Triplet Ground State Research Articles

Transient Triplet Metallopnictinidenes M-Pn (M = PdII, PtII; Pn = P, As, Sb): Characterization and Dimerization.

Nitrenes (R-N) have been subject to a large body of experimental and theoretical studies. The fundamental reactivity of this important class of transient intermediates has been attributed to their electronic structures, particularly the accessibility of triplet vs singlet states. In contrast, electronic structure trends along the heavier pnictinidene analogues (R-Pn; Pn = P-Bi) are much less systematically explored. We here report the synthesis of a series of metallodipnictenes, {M-Pn═Pn-M} (M = PdII, PtII; Pn = P, As, Sb, Bi) and the characterization of the transient metallopnictinidene intermediates, {M-Pn} for Pn = P, As, Sb. Structural, spectroscopic, and computational analysis revealed spin triplet ground states for the metallopnictinidenes with characteristic electronic structure trends along the series. In comparison to the nitrene, the heavier pnictinidenes exhibit lower-lying ground state SOMOs and singlet excited states, thus suggesting increased electrophilic reactivity. Furthermore, the splitting of the triplet magnetic microstates is beyond the phosphinidenes {M-P} dominated by heavy pnictogen atom induced spin-orbit coupling.

Ph3PC – A Monosubstituted C(0) Atom in Its Triplet State

This study introduces a novel class of carbon‐centered diradicals: a monosubstituted C‐atom stabilized by a phosphine. The diradical Ph3P→C was photochemically generated from a diazophosphorus ylide precursor (Ph3PCN2) and characterized by EPR and isotope‐sensitive ENDOR spectroscopy at low temperatures. Ph3P→C features an axial zero‐field splitting parameter D = 0.543 cm−1 with a vanishingly small rhombicity |E|/D = 0.002. Time‐ and temperature‐dependent measurements confirm a triplet ground state with a lifetime of approximately 10 min at 127 K in toluene‐d8. Multireference electronic structure calculations predict a clear triplet ground state with a singlet‐triplet gap greater than 20 kcal/mol. In contrast to divalent C(0) compounds, such as Ph3P→C←PPh3, in which carbon needs excitation into a highly‐excited closed‐shell 2s02p4 configuration, Ph3P→C can be explained by direct involvement of carbon in its natural 3P state arising from the 2s22p2 configuration.

Ph3PC - A Monosubstituted C(0) Atom in Its Triplet State.

This study introduces a novel class of carbon-centered diradicals: a monosubstituted C-atom stabilized by a phosphine. The diradical Ph3P→C was photochemically generated from a diazophosphorus ylide precursor (Ph3PCN2) and characterized by EPR and isotope-sensitive ENDOR spectroscopy at low temperatures. Ph3P→C features an axial zero-field splitting parameter D = 0.543 cm-1 with a vanishingly small rhombicity |E|/D = 0.002. Time- and temperature-dependent measurements confirm a triplet ground state with a lifetime of approximately 10 min at 127 K in toluene-d8. Multireference electronic structure calculations predict a clear triplet ground state with a singlet-triplet gap greater than 20 kcal/mol. In contrast to divalent C(0) compounds, such as Ph3P→C←PPh3, in which carbon needs excitation into a highly-excited closed-shell 2s02p4 configuration, Ph3P→C can be explained by direct involvement of carbon in its natural 3P state arising from the 2s22p2 configuration.

Triplet-ground-state nonalternant nanographene with high stability and long spin lifetimes

High-spin carbon-based polyradicals exhibit significant potential for applications in quantum information storage and sensing; however, their practical application is hampered by limited structural diversity and chemical instability. Here, we report a straightforward synthetic and isolation method for synthesizing a nonalternant nanographene (1) with a triplet ground state. Moving beyond the classic m-xylylene scaffold for high-spin organic molecules, seven-five-seven (7–5–7)-membered rings are introduced to create stable high-spin diradicals with half-lives (t1/2) as long as 101 days. Moreover, considering the spin relaxation of compound 1, with a spin–lattice relaxation time (T1) of 53.55 ms and a coherence time (Tm) of 3.41 μs at 10 K, the compound 1 shows great promise for applications in spin-based information retention and quantum computing.

A recyclable dynamic semiconducting polymer consisting of Pauli-paramagnetic diradicaloids promoted and stabilized by catechol-boron coordination.

