Spin Population Research Articles

Homogeneous Catalysts for Hydrogenative PHIP Used in Biomedical Applications

AbstractAt present, two competing hyperpolarization (HP) techniques, dissolution dynamic nuclear polarization (DNP) and parahydrogen (para‐H2) induced polarization (PHIP), can generate sufficiently high liquid state 13C signal enhancement for in vivo studies. PHIP utilizes the singlet spin state of para‐H2 to create non‐equilibrium spin populations. In hydrogenative PHIP, para‐H2 is irreversibly added to unsaturated precursors, typically in the presence of a homogeneous catalyst. The hydrogenation catalyst plays a crucial role in converting the singlet spin order of para‐H2 into detectable nuclear polarization. Currently, rhodium(I) bisphosphine complexes are the most widely employed catalysts for PHIP, capable of catalyzing the addition of para‐H2 to unsaturated precursors in organic solvents or aqueous media, depending on the ligand. Chiral catalysts enable the stereoselective production of hyperpolarized substrates. Ruthenium(II) piano stool complexes are capable of trans addition and are used to generate hyperpolarized fumarate. However, these catalysts systems are not optimal, and the greatest source of nuclear spin polarization loss is attributed to the mixing of singlet and triplet states of the protons derived from the para‐H2 during the hydrogenation process. Hence, future efforts should focus on enhancing the efficiency and kinetics of these catalysts.

Optical manipulation of spin states in ultracold magnetic atoms via an inner-shell hz transition

Lanthanides, like erbium and dysprosium, have emerged as powerful platforms for quantum-gas research due to their diverse properties, including a significant large spin manifold in their absolute ground state. However, effectively exploiting the spin richness necessitates precise manipulation of spin populations, a challenge yet to be fully addressed in this class of atomic species. In this work, we present an all-optical method for deterministically controlling the spin composition of a dipolar bosonic erbium gas, based on a clocklike transition in the telecom window at 1299nm. The atoms can be prepared in just a few tens of microseconds in any spin-state composition using a sequence of Rabi-pulse pairs, selectively coupling Zeeman sublevels of the ground state with those of the long-lived clocklike state. Finally, we demonstrate that this transition can also be used to create spin-selective light shifts, thus fully suppressing spin-exchange collisions. These experimental results unlock exciting possibilities for implementing advanced spin models in isolated, clean, and fully controllable lattice systems. Published by the American Physical Society 2024

The Role of a Redox‐Active Ligand in Cu(II)‐Nitrenoid Formation and Its Aziridination Reactivity

AbstractMetal‐nitrenoid species are the key intermediate in nitrene transfer reactions. The bond formation between the open‐shell transition metal and the nitrene species dictates their overall reactivity. Herein, we present an in‐depth electronic structure investigation to uncover how the formation of a Cu(II)‐nitrenoid species is assisted by redox‐active iminosemiquinone (ISQ) ligand in the [CuII(LISQ)2] complex and impacts the nitrene transfer reactivity. The reaction energetics of the aziridination reaction between N‐tosyliminophenyliodinane and 4‐chlorostyrene is established by DFT calculations, where the formation of the Cu(II)‐nitrenoid intermediate (Int1) is found to be the rate‐determining step. Based on the natural orbital and spin populations obtained from the ab initio multiconfigurational CASSCF(13,13) calculation, the electronic nature of the elusive Cu(II)‐nitrenoid intermediate (Int1) could be formulated as “imidyl” [(LISQ)(LIBQ)CuII─•NTos] (Tos = Tosyl) species. A weak σ interaction between the Cu(II) center and the N‐atom of the nitrene moiety (Cu 20% + N 29%) involving a Cu─N distance of 1.92 Å defines the Cu‐nitrenoid bonding. The interplay between elongated Cu─N bond and high‐spin population on N‐atom (+0.96) corroborates its high nitrene transfer ability. Furthermore, the intrinsic bond orbital (IBO) analysis unravels a ligand‐to‐substrate electron transfer mechanism that facilitates the formation of the crucial Cu(II)‐nitrenoid species.

