Spin-selective Electron Transfer Research Articles

Quenching controlled spin exchange interactions and spin selective electron transfer for oxygen evolution reactions

The utilization of magnetic field to enhance electrocatalytic activity has become an effective strategy garnering significant attention in the field of electrocatalysis. Herein, we employ a combination of electrospinning and quenching techniques to successfully synthesize high-entropy oxide (HEO) (FeCoNiCrMn)3O4. The HEO exhibited ferromagnetic properties and further increased the saturation magnetization after the quenching process, primarily due to temperature-mediated magnetic exchange interactions. The OER performance is significantly improved by employing a ferromagnetic ordered catalyst as a spin polarizer in the presence of a magnetic field. The introduction of an exterior magnetic field (∼130 mT) significantly improved the OER performance of HEO-LN (HEO quenching in liquid nitrogen), reducing the overpotential by 39.4 mV at 10 mA cm-2. The improved OER performance is due to the presence of the magnetic field-enhanced electron spin polarization, reducing electron repulsion and enhancing orbital hybridization. These findings not only provide crucial insights into understanding reactions involving magnetic field-induced electrochemistry but also contribute to the rational design of electrocatalysts with magnetic effect.

Electron Spin Selective Iridium Electrocatalysts for the Oxygen Evolution Reaction.

Highly efficient electrocatalysts for water electrolysis are crucial to the widespread commercialization of the technology and an important step forward toward a sustainable energy future. In this study, an alternative method for boosting the electrocatalytic activity toward the oxygen evolution reaction (OER) of a well-known electrocatalyst (iridium) is presented. Iridium nanoparticles (2.1 ± 0.2 nm in diameter) functionalized with chiral molecules were found to markedly enhance the activity of the OER when compared to unfunctionalized and achiral functionalized iridium nanoparticles. At a potential of 1.55 V vs Reference Hydrogen Electrode (RHE), chiral functionalized iridium nanoparticles exhibited an average 85% enhancement in activity with respect to unfunctionalized iridium nanoparticles compared to an average 13% enhancement for the achiral functionalized iridium nanoparticle. This activity enhancement is attributed to a spin-selective electron transfer mechanism taking place on the chiral functionalized catalysts, a characteristic induced by the chirality of the ligand. This alternative path for the OER drastically reduces the production of hydrogen peroxide, which was confirmed via a colorimetric method.

Coupling ferromagnetic ordering electron transfer channels and surface reconstructed active species for spintronic electrocatalysis of water oxidation

Sluggish reaction kinetics of oxygen evolution reaction (OER), resulting from multistep proton-coupled electron transfer and spin constriction, limits overall efficiency for most reported catalysts. Herein, using modeled ZnFe2−xNixO4 (0 ≤ x ≤ 0.4) spinel oxides, we aim to develop better OER electrocatalyst through combining the construction of ferromagnetic (FM) ordering channels and generation of highly active reconstructed species. The number of symmetry-breaking Fe–O–Ni structure links to the formation of FM ordering electron transfer channels. Meanwhile, as the number of Ni3+ increases, more ligand holes are formed, beneficial for redirecting surface reconstruction. The electro-activated ZnFe1.6Ni0.4O4 shows the highest specific activity, which is 13 and 2.5 times higher than that of ZnFe2O4 and unactivated ZnFe1.6Ni0.4O4, and even superior to the benchmark IrO2 under the overpotential of 350 mV. Applying external magnetic field can make electron spin more aligned, and the activity can be further improved to 39 times of ZnFe2O4. We propose that intriguing FM exchange-field interaction at FM/paramagnetic interfaces can penetrate FM ordering channels into reconstructed oxyhydroxide layers, thereby activating oxyhydroxide layers as spin-filter to accelerate spin-selective electron transfer. This work provides a new guideline to develop highly efficient spintronic catalysts for water oxidation and other spin-forbidden reactions.

