Self-Propulsion of a Light-Powered Microscopic Crystalline Flapper in Water.

A key goal in developing molecular microrobots that mimic real-world animal dynamic behavior is to understand better the self-continuous progressive motion resulting from collective molecular transformation. This study reports, for the first time, the experimental realization of directional swimming of a microcrystal that exhibits self-continuous reciprocating motion in a 2D water tank. Although the reciprocal flip motion of the crystals is like that of a fish wagging its tail fin, many of the crystals swam in the opposite direction to which a fish would swim. Here the directionality generation mechanism and physical features of the swimming behavior is explored by constructing a mathematical model for the crystalline flapper. The results show that a tiny crystal with a less-deformable part in its flip fin exhibits a pull-type stroke swimming, while a crystal with a fin that uniformly deforms exhibits push-type kicking motion.

Light-Driven Flipping of Azobenzene Assemblies-Sparse Crystal Structures and Responsive Behaviour to Polarised Light.

For creation of autonomous microrobots, which are able to move under conditions of a constant environment and a constant energy supply, a mechanism for maintenance of mechanical motion with a capacity for self-control is required. This requirement, known as self-organisation, represents the ability of a system to evade equilibrium through formation of a spatio-temporal pattern. Following our previous finding of a self-oscillatory flipping motion of an azobenzene-containing co-crystal, we present here regulation of the flipping motion by a light-receiving sensor molecule in relation to the alignment and role of azobenzene molecules in crystals. In the anisotropic structure, a specific azobenzene molecule acts as a reaction centre for the conversion of light to a mechanical function process, whereas the other molecules act as modulators for spatio-pattern regulation. The present results demonstrate that autonomously drivable molecular materials can exhibit information-responsive, self-sustainable motion by incorporating stimulus-responsive sensors.

Mechanism of Macroscopic Motion of Oleate Helical Assemblies: Cooperative Deprotonation of Carboxyl Groups Triggered by Photoisomerization of Azobenzene Derivatives.

Macroscopic and spatially ordered motions of self-assemblies composed of oleic acid and a small amount of an azobenzene derivative, induced by azobenzene photoisomerization, was previously reported. However, the mechanism of the generation of submillimeter-scale motions by the nanosized structural transition of azobenzene was not clarified. Herein, an underlying mechanism of the motions is proposed in which deprotonation of carboxyl groups in cooperation with azobenzene photoisomerization causes a morphological transition of the self-assembly, which in turn results in macroscopic forceful dynamics. The photoinduced deprotonation was investigated by potentiometric pH titration and FTIR spectroscopy. The concept of hierarchical molecular interaction generating macroscale function is expected to promote the next stage of supramolecular chemistry.

A Cyanide-Bridged Magnetically Switchable Cage with Encapsulated Water Molecules.

A cage complex, H2O@[Co5Fe4], was found to encapsulate two water molecules during its self-assembly. The complex exhibited remarkable multifunctionality, combining magnetic switching from the thermal electron-transfer-coupled spin transition of the cage host and dipolar reorientational motion of the confined water, as evidenced by permittivity measurements, density functional theory calculations, and solid-state 2H NMR spectra.

Dissipative and Autonomous Square-Wave Self-Oscillation of a Macroscopic Hybrid Self-Assembly under Continuous Light Irradiation.

Building a bottom-up supramolecular system to perform continuously autonomous motions will pave the way for the next generation of biomimetic mechanical systems. In biological systems, hierarchical molecular synchronization underlies the generation of spatio-temporal patterns with dissipative structures. However, it remains difficult to build such self-organized working objects via artificial techniques. Herein, we show the first example of a square-wave limit-cycle self-oscillatory motion of a noncovalent assembly of oleic acid and an azobenzene derivative. The assembly steadily flips under continuous blue-light irradiation. Mechanical self-oscillation is established by successively alternating photoisomerization processes and multi-stable phase transitions. These results offer a fundamental strategy for creating a supramolecular motor that works progressively under the operation of molecule-based machines.

Structure and growth behavior of centimeter-sized helical oleate assemblies formed with assistance of medium-length carboxylic acids.

