A Derivative of the Blatter Radical as a Potential Metal-Free Magnet for Stable Thin Films and Interfaces

Radical-Induced Changes in Transition Metal Interfacial Magnetic Properties: A Blatter Derivative on Polycrystalline Cobalt

In this work, we study the interface obtained by depositing a monolayer of a Blatter radical derivative on polycrystalline cobalt. By examining the occupied and unoccupied states at the interface, using soft X-ray techniques, combined with electronic structure calculations, we could simultaneously determine the electronic structure of both the molecular and ferromagnetic sides of the interface, thus obtaining a full understanding of the interfacial magnetic properties. We found that the molecule is strongly hybridized with the surface. Changes in the core level spectra reflect the modification of the molecule and the cobalt electronic structures inducing a decrease in the magnetic moment of the cobalt atoms bonded to the molecules which, in turn, lose their radical character. Our method allowed us to screen, beforehand, organic/ferromagnetic interfaces given their potential applications in spintronics.

Toward N-peri-Annulated Planar Blatter Radical through aza-Pschorr and Photocyclization

Preparation of the elusive N-peri-annulated planar Blatter radicals was attempted using aza-Pschorr and photocyclization methods. In both methods, substrates containing N-Me and N-Ac groups yielded a zwitterionic heterocycle lacking the N-substituent as the main product, while in one of them a carbazole derivative representing a new heterocyclic system was also obtained. The formation of the zwitterion and the carbazole suggests the formation of the desired planar Blatter radical, which undergoes facile fragmentation through homolysis of the N-R bond. This mechanism is supported by DFT computational results, which also suggest that N-Ar derivatives should be sufficiently stable for isolation. Electronic structures of three planar Blatter radicals annulated with the O, S, and N-Ph groups are compared.

From Stable Radicals to Thermally Robust High-Spin Diradicals and Triradicals

Stable radicals and thermally robust high-spin di- and triradicals have emerged as important organic materials due to their promising applications in diverse fields. New fundamental properties, such as SOMO/HOMO inversion of orbital energies, are explored for the design of new stable radicals, including highly luminescent ones with good photostability. A relation with the singlet-triplet energy gap in the corresponding diradicals is proposed. Thermally robust high-spin di- and triradicals, with energy gaps that are comparable to or greater than a thermal energy at room temperature, are more challenging to synthesize but more rewarding. We summarize a number of high-spin di- and triradicals, based on nitronyl nitroxides that provide a relation between the experimental pairwise exchange coupling constant J/k in the high-spin species vs experimental hyperfine coupling constants in the corresponding monoradicals. This relation allows us to identify outliers, which may correspond to radicals where J/k is not measured with sufficient accuracy. Double helical high-spin diradicals, in which spin density is delocalized over the chiral π-system, have been barely explored, with the sole example of such high-spin diradical possessing alternant π-system with Kekulé resonance form. Finally, we discuss a high-spin diradical with electrical conductivity and derivatives of triangulene diradicals.

High-Spin Blatter's Triradicals

Robust organic triradicals with high-spin quartet ground states provide promising applications in molecular magnets, spintronics, etc. In this context, a triradical based on Blatter's radical has been synthesized recently, having two low-lying non-degenerate doublet states with a quartet ground state. The traditional broken-symmetry (BS)-DFT computed doublet-quartet energy gaps are reported to be somewhat overestimated in comparison to the experimentally observed values. In this work, we have employed different ab initio methods on this prototypical system to obtain more accurate doublet-quartet energy gaps for this triradical. The spin-constraint broken-symmetry (CBS)-DFT method has been used to reduce the overestimation of energy gaps from BS-DFT. To address the issues of spin-contamination and the multireference nature of low-spin states affecting the DFT methods, we have computed the energy gaps using appropriately state-averaged CASSCF and NEVPT2 computations. Using a series of active spaces, our calculations are shown to provide quite accurate values in concordance with the experimentally observed results. Furthermore, we have proposed and modeled another two triradicals based on Blatter's radical, which are of interest for experimental synthesis and characterization. Our computations show that all these triradicals also have a quartet ground state with a similar energy difference between the excited doublet states.