Nuclear hyperpolarization in electron-transfer proteins: Revealing unexpected light-induced 15N signals with field-cycling magic-angle spinning NMR

The solid-state photo-CIDNP (photo-chemically induced dynamic nuclear polarization) effect allows for nuclear hyperpolarization, i.e., non-Boltzmann nuclear spin population. The effect relies on the light-induced formation of a spin-correlated radical pair (SCRP) and has been observed in various photosynthetic reaction center (RC) proteins and flavin-containing light, oxygen, voltage (LOV) proteins. Both systems exhibit strongly enhanced NMR signals originating from the electron transfer partners. Here, we present experimental data on the magnetic field dependence of the 15N solid-state photo-CIDNP effect in both phototropin LOV1 C57S from Chlamydomonas reinhardtii and the bacterial photosynthetic RC from Rhodobacter sphaeroides. Using a pneumatic field-cycling system, samples containing a frozen solution of the proteins are explored between 0.25 T and 9.4 T. Both systems yield hyperpolarized 15N NMR signals across the entire magnetic field range originating from the electron transfer moieties. Also, in both systems, hyperpolarized signals from unexpected positions are detected between 1.0 T and 2.0 T: position N-1 of the flavin in the LOV1 protein and the τ-N of the axial magnesium-coordinating histidine of the donor. A first attempt to explain the occurrence of these unexpected signals based on quantum chemical calculations is presented.

Boltzmann optical thermometry for cryogenics.

We propose and implement an optical technique to access the local temperature of an erbium doped crystal by probing the electron spin population under magnetic field. We reliably extract the sample temperature in the range 2-7K. We additionally discuss the suitability of our method as a primary standard for cryogenic thermometry. By adding an auxiliary heating laser, we are able to measure the interface conductance between the dielectric crystal and the cold plate of the cryostat by exploring different cooling configurations.

Quantum Spin Probe of Single Charge Dynamics.

Electronic defects in semiconductors form the basis for emerging quantum technologies, but many defect centers are difficult to access at the single-particle level. A method for probing optically inactive spin defects would reveal semiconductor physics at the atomic scale and advance the study of new quantum systems. We exploit the intrinsic correlation between the charge and spin states of defect centers to measure the charge populations and dynamics of single substitutional nitrogen spin defects in diamond. By probing their steady-state spin population, read out at the single-defect level with a nearby nitrogen vacancy center, we directly measure the defect ionization-corroborated by first-principles calculations-an effect we do not have access to with traditional coherence-based quantum sensing.

Graphene-Templated Achiral Hybrid Perovskite for Circularly Polarized Light Sensing.

This study points out the importance of the templating effect in hybrid organic-inorganic perovskite semiconductors grown on graphene. By combining two achiral materials, we report the formation of a chiral composite heterostructure with electronic band splitting. The effect is observed through circularly polarized light emission and detection in a graphene/α-CH(NH2)2PbI3 perovskite composite, at ambient temperature and without a magnetic field. We exploit the spin-charge conversion by introducing an unbalanced spin population through polarized light that gives rise to a spin photoconductive effect rationalized by Rashba-type coupling. The prepared composite heterostructure exhibits a circularly polarized photoluminescence anisotropy gCPL of ∼0.35 at ∼2.54 × 103 W cm-2 confocal power density of 532 nm excitation. A carefully engineered interface between the graphene and the perovskite thin film enhances the Rashba field and generates the built-in electric field responsible for photocurrent, yielding a photoresponsivity of ∼105 A W-1 under ∼0.08 μW cm-2 fluence of visible light photons. The maximum photocurrent anisotropy factor gph is ∼0.51 under ∼0.16 μW cm-2 irradiance. The work sheds light on the photophysical properties of graphene/perovskite composite heterostructures, finding them to be a promising candidate for developing miniaturized spin-photonic devices.

Comparison of optimized effective potential with inverse Kohn-Sham method for Hartree-Fock exchange energy.