The nonequilibrium organization of self-assemblies from small building-block molecules offers an attractive and essential means to develop advanced functional materials and to understand the intrinsic nature of life systems. Fatty acids are well-known amphiphiles that form self-assemblies of several shapes. Here, we found that the lengths of helical structures of oleic acid formed in a buffered aqueous solution are dramatically different by the presence or absence of certain amphiphilic carboxylic acids. For example, under the coexistence of a small amount of N-decanoyl-l-alanine, we observed the formation of over 1 centimeter-long helical assemblies of oleate with a regular pitch and radius, whereas mainly less than 100 µm-long helices formed without this additive. Such long helical assemblies are unique in terms of their highly dimensional helical structure and growth dynamics. Results from the real-time observation of self-assembly formation, site-selective small-angle X-ray scattering, high-performance liquid chromatography analysis, and pH titration experiments suggested that the coexisting carboxylates assist in elongation by supplying oleate molecules to a scaffold for oleate helical assembly.

Structural flexibilities and gas adsorption properties of one-dimensional copper(II) polymers with paddle-wheel units by modification of benzoate ligands.

CO2 and N2 gas adsorption/desorption properties of one-dimensional copper(II) polymers with paddle-wheel units [Cu(II)2(p-XBA)4(pyrazine)]∞ were successfully controlled through the tuning of interchain interactions by modification of para-substituent X groups on the benzoate (BA) ligands (X = Cl, Br, I, and OCH3). Although none of the four crystals had sufficient void space to integrate the crystallization solvents, gate-opening gas adsorption and desorption behaviors coupled with structural phase transitions were observed for CO2 (T = 195 K) and N2 (T = 77 K), with differences depending on the precise substituent. van der Waals interchain interactions, specifically π···π, halogen···π, and C-H···π contacts, were dominant in forming the crystal lattice; their magnitude was associated with gate-opening pressure and hysteresis behaviors. Both the type and magnitude of the interactions were evaluated by Hirshfeld surface analysis, which indicated that structural flexibility decreased as larger halogen atoms were included. Overall, weak interchain interaction and structural flexibility generated new void spaces to adsorb CO2 and N2 gases.

A tricky water molecule coordinated to a verdazyl radical-iron(II) complex: a multitechnique approach.

The first iron complexes of high-spin iron(II) species directly coordinated to verdazyl radicals, [Fe(II)(vdCOO)2(H2O)2]·2H2O (1; vdCOO(-) = 1,5-dimethyl-6-oxo-verdazyl-3-carboxylate) and [Fe(II)(vdCOO)2(D2O)2]·2D2O (2), were synthesized. The crystal structure of 1 was investigated by single-crystal X-ray diffraction at room temperature and at 90 K. The compound crystallizes in the P1 space group with no phase transition between 300 and 90 K. The crystals are composed of discrete [Fe(II)(vdCOO)2(H2O)2] complexes and crystallization water molecules. In the complex, two vdCOO(-) ligands coordinate to the iron(II) ion in a head-to-tail arrangement and two water molecules complete the coordination sphere. The Fe-X (X = O, N) distances vary in the 2.069-2.213 Å range at 300 K and in the 2.0679-2.2111 Å range at 90 K, indicating that the iron(II) ion is in its high-spin (HS) state at both temperatures. At 300 K, one of the coordinated water molecules is H-bonded to two crystallization water molecules whereas the second one appears as loosely H-bonded to the two oxygen atoms of the carboxylate group of two neighboring complexes. At 90 K, the former H-bonds remain essentially the same whereas the second coordinated water molecule reveals a complicated behavior appearing simultaneously as tightly H-bonded to two oxygen atoms and non-H-bonded. The (57)Fe Mössbauer spectra, recorded between 300 K and 10 K, give a clue to this situation. They show two sets of doublets typical of HS iron(II) species whose intensity ratio varies smoothly with temperature. It demonstrates the existence of an equilibrium between the high temperature and low temperature forms of the compounds. The solid-state magic angle spinning (2)H NMR spectra of 2 were recorded between 310 K and 193 K. The spectra suggest the existence of a strongly temperature-dependent motion of one of the coordinated water molecules in the whole temperature range. Variable-temperature magnetic susceptibility measurements indicate an antiferromagnetic interaction (J(Fe-vd) = -27.1 cm(-1); H = -J(ij)S(i)S(j)) of the HS iron(II) ion and the radical spins with high g(Fe) and D(Fe) values (g(Fe) = 2.25, D(Fe) = +3.37 cm(-1)) for the HS iron(II) ion. Moreover, the radicals are strongly antiferromagnetically coupled through the iron(II) center (J(vd-vd) = -42.8 cm(-1)). These last results are analysed based on the framework of the magnetic orbitals formalism.

Molecular rotors of coronene in charge-transfer solids.

