Takaaki Hiramatsu

Mapping molecular motions leading to charge delocalization with ultrabright electrons.

Ultrafast processes can now be studied with the combined atomic spatial resolution of diffraction methods and the temporal resolution of femtosecond optical spectroscopy by using femtosecond pulses of electrons or hard X-rays as structural probes. However, it is challenging to apply these methods to organic materials, which have weak scattering centres, thermal lability, and poor heat conduction. These characteristics mean that the source needs to be extremely bright to enable us to obtain high-quality diffraction data before cumulative heating effects from the laser excitation either degrade the sample or mask the structural dynamics. Here we show that a recently developed, ultrabright femtosecond electron source makes it possible to monitor the molecular motions in the organic salt (EDO-TTF)2PF6 as it undergoes its photo-induced insulator-to-metal phase transition. After the ultrafast laser excitation, we record time-delayed diffraction patterns that allow us to identify hundreds of Bragg reflections with which to map the structural evolution of the system. The data and supporting model calculations indicate the formation of a transient intermediate structure in the early stage of charge delocalization (less than five picoseconds), and reveal that the molecular motions driving its formation are distinct from those that, assisted by thermal relaxation, convert the system into a metallic state on the hundred-picosecond timescale. These findings establish the potential of ultrabright femtosecond electron sources for probing the primary processes governing structural dynamics with atomic resolution in labile systems relevant to chemistry and biology.

Dicyanoaurate(I) salts with 1-alkyl-3-methylimidazolium: luminescent properties and room-temperature liquid forming

Room-temperature ionic liquids containing dicyanoaurate(I) anions were prepared utilizing 1-alkyl-3-methylimidazolium cations, and display luminescence possibly due to the oligomerization of the significant fraction of Au(CN)2 anions in the liquids.

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.

Pressure-Tuned Exchange Coupling of a Quantum Spin Liquid in the Molecular Triangular Lattice κ-(ET)_{2}Ag_{2}(CN)_{3}.

The effects of pressure on a quantum spin liquid are investigated in an organic Mott insulator κ-(ET)_{2}Ag_{2}(CN)_{3} with a spin-1/2 triangular lattice. The application of negative chemical pressure to κ-(ET)_{2}Cu_{2}(CN)_{3}, which is a well-known sister Mott insulator, allows for extensive tuning of antiferromagnetic exchange coupling, with J/k_{B}=175-310  K, under hydrostatic pressure. Based on ^{13}C nuclear magnetic resonance measurements under pressure, we uncover universal scaling in the static and dynamic spin susceptibilities down to low temperatures ∼0.1k_{B}T/J. The persistent fluctuations and residual specific heat coefficient are consistent with the presence of gapless low-lying excitations. Our results thus demonstrate the fundamental finite-temperature properties of a quantum spin liquid in a wide parameter range.

Quantum spin liquid: design of a quantum spin liquid next to a superconducting state based on a dimer-type ET Mott insulator

The existence of a spin-disordered quantum state was predicted theoretically by Wannier in 1950 and Anderson in 1973. Various target materials had been considered before the discovery of the first quantum spin liquid (QSL) system in 2003: a Mott insulator κ-(ET)2Cu2(CN)3, where ET is bis(ethylenedithio)tetrathiafulvalene. The family of dimer-type ET conductors κ-(ET)2X (where X = an anion) exhibits various conduction profiles ranging from insulators to metals to superconductors depending on the counter anion. In κ-(ET)2X, the anion molecules form characteristic patterns of anion openings, on each of which an ET dimer corresponding to a single spin site is positioned, namely a key–keyhole relation. The topological consideration of the crystal structure affords the information on both a spin geometry (t′/t) and electron correlation (U/W), where t and t′ are interdimer transfer interactions with an isosceles triangular geometry, and U and W are the on-site Coulomb repulsion energy and bandwidth, respectively. The QSL system κ-(ET)2Cu2(CN)3 is characterized by a spin lattice containing nearly equilateral triangles (t′/t = 1.09) with strong electron correlations (U/W = 0.93) at room temperature. The temperature dependences of t′/t and U/W are bases to understand the transport and magnetic behaviors of κ-(ET)2X. κ-(ET)2Cu2(CN)3 has a superconducting state next to the QSL state under pressure without passing through an antiferromagnetic state. Here, the design of QSL systems next to a superconducting state is discussed based on the crystal and the electronic structures and physical properties of κ-(ET)2X using the key–keyhole relation and temperature variant band parameters t, t′, U, and W.

