Variable-temperature EPR Research Articles

Cationic Magnetically Active Nitrogen-Doped Polycyclic Aromatic Hydrocarbon with Record Low Band Gap.

Polycyclic aromatic hydrocarbons (PAHs) have attracted significant interest in material chemistry, particularly if they own extremely low band gaps and magnetic properties. However, challenges remain regarding the synthetic accessibility and energy saturation issues. In this study, we introduceNR-11, which consists of eleven aromatic rings in its main conjugation and is separately doped with two electron-rich nitrogen atoms. This unique structure imparts intriguing oxidation characteristics toNR-11. The cationic radicalNR-11+•exhibits enhanced stability and demonstrates strong absorption in the range of 1250 nm to 3000 nm, peaking at 2570 nm. As a result, the optical energy gap ofNR-11+•is one of the lowest reported to date. Additionally, X-ray crystal structure analysis reveals thatNR-11+•displays unusual symmetry-broken charge separation. For the dication, variable-temperature NMR and variable-temperature EPR studies indicate thatNR-112+exhibits a high diradical character with a ∆ES-Tof approximately-1 kcal/mol. Additionally, its spins are polarized at two ends of the PAH. Meanwhile, its strong absorption in the near-infrared II region suggests promise in photoacoustic (PA) conversion applications. This work underscores the significance of cationic species of extended long PAHs, highlighting their exceptional properties and potential applications.

Non-Kekulé meta-Quinodimethane Singlet Diradicals Based on Classical N-Heterocyclic Carbenes.

Diradicals based on a meta-quinodimethane (m-QDM) scaffold generally have a triplet ground state and are rather scarce. Herein, m-QDM-based non-Kekulé diradicals [3,3'-(NHC)2BP] (3-NHC) (NHC = SIPr = C{N(Dipp)CH2}2; IPr = C{N(Dipp)CH}2, Me-IPr = C{N(Dipp)CMe}2; Dipp = 2,6-iPr2C6H3; BP = 1,1'-C6H4C6H4) featuring N-heterocyclic carbene (NHC) pendants are reported as crystalline solids. The EPR spectra of 3-NHC show both allowed (Δms = 1) and forbidden (Δms = 2; 'half-field') transitions characteristic for triplet diradicals. Variable temperature EPR studies however reveal a singlet ground state for 3-SIPr. Consistent with the EPR spectra, calculations predict a remarkably small singlet-triplet energy gap (ΔEST ≤ 0.26 kcal/mol) for the 3-NHC compounds. The calculated singlet diradical character for the ground states of the 3-NHC compounds amounts to ~99 %.

Isolation of Elusive Fluoflavine Radicals in Two Differing Oxidation States.

Facile access and switchability between multiple oxidation states are key properties of many catalytic applications and spintronic devices yet poorly understood due to inherent complications arising from isolating a redox system in multiple oxidation states without drastic structural changes. Here, we present the first isolable, free fluoflavine (flv) radical flv(1-•) as a bottleable potassium compound, [K(crypt-222)](flv•), 1, and a new series of organometallic rare earth complexes [(Cp*2Y)2(μ-flvz)]X, (where Cp* = pentamethylcyclopentadienyl, X = [Al(OC{CF3}3)4]- (z = -1), 2; X = 0 (z = -2), 3; [K(crypt-222)]+ (z = -3), 4) comprising the flv ligand in three different oxidation states, two of which are paramagnetic flv1-• and flv3-•. Excitingly, 1, 2, and 4 constitute the first isolable flv1-• and flv3-• radical complexes and, to date, the only isolated flv radicals of any oxidation state. All compounds are accessible in good crystalline yields and were unambiguously characterized via single-crystal X-ray diffraction analysis, cyclic voltammetry, IR-, UV-vis, and variable-temperature EPR spectroscopy. Remarkably, the EPR spectra for 1, 2, and 4 are distinct and a testament to stronger spin delocalization onto the metal centers as a function of higher charge on the flv radical. In-depth analysis of the electron- and spin density via density functional theory (DFT) calculations utilizing NLMO, QTAIM, and spin density topology analysis confirmed the fundamental interplay of metal coordination, ligand oxidation state, aromaticity, covalency, and spin density transfer, which may serve as blueprints for the development of future spintronic devices, single-molecule magnets, and quantum information science at large.