Ten types of neutral charge transfer (CT) complexes of coronene (electron donor; D) were obtained with various electron acceptors (A). In addition to the reported 7,7,8,8-tetracyanoquinodimethane (TCNQ) complex of 1:1 stoichiometry with a DA-type alternating π column, TCNQ also afforded a 3:1 complex, in which a face-to-face dimer of parallel coronenes (Cor-As) is sandwiched between TCNQs to construct a DDA-type alternating π column flanked by another coronene (Cor-B). Whereas solid-state (2)H NMR spectra of the 1:1 TCNQ complex formed with deuterated coronene confirmed the single in-plane 6-fold flipping motion of the coronenes, two unsynchronized motions were confirmed for the 3:1 TCNQ complex, which is consistent with a crystallographic study. Neutral [Ni(mnt)2] (mnt: maleonitriledithiolate) as an electron acceptor afforded a 5:2 complex with a DDA-type alternating π column flanked by another coronene, similar to the 3:1 TCNQ complex. The fact that the Cor-As in the [Ni(mnt)2] complex arrange in a non-parallel fashion must cause the fast in-plane rotation of Cor-A relative to that of Cor-B. This is in sharp contrast to the 3:1 TCNQ complex, in which the dimer of parallel Cor-As shows inter-column interactions with neighboring Cor-As. The solid-state (1)H NMR signal of the [Ni(mnt)2] complex suddenly broadens at temperatures below approximately 60 K, indicating that the in-plane rotation of the coronenes undergoes down to approximately 60 K; the rotational rate reaches the gigahertz regime at room temperature. Rotational barriers of these CT complexes, as estimated from variable-temperature spin-lattice relaxation time (T1) experiments, are significantly lower than that of pristine coronene. The investigated structure-property relationships indicate that the complexation not only facilitates the molecular rotation of coronenes but also provides a new solid-state rotor system that involves unsynchronized plural rotators.

Huge dielectric response and molecular motions in paddle-wheel [Cu(II)2(adamantylcarboxylate)4(DMF)2]⋠(DMF)2.

The temperature-dependent dynamic properties of [Cu(II)(2)(ADCOO)(4)(DMF)(2)]⋅(DMF)(2) (1) and [Cu(II)(2)(ADCOO)(4)(AcOEt)(2)] (2) crystals were examined by X-ray crystallography, (1)H NMR spectroscopy, and measurements of the dielectric constants and magnetic susceptibilities (ADCOO = adamantane carboxylate, DMF = N,N-dimethylformamide, and AcOEt = ethyl acetate). In both crystals, four ADCOO groups bridged a binuclear Cu(II)-Cu(II) bond, forming a paddle-wheel [Cu(II)(2)(ADCOO)(4)] structure. The oxygen atoms of two DMF molecules in crystal 1 and two AcOEt molecules in crystal 2 were coordinated at axial positions of the [Cu(II)(2)(ADCOO)(4)] moiety, forming [Cu(II)(2)(ADCOO)(4)(DMF)(2)] and [Cu(II)(2)(ADCOO)(4)(AcOEt)(2)], respectively. Two additional DMF molecules were included in the unit cell of crystal 1, whereas AcOEt was not included in the unit cell of crystal 2. The structural analyses of crystal 1 at 300 K showed three-fold rotation of the adamantyl groups, whereas rotation of the adamantyl groups of crystal 2 at 300 K was not observed. Thermogravimetric measurements of crystal 1 indicated a gradual elimination of DMF upon increasing the temperature above 300 K. The dynamic behavior of the crystallized DMF yielded significant temperature-dependent dielectric responses in crystal 1, which showed a huge dielectric peak at 358 K in the heating process. In contrast, only small frequency-dependent dielectric responses were observed in crystal 2 because of the freezing of the molecular rotation of the adamantyl groups. The magnetic behavior was dominated by the strong antiferromagnetic coupling between the two S = 1/2 spins of the Cu(II)-Cu(II) site, with magnetic exchange energies (J) of -265 K (crystal 1) and -277 K (crystal 2).

Crystal transformation and host molecular motions in CO2 adsorption process of a metal benzoate pyrazine (M(II) = Rh, Cu).