Anion effects on the electronic structure and electrodynamic properties of the Mott insulator kappa

The Mott insulator κ-(BEDT-TTF)2Ag2(CN)3 forms a highly-frustrated triangular lattice of S = 1/2 dimers with a possible quantum-spin-liquid state. Ou

-(BEDT-TT

The preparation of charge-transfer (CT) solids using [Mo6X14]2− (X = Br, I) cluster units and several organic electron donor molecules (D) including a variety of tetrathiafulvalene (TTF) derivatives was studied. Galvanostatic electrochemical oxidation afforded single crystals of the cation radical salts of (tetrathionaphthacene)Mo6Br14(PhCN)3 (1, PhCN: benzonitrile), (hexamethylene-TTF)2Mo6I14(PhCN)4 (2), (bis(ethylenedioxy)-TTF)2Mo6Br14(PhCN)4 (3), (tetrakis(methylthio)-TTF)2Mo6Br14 (4), (perylene)6Mo6Br14 (5), and powders from several TTF derivatives combined with other kinds of electron donors. In 1, the donor molecules were in the diamagnetic dicationic state and formed two-dimensional D2+A2− (A = Mo6Br14) layers. Moreover, in 2–4, the donor molecules were in the monocationic radical state. The structural analysis of 3 indicated the presence of isolated cation radical molecules, whereas in 2 and 4, the donor cation radicals formed segregated stacks with dimerized units. Donor dimers and [Mo6I14]2− cluster units in 2 formed a CsCl-related structural pattern with solvent molecules occupying the free space in the crystal. The donor dimers and [Mo6Br14]2− cluster units in 4 formed infinite alternating stacks parallel to each other. The static magnetic susceptibility of 2 exhibited singlet–triplet behavior with |J|/kB = 190 K, whereas the radical spins in 4 were strongly coupled antiferromagnetically. There were two kinds of perylene molecules in 5: one formed segregated columns with weakly dimerized units having a charge of approximately +0.5, while the other enclosed the columns with parallel molecular planes having nearly zero charge. 5 was similar to that of a dimer-type Mott insulator. The electrochemical reaction of N-(3-perylenylmethyl)-N,N-bis(2-pyridylmethyl)amine (perbpa), which is a perylene-pendant tridentate ligand, and CuCl2 in the presence of (Bu4N)2Mo6Br14 in PhCN afforded a dinuclear Cu(II) complex [Cu2(μ-Cl)2(perbpa)2]Mo6Br14(PhCN)4 (6). The perylene groups in 6 were in a neutral electronic state. The temperature-dependent magnetic susceptibility measurement of 6 showed a weak ferromagnetic exchange interaction between the Cl-bridged Cu(II) cores with the S = 1 ground state (g = 2.14, J = +0.56 K) based on H = −2JS1·S2. The boundary between the neutral and ionic ground states of the CT solids of [Mo6Br14]2− shifted to the negative side by 0.5 V and 0.3–0.6 V from those of the TTF·p-quinone and TTF·TCNQ system, respectively, with respect to the redox potential differences between the donor and acceptor species.

_2Ag2(CN)3

The localization of charge carriers by electronic repulsion was suggested by Mott in the 1930s to explain the insulating state observed in supposedly metallic NiO. The Mott metal–insulator transition has been subject of intense investigations ever since1–3—not least for its relation to high-temperature superconductivity4. A detailed comparison to real materials, however, is lacking because the pristine Mott state is commonly obscured by antiferromagnetism and a complicated band structure. Here we study organic quantum spin liquids, prototype realizations of the single-band Hubbard model in the absence of magnetic order. Mapping the Hubbard bands by optical spectroscopy provides an absolute measure of the interaction strength and bandwidth—the crucial parameters that enter calculations. In this way, we advance beyond conventional temperature–pressure plots and quantitatively compose a generic phase diagram for all genuine Mott insulators based on the absolute strength of the electronic correlations. We also identify metallic quantum fluctuations as a precursor of the Mott insulator–metal transition, previously predicted but never observed. Our results suggest that all relevant phenomena in the phase diagram scale with the Coulomb repulsion U, which provides a direct link to unconventional superconductivity in cuprates and other strongly correlated materials.A thorough analysis of the optical and transport properties of several two-dimensional organic conductors and insulators with varying on-site correlation strengths and bandwidths led to a quantitative phase diagram for pristine Mott insulators.

