High-Spin Organic Molecules

An Air-Stable Carbon-Centered Triradical with a Well-Addressable Quartet Ground State

Organic polyradicals with a high-spin ground state and quantum magnetic properties suitable for spin manipulation are valuable materials for diverse innovative technologies, including quantum devices. However, the typically high reactivity and low stability of conventional polyradicals present a major obstacle to such applications. In this study, a highly stable carbon-centered triradical TR with a quartet ground state and excellent stability (τ1/2 of ∼90 days in air-saturated toluene at room temperature) is achieved, which shows apposite magnetic anisotropy and Zeeman splitting partition with favorable addressability. By virtue of the optimal stability, thorough structural and magnetic characterizations are realized. With X-ray crystallography unambiguously proving the molecular structure, the quartet ground state (ΔED-Q = 0.78 kcal/mol) is confirmed by the SQUID measurements, while the cw- and pulsed EPR techniques offer additional supportive evidence for the high-spin nature. Remarkably, owing to the easily attained magnetic anisotropy, selective excitations between different Zeeman splitting levels are successfully demonstrated with TR in its frozen toluene solution without the requirement for special alignment, which is unprecedented for organic polyradicals. Along with the millisecond spin-lattice relaxation and microsecond coherence time manifested by TR, this triradical is promising for potential coherent spin manipulation applications as a multienergy-level quantum information carrier.

Toward New Horizons in Verdazyl-Nitroxide High-Spin Systems: Thermally Robust Tetraradical with Quintet Ground State

High-spin organic tetraradicals with significant intramolecular exchange interactions have high potential for advanced technological applications and fundamental research, but examples reported to date exhibit limited stability and processability. In this work, we designed the first tetraradical based on an oxoverdazyl core and nitronyl nitroxide radicals and successfully synthesized it using a palladium-catalyzed cross-coupling reaction of an oxoverdazyl radical bearing three iodo-phenylene moieties with a gold(I) nitronyl nitroxide-2-ide complex in the presence of a recently developed efficient catalytic system. The molecular and crystal structures of the tetraradical were confirmed by single crystal X-ray diffraction analysis. The tetraradical possesses good thermal stability with decomposition onset at ∼125 °C in an inert atmosphere; in a toluene solution upon prolonged heating at 90 °C in air, no decomposition was observed. The resulting unique verdazyl-nitroxide conjugate was thoroughly studied using a range of experimental and theoretical techniques, such as SQUID magnetometry of polycrystalline powders, EPR spectroscopy in various matrices, cyclic voltammetry, and high-level quantum chemical calculations. All collected data confirm the high thermal stability of the resulting tetraradical and quintet multiplicity of its ground state, which makes the synthesis of this important paramagnet a new milestone in the field of creating high-spin systems.

N-Heterocyclic Carbene-Derived 1,3,5-Trimethylenebenzene: On-Surface Synthesis and Electronic Structure

1,3,5-Trimethylenebenzene (1,3,5-TMB), a 3-fold-symmetric triradical with a high-spin ground state, is an attractive platform for investigating the unique spin properties of π-conjugated triangular triradicals. Here, we report the on-surface synthesis of N-heterocyclic carbene (NHC)-derived 1,3,5-TMB (N-TMB) via surface-assisted C-C and C-N coupling reactions on Au(111). The chemical and electronic structures of N-TMB on the Au(111) surface are revealed with atomic precision using scanning tunneling microscopy and noncontact atomic force microscopy, combined with density functional theory (DFT) calculations. It is demonstrated that there is substantial charge transfer between N-TMB and the substrate, resulting in a positively charged N-TMB on Au(111). DFT calculations at the UB3LYP/def2-TZVP level of theory and multireference method, e.g., CASSCF/NEVPT2, indicate that N-TMB possesses a doublet ground state with reduced Cs symmetry in the gas phase, contrasting the quartet ground state of 1,3,5-TMB with D3h symmetry, and exhibits a doublet-quartet energy gap of -0.80 eV. The incorporation of NHC structures and the extended π-conjugation promote the spin-orbital overlaps in N-TMB, leading to Jahn-Teller distortion and the formation of a robust doublet state. Our results not only demonstrate the fabrication of polyradicals based on NHC but also shed light on the effect of NHC and π-conjugation on the electronic structure and spin coupling, which opens up new possibilities for precisely regulating the spin-spin exchange coupling of organic polyradicals.

N-Heterocyclic Carbene Analogues of Wittig Hydrocarbon

N-Heterocyclic carbene (NHC) analogues of Wittig hydrocarbon, [(NHC)(Stil)(NHC)] (3a-c) (NHC = SIPr (1a) = C[N(Dipp)CH2]2, Dipp = 2,6-iPr2C6H3; IPr (1b) = C[N(Dipp)CH]2; Me-IPr (1c) = C[N(Dipp)CMe]2 and Stil = C6H4CHCHC6H4) have been reported as crystalline solids. 3a-c are prepared by two-electron reductions of the corresponding bis-1,3-imidazoli(ni)um bromides [(NHC)(Stil)NHC)](Br)2 (2a-c) with KC8 in >94 % yields. 2a-c are accessible by the nickel catalyzed direct C-C coupling of NHCs (1a-c) with (E)-4,4'-dibromostilbene. One-electron oxidation of 3a,b yields the corresponding radical cations [(NHC)(Stil)NHC)]B(C6F5)4 4a,b. All compounds have been characterized by UV-Vis/NMR/EPR spectroscopy as well as 2a, 3a, and 3b by single crystal X-ray diffraction. The electronic structures of representative systems have been analyzed by quantum chemical calculations.

