Magnetic Bistability in a Discrete Organic Radical

Isolable GaN-based analogues of Thiele's and Chichibabin's hydrocarbons

Treatment of two equivalents of Trip2GaCl (Trip = 2,4,6-triisopropylphenyl) with pyrazine and 4,4'-bipyridine affords adducts 1 and 2 in high yields. The two-electron reduction of 1 and 2 using potassium graphite readily gives rise to the first GaN-based analogues of Thiele's and Chichibabin's hydrocarbons, Trip2Ga(C4N2H4)GaTrip2 (3) and Trip2Ga(C10N2H8)GaTrip2 (4), respectively, as crystalline solids. Structural analysis and DFT calculations suggest a closed-shell singlet ground state for both 3 and 4.

Bridge-Dependent Donor-Metal-Acceptor-Metal-Donor (D-M-A-M-D) Systems: From Charge Transfer to Electron Transfer in Dioxolene-Ge-Diimine Complexes

Synthesis and structural characterization of a family of germanium-dioxolene complexes with ditopic N-donor ligands (L1-L5) (L1=1,2-bis(pyridin-2-ylmethylene)hydrazine L2=1,6-bis-(pyridin-2-yl)-2,5-diaza-1,5-hexadiene, L3=N,N-bis(pyridin-2-ylmethylene)-1,4-benzenediamine, L4=N,N-bis(pyridin-2-ylmethylene)-(biphenyl)-4,4-diamine, L5=2,2'-azopyridine) is reported. The reaction of germanium bis-catecholate with bridging ligands L1 - L4, differing by the nature of the linker between pyridine sites gives rise to dinuclear digermanium complexes (36Cat2Ge)2L1-4 (36Cat=dianion of 3,6-di-tert-butylcatechol) 1-4 of DMAMD type (donor-metal-acceptor-metal-donor) with a charge transfer in the UV-Vis region. In opposite, the interaction of the 36Cat2Ge with 2,2'-azopyridine (L5) results in the two-electron transfer from the donor 36Cat2- ligands to the azopyridine bridge forming stable open-shell complex 5 [(36SQ)(36CatGe)]2(L5)2- (36SQ=radical-anionic semiquinonate ligand). Molecular structures of compounds 3 and 5 were determined by single crystal X-ray diffraction analysis. Electronic structures of complexes 1-5 were studied by means of DFT calculations.

Isolation and characterization of a triplet nitrene

Nitrene radical compounds are short-lived intermediates in a variety of nitrogen-involved transformations. They feature either a singlet or a triplet ground state, depending on the electronic properties of the substituents. Triplet nitrenes are highly reactive and their isolation in the condensed phase under ambient conditions is challenging. Here we report the synthesis and isolation of a triplet arylnitrene supported by a bulky hydrindacene ligand. The arylnitrene is fully characterized by various spectroscopic and structural techniques including electron paramagnetic resonance spectroscopy and single-crystal X-ray diffraction. Its high stability is largely attributed to the steric hindrance and effective electron delocalization provided by the supporting ligand. Electron paramagnetic resonance spectroscopy in conjunction with highly correlated wavefunction-based ab initio calculations provides support for a triplet ground state nitrene with axial zero-field splitting D = 0.92 cm-1 and vanishing rhombicity E/D = 0.002.

Hysteretic Room-Temperature Magnetic Bistability of the Crystalline 4,7-Difluoro-1,3,2-Benzodithiazolyl Radical

The title radical R⋅, synthesized by reduction of the corresponding cation R+, is thermally stable up to ~380 K in the crystalline state under anaerobic conditions. With SQUID magnetometry, single-crystal and powder XRD, solid-state EPR and TG-DSC, reversible spin-Peierls transition between diamagnetic and paramagnetic states featuring ~10 K hysteretic loop is observed for R⋅ in the temperature range ~310-325 K; ΔH=~2.03 kJ mol-1 and ΔS=~6.23 J mol-1 K-1. The transition is accompanied by mechanical movement of the crystals, i. e., by thermosalient behavior. The low-temperature diamagnetic P-1 polymorph of R⋅ consists of R⋅2 π-dimers arranged in (…R⋅2…)n π-stacks; whereas the high-temperature paramagnetic P21/c polymorph, of uniform (…R⋅…)n π-stacks. With the XRD geometries, CASSCF and broken-symmetry DFT jointly suggest strong antiferromagnetic (AF) interactions within R⋅2 and weak between R⋅2 for the (…R⋅2…)n stacks; and moderate AF interactions between R⋅ for the (…R⋅…)n stacks. The fully hydrocarbon archetype of R⋅ does not reveal the aforementioned properties. Thus, the fluorinated 1,3,2-benzodithiazolyls pave a new pathway in the design and synthesis of metal-less magnetically-bistable materials.