The inverse Kohn-Sham (inv-KS) density-functional theory for the electron density of the Hartree-Fock (HF) wave function was revisited within the context of the optimized effective potential (HF-OEP). First, we clarify the relationship between the inv-KS and the HF-OEP within the framework of the potential-functional theory. The similarities and the differences of the approaches are then discussed on the basis of their methodological details, which motivates comparisons of the wave function provided by each method. Next, the real-space grid implementations of the inv-KS and the HF-OEP are addressed for the comparisons. The total HF energies EHF[{φiinv-KS}] for the wave functions φiinv-KS on the effective potentials optimized by the inv-KS are computed for a set of small molecules. It is found that the mean absolute deviation (MAD) of EHF[{φiinv-KS}] from the HF energy is clearly smaller than the MAD of EHF[{φiOEP}], demonstrating that the inv-KS is advantageous in constructing the detailed structure of the exchange potential υx as compared with the HF-OEP. The inv-KS method is also applied to an ortho-benzyne radical known as a strongly correlated polyatomic molecule. It is revealed that the spin populations on the atomic sites computed by the UHF calculation can be faithfully reproduced by the wave functions on the inv-KS potential.

Chiral Perovskite Heterostructure Films of CsPbBr3 Quantum Dots and 2D Chiral Perovskite with Circularly Polarized Luminescence Performance and Energy Transfer.

This work reports the synthesis of chiral perovskite heterostructure films by combining a two-dimensional (2D) chiral (R-/S-MBA)2PbI4 perovskite with CsPbBr3 quantum dots (QDs). The as-synthesized chiral heterostructure films exhibit obvious circularly polarized luminescence (CPL) properties, even though pure 2D chiral perovskite cannot present photoluminescence. It indicates that the chirality of the excited state of the QDs originates from the 2D chiral perovskite. The circular polarization-resolved transient absorption (TA) spectra further demonstrate that the CPL response of heterostructure films originates from the energy transfer between the chiral perovskite layer and QDs layer and the suppression of spin relaxation, which induces the imbalance of the spin population of excited states in QDs layer. In addition, the photoluminescence (PL), circular dichroism (CD), and CPL spectra of these heterostructure films can be controlled by varying the thickness and component of the chiral perovskite layer, which demonstrates that the anion exchange between chiral perovskite and CsPbBr3 QDs can tune the chemical composition and optoelectronic properties due to the low bonding energy difference between them and decrease the strain within the QDs layer to reduce the radiative recombination lifetime. This work provides guidance for the synthesis of chiral perovskites with a strong CPL response and further provides insight into the origination of CPL.

Oblique spin injection to graphene via geometry controlled magnetic nanowires

We exploit the geometry of magnetic nanowires, which define 1D contacts to an encapsulated graphene channel, to introduce an out-of-plane component in the polarisation of spin carriers. By design, the magnetic nanowires traverse the angled sides of the 2D material heterostructure. Consequently, the easy axis of the nanowires is inclined, and so the local magnetisation is oblique at the injection point. As a result, when performing non-local spin valve measurements we simultaneously observe both switching and spin precession phenomena, implying the spin population possesses both in-plane and out-of-plane polarisation components. By comparing the relative magnitudes of these components, we quantify the angle of the total spin polarisation vector. The extracted angle is consistent with the angle of the nanowire at the graphene interface, evidencing that the effect is a consequence of the device geometry. This simple method of spin-based vector magnetometry provides an alternative technique to define the spin polarisation in 2D spintronic devices.

Efficient and low-energy consumption degradation of bisphenol A in aqueous solutions and its density functional theory calculation

An advanced Fenton technique using urea peroxide (UHP) as an oxidant was studied for the degradation of bisphenol A (BPA) in aqueous solution. Compared to the H2O2/Fe(II) system, the UHP/Fe(II) system performs noticeably better in terms of degradation and requires less catalyst. The mechanism by which UHP exhibits unique and remarkable properties has been elucidated at the molecular level (bond order, AIM analysis, ELF, Spin Population number, electron difference density maps) by density functional theory (DFT) calculations. The ELF and the electron density difference revealed that UHP created an intermediate coordination compound with six-coordinated iron (II), facilitating electron transfer and the disruption of O-O bonds because of the feeble O-O strength, leading to an increase in hydroxyl radicals. Consequently, UHP increases the rate of electron usage and targeting efficiency, which accelerates the breakdown of BPA. targeting efficiency, which accelerates the breakdown of BPA. Based on the intermediates determined by LC-MS/MS, the possible degradation pathway of BPA in the UHP/Fe(II) system were proposed, which was verified to be a lower energy consumption pathway by calculating the reaction energy barriers. It is expected to be an advanced Fenton technology with practical engineering applications potential, due to the advantages of wider pH window, lower energy consumption and higher electron utilization.

