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The design of high-performance optical materials can be realized using coordination polymers (CPs) often supported by non-covalent interactions, such as metallophilicity. The challenge is to control two or more optical effects, e.g., non-linear optics (NLO) and photoluminescence (PL). We present a new strategy for the combination of the NLO effect of second-harmonic generation (SHG) and the visible PL achieved by linking dicyanidoaurate(i) ions, which form luminescent metallophilic stacks, with cadmium(ii) complexes bearing chiral amine ligands, used to break the crystal's symmetry. We report a family of NLO- and PL-active materials based on heterometallic Cd(ii)-Au(i) coordination systems incorporating enantiopure propane-1,2-diamine (pda) ligands (1-S, 1-R), their racemate (2), and enantiopure trans-cyclopentane-1,2-diamine (cpda) ligands (3-S, 3-R). Due to acentric space groups, they exhibit the SHG signal, tunable within the range of 11-24% of the KDP reference, which was correlated with the dipole moments of Cd(ii) units. They show efficient blue PL whose energy and quantum yield, the latter ranging from 0.40 to 0.83, are controlled by Cd(ii) complexes affecting the Au-Au distances and vibrational modes. We prove that chiral Cd(ii)-amine complexes play the role of molecular agents for the stimulation of both the NLO and PL of the materials based on aurophilic stacks.
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Multifunctional optical materials can be realized by combining stimuli-responsive photoluminescence (PL), e.g., optical thermometry, with non-linear optical (NLO) effects, such as second-harmonic generation (SHG). We report a novel approach towards SHG-active luminescent thermometers achieved by constructing unique iridium(iii) complexes, cis-[IrIII(CN)2(R,R-pinppy)2]- (R,R-pinppy = (R,R)-2-phenyl-4,5-pinenopyridine), bearing both a chiral 2-phenylpyridine derivative and cyanido ligands, the latter enabling the formation of a series of molecular materials: (TBA)[IrIII(CN)2(R,R-pinppy)2]·2MeCN (1) (TBA+ = tetrabutylammonium) and (nBu-DABCO)2[IrIII(CN)2(R,R-pinppy)2](i)·MeCN (2) (nBu-DABCO+ = 1-(n-butyl)-1,4-diazabicyclo-[2.2.2]octan-1-ium) hybrid salts, (TBA)2{[LaIII(NO3)3(H2O)0.5]2[IrIII(CN)2(R,R-pinppy)2]2} (3) square molecules, and {[LaIII(NO3)2(dmf)3][IrIII(CN)2(R,R-pinppy)2]}·MeCN (4) coordination chains. Thanks to the chiral pinene group, 1-4 crystallize in non-centrosymmetric space groups leading to SHG activity, while the N,C-coordination of ppy-type ligands to Ir(iii) centers generates visible charge-transfer (CT) photoluminescence. The PL characteristics are distinctly temperature-dependent which was utilized in achieving ratiometric optical thermometry below 220 K. The PL phenomena were rationalized by DFT/TD-DFT calculations indicating an MLCT-type of the emission in obtained Ir(iii) complexes with the rich vibronic structure providing a few emission bands that variously depend on temperature due to the role of thermally activated vibrations. As these crucial vibrational modes depend on the crystal lattice, the thermometry performance differs within 1-4 being the most efficient in 4 while the SHG is by far the best also for 4. This proves that pinene-functionalized cyclometalated dicyanidoiridates(iii) are great prerequisites for tunable PL-NLO conjunction with the most effective multifunctionality ensured by the insertion of these anions into bimetallic frameworks.
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Ultrafast photoinduced phase transitions at room temperature, driven by a single laser shot and persisting long after stimuli, represent emerging routes for ultrafast control over materials' properties. Time-resolved studies provide fundamental mechanistic insight into far-from-equilibrium electronic and structural dynamics. Here we study the photoinduced phase transformation of the Rb0.94Mn0.94Co0.06[Fe(CN)6]0.98 material, designed to exhibit a 75 K wide thermal hysteresis around room temperature between MnIIIFeII tetragonal and MnIIFeIII cubic phases. We developed a specific powder sample streaming technique to monitor by ultrafast X-ray diffraction the structural and symmetry changes. We show that the photoinduced polarons expand the lattice, while the tetragonal-to-cubic photoinduced phase transition occurs within 100 ps above threshold fluence. These results are rationalized within the framework of the Landau theory of phase transition as an elastically-driven and cooperative process. We foresee broad applications of the streaming powder technique to study non-reversible and ultrafast dynamics.