Magnetic Bistability in an Organic Radical-Based Charge Transfer Cocrystal

We report herein an organic charge transfer cocrystal complex, consisting of a stable radical TPVr and an electron acceptor TCNQF4, as a rare sort of all-organic-based magnetic bistable materials with a thermally activated magnetic hysteresis loop over the temperature range from 170 to 260 K. Detailed X-ray crystallographic studies and theoretical calculations revealed that while a π-associated radical anion dimer was formed upon an integer charge transfer process from TPVr to the TCNQF4 molecules within the cocrystal lattice, the resulting TCNQF4·- π-dimers were found to exhibit varied intradimer π-stacking distances and singly occupied molecular orbital overlaps at different temperatures, thus yielding two different singlet states with distinct singlet-triplet gaps above and below the loop, which eventually contributed to the thermally excited molecular magnetic bistability.

Organic radical ferroelectric crystals with martensitic phase transition

Organic martensitic compounds are an emerging type of smart material with intriguing physical properties including thermosalient effect, ferroelasticity, and shape memory effect. However, due to the high structural symmetry and limited design theories for these materials, the combination of ferroelectricity and martensitic transformation has rarely been found in organic systems. Here, based on the chemical design strategies for molecular ferroelectrics, we show a series of asymmetric 1,4,5,8-naphthalenediimide derivatives with the homochiral amine and 2,2,6,6-tetramethylpiperidine-N-oxyl components, which adopt the low-symmetric polar structure and so allow ferroelectricity. Upon H/F substitution, the fluorinated compounds exhibit reversible ferroelectric and martensitic transitions at 399 K accompanied by a large thermal hysteresis of 132 K. This large thermal hysteresis with two competing (meta)-stable phases is further confirmed by density functional theory calculations. The rare combination of martensitic phase transition and ferroelectricity realizes the bistability with two different ferroelectric phases at room temperature. Our finding provides insight into the exploration of martensitic ferroelectric compounds with potential applications in switchable memory devices, soft robotics, and smart actuators.

Multimodal Reactivity of N-H Bonds in Triazanes and Isolation of a Triazinyl Radical

The employment of nitrogen Lewis acids based on nitrenium cations has been increasingly featured in the fields of main group chemistry and catalysis. A formally reduced form of nitrenium D─cyclic triazanes E─are intriguing chemical compounds, the chemistry of which is completely unexplored. In this work, we reveal that N-H-triazanes exhibit unusual N-H bond properties; namely, they can serve as protons, hydrides, or hydrogen atom donors. This unique multimodal reactivity provides an N-cation, N-anion, or N-radical from the same species. It allowed us to isolate, for the first time, a stable naphto[1,2,3]triazinyl radical, which was fully characterized both computationally and experimentally, including its monomeric X-ray structure. Moreover, this radical can be prepared directly from the nitrenium cation by a single electron reduction (E = -0.46 V), and this process is reversible. We envision versatile uses of this radical in synthetic and materials chemistry.

Multi-step phase transition crystal with dielectric constant bistability and temperature-dependent conductivity

Here, we report the crystal structures, phase transitions, and thermal, dielectric, and conducting properties of an ion-pair compound [C4-bmim][Ni(mnt)2] (1). 1 undergoes a three-step phase transition with four phases before melting. A two-step dielectric constant bistability is also realized by the structural phase transition in 1 occurring among phases I, II, and III, which is due to the in-plane oscillations of the alkyl chain and crystal-to-mesophase transition, respectively. Moreover, 1 exhibits rare temperature-dependent conducting properties accompanying structural phase transitions, and conductivity is very high with 0.00186 S cm-1 at 413 K. The conduction properties of phase III (mesophase) arise from the dipole molecular motion.