Thermally induced dimensionality changes in derivatives of a "super stable" Blatter radical

Two derivatives of a "super stable" Blatter radical (1,3-diphenyl-7-trifluoromethyl-1,4-dihydrobenzo[e][1,2,4]triazin-4-yl) with N(1)-Ar = 2-CF3C6H4 and 2-MeOC6H4 were obtained and investigated using XRD and SQUID magnetometry methods. The investigation revealed strong antiferromagnetic interactions in both radicals, which are described using the Hatfield model. For the latter radical, an abrupt and reversible change in the χ(T) plot was observed at 29 K. It was ascribed to a structural transition, consistent with a two-dimensional to one-dimensional thermally activated crossover, as supported by specific heat measurements (CvHvs. T). It is suggested that the transition is related to an order-disorder transition of the CF3 group, which is corroborated using XRD structural analysis.

Thermally Ultrarobust S = 1/2 Tetrazolinyl Radicals: Synthesis, Electronic Structure, Magnetism, and Nanoneedle Assemblies on Silicon Surface

Open-shell organic molecules, including S = 1/2 radicals, may provide enhanced properties for several emerging technologies; however, relatively few synthesized to date possess robust thermal stability and processability. We report the synthesis of S = 1/2 biphenylene-fused tetrazolinyl radicals 1 and 2. Both radicals possess near-perfect planar structures based on their X-ray structures and density-functional theory (DFT) computations. Radical 1 possesses outstanding thermal stability as indicated by the onset of decomposition at 269 °C, based on thermogravimetric analysis (TGA) data. Both radicals possess very low oxidation potentials <0 V (vs. SCE) and their electrochemical energy gaps, Ecell ≈ 0.9 eV, are rather low. Magnetic properties of polycrystalline 1 are characterized by superconducting quantum interference device (SQUID) magnetometry revealing a one-dimensional S = 1/2 antiferromagnetic Heisenberg chain with exchange coupling constant J'/k ≈ -22.0 K. Radical 1 in toluene glass possesses a long electron spin coherence time, Tm ≈ 7 μs in the 40-80 K temperature range, a property advantageous for potential applications as a molecular spin qubit. Radical 1 is evaporated under ultrahigh vacuum (UHV) forming assemblies of intact radicals on a silicon substrate, as confirmed by high-resolution X-ray photoelectron spectroscopy (XPS). Scanning electron microscope (SEM) images indicate that the radical molecules form nanoneedles on the substrate. The nanoneedles are stable for at least 64 hours under air as monitored by using X-ray photoelectron spectroscopy. Electron paramagnetic resonance (EPR) studies of the thicker assemblies, prepared by UHV evaporation, indicate radical decay according to first-order kinetics with a long half-life of 50 ± 4 days at ambient conditions.

Long-Term Degradation Mechanisms in Application-Implemented Radical Thin Films

Blatter radical derivatives are very attractive due to their potential applications, ranging from batteries to quantum technologies. In this work, we focus on the latest insights regarding the fundamental mechanisms of radical thin film (long-term) degradation, by comparing two Blatter radical derivatives. We find that the interaction with different contaminants (such as atomic H, Ar, N, and O and molecular H₂, N₂, O₂, H₂O, and NH₂) affects the chemical and magnetic properties of the thin films upon air exposure. Also, the radical-specific site, where the contaminant interaction takes place, plays a role. Atomic H and NH₂ are detrimental to the magnetic properties of Blatter radicals, while the presence of molecular water influences more specifically the magnetic properties of the diradical thin films, and it is believed to be the major cause of the shorter diradical thin film lifetime in air.

Metal-Free Organic Radical Spin Source

Organic radicals have long been suggested as candidates for organic magnets and components in organic spintronic devices. Herein, we demonstrate spin current emission from an organic radical film via spin pumping at room temperature. We present the synthesis and the thin film preparation of a Blatter-type radical with outstanding stability and low roughness. These features enable the fabrication of a radical/ferromagnet bilayer, in which the spin current emission from the organic radical layer can be reversibly reduced when the ferromagnetic film is brought into simultaneous resonance with the radical. The results provide an experimental demonstration of a metal-free organic radical layer operating as a spin source, opening a new avenue for the development of purely organic spintronic devices and bridging the gap between potential and real applications.