Coordination between 5,5',6,6'-tetrahydroxyindigo (4OH-ID) and boron tribromide unexpectedly affords a novel dynamic covalent polymer, namely P(ID-O-B), consisting of alternating indigo and indigo diradicaloid units. The catechol-boron dynamic bond plays a vital role in promoting the diradicaloid formation and stabilizing the formed diradicaloid segments. The diradicaloid segment in the polymer has a triplet ground state and a thermally populated doublet state, which has been confirmed by the EPR study. Although not conjugated, the polymer still exhibits an electrical conductivity over 10-6 S cm-1. The SQUID study shows that the polymer is Pauli paramagnetic, indicating that the metallic domain exists in this non-conjugated polymer. This diradicaloid-containing polymer is stable toward long-term storage (over 6 months) and thermal treatment over 200 °C, but can be easily depolymerized when treated with methanol.

Nickelocene SPM tip as a molecular spin sensor

Functionalization of a scanning microscopy probe with a single nickelocene allows reproducible spin-sensitive measurements of magnetic systems on surfaces. The triplet ground state of the nickelocene tip gives rise to a characteristic inelastic electron spin-flip excitation, which can change upon interaction with spin systems on the surface. These changes, together with theoretical simulations, enable us to determine the local spin moment on the surface. In this paper, we discuss the experimental and theoretical aspects of nickelocene-tip measurements. We rationalize the interactions between the nickelocene spin and the magnetic centers using a spin Heisenberg and dipole model, complemented by cotunneling theory, and compare the simulated dI/dV with selected experimental results. We developed a Python script to simulate this magnetic interaction, which is available on GitHub.

Coordinative Self‐assembly of π‐Electron Magnetic Porphyrins

Abstractπ‐Electron magnetic compounds on surfaces have emerged as a powerful platform to interrogate spin interactions at the atomic scale, with great potential in spintronics and quantum technologies. A key challenge is organizing these compounds over large length scales, while elucidating their resulting magnetic properties. Herein, we offer a relevant contribution toward this objective, which consists of using on‐surface synthesis coupled with coordination chemistry to promote the self‐assembly of π‐electron magnetic porphyrin species. A porphyrin precursor equipped with carbonitrile moieties in a trans arrangement was prepared by solution synthesis and deposited on Au(111)/mica. Depending on the specific growth protocol, surface‐promoted reactions led to the transformation of the precursor into non‐magnetic Au‐CN coordinated porphyrin monomers, covalent porphyrin dimers, and one‐dimensional porphyrin polymers (based on porphyrin monomers or covalent porphyrin dimers), as revealed by scanning probe microscopy studies combined with theoretical calculations. Interestingly, the scanning tunneling microscopy tip could convert such closed‐shell porphyrin units into open‐shell species by the removal of some peripheral hydrogen atoms. The magnetic features (i.e., singlet or triplet ground state) of the porphyrin units comprising the polymers were investigated for polymers of different lengths. No magnetic exchange coupling between adjacent units was observed, suggesting protection of the magnetic entities.

Direct Determination of a Giant Zero-Field Splitting of 5422 cm-1 in a Triplet Organobismuthinidene by Infrared Electron Paramagnetic Resonance.

Stable monocoordinated organobismuthinidenes were only recently isolated and analyzed toward their chemical and electronic structure. Quantum chemical calculations on tBu-MSFluind-Bi(I) (2) predicted an unusual electronic structure dominated by a triplet ground state and a spectacular zero-field splitting (ZFS) > 4500 cm-1. However, experimental evidence for these predictions remained elusive due to limitations in the available magnetic characterization techniques. Herein, we determine an axial ZFS of D = 5422 cm-1 for 2, by direct detection of triplet electron paramagnetic resonance using magneto-optical infrared spectroscopy. To date, this represents the largest ZFS experimentally measured.

A Kinetically Stabilized Dianthraceno[2,3-a:3',2'-h]-s-Indacene: Stable Kekulé Diradical Polycyclic Hydrocarbon with Triplet Ground State.

High-spin polycyclic hydrocarbons (PHs) hold significant potential in organic spintronics and organic magnets. However, their synthesis is very challenging due to their extremely high reactivity. Herein, we report the successful synthesis and isolation of a kinetically blocked derivative (1) of dianthraceno[2,3-a : 3',2'-h]-s-indacene, which represents a rare persistent triplet diradical of a Kekulé PH. Its triplet ground state was unambiguously confirmed by electron paramagnetic resonance and superconducting quantum interference device measurements. Its structure was also unequivocally confirmed through X-ray crystallographic analysis, and its electronic properties were systematically investigated by both experiments and theoretical calculations. The key design principle is to extend the π-conjugation for achieving the decrease of the bonding interaction and the increase of the exchange interaction between unpaired electrons, which are essential for accessing the stable triplet ground state. Due to kinetic blocking, 1 shows a reasonable stability with a half-life time of 64 h under ambient conditions. It has a narrow HOMO-LUMO energy gap and displays amphoteric redox behavior. Notably, its dication and dianion exhibit a closed-shell ground state and near-infrared absorption, and the structures were identified by X-ray crystallographic analysis. This study will shed new light on the design and synthesis of novel stable PHs with high-spin multiplicity.

