹ H NMR and fluorescence spectroscopy. The association constant of the mesityl and 2,4,6-tributoxyphenyl derivatives with C₆₀ was determined to be 2.2 × 10⁴  L mol^-1 , which was larger than that of the pentafluorophenyl derivative. Direct experimental evidence of the complexation was obtained by X-ray diffraction analysis: the guest C₆₀ molecule was included in the cavity via multipoint CH⋅⋅⋅π interactions. Dynamic disorders of the included C₆₀ molecule in variable-temperature X-ray analysis indicated uniaxial motion, such as gyroscopic motion. The unique dynamic behavior of the spherical C₆₀ rotor anchored by the cage stator via CH⋅⋅⋅π interactions in the crystal, as well as substituent effects on the association properties, are discussed with the aid of DFT calculations.

Molecular solid solutions for advanced materials – homeomorphic or heteromorphic

Crystalline molecular solid solutions are discussed on the basis of homeomorphism and heteromorphism of blended molecules.

Solvent Dependence of Crystal Structure and Dielectric Relaxation in Ferromagnetic [MnCr(oxalate)3]− Salt

[MnCr(oxalate)3]− possesses a two-dimensional ferromagnetic network that is an ideal system for the construction of multifunctional molecular materials based on ferromagnetism. This is because additional functions, such as ferroelectricity, can...

Dynamics of proton, ion, molecule, and crystal lattice in functional molecular assemblies

Dynamic molecular processes, such as short- or long-range proton (H⁺) and ion (M⁺) motions, and molecular rotations in electrical conducting and magnetic molecular assemblies enable the fabrication of electron-H⁺ (or M⁺) coupling systems, while crystal lattice dynamics and molecular conformation changes in hydrogen-bonded molecular crystals have been utilised in external stimuli responsive reversible gas-induced gate opening and molecular adsorption/desorption behavior. These dynamics of the polar structural units are responsible for the dielectric measurements. The H⁺ dynamics are formed from ferroelectrics and H⁺ conductors, while the dynamic M⁺ motions of Li⁺ and Na⁺ involve ionic conductors and coupling to the conduction electrons. In n-type organic semiconductors, the crystal lattices are modulated by replacing M⁺ cations, with cations such as Li⁺, Na⁺, K⁺, Rb⁺, and Cs⁺. The use of polar rotator or inversion structures such as alkyl amides, m-fluoroanilinium cations, and bowl-shaped trithiasumanene π-cores enables the formation of ferroelectric molecular assemblies. The host-guest molecular systems of ESIPT fluorescent chromic molecules showed interesting molecular sensing properties using various bases, where the dynamic transformation of the crystal lattice and the molecular conformational change were coupled to each other.

Record Enhancement of Curie Temperature in Host-Guest Inclusion Ferroelectrics

Solid-state molecular rotor-type materials such as host-guest inclusion compounds are very desirable for the construction of molecular ferroelectrics. However, they usually have a low Curie temperature (T_c) and uniaxial nature, severely hindering their practical applications. Herein, by regulating the anion to control "momentum matching" in the crystal structure, we successfully designed a high-temperature multiaxial host-guest inclusion ferroelectric [(MeO-C₆H₄-NH₃)(18-crown-6)][TFSA] (MeO-C₆H₄-NH₃ = 4-methoxyanilinium, TFSA = bis(trifluoromethanesulfonyl)ammonium) with the Aizu notation of mmmFm. Compared to the parent uniaxial ferroelectric [(MeO-C₆H₄-NH₃)(18-crown-6)][BF₄] with a T_c of 127 K, the introduction of larger TFSA anions brings a lower crystal symmetry at room temperature and a higher energy barrier of molecular motions in phase transition, giving [(MeO-C₆H₄-NH₃)(18-crown-6)][TFSA] multiaxial ferroelectricity and a high T_c up to 415 K (above that of BaTiO₃). To our knowledge, such a record temperature enhancement of 288 K makes its T_c the highest among the reported crown-ether-based ferroelectrics, giving a wide working temperature range for applications in data storage, temperature sensing, actuation, and so on. This work will provide guidance and inspiration for designing high-T_c host-guest inclusion ferroelectrics.

Molecular motion of halogenated ethylammonium/[18]crown-6 supramolecular ions in nickel dithiolate magnetic crystals

Supramolecular cations, consisting of ethylammonium derivatives (X–CH₂CH₂–NH₃⁺) complexed with [18]crown-6, were incorporated into [Ni(dmit)₂]⁻ crystals in order to promote molecular motion.