Molecular qubits based on photogenerated spin-correlated radical pairs for quantum sensing

Photogenerated spin-correlated radical pairs (SCRPs) in electron donor–bridge–acceptor (D–B–A) molecules can act as molecular qubits and inherently spin qubit pairs. SCRPs can take singlet and triplet spin states, comprising the quantum superposition state. Their synthetic accessibility and well-defined structures, together with their ability to be prepared in an initially pure, entangled spin state and optical addressability, make them one of the promising avenues for advancing quantum information science. Coherence between two spin states and spin selective electron transfer reactions form the foundation of using SCRPs as qubits for sensing. We can exploit the unique sensitivity of the spin dynamics of SCRPs to external magnetic fields for sensing applications including resolution-enhanced imaging, magnetometers, and magnetic switch. Molecular quantum sensors, if realized, can provide new technological developments beyond what is possible with classical counterparts. While the community of spin chemistry has actively investigated magnetic field effects on chemical reactions via SCRPs for several decades, we have not yet fully exploited the synthetic tunability of molecular systems to our advantage. This review offers an introduction to the photogenerated SCRPs-based molecular qubits for quantum sensing, aiming to lay the foundation for researchers new to the field and provide a basic reference for researchers active in the field. We focus on the basic principles necessary to construct molecular qubits based on SCRPs and the examples in quantum sensing explored to date from the perspective of the experimentalist.

Oxygen evolution in spin-sensitive pathways

The overall performance of water electrolysis suffers from the high kinetic barrier in the oxygen evolution reaction (OER) at the anode. Considerable effort has been made on the fundamental understandings of the reaction mechanisms of OER. Recently, the attention has been given to the OER on magnetic catalysts, which is believed being able to promote the kinetics of an OER process from singlet reactants to triplet oxygen. The process in principle involves spin selective electron transfer. Here, we discuss the effects of spin in OER based on the recent advances and summarize our recently proposed mechanisms of the OER in spin-sensitive pathways under the lattice oxygen oxidation mechanism, the interaction of two M−O entity mechanism, and the adsorbate evolution mechanism.

Radiation-Induced Effect on Spin-Selective Electron Transfer through Self-Assembled Monolayers of ds-DNA

Stability of the DNA molecule is essential for the proper functioning and sustainability of all living organisms. In this study, we investigate the effect of gamma radiation (γ-radiation) on spin-selective electron transfer through double strand (ds)DNA molecules. Self-assembled monolayers (SAMs) of 21-base long DNA are prepared on Au-coated Ni thin film. We measure the spin polarization (%) of the SAMs of ds-DNA using the spin-dependent electrochemical technique. We use a Cs-based γ-radiation source to expose the SAMs of ds-DNA immobilized on thin films for various time intervals ranging from 0–30 min. The susceptibility of DNA to γ-radiation is measured by spin-dependent electrochemistry. We observe that the efficiency of spin filtering by ds-DNA gradually decreases when exposure (to γ-radiation) time increases, and drops below 1% after 30 min of exposure. The change in spin polarization value is related either to the conformational perturbation in DNA or to structural damage in DNA molecules caused by ionizing radiation.

Spin Dependent Transport through Driven Magnetic System with Aubry-Andre-Harper Modulation

In this work, we put forward a prescription of achieving spin selective electron transfer by means of light irradiation through a tight-binding (TB) magnetic chain whose site energies are modulated in the form of well known Aubry–Andre–Harper (AAH) model. The interaction of itinerant electrons with local magnetic moments in the magnetic system provides a misalignment between up and down spin channels which leads to a finite spin polarization (SP) upon locating the Fermi energy in a suitable energy zone. Both the energy channels are significantly affected by the irradiation which is directly reflected in degree of spin polarization as well as in its phase. We include the irradiation effect through Floquet ansatz and compute spin polarization coefficient by evaluating transmission probabilities using Green’s function prescription. Our analysis can be utilized to investigate spin dependent transport phenomena in any driven magnetic system with quasiperiodic modulations.

Interaction of spin-correlated radical pair with a third radical: Combined effect of spin-exchange interaction and spin-selective reaction.