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Solid refrigerants exhibiting a caloric effect upon applying external stimuli are receiving attention as one of the next-generation refrigeration technologies. Herein, we report a new inorganic refrigerant, rubidium cyano-bridged manganese-iron-cobalt ternary metal assembly (cyano-RbMnFeCo). Cyano-RbMnFeCo shows a reversible barocaloric effect with large reversible adiabatic temperature changes of 74 K (from 57 °C to -17 °C) at 340 MPa, and 85 K (from 88 °C to 3 °C) at 560 MPa. Such large reversible adiabatic temperature changes have yet to be reported among caloric effects in solid-solid phase transition refrigerants. The reversible refrigerant capacity is 26000 J kg-1 and the temperature window is 142 K. Additionally, cyano-RbMnFeCo shows barocaloric effects even at low pressures, e.g., reversible adiabatic temperature change is 21 K at 90 MPa. Furthermore, direct measurement of the temperature change using a thermocouple shows +44 K by applying pressure. The temperature increase and decrease upon pressure application and release are repeated over 100 cycles without any degradation of the performance. This material series also possesses a high thermal conductivity value of 20.4 W m-1 K-1. The present barocaloric material may realize a high-efficiency solid refrigerant.
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The development of nanolayered materials is one of the greatest challenges in nanoscience. Until now, pseudohalogen-bridged nanosheets using the mechanical exfoliation method have not been reported. A state-of-the-art material, {[FeII(3-acetylpyridine)2][HgII(µ-SCN)4]}n (1), has been developed to achieve the goal. The compound forms a two-dimensional (2D) coordination polymer with weak out-of-plane van der Waals interactions and has an intrinsic tendency to form shear planes perpendicular to the crystallographic c-direction. These structural features predispose 1 to mechanical exfoliation realized by employing the "Scotch-tape method". As a result, nanosheets were fabricated and characterized by digital optical microscopy, scanning electron microscopy, and atomic force microscopy. The nanosheets were found to have a minimum thickness of â¼15 nm and a lateral size of several micrometers. As the first example of thiocyanato-bridged coordination nanosheets, these materials extend the scope of 2D materials and potentially pave the way toward developing nanolayered materials.
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Knowledge of the magnetic domain is indispensable for understanding the magnetostatic properties of magnets. However, to date, the magnetic domain has not yet been reported in the field of molecule-based magnets. Herein, we study the magnetic domains of molecule-based magnets. Two magnetic films of iron/chromium hexacyanidochromate FexCr1-x[Cr(CN)6]2/3·5H2O (x = 0; Film 1 and x = 0.2; Film 2) were prepared for investigation. The temperature evolution of surface magnetization was measured using magnetic force microscopy. Film 1 showed a magnetic domain below Curie temperature (TC) and its positive-magnetic polarization increased monotonously with decreasing temperature, while Film 2 showed positive magnetic polarization below TC and switches from positive to negative magnetization through a demagnetization state at 146 K. This study originally reports the temperature variation of the magnetization state at the magnetization reversal. The magnetic domains appeared as a maze pattern with an approximate domain size of one-to-several micrometers. This work shows that research on molecule-based magnets can be expanded from magnetochemistry to the magnetostatic engineering of bulk magnets, molecule-based magnetostatic engineering.