Achieving large thermal hysteresis in an anthracene-based manganese(II) complex via photo-induced electron transfer

Achieving magnetic bistability with large thermal hysteresis is still a formidable challenge in material science. Here we synthesize a series of isostructural chain complexes using 9,10-anthracene dicarboxylic acid as a photoactive component. The electron transfer photochromic Mn²⁺ and Zn²⁺ compounds with photogenerated diradicals are confirmed by structures, optical spectra, magnetic analyses, and density functional theory calculations. For the Mn²⁺ analog, light irradiation changes the spin topology from a single Mn²⁺ ion to a radical-Mn²⁺ single chain, further inducing magnetic bistability with a remarkably wide thermal hysteresis of 177 K. Structural analysis of light irradiated crystals at 300 and 50 K reveals that the rotation of the anthracene rings changes the Mn1–O2–C8 angle and coordination geometries of the Mn²⁺ center, resulting in magnetic bistability with this wide thermal hysteresis. This work provides a strategy for constructing molecular magnets with large thermal hysteresis via electron transfer photochromism.

Achieving magnetic bistability with large thermal hysteresis is still a challenge in material science. Here, the authors report a Mn(II) chain complex that enables light-induced magnetic bistability with a 177 K thermal hysteresis loop.

The Lewis Acid Induced Formation of a Stable Diradical with an Intramolecular Ion Pairing State

A stable cross-conjugated diradical was prepared by the reaction of a donor-acceptor-donor (D-A-D) molecule with B(C₆F₅)₃. Its geometry and electronic structure were characterized by single crystal X-ray diffraction, EPR spectroscopy, SQUID measurement, UV/vis spectroscopy, and DFT calculation. It has an open-shell singlet ground state with a thermally excited triplet state. It can be viewed as an intramolecular radical ion pair, and the formation mechanism is proposed as an intramolecular single electron transfer that occurs from the bis(triarylamine) donor fragment to the central dioxophenyl acceptor moiety, induced by the acidic boron atom. This work provides a Lewis acid induced approach to the formation of neutral and cross-conjugated diradicals.

Crystalline radical cations of bis-BN-based analogues of Thiele's hydrocarbon

Two radical cations of bis-BN-based analogues of Thiele's hydrocarbons were facilely synthesized, fully characterized, and theoretically investigated. One-electron oxidation leads to the reduced bond length alternation and NICS values of the central C₄N₂ rings, suggesting the decreasing antiaromatic character. The spin density of the radical cations is significantly delocalized over the central linkers with a small contribution from two terminal N-heterocyclic boryl units.

Merging Charge Transfer into Metal-Organic Frameworks to Achieve High Reduction Potentials via Multiphoton Excitation

Utilization of multiphotons to achieve high reduction potentials is a highly demanding but still challenging task for reductive cleavage of inert bonds. Herein, we report a new charge transfer approach that simultaneously excites the electron-rich dye and the radical anionic of the electron-deficient one for photocatalytic activation of aryl chlorides with high reduction potentials (E_red ≈ -1.9 to -2.9 V). Interactions between the tetraphenylbenzene-1,4,-diamine dyes in the large pores of metal-organic frameworks and the adsorbed 9,10-dicyanoanthracene partly endows charge transfer in the ground state. The first photoexcitation led to the formation charge separation pairs containing both radical cation and anion for second photon excitation. The possibility of modifying each absorption band of the two dyes independently innovated the resultant aryl radicals applied in various useful transformations, expanding multiphoton manifolds on both the dye scopes and reaction versions. A comparison of the catalytic performance between different structural patterns of metal-organic frameworks with the same ligand demonstrated that the incorporating of the organic dyes within the pores of the frameworks was essential to form charge-transfer species and accelerate the interesting chemical conversion.

Accessing Highly Efficient Photothermal Conversion with Stable Open-Shell Aromatic Nitric Acid Radicals

It is very challenging to prepare stable radicals as they are usually thermodynamically or kinetically unstable in air. Herein, a series of star-shaped aromatic nitric acid radicals were prepared via facile demethylation and consequent oxidation. As phenol radicals without steric hindrance group protection, they exhibit high electrochemical and thermal stability due to their rich resonance structures including closed-shell nitro-like and open-shell nitroxide structure with unpaired electrons delocalized in conjugated backbones. Among them, TPA-TPA-O₆ powder exhibited extremely wide absorption from 300 to 2000 nm covering the whole solar spectral irradiance, high photothermal conversion efficiency, and negligible photobleaching effect in seawater desalination. Under the irradiation of one sunlight, the water evaporation efficiency of TPA-TPA-O₆ is recorded to be as high as 89.41 % and the water evaporation rate is 1.293 kg m^-2  h^-1 , which represents the top performance in pure organic small molecule photothermal materials.