Room-temperature dielectric switching in a host–guest crown ether inclusion complex

Using the “momentum matching” theory, we have designed a new host–guest crown ether inclusion complex, which exhibits prominent room temperature bistable dielectric switching.

On Librational and Rotational Motions of Aromatic Rings in Layered Sn(IV) and Zr(IV) Phosphonate Materials: A Variable-Temperature 13C, 31P Solid-State NMR Study

According to the solid-state ¹³C, ³¹P NMR study and ¹³C chemical shift anisotropy (CSA) measurements, aromatic rings in the layered metal(IV) phosphonate materials behave as low-energy rotors at rotation activation energy, E_act, of 1.4-3.0 kcal/mol. The rotational mechanism consists of 180° flips and librations around C(1)-C(4) axis. The amplitude of the librations, added to the flips, grows with temperature, shifting the reorientations toward rotational diffusion at high temperatures.

Anion-Regulated Molecular Rotor Crystal: The First Case of a Stator-Rotator Double Switch with Relaxation Behavior

Molecular rotational motion is crucial in artificial molecular machines and is expected to be very significant for the development of an electronic information molecular machine as mentioned in the 2016 Nobel Prize. However, controlling multiple motor modes is a huge challenge. Here, we report a case in which the structural phase transition effectively triggers multiple motor modes by regulating the rotational speed of the cation and/or anion. A novel switchable crystalline supramolecular rotor, [(cyclohexylammonium)(18-crown-6)] FSO₃ (1), exhibits prominent temperature-dependent double switching behavior at 157.9 and 389.1 K induced by the variation of the rotational speed of the FSO₃^- anion (which acts as a super miniature rotator) in response to temperature. Moreover, it exhibits significant relaxation behavior and excellent pyroelectric switch characteristics. To the best of our knowledge, this might be the first discovery of the stator-rotator double switch with a relaxation effect, which could be a promising candidate for a slow/fast responsive double switch over a wide temperature range.

Libration of phenyl groups detected by VT-SSNMR: Comparison with X-ray crystallography

The X-ray crystal structure of 2-benzyl-1H-benzimidazole, 2BnBzIm, was determined at 293 K showing no dynamic phenomena (disorder) of any class. On the other hand, some ¹³ C NMR signals were absent in the CPMAS spectrum (100 MHz, 300 K). We decided to carry out variable-temperature SSNMR and discovered that the missing signals are ortho and meta carbons of the phenyl ring of the benzyl group. Line-shape analysis and the Eyring equation were used to determine the barrier, which was compared with the calculated DFT for the gas phase that it is much lower.

Supramolecular self-assembly for designing non-centrosymmetric crystals based on Keggin polyoxometallates and crown ether

POM based non-centrosymmetric inorganic–organic hybrids are designed by gradually introducing asymmetry into the building units.

A room temperature reversible phase transition containing dielectric switching of a host-guest supramolecular metal-halide compound

Following our recent findings on dielectric materials, we synthesized a new host-guest supramolecular metal-halide compound, [(2-AMPD)(18-crown-6)]CuCl₄ (1, 2-AMPD = 2-aminomethylpiperidinium). Systematic characterization techniques such as variable-temperature crystal structure analyses, differential scanning calorimetry (DSC) measurements, temperature-dependent dielectric measurements and powder X-ray diffraction (PXRD) measurements demonstrate that 1 undergoes a reversible phase transition at room temperature, accompanied by switchable dielectric responses and remarkable anisotropy along three different crystallographic axes. The structural phase transition mechanism is triggered by the order-disorder transition of the 18-crown-6 molecules. We believe that these findings might further promote the application of a host-guest inclusion compound in the field of switchable dielectric materials.

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.

Conformational Transformations of (C12H25NH3(+))(Pyridinesulfonate) in the Solid State

The phase-transition behaviors, crystal structures, and dielectric properties of four kinds of simple 1:1 organic salts of (C12H25NH3(+))(benzenesulfonate) and (C12 H25NH3(+))(pyridine sulfonates) were examined from the viewpoint of intermolecular hydrogen-bonding interactions and dynamic conformational transformation in molecular assemblies. Crystals of (C12H25NH3(+))(benzenesulfonate) and (C12H25NH3(+))(3-pyridinesulfonate) were isostructural and solid-solid and solid-liquid-crystal smectic A (SmA) phase transitions were observed. These two crystals formed rodlike cation-anion assemblies. However, the two salts, (C12H25NH3(+))(2-pyridinesulfonate) and (C12H25NH3(+))(4-pyridinesulfonate), formed largely bent L-shaped cation-anion conformations. Interesting conformational transformations from rodlike to L-shaped assemblies were observed in (C12H25NH3(+))(2-pyridinesulfonate) and (C12H25NH3(+))(3-pyridinesulfonate).