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A conjunction of Single-Molecule Magnet (SMM) behavior and luminescence thermometry is an emerging research line aiming at contactless read-out of temperature in future SMM-based devices. The shared working range between slow magnetic relaxation and the thermometric response is typically narrow or absent. We report TbIII -based emissive SMMs formed in a cyanido-bridged framework whose properties are governed by the reversible structural transformation from [TbIII (H2 O)2 ][CoIII (CN)6 ] â 2.7H2 O (1) to its dehydrated phase, TbIII [CoIII (CN)6 ] (2). The 8-coordinated complexes in 1 show the moderate SMM effect but it is enhanced for trigonal-prismatic TbIII complexes in 2, showing the SMM features up to 42â K. They are governed by the combination of QTM, Raman, and Orbach relaxation with the energy barrier of 594(18)â cm-1 (854(26) K), one of the highest among the TbIII -based molecular nanomagnets. Both systems exhibit emission related to the f-f electronic transitions, with the temperature variations resulting in the optical thermometry below 100â K. The dehydration leads to a wide temperature overlap between the SMM behavior and thermometry, from 6â K to 42â K. These functionalities are further enriched after the magnetic dilution. The role of post-synthetic formation of high-symmetry TbIII complexes in achieving the SMM effect and hot-bands-based optical thermometry is discussed.
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Effective reuse of waste heat energy is an important energy savings issue for green transformation. In general, phase-change heat storage materials cannot store energy for a prolonged period. If a solid material could conserve the accumulated thermal energy and release it only on demand, then its heat-storage application potential is considerably widened. From this angle, in 2015, we proposed the concept of a long-term heat-storage material, in which latent heat is preserved until the material is triggered by an external stimulus. This feature article describes long-term heat-storage ceramics composed of lambda-trititanium-pentoxide (λ-Ti3O5) from their discovery to heat-storage properties and future applications.
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Calor , TitanioRESUMEN
We investigated the vibrational density of states of sodium carboxymethyl starch (CM-starch) by terahertz (THz) time-domain spectroscopy. The CM-starch showed a broad peak at â¼3 THz. The structure of the peak was similar to those corresponding to glucose-based polymer glasses possessing hydrogen bonds. The boson peak (BP) appeared at 1.16 THz at the lowest temperature and disappeared because of the existence of excess wing at higher temperatures. However, based on our novel BP frequency determination method using the inflection point of the extinction coefficient, the BP frequency showed almost no dependence on temperature. Further, the chain length dependence of the BP frequency of the glucose-based glasses showed that the BP frequency of the polymer glass was slightly lower than that of the monomer glass. The power law behaviour of the absorption coefficient suggested the existence of fractons, and the fractal dimension was estimated to be 2.33.
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Espectroscopía de Terahertz , Enlace de Hidrógeno , Almidón/análogos & derivados , VibraciónRESUMEN
We study by femtosecond optical pump-probe spectroscopy the photoinduced charge transfer (CT) in the RbMnFe Prussian blue analogue. Previous studies evidenced the local nature of the photoinduced MnIII FeII â MnII FeIII process, occurring within less than 1â ps. Here we show experimentally that two photoswitching pathways exist, depending on the excitation pump wavelength, which is confirmed by band structure calculations. Photoexcitation of α spins corresponds to the Mn(d-d) band, which drives reverse Jahn-Teller distortion through the population of antibonding Mn-N orbitals, and induces CT within ≈190â fs. The process launches coherent lattice torsion during the self-trapping of the CT small-polaron. Photoexcitation of ß spins drives intervalence FeâMn CT towards non-bonding states and results in a slower dynamic.
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Iron oxide magnets, which are composed of the common elements iron and oxygen, are called ferrite magnets. They have diverse applications because they are chemically stable and inexpensive. Epsilon-iron oxide (ε-Fe2O3) is a polymorph that shows an extremely large coercive field as a magnetic oxide. It maintains its ferromagnetic ordering even when downsized to a single nano-sized scale (i.e.,<10 nm). Due to these characteristics, ε-Fe2O3 is highly expected to be used for high-density magnetic recording media in the big data era. Here, we describe the recent developments of magnetic films composed of metal-substituted ε-iron oxide, ε-MxFe2-xO3 (M: substitution metal), toward the next-generation of magnetic media.
