Spin-Correlated Luminescence of a Carbazole-Containing Diradical Emitter: Single-Molecule Magnetoluminescence and Thermally Activated Emission.

Luminescent radicals have been intensively studied as a new class of materials exhibiting novel photofunctions unique to open-shell systems. When luminescent radicals are assembled, intriguing spin-correlated luminescence phenomena emerge, including excimer-like emission and magnetic-field effects on luminescence (i.e., magnetoluminescence, MagLum). However, the underlying mechanisms of these phenomena arising from spin multiplicity and spin-dependent excited-state dynamics are poorly understood due to the limited number of luminescent polyradical systems available for study. In particular, the correlation between stronger intramolecular exchange interactions (|2J/kB| > ∼10 K, where J and kB are the intramolecular exchange coupling constant and the Boltzmann constant, respectively) and luminescence properties has not been fully explained. In this study, a novel carbazole-containing diradical emitter (1) and the corresponding monoradical (2) were prepared for the in-depth study of spin-correlated luminescence properties, with luminescence measurements under magnetic fields of up to 18 T. Diradical 1 has a negative 2J/kB value of several tens of kelvin and exhibits a single-molecule MagLum and thermally activated luminescence, whereas 2 does not. Detailed quantitative analyses revealed that both the spin-correlated luminescence properties of 1 are strongly dominated by ground-state spin statistics based on the Boltzmann distribution (i.e., 2J/kB values). Furthermore, diradical 1 exhibits external heavy-atom effects in heavy-atom-containing solvents such as iodobenzene, whereas monoradical 2 does not. This is the first experimental verification of external heavy-atom effects in polyradical emitters. This work demonstrates that polyradical emitters can be designed based on spin degrees of freedom in both ground and excited states.

Pressure-tuned quantum criticality in the large-D antiferromagnet DTN.

Strongly correlated spin systems can be driven to quantum critical points via various routes. In particular, gapped quantum antiferromagnets can undergo phase transitions into a magnetically ordered state with applied pressure or magnetic field, acting as tuning parameters. These transitions are characterized by z = 1 or z = 2 dynamical critical exponents, determined by the linear and quadratic low-energy dispersion of spin excitations, respectively. Employing high-frequency susceptibility and ultrasound techniques, we demonstrate that the tetragonal easy-plane quantum antiferromagnet NiCl2 ⋅ 4SC(NH2)2 (aka DTN) undergoes a spin-gap closure transition at about 4.2 kbar, resulting in a pressure-induced magnetic ordering. The studies are complemented by high-pressure-electron spin-resonance measurements confirming the proposed scenario. Powder neutron diffraction measurements revealed that no lattice distortion occurs at this pressure and the high spin symmetry is preserved, establishing DTN as a perfect platform to investigate z = 1 quantum critical phenomena. The experimental observations are supported by DMRG calculations, allowing us to quantitatively describe the pressure-driven evolution of critical fields and spin-Hamiltonian parameters in DTN.

Optically Electroactive Polymer Synthesized in a Liquid Crystal with Cyclosporin A─Circularly Polarized Electron Spin Resonance.

Cyclosporine A (CsA), a naturally derived biomaterial and physiologically active substance, is commonly used as an immunosuppressant. In this study, CsA was revealed to function as a chiral inducer of cholesteric liquid crystals (CLCs) with a high helical twisting power. CsA induced helical structures in 4-cyano-4'-pentylbiphenyl, a synthetic liquid crystal (LC) used for general purposes. Electrochemical polymerization in CLC with CsA was also performed. The polymer prepared in CLC showed electro-optical activity via chiral induction by CsA. Synchrotron X-ray diffraction measurements indicated that the polymer film prepared in the CLC formed in the manner of LC molecular arrangement through molecular form imprinting from the LC order, although the polymer exhibited no liquid crystallinity. The polymer showed structural color and laser light oscillation diffraction derived from its periodic structure. The anisotropy of the circularly polarized electron spin resonance signals for the resulting polymer with respect to the magnetic field was observed.

