Dual Structure of a Vanadyl-Based Molecular Qubit Containing a Bis(ß-diketonato) Ligand.

We designed [VO(bdhb)] (1') as a new electronic qubit containing an oxovanadium(IV) ion (S = 1/2) embraced by a single bis(ß-diketonato) ligand [H2bdhb = 1,3-bis(3,5-dioxo-1-hexyl)benzene]. The synthesis afforded three different crystal phases, all of which unexpectedly contain dimers with formula [(VO)2(bdhb)2] (1). A trigonal form (1h) with a honeycomb structure and 46% of solvent-accessible voids quantitatively transforms over time into a monoclinic solvatomorph 1m and minor amounts of a triclinic solventless phase (1a). In a static magnetic field, 1h and 1m have detectably slow magnetic relaxation at low temperatures through quantum tunneling and Raman mechanisms. Angle-resolved electron paramagnetic resonance (EPR) spectra on single crystals revealed signatures of low-dimensional magnetic behavior, which is solvatomorph-dependent, being the closest interdimer V···V separations (6.7-7.5 Å) much shorter than intramolecular V···V distances (11.9-12.1 Å). According to 1H diffusion ordered spectroscopy (DOSY) and EPR experiments, the complex adopts the desired monomeric structure in organic solution and its geometry was inferred from density functional theory (DFT) calculations. Spin relaxation measurements in a frozen toluene-d8/CD2Cl2 matrix yielded Tm values reaching 13 µs at 10 K, and coherent spin manipulations were demonstrated by Rabi nutation experiments at 70 K. The neutral quasi-macrocyclic structure, featuring nuclear spin-free donors and additional possibilities for chemical functionalization, makes 1' a new convenient spin-coherent building block in quantum technologies.

A Heterometallic Porphyrin Dimer as a Potential Quantum Gate: Magneto-Structural Correlations and Spin Coherence Properties.

In the development of two-qubit quantum gates, precise control over the intramolecular spin-spin interaction between molecular spin units plays a pivotal role. A weak but measurable exchange coupling is especially important for achieving selective spin addressability that allows controlled manipulation of the computational basis states |00⟩ |01⟩ |10⟩ |11⟩ by microwave pulses. Here, we report the synthesis and Electron Paramagnetic Resonance (EPR) study of a heterometallic meso-meso (m-m) singly-linked VIV O-CuII porphyrin dimer. X-band continuous wave EPR measurements in frozen solutions suggest a ferromagnetic exchange coupling of ca. 8 ⋅ 10-3  cm-1 . This estimation is supported by Density Functional Theory calculations, which also allow disentangling the ferro- and antiferromagnetic contributions to the exchange. Pulsed EPR experiments show that the dimer maintains relaxation times similar to the monometallic CuII porphyrins. The addressability of the two individual spins is made possible by the different g-tensors of VIV and CuII -ions, in contrast to homometallic dimers where tilting of the porphyrin planes plays a key role. Therefore, single-spin addressability in the heterometallic dimer can be maintained even with small tilting angles, as expected when deposited on surface, unlocking the full potential of molecular quantum gates for practical applications.

Singlet-Triplet Energy Inversion in Carbon Nitride Photocatalysts.

Time-resolved EPR (TR-EPR) demonstrates the formation of well-defined spin triplet excitons in carbon nitride. This permits to experimentally probe the extent of the triplet wavefunction which delocalizes over several tri-s-triazine units. Analysis of the temperature dependence of the TR-EPR signal reveals the mobility of the triplet excitons. By employing monochromatic light excitation in the range 430-600 nm, the energy of the spin triplet is estimated to be ≈0.2 eV above the conduction band edge, proving that the triplet exciton lies above the corresponding singlet. Comparison between amorphous and graphitic forms establishes the singlet-triplet inversion as a general feature of carbon nitride materials.

Challenges in the Direct Detection of Chirality-induced Spin Selectivity: Investigation of Foldamer-based Donor-acceptor Dyads.

