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.

Mechanistic origins of methyl-driven Overhauser DNP.

The Overhauser effect in the dynamic nuclear polarization (DNP) of non-conducting solids has drawn much attention due to the potential for efficient high-field DNP as well as a general interest in the underlying principles that enable the Overhauser effect in small molecules. We recently reported the observation of 1H and 2H Overhauser effects in H3C- or D3C-functionalized Blatter radical analogs, which we presumed to be caused by methyl rotation. In this work, we look at the mechanism for methyl-driven Overhauser DNP in greater detail, considering methyl librations and tunneling in addition to classical rotation. We predict the temperature dependence of these mechanisms using density functional theory and spin dynamics simulations. Comparisons with results from ultralow-temperature magic angle spinning-DNP experiments revealed that cross-relaxation at temperatures above 60 K originates from both libration and rotation, while librations dominate at lower temperatures. Due to the zero-point vibrational nature of these motions, they are not quenched by very low temperatures, and methyl-driven Overhauser DNP is expected to increase in efficiency down to 0 K, predominantly due to increases in nuclear relaxation times.

Crystal Structure and Solid-State Packing of 4-Chloro-5H-1,2,3-dithiazol-5-one and 4-Chloro-5H-1,2,3-dithiazole-5-thione.

The crystal structure and solid-state packing of 4-chloro-5H-1,2,3-dithiazol-5-one and two polymorphs of 4-chloro-5H-1,2,3-dithiazole-5-thione were analyzed and compared to structural data of similar systems. These five-membered S,N-rich heterocycles are planar with considerable bond localization. All three structures demonstrate tight solid-state packing without voids which is attributed to a rich network of short intermolecular electrostatic contacts. These include Sδ+…Nδ-, Sδ+…Oδ-, Sδ+…Clδ- and Sδ+…Sδ- interactions that are well within the sum of their van der Waals radii (∑VDW). B3LYP, BLYP, M06, mPW1PW, PBE and MP2 were employed to calculate their intramolecular geometrical parameters, the Fukui condensed functions to probe their reactivity, the bond order, Bird Index and NICS(1) to establish their aromaticity.

Multifunctional Dithiadiazolyl Radicals: Fluorescence, Electroluminescence, and Photoconducting Behavior in Pyren-1'-yl-dithiadiazolyl.

The pyren-1'-yl-functionalized dithiadiazolyl (DTDA) radical, C16H9CNSSN (1), is monomeric in solution and exhibits fluorescence in the deep-blue region of the visible spectrum (440 nm) upon excitation at 241 nm. The salt [1][GaCl4] exhibits similar emission, reflecting the largely spectator nature of the radical in the fluorescence process, although the presence of the radical leads to a modest quenching of emission (ΦF = 98% for 1+ and 50% for 1) through enhancement of non-radiative decay processes. Time-dependent density functional theory studies on 1 coupled with the similar emission profiles of both 1+ and 1 are consistent with the initial excitation being of predominantly pyrene π-π* character. Spectroscopic studies indicate stabilization of the excited state in polar media, with the fluorescence lifetime for 1 (τ = 5 ns) indicative of a short-lived excited state. Comparative studies between the energies of the frontier orbitals of pyren-1'-yl nitronyl nitroxide (2, which is not fluorescent) and 1 reveal that the energy mismatch and poor spatial overlap between the DTDA radical SOMO and the pyrene π manifold in 1 efficiently inhibit the non-radiative electron-electron exchange relaxation pathway previously described for 2. Solid-state films of both 1 and [1][GaCl4] exhibit broad emission bands at 509 and 545 nm, respectively. Incorporation of 1 within a host matrix for OLED fabrication revealed electroluminescence, with CIE coordinates of (0.205, 0.280) corresponding to a sky-blue emission. The brightness of the device reached 1934 cd/m2 at an applied voltage of 16 V. The crystal structure of 1 reveals a distorted π-stacked motif with almost regular distances between the pyrene rings but alternating long-short contacts between DTDA radicals. Solid state measurements on a thin film of 1 reveal emission occurs at shorter wavelengths (375 nm) whereas conductivity measurements on a single crystal of 1 show a photoconducting response at longer wavelength excitation (455 nm).