For the purpose of investigating the correlation between host gas adsorption ability and structural flexibility, the combination of metal benzoate complexes [M(II)(2)(bza)(4)] (M(II) = Rh (a), Cu (b); bza = benzoate) and pyrazine derivatives (pyz = pyrazine (1), 2-mpyz = 2-methylpyrazine (2), 2,3-dmpyz = 2,3-dimethylpyrazine (3)) yields a series of one-dimensionally assembled complexes. The study of the adsorption properties of this series was examined for CO(2), H(2), N(2), O(2), and Ar gases at 195 K (CO(2)) or at 77 K (all others). The adsorption manners of these complexes are similar for each gas, while the pressure at which adsorption started or rapidly grew increased with a rise in the number of methyl groups in the case of adsorbable gases. The maximum amount of adsorption was a positive integer, e.g., 3 molecules per M(2) unit for 1 and 2 and 2 molecules per M(2) unit for 3 in the case of CO(2) adsorption for all complexes at 0.1 MPa of adsorbable gases. Structural transformation was observed accompanying gas adsorption. This transformation was observed when the adsorption amount reached 1 molecule per M(2) unit, suggesting a correlation of the adsorption amount and dynamic adsorption behavior. Single-crystal X-ray analyses of nonincluded crystals and CO(2) inclusions for all hosts (1-3) revealed that large structural changes occurred through CO(2) adsorption to increase the inner space for adsorption gases, depending on the substituents on the pyrazine ring. These facts were confirmed as a transition by DSC measurements using a mixed CO(2)/N(2) gas atmosphere. Solid-state (1)H and (2)H NMR studies of the crystalline sample of 1a and its partially deuterated samples of 1a' (deuterated phenyl group) and 1a'' (deuterated pyrazine) revealed rapid 180 degree-flip motions of the aromatic rings of the host skeletons, which form the walls of the channels. These "rotating" motions would help the diffusion of CO(2) molecules through a narrow channel at relatively low pressure. Indeed, the motions of phenyl groups and methyl-substituted pyrazine moieties of phenyl deuterated 3a were confirmed to be very slow by solid-state (1)H and (2)H NMR spectra, where the amount of adsorbed gas molecules was small for 3a at 0.1 MPa of CO(2).

Ferroelectricity and polarity control in solid-state flip-flop supramolecular rotators.

Molecular rotation has attracted much attention with respect to the development of artificial molecular motors, in an attempt to mimic the intelligent and useful functions of biological molecular motors. Random motion of molecular rotators--for example the 180 degree flip-flop motion of a rotatory unit--causes a rotation of the local structure. Here, we show that such motion is controllable using an external electric field and demonstrate how such molecular rotators can be used as polarization rotation units in ferroelectric molecules. In particular, m-fluoroanilinium forms a hydrogen-bonding assembly with dibenzo[18]crown-6, which was introduced as the counter cation of [Ni(dmit)(2)](-) anions (dmit(2-) = 2-thioxo-1,3-dithiole-4,5-dithiolate). The supramolecular rotator of m-fluoroanilinium exhibited dipole rotation by the application of an electric field, and the crystal showed a ferroelectric transition at 348 K. These findings will open up new strategies for ferroelectric molecules where a chemically designed dipole unit enables control of the nature of the ferroelectric transition temperature.

Solid-state molecular rotators of anilinium and adamantylammonium in [Ni(dmit)2](-) salts with diverse magnetic properties.

Supramolecular rotators of hydrogen-bonding assemblies between anilinium (Ph-NH 3 (+)) or adamantylammonium (AD-NH 3 (+)) and dibenzo[18]crown-6 (DB[18]crown-6) or meso-dicyclohexano[18]crown-6 (DCH[18]crown-6) were introduced into [Ni(dmit) 2] salts (dmit (2-) is 2-thioxo-1,3-dithiole-4,5-dithiolate). The ammonium moieties of Ph-NH 3 (+) and AD-NH 3 (+) cations were interacted through N-H (+) approximately O hydrogen bonding with the six oxygen atoms of crown ethers, forming 1:1 supramolecular rotator-stator structures. X-ray crystal-structure analyses revealed a jackknife-shaped conformation of DB[18]crown-6, in which two benzene rings were twisted along the same direction, in (Ph-NH 3 (+))(DB[18]crown-6)[Ni(dmit) 2] (-) ( 1) and (AD-NH 3 (+))(DB[18]crown-6)[Ni(dmit) 2] (-) ( 3), whereas the conformational flexibility of two dicyclohexyl rings was observed in (Ph-NH 3 (+))(DCH[18]crown-6)[Ni(dmit) 2] (-) ( 2) and (AD-NH 3 (+))(DCH[18]crown-6)[Ni(dmit) 2] (-) ( 4). Sufficient space for the molecular rotation of the adamantyl group was achieved in the crystals of salts 3 and 4, whereas the rotation of the phenyl group in salts 1 and 2 was rather restricted by the nearest neighboring molecules. The rotation of the adamantyl group in salts 3 and 4 was evidenced from the temperature-dependent wide-line (1)H NMR spectra, dielectric properties, and X-ray crystal structure analysis. ab initio calculations showed that the potential energy barriers for the rotations of adamantyl groups in salts 3 (Delta E approximately 18 kJmol (-1)) and 4 (Delta E approximately 15 kJmol (-1)) were similar to those of ethane ( approximately 12 kJmol (-1)) and butane (17-25 kJmol (-1)) around the C-C single bond, which were 1 order of magnitude smaller than those of phenyl groups in salts 1 (Delta E approximately 180 kJmol (-1)) and 2 (Delta E approximately 340 kJmol (-1)). 1D or 2D [Ni(dmit) 2] (-) anion arrangements were observed in the crystals according to the shape of crown ether derivatives. The 2D weak intermolecular interactions between [Ni(dmit) 2] (-) anions in salts 1 and 3 led to Curie-Weiss behavior with weak antiferromagnetic interaction, whereas 1D interactions through lateral sulfur-sulfur atomic contacts between [Ni(dmit) 2] (-) anions were observed in salts 2 and 4, whose magnetic behaviors were dictated by ferromagnetic (salt 2) and singlet-triplet (salt 4) intermolecular magnetic interactions, respectively.