Single-Molecule Magnetoluminescence from a Spatially Confined Persistent Diradical Emitter.

Luminescent radicals are an emerging class of materials that exhibit unique photofunctions not found in closed-shell molecules due to their open-shell electronic structure. Particularly promising are photofunctions in which radical's spin and luminescence are correlated; for example, when a magnetic field can affect luminescence (i.e., magnetoluminescence, ML). These photofunctions could be useful in the new science of spin photonics. However, previous observations of ML in radicals have been limited to systems in which radicals are randomly doped in host crystals or polymerized through metal complexation. This study shows that a covalently linked luminescent radical dimer (diradical) can exhibit ML as a single-molecular property. This facilitates detailed elucidation of the requirements for and mechanisms of ML in radicals and can aid the rational design of ML-active radicals based on synthetic chemistry.

Coexistence of Urbach-Tail-Like Localized States and Metallic Conduction Channels in Nitrogen-Doped 3D Curved Graphene.

Nitrogen (N) doping is one of the most effective approaches to tailor the chemical and physical properties of graphene. By the interplay between N dopants and 3D curvature of graphene lattices, N-doped 3D graphene displays superior performance in electrocatalysis and solar-energy harvesting for energy and environmental applications. However, the electrical transport properties and the electronic states, which are the key factors to understand the origins of the N-doping effect in 3D graphene, are still missing. The electronic properties of N-doped 3D graphene are systematically investigated by an electric-double-layer transistor method. It is demonstrated that Urbach-tail-like localized states are located around the neutral point of N-doped 3D graphene with the background metallic transport channels. The dual nature of electronic states, generated by the synergistic effect of N dopants and 3D curvature of graphene, can be the electronic origin of the high electrocatalysis, enhanced molecular adsorption, and light absorption of N-doped 3D graphene.

Chiral weak ferromagnets formed in one-dimensional organic-inorganic hybrid manganese chloride hydrates.

Chiral weak ferromagnets were developed in organic-inorganic hybrid systems of one-dimensional manganese(II) chloride hydrate with chiral organic cations of (S)- and (R)-ß-methylphenethylammonium (= (S)- and (R)-MPA+), [(S)/(R)-MPA]2[MnCl4(H2O)]. The weak ferromagnetic phase found below 3.2 K could be attributed to a spin canting antiferromagnetic state induced by Dzyaloshinskii-Moriya interactions owing to spatial inversion symmetry breaking in chirality introduced systems. This is the first chiral weak ferromagnet found in a spin chain of deformed perovskite derivatives with a corner-sharing structure of transition metal halide hydrates.

Enhancement of the Magnetoelectric Effect Using the Dynamic Jahn-Teller Effect in a Transition-Metal Complex.

In this Letter, a novel mechanism to enhance the magnetoelectric (ME) coupling between electric polarization and magnetism using the dynamic Jahn-Teller (JT) effect is demonstrated. Electric polarization of over 100 µC/m^{2} is induced by the magnetic field owing to the second-order ME effect in the noncentrosymmetric transition metal complex [Mn^{III}(taa)]. This appearance of electric polarization does not require magnetic order in contrast to the linear ME effect in ME multiferroic materials. The value of the electric polarization is 1 order larger than that induced by the second-order ME effect, which originates from the p-d hybridization. Our calculation, taking into account the single-ion-type magnetic anisotropy originating from the spin-orbit interaction and ferrodistortive intermolecular interaction, verifies that the alignment of the JT distortion by the magnetic field results in the large electric polarization observed. Thus, our results provide a new method to gain strong ME coupling by tuning the atomic displacement using a magnetic field.

A ground-state-dominated magnetic field effect on the luminescence of stable organic radicals.