Perylene with Split‐Azulene Embedding

AbstractSplitting the five and seven‐membered rings of azulene and embedding them separately into a conjugated backbone provides azulene‐like polycyclic aromatic hydrocarbons (PAHs), which are of great interest in quantum and material chemistry. However, the synthetic accessibility poses a significant challenge. In this study, we present the synthesis of a novel azulene‐like PAH, Pery‐57, which can be viewed as the integration of a perylene framework into the split azulene. The compact structure of Pery‐57 displays several intriguing characteristics, including NIR II absorption at 1200 nm, a substantial dipole moment of 3.5 D, and head‐to‐tail alternating columnar packing. Furthermore, Pery‐57 exhibits remarkable redox properties. The cationic radical Pery‐57⋅+ readily captures a hydrogen atom. Variable‐temperature NMR (VT NMR ) and variable‐temperature EPR (VT‐EPR) studies reveal that the dianion Pery‐572− possesses an open‐shell singlet ground state and demonstrates significant global anti‐aromaticity. The dication Pery‐572+ is also predicted to exhibit diradical character. Despite bearing three bulky substituents, Pery‐57 displays p‐type transport characteristics with a mobility of 0.03 cm2 V−1 s−1, attributed to its unique azulene‐like structure. Overall, this work directs interest in azulene‐like PAHs, a unique member of nonalternant PAHs showcasing exceptional properties and applications.

Nonalternating π-System Mediated Spin Coupling in Azulene Nitronyl Nitroxide Diradicals.

To explore the distinctions in spin coupling between the molecular bridges of alternating and nonalternating π-systems, we synthesized a pair of isoelectronic compounds, namely, 2,6-Na-NN and 2,6-Az-NN, by utilizing naphthalene and azulene (naphthalene = Na and azulene = Az) as the bridges, respectively. Moreover, we conducted assessments to predict the coupling paths for nonalternating azulene. Variable-temperature EPR (VT-EPR) and SQUID results consistently reveal that both 2,6-Na-NN and 2,6-Az-NN exhibit antiferromagnetic coupling interactions, with coupling constants of J(2,6-Na-NN) = -22.3 cm-1 and J(2,6-Az-NN) = -30.1 cm-1, respectively. Density functional theory computations support these discoveries by revealing negative coupling constants (J < 0) and the spin densities population of the diradicals are observed to delocalize into the molecular bridges. This work suggests the most suitable coupling path for 2,6-Az-NN. In addition, we have investigated the potential spatial resistance of the diradicals in conjunction with single-crystal data. Theoretical calculations underestimating the torsion angle of the diradicals and overestimating the value of the magnetic coupling provide an explanation for this phenomenon. The final experimental results and theoretical calculations show that the 2,6-Az-NN coupling path prefers short paths.

Charge and Spin Transfer Dynamics in a Weakly Coupled Porphyrin Dimer.

The dynamics of electron and spin transfer in the radical cation and photogenerated triplet states of a tetramethylbiphenyl-linked zinc-porphyrin dimer were investigated, so as to test the relevant parameters for the design of a single-molecule spin valve and the creation of a novel platform for the photogeneration of high-multiplicity spin states. We used a combination of multiple techniques, including variable-temperature continuous wave EPR, pulsed proton electron-nuclear double resonance (ENDOR), transient EPR, and optical spectroscopy. The conclusions are further supported by density functional theory (DFT) calculations and comparison to reference compounds. The low-temperature cw-EPR and room-temperature near-IR spectra of the dimer monocation demonstrate that the radical cation is spatially localized on one side of the dimer at any point in time, not coherently delocalized over both porphyrin units. The EPR spectra at 298 K reveal rapid hopping of the radical spin density between both sites of the dimer via reversible intramolecular electron transfer. The hyperfine interactions are modulated by electron transfer and can be quantified using ENDOR spectroscopy. This allowed simulation of the variable-temperature cw-EPR spectra with a two-site exchange model and provided information on the temperature-dependence of the electron transfer rate. The electron transfer rates range from about 10.0 MHz at 200 K to about 53.9 MHz at 298 K. The activation enthalpies Δ‡H of the electron transfer were determined as Δ‡H = 9.55 kJ mol-1 and Δ‡H = 5.67 kJ mol-1 in a 1:1:1 solvent mixture of CD2Cl2/toluene-d8/THF-d8 and in 2-methyltetrahydrofuran, respectively, consistent with a Robin-Day class II mixed valence compound. These results indicate that the interporphyrin electronic coupling in a tetramethylbiphenyl-linked porphyrin dimer is suitable for the backbone of a single-molecule spin valve. Investigation of the spin density distribution of the photogenerated triplet state of the Zn-porphyrin dimer reveals localization of the triplet spin density on a nanosecond time scale on one-half of the dimer at 20 K in 2-methyltetrahydrofuran and at 250 K in a polyvinylcarbazole film. This establishes the porphyrin dimer as a molecular platform for the formation of a localized, photogenerated triplet state on one porphyrin unit that is coupled to a second redox-active, ground-state porphyrin unit, which can be explored for the formation of high-multiplicity spin states.