Mechanisms of Ligand Hyperfine Coupling in Transition-Metal Complexes: σ and π Transmission Pathways.

Theoretical interpretation of hyperfine interactions was pioneered in the 1950s-1960s by the seminal works of McConnell, Karplus, and others for organic radicals and by Watson and Freeman for transition-metal (TM) complexes. In this work, we investigate a series of octahedral Ru(III) complexes with aromatic ligands to understand the mechanism of transmission of the spin density from the d-orbital of the metal to the s-orbitals of the ligand atoms. Spin densities and spin populations underlying ligand hyperfine couplings are analyzed in terms of π-conjugative or σ-hyperconjugative delocalization vs spin polarization based on symmetry considerations and restricted open-shell vs unrestricted wave function analysis. The transmission of spin density is shown to be most efficient in the case of symmetry-allowed π-conjugative delocalization, but when the π-conjugation is partially or fully symmetry-forbidden, it can be surpassed by σ-hyperconjugative delocalization. Despite a lower spin population of the ligand in σ-hyperconjugative transmission, the hyperfine couplings can be larger because of the direct involvement of the ligand s-orbitals in this delocalization pathway. We demonstrate a quantitative correlation between the hyperfine couplings of aromatic ligand atoms and the characteristics of the metal-ligand bond modulated by the trans substituent, a hyperfine trans effect.

Free radical production induced by visible light in live fruit flies

Visible light triggers free radical production in alive and intact Drosophila melanogaster. We exposed fruit flies to red (613–631 nm), green (515–535 nm), and blue (455–475 nm) light while we monitored changes in unpaired electron content with an electron spin resonance spectrometer (ESR/EPR). The immediate response to light is a rapid increase in spin content lasting approximately 10 s followed by a slower, linear increase for approximately 170 s. When the light is turned off, the spin population promptly decays with a similar time course, though never fully returning to baseline. The magnitude and time course of the spin production depends on the wavelength of the light. Initially, we surmised that eumelanin might be responsible for the spin change because of its documented ability for visible light absorption and its highly stable free radical content. To explore this, we utilized different fruit fly strains with varying eumelanin content and clarified the relation of melanin types with the spin response. Our findings revealed that flies with darker cuticle have at least three-fold more unpaired electrons than flies with yellow cuticle. However, to our surprise, the increase in unpaired electron population by light was not drastically different amongst the genotypes. This suggests that light-induced free radical production may not exclusively rely on the presence of black melanin, but may instead be dependent on light effects on quinone-based cuticular polymers.

Molybdenum(VI) Nitrido Complexes with Tripodal Silanolate Ligands. Structure and Electronic Character of an Unsymmetrical Dimolybdenum μ-Nitrido Complex Formed by Incomplete Nitrogen Atom Transfer.

In contrast to a tungsten nitrido complex endowed with a tripodal silanolate ligand framework, which was reported in the literature to be a dimeric species with a metallacyclic core, the corresponding molybdenum nitrides 3 are monomeric entities comprising a regular terminal nitride unit, as proven by single-crystal X-ray diffraction (SC-XRD). Their electronic character is largely determined by the constraints imposed on the metal center by the podand ligand architecture. 95Mo nuclear magnetic resonance (NMR) and, to a lesser extent, 14N NMR spectroscopy allow these effects to be studied, which become particularly apparent upon comparison with the spectral data of related molybdenum nitrides comprising unrestrained silanolate, alkoxide, or amide ligands. Attempted nitrogen atom transfer from these novel terminal nitrides to [(tBuArN)3Mo] (Ar = 3,5-dimethylphenyl) as the potential acceptor stopped at the stage of unsymmetric dimolybdenum μ-nitrido complex 13a as the first intermediate along the reaction pathway. SC-XRD, NMR, electron paramagnetic resonance, and ultraviolet-visible spectroscopy as well as magnetometry in combination with density functional theory allowed a clear picture of the geometric and electronic structure of this mixed-valent species to be drawn. 13a is formally best described as an adduct of the type [(Mo[O])+III-(μN)-III-(Mo[N])+VI], S = 1/2 complex with (Mo[O])+III in the low-spin configuration, whereas related complexes such as [(AdS)3Mo-(μN)-Mo(NtBuAr)3] (19; Ad = 1-adamantyl) have previously been regarded in the literature as mixed-valent Mo+IV/Mo+V species. The spin population at the two Mo centers is uneven and notably larger at the more reduced Mo[O] atom, whereas the only spin present at the (μN) bridge is derived from spin polarization.