Stable Mono‐Radical and Triplet Diradicals Based on Allylic Radical‐Embedded All‐Benzenoid Polycyclic Hydrocarbons

AbstractHigh‐spin organic radicals are notable for their unique optical, electronic, and magnetic properties, but synthesizing stable high‐spin systems is challenging due to their inherent reactivity. This study presents a novel strategy for designing stable high‐spin polycyclic hydrocarbons (PHs) by incorporating allylic radical into fused aromatic benzenoid rings. To enhance stability, large steric hindrance groups with a synergistic captodative effect were added to the allylic radical centers. This approach was applied to synthesize derivatives of two open‐shell all‐benzenoid PHs: benzo[fg]tetracene (1) and dibenzo[fg,lm]heptacene (2). Both compounds exhibited excellent stability, with diradical 2 showing a half‐life of up to 17.2 days under ambient conditions. Bond length analysis and theoretical calculations suggest that 1 and 2 predominantly feature Clar's aromatic sextet structures with embedded allylic radicals. Compound 2 was confirmed to have a triplet ground state through DFT calculations and experimental methods, including pulse EPR spectroscopy and SQUID measurements. This work introduces a new design strategy for stable high‐spin hydrocarbons, paving the way for future developments in high‐spin organic materials.

Stable Mono-Radical and Triplet Diradicals Based on Allylic Radical-Embedded All-Benzenoid Polycyclic Hydrocarbons.

High-spin organic radicals are notable for their unique optical, electronic, and magnetic properties, but synthesizing stable high-spin systems is challenging due to their inherent reactivity. This study presents a novel strategy for designing stable high-spin polycyclic hydrocarbons (PHs) by incorporating allylic radical into fused aromatic benzenoid rings. To enhance stability, large steric hindrance groups with a synergistic captodative effect were added to the allylic radical centers. This approach was applied to synthesize derivatives of two open-shell all-benzenoid PHs: benzo[fg]tetracene (1) and dibenzo[fg,lm]heptacene (2). Both compounds exhibited excellent stability, with diradical 2 showing a half-life of up to 17.2 days under ambient conditions. Bond length analysis and theoretical calculations suggest that 1 and 2 predominantly feature Clar's aromatic sextet structures with embedded allylic radicals. Compound 2 was confirmed to have a triplet ground state through DFT calculations and experimental methods, including pulse EPR spectroscopy and SQUID measurements. This work introduces a new design strategy for stable high-spin hydrocarbons, paving the way for future developments in high-spin organic materials.

Kinetic Study of the Reactions of Ground State Atomic Carbon and Oxygen with Nitrogen Dioxide over the 50-296 K Temperature Range.

The kinetics of the reactions of nitrogen dioxide, NO2, with atomic oxygen and atomic carbon in their ground triplet states (3P) have been studied at room temperature and below using a supersonic flow (Laval nozzle) reactor. O(3P) and C(3P) atoms (hereafter O and C respectively) were created in situ by the pulsed laser photolysis of the precursor molecules NO2 at 355 nm and CBr4 at 266 nm, respectively. While the progress of the O + NO2 reaction was followed by detecting O atoms by a chemiluminescent tracer method, progress of the C + NO2 reaction was followed by detecting C atoms directly by vacuum ultraviolet laser-induced fluorescence at 116 nm. The measured rate constants for the O + NO2 reaction are found to be in excellent agreement with earlier work at higher temperatures and extend the available kinetic data for this process down to 50 K. The present work represents the first kinetics study of the C + NO2 reaction. Although both reactions display rate constants that increase as the temperature falls, a more substantial rate increase is observed for the O + NO2 reaction. The effects of these reactions on the simulated abundances of interstellar NO2 and related compounds were tested using a gas-grain model of the dense interstellar medium, employing expressions for the rate constants of the form, k(T) = α(T/300)β, with α = 1 × 10-11 cm3 s-1 and β = -0.65 for the O + NO2 reaction and α = 2 × 10-10 cm3 s-1 and β = -0.11 for the C + NO2 reaction. Although these simulations predict that gas-phase NO2 abundances are low in dense interstellar clouds, NO2 abundances on interstellar dust grains are predicted to reach reasonably high levels, indicating the potential for detection of this species in warmer regions.

