Blatter Radical Research Articles

Discs, dumbbells and superdiscs: molecular and supermolecular architecture dependent magnetic behavior of mesogenic Blatter radical derivatives

A series of isomeric supermolecular bi-and tri-radicals, in which “half disc” mesogens are connected 1,4- and 1,3,5-through benzene, respectively, are investigated for liquid crystalline and magnetic properties.

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.

Effect of π-System Extension on the Ionization Energy of the Planar Blatter Radical: Experimental and Theoretical Studies.

Fusion of benzene, naphthalene, and phenalene rings with the D ring of the planar Blatter radical leads to extension of the π-system and increased spin delocalization. The effect of this π-extension and the position of the ring fusion on the electronic structure of the radicals was investigated by UV-photoelectron spectroscopy and DFT CAM-B3LYP/6-311G(d,p) method. The experimental data obtained for 3 out of 8 derivatives were correlated with DFT-derived ionization energies.

Controlled polymerization of styrene in the presence of Blatter’s radicals

Controlled polymerization of styrene (both self-initiated and initiated with azobisisobutyronitrile) in the presence of Blatter’s radical at 125 °C was studied. When using the radical initiator, there is no induction period. On the basis of UV spectroscopy and MALDI massspectrometry studies, it was established that the Blatter radical is completely consumed at the initial stage of the polymerization, inserting into almost all macromolecules. The prepared polystyrene exhibits the ability to reinitialization in post-polymerization and the synthesis of block copolymers.

Computational Optimization of Alkoxyamine-based Electrochemical Methylation.

Computational chemistry at the G3(MP2)-RAD//M06-2X/6-31+G(d,p)//SMD level of theory was used to study the oxidation of a test set of methyl adducts of nitroxide radicals and methyl adducts of Blatter's radical, a Kuhn verdazyl and two oxo-verdazyls. The barriers and the reaction energies of the SN2 reactions of the oxidized species with pyridine were also studied with a view to identify species with both low oxidation potentials and low SN2 barriers, so as to broaden the functional group tolerance of in situ electrochemical methylation compared with TEMPO-Me (1-methoxy-2,2,6,6-tetramethylpiperidine). Within the alkoxyamines, the oxidation potentials covered a range of 0.5 V, with trends explicable in terms of electrostatics, ring strain, and charge transfer. The oxidation potentials of oxo-verdazyl adducts, verdazyl adducts, and particularly the methyl adducts of Blatter's radical were considerably low due to the ability of their extensive π-systems to stabilize a positive charge. As expected, the SN2 reaction energies of the oxidized substrate became less favorable as the oxidation potential decreases. Unfortunately, this also meant that the barriers increased due to the excellent Evans-Polanyi correlation (R2 = 0.92). Nonetheless, 7-methoxy-7-azadispiro[5.1.5.836]hexadecane, N,N-di-tert-butyl-O-methylhydroxylamine, and particularly 1-methoxy-2,2,5,5-tetramethylpyrrolidine were identified as suitable candidates for broadening the scope of in situ electrochemical methylation while maintaining comparable kinetics to known reagents.

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.

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, first reported by Blatter in 1968. In contrast to their nitroxide counterparts, their properties can be modified more widely and more easily through simple substitution changes. This, together with recent developments in their synthesis, now places them at the forefront of developing applications in functional materials. Herein, we survey the various methods to synthesise and customise Blatter radicals, highlighting key developments in the last decade that have transformed their utility. We then outline their important spectroscopic, structural, electrochemical, magnetic and chemical properties and how these depend on their chemical structure and morphology. Finally, we review their growing list of applications including as sensors, spin labels, magnetic materials, liquid crystals and in polymer and small molecule synthesis.

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.

Recent advances of stable Blatter radicals: synthesis, properties and applications

Radicals, organic molecules with unpaired electrons, are applied across different scientific disciplines such as electronics, energy storage and biochemistry.