Control of molecular rotor rotational frequencies in porous coordination polymers using a solid-solution approach

Rational design to control the dynamics of molecular rotors in crystalline solids is of interest because it offers advanced materials with precisely tuned functionality. Herein, we describe the control of the rotational frequency of rotors in flexible porous coordination polymers (PCPs) using a solid-solution approach. Solid-solutions of the flexible PCPs [{Zn(5-nitroisophthalate)x(5-methoxyisophthalate)1-x(deuterated 4,4'-bipyridyl)}(DMF·MeOH)]n allow continuous modulation of cell volume by changing the solid-solution ratio x. Variation of the isostructures provides continuous changes in the local environment around the molecular rotors (pyridyl rings of the 4,4'-bipyridyl group), leading to the control of the rotational frequency without the need to vary the temperature.

A host–guest inclusion compound for reversible switching of quadratic nonlinear optical properties

A host–guest inclusion compound [(DPA)(18-crown-6)]ClO₄ (DPA = dipropylamine) can be capable of switching the NLO response between SHG-ON and SHG-OFF states.

Crystal-to-crystal structural transformation of hydrogen-bonding molecular crystals of (imidazolium)(3-hydroxy-2-quinoxalinecarboxylate) through H2O adsorption–desorption

Crystal-to-crystal structural transformation was observed following H₂O adsorption–desorption of hydrogen-bonding molecular crystals.

Crystal structures, CO₂ adsorption, and dielectric properties of [Cu(II)₂(R-benzoate)₄(pyrazine)]∞ polymers (R = m-F, 2,3-F₂, m-Cl, and m-CH₃)

m-Fluorobenzoate (m-FBA), 2,3-difluorobenzoate (2,3-F2BA), m-methylbenzoate (m-MBA), and m-chlorobenzoate (m-ClBA) were introduced into the Cu(II) binuclear unit as bridging ligands between two Cu(II) sites, which were further connected by an axial pyrazine (pz) ligand to form one-dimensional coordination polymers of [Cu(II)2(m-FBA)4(pz)]∞ (1), [Cu(II)2(2,3-F2BA)4(pz)]∞ (2), [Cu(II)2(m-MBA)4(pz)]∞ (3), and [Cu(II)2(m-ClBA)4(pz)]∞ (4), respectively. The parallel arrangements of one-dimensional (1D) polymers result in 1D channels between the polymers that crystallization CH3CN molecules can occupy to form single crystals of 1·4CH3CN, 2·4CH3CN, 3·2CH3CN, and 4·2CH3CN. Both π-dimer and dipole-dipole interactions were simultaneously observed in the interchain interactions of m-FBA and/or 2,3-F2BA ligands in crystals 1 and 2. The sizes of the one-dimensional channels between the polymers are thus modulated according to the interchain interactions between the polar BA ligands. CH3CN molecules within the channels were easily replaced by H2O under ambient conditions. CO2 adsorption-desorption isotherms of crystals 1, 2, and 3 at 195 K indicated gate-adsorption with a hysteresis, whereas two-step gate-adsorption behavior was observed for CO2 in crystal 4. Temperature- and frequency-dependent dielectric responses were not observed in crystals 1-4 under vacuum conditions, whereas dielectric anomalies were observed around 290 K for crystals 1 and 2 with adsorbed CO2. CO2 desorption from the channels in crystals 1 and 2 activated the molecular motions of polar BA ligands and dielectric responses around 290 K, which were confirmed from CO2 adsorption-desorption isotherms around 290 K and differential scanning calorimetries under CO2 conditions.