Deciphering the ground state of a C3-symmetrical Blatter-type triradical by CW and pulse EPR spectroscopy

3,3',3''-(Benzene-1,3,5-triyl)tris(1-phenyl-1H-benzo[e][1,2,4]triazin-4-yl) (1) is a C₃-symmetrical triradical comprised of three Blatter radical units connected at the 1, 3, 5 positions of a central trimethylenebenzene core. This triradical has an excellent air, moisture, and thermal stability. Single-crystal XRD indicates that triradical 1 adopts a propeller-like geometry with the benzotriazinyl moieties twisted by 174.1(2)° and packs in 1D chains along the c axis to form an extensive network of weak intermolecular interactions. Frozen solution continuous wave (CW) EPR spectra and variable-temperature field-sweep echo-detected (FSED) spectra revealed an intramolecular ferromagnetic exchange within the spin system, supporting a quartet S = 3/2 ground state. DFT calculations further supported these experimental findings.

"Super stable" Blatter radicals through ArLi addition: surprising chemistry of 7-(trifluoromethyl)benzo[e][1,2,4]triazine

Addition of PhLi to 7-(CF₃)benzo[e][1,2,4]triazine at -78 °C gives the "super stable" Blatter radical in high yields, while above -5 °C two additional products are formed. XRD analysis revealed the formation of a "trimer" and a benzo[f][1,2,4]triazepine via a novel mechanism. The latter is formed from the anion generated from the isolated radical, which suggests its instability in organic batteries.

Blatter Radical-Decorated Silica as a Prospective Adsorbent for Selective NO Capture from Air

Nitrogen oxides are adverse poisonous gases present in the atmosphere and having detrimental effects on the human health and environment. In this work, we propose a new type of mesoporous materials capable of capturing nitrogen monoxide (NO) from air. The designed material combines the robust Santa Barbara Amorphous-15 silica scaffold and ultrastable Blatter-type radicals acting as NO traps. Using in situ electron paramagnetic resonance spectroscopy, we demonstrate that NO capture from air is selective and reversible at practical conditions, thus making Blatter radical-decorated silica highly promising for environmental applications.

Revisiting the Synthesis of Functionally Substituted 1,4-Dihydrobenzo[e][1,2,4]triazines

A series of novel 1,4-dihydrobenzo[1,2,4][e]triazines bearing an acetyl or ester moiety as a functional group at the C(3) atom of the 1,2,4-triazine ring were synthesized. The synthetic protocol is based on an oxidative cyclization of functionally substituted amidrazones in the presence of DBU and Pd/C. It was found that the developed approach is suitable for the preparation of 1,4-dihydrobenzo[e][1,2,4]triazines, but the corresponding Blatter radicals were isolated only in few cases. In addition, a previously unknown dihydrobenzo[e][1,2,4]triazolo[3,4-c][1,2,4]triazine tricyclic open-shell derivative was prepared. Studies of thermal behavior of the synthesized 1,4-dihydrobenzo[1,2,4][e]triazines revealed their high thermal stability (up to 240–250 °C), which enables their application potential as components of functional organic materials.

High-Spin (S = 1) Blatter-Based Diradical with Robust Stability and Electrical Conductivity

Triplet ground-state organic molecules are of interest with respect to several emerging technologies but usually show limited stability, especially as thin films. We report an organic diradical, consisting of two Blatter radicals, that possesses a triplet ground state with a singlet-triplet energy gap, ΔEST ≈ 0.4-0.5 kcal mol-1 (2J/k ≈ 220-275 K). The diradical possesses robust thermal stability, with an onset of decomposition above 264 °C (TGA). In toluene/chloroform, glassy matrix, and fluid solution, an equilibrium between two conformations with ΔEST ≈ 0.4 kcal mol-1 and ΔEST ≈ -0.7 kcal mol-1 is observed, favoring the triplet ground state over the singlet ground-state conformation in the 110-330 K temperature range. The diradical with the triplet ground-state conformation is found exclusively in crystals and in a polystyrene matrix. The crystalline neutral diradical is a good electrical conductor with conductivity comparable to the thoroughly optimized bis(thiazolyl)-related monoradicals. This is surprising because the triplet ground state implies that the underlying π-system is cross-conjugated and thus is not compatible with either good conductance or electron delocalization. The diradical is evaporated under ultra-high vacuum to form thin films, which are stable in air for at least 18 h, as demonstrated by X-ray photoelectron and electron paramagnetic resonance (EPR) spectroscopies.