Over the past two decades, the chirality-induced spin selectivity (CISS) effect was reported in several experiments disclosing a unique connection between chirality and electron spin. Recent theoretical works highlighted time-resolved Electron Paramagnetic Resonance (trEPR) as a powerful tool to directly detect the spin polarization resulting from CISS. Here, we report a first attempt to detect CISS at the molecular level by linking the pyrene electron donor to the fullerene acceptor with chiral peptide bridges of different length and electric dipole moment. The dyads are investigated by an array of techniques, including cyclic voltammetry, steady-state and transient optical spectroscopies, and trEPR. Despite the promising energy alignment of the electronic levels, our multi-technique analysis reveals no evidence of electron transfer (ET), highlighting the challenges of spectroscopic detection of CISS. However, the analysis allows the formulation of guidelines for the design of chiral organic model systems suitable to directly probe CISS-polarized ET.

Carbon Vacancies Steer the Activity in Dual Ni Carbon Nitride Photocatalysis.

The manipulation of carbon nitride (CN) structures is one main avenue to enhance the activity of CN-based photocatalysts. Increasing the efficiency of photocatalytic heterogeneous materials is a critical step toward the realistic implementation of sustainable schemes for organic synthesis. However, limited knowledge of the structure/activity relationship in relation to subtle structural variations prevents a fully rational design of new photocatalytic materials, limiting practical applications. Here, the CN structure is engineered by means of a microwave treatment, and the structure of the material is shaped around its suitable functionality for Ni dual photocatalysis, with a resulting boosting of the reaction efficiency toward many CX (X = N, S, O) couplings. The combination of advanced characterization techniques and first-principle simulations reveals that this enhanced reactivity is due to the formation of carbon vacancies that evolve into triazole and imine N species able to suitably bind Ni complexes and harness highly efficient dual catalysis. The cost-effective microwave treatment proposed here appears as a versatile and sustainable approach to the design of CN-based photocatalysts for a wide range of industrially relevant organic synthetic reactions.

Short-Range Electronic Interactions between Vanadium and Molybdenum in Bimetallic SAPO-5 Catalysts Revealed by Hyperfine Spectroscopy.

Engineering two cooperative sites into a catalyst implies the onset of synergistic effects related to the existence of short-range electronic interactions between two metal components. However, these interactions and the relative structure-property correlations are often difficult to obtain. Here we show that hyperfine spectroscopy has the potential to reveal the presence of V4+-O-Mo6+ linkages assessing the degree of spin density transfer from paramagnetic V4+ species to proximal oxo-bridged Mo6+ metal ions. The dimer species were prepared by adsorption of Mo(CO)6 in the pores of SAPO-5, followed by thermal decomposition and oxidation and subsequent grafting of anhydrous VCl4(g) followed by hydrolysis and dehydration. The metal species react with SAPO protons during the exchange process and generate new Lewis acid sites, which act as redox centers. X- and Q-band EPR and HYSCORE experiments have been employed to monitor the local environment of V4+ species obtaining direct evidence for spin delocalization over 27Al, 31P, 95Mo, and 97Mo nuclei, demonstrating the presence of bimetallic V-O-Mo well-defined structures.

VdW Mediated Strong Magnetic Exchange Interactions in Chains of Hydrogen-Free Sublimable Molecular Qubits.

Sulfur-rich molecular complexes of dithiolene-like ligands are appealing candidates as molecular spin qubits because spin coherence properties are enhanced in hydrogen-free environments. Herein, we employ the hydrogen-free mononegative 1,3,2-dithiazole-4-thione-5-thiolate (dttt-) ligand as an alternative to common dinegative dithiolate ligands. We report the first synthesis and structural characterization of its Cu2+, Ni2+, and Pt2+ neutral complexes. The XPS analysis of thermal deposition of [Cu(dttt)2] in UHV conditions indicates that films of intact molecules can be deposited on surfaces by sublimation. Thanks to a combined approach employing DC magnetometry and DFT calculations, we highlighted AF exchange interactions of 108 cm-1 and 36 cm-1 attributed to the two different polymorph phases. These couplings are exclusively mediated by S···S VdW interactions, which are facilitated by the absence of counterions and made particularly efficient by the diffuse electron density on S atoms. Furthermore, the spin dynamics of solid-state magnetically diluted samples was investigated. The longest observed T m is 2.3 µs at 30 K, which significantly diverges from the predicted T m > 100 µs. These results point to the diluting matrix severely affecting the coherence lifetime of Cu2+ species via different factors, such as the contributions of neighboring 14N nuclei and the formation of radical impurities in a non-completely controllable way. However, the ease of processing [Cu(dttt)2] via thermal sublimation can allow dispersion in matrices better suited for coherent spin manipulation of isolated molecules and the realization of AF-coupled VdW structures on surfaces.