High CO2 /CH4 Selectivity of a Flexible Copper(II) Porous Coordination Polymer under Humid Conditions.

Invited for this month's cover is the group of Prof. Shin-ichiro Noro from Hokkaido University, Japan. The cover picture shows a copper(II) porous coordination polymer that can adsorb CO2 over CH4 at high selectivity under CO2 /CH4 mixed gas conditions, even with the coexistence of water. Read the full text of the article at 10.1002/cplu.201500278.

High CO2 /CH4 Selectivity of a Flexible Copper(II) Porous Coordination Polymer under Humid Conditions.

The development of highly efficient CO2 separation materials is very important for environmental preservation and energy conservation. Crystalline porous coordination polymers (PCPs)/metal-organic frameworks are one of a number of promising types of porous materials for CO2 separation because of their controllable pore size, shape and surface function. Simultaneously, the unique structural flexibility of PCPs affords both high CO2 selectivity and inexpensive regeneration. However, this family of materials suffers from the coexistence of water that destroys the framework of PCPs and its adsorption in the pores is greater than that of CO2 , which results in a deterioration in CO2 -separation performance. Herein, a flexible and hydrophobic CuII PCP that is stable towards water has been designed and synthesised. This PCP has extremely high adsorption selectivity for CO2 over CH4 , derived from its structural flexibility. Furthermore, the obtained water-tolerant flexible PCP, under CO2 /CH4 mixed-gas conditions, exhibits highly selective CO2 adsorption over CH4 , even in the presence of water.

Assembling an alkyl rotor to access abrupt and reversible crystalline deformation of a cobalt(II) complex.

Harnessing molecular motion to reversibly control macroscopic properties, such as shape and size, is a fascinating and challenging subject in materials science. Here we design a crystalline cobalt(II) complex with an n-butyl group on its ligands, which exhibits a reversible crystal deformation at a structural phase transition temperature. In the low-temperature phase, the molecular motion of the n-butyl group freezes. On heating, the n-butyl group rotates ca. 100° around the C-C bond resulting in 6-7% expansion of the crystal size along the molecular packing direction. Importantly, crystal deformation is repeatedly observed without breaking the single-crystal state even though the shape change is considerable. Detailed structural analysis allows us to elucidate the underlying mechanism of this deformation. This work may mark a step towards converting the alkyl rotation to the macroscopic deformation in crystalline solids.

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.

Macroscopic motion of supramolecular assemblies actuated by photoisomerization of azobenzene derivatives.

Submillimetre size self-assemblies composed of oleate and azobenzene derivatives show forceful motions such as screw-type coiling-recoiling motion by photoirradiation.

Reversible solid-state structural conversion between a three-dimensional network and a one-dimensional chain of Cu(II) triazole coordination polymers in acidic/basic-suspensions or vapors.

New Cu(II) triazole coordination polymers with a 3D network were synthesized and reversible structural conversion between a 3D network and a 1D chain with color change was realized by pH controlled acidic and basic-suspensions or vapors. For each conversion process of decreasing and increasing pH, conversion was accomplished with high yield, in which the crystal before conversion played the role of a solid-state crystal template.