Perylene with Split-Azulene Embedding.

Splitting the five and seven-membered rings of azulene and embedding them separately into a conjugated backbone provides azulene-like polycyclic aromatic hydrocarbons (PAHs), which are of great interest in quantum and material chemistry. However, the synthetic accessibility poses a significant challenge. In this study, we present the synthesis of a novel azulene-like PAH, Pery-57, which can be viewed as the integration of a perylene framework into the split azulene. The compact structure of Pery-57 displays several intriguing characteristics, including NIR II absorption at 1200 nm, a substantial dipole moment of 3.5 D, and head-to-tail alternating columnar packing. Furthermore, Pery-57 exhibits remarkable redox properties. The cationic radical Pery-57⋅+ readily captures a hydrogen atom. Variable-temperature NMR (VT NMR ) and variable-temperature EPR (VT-EPR) studies reveal that the dianion Pery-572- possesses an open-shell singlet ground state and demonstrates significant global anti-aromaticity. The dication Pery-572+ is also predicted to exhibit diradical character. Despite bearing three bulky substituents, Pery-57 displays p-type transport characteristics with a mobility of 0.03 cm2 V-1 s-1, attributed to its unique azulene-like structure. Overall, this work directs interest in azulene-like PAHs, a unique member of nonalternant PAHs showcasing exceptional properties and applications.

A Stable Aluminum Tris(dithiolene) Triradical.

A stable aluminum tris(dithiolene) triradical (3) was experimentally realized through a low-temperature reaction of the sterically demanding lithium dithiolene radical (2) with aluminum iodide. Compound 3 was characterized by single-crystal X-ray diffraction, UV-vis and EPR spectroscopy, SQUID magnetometry, and theoretical computations. The quartet ground state of triradical 3 has been unambiguously confirmed by variable-temperature continuous wave EPR experiments and SQUID magnetometry. Both SQUID magnetometry and broken-symmetry DFT computations reveal a small doublet-quartet energy gap [ΔEDQ = 0.18 kcal mol-1 (SQUID); ΔEDQ = 0.14 kcal mol-1 (DFT)]. The pulsed EPR experiment (electron spin echo envelop modulation) provides further evidence for the interaction of these dithiolene-based radicals with the central aluminum nucleus of 3.

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.

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.

Symmetry-Broken Intermolecular Charge Separation of Cationic Radicals.

A quadrupolar compound Pyr-BA with two pyrrole-type nitrogen atoms doped externally was prepared in this work. In high contrast with other π ionic radicals, its cationic radical Pyr-BA⋅+ undergoes unusual symmetry-broken charge separation (SB-CS), generating the mixed valence complex of Pyr-BA+1-q ⋅⋅⋅Pyr-BA+1+q , where q is the degree of charge transfer. Variable-temperature (VT) single-crystal analysis, absorption and EPR experiments all confirmed that aggregation and lower temperature would help to facilitate this SB-CS process. Gibbs energy calculations and gauge-including magnetically induced current simulation both validate that, for Pyr-BA⋅+ , SB-CS behavior is more favorable than the conventional dimerization mode. To the best of our knowledge, this is the first study that shows solid single-crystal evidence for spontaneous SB-CS between identical ionic radicals. Such a unique phenomenon is of great significance both in terms of fundamental aspects and uncharted material science.

Meso-Carbon Atom Nucleophilic Attack Susceptibility in the Sterically Strained Antiaromatic Bis-BODIPY Macrocycle and Extended Electron-Deficient BODIPY Precursor.