Quantum spin coherence and electron spin distribution channels in vanadyl-containing lantern complexes.

We herein investigate the heterobimetallic lantern complexes [PtVO(SOCR)4] as charge neutral electronic qubits based on vanadyl complexes (S = 1/2) with nuclear spin-free donor atoms. The derivatives with R = Me (1) and Ph (2) give highly resolved X-band EPR spectra in frozen CH2Cl2/toluene solution, which evidence the usual hyperfine coupling with the 51V nucleus (I = 7/2) and an additional superhyperfine interaction with the I = 1/2 nucleus of the 195Pt isotope (natural abundance ca. 34%). DFT calculations ascribe the spin density delocalization on the Pt2+ ion to a combination of π and δ pathways, with the former representing the predominant channel. Spin relaxation measurements in frozen CD2Cl2/toluene-d8 solution between 90 and 10 K yield Tm values (1-6 µs in 1 and 2-11 µs in 2) which compare favorably with those of known vanadyl-based qubits in similar matrices. Coherent spin manipulations indeed prove possible at 70 K, as shown by the observation of Rabi oscillations in nutation experiments. The results indicate that the heavy Group 10 metal ion is not detrimental to the coherence properties of the vanadyl moiety and that Pt-VO lanterns can be used as robust spin-coherent building blocks in materials science and quantum technologies.

Single Metal Atoms on Oxide Surfaces: Assessing the Chemical Bond through 17O Electron Paramagnetic Resonance.

ConspectusEven in the gas phase single atoms possess catalytic properties, which can be crucially enhanced and modulated by the chemical interaction with a solid support. This effect, known as electronic metal-support interaction, encompasses charge transfer, orbital overlap, coordination structure, etc., in other words, all the crucial features of the chemical bond. These very features are the object of this Account, with specific reference to open-shell (paramagnetic) single metal atoms or ions on oxide supports. Such atomically dispersed species are part of the emerging class of heterogeneous catalysts known as single-atom catalysts (SACs). In these materials, atomic dispersion ensures maximum atom utilization and uniform active sites, whereby the nature of the chemical interaction between the metal and the oxide surface modulates the catalytic activity of the metal active site by tuning the energy of the frontier orbitals. A comprehensive set of examples includes fourth period metal atoms and ions in zeolites on insulating (e.g., MgO) or reducible (e.g., TiO2) oxides and are among the most relevant catalysts for a wealth of key processes of industrial and environmental relevance, from the abatement of NOx to the selective oxidation of hydrocarbons and the conversion of methane to methanol.There exist several spectroscopic techniques able to inform on the geometric and electronic structure of isolated single metal ion sites, but either they yield information averaged over the bulk or they lack description of the intimate features of chemical bonding, which include covalency, ionicity, electron and spin delocalization. All of these can be recovered at once by measuring the magnetic interactions between open-shell metals and the surrounding nuclei with Electron Paramagnetic Resonance (EPR) spectroscopy. In the case of oxides, this entails the synthesis of 17O isotopically enriched materials. We have established 17O EPR as a unique source of information about the local binding environment around oxygen of magnetic atoms or ions on different oxidic supports to rationalize structure-property relationships. Here, we will describe strategies for 17O surface enrichments and approaches to monitor the state of charge and spin delocalization of atoms or ions from K to Zn dispersed on oxide surfaces characterized by different chemical properties (i.e., basicity or reducibility). Emphasis is placed on chemical insight at the atomic-scale level achieved by 17O EPR, which is a crucial step in understanding the structure-property relationships of single metal atom catalysts and in enabling efficient design of future materials for a range of end uses.

Direct detection of spin polarization in photoinduced charge transfer through a chiral bridge.

It is well assessed that the charge transport through a chiral potential barrier can result in spin-polarized charges. The possibility of driving this process through visible photons holds tremendous potential for several aspects of quantum information science, e.g., the optical control and readout of qubits. In this context, the direct observation of this phenomenon via spin-sensitive spectroscopies is of utmost importance to establish future guidelines to control photo-driven spin selectivity in chiral structures. Here, we provide direct proof that time-resolved electron paramagnetic resonance (EPR) can be used to detect long-lived spin polarization generated by photoinduced charge transfer through a chiral bridge. We propose a system comprising CdSe quantum dots (QDs), as a donor, and C60, as an acceptor, covalently linked through a saturated oligopeptide helical bridge (χ) with a rigid structure of ∼10 Å. Time-resolved EPR spectroscopy shows that the charge transfer in our system results in a C60 radical anion, whose spin polarization maximum is observed at longer times with respect to that of the photogenerated C60 triplet state. Notably, the theoretical modelling of the EPR spectra reveals that the observed features may be compatible with chirality-induced spin selectivity, but the electronic features of the QD do not allow the unambiguous identification of the CISS effect. Nevertheless, we identify which parameters need optimization for unambiguous detection and quantification of the phenomenon. This work lays the basis for the optical generation and direct manipulation of spin polarization induced by chirality.

