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.

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.

Charge-transfer solids using nucleobases: supramolecular architectures composed of cytosine and [Ni(dmit)2] assembled by multiple hydrogen bonds and heteroatomic contacts.

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.

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.

New insights into the structural properties of κ-(BEDT-TTF)2Ag2(CN)3 spin liquid.

Here, the first accurate study is presented of the room-temperature and 100 K structures of one of the first organic spin liquids, κ-(BEDT-TTF)2Ag2(CN)3. It is shown that the monoclinic structure determined previously is only the average one. It is shown that the exact structure presents triclinic symmetry with two non-equivalent dimers in the unit cell. But surprisingly this does not lead to a sizeable charge disproportionation between dimers. The difference from the analogue compound κ-(BEDT-TTF)2Cu2(CN)3 which also presents a spin liquid phase is discussed in detail. The data provided here show the importance of the anionic layer and in particular the transition metal position in the process of symmetry breaking. The possible impact of the symmetry breaking, albeit weak, on the spin-liquid mechanism and the influence of various disorders on the physical properties of this system is also discussed.

Solvent-dependent structural and electronic behaviors of a push-pull molecule: {4-[4,5-bis(methylsulfanyl)-1,3-dithiol-2-ylidene]cyclohexa-2,5-dien-1-ylidene}malononitrile.

We examined the solvent effect on the electronic absorption and infrared absorption spectra of a push-pull molecule fabricated by connecting donor and acceptor segments linked with a quinoid bridge, {4-[4,5-bis(methylsulfanyl)-1,3-dithiol-2-ylidene]cyclohexa-2,5-dien-1-ylidene}malononitrile (BMDCM), in liquid solutions. A BMDCM molecule represented by a resonance hybrid of the two canonical structures, the quinoid and benzenoid forms, showed remarkable solvatochromism. By analyzing the vibrational progressions in the electronic absorption spectra, we have grasped the behaviors of molecular structure and intramolecular charge distribution for the ground and excited states in reference to the solvent polarity.