Isolation and characterization of a triplet nitrene.

Nitrene radical compounds are short-lived intermediates in a variety of nitrogen-involved transformations. They feature either a singlet or a triplet ground state, depending on the electronic properties of the substituents. Triplet nitrenes are highly reactive and their isolation in the condensed phase under ambient conditions is challenging. Here we report the synthesis and isolation of a triplet arylnitrene supported by a bulky hydrindacene ligand. The arylnitrene is fully characterized by various spectroscopic and structural techniques including electron paramagnetic resonance spectroscopy and single-crystal X-ray diffraction. Its high stability is largely attributed to the steric hindrance and effective electron delocalization provided by the supporting ligand. Electron paramagnetic resonance spectroscopy in conjunction with highly correlated wavefunction-based ab initio calculations provides support for a triplet ground state nitrene with axial zero-field splitting D = 0.92 cm-1 and vanishing rhombicity E/D = 0.002.

First excited singlet state aromaticity of macroheterocycles

The ground and first excited singlet and triplet states of porphine, tetraoxa-isophlorin, and tetraoxa[8]circulene have been studied in terms of aromaticity. The magnetically induced ring-currents have been calculated at the density functional theory and ab initio (CASSCF) levels of theory to assess aromaticity. Based on the analysis of the expression for the ring-current density in the first excited singlet state, a spectroscopic criterion for antiaromaticity in the S1 state has been proposed. The antiaromatic S1 state is expected for molecules with low-lying magnetic dipole-allowed electronic transitions S1 → Sn, where the transition energy is less than 1.0 eV.

Block-Correlated Coupled Cluster Theory Based on the Generalized Valence Bond Reference for Singlet-Triplet Energy Gaps of Strongly Correlated Systems.

A block-correlated coupled cluster (BCCC) method based on the triplet generalized valence bond (GVB) wave function (GVB-BCCC) has been implemented for the first time. By introducing several techniques, we have developed a practical and efficient GVB-BCCC code. The GVB-BCCC3 method (with up to three-pair correlation) can be used to deal with strongly correlated (SC) systems with triplet or singlet ground states, allowing singlet-triplet (S-T) energy gaps in the active space of SC systems computationally available. For selected SC systems, our calculations show that GVB-BCCC3 can always provide correct ground-state spin multiplicity as the complete active space configuration interaction (CASCI) or density matrix renormalization group (DMRG). Furthermore, we found that the S-T energy gaps from GVB-BCCC3 are quite consistent with CASCI or DMRG results. This work demonstrates that GVB-BCCC3 is a promising theoretical tool for describing S-T energy gaps within the active space of SC systems with large active spaces.

Charge-driven first-order magnetic transition in NiPS3.

Cross-coupling among the fundamental degrees of freedom in solids has been a long-standing problem in condensed matter physics. Despite its progress using predominantly three-dimensional materials, how the same physics plays out for two-dimensional materials is unknown. Here, we show that using31P nuclear magnetic resonance (NMR), the van der Waals antiferromagnet NiPS3undergoes a first-order magnetic phase transition due to the strong charge-spin coupling in a honeycomb lattice. Our31P NMR spectrum near the N´eel ordering temperature TN= 155 K exhibits the coexistence of paramagnetic and antiferromagnetic phases within a finite temperature range. Furthermore, we observed a discontinuity in the order parameter at TNand the complete absence of critical behavior of spin fluctuations above TN, decisively establishing the first-order nature of the magnetic transition. We propose that a charge stripe instability arising from a Zhang-Rice triplet ground state triggers the first-order magnetic transition.

Chirality Engineering of Nanostructured Copper Oxide for Enhancing Oxygen Evolution from Water Electrolysis.

The exploration of a new conceptual strategy for improving the oxygen evolution reaction (OER) of earth-abundant electrocatalysts is critical. In this study, chiral copper oxide nanoflower is explored by a self-assembly method. The characterization suggests the chiral structure originates from the crystal plane-level helical stack of the secondary nanosheets. Of note, the assembly illustrates a record-high degree of spin polarization of 96%, indicating the ideal alignment of electron spin. Moreover, density function theory calculations show the chiral structure reducing the reaction energy barrier (REB) while switching the potential-determining step from *O→*OOH to *OH→*O. Together with the enhanced electrochemical active surface area and accelerated charge transfer, the production of ground-state triplet O2 is improved via a spin-forbidden route that involves the singlet H2O/OH•. Consequently, the chiral nanoflower shows a overpotential of 308mV at 10mA cm-2 and a Tafel slope of 93.5mV dec-1, which is even superior to the commercial RuO2 (310mV, 101mV dec-1). This study presents a new strategy for improving the OER activity by simultaneously enhancing electronic properties and lowering the REB of an non-noble electrocatalyst via chirality engineering.