Organic radicals are an emerging class of luminophores possessing multiplet spin states and potentially showing spin-luminescence correlated properties. We investigated the mechanism of recently reported magnetic field sensitivity in the emission of a photostable luminescent radical, (3,5-dichloro-4-pyridyl)bis(2,4,6-trichlorophenyl)methyl radical (PyBTM) doped into host αH-PyBTM molecular crystals. The magnetic field (0-14 T), temperature (4.2-20 K), and the doping concentration (0.1, 4, 10, and 22 wt%) dependence on the time-resolved emission were examined by measuring emission decays of the monomer and excimer. Quantum mechanical simulations on the decay curves disclosed the role of the magnetic field; it dominantly affects the spin sublevel population of radical dimers in the ground states. This situation is distinctly different from that in conventional closed-shell luminophores, where the magnetic field modulates their excited-state spin multiplicity. Namely, the spin degree of freedom of ground-state open-shell molecules is a new key for achieving magnetic-field-controlled molecular photofunctions.

Magneto-Electric Directional Anisotropy in Polar Soft Ferromagnets of Two-Dimensional Organic-Inorganic Hybrid Perovskites.

Two-dimensional organic-inorganic hybrid perovskites (2D-OIHPs) are attracting interest due to their structural tunability and rich functional characteristics, such as ferroelectricity and ferromagnetism. Here, we report the chiral-polar ferromagnetic 2D-OIHP copper chlorides with discernable electric polarization in the inorganic layers. In these systems, the magneto-electric (ME) correlation has been clearly observed by measuring a magneto-electric directional anisotropy (MEA), in which an optical absorption coefficient changes with reversal of the light propagating direction. We have found that the MEA can be induced by a low magnetic field of about 50 mT, reflecting soft magnetic nature. The present results suggest a new paradigm for designing functional ME multiferroics, which effectively couples magnetic and electric properties.

Radical-Based Coordination Polymers as a Platform for Magnetoluminescence.

Spin-correlated electronic and magnetic properties of organic radicals have been developed, but luminescence properties, based on interplay with spins, have rarely been reported. The effect of magnetic fields on luminescence (i.e., magnetoluminescence) is a rare example of such properties, observed to date only in radicals dispersed in host matrices. We now report a novel method for achieving radical magnetoluminescence involving radical-based coordination polymers (CPs). The luminescence properties of the bis(3,5-dichloro-4-pyridyl)(2,4,6-trichlorophenyl)methyl (bisPyTM) and tris(3,5-dichloro-4-pyridyl)methyl (trisPyM) radicals and their 1D and 2D ZnII CPs were investigated. Although solid-state emissions of bisPyTM and trisPyM were not affected significantly by external magnetic fields at 4.2 K, those of CPs were greatly modulated. Studies of the crystal structures, magnetic properties, and the temperature-dependence and time-resolved properties of the magnetoluminescence indicate that the reduction of radical-radical interactions in CPs would be a key method for achieving magnetoluminescence.

Dirac Fermion Kinetics in 3D Curved Graphene.

3D integration of graphene has attracted attention for realizing carbon-based electronic devices. While the 3D integration can amplify various excellent properties of graphene, the influence of 3D curved surfaces on the fundamental physical properties of graphene has not been clarified. The electronic properties of 3D nanoporous graphene with a curvature radius down to 25-50 nm are systematically investigated and the ambipolar electronic states of Dirac fermions are essentially preserved in the 3D graphene nanoarchitectures, while the 3D curvature can effectively suppress the slope of the linear density of states of Dirac fermion near the Fermi level are demonstrated. Importantly, the 3D curvature can be utilized to tune the back-scattering-suppressed electrical transport of Dirac fermions and enhance both electron localization and electron-electron interaction. As a result, nanoscale curvature provides a new degree of freedom to manipulate 3D graphene electrical properties, which may pave a new way to design new 3D graphene devices with preserved 2D electronic properties and novel functionalities.

Excimer emission and magnetoluminescence of radical-based zinc(II) complexes doped in host crystals.