Electronic and Magnetic Interactions in 6-Oxoverdazyl Diradicals: Connection through N(1) vs C(3) Revisited

Four isomeric di-6-oxoverdazyl diradicals connected at their N(1) or C(3) positions with either 1,3- or 1,4-phenylene linkers were obtained and characterized by spectroscopic, electrochemical, magnetic, and structural methods. These results were compared to those for the corresponding 6-oxoverdazyl monoradicals. UV-vis spectroscopy demonstrated that only the N(1)-connected para-through-benzene diradical has a distinct spectrum with significant bathochromic and hypsochromic shifts relative to the remaining species. Electrochemical analysis revealed two one-electron reduction processes in all diradiacals, while only the N(1)-connected para-through-benzene diradical exhibits two one-electron oxidation processes separated by 0.10 V. Variable temperature EPR measurements in polystyrene solid solutions gave negative mean exchange interaction energies J for all diradicals, suggesting the dominance of conformers with significant intramolecular antiferromagnetic interactions for the meta-through-benzene isomers. DFT calculations predict a small preference for the triplet state with the ΔES-T of about 0.25 kcal mol-1 for both meta-through-benzene connected diradicals.

Chiral π-Conjugated Double Helical Aminyl Diradical with the Triplet Ground State

We report a neutral high-spin diradical of chiral C2-symmetric bis[5]diazahelicene with ΔEST ≈ 0.4 kcal mol-1, as determined by EPR spectroscopy/SQUID magnetometry. The diradical is the most persistent among all high-spin aminyl radicals reported to date by a factor of 20, with a half-life of up to 6 days in 2-MeTHF at room temperature. Its triplet ground state and excellent persistence may be associated with the unique spin density distribution within the dihydrophenazine moiety, which characterizes two effective 3-electron C-N bonds analogous to the N-O bond of a nitroxide radical. The enantiomerically enriched (ee ≥ 94%) (MM)- and (PP)-enantiomers of the precursors to the diradicals are obtained by either preparative chiral supercritical fluid chromatography or resolution via functionalization with the chiral auxiliary of the C2-symmetric racemic tetraamine. The barrier for the racemization of the solid tetraamine is ΔG‡ = 43 ± 0.01 kcal mol-1 in the 483-523 K range. The experimentally estimated lower limit of the barrier for the racemization of a diradical, ΔG‡ ≥ 26 kcal mol-1 in 2-MeTHF at 293 K, is comparable to the DFT-determined barrier of ΔG‡ = 31 kcal mol-1 in the gas phase at 298 K. While the enantiomerically pure tetraamine displays strong chiroptical properties, with anisotropy factor |g| = |Δε|/ε = 0.036 at 376 nm, |g| ≈ 0.005 at 548 nm of the high-spin diradical is comparable to that recently reported triplet ground-state diradical dication. Notably, the radical anion intermediate in the generation of diradical exhibits a large SOMO-HOMO inversion, SHI = 35 kcal mol-1.

Elucidating the structure of donor-acceptor conjugated polymer aggregates in liquid solution

High-spin donor-acceptor conjugated polymers are extensively studied for their potential applications in magnetic and spintronic devices. Inter-chain charge transfer among these high-spin polymers mainly depends on the nature of the local structure of the thin film and π-stacking between the polymer chains. However, the microscopic structural details of high-spin polymeric materials are rarely studied with an atomistic force field, and the molecular-level local structure in the liquid phase remains ambiguous. Here, we have examined the effects of oligomer chain length, side chain, and processing temperature on the organization of the high-spin cyclopentadithiophene-benzobisthiadiazole donor-acceptor conjugated polymer in chloroform solvent. We find that the oligomers display ordered aggregates whose structure depends on their chain length, with an average π-stacking distance of 3.38 ± 0.03 Å (at T = 298 K) in good agreement with the experiment. Also, the oligomers with longer alkyl side chains show better solvation and a shorter π-stacking distance. Furthermore, the clusters grow faster at higher temperature with more ordered aggregation between the oligomer chains.

A Nitronyl Nitroxide-Substituted Benzotriazinyl Tetraradical

High-spin organic tetraradicals with significant intramolecular exchange interactions have high potential for advanced technological applications and fundamental research, but those synthesized to date possess limited stability and processability. In this work, we have designed a tetraradical based on the Blatter's radical and nitronyl nitroxide radical moieties and successfully synthesized it by using the palladium-catalyzed cross-coupling reaction of a triiodo-derivative of the 1,2,4-benzotriazinyl radical with gold(I) nitronyl nitroxide-2-ide complex in the presence of a newly developed efficient catalytic system. The molecular and crystal structure of the tetraradical was confirmed by X-ray diffraction analysis. The tetraradical possesses good thermal stability with decomposition onset at ∼150 °C under an inert atmosphere and exhibits reversible redox waves at -0.54 and 0.45 V versus Ag/AgCl. The magnetic properties of the tetraradical were characterized by SQUID magnetometry of polycrystalline powders and EPR spectroscopy in various matrices. The collected data, analyzed by using high-level quantum chemical calculations, confirmed that the tetraradical has a triplet ground state and a nearby excited quintet state. The unique high stability of the prepared triazinyl-nitronylnitroxide tetraradical is a new milestone in the field of creating high-spin systems.

A Rational Way to Control the Triplet State Wave Function Confinement of Organic Chromophores: Effect of the Connection Sites and Spin Density Distribution-Guided Molecular Structure Design Principles in Bodipy Dimers

To study the intersystem crossing (ISC) and the spatial confinement of the triplet excited states of organic chromophores, we prepared a series of Bodipy dimers. We found that the connection position of the two units has a significant effect on the absorption and fluorescence. Singlet oxygen quantum yields of 3.8-12.4% were observed for the dimers, which are independent of solvent polarity. Nanosecond transient absorption spectra indicate the population of long-lived triplet excited states with lifetimes (τT) of 45-454 μs. Pulsed laser-excited time-resolved electron paramagnetic resonance (TREPR) spectra show that the T1 triplet states are essentially delocalized, which is different from the case for the previously reported Bodipy dimers. The TREPR spectra of the triplet states imply that the delocalization over the whole dimer essentially depends on the electron density of the carbon atoms at the connection sites. This property may become a universal rule for controlling the T1 state confinement in multichromophore organic molecules.