Morphology and Light-Dependent Spatial Distribution of Spin Defects in Carbon Nitride.

Carbon nitride (CN) is a heterogeneous photocatalyst that combines good structural properties and a broad scope. The photocatalytic efficiency of CN is associated with the presence of defective and radical species. An accurate description of defective states-both at a local and extended level-is key to develop a thorough mechanistic understanding of the photophysics of CN. In turn, this will maximise the generation and usage of photogenerated charge carriers and minimise wasteful charge recombination. Here the influence of morphology and light-excitation on the number and chemical nature of radical defects is assessed. By exploiting the magnetic dipole-dipole coupling, the spatial distribution of native radicals in CN is derived with high precision. From the analysis an average distance in the range 1.99-2.34 nm is determined, which corresponds to pairs of radicals located approximately four tri-s-triazine units apart.

Charge transfer modulation in charge transfer co-crystals driven by crystal structure morphology.

The electronic properties of a charge-transfer (donor-acceptor) semiconducting organic co-crystal, Perylene:F4-TCNQ (PE:F4) (the donor, D, is PE and the acceptor, A, is 2,3,5,6-tetrafluoro-7,7,8,8 tetracyanoquinodimethane (F4)) in its 3 : 2 stoichiometry, are experimentally and theoretically studied. This is performed by means of electron paramagnetic resonance (EPR) and solid state electrochemical techniques, such as cyclic voltammetry (CV) measurements on single crystals. In particular, solid state electrochemistry proves to be an effective tool to probe, on a macroscopic scale, the electronic characteristics of the co-crystal. However, EPR highlights the presence of spin ½ radicals localized on F4 molecules, possibly linked to defects. The experimental findings are discussed on the basis of density functional theory (DFT) based calculations, carried out using both the projector augmented wave (PAW), with "periodic boundary conditions" (pbc), method and the localized orbitals, molecular cluster, approach. In particular, a satisfying agreement is found between the experimental, 0.336 eV (electrochemical), and theoretical, 0.303 eV (PAW), band gaps. Differences with the reported optical bandgap are discussed considering excitonic effects.

The Structure of Monomeric Hydroxo-CuII Species in Cu-CHA. A Quantitative Assessment.

Using EPR and HYSCORE spectroscopies in conjunction with ab initio calculations, we assess the structure of framework-bound monomeric hydroxo-CuII in copper-loaded chabazite (CHA). The species is an interfacial distorted square-planar [CuIIOH(O-8MRs)3] complex located at eight-membered-ring windows, displaying three coordinating bonds with zeolite lattice oxygens and the hydroxo ligand hydrogen-bonded to the cage. The complex has a distinctive EPR signature with g = [2.072 2.072 2.290], CuA= [30 30 410] MHz, and HA = [-13.0 -4.5 +11.5] MHz, distinctively different from other CuII species in CHA.

Nitrogen-Doped Zinc Oxide for Photo-Driven Molecular Hydrogen Production.

Due to its thermal stability, conductivity, high exciton binding energy and high electron mobility, zinc oxide is one of the most studied semiconductors in the field of photocatalysis. However, the wide bandgap requires the use of UV photons to harness its potential. A convenient way to appease such a limitation is the doping of the lattice with foreign atoms which, in turn, introduce localized states (defects) within the bandgap. Such localized states make the material optically active in the visible range and reduce the energy required to initiate photo-driven charge separation events. In this work, we employed a green synthetic procedure to achieve a high level of doping and have demonstrated how the thermal treatment during synthesis is crucial to select specific the microscopic (molecular) nature of the defect and, ultimately, the type of chemistry (reduction versus oxidation) that the material is able to perform. We found that low-temperature treatments produce material with higher efficiency in the water photosplitting reaction. This constitutes a further step in the establishment of N-doped ZnO as a photocatalyst for artificial photosynthesis.