"Upper" Ring Expansion of the Planar Blatter Radical via Photocyclization: Limitations and Impact on the Electronic Structure

Photocyclization of 8-aryloxybenzo[e][1,2,4]triazines leads to the formation of π-expanded flat Blatter radicals for three phenanthryloxy and pyren-1-yloxy derivatives, whereas no photoreaction is observed for the perylen-3-yloxy precursor. Two of the new radicals are nonplanar, out of which one is unstable to isolation. The radical with the fused pyrene ring constitutes the largest thus far paramagnetic polycyclic π-system containing seven fused rings with 27 sp²-hybridized atoms and 29 π-delocalized electrons. The investigation of the reaction conditions demonstrated the higher efficiency of photoformation of the parent radical in polar solvents, which suggests a polar transition state and the S₁ photoreactive state. The effect of π expansion on the electronic structure was investigated with spectroscopic (UV-vis, electron paramagnetic resonance) and electrochemical methods augmented with density functional theory computational studies. The molecular structure of one of the radicals was determined with a single-crystal X-ray diffraction method.

Tethered Blatter Radical for Molecular Grafting: Synthesis of 6-Hydroxyhexyloxy, Hydroxymethyl, and Bis(hydroxymethyl) Derivatives and Their Functionalization

Synthetic access to 7-CF₃-1,4-dihydrobenzo[e][1,2,4]triazin-4-yl radicals containing 4-(6-hydroxyhexyloxy)phenyl, 4-hydroxymethylphenyl or 3,5-bis(hydroxymethyl)phenyl groups at the C(3) position and their conversion to tosylates and phosphates are described. The tosylates were used to obtain disulfides and an azide with good yields. The Blatter radical containing the azido group underwent a copper(I)-catalyzed azide-alkyne cycloaddition with phenylacetylene under mild conditions, giving the [1,2,3]triazole product in 84% yield. This indicates the suitability of the azido derivative for grafting Blatter radical onto other molecular objects via the CuAAC "click" reaction. The presented derivatives are promising for accessing surfaces and macromolecules spin-labeled with the Blatter radical.

Tuning the magnetic properties of a diamagnetic di-Blatter's zwitterion to antiferro- and ferromagnetically coupled diradicals

In the quest of obtaining organic molecular magnets based on stable diradicals, we have tuned the inherent zwitterionic ground state of tetraphenylhexaazaanthracene (TPHA), a molecule containing two Blatter's moieties, by adopting two different strategies. In the first strategy, we have increased the length of the coupler between the two radical moieties and observed a transition from the zwitterionic ground state to the diradicalized state. With a larger coupler, ferromagnetic interactions are realized based on density functional theory (DFT) and wave-function theory (WFT) based complete active space self-consistent field (CASSCF)-N-electron valence state perturbation theory (NEVPT2) methods. An analysis based on the extent of spin contamination, diradical character, CASSCF orbital occupation number, Head-Gordon's index, HOMO-LUMO and SOMOs energy gaps is demonstrated that marks the transition of the ground state in these systems. In another approach, we systematically explore the effect of push-pull substitution on the way to obtain molecules based on a TPHA skeleton with diradicaloid state and, in some cases, even a triplet ground state.

Planar Blatter radicals through Bu3SnH- and TMS3SiH-assisted cyclization of aryl iodides: azaphilic radical addition

Radical chain cyclization of aryl iodides provides an efficient synthesis of planar Blatter radicals, and, for the first time, access to functionalized sulphur-containing analogues.

The synthesis and magnetic properties of carboxylic acid-derived 1,2,4-benzotriazinyl radicals and their coordination particles

Stable magnetic coordination particles based on π-conjugated 1,2,4-benzotriazinyl radical ligands were synthesized using a sonochemical method.