A sterically strained 32π-electron antiaromatic bis-BODIPY macrocycle in which two BODIPY fragments are linked by p-divinylbenzene groups was prepared and characterized. Unlike regular BODIPYs, the fluorescence in this macrocycle is quenched. The broad signals in the NMR spectra of the macrocycle were explained by the vibronic freedom of the p-divinylbenzene fragments. The possible diradicaloid nature of the macrocycle was excluded on the basis of variable-temperature EPR spectra in solution and in solid state, which is indicative of its closed-shell quinoidal structure. The meso-C-H bond in the macrocycle and its precursor BODIPY dialdehyde 3 forms a weak hydrogen bond with THF and is susceptible for the nucleophilic attack by organic amines and cyanide anion. The reaction products of such a nucleophilic attack have meso-sp3 carbon atoms and were characterized by NMR, mass spectrometry and, in one case, X-ray crystallography. Unlike the initial bis-BODIPY macrocycle, the adducts have strong fluorescence in the 400 nm region. The electronic structure and spectroscopic properties of new chromophores were probed by density functional theory (DFT) and time-dependent DFT (TDDFT) calculations and correlate well with the experimental data.

Synergistic effect of underwater arc discharge plasma and Fe2O3-CoFe2O4 enhanced PMS activation to efficiently degrade refractory organic pollutants

This study employed a self-designed underwater arc discharge (UAD) plasma reactor, which combined advanced oxidation process (Fe2O3-CoFe2O4/PMS) and low-temperature plasma technology to efficiently degrade refractory organic pollutants (i.e. phenol, p-chlorophenol, p-nitrochlorobenzene). The results illustrated that the plasma/PMS/Fe2O3-CoFe2O4 system possessed high degradation efficiency and TOC removal rate towards three target pollutants (phenol: 99.8%, 96.12%; 4-CP: 98.4%, 86.48%; p-NCB: 99.9%, 88.31%). With the help of in-situ variable temperature EPR and Indigo colorimetry method, ·OH, SO4·-, O2·-, 1O2 and O3 were identified as the main reactive oxygen species (ROS) for the degradation of target pollutants, revealing the synergistic catalytic mechanism between plasma and Fe2O3-CoFe2O4 for PMS activation and inferring the degradation pathways of target pollutants. The results confirmed that the plasma/PMS/Fe2O3-CoFe2O4 system was feasible for the degradation of refractory organic pollutants, has the advantages of high efficiency and low energy consumption, and could provide an effective strategy for the treatment of refractory organic pollutants in groundwater/soil.

Efficient Dynamic Nuclear Polarization up to 230 K with Hybrid BDPA-Nitroxide Radicals at a High Magnetic Field.

Pairing the spectral resolution provided by high magnetic fields at ambient temperature with the enhanced sensitivity offered by dynamic nuclear polarization (DNP) is a major goal of modern solid-state NMR spectroscopy, which will allow one to unlock ever-challenging applications. This study demonstrates that, by combining HyTEK2, a hybrid BDPA-nitroxide biradical polarizing agent, with ortho-terphenyl (OTP), a rigid DNP matrix, enhancement factors as high as 65 can be obtained at 230 K, 40 kHz magic angle spinning (MAS), and 18.8 T. The temperature dependence of the DNP enhancement and its behavior around the glass transition temperature (Tg) of the matrix is investigated by variable-temperature EPR measurements of the electron relaxation properties and numerical simulations. A correlation is suggested between the decrease in enhancement at the passage of the Tg and the concomitant drop of both transverse electron relaxation times in the biradical.

Reversible Switching of Organic Diradical Character via Iron-Based Spin-Crossover.

Organic diradicals are uncommon species that have been intensely studied for their unique properties and potential applicability in a diverse range of innovative fields. While there is a growing class of stable and well-characterized organic diradicals, there has been recent focus on how diradical character can be controlled or modulated with external stimuli. Here we demonstrate that a diiron complex bridged by the doubly oxidized ligand tetrathiafulvalene-2,3,6,7-tetrathiolate (TTFtt2-) undergoes a thermally induced Fe-centered spin-crossover which yields significant diradical character on TTFtt2-. UV-vis-near-IR, Mössbauer, NMR, and EPR spectroscopies with magnetometry, crystallography, and advanced theoretical treatments suggest that this diradical character arises from a shrinking TTFtt2- π-manifold from the Fe(II)-centered spin-crossover. The TTFtt2--centered diradical is predicted to have a singlet ground state by theory and variable temperature EPR. This unusual phenomenon demonstrates that inorganic spin transitions can be used to modulate organic diradical character.

Exchange interactions in photoinduced magnetostructural states of copper(ii)-nitroxide spin dyads.