Reversal of magnetic exchange coupling between copper(II) and Blatter radical depending on the coordination environment

A neutral Blatter radical, 1-phenyl-3-(pyridin-2-yl)-7-(trifluoromethyl)-1,4- dihydrobenzo[e][1,2,4]triazine (L1′) was synthesized and employed as a bidentate ligand to synthesize complexes of Cu(II). Variation of molar ratio of this radical L1′ and Cu(II) precursor salts from 1:1 to 2:1 in a reaction yielded two complexes having formula [Cu(L1′)(hfac)2] (2a) and [Cu(L1′)2](ClO4)2·MeOH (2b) respectively. To the best of our knowledge, complex 2b is the first 1:2 metal-Blatter complex. Magnetic measurements showed a substantial metal-radical ferromagnetic exchange (+199 cm−1) and weak radical…radical antiferromagnetic exchange (−0.45 cm−1) between the dimeric units in complex 2a. Complex 2b exhibits weak metal-radical (−5.13 cm−1) as well as intramolecular radical…radical antiferromagnetic exchange (−6.31 cm−1) between the radicals, with no interaction between the dimeric units. Through-bond, as well as through-space exchange interactions, were quantified by DFT calculations for complex 2a. Such complexes possessing moderate to high exchange interactions are important constituents of molecular magnetic materials.

Functional Planar Blatter Radical through Pschorr-Type Cyclization.

A series of functional 2-phenyl-3H-[1,2,4]triazino[5,6,1-kl]phenoxazin-3-yl (planar Blatter) radicals containing CO2Me, CN, and NO2 groups at the C(10) position and 2-phenyl-3H-pyrido[3,2-b][1,2,4]triazino[1,4]benzoxazin-3-yl have been obtained in yields of 40-80% by using the Pschorr-type cyclization and characterized by spectroscopic (ultraviolet-visible and electron paramagnetic resonance), electrochemical, and density functional theory methods. Two of these groups, CO2Me and NO2, have been transformed into CO2H and NH2, respectively, and conjugated with amino acids. One of the derivatives was analyzed by single-crystal X-ray diffraction methods.

C(3) Functional Derivatives of the Blatter Radical.

A series of 3-substituted 1-phenyl-1,4-dihydrobenzo[e][1,2,4]triazin-4-yls 1 was prepared by addition of PhLi to 3-substituted benzo[e][1,2,4]triazines 2 followed by aerial oxidation. The scope of the C(3) substituents in the reaction was investigated, and 10 structurally diverse radicals 1 were isolated, their stability was assessed and properties were investigated with spectroscopic and electrochemical methods. Two radicals were analyzed with single-crystal XRD methods. Experimental data are compared to DFT results and correlated with Hammett substituent parameters.

Resolving the Unpaired-Electron Orbital Distribution in a Stable Organic Radical by Kondo Resonance Mapping.

The adsorption geometry and the electronic structure of a Blatter radical derivative on a gold surface were investigated by a combination of high-resolution noncontact atomic force microscopy and scanning tunneling microscopy. While the hybridization with the substrate hinders direct access to the molecular states, we show that the unpaired-electron orbital can be probed with Ångström resolution by mapping the spatial distribution of the Kondo resonance. The Blatter derivative features a peculiar delocalization of the unpaired-electron orbital over some but not all moieties of the molecule, such that the Kondo signature can be related to the spatial fingerprint of the orbital. We observe a direct correspondence between these two quantities, including a pronounced nodal plane structure. Finally, we demonstrate that the spatial signature of the Kondo resonance also persists upon noncovalent dimerization of molecules.

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.

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.

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.

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.

The Planar Blatter Radical: Structural Chemistry of 1,4‐Dihydrobenzo[e][1,2,4]triazin‐4‐yls

AbstractTwo planarized analogues of the prototypical Blatter radical (1), peri‐annulated 1S and 1O, are demonstrated and provide a new platform for molecular and supramolecular engineering, and for tuning electronic and magnetic properties of the radical. Planarization of 1 results in bathochromic shift to the near‐IR region, greater spin delocalization, and anodic shift of the reduction potential only for 1S. Magnetization studies revealed nearly ideal paramagnetic behavior at high temperatures for both radicals 1S and 1O with one‐dimensional ferromagnetic interaction in the former (2J=14.4 cm−1) and antiferromagnetic interactions in 1O at low temperatures.

The Planar Blatter Radical: Structural Chemistry of 1,4-Dihydrobenzo[e][1,2,4]triazin-4-yls.

Two planarized analogues of the prototypical Blatter radical (1), peri-annulated 1S and 1O , are demonstrated and provide a new platform for molecular and supramolecular engineering, and for tuning electronic and magnetic properties of the radical. Planarization of 1 results in bathochromic shift to the near-IR region, greater spin delocalization, and anodic shift of the reduction potential only for 1S . Magnetization studies revealed nearly ideal paramagnetic behavior at high temperatures for both radicals 1S and 1O with one-dimensional ferromagnetic interaction in the former (2J=14.4 cm(-1) ) and antiferromagnetic interactions in 1O at low temperatures.