Synthesis and Thin Films of Thermally Robust Quartet (S = 3/2) Ground State Triradical

High-spin (S = 3/2) organic triradicals may offer enhanced properties with respect to several emerging technologies, but those synthesized to date typically exhibit small doublet quartet energy gaps and/or possess limited thermal stability and processability. We report a quartet ground state triradical 3, synthesized by a Pd(0)-catalyzed radical-radical cross-coupling reaction, which possesses two doublet-quartet energy gaps, ΔE_DQ ≈ 0.2-0.3 kcal mol^-1 and ΔE_DQ2 ≈ 1.2-1.8 kcal mol^-1. The triradical has a 70+% population of the quartet ground state at room temperature and good thermal stability with onset of decomposition at >160 °C under an inert atmosphere. Magnetic properties of 3 are characterized by SQUID magnetometry in polystyrene glass and by quantitative EPR spectroscopy. Triradical 3 is evaporated under ultrahigh vacuum to form thin films of intact triradicals on silicon substrate, as confirmed by high-resolution X-ray photoelectron spectroscopy. AFM and SEM images of the ∼1 nm thick films indicate that the triradical molecules form islands on the substrate. The films are stable under ultrahigh vacuum for at least 17 h but show onset of decomposition after 4 h at ambient conditions. The drop-cast films are less prone to degradation in air and have a longer lifetime.

Recent advances in chemistry of high-spin nitrenes

Experimental and theoretical studies on aromatic nitrenes bearing from three to six unpaired electrons and having quartet, quintet, sextet or septet ground spin states, published in the last 15 years are analyzed. A comparative analysis of the magnetic properties of high-spin nitrenes and all other known high-spin organic molecules is performed. Promising areas of practical application of high-spin nitrenes as molecular magnets and as qubits and qudits for quantum computations are discussed.

The bibliography includes 214 references.

Substituent effects on the electronic structure of the flat Blatter radical: correlation analysis of experimental and computational data

Functionalized flat Blatter radicals were obtained and substituent effects on spectroscopy, electrochemistry, and stability were investigated by correlation and DFT methods.

UV-photoelectron spectroscopy of stable radicals: the electronic structure of planar Blatter radicals as materials for organic electronics

The electronic structure of Blatter radicals and a series of C(10)-substituted derivatives of 2-phenyl-3H-[1,2,4]triazino[5,6,1-kl]phenoxazin-3-yl (planar Blatter radicals) containing H, F, Cl, Br, CN, CF3 and OMe substituents was investigated by gas phase UV-photoelectron spectroscopy. The energy of the SOMO of the radicals, determined to be about 6.5 eV, was correlated with their electrochemical oxidation potentials, E0/+11/2, relative to the Fc/Fc+ couple in CH2Cl2 giving the correction of 6.60(1) eV. The optical band gap Eoptg ∼ 1.7 eV of the radicals yielded the electronic transport gap, Eelg, of about 2.1 eV, which is similar to the electronic parameters of pentacene. The radicals were analyzed by EPR spectroscopy and single crystal XRD methods, and all experimental data were compared to DFT computational results obtained at the CAM-B3LYP/6-311G(d,p) level of theory.

Stability of radical-functionalized gold surfaces by self-assembly and on-surface chemistry

We have investigated the radical functionalization of gold surfaces with a derivative of the perchlorotriphenylmethyl (PTM) radical using two methods: by chemisorption from the radical solution and by on-surface chemical derivation from a precursor. We have investigated the obtained self-assembled monolayers by photon-energy dependent X-ray photoelectron spectroscopy. Our results show that the molecules were successfully anchored on the surfaces. We have used a robust method that can be applied to a variety of materials to assess the stability of the functionalized interface. The monolayers are characterized by air and X-ray beam stability unprecedented for films of organic radicals. Over very long X-ray beam exposure we observed a dynamic nature of the radical-Au complex. The results clearly indicate that (mono)layers of PTM radical derivatives have the necessary stability to withstand device applications.

A practical method for fabricating superparamagnetic films and the mechanism involved

Due to the widespread applications of biosensors, such as in magnetic resonance imaging, cancer detection and drug delivery, the use of superparamagnetic materials for preparing biosensors has increased greatly. We report herein on a strategy toward fabrication of a nanoscale biosensor composed of superparamagnetic films. On increasing the film thickness of magnetic layers, a phase transition typically occurs from either a low-Curie-temperature state or a superparamagnetic state to a ferromagnetic state. A new finding is demonstrated wherein a phase transition of such a superparamagnetic phase can be induced by controlling the thickness of ultrathin ferromagnetic layers with perpendicular magnetic anisotropy. Both the M-H curve with zero coercive force at 300 K and deviations of the normalized hysteresis loop at 2 K confirm the superparamagnetic state of Co/Ir(111) at room temperature. An overstrained film transforming into clusters (OFTC) model based on the new finding and our experimental evidence is proposed for modeling this phenomenon. From the energetic point of view of the OFTC model, we propose a limited distortion mechanism that can be useful in determining the critical thickness for the phase transition. This mechanism considers the balance between interfacial strain energy and surface free energy. A method for producing superparamagnetic films by taking advantage of the accumulation of strain and relaxation is reported.