A Triplet/Singlet Ground-State Switch via the Steric Inhibition of Conjugation in 4,6-Bis(trifluoromethyl)-1,3-phenylene Bisnitroxide

Ground triplet 4,6-bis(trifluoromethyl)-1,3-phenylene bis(tert-butyl nitroxide) (TF2PBN) reacted with [Y(hfac)3(H2O)2] (hfac = 1,1,1,5,5,5-hexafluoropentane-2,4-dionate), affording a doubly hydrogen-bonded adduct [Y(hfac)3(H2O)2(TF2PBN)]. The biradical was recovered from the adduct through recrystallization. Crystallographic analysis indicates that the torsion angles (|θ| ≤ 90°) between the benzene ring and nitroxide groups were 74.9 and 84.8° in the adduct, which are larger than those of the starting material TF2PBN. Steric congestion due to o-trifluoromethyl groups gives rise to the reduction of π-conjugation. Two hydrogen bonds enhance this deformation. Susceptometry of the adduct indicates a ground singlet with 2J/kB = -128(2) K, where 2J corresponds to the singlet-triplet gap. The observed magneto-structure relation is qualitatively consistent with Rajca's pioneering work. A density functional theory calculation at the UB3LYP/6-311+G(2d,p) level using the atomic coordinates determined provided a result of 2J/kB = -162.3 K for the adduct, whilst the corresponding calculation on intact TF2PBN provided +87.2 K. After a comparison among a few known compounds, the 2J vs. |θ| plot shows a negative slope with a critical torsion of 65(3)°. The ferro- and antiferromagnetic coupling contributions are balanced in TF2PBN, being responsible for ground-state interconversion by means of small structural perturbation like hydrogen bonds.

Rational Design of an Air-Stable, High-Spin Diradical with Diazapyrene

Stable carbon-based polyradicals exhibiting strong spin-spin coupling and slow depolarization processes are particularly attractive functional materials. A new molecular motif synthesized by a convenient method that allows the integration of stable, high-spin radicals to (hetero)aromatic polycycles has been developed, as illustrated by a non-Kekulé diradical showing a triplet ground state with long persistency (τ1/2 ≈31 h) in air. Compared to the widely used 1,3-phenylene, the newly designed (diaza)pyrene-4,10-diyl moiety is for the first time demonstrated to confer ferromagnetic (FM) spin coupling, allowing delocalized non-disjoint SOMOs. With the X-ray crystallography unambiguously proving the diradical structure, the triplet ground state was thoroughly characterized. A large ΔES-T of 1.1 kcal/mol, proving the strong FM coupling effect, was revealed consistently by superconducting quantum interference device (SQUID) measurements and variable-temperature electron paramagnetic resonance (EPR) spectroscopy, while the zero-field splitting and triplet nutation characters were examined by continuous-wave and pulsed EPR spectroscopy. A millisecond spin-lattice relaxation time was also detected. The current study not only offers a new molecular motif enabling FM coupling between carbon-based spins, but more importantly presents a general method for installing stable polyradicals into functional π-systems.

A highly efficient open-shell singlet luminescent diradical with strong magnetoluminescence properties

Developing open-shell singlet (OS) diradicals with high luminescent properties and exceptional single-molecule magnetoluminescence (ML) performance is extremely challenging. Herein, we propose a concept to enhance luminescent efficiency by adjusting the donor conjugation of OS diradicals, thereby achieving a highly luminescent diradical, DR1, with outstanding stability and making it a viable option for use in the emitting layer of organic light-emitting diodes (OLEDs). More importantly, the 0.5 wt%-DR1 doped film demonstrates significant single-molecule magnetoluminescence (ML) properties. A giant ML value of 210% is achieved at a magnetic field of 7 T, showing the great potential of DR1 in magneto-optoelectronic devices.

Blatter Diradicals with a Spin Coupler at the N(1) Position

Reactions of a benzo[e][1,2,4]triazine with dilithiobenzenes lead to di-Blatter diradicals connected at the N(1) positions via a spin coupling unit, 1,4-phenylene or 1,3-phenylene. Electrochemical analysis in MeCN revealed four one-electron redox processes separated by 0.1-0.3 V in both diradicals. Variable temperature EPR measurements in polystyrene (PS) solid solutions gave the singlet-triplet energy gaps ΔES-T =2 J of -3.02(11) and -0.16(1) kcal mol-1 for 1,4-phenylene and 1,3-phenylene derivatives, respectively. The latter negative value was attributed to conformational properties of the diradical in the PS solid solution. Results suggest a simple and efficient access to a family of stable Blatter diradicals with a controllable S-T gap through a judicious choice of the arylene coupling unit. DFT calculations indicate that the triplet state is stabilized by (het)arylenes with low LUMO.

Bis-Olefin Based Crystalline Schlenk Hydrocarbon Diradicals with a Triplet Ground State

A modular approach for the synthesis of isolable crystalline Schlenk hydrocarbon diradicals from m-phenylene bridged electron-rich bis-triazaalkenes as synthons is reported. EPR spectroscopy confirms their diradical nature and triplet electronic structure by revealing a half-field signal. A computational analysis confirms the triplet state to be the ground state. As a proof-of-principle for the modular methodology, the 4,6-dimethyl-m-phenylene was further utilized as a coupling unit between two alkene motifs. The steric conjunction of the 4,6-dimethyl groups substantially twists the substituents at the nonbonding electron bearing centers relative to the central coupling m-phenylene motif. As a result, the spin delocalization is decreased and the exchange coupling between the two unpaired spins, hence, significantly reduced. Notably, 108 years after Schlenk's m-phenylene-bis(diphenylmethyl) synthesis as a diradical, for the first time we were able to isolate its derivative with the same spacer, i.e. m-phenylene, between two radical centers in a crystalline form.

