Conformationally constrained, stable, triplet ground state (S = 1) nitroxide diradicals. Antiferromagnetic chains of S = 1 diradicals

Nitroxide diradicals, in which nitroxides are annelated to m-phenylene forming tricyclic benzobisoxazine-like structures, have been synthesized and characterized by X-ray crystallography, magnetic resonance (EPR and 1H NMR) spectroscopy, as well as magnetic studies in solution and in solid state. For the octamethyl derivative of benzobisoxazine nitroxide diradical, the conformationally constrained nitroxide moieties are coplanar with the m-phenylene, leading to large values of 2J (2J/k > 200 K in solution and 2J/k > 300 K in the solid state). For the diradical, in which all ortho and para positions of the m-phenylene are sterically shielded, distortion of the nitroxide moieties from coplanarity is moderate, such that the singlet-triplet gaps remain large in both solution (2J/k > 200 K) and the solid state (2J/k approximately 400-800 K), though an onset of thermal depopulation of the triplet ground state is detectable near room temperature. These diradicals have robust triplet ground states with strong ferromagnetic coupling and good stability at ambient conditions. Magnetic behavior of the nitroxide diradicals at low temperature is best fit to the model of one-dimensional S = 1 Heisenberg chains with intrachain antiferromagnetic coupling. The antiferromagnetic coupling between the S = 1 diradicals may be associated with the methyl nitroxide C-H- - -O contacts, including nonclassical hydrogen bonds. These unprecedented organic S = 1 antiferromagnetic chains are highly isotropic, compared to those of the extensively studied Ni(II)-based chains.     

A Highly Stable Organic Luminescent Diradical

It is very challenging to obtain stable room-temperature luminescent open-shell singlet diradicals. Herein we report the first stable Müller's hydrocarbon TTM-PhTTM with luminescent properties. Variable-temperature electron paramagnetic resonance spectroscopy measurements and theoretical calculations show that TTM-PhTTM has an open-shell singlet ground state with a diradical character of 90 %. Because of a small singlet-triplet energy gap, the open-shell singlet ground state can be thermally excited to a triplet state. TTM-PhTTM shows room-temperature deep-red emission in various solutions. Unusually high stability of TTM-PhTTM was also observed owing to effective steric hindrance and spin delocalization. Our results are beneficial to the rational design and discovery of more stable luminescent diradical materials.     

A stable triplet diradical emitter

Molecules with luminescence have been extensively investigated, but the luminescence of a stable molecule with a triplet ground state has not been observed. Synthesis of boron-containing radicals has attracted lots of interest because of their unique electronic structures and potential applications in organic semiconductors. Though some boron-based diradicals have been reported, neutral boron-containing diradicals with triplet ground states are rare. Herein two borocyclic diradicals with different substituents (3 and 4) have been isolated. Their electronic structures were investigated by EPR and UV spectroscopy, and SQUID magnetometry, in conjunction with DFT calculations. Both experiment and calculation suggest that 3 is an open shell singlet diradical while 4 is a triplet ground state diradical with a large singlet–triplet gap (0.25 kcal mol⁻¹). Both diradicals show multi fluorescence peaks (3: 414, 431, and 470 nm; 4: 420, 433, and 495 nm). 3 displays multiple redox steps and is a potential material towards the design of high-density memory devices. 4 represents the first example of a neutral triplet boron-containing diradical with a strong ferromagnetic interaction, and also is the first stable triplet diradical emitter.

Stable borocyclic diradical emitters with a tunable ground state.     

Ferromagnetically Coupled S=1 Chains in Crystals of Verdazyl-Nitronyl Nitroxide Diradicals

Thermally stable organic diradicals with a triplet ground state along with large singlet-triplet energy gap have significant potential for advanced technological applications. A series of phenylene-bridged diradicals with oxoverdazyl and nitronyl nitroxide units were synthesized via a palladium-catalyzed cross-coupling reaction of iodoverdazyls with a nitronyl nitroxide-2-ide gold(I) complex with high yields. The diradicals exhibit high stability and do not decompose in an inert atmosphere up to 180 °C. For the diradicals, both substantial AF (ΔE_ST≈−64 cm⁻¹) and FM (ΔE_ST≥25 and 100 cm⁻¹) intramolecular exchange interactions were observed. The sign of the exchange interaction is determined both by the bridging moiety (para- or meta-phenylene) and by the type of oxoverdazyl block (C-linked or N-linked). Upon crystallization, diradicals with the triplet ground state form unique one-dimensional exchange-coupled chains with strong intra- and weak inter-diradical ferromagnetic coupling.     

Is tetramethyleneethane a ground state triplet?