π‐Extended 4,5‐Fused Bis‐Fluorene: Highly Open‐Shell Compounds and their Cationic Tetrathiafulvalene Derivatives

AbstractThe synthesis of diradical organic compounds has garnered significant attention due to their thermally accessible spin inversion and optoelectronic properties. Yet, preparing such stable structures with high open‐shell behavior remains challenging. Herein, we report the synthesis and properties of four π‐extended, fused fluorene derivatives with high diradical character, taking advantage of a molecular design where the closed‐shell does not include any Clar sextet, comparatively to a maximum of 5 in the corresponding open‐shell state. This led to an unusual open‐shell triplet ground state with an outstanding singlet‐triplet energy difference (ΔEST) of ca. 19 kcal/mol, one of the highest values reported to date for an all‐carbon conjugated scaffold. Incorporation of dithiafulvene units at each end of the molecule (at the five‐membered rings) furnishes extended tetrathiafulvalenes (TTFs) undergoing reversible oxidations to the radical cation and diradical dication. The various pro‐aromatic structures presented herein show highly localized spin density and a limited conjugation due to the confined π‐electrons in the aromatic cycles, as supported by 1H NMR, UV/Visible, EPR spectroscopy and DFT calculations.

Stereoelectronic Tuning of Bioinspired Nonheme Iron(IV)-Oxo Species by Amide Groups in Primary and Secondary Coordination Spheres for Selective Oxygenation Reactions.

Two mononuclear iron(II) complexes, [(6-amide2-BPMEN)FeII](OTf)2 (1) and [(6-amide-Me-BPMEN)FeII(OTf)](OTf) (2), supported by two BPMEN-derived (BPMEN = N1,N2-dimethyl-N1,N2-bis(pyridine-2-yl-methyl)ethane-1,2-diamine) ligands bearing one or two amide functionalities have been isolated to study their reactivity in the oxygenation of C-H and C═C bonds using isopropyl 2-iodoxybenzoate (iPr-IBX ester) as the oxidant. Both 1 and 2 contain six-coordinate high-spin iron(II) centers in the solid state and in solution. The 6-amide2-BPMEN ligand stabilizes an S = 1 iron(IV)-oxo intermediate, [(6-amide2-BPMEN)FeIV(O)]2+ (1A). The oxidant (1A) oxygenates the C-H and C═C bonds with a high selectivity. Oxidant 1A, upon treatment with 2,6-lutidine, is transformed into another oxidant [{(6-amide2-BPMEN)-(H)}FeIV(O)]+ (1B) through deprotonation of an amide group, resulting in a stronger equatorial ligand field and subsequent stabilization of the triplet ground state. In contrast, no iron-oxo species could be observed from complex 2 and [(6-Me2-BPMEN)FeII(OTf)2] (3) under similar experimental conditions. The iron(IV)-oxo oxidant 1A shows the highest A/K selectivity in cyclohexane oxidation and 3°/2° selectivity in adamantane oxidation reported for any synthetic nonheme iron(IV)-oxo complexes. Theoretical investigation reveals that the hydrogen bonding interaction between the -NH group of the noncoordinating amide group and Fe═O core smears out the equatorial charge density, reducing the triplet-quintet splitting, and thus helping complex 1A to achieve better reactivity.

π-Extended 4,5-Fused Bis-Fluorene: Highly Open-Shell Compounds and their Cationic Tetrathiafulvalene Derivatives.

The synthesis of diradical organic compounds has garnered significant attention due to their thermally accessible spin inversion and optoelectronic properties. Yet, preparing such stable structures with high open-shell behavior remains challenging. Herein, we report the synthesis and properties of four π-extended, fused fluorene derivatives with high diradical character, taking advantage of a molecular design where the closed-shell does not include any Clar sextet, comparatively to a maximum of 5 in the corresponding open-shell state. This led to an unusual open-shell triplet ground state with an outstanding singlet-triplet energy difference (ΔEST) of ca. 19 kcal/mol, one of the highest values reported to date for an all-carbon conjugated scaffold. Incorporation of dithiafulvene units at each end of the molecule (at the five-membered rings) furnishes extended tetrathiafulvalenes (TTFs) undergoing reversible oxidations to the radical cation and diradical dication. The various pro-aromatic structures presented herein show highly localized spin density and a limited conjugation due to the confined π-electrons in the aromatic cycles, as supported by 1H NMR, UV/Visible, EPR spectroscopy and DFT calculations.