Long-Range Spatial Distribution of Single Aluminum Sites in Zeolites.

How aluminum distributes during synthesis and rearranges after processing within the zeolite framework is a central question in heterogeneous catalysis, as it determines the structure and location of the catalytically active sites of the one of the most important classes of industrial catalysts. Here, exploiting the dipolar interaction between paramagnetic metal ions, we derive the spatial distribution of single aluminum sites within the ZSM-5 zeolite framework in the nanometer range, in polycrystalline samples lacking long-range order. We use a Monte Carlo approach to validate the findings on a pristine ZSM-5 sample and demonstrate that the method is sensitive enough to monitor aluminum redistribution induced in the framework by chemical stress.

An exchange coupled meso-meso linked vanadyl porphyrin dimer for quantum information processing.

We report here the synthesis of a new meso-meso (m-m) singly linked vanadyl-porphyrin dimer that crystallizes in two different pseudo-polymorphs. The single crystal continuous-wave electron paramagnetic resonance investigation evidences a small but crucial isotropic exchange interaction, J, between the two tilted, and thus distinguishable, spin centers of the order of 10-2 cm-1. The experimental and DFT studies evidence a correlation between J values and porphyrin plane tilting angle and distortion. Pulsed EPR analysis shows that the two vanadyl dimers maintain the coherence time of the monomer. With the obtained spin Hamiltonian parameters, we identify suitable transitions that could be used as computational basis states. Our results, coupled with the evaporability of porphyrin systems, establish this class of dimers as extremely promising for quantum information processing applications.

Correction: An exchange coupled meso-meso linked vanadyl porphyrin dimer for quantum information processing.

[This corrects the article DOI: 10.1039/D2SC04969D.].

Zinc oxide hollow spheres decorated with cerium dioxide. The role of morphology in the photoactivity of semiconducting oxides.

The photochemical activity of the recently proposed CeO2-ZnO photocatalytic material active under visible light has been improved by means of significant modifications of its morphology. A polymeric templating agent (Pluronic) has been used in the synthesis obtaining a particle morphology based on hollow spheres that is better defined in the case of high template concentration. The charge separation ability and the light-induced surface electron transfer under irradiation with visible polychromatic light in various ranges of wavelengths has been investigated by electron paramagnetic resonance. The reactivity of the photogenerated holes has been monitored by the spin trapping technique in the presence of DMPO. The hollow spheres morphology achieved through the synthesis here reported leads to systems with a higher photoactivity under visible irradiation than the same system displaying the classic platelets morphology. A parallel increase of the photocatalytic activity of this novel system in pollution remediation reactions is therefore predictable.

17O-EPR determination of the structure and dynamics of copper single-metal sites in zeolites.

The bonding of copper ions to lattice oxygens dictates the activity and selectivity of copper exchanged zeolites. By 17O isotopic labelling of the zeolite framework, in conjunction with advanced EPR methodologies and DFT modelling, we determine the local structure of single site CuII species, we quantify the covalency of the metal-framework bond and we assess how this scenario is modified by the presence of solvating H216O or H217O molecules. This enables to follow the migration of CuII species as a function of hydration conditions, providing evidence for a reversible transfer pathway within the zeolite cage as a function of the water pressure. The results presented in this paper establish 17O EPR as a versatile tool for characterizing metal-oxide interactions in open-shell systems.

Radiofrequency to Microwave Coherent Manipulation of an Organometallic Electronic Spin Qubit Coupled to a Nuclear Qudit.

We report here a comprehensive characterization of a 3d organometallic complex, [V(Cp)2Cl2] (Cp = cyclopentadienyl), which can be considered as a prototypical multilevel nuclear qudit (nuclear spin I = 7/2) hyperfine coupled to an electronic qubit (electronic spin S = 1/2). By combining complementary magnetic resonant techniques, such as pulsed electron paramagnetic resonance (EPR) and broadband nuclear magnetic resonance (NMR), we extensively characterize its Spin Hamiltonian parameters and its electronic and nuclear spin dynamics. Moreover, we demonstrate the possibility to manipulate the qubit-qudit multilevel structure by resonant microwave and radiofrequency pulses, driving coherent Rabi oscillations between targeted electronuclear states. The obtained results demonstrate that this simple complex is a promising candidate for quantum computing applications.