We have examined a number of possible ways by which tetramethyleneethane (TME) can be a ground state triplet, as claimed by experimental studies, in violation of Ovchinnikov’s theorem for alternant hydrocarbons of equal bond lengths. Model exact π calculations of the low-lying states of TME, 3,4-dimethylenefuran and 3,4-dimethylenepyrrole were carried out using a diagrammatic valence bond approach. The calculations failed to yield a triplet ground state even after (a) tuning of electron correlation, (b) breaking alternancy symmetry, and (c) allowing for geometric distortions. In contrast to earlier studies of fine structure constants in other conjugated systems, the computedD andE values of all the low-lying triplet states of TME for various geometries are at least an order of magnitude different from the experimentally reported values. Incorporation of σ-π mixing by means of UHF MNDO calculations is found to favour a singlet ground state even further. A reinterpretation of the experimental results of TME is therefore suggested to resolve the conflict.     

Reversible Self-Assembling of Boryl Radical Anions to Their Diradicals with Tunable Singlet Ground States

Two novel boron-centered diradicals based on dimesitylpyridine borane ( 1 ) were synthesized by the self-assembling of the corresponding radical sodium and potassium salts, respectively. The sodium diradical was obtained by re-dissolving the crystals of the radical salt 1Na in toluene, while the potassium diradical was directly obtained by the reduction of 1 with potassium in THF. The diradicals could be converted back to their radical anions in THF solution, forming a reversible process. EPR spectroscopy and SQUID measurements, together with theoretical calculations, show that the diradicals have singlet ground states with excited triplet states. Their singlet–triplet energy gaps are tunable with metals.     

A Dicyanomethylene‐Substituted Triangulene: Effects of Molecular‐Symmetry Reduction and Electron‐Accepting Substituents on a Fused Polycyclic Neutral π‐Radical System

A triangulene‐based C₂‐symmetric 33 π‐conjugated stable neutral π‐radical, 2^. , which possesses two dicyanomethylene groups and one oxo group, has been designed, synthesized, and isolated as an analogue of tris(dicyanomethylene) derivative 1^. and trioxo derivative TOT^. with C₃ symmetry. Effects of molecular‐symmetry reduction and electron‐accepting substituents on this fused polycyclic neutral π‐radical system were studied in terms of their molecular structure, electronic‐spin structure, and electrochemical and optical properties with the help of theoretical calculations. Interestingly, this system ( 2^. ) has a four‐stage redox ability, like TOT^. , as well as low frontier energy levels and a small SOMO–LUMO gap, similar to 1^. , in spite of the loss of the degenerate LUMOs in symmetry‐lowered 2^. , which is associated with the attachment of the weaker electron‐accepting oxo group instead of the dicyanomethylene group in 1^. . These prominent results are attributable to the structural and electronic properties in the triangulene‐based highly delocalized fused polycyclic neutral π‐radical system.     

Room-Temperature Phosphorescence Emitters Exhibiting Red to Near-Infrared Emission Derived from Intermolecular Charge-Transfer Triplet States of Naphthalenediimide−Halobenzoate Triad Molecules

Room-temperature phosphorescence (RTP) emitters have attracted significant attention. However, purely organic RTP emitters in red to near-infrared region have not been properly investigated. In this study, a series of naphthalenediimide−halobenzoate-linked molecules are synthesized, one of which exhibits efficient RTP properties, showing red to near-infrared emission in solid and aqueous dispersion. Spectroscopic studies and single-crystal X-ray diffraction analysis have shown that the difference in the stacking modes of compounds affects the optical properties, and the formation of intermolecular charge-transfer complexes of naphthalenediimide−halobenzoate moiety results in a bathochromic shift of absorption and RTP properties. The time-dependent density functional theory calculations showed that the formation of charge-transfer triplet states and the external heavy atom effect of the halogen atom enhance the intersystem crossing between excited singlet and triplet states.     

Metal bacteriochlorins which act as dual singlet oxygen and superoxide generators

A series of stable free-base, Zn(II) and Pd(II) bacteriochlorins containing a fused six- or five-member diketo- or imide ring have been synthesized as good candidates for photodynamic therapy sensitizers, and their electrochemical, photophysical, and photochemical properties were examined. Photoexcitation of the palladium bacteriochlorin affords the triplet excited state without fluorescence emission, resulting in formation of singlet oxygen with a high quantum yield due to the heavy atom effect of palladium. Electrochemical studies revealed that the zinc bacteriochlorin has the smallest HOMO-LUMO gap of the investigated compounds, and this value is significantly lower than the triplet excited-state energy of the compound in benzonitrile. Such a small HOMO-LUMO gap of the zinc bacteriochlorin enables intermolecular photoinduced electron transfer from the triplet excited state to the ground state to produce both the radical cation and the radical anion. The radical anion thus produced can transfer an electron to molecular oxygen to produce superoxide anion which was detected by electron spin resonance. The same photosensitizer can also act as an efficient singlet oxygen generator. Thus, the same zinc bacteriochlorin can function as a sensitizer with a dual role in that it produces both singlet oxygen and superoxide anion in an aprotic solvent (benzonitrile).     