The cis/trans conformation approach for tuning the magnetic coupling in a diradical: isolation of pure pyridine-based diradical dianions

Two-electron reductions of 3,3'-bis(2,6-dimesitylpyridin-4-yl)-1,1'-biphenyl 1 with elemental potassium in the absence and presence of 18-c-6 afforded the diradical dianion salts [K⁺]₂˙[trans-1]˙˙^2- and [K(18-c-6)]⁺₂˙[cis-1]˙˙^2-, which exhibit trans and cis configurations, respectively. The transoid conformer could be converted to the cisoid one through reacting with 18-c-6.

The catenation of a singlet diradical dication and modulation of diradical character by metal coordination

A singlet bis(triarylamine) diradical dication and its zigzag 1D magnetic chain catenated by silver cations were isolated and characterized by single-crystal X-ray crystallography, EPR spectroscopy, SQUID measurements, cyclic voltammetry and...

One- and Two-Electron Transfer Oxidation of 1,4-Disilabenzene with Formation of Stable Radical Cations and Dications

Electron‐transferable oxidants such as B(C₆F₅)₃/nBuLi, B(C₆F₅)₃/LiB(C₆F₅)₄, B(C₆F₅)₃/LiHBEt₃, Al(C₆F₅)₃/(o‐RC₆H₄)AlH₂ (R=N(CMe₂CH₂)₂CH₂), B(C₆F₅)₃/AlEt₃, Al(C₆F₅)₃, Al(C₆F₅)₃/nBuLi, Al(C₆F₅)₃/AlMe₃, (CuC₆F₅)₄, and Ag₂SO₄, respectively were employed for reactions with (L)₂Si₂C₄(SiMe₃)₂(C₂SiMe₃)₂ (L=PhC(NtBu)₂, 1). The stable radical cation [1]^+. was formed and paired with the anions [nBuB(C₆F₅)₃]⁻ (in 2), [B(C₆F₅)₄]⁻ (in 3), [HB(C₆F₅)₃]⁻ (in 4), [EtB(C₆F₅)₃]⁻ (in 5), {[(C₆F₅)₃Al]₂(μ‐F)]⁻ (in 6), [nBuAl(C₆F₅)₃]⁻ (in 7), and [Cu(C₆F₅)₂]⁻ (in 8), respectively. The stable dication [1]²⁺ was also generated with the anions [EtB(C₆F₅)₃]⁻ (9) and [MeAl(C₆F₅)₃]⁻ (10), respectively. In addition, the neutral compound [(L)₂Si₂C₄(SiMe₃)₂(C₂SiMe₃)₂][μ‐O₂S(O)₂] (11) was obtained. Compounds 2–11 are characterized by UV‐vis absorption spectroscopy, X‐ray crystallography, and elemental analysis. Compounds 2–8 are analyzed by EPR spectroscopy and compounds 9–11 by NMR spectroscopy. The structure features are discussed on the central Si₂C₄‐rings of 1, [1]^+., [1]²⁺, and 11, respectively.

Room-Temperature Magnetic Bistability in a Salt of Organic Radical Ions

Cocrystallization of 7,7',8,8'-tetracyanoquinodimethane radical anion (TCNQ^-•) and 3-methylpyridinium-1,2,3,5-dithiadiazolyl radical cation (3-MepyDTDA^+•) afforded isostructural acetonitrile (MeCN) or propionitrile (EtCN) solvates containing cofacial π dimers of homologous components. Loss of lattice solvent from the diamagnetic solvates above 366 K affords a high-temperature paramagnetic phase containing discrete TCNQ^-• and weakly bound π dimers of 3-MepyDTDA^+•, as evidenced by X-ray diffraction methods and magnetic susceptibility measurements. Below 268 K, a first-order phase transition occurs, leading to a low-temperature diamagnetic phase with TCNQ^-• σ dimer and π dimers of 3-MepyDTDA^+•. This study reveals the first example of cooperative interactions between two different organic radical ions leading to magnetic bistability, and these results are central to the future design of multicomponent functional molecular materials.