Tandem Electrochemical Oxidation of Polycyclic Aromatic Amines Towards Carbazoles and Phenazine-based Aza-helicenes.

Experimental and theoretical study of the regioselectivity and mechanism of polycyclic aromatic amine (PAA) electrochemical oxidation is important for designing nitrogen doped large π-conjugated functional molecules. Herein, we used binary-, ternary-, and quaternary-fused PAAs as electro-oxidative reaction substrates to investigate the yield changes of carbazole and phenazine based aza-helicene other than oligomers, which were obtained through pyrrole and pyrazine annulation pathways. Combined with the restrained electrostatic potential (RESP) and steric hindrance factor analysis of the substrate, the electron spin density distribution of free radical resonance hybrid and the spin population analysis of the atoms in the structure of each free radical tautomer indicate that the degree of delocalized dispersion of N free radical and the resulting change in the spin density distribution of C free radical tautomers determine the reaction regioselectivity. The potential charge of the K-region, Bay-region, and L-region adjacent to the C(α)-C(β1) bond is higher than that of other regions within the molecule, and the charge in these high RESP regions tends to delocalize more strongly toward electron-deficient N free radicals. Thus, the activity of N-C(α)-C(β1) region is increased, which supports the proposed free radical addition and free radical coupling mechanism for the electro-oxidative reaction of PAA.

Metal-Ligand Exchange Coupling Alters the Open-Shell Ligand Electronic Structure in a Bis(semiquinone) Complex.

The electronic structure of the bis(dioxolene) bridging ligand -SQ2Th2- is responsive to metal-ligand magnetic exchange coupling. Comparison of the crystal structure of (NiSQ)2Th2 to that of (ZnSQ)2Th2 indicates an open-shell biradical ground state for the dinuclear Ni(II) complex compared to the closed-shell quinoidal character found in the dinuclear Zn(II) complex. Consistent with a comparison of bond lengths obtained by X-ray diffraction, the analysis of the variable-temperature magnetic susceptibility data for crystalline (NiSQ)2Th2 yields reduced SQ-SQ radical-radical magnetic exchange coupling (JSQ-SQ = -203 cm-1) compared to that of (ZnSQ)2Th2 (JSQ-SQ = -321 cm-1). The reduced SQ-SQ exchange coupling in (NiSQ)2Th2 derives from an attenuation of the SQ spin densities, which in turn is derived from the Ni-SQ antiferromagnetic exchange interactions. This reduction in SQ--SQ exchange that we observe for (NiSQ)2Th2 correlates with an effective lengthening of the bridge unit by ∼2.1 Å relative to that of (ZnSQ)2Th2. This magnitude of the effective increase in the bridge distance is consistent with the (NiSQ)2Th2 JSQ-SQ value lying between those of (ZnSQ)2Th2 and (ZnSQ)2Th3. The ability to modulate spin populations on an organic radical via pairwise Ni-SQ magnetic exchange interactions is a general way to affect electronic coupling in the Th-Th bridge. Our results suggest that metal-radical exchange coupling represents a powerful mechanism for tuning organic molecular electronic structure, with important implications for molecular electronics and molecular electron transport.

Motional Narrowing Effects in the Excited State Spin Populations of Mn-Doped Hybrid Perovskites.

Spin-orbit coupling in the electronic states of solution-processed hybrid metal halide perovskites forms complex spin-textures in the band structures and allows for optical manipulation of the excited state spin-polarizations. Here, we report that motional narrowing acts on the photoexcited spin-polarization in CH3NH3PbBr3 thin films, which are doped at percentage-level with Mn2+ ions. Using ultrafast circularly polarized broadband transient absorption spectroscopy at cryogenic temperatures, we investigate the spin population dynamics in these doped hybrid perovskites and find that spin relaxation lifetimes are increased by a factor of 3 compared to those of undoped materials. Using quantitative analysis of the photoexcitation cooling processes, we reveal increased carrier scattering rates in the doped perovskites as the fundamental mechanism driving spin-polarization-maintaining motional narrowing. Our work reports transition-metal doping as a concept to extend spin lifetimes of hybrid perovskites.