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In the era of Big Data and the Internet of Things, data archiving is a key technology. From this viewpoint, magnetic recordings are drawing attention because they guarantee long-term data storage. To archive an enormous amount of data, further increase of the recording density is necessary. Herein a new magnetic recording methodology, "focused-millimeter-wave-assisted magnetic recording (F-MIMR)," is proposed. To test this methodology, magnetic films based on epsilon iron oxide nanoparticles are prepared and a focused-millimeter-wave generator is constructed using terahertz (THz) light. Irradiating the focused millimeter wave to epsilon iron oxide instantly switches its magnetic pole direction. The spin dynamics of F-MIMR are also calculated using the stochastic Landau-Lifshitz-Gilbert model considering all of the spins in an epsilon iron oxide nanoparticle. In F-MIMR, the heat-up effect of the recording media is expected to be suppressed. Thus, F-MIMR can be applied to high-density magnetic recordings.
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The boson peak is a largely unexplained excitation found universally in the terahertz vibrational spectra of disordered systems; the so-called fracton is a vibrational excitation associated with the self-similar structure of monomers in polymeric glasses. We demonstrate that such excitations can be detected using terahertz spectroscopy. In the case of fractal structures, we determine the infrared light-vibration coupling coefficient for the fracton region and show that information concerning the fractal and fracton dimensions appears in the exponent of the absorption coefficient. Finally, using terahertz time-domain spectroscopy and low-frequency Raman scattering, we experimentally observe these universal excitations in a protein (lysozyme) system that has an intrinsically disordered and fractal structure and argue that the system should be considered a single supramolecule. These findings are applicable to amorphous and fractal objects in general and will be valuable for understanding universal dynamics of disordered systems via terahertz light.
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Ionic conductors serve as solid electrolytes for fuel cells and batteries, whereas polar crystals such as ferroelectrics and pyroelectrics-which are typically insulating materials-are used in electronic devices. Here we show a material that combines superionic conductivity with a polar crystal structure at room temperature. This three-dimensional anionic network is based on -Fe-N≡C-Mo- units, with Cs cations hosted in every other pore. In the resulting Cs1.1Fe0.95[Mo(CN)5(NO)]·4H2O material, the negative and positive charges of the framework and Cs+ ions, respectively, are non-symmetrically shifted in the c-axis direction of the unit cell, and spontaneous electric polarization is generated, in turn leading to second harmonic generation (SHG). Additionally, this material is a superionic conductor (with an ionic conductivity value of 4 × 10-3 S cm-1 at 318 K). Furthermore, the ionic conductivity significantly decreases under 532 nm light irradiation (from 1 × 10-3 S cm-1 to 6 × 10-5 S cm-1 at room temperature) and, when irradiation stops, returns to its original value within ~1 h.
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Herein we report crystal growth control of rod-shaped ε-Fe2O3 nanocrystals by developing a synthesis based on the sol-gel technique using ß-FeO(OH) as a seed in the presence of a barium cation. ε-Fe2O3 nanocrystals are obtained over a wide calcination temperature range between 800 °C and 1000 °C. A low calcination temperature (800 °C) provides an almost cubic rectangular-shaped ε-Fe2O3 nanocrystal with an aspect ratio of 1.4, whereas a high calcination temperature (1000 °C) provides an elongated rod-shaped ε-Fe2O3 nanocrystal with an aspect ratio of 3.3. Such systematic anisotropic growth of ε-Fe2O3 is achieved due to the wide calcination temperature in the presence of barium cations. The surface energy and the anisotropic adsorption of barium on the surface of ε-Fe2O3 can explain the anisotropic crystal growth of rod-shaped ε-Fe2O3 along the crystallographic a-axis. The present work may provide important knowledge about how to control the anisotropic crystal shape of nanomaterials.
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Atomic vibrations due to stretching or bending modes cause optical phonon modes in the solid phase. These optical phonon modes typically lie in the frequency range of 102 to 104 cm-1. How much can the frequency of optical phonon modes be lowered? Herein we show an extremely low-frequency optical phonon mode of 19 cm-1 (0.58 THz) in a Rb-intercalated two-dimensional cyanide-bridged Co-W bimetal assembly. This ultralow frequency is attributed to a millefeuille-like structure where Rb ions are very softly sandwiched between the two-dimensional metal-organic framework, and the Rb ions slowly vibrate between the layers. Furthermore, we demonstrate temperature-induced and photo-induced switching of this low-frequency phonon mode. Such an external-stimulation-controllable sub-terahertz (sub-THz) phonon crystal, which has not been reported before, should be useful in devices and absorbers for high-speed wireless communications such as beyond 5G or THz communication systems.