Laser-Induced Cooperative Transition in Molecular Electronic Crystal

The competing and non-equilibrium phase transitions, involving dynamic tunability of cooperative electronic and magnetic states in strongly correlated materials, show great promise in quantum sensing and information technology. To date, the stabilization of transient states is still in the preliminary stage, particularly with respect to molecular electronic solids. Here, a dynamic and cooperative phase in potassium-7,7,8,8-tetracyanoquinodimethane (K-TCNQ) with the control of pulsed electromagnetic excitation is demonstrated. Simultaneous dynamic and coherent lattice perturbation with 8 ns pulsed laser (532 nm, 15 MW cm^-2 , 10 Hz) in such a molecular electronic crystal initiates a stable long-lived (over 400 days) conducting paramagnetic state (≈42 Ωcm), showing the charge-spin bistability over a broad temperature range from 2 to 360 K. Comprehensive noise spectroscopy, in situ high-pressure measurements, electron spin resonance (ESR), theoretical model, and scanning tunneling microscopy/spectroscopy (STM/STS) studies provide further evidence that such a transition is cooperative, requiring a dedicated charge-spin-lattice decoupling to activate and subsequently stabilize nonequilibrium phase. The cooperativity triggered by ultrahigh-strain-rate (above 10⁶ s^{- 1} ) pulsed excitation offers a collective control toward the generation and stabilization of strongly correlated electronic and magnetic orders in molecular electronic solids and offers unique electro-magnetic phases with technological promises.

Synthesis, Structures, and Electronic Properties of 2,7-Anthrylene-Based Azacyclophanes Bearing o-, m-, and p-Phenylenediamine Linkers

A series of novel azacyclophanes consisting of 2,7-anthrylene and phenylene units were designed and synthesized by the Buchwald-Hartwig coupling reaction to investigate their unique electronic properties in multiple oxidized states. Cyclic voltammetry showed that the p-phenylene derivative exhibited three reversible oxidation waves, whereas the o- and m-phenylene derivatives showed two quasi-reversible oxidation waves due to the complicated intramolecular interaction between the oxidized units and neutral units. Moreover, the absorption spectra of the p-phenylene derivative in different oxidation states showed absorption bands at 865 and 1025 nm, which were attributed to intramolecular charge-transfer interactions. The photophysical and electrochemical properties of the p-phenylene analog were also compared with those of the o- and m-phenylene derivatives based on theoretical calculations for further evaluation of the intramolecular electronic interactions.

Controlling pairing of π-conjugated electrons in 2D covalent organic radical frameworks via in-plane strain

Controlling the electronic states of molecules is a fundamental challenge for future sub-nanoscale device technologies. π-conjugated bi-radicals are very attractive systems in this respect as they possess two energetically close, but optically and magnetically distinct, electronic states: the open-shell antiferromagnetic/paramagnetic and the closed-shell quinoidal diamagnetic states. While it has been shown that it is possible to statically induce one electronic ground state or the other by chemical design, the external dynamical control of these states in a rapid and reproducible manner still awaits experimental realization. Here, via quantum chemical calculations, we demonstrate that in-plane uniaxial strain of 2D covalently linked arrays of radical units leads to smooth and reversible conformational changes at the molecular scale that, in turn, induce robust transitions between the two kinds of electronic distributions. Our results pave a general route towards the external control, and thus technological exploitation, of molecular-scale electronic states in organic 2D materials.

Controlling the electronic states of molecules is a fundamental challenge for future sub-nanoscale device technologies but the external dynamical control of these states still awaits experimental realization. Here, via quantum chemical calculations, the authors demonstrate that in-plane uniaxial strain of 2D covalently linked arrays of radical units induces controlled pairing of π-conjugated electrons in a reversible way.

Rational design and syntheses of aniline-based diradical dications: isolable congeners of quinodimethane diradicals

Two-electron oxidation of five aniline-based compounds 4,4′′-p/m-terphenyldiamines afforded the first isolable aniline-based diradical dications 1²⁺–5²⁺.