Copper(ii) complexes with stable nitroxide radicals are capable of magnetostructural spin-crossover like anomalies induced by external stimuli. Photoswitching in such systems is particularly important; however, retrieving the properties of photoinduced states is challenging and requires development of novel approaches. In this work, we investigate the exchange interactions in metastable photoinduced states of two compounds containing copper(ii)-nitroxide dyads. Using Electron Paramagnetic Resonance (EPR) with photoexcitation we obtain temperature dependence of magnetic susceptibility in the photoinduced state and estimates for the corresponding values of exchange coupling in the studied complexes. The interplay between intra- and inter-cluster exchange couplings is considered and analyzed. The proposed methodology is applicable also to other photoswitchable exchange-coupled systems.

Unusual coordination behavior by a hydroxypyridine/pyridone ligand: Synthesis and structure of [(2-bromo-4-hydroxypyridine)2(2-bromo-1(H)-4-pyridone)2copper(II)] perchlorate • 2(2-bromo-4-hydroxypyridine) • 2(2-bromo-1(H)-4-pyridone)

The synthesis and structure of [Cu(2-Br-4-HOpy)2(2-Br-4-pyone)2](ClO4)2 • 2(2-Br-4-HOpy) • 2(2-Br-4-pyone) is reported [2-Br-4-HOpy = 2-bromo-4-hydroxypyridine; 2-Br-4-pyone = 2-bromo-(1H)4-pyridone]. The compound forms as a co-crystal in the triclinic P-1 space group with four-coordinate Cu(II) ions and perchlorate geganions interacting with both the hydroxypyridine and pyridone forms of the organic molecule through hydrogen and halogen bonding. The coordination complex exhibits the unusual coordination motif of having both the hydroxypyridine and pyridone tautomers of the ligand coordinated to the same metal ion. The isolation of the Cu complex by the organic groups suggests an absence of magnetic exchange within the sample which is confirmed by variable temperature magnetization measurements and EPR data.

Front Cover: Bis‐[3]Ferrocenophanes with Central &gt;E−E’&lt; Bonds (E, E’=P, SiH): Preparation, Properties, and Thermal Activation (ChemistryOpen 10/2019)

The Front Cover visualizes the thermally induced homolytic cleavage of the central P-P bond in a phosphorus–rich bis-ferrocenophane furnishing P-centered radicals. The bond fission process and conformational dynamics in this compound as well as isolobal analogues obtained by formal replacement of individual P-atoms by nitrogen atoms or SiH fragments is elucidated from analysis of their variable temperature NMR and EPR spectra and simulated by DFT calculations. The central P6 unit in the title compound is a structural analog of the connecting unit in Hittorf's violet phosphorus, which links the orthogonally arranged tubular entities. More information can be found in the Full Paper by S. Isenberg et al. on page 1235 in Issue 10, 2019 (DOI: 10.1002/open.201900182).

A variable-temperature Q- and X-band EPR study of spin-crossover iron(III) Schiff base complex

The mononuclear iron(III) Schiff base complex was analysed by EPR spectroscopy. The first derivative Q- and X-band EPR spectra of spin-crossover iron(III) complex exhibit the presence of the high spin (S = 5/2) and low spin (S = 1/2) iron(III) state. The variable-temperature EPR measurement confirmed a gradual spin-crossover transition (S = 5/2 ↔ S = 1/2). The possible hysteresis in the thermally induced spin-crossover effect was not detected. The EPR data are in agreement with observed trends in the magnetic measurements reported previously. Note that the EPR studies of this complex have not been performed before. It is obvious that EPR spectroscopy is a powerful method for investigation of the spin-crossover transition and their temperature evolution in the iron(III) complexes.

Stable radical versus reversible σ-bond formation of (porphyrinyl)dicyanomethyl radicals.

(Porphyrinyl)dicyanomethyl radicals were produced by oxidation of dicyanomethyl-substituted porphyrins with PbO2. These radicals constitute a rare example displaying stable radical versus dynamic covalent chemistry (DCC) depending upon the substitution position of the dicyanomethyl radical. meso-Dicyanomethyl-substituted radicals exist as stable monomeric species and do not undergo any dimerization processes either in the solid state or in solution. In contrast, β-dicyanomethyl-substituted radicals are isolated as σ-dimers that are stable in the solid-state but display reversible σ-dimerization behavior in solution; monomeric radical species exist predominantly at high temperatures, while σ-dimerization is favoured at low temperatures. This dynamic behaviour has been confirmed by variable-temperature 1H NMR, UV-vis and EPR measurements. The structures of the stable radical and σ-dimer have been revealed by single-crystal X-ray diffraction analysis. The observed different reactivities of the two (porphyrinyl)dicyanomethyl radicals have been rationalized in terms of their spin delocalization behaviours.