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.

The Suppression of Columnar π-Stacking in 3-Adamantyl-1-phenyl-1,4-dihydrobenzo[e][1,2,4]triazin-4-yl.

3-Adamantyl-1-phenyl-1,4-dihydrobenzo[e][1,2,4]triazin-4-yl (4) crystallizes as chains of radicals where the spin bearing benzotriazinyl moieties are isolated from each other. Magnetic susceptibility studies in the 5-300 K temperature region indicate that radical 4 demonstrates typical paramagnetic behavior stemming from non-interacting S = ½ spins.

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.

A magnetostructural investigation of an abrupt spin transition for 1-phenyl-3-trifluoromethyl-1,4-dihydrobenzo[e][1,2,4]triazin-4-yl.

1-Phenyl-3-trifluoromethyl-1,4-dihydrobenzo[e][1,2,4]triazin-4-yl is the first example of a hydrazyl radical that shows a reversible sharp spin transition fully completed within 5(1) K. The nominally first-order transition takes place at ca. 58(2) K and proceeds via subtle changes of intra- and interstack interactions between two similar structural phases. The low-temperature phase (5-60 K) is diamagnetic and has a singlet ground state (2Jexp = -166.8 cm(-1), gsolid = 2.0042, ρ = 0.2%) stemming from a multicenter two-electron interaction. The high-temperature phase (60-300 K) is paramagnetic as a result of noninteracting S = 1/2 spins arising from weakly bound dimers.

Structural, magnetic, and computational correlations of some imidazolo-fused 1,2,4-benzotriazinyl radicals.

1,3,7,8-Tetraphenyl-4,8-dihydro-1H-imidazolo[4,5g][1,2,4]benzotriazin-4-yl (5), 8-(4-bromophenyl)-1,3,7-triphenyl-4,8-dihydro-1H-imidazolo[4,5g][1,2,4]benzotriazin-4-yl (6), and 8-(4-methoxyphenyl)-1,3,7-triphenyl-4,8-dihydro-1H-imidazolo[4,5g][1,2,4]benzotriazin-4-yl (7) were characterized by using X-ray diffraction crystallography, variable-temperature magnetic susceptibility studies, and DFT calculations. Radicals 5-7 pack in 1 D π stacks made of radical pairs with alternate short and long interplanar distances. The magnetic susceptibility (χ vs. T) of radicals 5 and 6 exhibit broad maxima at (50±2) and (50±4) K, respectively, and are interpreted in terms of an alternating antiferromagnetic Heisenberg linear chain model with average exchange-interaction values of J = -31.3 and -35.4 cm(-1) (gsolid = 2.0030 and 2.0028) and an alternation parameter a = 0.15 and 0.38 for 5 and 6, respectively. However, radical 7 forms 1 D columns of radical pairs with alternating distances; one of the interplanar distances is significantly longer than the other, which decreases the magnetic dimensionality and leads to discrete dimers with a ferromagnetic exchange interaction between the radicals (2J = 23.6 cm(-1) , 2zJ' = -2.8 cm(-1) , gsolid = 2.0028). Magnetic exchange-coupling interactions in 1,2,4-benzotriazinyl radicals are sensitive to the degree of slippage and inter-radical separation, and such subtle changes in structure alter the fine balance between ferro- and antiferromagnetic interactions.

Route to benzo- and pyrido-fused 1,2,4-triazinyl radicals via N'-(het)aryl-N'-[2-nitro(het)aryl]hydrazides.

A two-step route to 1,3-disubstituted benzo- and pyrido-fused 1,2,4-triazinyl radicals is presented. The route involves the N'-(2-nitroarylation) of easily prepared N'-(het)arylhydrazides via nucleophilic aromatic substitution of 1-halo-2-nitroarenes, which in most cases gives N'-(het)aryl-N'-[2-nitro(het)aryl]hydrazides in good yields. Mild reduction of the nitro group followed by an acid-mediated cyclodehydration gives the fused triazines, which upon alkali treatment afford the desired radicals. Fifteen examples of radicals are presented bearing a range of substituents at N-1, C-3, and C-7, including the pyrid-2-yl and 8-aza analogues. This route to the N'-(het)aryl-N'-[2-nitro(het)aryl]hydrazides, which works well with benzo- and picolinohydrazides, required a modification for aceto- and trifluoroacetohydrazides that involved a multistep synthesis of asymmetrically 1,1-diaryl-substituted hydrazines.