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.

Redox-Regulated Magnetic Conversions between Ferro- and Antiferromagnetism in Organic Nitroxide Diradicals

Redox-induced magnetic transformation in organic diradicals is an appealing phenomenon. In this study, we theoretically designed twelve couples of diradicals in which two nitroxide (NO) radical groups are connected to the redox-active couplers including p-benzoquinonyl, 1,4-naphthoquinyl, 9,10-anthraquinonyl, naphthacene-5,12-dione, pentacene-6,13-dione, hexacene-6,15-dione, pyrazinyl, quinoxalinyl, phenazinyl, 5,12-diazanaphthacene, 6,13-diazapentacene, and 6,15-diazahexacene. As evidenced at both the B3LYP and M06-2X levels of theory, the calculations reveal that the magnetic reversal can take place from ferromagnetism to antiferromagnetism, or vice versa, by means of redox method in these designed organic magnetic molecules. It was observed that p-benzoquinonyl, 1,4-naphthoquinyl, 9,10-anthraquinonyl, naphthacene-5,12-dione, pentacene-6,13-dione, and hexacene-6,15-dione-bridged NO diradicals produce antiferromagnetism while their dihydrogenated counterparts exhibit ferromagnetism. Similarly, pyrazinyl, quinoxalinyl, phenazinyl, 5,12-diazanaphthacene, 6,13-diazapentacene, and 6,15-diazahexacene-bridged NO diradicals present ferromagnetism while their dihydrogenated counterparts show antiferromagnetism. The differences in the magnetic behaviors and magnetic magnitudes of each of the twelve couples of diradicals could be attributed to their distinctly different spin-interacting pathways. It was found that the nature of the coupler and the length of the coupling path are important factors in controlling the magnitude of the magnetic exchange coupling constant J. Specifically, smaller HOMO-LUMO (HOMO: highest occupied molecular orbital, LUMO: lowest unoccupied molecular orbital) gaps of the couplers and shorter coupler lengths, as well as shorter linking bond lengths, can attain stronger magnetic interactions. In addition, a diradical with an extensively π-conjugated structure is beneficial to spin transport and can effectively promote magnetic coupling, yielding a large |J| accordingly. That is, a larger spin polarization can give rise to a stronger magnetic interaction. The sign of J for these studied diradicals can be predicted from the spin alternation rule, the shape of the singly occupied molecular orbitals (SOMOs), and the SOMO-SOMO energy gaps of the triplet state. This study paves the way for the rational design of magnetic molecular switches.

Heteroatom-Based Diradical(oid)s

Heteroatom-centered diradical(oid)s have been in the focus of molecular main group chemistry for nearly 30 years. During this time, the diradical concept has evolved and the focus has shifted to the rational design of diradical(oid)s for specific applications. This review article begins with some important theoretical considerations of the diradical and tetraradical concept. Based on these theoretical considerations, the design of diradical(oid)s in terms of ligand choice, steric, symmetry, electronic situation, element choice, and reactivity is highlighted with examples. In particular, heteroatom-centered diradical reactions are discussed and compared with closed-shell reactions such as pericyclic additions. The comparison between closed-shell reactivity, which proceeds in a concerted manner, and open-shell reactivity, which proceeds in a stepwise fashion, along with considerations of diradical(oid) design, provides a rational understanding of this interesting and unusual class of compounds. The application of diradical(oid)s, for example in small molecule activation or as molecular switches, is also highlighted. The final part of this review begins with application-related details of the spectroscopy of diradical(oid)s, followed by an update of the heteroatom-centered diradical(oid)s and tetraradical(oid)s published in the last 10 years since 2013.

Spin-Frustrated Trisradical Trication of PrismCage

Organic trisradicals featuring threefold symmetry have attracted significant interest because of their unique magnetic properties associated with spin frustration. Herein, we describe the synthesis and characterization of a triangular prism-shaped organic cage for which we have coined the name PrismCage6+ and its trisradical trication─TR3(•+). PrismCage6+ is composed of three 4,4'-bipyridinium dications and two 1,3,5-phenylene units bridged by six methylene groups. In the solid state, PrismCage6+ adopts a highly twisted conformation with close to C3 symmetry as a result of encapsulating one PF6- anion as a guest. PrismCage6+ undergoes stepwise reduction to its mono-, di-, and trisradical cations in MeCN on account of strong electronic communication between its 4,4'-bipyridinium units. TR3(•+), which is obtained by the reduction of PrismCage6+ employing CoCp2, adopts a triangular prism-shaped conformation with close to C2v symmetry in the solid state. Temperature-dependent continuous-wave and nutation-frequency-selective electron paramagnetic resonance spectra of TR3(•+) in frozen N,N-dimethylformamide indicate its doublet ground state. The doublet-quartet energy gap of TR3(•+) is estimated to be -0.08 kcal mol-1, and the critical temperature of spin-state conversion is found to be ca. 50 K, suggesting that it displays pronounced spin frustration at the molecular level. To the best of our knowledge, this example is the first organic radical cage to exhibit spin frustration. The trisradical trication of PrismCage6+ opens up new possibilities for fundamental investigations and potential applications in the fields of both organic cages and spin chemistry.

Equation-of-Motion Block-Correlated Coupled Cluster Method for Excited Electronic States of Strongly Correlated Systems

An equation-of-motion block-correlated coupled cluster method based on the generalized valence bond wave function (EOM-GVB-BCCC) is proposed to describe low-lying excited states for strongly correlated systems. The EOM-GVB-BCCC2b method with up to two-pair correlation has been implemented and tested for a few strongly correlated systems. For a water hexamer with stretched O-H bonds, which is beyond the capability of the CASSCF method, EOM-GVB-BCCC2b provides very close results as the density matrix renormalization group (DMRG). For four conjugated diradical species with triplet ground states, we found that their vertical S-T gaps from EOM-GVB-BCCC2b are also quite consistent with the DMRG results. This new method is expected to be a promising theoretical tool for describing the low-lying excited states of strongly correlated systems with large active spaces.