Density Functional Theory (DFT) studies on the ground state of NO_3(~2A''''_2) radical and the first triplet state of NO_3~ cation

Density Functional Theory (DFT) studies on the ground states (2A'2) of NO3 radical and on the ground state (1A1') and the first triplet state (3E") of NO3 cation provide an unambiguous prediction about their geometrical structure-, the ground states of both NO3 radical and NO3 cation have D3h symmetry and the geometrical configuration of the first triplet state 3E" of NO3 cation has C2v symmetry. It is shown that as far as the ionization energy calculations on NO, radical are concerned, the results are only slightly different, no mater that gradient corrections of the exchange-correlation energy are included during self-consistent iterations or they are included as perturbations after the self-consistent iterations.     

A New Class of Neutral Boron‐Based Diradicals Spanned by a Two‐Carbon‐Atom Bridge

A new structural class of boron‐based diradicals is prepared by Lewis base addition to and reduction of singly cyclic (alkyl)(amino)carbene (CAAC) stabilised diborylalkenes. The diradicals feature a perpendicular olefinic bridge preventing delocalization between the B(CAAC) π systems, making the coupling between the spin centres very weak. DFT calculations indicate that the compounds are ground‐state (open‐shell) singlets, with triplet states negligibly higher in energy, thus making the triplet states easily populated thermally.     

Benzylic cations with triplet ground states: computational studies of aryl carbenium ions, silylenium ions, nitrenium ions, and oxenium ions substituted with meta pi donors

Density functional theory (B3LYP/6-31G(d,p)) was used to predict the effect of meta substitution on aryl cationic (Ar-X+) species, including aryloxenium ions, arylsilylenium ions, arylnitrenium ions, and arylcarbenium ions. Multireference second-order perturbation theory (CASPT2) calculations were used to benchmark the quantitative accuracy of the DFT calculations for representative systems. Substituting the meta positions on these species with pi donors stabilizes a pi,pi* diradical state analogous to the well-known m-xylylene diradical. Notably, the 3,5-bis(N,N-dimethylamino)benzyl cation is predicted to have a triplet ground state by 1.9 kcal/mol by DFT and to have essentially degenerate singlet-triplet states at the CASPT2(10,9) level of theory. Adding electron-withdrawing CF3 groups to the exocyclic carbon of this meta-disubstituted benzyl cation further increases the predicted singlet-triplet gap in favor of the triplet. Other aryl cationic species substituted with strong pi electron-donating groups in the meta positions are predicted to have low-energy or ground-state triplet states. Systems analogous to the naphthaquinodimethane diradicals are also reported.     

C3H4: Theoretical study of structures and stabilities of isomers

The various isomers including stable structures, carbenes, and diradicals on the C₃H₄ surface have been investigated. The two carbenes propenylidene and cyclopropylidene have been found to have singlet ground states. Vinylmethylene is predicted to have a triplet ground state with a planar diradical type of structure. The syn and anti forms of this state are degenerate. This is in agreement with the observation of two triplet states in the electron spin resonance (ESR ) spectra. The π electrons are found to be delocalized over the three carbons. The singlet diradical structures are found to be more stable than the carbene structures, which retain the CH₂ (DOUBLE BOND) CH allylic structures. The orbital compositions of the frontier orbitals of all systems have been determined to examine the nature of these orbitals. © 1996 John Wiley & Sons, Inc.     

Magnetic phase transition in a heteromolecular hydrogen-bonded complex of nitronylnitroxide radicals

An organic molecular acid-base complex has been synthesized from pyridine-substituted biradical 2 in a triplet (S = 1) ground state and a benzoic acid derivative of monoradical 3 with S = 1/2. The two constituent molecules are bound by an OH-N hydrogen bond in a crystalline solid state. The complex has been found to exhibit an antiferromagnetic phase transition at 5 K. The complex is the first example of a hydrogen-bonded heterospin, heteromolecular complex exhibiting a magnetic phase transition in purely organic molecule-based materials.     