Spin population determines whether antiaromaticity can increase or decrease radical stability.

Aromaticity, as a classical and fundamental concept in chemistry, can enhance thermodynamic stability. In sharp contrast, a previous study showed that antiaromaticity rather than aromaticity can enhance the radical stability of α-methyl heterocyclic compounds. Here, we demonstrate a similar antiaromaticity-promoted radical stability when the methyl group is replaced by five-membered (alkyl)(amino)cyclics (AACs). More interestingly, when an AAC is fused with an antiaromatic ring, the radical stability could be either reduced or enhanced, depending on the spin population. Specifically, when the spin density is populated on an incoming antiaromatic 1,4-dihydro-1,4-diborinine moiety, the radical stability is enhanced whereas when the spin density is maintained on the original five-membered ring, the radical stability is reduced. Our findings highlight the importance of spin density in tuning the radical stability, inviting experimental chemists' verification.

Dual-ligand quasi-2D perovskites with chiral-induced spin selectivity for room temperature spin-LEDs.

Spin-LEDs have been a central topic in semiconductor spintronics research and represent a promising avenue for advanced optoelectronic devices and applications. The future advancements of spin-LEDs will undoubtedly hinge on the generation and manipulation of spin-polarized population at room temperature. In this research, we elucidate the development of room-temperature spin-LEDs using quasi-2D perovskites, based on the chiral-induced spin selectivity (CISS) effect. During the carrier transfer from the chiral n2 phase to the randomly oriented high-n phase caused by the bandgap gradient distribution, CISS works to generate non-equilibrium spin population, leading to room-temperature spin-polarized fluorescence. A spin-polarization of ∼93% is observed for the films. Finally, we realize spin-LEDs at room temperature, exhibiting a |gCP-EL| value of 0.05 and an EQE of 3.8%. This work highlights the potential of integrating dual ligands to optimize the phase distribution and crystalline orientation in quasi-2D films to achieve efficient CISS for spin-LED applications.

Insights into dioxygen binding on metal centers: an ab initio multireference electronic structure analysis.

Why does binding of dioxygen (O2) to metal centers, the initial step of O2 storage, transportation, and activation, almost inevitably induce metal-to-O2 single-electron transfer and generate superoxo (O2-˙) species, instead of genuine O02 adducts? To address this question, this study describes highly correlated wavefunction-based ab initio calculations using CASSCF/NEVPT2 (CASSCF = complete active space self-consistent field, and NEVPT2 = N-electron valence state second-order perturbation theory) approaches to explore the electronic-structure evolution of O2 association on Fe(II)(BDPP) (H2BDPP = 2,6-bis((2-(S)-diphenylhydroxylmethyl-1-pyrrolidinyl)methyl)pyridine) and Co(II)(BDPP) to produce S = 3 Fe(III)(BDPP)(O2-˙) (1) and Co(III)(BDPP)(O2-˙) (2). CASSCF/NEVPT2 calculations suggest that the processes furnishing 1 and 2 feature an avoided crossing resulting from interactions of two diabatic curves, of which one is characterized as Co(II) and Fe(II) centers interacting with a triplet O2 ligand and the other as Co(III) and Fe(III) centers bound to a superoxo ligand. In both cases, the avoided crossing induces a one-electron transfer from the divalent metal center to the incoming O2 ligand and leads to formation of trivalent metal-O2-˙ complexes. To facilitate the interpretation of complicated multireference wavefunctions, we formulated two-fragment spin eigenfunctions utilizing Clebsch-Gordan coefficients (CGCs) to rationalize computed spin populations on the metal centers and the O2 ligand and compared these results with usual valence bonding (VB) analyses. It turns out that both methods give the same results and are complementary to each other. Finally, the limitation of DFT approaches in describing complex electronic structures involving metal-ligand magnetic couplings is delineated.