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.

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.

High-Spin S = 3/2 Ground-State Aminyl Triradicals: Toward High-Spin Oligo-Aza Nanographenes

We report high-spin aminyl triradicals with near-planar triphenylene backbones. Near-planarity of the fused aminyl radicals and the 2,6,10-triphenylene ferromagnetic coupling unit (FCU), magnetically equivalent to three fused 3,4'-biphenyl FCUs, assures an effective 2pπ-2pπ overlap within the cross-conjugated π-system, leading to an S = 3/2 (quartet) ground state that is well separated from low-spin excited doublet states. Thermal populations of the low-spin (S = 1/2) excited states are detectable both by SQUID magnetometry and electron paramagnetic resonance (EPR) spectroscopy, providing doublet-quartet energy gaps, ΔEDQ, corresponding to >85% population of the quartet ground states at room temperature. Notably, EPR-based determination of ΔEDQ relies on direct detection of the quartet ground state and doublet excited states. The ΔEDQ values are 1.0-1.1 kcal mol-1, with the more sterically shielded triradical having the larger value. The half-life of the more sterically shielded triradical in 2-methyltetrahydrofuran (2-MeTHF) is about 6 h at room temperature. The less sterically shielded triradical in 2-MeTHF decomposes at 158 K with a half-life of about 4 h, while at 195 K, the half-life is still about 2 h. The dominant products of the decay of triradicals are the corresponding triamines, suggesting hydrogen atom abstraction from the solvent as the primary mechanism. This study expands the frontier of the open-shell PAHs/nanographenes, of which the unique electronic, nonlinear optical, and magnetic properties could be useful in the development of novel organic electronics, photonics, and spintronics.

Non-Aufbau orbital ordering and spin density modulation in high-spin donor-acceptor conjugated polymers

High-spin ground-state organic materials with unique spin topology can significantly impact molecular magnetism, spintronics, and quantum computing devices. However, strategies to control the spin topology and alignment of the unpaired spins in different molecular orbitals are not well understood. Here, we report modulating spin distribution along the molecular backbone in high-spin ground-state donor-acceptor (D-A) conjugated polymers. Density functional theory calculations indicate that substitution of different heteroatoms (such as C, Si, N, and Se) alters the aromatic character in the thiadiazole unit of the benzobisthiadiazole (BBT) acceptor and modulates the oligomer length to result in high-spin triplet ground-state, orbital and spin topology. The C, Si, and Se atom substituted polymers show a localized spin density at the two opposite ends of the polymers. However, a delocalized spin distribution is observed in the N substituted polymer. We find that the hybridization (sp³vs. sp²) of the substituent atom plays an important role in controlling the electronic structure of these materials. This study shows that atomistic engineering is an efficient technique to tune the spin topologies and electronic configurations in the high-spin ground-state donor-acceptor conjugated polymers, compelling synthetic targets for room-temperature magnetic materials.

Organic radicals with inversion of SOMO and HOMO energies and potential applications in optoelectronics

Organic radicals possessing an electronic configuration in which the energy of the singly occupied molecular orbital (SOMO) is below the highest doubly occupied molecular orbital (HOMO) level have recently attracted significant interest, both theoretically and experimentally. The peculiar orbital energetics of these SOMO-HOMO inversion (SHI) organic radicals set their electronic properties apart from the more common situation where the SOMO is the highest occupied orbital of the system. This review gives a general perspective on SHI, with key fundamental aspects regarding the electronic and structural factors that govern this particular electronic configuration in organic radicals. Selected examples of reported compounds with SHI are highlighted to establish molecular guidelines for designing this type of radical, and to showcase the potential of SHI radicals in organic spintronics as well as for the development of more stable luminescent radicals for OLED applications.

Ferromagnetic 2p-2p and 4f-2p Couplings in a Macrocycle from Two Biradicals and Two Gadolinium(III) Ions

A new ground triplet biradical 2′,4′,6′-triisopropylbiphenyl-3,5-diyl bis(tert-butyl nitroxide) (iPr3BPBN) was prepared and characterized by means of room-temperature ESR spectroscopy displaying a zero-field splitting pattern together with a half-field signal. Complex formation with gadolinium(III) 1,1,1,5,5,5-hexafluoropentane-2,4-dionate (hfac) afforded a macrocycle [{Gd(hfac)3(μ-iPr3BPBN)}2]. As the X-ray crystallographic analysis clarified, the biradical serves as a bridging ligand, giving a 16-membered ring, where each nitroxide radical oxygen atom is directly bonded to a Gd3+ ion. The magnetic study revealed that the iPr3BPBN bridge behaved as a practically triplet biradical and that the Gd3+-radical magnetic coupling was weakly ferromagnetic. The exchange parameters were estimated as 2jrad-rad/kB > 300 K and 2JGd-rad/kB = 1.2 K in the H = −2J S1•S2 convention. The DFT calculation based on the atomic coordinates clarified the ground triplet nature in metal-free iPr3BPBN and the enhanced triplet character upon coordination. The calculation also suggests that ferromagnetic coupling would be favorable when the Gd-O-N-C(sp2) torsion comes around 100°. The present results are compatible with the proposed magneto-structure relationship on the nitroxide-Gd compounds.

Rare-Earth (RE = Y, Gd, Tb, Dy, Ho, and Er) Chains Bridged with a Triplet Biradical and Magnetic Hysteresis Recorded for RE = Tb

Complex formation of 5-tert-butyl-1,3-phenylene bis(tert-butyl nitroxide) and rare-earth (RE) metal ions gave a linear chain where each nitroxide O atom is directly bonded to the RE ion. The bridge was proven to be a ground triplet molecule in the complexes. A hysteresis loop was recorded below 2.8 K as a single-chain magnet for the RE = Tb derivative.