The reaction of electron transfer between two paramagnetic particles may be strongly dependent on the total spin state of the pair. Such dependence can be used to control electron transfer in a molecular medium via the control of the spin degrees of freedom. In this work, the spin-selective electron transfer has been studied in a three-spin system composed of a spin-correlated radical ion pair (RIP) and the nitroxide radical, TEMPONE. The RIPs were created in an n-hexane solution of tetramethylpiperidine (TMP) and para-terphenyl (p-TP) using X-rays. To monitor the spin evolution of the RIPs with a nanosecond time resolution, the method of time-resolved magnetic field effect in the RIP recombination fluorescence was applied. It was found that increasing the TEMPONE concentration increased the rate of both the radiation-induced fluorescence intensity decay and the paramagnetic relaxation of the spin-correlated RIP. For the three-spin system studied, we developed a theoretical model to calculate the singlet state population of the spin-correlated RIP that described both the spin-selective reaction and the spin-exchange interaction during an encounter between RIP partners and a third radical. It was found that the effect of the spin exchange could be neglected if the rate of the spin-selective reaction is high enough. Based on quantum chemical calculations and experiments, we found that there was a spin-selective distant electron transfer from p-TP radical anions to the TEMPONE radical. Another partner of the RIP, the radical cation formed from TMP, was only involved in the spin exchange interaction with TEMPONE radicals.

Electron spin relaxation in radical pairs: Beyond the Redfield approximation.

Relaxation processes can have a large effect on the spin selective electron transfer reactions of radical pairs. These processes are often treated using phenomenological relaxation superoperators or with some model for the microscopic relaxation mechanism treated within Bloch-Redfield-Wangsness theory. Here, we demonstrate that an alternative perturbative relaxation theory, based on the Nakajima-Zwanzig equation, has certain advantages over Redfield theory. In particular, the Nakajima-Zwanzig equation does not suffer from the severe positivity problem of Redfield theory in the static disorder limit. Combining the Nakajima-Zwanzig approach consistently with the Schulten-Wolynes semiclassical method, we obtain an efficient method for modeling the spin dynamics of radical pairs containing many hyperfine-coupled nuclear spins. This is then used to investigate the spin-dependent electron transfer reactions and intersystem crossing of dimethyljulolidine-naphthalenediimide (DMJ-NDI) radical ion pairs. By comparing our simulations with experimental data, we find evidence for a field-independent contribution to the triplet quantum yields of these reactions which cannot be explained by electron spin relaxation alone.

Spin-selective electron transfer reactions of radical pairs: Beyond the Haberkorn master equation

Radical pair recombination reactions are normally described using a quantum mechanical master equation for the electronic and nuclear spin density operator. The electron spin state selective (singlet and triplet) recombination processes are described with a Haberkorn reaction term in this master equation. Here we consider a general spin state selective electron transfer reaction of a radical pair and use Nakajima-Zwanzig theory to derive the master equation for the spin density operator, thereby elucidating the relationship between non-adiabatic reaction rate theory and the Haberkorn reaction term. A second order perturbation theory treatment of the diabatic coupling naturally results in the Haberkorn master equation with an additional reactive scalar electron spin coupling term. This term has been neglected in previous spin chemistry calculations, but we show that it will often be quite significant. We also show that beyond the second order in perturbation theory, i.e., beyond the Fermi golden rule limit, an additional reactive singlet-triplet dephasing term appears in the master equation. A closed form expression for the reactive scalar electron spin coupling in terms of the Marcus theory parameters that determine the singlet and triplet recombination rates is presented. By performing simulations of radical pair reactions with the exact hierarchical equations of motion method, we demonstrate that our master equations provide a very accurate description of radical pairs undergoing spin-selective non-adiabatic electron transfer reactions. The existence of a reactive electron spin coupling may well have implications for biologically relevant radical pair reactions such as those which have been suggested to play a role in avian magnetoreception.

Magnetless Device for Conducting Three‐Dimensional Spin‐Specific Electrochemistry

AbstractElectron spin states play an important role in many chemical processes. Most spin‐state studies require the application of a magnetic field. Recently it was found that the transport of electrons through chiral molecules also depends on their spin states and may also play a role in enantiorecognition. Electrochemistry is an important tool for studying spin‐specific processes and enantioseparation of chiral molecules. A new device is presented, which serves as the working electrode in electrochemical cells and is capable of providing information on the correlation of spin selectivity and the electrochemical process. The device is based on the Hall effect and it eliminates the need to apply an external magnetic field. Spin‐selective electron transfer through chiral molecules can be monitored and the relationship between the enantiorecognition process and the spin of electrons elucidated.