CAAC-Based Thiele and Schlenk Hydrocarbons

Diradicals have been of tremendous interest for over a century ever since the first reports of p- and m-phenylene-bridged diphenylmethylradicals in 1904 by Thiele and 1915 by Schlenk. Reported here are the first examples of cyclic(alkyl)(amino)carbene (CAAC) analogues of Thiele's hydrocarbon, a Kekulé diradical, and Schlenk's hydrocarbon, a non-Kekulé diradical, without using CAAC as a precursor. The CAAC analogue of Thiele's hydrocarbon has a singlet ground state, whereas the CAAC analogue of Schlenk's hydrocarbon contains two unpaired electrons. The latter forms a dimer, by an intermolecular double head-to-tail dimerization. This straightforward synthetic methodology is modular and can be extended for the generation of redox-active organic compounds.

Serendipitous isolation of a triazinone-based air stable organic radical: synthesis, crystal structure, and computation

Here we report the synthesis, isolation, and characterization of a dication salt, namely 4,6-bis(4,4′-bipyridinium)-1,3,5-triazin-2-one {1²⁺(PF₆)₂²⁻·2H₂O1(PF6)2·2H2O}, and its radical cation salt, namely 4,6-bis(4,4′-bipyridinium)-1,3,5-triazin-2-one (1+˙PF₆⁻1˙PF6).

Conformational Spin Switching and Spin-Selective Hydrogenation of a Magnetically Bistable Carbene

The control of the spin states of molecules opens the path to tuning selectivity in chemical reactions and to developing novel magnetically switchable materials. 3-Methoxy-9-fluorenylidene is a carbene that is generated in cryogenic matrices both in its lowest energy singlet and triplet states, and the ratio of these states can be shifted by selective irradiation. The interconversion of the nearly degenerate spin states is induced by a conformational change of the methoxy group: switching the methoxy group into the "up" position results in the singlet state and switching into the "down" position in the triplet state. The spin control via a remote functional group makes this carbene unique for the study of spin-specific reactions, which is demonstrated for the hydrogenation reaction. Spin switching by switching the conformation of a remote functional group is a novel phenomenon with potential applications in the design of functional materials.

Effect of Substitution on the Hysteretic Phase Transition in a Bistable Phenalenyl-Based Neutral Radical Molecular Conductor

The ability to tune the physical properties of bistable organic functional materials by means of chemistry can facilitate their development for molecular electronic switching components. The butylamine-containing biphenalenyl boron neutral radical, [Bu]₂ B, crystalline compound has recently attracted significant attention by displaying a hysteretic phase transition accompanied by simultaneous bistability in magnetic, electrical, and optical properties close to room temperature. In this report, substitutional doping was applied to [Bu]₂ B by crystallizing solid solutions of bistable [Bu]₂ B and its non-radical-containing counterpart [Bu]₂ Be. With increasing doping degree, the hysteretic phase transition is gradually suppressed in terms of reducing the height, but conserves the width of the hysteresis loop as observed through magnetic susceptibility and electrical conductivity measurements. At the critical doping level of about 6 %, the abrupt transformation of the crystal structure to that of the pure [Bu]₂ Be crystal packing was observed, accompanied by a complete collapse of the hysteresis loop. Further study of the structure-properties relationships of bistable neutral radical conductors based on the [Bu]₂ B host can be conducted utilizing a variety of biphenalenyl-based molecular conductors.

Wide Magnetic Thermal Memory Effect (∼55 K) Above Room Temperature Coupled to a Structure Phase Transition of Lattice Symmetry Reduction in High-Temperature Phase in an S = 1/2 Spin Chain Molecule Crystal

One-dimensional (1D) S = 1/2 Heisenberg antiferromagnetic (AFM) chain system shows frequently a spin-Peierls-type transition owing to strong spin-lattice coupling. From high-temperature phase (HTP) to low-temperature phase (LTP), the spin chain distortion leads to the reduction in lattice symmetry in LTP, called the symmetry breaking (SB) phase transition. Herein, we report the first example of 1D S = 1/2 AFM molecular crystal, [Et₃( n-Pr)N][Ni(dmit)₂] (Et₃( n-Pr)N⁺ = triethylpropylammonium, dmit^2- = 2-thioxo-1,3-dithiole-4,5-dithiolate), which shows a structural phase transition with lattice symmetry increase in LTP, which is contrary to the SB phase transition. Particularly, the structure phase transition leads to magnetically bistable state with T_C↑ ≈ 375 K, T_C↓ ≈ 320 K, and surprisingly large thermal hysteresis (∼55 K). Additionally, LTP and HTP coexist in a temperature region near T_C but not at T_C in this 1D spin system. The large hysteresis is related to the huge deformation of anion stack, which needs high activation energy for the structure transformation and magnetic transition between LTP and HTP. This study would not only provide new insight into the relationship of spin-Peierls-type transition and structure phase transition but also offer a roadmap for searching molecular-scale magnetic bistable materials, which are in huge demand in future electronic, magnetic, and photonic technologies.