Synthesis and properties of imidazolo-fused benzotriazinyl radicals.

1,3-Diphenylbenzo[e][1,2,4]triazin-7(1H)-one, the oxidation product of 1,3-diphenyl-1,4-dihydro-1,2,4-benzotriazin-4-yl (Blatter's radical), reacts with N'-arylbenzamidines in PhMe at ca. 100 °C in the presence of N,N-diisopropylethylamine (Hünig's base) (1 equiv.) to give N'-aryl-N-(1,7-dihydro-7-oxo-1,3-diphenylbenzo[e][1,2,4]triazin-6-yl)benzimidamides in 49-95% yield. In neat AcOH heated at ca. 120 °C, N'­aryl-N-(1,7-dihydro-7-oxo-1,3-diphenylbenzo[e][1,2,4]triazin-6-yl)benzimidamides cyclodehydrate to give the novel 8-substituted 1,3,7-triphenyl-4,8-dihydro-1H-imidazo[4,5-g][1,2,4]benzotriazin-4-yls in 13-81% yield. During the optimization of this cyclodehydration an additional oxazole fused benzotriazinyl radical 1,3,7-triphenyl-1,4-dihydro[1,3]oxazolo[4,5-g][1,2,4]benzotriazin-4-yl was isolated as a side product and characterized. The CV and EPR data of the imidazolo- and oxazolo-fused radicals are presented as well as single crystal X-ray structures of 1,3,7-triphenyl-1,4-dihydro-[1,3]oxazolo[4,5-g][1,2,4]benzotriazin-4-yl and 1,3,7,8-tetraphenyl-4,8-dihydro-1H-imidazo[4,5-g][1,2,4]benzotriazin-4-yl.

"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.

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.

The Effect of High Pressure on Polymorphs of a Derivative of Blatter's Radical: Identification of the Structural Signatures of Subtle Phase Transitions.

The effect of pressure on the α and ß polymorphs of a derivative of Blatter's radical, 3-phenyl-1-(pyrid-2-yl)-1,4-dihydrobenzo[e][1,2,4]triazin-4-yl, has been investigated using single-crystal X-ray diffraction to maximum pressures of 5.76 and 7.42 GPa, respectively. The most compressible crystallographic direction in both structures lies parallel to π-stacking interactions, which semiempirical Pixel calculations indicate are also the strongest interactions present. The mechanism of compression in perpendicular directions is determined by void distributions. Discontinuities in the vibrational frequencies observed in Raman spectra measured between ambient pressure and ∼5.5 GPa show that both polymorphs undergo phase transitions, the α phase at 0.8 GPa and the ß phase at 2.1 GPa. The structural signatures of the transitions, which signal the onset of compression of initially more rigid intermolecular contacts, were identified from the trends in the occupied and unoccupied volumes of the unit cell with pressure and in the case of the ß phase by deviations from an ideal model of compression defined by Birch-Murnaghan equations of state.

Ferromagnetic interactions in a 1D alternating linear chain of π-stacked 1,3-diphenyl-7-(thien-2-yl)-1,4-dihydro-1,2,4-benzotriazin-4-yl radicals.