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Vapochromic behaviour of porous crystals is beneficial for facile and rapid detection of gaseous molecules without electricity. Toward this end, tailored molecular designs have been established for metal-organic, covalent-bonded and hydrogen-bonded frameworks. Here, we explore the hydrochromic chemistry of a van der Waals (VDW) porous crystal. The VDW porous crystal VPC-1 is formed from a novel aromatic dendrimer having a dibenzophenazine core and multibranched carbazole dendrons. Although the constituent molecules are connected via VDW forces, VPC-1 maintains its structural integrity even after desolvation. VPC-1 exhibits reversible colour changes upon uptake/release of water molecules due to the charge transfer character of the constituent dendrimer. Detailed structural analyses reveal that the outermost carbazole units alone are mobile in the crystal and twist simultaneously in response to water vapour. Thermodynamic analysis suggests that the sigmoidal water sorption is induced by the affinity alternation of the pore surface from hydrophobic to hydrophilic.
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The accumulated heat energy of a heat-storage material is typically released over time. If a heat-storage material could preserve its accumulated heat energy for a prolonged period, the applicability of such materials would be expanded greatly. Herein we report a newly fabricated heat-storage material that can store latent heat energy for a long period and release the heat energy upon demand by applying an extremely low pressure. This material is a block-type lambda trititanium pentoxide (block-type λ-Ti3O5). The block-type λ-phase accumulates a large heat energy of 237 kJ L-1 and exhibits a pressure-induced phase transition to beta trititanium pentoxide. The pressure-induced phase transition occurs by applying only several tens of bars, and half of the fraction transforms by 7 MPa (70 bar). Such a low-pressure-responsive heat-storage ceramic is effective to reuse excessive heat in automobiles or waste heat at industrial factories.
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Phase transition materials are attractive from the viewpoints of basic science as well as practical applications. For example, optical phase transition materials are used for optical recording media. If a phase transition in condensed matter could be predicted or designed prior to synthesizing, the development of phase transition materials will be accelerated. Herein we show a logical strategy for designing a phase transition accompanying a thermal hysteresis loop. Combining first-principles phonon mode calculations and statistical thermodynamic calculations considering cooperative interaction predicts a charge-transfer phase transition between the A-B and A+-B- phases. As an example, we demonstrate the charge-transfer phase transition on rubidium manganese hexacyanoferrate. The predicted phase transition temperature and the thermal hysteresis loop agree well with the experimental results. This approach will contribute to the rapid development of yet undiscovered phase transition materials.
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A novel three-dimensional (3D) coordination polymer with the formula (C3N2H5)4[MnCr2(ox)6]·5H2O (2), where ox = oxalate and C3N2H5 = imidazolium cation, is reported. Single crystal X-ray diffraction reveals that this porous coordination polymer adopts a chiral three-dimensional quartz-like architecture, with the guest imidazolium cations and water molecules being hosted in its pores. This novel multifunctional material exhibits both a ferromagnetic ordering at TC = 3.0 K, related to the host MnCr2 network, and high proton conductivity [1.86 × 10-3 S cm-1 at 295 K and 88% relative humidity (RH)] due to the presence of the acidic imidazolium cations and free water molecules. The similarity of the structure of compound 2 to that of the previously reported analogous compound (NH4)4[MnCr2(ox)6]·4H2O, (1), also allows us to analyse, to a certain extent, the effect of the acidity of the proton donating guest molecules on proton conduction properties. 2 hosts, in one-dimensional (1D) channels, imidazolium cations, which are more acidic than the ammonium ones in 1 and, as a consequence, 2 shows higher proton conduction than 1, highlighting the effect of the pKa of the proton donating guest molecules on proton conductivity.