The Diradical-Dication Strategy for BODIPY- and Porphyrin-Based Dyes with Near-Infrared Absorption Maxima from 1070 to 2040 nm

Four stable boron dipyrromethene (BODIPY)- and porphyrin-based bis-arylamine diradical dications were synthesized by two-electron oxidation of their neutral molecules. The two BODIPY-based dications have open-shell singlet ground states. UV/Vis absorption spectra of all four dications showed large redshifts in the NIR region compared to their neutral precursors with absorption maxima at 1274 and 1068 nm for the two BODIPY-based dications and 1746 and 2037 nm for the two porphyrin-based dications. Thus, two new types of NIR dyes with longer wavelengths are provided by the diradical-dication strategy, which can be applied for the generation of other NIR dyes with a range of different chromophores and auxochromes.

Carbene derived diradicaloids - building blocks for singlet fission?

Organic singlet diradicaloids promise application in non-linear optics, electronic devices and singlet fission. The stabilization of carbon allotropes/cumulenes (C1, C2, C4) by carbenes has been equally an area of high activity. Combining these fields, we showed recently that carbene scaffolds allow as well for the design of diradicaloids. Herein, we report a comprehensive computational investigation (CASSCF/NEVPT2; fractional occupation DFT) on the electronic properties of carbene-bridge-carbene type diradicaloids. We delineate how to adjust the properties of these ensembles through the choice of carbene and bridge and show that already a short C2 bridge results in remarkable diradicaloid character. The choice of the carbene separately tunes the energies of the S1 and T1 excited states, whereas the bridge adjusts the overall energy level of the excited states. Accordingly, we develop guidelines on how to tailor the electronic properties of these molecules. Of particular note, fractional occupation DFT is an excellent tool to predict singlet-triplet gaps.

Isomerism, Diradical Signature, and Raman Spectroscopy: Underlying Connections in Diamino Oligophenyl Dications

A diradical dication of a 4,4'-di(bis(1,4-methylphenyl)amino)-p-terphenyl oligomer has been characterized in solid-state by Raman spectroscopy and thermo-spectroscopy together with quantum chemical calculations. The diradical character has been evaluated on the basis of the Raman spectra and as a function of temperature. A complete understanding of the nature of the changes in solid state has been provided based on a pseudo-Jahn-Teller effect, which is feasible owing to the fine balance between quinoidal/aromatic extension among consecutive rings and steric crowding. This study contributes to the further comprehension of the molecular and electronic structures of these particular diradical molecules with strong implications on the understanding of the nature of chemical bonds in the limits of high electronic correlation or π-conjugation.

S = 1 Tetraazacyclophane Diradical Dication with Robust Stability: A Case of Low-Temperature One-Dimensional Antiferromagnetic Chain

One-dimensional (1D) spin-1 ( S = 1) chain of organic radicals with low local magnetic anisotropy may provide a better understanding of the low-dimensional magnetism. We report solid-state studies, including single crystal X-ray crystallography, of air-stable tetraazacyclophane diradical dication salt 1^2·2+·2[Al(OC(CF₃)₂CH₃)₄]^- with a triplet ground state (Δ E_ST ≈ 0.5 kcal mol^-1). The magnetic behavior for 1^2·2+ at low temperature is best modeled by 1D spin S = 1 Heisenberg chain with intrachain antiferromagnetic coupling of J'/ k = -5.4 K, which is associated with the interaryl C···C contacts, including π-π interactions. Zero-field splitting value, | D/ hc| ≈ 5.6 × 10^-3 cm^-1, for 1^2·2+ is rather small; thus, the 1D chains are characterized by the high degree of isotropicity | D/2 J'| ≈ 7.5 × 10^-4. The diradical dication salt possesses extraordinary stability with onset of decomposition at temperature of about 180 °C (∼450 K), based on thermogravimetric analysis and EPR spectroscopy.