A ZnII complex based on a luminescent organic radical was doped into host molecular crystals. The 5, 10, and 20 wt%-doped crystals showed excimer emissions and their luminescent behaviours were significantly modulated by an external magnetic field. These are the first examples showing excimer emissions and magnetic-field-sensitive luminescent properties for complexes based on luminescent radicals. The excimer species contributing to magnetoluminescence was determined by analyzing the emission spectra and their magnetic-field dependencies. These results suggest the general nature of magnetic field effects on the luminescence of radicals as well as the importance of the type of interaction between radicals.

Electrical Switching of the Nonreciprocal Directional Microwave Response in a Triplon Bose-Einstein Condensate.

We present a microwave electron spin resonance study of the quantum spin dimer system TlCuCl_{3}, which shows the magnetic-field-induced ordering with both antiferromagnetic spin order and ferroelectricity by the Bose-Einstein condensation (BEC) of triplon quasiparticles. Our main achievement is an electrical switching of the nonreciprocal directional microwave response in the triplon BEC phase. High-speed directional control of microwave absorption by applying an electric field has been accomplished in this Letter. The strength of the observed nonreciprocal microwave response well agrees with the calculation based on Kubo theory with the parameters, evaluated from the static electric polarization in this material.

Magnetochiral Dichroism in a Collinear Antiferromagnet with No Magnetization.

We show the directional dichroism in a collinear antiferromagnet MnTiO_{3}. The dichroism between two distinctive antiferromagnetic states with opposite signs of staggered magnetic moments can be regarded as magnetochiral dichroism in the absence of external fields. Electric-field reversal of antiferromagnetic domain causes a change in the absorption intensity of unpolarized light around 2.15 eV. The difference in optical absorption between two antiferromagnetic states is reversed for the light propagating in the opposite direction. The absorption coefficient displays a hysteretic behavior for a cycle of sweeping the external electric or magnetic field.

Magnetic Tunnel Junctions with a Nearly Zero Moment Manganese Nanolayer with Perpendicular Magnetic Anisotropy.

A magnetic nanolayer with a perpendicular magnetic easy axis and negligible magnetization is demonstrated. Even though a manganese metal is antiferromagnetic in bulk form, a few manganese monolayers grown on a paramagnetic ordered alloy template and capped by an oxide layer exhibit a strong perpendicular magnetic anisotropy field exceeding 19 T as well as a negligible magnetization of 25 kA/m. The nanolayer shows tunnel magnetoresistance. Moreover, the perpendicular magnetic anisotropy for the nanolayer can be reduced by applying an electric voltage. These findings will provide new insight into a creation of new nanolayer magnets.

Magnetoluminescence in a Photostable, Brightly Luminescent Organic Radical in a Rigid Environment.

We investigated the emission properties of a photostable luminescent organic radical, (3,5-dichloro-4-pyridyl)bis(2,4,6-trichlorophenyl)methyl radical (PyBTM), doped into host molecular crystals. The 0.05 wt %-doped crystals displayed luminescence attributed to a PyBTM monomer with a room-temperature emission quantum yield of 89 %, which is exceptionally high among organic radicals. The 10 wt %-doped crystals exhibited both PyBTM monomer and excimer-centered emission bands, and the intensity ratio of these two bands was modulated drastically by applying a magnetic field of up to 18 T at 4.2 K. This is the first observation of a magnetic field affecting the luminescence of organic radicals, and we also proposed a mechanism for this effect.

Magnetic Switching by the In Situ Electrochemical Control of Quasi-Spin-Peierls Singlet States in a Three-Dimensional Spin Lattice Incorporating TTF-TCNQ Salts.