Magnetless Device for Conducting Three‐Dimensional Spin‐Specific Electrochemistry

Electron spin states play an important role in many chemical processes. Most spin-state studies require the application of a magnetic field. Recently it was found that the transport of electrons through chiral molecules also depends on their spin states and may also play a role in enantiorecognition. Electrochemistry is an important tool for studying spin-specific processes and enantioseparation of chiral molecules. A new device is presented, which serves as the working electrode in electrochemical cells and is capable of providing information on the correlation of spin selectivity and the electrochemical process. The device is based on the Hall effect and it eliminates the need to apply an external magnetic field. Spin-selective electron transfer through chiral molecules can be monitored and the relationship between the enantiorecognition process and the spin of electrons elucidated.

The sensitivity of a radical pair compass magnetoreceptor can be significantly amplified by radical scavengers

Birds have a remarkable ability to obtain navigational information from the Earth’s magnetic field. The primary detection mechanism of this compass sense is uncertain but appears to involve the quantum spin dynamics of radical pairs formed transiently in cryptochrome proteins. We propose here a new version of the current model in which spin-selective recombination of the radical pair is not essential. One of the two radicals is imagined to react with a paramagnetic scavenger via spin-selective electron transfer. By means of simulations of the spin dynamics of cryptochrome-inspired radical pairs, we show that the new scheme offers two clear and important benefits. The sensitivity to a 50 μT magnetic field is greatly enhanced and, unlike the current model, the radicals can be more than 2 nm apart in the magnetoreceptor protein. The latter means that animal cryptochromes that have a tetrad (rather than a triad) of tryptophan electron donors can still be expected to be viable as magnetic compass sensors. Lifting the restriction on the rate of the spin-selective recombination reaction also means that the detrimental effects of inter-radical exchange and dipolar interactions can be minimised by placing the radicals much further apart than in the current model.

Spin Chemistry: Coherent Spin Dynamics Rules Chemical Reactions.

It is a great honor to edit a special issue of Applied Magnetic Resonance, in particular if the area presented is not magnetic resonance spectroscopy itself. The topic of this special issue, spin chemistry [1], can be considered as upspring of magnetic resonance spectroscopy. On the other hand, the field of spin chemistry contributed significantly to the methodological improvement of both NMR and EPR. Indeed, spin chemistry was born with the discovery of the CIDEP (chemically induced dynamic electron polarization) effect by EPR spectroscopy in 1963 [2], followed soon by the observation of CIDNP (chemically induced dynamic nuclear polarization) in chemical dark [3–6] and light [7] reactions. The explanation of these phenomena was presented as so-called ‘radical pair mechanism’ (RPM) in 1969 [8, 9]. That revolutionary reaction scheme allowed for the first time nuclear spin states to decide on the fate of a chemical reaction. Since reaction enthalpies are dramatically larger than spin energies, such kinetic control mechanism was a big surprise, in particular for chemists. Soon magnetic field effects (MFEs) were observed on chemical reaction systems [10, 11] and discussed in particular on photosynthetic systems [12–14] which led to the theory of spin-correlated radical pairs [15, 16]. Upon discovery of the solid-state photo-CIDNP effect [17], the concept of spin-correlated radical pairs was extended to solid-state conditions by introduction of coherent anisotropic interactions [18–23] and to natural low-field conditions by allowing for S–T mixing [24]. Such coherent spin dynamics at natural conditions might explain the persistence of the solid-state photo-CIDNP effect in natural photosynthesis [25]. Another hot field in which spin-chemical research aims for understanding of natural phenomena is dedicated to light-dependent magnetosensing by animals via a RPM [26–28]. In this special issue of Applied Magnetic Resonance, a range of spin-chemical effects on chemical and biological electron transfer systems and complementary experimental methods, i.e. EPR and NMR, is reported. First, spin-chemical effects in chemical systems are presented: Basu and coworkers [29] observe an MFE on photoinduced electron transfer reactions between three aromatic amines and acridone in ethanol at low fields. Joonwichien et al. [30] describe MFEs on the photodegeneration of methylene blue dye. Bagryanskaya et al. [31] study spin-selective excited state relaxation and reactivity of deoxybenzoin (DOB) and benzophenon (BP) molecules by time-resolved EPR spectroscopy. Van der Est and coworkers [32] discuss the time-resolved EPR signature of spin-selective electron transfer in series of metalloporphyrin pyridyl-linked naphthalene diimides. Second, spin-chemical effects on biological systems are described: Alia and coworkers [33] present photo-CIDNP data on photosystem I of plants. Hirayama et al. [34] discuss whether MFEs affect the light-dependent circadian clock in zebrafish. Third, an improved technique in solid-state NMR, a main tool in spin chemistry, is presented. Song et al. [35] show a new technique allowing for improvement of proton resolution in proteins. Hence, the present issue demonstrates that nowadays spin chemistry is a matured and lively field with increasing impact on both, magnetic resonance spectroscopy as well as general chemistry. I express my gratitude to the authors who have contributed to this special issue. I am also grateful to Prof. K. Salikhov and Dr. L. Mosina. Without their efforts, the publication of this special issue would not have been possible.