High-mobility semiconducting polymers with different spin ground states

Organic semiconductors with high-spin ground states are fascinating because they could enable fundamental understanding on the spin-related phenomenon in light element and provide opportunities for organic magnetic and quantum materials. Although high-spin ground states have been observed in some quinoidal type small molecules or doped organic semiconductors, semiconducting polymers with high-spin at their neutral ground state are rarely reported. Here we report three high-mobility semiconducting polymers with different spin ground states. We show that polymer building blocks with small singlet-triplet energy gap (ΔE_S-T) could enable small ΔE_S-T gap and increase the diradical character in copolymers. We demonstrate that the electronic structure, spin density, and solid-state interchain interactions in the high-spin polymers are crucial for their ground states. Polymers with a triplet ground state (S = 1) could exhibit doublet (S = 1/2) behavior due to different spin distributions and solid-state interchain spin-spin interactions. Besides, these polymers showed outstanding charge transport properties with high hole/electron mobilities and can be both n- and p-doped with superior conductivities. Our results demonstrate a rational approach to obtain high-mobility semiconducting polymers with different spin ground states.

Semiconducting polymers with high-spin at their neutral ground state are rarely reported. Here the authors synthesize three semiconducting polymers with different spin ground states and high hole/electron mobility, by appropriate choice of the building blocks’ singlet-triplet energy gap, spin distributions and solid-state interchain interactions.

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.

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.

Electronic and Spintronic Open-Shell Macromolecules, Quo Vadis?

Open-shell macromolecules (i.e., polymers containing radical sites either along their backbones or at the pendant sites of repeat units) have attracted significant attention owing to their intriguing chemical and physical (e.g., redox, optoelectronic, and magnetic) properties, and they have been proposed and/or implemented in a wide range of potential applications (e.g., energy storage devices, electronic systems, and spintronic modules). These successes span multiple disciplines that range from advanced macromolecular chemistry through nanoscale structural characterization and on to next-generation solid-state physics and the associated devices. In turn, this has allowed different scientific communities to expand the palette of radical-containing polymers relatively quickly. However, critical gaps remain on many fronts, especially regarding the elucidation of key structure-property-function relationships that govern the underlying electrochemical, optoelectronic, and spin phenomena in these materials systems. Here, we highlight vital developments in the history of open-shell macromolecules to explain the current state of the art in the field. Moreover, we provide a critical review of the successes and bring forward open opportunities that, if solved, could propel this class of materials in a meaningful manner. Finally, we provide an outlook to address where it seems most likely that open-shell macromolecules will go in the coming years. Our considered view is that the future of radical-containing polymers is extremely bright and the addition of talented researchers with diverse skills to the field will allow these materials and their end-use devices to have a positive impact on the global science and technology enterprise in a relatively rapid manner.

Rules for Magnetic Exchange in Azulene-Bridged Biradicals: Quo Vadis?

Electronic coupling through organic bridges facilitates magnetic exchange interactions and controls electron transfer and single-molecule device electron transport. Electronic coupling through alternant π-systems (e.g., benzene) is better understood than the corresponding coupling through nonalternant π-systems (e.g., azulene). Herein, we examine the structure, spectroscopy, and magnetic exchange coupling in two biradicals (1,3-SQ₂Az and 1,3-SQ-Az-NN; SQ = the zinc(II) complex of spin-1/2 semiquinone radical anion, NN = spin-1/2 nitronylnitroxide; Az = azulene) that possess nonalternant azulene π-system bridges. The SQ radical spin density in both molecules is delocalized into the Az π-system, while the NN spin is effectively localized onto the five-atom ONCNO π-system of NN radical. The spin distributions and interactions are probed by EPR spectroscopy and magnetic susceptibility measurements. We find that J = +38 cm^-1 for 1,3-SQ₂Az and J = +9 cm^-1 for 1,3-SQ-Az-NN (H=-2JS^SQ·S^SQorNN). Our results highlight the differences in exchange coupling mediated by azulene compared to exchange coupling mediated by alternant π-systems.

Bis-Spiro-Oxetane and Bis-Spiro-Tetrahydrofuran Pyrroline Nitroxide Radicals: Synthesis and Electron Spin Relaxation Studies

Synthesis of bis-spiro-oxetane and bis-spiro-tetrahydrofuran pyrroline nitroxide radicals relies on the Mitsunobu reaction-mediated double cyclizations of N-Boc protected pyrroline tetraols. Structures of the nitroxide radicals are supported by X-ray crystallography. In a trehalose/sucrose matrix at room temperature, the bis-spiro-oxetane nitroxide radical possesses electron spin coherence time, Tm ≈ 0.7 μs. The observed enhanced Tm is most likely associated with strong hydrogen bonding of oxetane moieties to the trehalose/sucrose matrix.

An Open-Shell Coronoid with Hybrid Chichibabin-Schlenk Conjugation

A hexaradicaloid molecule with alternating Kekulé and non-Kekulé connectivities between adjacent spin centers was obtained by fusing two classic conjugation motifs, found respectively in the Chichibabin and Schlenk hydrocarbons, into a coronoid structure. 1 H NMR, ESR and SQUID experiments, combined with computational analyses reveal that the system has a singlet ground state, characterized by a significant hexaradicaloid character ( γ 0 = 0.826, γ 1 = γ 2 = 0.773). It possesses multiple thermally accessible high-spin states (up to the septet), with uniform energy gaps of ca 1.0 kcal/mol between consecutive multiplicities. In line with its open-shell character, the coronoid has a small electronic bandgap of ca. 0.8 eV and undergoes two consecutive one-electron oxidations at low potentials, yielding cationic forms with extended near-infrared absorption. The hexaradicaloid, which combines open-shell and macrocyclic contributions to its π conjugation, provides an example of a design strategy for multistate spin switches and redox-amphoteric NIR dyes.