Magnetic phase switching in a coordination polymer is reported, which is demonstrated by combining two processes: (A) the pre-organization of magnetic/redox-active molecules into a framework, and (B) a post-treatment through electrochemical tuning of the pre-organized molecules. A TTF.+ -TCNQ.- salt (TTF=tetrathiafulvalene; TCNQ=7,7,8,8-tetracyano-p-quinodimethane) was incorporated into a three-dimensional framework with paddlewheel-type dimetal(II, II) units ([M2II,II ]; M=Ru with S=1, 1; and Rh with S=0, 2), where the [M2II,II ] and TCNQ.- units form the coordinating framework, and TTF.+ is located in the pores of framework, forming an irregular π-stacking alternating column with the TCNQ.- in the framework. In 1, the spins of [Ru2II,II ] and TCNQ.- units make a magnetic correlation through the framework upon decreasing the temperature from 300 K, which is, however, suddenly suppressed below 137 K (=Td (1)) by the formation of a spin singlet in the TTF.+ -TCNQ.- columns, as seen in the spin-Peierls transition (Td (2)=200 K). This material was incorporated as a cathode in a Li-ion battery (LIB); a long-range ferrimagnetic correlation was formed through the three-dimensional [{Ru2II,II }2 TCNQ]- framework at Tc =78 K in the discharge process. The reversible magnetic phase switching between the non-volatile ferrimagnetic and paramagnetic states, resulting from the local spin tuning of quasi-spin-Peierls singlet, is demonstrated through the discharge/charge cycling of the LIB.

Electron spin resonance spectral analysis of irradiated royal jelly.

The analysis of unpaired electron components in royal jelly was carried out using electron spin resonance (ESR) with the aim to develop a detection method for irradiated royal jelly. The ESR spectrum of royal jelly had natural signals derived from transition metals, including Fe(3+) and Cu(2+), and a signal line near g=2.00. After irradiation, a new splitting asymmetric spectrum with overall spectrum width ca. 10mT at g=2.004 was observed. The intensities of the signals at g=2.004 increased in proportion to the absorbed dose in samples under different storage conditions: fresh frozen royal jelly and dried royal jelly powder at room temperature. The signal intensity of the fresh frozen sample was stable after irradiation. One year after 10kGy irradiation of dried powder, the signal intensity was sevenfold greater than before irradiation, although the intensity continued to steadily decrease with time. This stable radiation-induced radical component was derived from the poorly soluble constituent of royal jelly.

Construction of a novel topological frustrated system: a frustrated metal cluster in a helical space.

A novel topologically frustrated pentanuclear cluster helicate [{Cu(II)(µ-L)(3)}(2)Cu(II) (3)(µ(3)-OH)](3+) (L(-)=3,5-bis(2-pyridyl)pyrazolate) has been synthesized and characterized. This cluster has a helical arrangement of ligands around the central metal core. Dzyaloshinsky-Moriya interactions are essential components to observe a gradual magnetization and forbidden transitions of high-field/multi-frequency (HF/MF)-ESR. The origin of the magnetic anisotropy of this compound is influenced by its helical spin structure, and consequently, the Cu(5)-cluster helicate introduces a unique magnetic anisotropy. This observation is a direct evidence of the topological part of the new spin phase in a magnetic system.

Detection of organic free radicals in irradiated Foeniculi fructus by electron spin resonance spectroscopy.

Foeniculi fructus were irradiated with an electron beam and organic free radicals were detected by electron spin resonance (ESR) spectroscopy for the purpose of identifying radio-disinfected and sterilized herbal drugs. An ESR single-line spectrum near g = 2.005 was observed in the sample before irradiation. After irradiation, the intensity of the signal near g = 2.005 increased. In addition, two subsignals derived from cellulose radicals were observed approximately 3 mT to either side of the main signal, at g = 2.023 and g = 1.987. The intensity of the subsignal at g = 2.023 was proportional to the absorbed dose of radiation. The decrease in intensity of the signals was considerable 2 weeks after irradiation, and continued to decrease steadily thereafter. Among the signals, the fading of the subsignal at g = 2.023 was relatively small. The intensity of the subsignal at g = 2.023 was detectable for over 1 year in the sample that had been irradiated to the level of disinfection and sterilization. Therefore, organic free radicals in irradiated Foeniculi fructus can be measured rapidly and with high sensitivity by ESR spectroscopy. The stable signal at g = 2.023 is a promising indicator of the detection of irradiated herbal drugs.