1-(2-Methoxyphenyl)-3-phenyl-1,4-dihydro-1,2,4-benzotriazin-4-yl: a tricky “structure-to-magnetism” correlation aided by DFT calculations

1-(2-Methoxyphenyl)-3-phenyl-1,4-dihydro-1,2,4-benzotriazin-4-yl (2) is a Blatter radical with a challenging structure-to-magnetism correlation.

Recent advances in the chemistry of benzo[e][1,2,4]triazinyl radicals

Benzo[e][1,2,4]triazinyl, or Blatter radicals, are stable free radicals with customisable magnetic, spectroscopic and electrochemical properties, and wide-ranging applications in synthesis and functional materials.

Polymorphism in a π stacked Blatter radical: structures and magnetic properties of 3-(phenyl)-1-(pyrid-2-yl)-1,4-dihydrobenzo[e][1,2,4]triazin-4-yl

The first polymorphism example in Blatter radicals, is reported.

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).

Air-Stable Thin Films with High and Anisotropic Electrical Conductivities Composed of a Carbon-Centered Neutral π-Radical

Air-stable thin films (50-720 nm thickness) composed of a carbon-centered neutral π-radical with high and anisotropic electrical conductivities were fabricated by vapor deposition of 4,8,12-trioxotriangulene (TOT). The thin films were air-stable over 15 months and were the aggregate of TOT microcrystals, in which a one-dimensional π-stacking column was formed through the strong singly occupied molecular orbital (SOMO)-SOMO interaction with two-electron-multicenter bond among the spin-delocalized π-planes. The orientations of the one-dimensional column of TOT were changed depending on the deposition rate and substrates, where face-on-oriented thin films were epitaxially grown on the graphite 0001 surface, and edge-on-oriented thin films were grown on glass, SiO2, and indium tin oxide substrates under a high-deposition rate condition. The films showed high electrical conductivities of 2.5 × 10-2 and 5.9 × 10-5 S cm-1 along and perpendicular to the π-stacking column, respectively, for an edge-on oriented thin film.

The Environment-Dependent Behavior of the Blatter Radical at the Metal-Molecule Interface

Stable organic radicals have potential applications for building organic spintronic devices. To fulfill this potential, the interface between organic radicals and metal electrodes must be well characterized. Here, through a combined effort that includes synthesis, scanning tunneling microscopy, X-ray spectroscopy, and single-molecule conductance measurements, we comprehensively probe the electronic interaction between gold metal electrodes and a benchtop stable radical-the Blatter radical. We find that despite its open-shell character and having a half-filled orbital close to the Fermi level, the radical is stable on a gold substrate under ultrahigh vacuum. We observe a Kondo resonance arising from the radical and spectroscopic signatures of its half-filled orbitals. By contrast, in solution-based single-molecule conductance measurements, the radical character is lost through oxidation with charge transfer occurring from the molecule to metal. Our experiments show that the stability of radical states can be very sensitive to the environment around the molecule.

Thermally and Magnetically Robust Triplet Ground State Diradical

High spin ( S = 1) organic diradicals may offer enhanced properties with respect to several emerging technologies, but typically exhibit low singlet triplet energy gaps and possess limited thermal stability. We report triplet ground state diradical 2 with a large singlet-triplet energy gap, Δ E_ST ≥ 1.7 kcal mol^-1, leading to nearly exclusive population of triplet ground state at room temperature, and good thermal stability with onset of decomposition at ∼160 °C under inert atmosphere. Magnetic properties of 2 and the previously prepared diradical 1 are characterized by SQUID magnetometry of polycrystalline powders, in polystyrene glass, and in other matrices. Polycrystalline diradical 2 forms a novel one-dimensional (1D) spin-1 ( S = 1) chain of organic radicals with intrachain antiferromagnetic coupling of J'/ k = -14 K, which is associated with the N···N and N···O intermolecular contacts. The intrachain antiferromagnetic coupling in 2 is by far strongest among all studied 1D S = 1 chains of organic radicals, which also makes 1D S = 1 chains of 2 most isotropic, and therefore an excellent system for studies of low-dimensional magnetism. In polystyrene glass and in frozen benzene or dibutyl phthalate solution, both 1 and 2 are monomeric. Diradical 2 is thermally robust and is evaporated under ultrahigh vacuum to form thin films of intact diradicals on silicon substrate, as demonstrated by X-ray photoelectron spectroscopy. Based on C-K NEXAFS spectra and AFM images of the ∼1.5 nm thick films, the diradical molecules form islands on the substrate with molecules stacked approximately along the crystallographic a-axis. The films are stable under ultrahigh vacuum for at least 60 h but show signs of decomposition when exposed to ambient conditions for 7 h.