Dynamics of molecular rotors confined in two dimensions: transition from a 2D rotational glass to a 2D rotational fluid in a periodic mesoporous organosilica

The motional behavior of p-phenylene-d(4) rotators confined within the 2D layers of a hierarchically ordered periodic mesoporous p-divinylbenzenesilica has been elucidated to evaluate the effects of reduced dimensionality on the engineered dynamics of artificial molecular machines. The hybrid mesoporous material, characterized by a honeycomb lattice structure, has arrays of alternating p-divinylbenzene rotors and siloxane layers forming the molecularly ordered walls of the mesoscopic channels. The p-divinylbenzene rotors are strongly anchored between two adjacent siloxane sheets, so that the p-phenylene rotators are unable to experience translational diffusion and are allowed to rotate about only one fixed axis. Variable-temperature (2)H NMR experiments revealed that the p-phenylene rotators undergo an exchange process between sites related by 180° and a non-Arrhenius temperature dependence of the dynamics, with reorientational rates ranging from 10(3) to 10(8) Hz between 215 to 305 K. The regime of motion changes rapidly at about 280 K indicating the occurrence of a dynamical transition. The transition was also recognized by a steep change in the heat capacity at constant pressure. As a result of the robust lamellar architecture comprising the pore walls, the orientational dynamic disorder related to the phase transition is only realized in two dimensions within the layers, that is in the plane perpendicular to the channel axis. Thus, the aligned rotors that form the organic layers exhibit unique anisotropic dynamical properties as a result of the architecture's reduced dimensionality. The dynamical disorder restricted to two dimensions constitutes a highly mobile fluidlike rotational phase at room temperature, which upon cooling undergoes a transition to a more rigid glasslike phase. Activation energies of 5.9 and 9.5 kcal/mol respectively have been measured for the two dynamical regimes of rotation. Collectively, our investigation has led to the discovery of an orientationally disordered 2D rotational glass and its transition from rigid to soft at increasing temperature. The spectral narrowing observed in the (2)H NMR experiments at higher temperatures (310-420 K) is consistent with fast rotational dynamics, which remain anisotropic in nature within the robust lamellar architecture. This study suggests that exploiting reduced dimensionality in the design of solid-state artificial molecular machines and functional materials may yield access to behavior previously unrealized in 3D materials.

Crystalline molecular machines: function, phase order, dimensionality, and composition

The design of molecular machines is stimulated by the possibility of developing new materials with complex physicochemical and mechanical properties that are responsive to external stimuli. Condensed-phase matter with anisotropic molecular order and controlled dynamics, also defined as amphidynamic crystals, offers a promising platform for the design of bulk materials capable of performing such functions. Recent studies have shown that it is possible to engineer molecular crystals and extended solids with Brownian rotation about specific axes that can be interfaced with external fields, which may ultimately be used to design novel optoelectronic materials. Structure/function relationships of amphidynamic materials have been characterized, establishing the blueprints to further engineer sophisticated function. However, the synthesis of amphidynamic molecular machines composed of multiple "parts" is essential to realize increasingly complex behavior. Recent progress in amphidynamic multicomponent systems suggests that sophisticated functions similar to those of simple biomolecular machines may eventually be within reach.

Supramolecular cations of the m-fluoroanilinium(dibenzo[18]crown-6) in ferromagnetic salt

A supramolecular cation of (m-FAni(+))(DB[18]crown-6), where m-FAni(+) and DB[18]crown-6 denote m-fluoroanilinium(+) and dibenzo[18]crown-6, respectively, which is the polar unit rotating in the ferroelectric crystal of (m-FAni(+))(DB[18]crown-6)[Ni(dmit)(2)](-), was introduced into a ferromagnetic [MnCr(oxalate)(3)](-) salt as the counter cation. The crystal structure of (m-FAni(+))(DB[18]crown-6)[MnCr(oxalate)(3)](-)(CH(3)OH)(CH(3)CN) (1) is constructed from alternating layers of a two-dimensional honeycomb layer of [MnCr(oxalate)(3)](-) and (m-FAni(+))(DB[18]crown-6) supramolecular cations. The anionic layer is composed of Mn(II) and Cr(III) ions with S = 5/2 and S = 3/2 spins, respectively, bridged by the oxalate anions, which show ferromagnetic ordering at 5.5 K. The supramolecular structure is formed through the formation of hydrogen bonds between the ammonium hydrogen atoms of the m-FAni(+) cations and the oxygen atoms of the DB[18]crown-6 cavity. No orientational disorder of the fluorine atoms was observed in our X-ray structural analysis, suggesting that a two-fold flip-flop motion of the m-FAni(+) cations does not occur in the salt. The rotational freedom of the m-FAni(+) cations in the salt is restricted by the steric hindrance from neighbouring DB[18]crown-6 molecules. A design strategy for the rotation in a salt is discussed, based on the volume that the supramolecular cations occupy in the unit cell.