A high-spin diradical dianion and its bridged chemically switchable single-molecule magnet

Triplet diradicals have attracted tremendous attention due to their promising application in organic spintronics, organic magnets and spin filters. However, very few examples of triplet diradicals with singlet–triplet energy gaps (ΔE_ST) over 0.59 kcal mol⁻¹ (298 K) have been reported to date. In this work, we first proved that the dianion of 2,7-di-tert-butyl-pyrene-4,5,9,10-tetraone (2,7-tBu₂-PTO) was a triplet ground state diradical in the magnesium complex 1 with a singlet–triplet energy gap ΔE_ST = 0.94 kcal mol⁻¹ (473 K). This is a rare example of stable diradicals with singlet–triplet energy gaps exceeding the thermal energy at room temperature (298 K). Moreover, the iron analog 2 containing the 2,7-tBu₂-PTO diradical dianion was isolated, which was the first single-molecule magnet bridged by a diradical dianion. When 2 was doubly reduced to the dianion salt 2K2, single-molecule magnetism was switched off, highlighting the importance of diradicals in single-molecule magnetism.

We report a triplet diradical dianion in magnesium complex with ΔE_ST = 0.94 kcal mol⁻¹ (473 K). Its iron analog is the first single-molecule magnet bridged by a diradical dianion, and the SMM property is switched off through two-electron reduction.

Platform for High-Spin Molecules: A Verdazyl-Nitronyl Nitroxide Triradical with Quartet Ground State

Thermally resistant air-stable organic triradicals with a quartet ground state and a large energy gap between spin states are still unique compounds. In this work, we succeeded to design and prepare the first highly stable triradical, consisting of oxoverdazyl and nitronyl nitroxide radical fragments, with a quartet ground state. The triradical and its diradical precursor were synthesized via a palladium-catalyzed cross-coupling reaction of diiodoverdazyl with nitronyl nitroxide-2-ide gold(I) complex. Both paramagnetic compounds were fully characterized by single-crystal X-ray diffraction analysis, superconducting quantum interference device magnetometry, EPR spectroscopy in various matrices, and cyclic voltammetry. In the diradical, the verdazyl and nitronyl nitroxide centers demonstrated full reversibility of redox process, while for the triradical, the electrochemical reduction and oxidation proceed at practically the same redox potentials, but become quasi-reversible. A series of high-level CASSCF/NEVPT2 calculations was performed to predict inter- and intramolecular exchange interactions in crystals of di- and triradicals and to establish their magnetic motifs. Based on the predicted magnetic motifs, the temperature dependences of the magnetic susceptibility were analyzed, and the singlet-triplet (135 ± 10 cm^-1) and doublet-quartet (17 ± 2 and 152 ± 19 cm^-1) splitting was found to be moderate. Unique high stability of synthesized verdazyl-nitronylnitroxide triradical opens new perspectives for further functionalization and design of high-spin systems with four or more spins.

Auxiliary Atomic Relay Center Facilitates Enhanced Magnetic Couplings in Blatter's Radical

The recent accomplishments in obtaining strong ferromagnetic exchange interactions in organic diradicals have made the field quite fascinating and even more promising toward its technological applications. In this context, herein, we report a unique combination of remarkably strong ferromagnetic exchange interactions coupled with molecular rigidity, utilizing superstable Blatter's radical as a spin source. The planar analogues of the parent Blatter's radical obtained by annulation with a chalcogen coupled to nitronyl nitroxide (NN) are investigated using density functional theory along with the wave function-based multiconfigurational self-consistent field methods, for example, complete active space self-consistent field (CASSCF)-N-electron valence state perturbation theory (NEVPT2). The calculations reveal phenomenal modulation in exchange couplings upon annulation such that remarkably strong ferromagnetic interactions are realized especially for a certain class of the Blatter-NN diradicals. The modulation of spin-spin interactions is rationalized by variation in spin density distribution and molecular torsional angles. We demonstrate that annulation in OMMs opens an additional coupling pathway via auxiliary X-atom acting as the atomic relay center which strongly manipulates the magnitude of exchange couplings.

1,3-Diradicals Embedded in Curved Paraphenylene Units: Singlet versus Triplet State and In-Plane Aromaticity

Curved π-conjugated molecules and open-shell structures have attracted much attention from the perspective of fundamental chemistry, as well as materials science. In this study, the chemistry of 1,3-diradicals (DRs) embedded in curved cycloparaphenylene (CPPs) structures, DR-(n+3)CPPs (n = 0-5), was investigated to understand the effects of the curvature and system size on the spin-spin interactions and singlet versus triplet state, as well as their unique characteristics such as in-plane aromaticity. A triplet ground state was predicted for the larger 1,3-diradicals, such as the seven- and eight-paraphenylene-unit-containing diradicals DR-7CPP (n = 4) and DR-8CPP (n = 5), by quantum chemical calculations. The smaller-sized diradicals DR-(n+3)CPPs (n = 0-3) were found to possess singlet ground states. Thus, the ground-state spin multiplicity is controlled by the size of the paraphenylene cycle. The size effect on the ground-state spin multiplicity was confirmed by the experimental generation of DR-6CPP in the photochemical denitrogenation of its azo-containing precursor (AZ-6CPP). Intriguingly, a unique type of in-plane aromaticity emerged in the smaller-sized singlet states such as S-DR-4CPP (n = 1), as proven by nucleus-independent chemical shift calculations (NICS) and an analysis of the anisotropy of the induced current density (ACID), which demonstrate that homoconjugation between the 1,3-diradical moiety arises because of the curved and distorted bonding system.

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.