Interfacing a Potential Purely Organic Molecular Quantum Bit with a Real-Life Surface

By using a multidisciplinary and multitechnique approach, we have addressed the issue of attaching a molecular quantum bit to a real surface. First, we demonstrate that an organic derivative of the pyrene-Blatter radical is a potential molecular quantum bit. Our study of the interface of the pyrene-Blatter radical with a copper-based surface reveals that the spin of the interface layer is not canceled by the interaction with the surface and that the Blatter radical is resistant in presence of molecular water. Although the measured pyrene-Blatter derivative quantum coherence time is not the highest value known, this molecule is known as a "super stable" radical. Conversely, other potential qubits show poor thin film stability upon air exposure. Therefore, we discuss strategies to make molecular systems candidates as qubits competitive, bridging the gap between potential and real applications.

Nitrogen analogues of Chichibabin's and Müller's hydrocarbons with small singlet-triplet energy gaps

Nitrogen analogues of Chichibabin's and Müller's hydrocarbons, DPh-D and TPh-D, based on 1,2,4-benzotriazinyl (Blatter) were studied. The two diradicaloids with good chemical and thermal stability exhibit smaller singlet-triplet energy gaps (ΔES-T from -1.05 to -1.27 kcal mol-1) than the hydrocarbon diradicaloids with the same bridges.

Ferromagnetic interactions in a 1D Heisenberg linear chain of 1-phenyl-3,7-bis(trifluoromethyl)-1,4-dihydro-1,2,4-benzotriazin-4-yls

1-Phenyl-3,7-bis(trifluoromethyl)-1,4-dihydro-1,2,4-benzotriazin-4-yl (2), was characterized by single crystal X-ray diffractometry and variable temperature SQUID magnetometry to investigate its structure-magnetism correlation.

Modern Spin on the Electrochemical Persistence of Heteroatom-Bridged Triphenylmethyl-Type Radicals

Herein we present a clarification of the ambiguous persistence of the 10-methyl-9-phenylacridanyl, 9-phenylxanthenyl, and 9-phenylthioxanthenyl radicals in electrochemical experiments. Each of these radicals has separately been the subject of conflicting literature results for decades with publications claiming both their chemical inertness and propensity to dimerize. We assert that each radical is persistent at conventional electrochemical time scales up to several minutes based on reversible redox couples and cyclic voltammogram simulations of the radicals and their respective cations. All three radicals are rapidly consumed by aerial O₂, which lends irreversibility to the redox couples after fewer than 20 s of exposure to air. With appreciation for the O₂ sensitivity of these radicals, their electrochemically generated UV-visible absorption spectra have been acquired and matched to predictions made by TD-DFT calculations. Further, we propose that previous claims to have electrochemically measured radical-radical dimerizations have only observed reaction of these radicals with dissolved O₂.