Modulative magnetic interaction in π-diradicals of directly meso-linked porphyrin dimers

A series of directly meso-linked Zn(II) porphyrin dimers were prepared, and properties of their bis(π-radical cation)s were investigated. The redox potentials of the dimers bearing various meso-substituents are distributed in wide range although the potential difference (ΔE) between the first and the second oxidations were similar values. Among these dimers, di-tert-butylphenyl derivative and octyloxyphenyl derivative were chemically oxidized by the treatment with NaAuCl₄ in chloroform to afford mono(π-radical cation)s and bis(π-radical cation)s. In the mono(π-radical cation), the π-radical resides one of the porphyrin rings in view of the ESR hyperfine structure. The bis(π-radical cation)s of these dimers showed fine-structured ESR spectra due to triplet species, indicating that the orthogonal alignment of π-radicals leads a stable triplet state. The plots of the triplet ESR signal intensity of the bis(π-radical cation)s versus temperature showed an anomalous feature, in which slight change at lower temperatures and drastic change at higher temperatures were observed. The spectral behavior at lower temperatures was elucidated to be derived from the existence of several kinds of frozen dimers with various dihedral angles between two porphyrin rings. An abrupt increase of triplet signal intensity around 130–160 K can be rationalized by molecular motion around the meso–meso linkage. Such phenomena are comparable with spin-crossover. It also proved that the magnetic interaction in the π-diradical is modulated by the electron-donating ability of the meso-aryl substituents.     

理论研究V2O6-的结构和电子性质

The geometrical and electronic properties of the anionic and neutral V2O6 clusters were studied with the spin unrestricted hybrid density functional B3LYP method. The calculated ground states of both clusters are different from the previous theoretical results. The ground state of V2O6- is found to be a doublet with C2v symmetry, while a doublet with D2h symmetry was previously obtained by Vyboishchikov and Sauer. For neutral V2O6, the ground state is an open-shell singlet with D2h symmetry whose energy is very close to that of the triplet state. In contrast, a closed-shell singlet with D2h symmetry was obtained by Vyboishchikov and Sauer, and Calatayud et al. found a triplet ground state with Cs symmetry. Moreover,the calculated adiabatic and vertical detachment energies of the anion cluster are in much better agreement with the experimental results of photoelectron spectroscopy than previous theoretical values.     

Fused Triangulene Dimers: Facile Synthesis by Intramolecular Radical-Radical Coupling and Application for Near-Infrared Lasers

Large graphene-like molecules with four zigzag edges are ideal gain medium materials for organic near-infrared (NIR) lasers. However, synthesizing them becomes increasingly challenging as the molecular size increases. In this study, we introduce a new intramolecular radical-radical coupling approach and successfully synthesize two fused triangulene dimers ( 1 a / 1 b ) efficiently. X-ray crystallographic analysis of 1 a indicates that there is no intermolecular π-π stacking in the solid state. When the more soluble derivative 1 b is dispersed in polystyrene thin films, amplified spontaneous emission in the NIR region is observed. Using 1 b as the active gain material, we fabricate solution-processed distributed feedback lasers that exhibit a narrow emission linewidth at around 790 nm. The laser devices also exhibit low thresholds with high photostability. Our study provides a new synthetic strategy for extended nanographenes, which have diverse applications in electronics and photonics.     

(NO)2+复合物阳离子的结构和性质研究

用B3LYP/6-311++G**方法对NO二聚体阳离子(NO)2+进行了研究.几何优化并结合振动分析表明,该复合物存在5种可能的稳定构型.能量最低的是N-N相连的反式异构体,具有C2h对称性.分析了各稳定构型的相对稳定性及成键特征.建立了态态相关并给出异构化过渡态,分析了各构型之间转化的途径.     

Collective All-Carbon Magnetism in Triangulene Dimers**

Triangular zigzag nanographenes, such as triangulene and its π-extended homologues, have received widespread attention as organic nanomagnets for molecular spintronics, and may serve as building blocks for high-spin networks with long-range magnetic order, which are of immense fundamental and technological relevance. As a first step towards these lines, we present the on-surface synthesis and a proof-of-principle experimental study of magnetism in covalently bonded triangulene dimers. On-surface reactions of rationally designed precursor molecules on Au(111) lead to the selective formation of triangulene dimers in which the triangulene units are either directly connected through their minority sublattice atoms, or are separated via a 1,4-phenylene spacer. The chemical structures of the dimers have been characterized by bond-resolved scanning tunneling microscopy. Scanning tunneling spectroscopy and inelastic electron tunneling spectroscopy measurements reveal collective singlet–triplet spin excitations in the dimers, demonstrating efficient intertriangulene magnetic coupling.     

Silicon Tris(perchloro)dioxolene: A Neutral Triplet Diradical

The reaction of ortho‐quinones with silicon tetraiodide leads to neutral silicon trisdioxolenes in high yield, delivering the unknown oxidized form of triscatecholatosilicate dianions and the first example of open‐shell semiquinonates connected via a single non‐metal center. Silicon tris(perchloro)dioxolene is a stable diradical with a triplet ground state, as supported by X‐ray diffraction; IR, resonance Raman, UV/Vis, and (VT)EPR spectroscopy; and Kohn–Sham broken‐symmetry computations. Preliminary results suggest that the preferred magnetic ground state can be altered through variation of the substituents.