A Robust Porphyrin-Stabilized Triplet Carbon Diradical

The synthesis of robust high-spin carbon radicals is an important topic in organic chemistry. Toward this end, several porphyrin-stabilized radicals have been systematically explored. A singly naphthalene-fused porphyrin radical was synthesized by a reaction sequence consisting of a Suzuki-Miyaura coupling of β-borylated porphyrin with 2-bromobenzaldehyde, addition of mesityl Grignard reagent, intramolecular Friedel-Crafts alkylation, and final oxidation with DDQ or tBuOK/O₂ . This strategy was also used to synthesize doubly naphthalene-fused porphyrins and syn- and anti-fused-anthracene-bridged porphyrin dimers. While singly naphthalene-fused porphyrin radical has been shown to be a stable monoradical, doubly naphthalene-fused porphyrins and anti-fused-anthracene-bridged porphyrin dimers have been shown to be closed-shell molecules. Finally, the syn-dimer was characterized as a surprisingly stable radical (t_1/2 =28 days under ambient air and at 80 °C) that is storable for more than several months, despite its high-spin triplet ground-state carbon diradical.

Nitronyl Nitroxide Bifunctionalized Electron-Poor Chromophores: Synthesis of Stable Dye Biradicals by Lewis Acid Promoted Desilylation

Open shell organic molecules bearing π-cores are of great interest for optical, electronic, and magnetic applications but frequently suffer fast decomposition or lack synthetic accessibility. In this regard, nitronyl nitroxides are promising candidates for stable (bi-)radicals due to their high degree of spin delocalization along the O-N-C-N-O pentad unit. Unfortunately, they are limited to electron-rich systems so far. To overcome this limitation, we developed a synthetic procedure for the twofold spin decoration of electron-poor chromophores (E_red = -1158 mV) with nitronyl nitroxide radical moieties via selective deprotection/oxidation of the respective silylated precursors with boron fluoride and subsequent quenching with tetraethyl orthosilicate. Nitronyl nitroxide biradicals PBI-NN, IIn-NN, PhDPP-NN, ThDPP-NN, and FuDPP-NN bridged by perylene bisimide (PBI), isoindigo (IIn), and diketopyrrolopyrrole (DPP) pigment colorants were finally obtained as bench stable compounds after periodate oxidation with yields of 60-81%. The absorption spectral signatures of the chromophores remain preserved in the open shell state and match the ones of the pristine parent compounds, which allowed an a priori prediction of their optical properties. Consequently, we achieved twofold spin labeling while keeping the intrinsic properties of the electron deficient chromophores intact.

Molecular S = 2 High-Spin, S = 0 Low-Spin and S = 0 ⇄ 2 Spin-Transition/-Crossover Nickel(II)-Bis(nitroxide) Coordination Compounds

Heterospin systems have a great advantage in frontier orbital engineering since they utilize a wide diversity of paramagnetic chromophores and almost infinite combinations and mutual geometries. Strong exchange couplings are expected in 3d–2p heterospin compounds, where the nitroxide (aminoxyl) oxygen atom has a direct coordination bond with a nickel(II) ion. Complex formation of nickel(II) salts and tert-butyl 2-pyridyl nitroxides afforded a discrete 2p–3d–2p triad. Ferromagnetic coupling is favored when the magnetic orbitals, nickel(II) dσ and radical π*, are arranged in a strictly orthogonal fashion, namely, a planar coordination structure is characterized. In contrast, a severe twist around the coordination bond gives an orbital overlap, resulting in antiferromagnetic coupling. Non-chelatable nitroxide ligands are available for highly twisted and practically diamagnetic complexes. Here, the Ni–O–N–Csp2 torsion (dihedral) angle is supposed to be a useful metric to describe the nickel ion dislocated out of the radical π* nodal plane. Spin-transition complexes exhibited a planar coordination structure in a high-temperature phase and a nonplanar structure in a low-temperature phase. The gradual spin transition is described as a spin equilibrium obeying the van’t Hoff law. Density functional theory calculation indicates that the energy level crossing of the high- and low-spin states. The optimized structures of diamagnetic and high-spin states well agreed with the experimental large and small torsions, respectively. The novel mechanism of the present spin transition lies in the ferro-/antiferromagnetic coupling switch. The entropy-driven mechanism is plausible after combining the results of the related copper(II)-nitroxide compounds. Attention must be paid to the coupling parameter J as a variable of temperature in the magnetic analysis of such spin-transition materials. For future work, the exchange coupling may be tuned by chemical modification and external stimulus, because it has been clarified that the parameter is sensitive to the coordination structure and actually varies from 2J/kB = +400 K to −1400 K.

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.

A divergent strategy for the synthesis of redox-active verdazyl radical polymers

We describe a divergent synthetic strategy based on ATRP and CuAAC chemistry for the production of stable radical polymers. As a proof of concept, we prepare verdazyl radical polymers with properties suitable for use in organic electronics.

Axially and Helically Chiral Cationic Radical Bicarbazoles: SOMO-HOMO Level Inversion and Chirality Impact on the Stability of Mono- and Diradical Cations

We report persistent chiral organic mono- and diradical cations based on bicarbazole molecular design with an unprecedented stability dependence on the type of chirality, namely, axial versus helical. An unusual chemical stability was observed for sterically unprotected axial bicarbazole radical in comparison with monocarbazole and helical bicarbazole ones. Such results were experimentally and theoretically investigated, revealing an inversion in energy of the singly occupied molecular orbital (SOMO) and the highest (doubly) occupied molecular orbital (HOMO) in both axial and helical bicarbazole monoradicals along with a subtle difference of electronic coupling between the two carbazole units, which is modulated by their relative dihedral angle and related to the type of chirality. Such findings allowed us to explore in depth the SOMO-HOMO inversion (SHI) in chiral radical molecular systems and provide new insights regarding its impact on the stability of organic radicals. Finally, these specific electronic properties allowed us to prepare a persistent, intrinsically chiral, diradical which notably displayed near-infrared electronic circular dichroism responses up to 1100 nm and almost degenerate singlet-triplet ground states with weak antiferromagnetic interactions evaluated by magnetometry experiments.