X-ray studies show that 1,3-diphenyl-7-(thien-2-yl)-1,4-dihydro-1,2,4-benzotriazin-4-yl (6) adopts a distorted, slipped π-stacked structure of centrosymmetric dimers with alternate short and long interplanar distances (3.48 and 3.52 Å). Cyclic voltammograms of 7-(thien-2-yl)benzotriazin-4-yl 6 show two fully reversible waves that correspond to the -1/0 and 0/+1 processes. EPR and DFT studies on radical 6 indicate that the spin density is mainly delocalized over the triazinyl fragment. Magnetic susceptibility measurements show that radical 6 obeys Curie-Weiss behavior in the 5-300 K region with C=0.378 emu K mol(-1) and θ=+4.72 K, which is consistent with ferromagnetic interactions between S=1/2 radicals. Fitting the magnetic susceptibility revealed the behavior is consistent with an alternating ferromagnetic chain (g=2.0071, J(1) =+7.12 cm(-1), J(2) =+1.28 cm(-1)).

Antiferromagnetic interactions in 1D Heisenberg linear chains of 7-(4-fluorophenyl) and 7-phenyl-substituted 1,3-diphenyl-1,4-dihydro-1,2,4-benzotriazin-4-yl radicals.

7-(4-Fluorophenyl) and 7-phenyl-substituted 1,3-diphenyl-1,4-dihydro-1,2,4-benzotriazin-4-yl radicals were characterized by X-ray diffraction analysis and variable-temperature magnetic susceptibility studies. The radicals pack in 1D π stacks of equally spaced slipped radicals with interplanar distances of 3.59 and 3.67 Å and longitudinal angles of 40.97 and 43.47°, respectively. Magnetic-susceptibility studies showed that both radicals exhibit antiferromagnetic interactions. Fitting the magnetic data revealed that the behavior is consistent with 1D regular linear antiferromagnetic chain with J=-12.9 cm(-1), zJ'=-0.4 cm(-1), g=2.0069 and J=-11.8 cm(-1), zJ'=-6.5 cm(-1), g=2.0071, respectively. Magnetic-exchange interactions in benzotriazinyl radicals are sensitive to the degree of slippage, and inter-radical separation and subtle changes in structure alter the fine balance between ferro- and antiferromagnetic interactions.

A first-order phase transition in Blatter's radical at high pressure.

The crystal structure of Blatter's radical (1,3-diphenyl-1,4-dihydrobenzo[e][1,2,4]triazin-4-yl) has been investigated between ambient pressure and 6.07 GPa. The sample remains in a compressed form of the ambient-pressure phase up to 5.34 GPa, the largest direction of strain being parallel to the direction of π-stacking interactions. The bulk modulus is 7.4 (6) GPa, with a pressure derivative equal to 9.33 (11). As pressure increases, the phenyl groups attached to the N1 and C3 positions of the triazinyl moieties of neighbouring pairs of molecules approach each other, causing the former to begin to rotate between 3.42 to 5.34 GPa. The onset of this phenyl rotation may be interpreted as a second-order phase transition which introduces a new mode for accommodating pressure. It is premonitory to a first-order isosymmetric phase transition which occurs on increasing pressure from 5.34 to 5.54 GPa. Although the phase transition is driven by volume minimization, rather than relief of unfavourable contacts, it is accompanied by a sharp jump in the orientation of the rotation angle of the phenyl group. DFT calculations suggest that the adoption of a more planar conformation by the triazinyl moiety at the phase transition can be attributed to relief of intramolecular H...H contacts at the transition. Although no dimerization of the radicals occurs, the π-stacking interactions are compressed by 0.341 (3) Šbetween ambient pressure and 6.07 GPa.

Methyl-Driven Overhauser Dynamic Nuclear Polarization.

The Overhauser effect is unique among DNP mechanisms in that it requires the modulation of the electron-nuclear hyperfine interactions. While it dominates DNP in liquids and metals, where unpaired electrons are highly mobile, Overhauser DNP is possible in insulating solids if rapid structural modulations are linked to a modulation in hyperfine coupling. Herein, we report that Overhauser DNP can be triggered by the strategic addition of a methyl group, demonstrated here in a Blatter's radical. The rotation of the methyl group leads to a modulation of the hyperfine coupling to its protons, which in turn facilitates electron-nuclear cross-relaxation. Removal of the methyl protons, through deuteration, quenches the process, as does the reduction of the hyperfine coupling strength. This result suggests the possibility for the design of tailor-made Overhauser DNP polarizing agents for high-field MAS-DNP.