Nanoscale Studies of Organic Radicals: Surface, Interface, and Spinterface

In the last decade technology has brought significant changes to our lives, including new habits and a new view on social relationships. These technological innovations are based on several factors, one of which is miniaturization. This was made possible also due to the discovery and synthesis of new materials with characteristics at the nanoscale that are designed for specific purposes. This "on purpose" approach, joined to the development of preparation and growth methods, has led to use of thin films rather than bulk materials in devices. Using thin films makes devices easier to produce, and using films for coating protects the devices and gives specific properties to surfaces. For several decades thin films, surfaces, and interfaces have been intensively investigated. Indeed, device performances rely on the optimized match of thin films of different natures, such as organic and inorganic semiconductors and metals for contacts. Surprisingly, in comparison, little attention has been devoted to the deposition of organic radicals on a substrate. This might be because these materials are considered not stable enough for evaporation. In this work, we demonstrate that it is possible to evaporate and deposit organic radicals onto well-defined surfaces under controlled conditions, without degradation. Using soft X-ray spectroscopies, performed also at synchrotrons, we investigate thin film processes, surfaces, and interfaces at the nanoscale, when organic radicals are deposited on metal and metal oxide surfaces. We suggest how to design organic radicals bearing in mind the thermodynamic factors that govern thin film stability, with the purpose of obtaining not only a chemically stable radical, but also stable thin films. We investigate the thermal and air stability of the deposited films, and we explore the influence of the surface/radical chemical bond and the role of surface defects on the magnetic moment at the interface. We find that organic radicals are physisorbed and keep their magnetic moment on inert and passivated surfaces such as Au(111) and Al₂O₃(112̅0) single crystals, SiO₂, and ideal TiO₂(110) single crystals, while defective sites such as oxygen vacancies or the presence of OH groups lead to chemisorption of the organic radicals on the surface with quenching of their magnetic moment. Our work shows that the use of X-ray based techniques represents a powerful approach to reveal the mechanisms governing complex interfaces, such as radical/metal and radical/metal-oxide, where it is important to describe both charge and spin behavior (spinterfaces). It also makes it possible to conceive new experiments to investigate the magnetic character of the thin films versus their structural properties, toward tuning the arrangement of the molecules in films. Controlling the molecular arrangement will give the opportunity to tune the mutual position and orientation of the molecules, that is, of the single magnetic moments in the films, "imprinting" their magnetic properties. A deep understanding of stable radical/inorganic spinterfaces may open the way to use radicals in solid state devices or as quantum bits with dedicated configurations, as proposed for other molecular quantum bits, and in spin-based electronics.

Magneto-structural correlation of cyano-substituted 3-tert-butyl-1-phenyl-1,2,4-benzotriazin-4-yl: spin transition behaviour observed in a 6-cyano derivative

New cyano-substituted 3-tert-butylbenzotriazinyls, 7CN and 6CN, were synthesized and their spectroscopic properties and magneto-structural correlation were investigated.

Preparation of Blatter Radicals via Aza-Wittig Chemistry: The Reaction of N-Aryliminophosphoranes with 1-(Het)aroyl-2-aryldiazenes

Reacting N-aryliminophosphoranes with 1-(het)aroyl-2-aryldiazenes in preheated diphenyl ether at ca. 150-250 °C for 5-25 min affords in most cases the 1,3-diaryl-1,4-dihydrobenzo[e][1,2,4]triazin-4-yls (aka Blatter radicals) in moderate to good yields. All new compounds are fully characterized, including EPR and CV studies for the radicals. Single-crystal X-ray structures of 1-benzoyl-2-(perfluorophenyl)diazene and 1-(perfluorophenyl)-3-phenyl-1,4-dihydrobenzo[e][1,2,4]triazinyl are also presented.

New Blatter-type radicals from a bench-stable carbene

Stable benzotriazinyl radicals (Blatter's radicals) recently attracted considerable interest as building blocks for functional materials. The existing strategies to derivatize Blatter's radicals are limited, however, and synthetic routes are complex. Here, we report that an inexpensive, commercially available, analytical reagent Nitron undergoes a previously unrecognized transformation in wet acetonitrile in the presence of air to yield a new Blatter-type radical with an amide group replacing a phenyl at the C(3)-position. This one-pot reaction of Nitron provides access to a range of previously inaccessible triazinyl radicals with excellent benchtop stabilities. Mechanistic investigation suggests that the reaction starts with a hydrolytic cleavage of the triazole ring followed by oxidative cyclization. Several derivatives of Nitron were prepared and converted into Blatter-type radicals to test the synthetic value of the new reaction. These results significantly expand the scope of using functionalized benzotriazinyls as stable radical building blocks.

Blatter's radicals are highly valuable stable organic free radicals, comprising a heterocyclic ring system functionalized with a heteroaryl or alkyl group at the C-3 position. Here, the authors report a synthetic strategy to obtain Blatter's radical substituted with an amine and amide group.

Emission from the stable Blatter radical

Spectroscopic studies reveals broadband emission that spans the visible range originating from excited electronic states of the stable Blatter radical.