Spin-Selective Electron Transfer and Charge Recombination in Self-Assembled Porphyrin Naphthalenediimide Dyads

The spin selectivity of electron transfer in a series of metalloporphyrin pyridyl-linked naphthalenediimides (MTPP-Pyr(CH2)nNDI, where M = Zn, n = 2, 4, 7, and M = Al(OCOPh), n = 7) is studied by time-resolved electron paramagnetic resonance (TREPR) spectroscopy in the nematic liquid crystal 4-cyano-4′-pentylbiphenyl (5CB). Following pulsed laser excitation, all of the complexes show a narrow antiphase doublet that is assigned to the triplet state of the radical pair MTPP•+NDI•−. Initially, the antiphase doublet has an emission/absorption (E/A) polarization pattern characteristic of singlet electron transfer. At later times the polarization inverts to an A/E pattern. The intensity of the late signal depends very strongly on the nature of the metal in the porphyrin. A qualitative model that rationalizes this result is presented. It is proposed that both singlet and triplet electron transfer occur in the dyads and that the differences in the intensity of the polarization are the result of differences in the spin selectivity of intersystem crossing for the different metals. The consequences of this model for magnetic field effects in such systems are briefly discussed.

On the electron spin polarization observed in TREPR experiments involving hydroxyl and sulfate radicals

Hydrogen peroxide and the persulfate dianion both undergo photochemical cleavage reactions in aqueous solution, creating a pair of hydroxyl (HO•) or sulfate anion () radicals, respectively. These radicals can abstract H-atoms or undergo electron transfer reactions in the presence of organic substrates, leading to carbon-centred free radicals. The chemically induced electron spin polarization (CIDEP) pattern created during time-resolved electron paramagnetic resonance (TREPR) experiments involving these species is always observed to be low field emissive/high field absorptive (E/A), coupled with net absorption. There are numerous suggestions in the literature regarding the origin of the E/A* pattern: Contributions from thermally equilibrated radicals, concentration-dependent spin relaxation, or the result of a spin–selective electron transfer reaction to create closed shell ionic structures. Here, it is proposed that E/A and E/A* contributions arise from two different F-pairs that must be present in these solutions. Examples are presented and the origin of the effect is explained in terms of well-established CIDEP theories for freely diffusing radical pairs. The importance of the A* contribution is discussed with regard to the recently reported observation of peroxyl radicals from addition of oxygen to amino acid radicals at room temperature.

Cidnp study of reactions of ketyls and dianions derived from benzophenone and fluorenone with acetic anhydride

A study is presented of 1H and 13C CIDNP effects in the reactions of Na and Li salts of ketyls and dianions derived from benzophenone and fluorenone with acetic anhydride in tetrahydrofuran and dimethoxyethane. CIDNP effects were observed for products obtained by mixing of reactants both at high and at low field. Interpretation of the CIDNP effects (mixing at high field) indicates that in reactions of ketyls with acetic anhydride the primary step is O-acylation followed by spin-selective electron transfer between ketyl and O-acylated ketyl. At higher dilution or in the presence of strongly coordinating agents, heterolytic deprotonation of acetic anhydride by ketyl is also observed. CIDNP effects depend on the presence of ketyl which affects the relaxation of sterically accessible nuclei and also suppresses the intensity of ketone signals by rapid electron transfer. In reactions of dianions, electron transfer between dianion and acetic anhydride partly takes place, and the ketyl formed in this way reacts with a further molecule of acetic anhydride.