Synthesis and properties of charge-transfer solids with cluster units [Mo6X14]2− (X = Br, I)

Two novel antiperovskite charge-transfer (CT) solids composed of a tetraselenafulvalene radical cation (TSF˙+), a dianionic molybdenum cluster unit [Mo6X14]2−, and a halogen anion (Y−) (X, Y = Cl, Br) were prepared by electrocrystallization. Their crystal structures and magnetic properties with regard to spin frustration are discussed together with those of isostructural tetrathiafulvalene (TTF) CT solids previously reported. Both TSF and TTF salts have an apex sharing distorted octahedral spin lattice with a rhombohedral R space group. The calculated overlap integrals based on the crystal structures and insulating nature of the TSF salts indicate that they are Mott insulators. Their spin susceptibilities obeyed the Curie–Weiss law and exhibited an antiferromagnetic ordering at lower temperatures for the TSF salts (Neel temperature, TN = 3.0 K for X = Y = Cl and 5.5 K for X = Y = Br) than the TTF salts. The Curie–Weiss temperatures (\textbarΘCW\textbar ∼ 1.6–6.3 K) for the TSF salts are lower than those of the TTF salts. For the TSF salts, spin-flop behavior was detected at 3.2 T for X = Y = Cl and 1.5 T for X = Y = Br at 1.9 K. Due to both the distortion of the octahedral geometry of the spin lattice and the anisotropic molecular orientation, the geometrical spin frustrations in TSF and TTF systems are weakened

Quantum spin liquids unveil the genuine Mott state

Protonated species of the nucleobase cytosine (C), namely the monoprotonated CH(+) and the hemiprotonated CHC(+), were used to obtain four charge-transfer complexes of [Ni(dmit)2] (dmit: 1,3-dithiole-2-thione-4,5-dithiolate). Diffusion methods afforded two semiconducting [Ni(dmit)2](-) salts; (CH)[Ni(dmit)2](CH3CN) (1) and (CHC)[Ni(dmit)2] (2). In salt 1, the [Ni(dmit)2](-) ions with a S = 1/2 spin construct a uniform one-dimensional array along the molecular long axis, and the significant intermolecular interaction along the face-to-face direction results in a spin-singlet ground state. In contrast, salt 2 exhibits the Mott insulating behavior associated with uniform 1D arrays of [Ni(dmit)2](-), which assemble a two-dimensional layer that is sandwiched between the layers of hydrogen-bonded CHC(+) ribbons. Multiple hydrogen bonds between CHC(+) and [Ni(dmit)2](-) seem to result in the absence of structural phase transition down to 0.5 K. Electrooxidation of [Ni(dmit)2](-) afforded the polymorphs of the [Ni(dmit)2](0.5-) salts, (CHC(+))[{Ni(dmit)2}(0.5-)]2 (3 and 4), which are the first mixed-valence salts of nucleobase cations with metal complex anions. Similar to 2, salt 3 contains CHC(+) ribbons that are sandwiched between the 2D [Ni(dmit)2](0.5-) layers. In the layer, the [Ni(dmit)2](0.5-) ions form dimers with a S = 1/2 spin and the narrow electronic bandwidth causes a semiconducting behavior. In salt 4, the CHC(+) units form an unprecedented corrugated 2D sheet, which is sandwiched between the 2D [Ni(dmit)2](0.5-) layers that involve ring-over-atom and spanning overlaps. In contrast to 3, salt 4 exhibits metallic behavior down to 1.8 K, associated with a wide bandwidth and a 2D Fermi surface. The ability of hydrogen-bonded CHC(+) sheets as a template for the anion radical arrangements is demonstrated.