Spin Signature of the C₆₀ Fullerene Anion: A Combined X- and D‑Band EPR and DFT Study

Fullerenes attract much attention in various scientific fields, but their electronic properties are still not completely understood. Here we report on a combined EPR and DFT study of the fullerene anion C₆₀^– in solid glassy environment. DFT calculations were used to characterize its electronic structure through spin density distribution and magnetic resonance parameters. The electron spin density is not uniformly distributed throughout the C₆₀^– cage but shows a pattern similar to PC₆₁BM^–. EPR spectroscopy reveals a rhombic g-tensor sensitive to the environment in the frozen glassy solutions, which can be rationalized by deformation of the fullerenes along low-frequency vibrational modes upon cooling. DFT modeling confirms that these deformations lead to variation in the C₆₀^– <i>g</i> values. The decrease in g-tensor anisotropy with sample annealing is related to the lessening of g-tensor strain upon temperature relaxation of the most distorted sites in the glassy state

Millisecond Coherence Time in a Tunable Molecular Electronic Spin Qubit

Quantum information processing (QIP) could revolutionize areas ranging from chemical modeling to cryptography. One key figure of merit for the smallest unit for QIP, the qubit, is the coherence time (<i>T</i>₂), which establishes the lifetime for the qubit. Transition metal complexes offer tremendous potential as tunable qubits, yet their development is hampered by the absence of synthetic design principles to achieve a long <i>T</i>₂. We harnessed molecular design to create a series of qubits, (Ph₄P)₂[V­(C₈S₈)₃] (<b>1</b>), (Ph₄P)₂[V­(β-C₃S₅)₃] (<b>2</b>), (Ph₄P)₂[V­(α-C₃S₅)₃] (<b>3</b>), and (Ph₄P)₂[V­(C₃S₄O)₃] (<b>4</b>), with <i>T</i>₂s of 1–4 μs at 80 K in protiated and deuterated environments. Crucially, through chemical tuning of nuclear spin content in the vanadium­(IV) environment we realized a <i>T</i>₂ of ∼1 ms for the species (<i>d</i>₂₀-Ph₄P)₂[V­(C₈S₈)₃] (<b>1</b>′) in CS₂, a value that surpasses the coordination complex record by an order of magnitude. This value even eclipses some prominent solid-state qubits. Electrochemical and continuous wave electron paramagnetic resonance (EPR) data reveal variation in the electronic influence of the ligands on the metal ion across <b>1</b>–<b>4</b>. However, pulsed measurements indicate that the most important influence on decoherence is nuclear spins in the protiated and deuterated solvents utilized herein. Our results illuminate a path forward in synthetic design principles, which should unite CS₂ solubility with nuclear spin free ligand fields to develop a new generation of molecular qubits

Multiple Quantum Coherences from Hyperfine Transitions in a Vanadium(IV) Complex

We report a vanadium complex in a nuclear-spin free ligand field that displays two key properties for an ideal candidate qubit system: long coherence times that persist at high temperature, <i>T</i>₂ = 1.2 μs at 80 K, and the observation of quantum coherences from multiple transitions. The electron paramagnetic resonance (EPR) spectrum of the complex [V­(C₈S₈)₃]^2– displays multiple transitions arising from a manifold of states produced by the hyperfine coupling of the <i>S</i> = ¹/₂ electron spin and <i>I</i> = ⁷/₂ nuclear spin. Transient nutation experiments reveal Rabi oscillations for multiple transitions. These observations suggest that each pair of hyperfine levels hosted within [V­(C₈S₈)₃]^2– are candidate qubits. The realization of multiple quantum coherences within a transition metal complex illustrates an emerging method of developing scalability and addressability in electron spin qubits. This study presents a rare molecular demonstration of multiple Rabi oscillations originating from separate transitions. These results extend observations of multiple quantum coherences from prior reports in solid-state compounds to the new realm of highly modifiable coordination compounds

Electron Paramagnetic Resonance Characterization of the Triheme Cytochrome from <i>Geobacter sulfurreducens</i>

Periplasmic cytochrome A (PpcA) is a representative of a broad class of multiheme cytochromes functioning as protein “nanowires” for storage and extracellular transfer of multiple electrons in the δ-proteobacterium <i>Geobacter sulfurreducens</i>. PpcA contains three bis-His coordinated hemes held in a spatial arrangement that is highly conserved among the multiheme cytochromes c₃ and c₇ families, carries low potential hemes, and is notable for having one of the lowest number of amino acids utilized to maintain a characteristic protein fold and site-specific heme function. Low temperature X-band electron paramagnetic resonance (EPR) spectroscopy has been used to characterize the electronic configuration of the Fe­(III) and the ligation mode for each heme. The three sets of EPR signals are assigned to individual hemes in the three-dimensional crystal structure. The relative energy levels of the Fe­(III) 3d orbitals for individual hemes were estimated from the principal <i>g</i>-values. The observed <i>g</i>-tensor anisotropy was used as a probe of electronic structure of each heme, and differences were determined by specifics of axial ligation. To ensure unambiguous assignment of highly anisotropic low-spin (HALS) signal to individual hemes, EPR analyses of iron atom electronic configurations have been supplemented with investigation of porphyrin macrocycles by one-dimensional ¹H NMR chemical shift patterns for the methyl substituents. Within optimized geometry of hemes in PpcA, the magnetic interactions between hemes were found to be minimal, similar to the c₃ family of tetraheme cytochromes

Pulse Q‑Band EPR and ENDOR Spectroscopies of the Photochemically Generated Monoprotonated Benzosemiquinone Radical in Frozen Alcoholic Solution

Quinones are essential cofactors in many physiological processes, among them proton-coupled electron transfer (PCET) in photosynthesis and respiration. A key intermediate in PCET is the monoprotonated semiquinone radical. In this work we produced the monoprotonated benzosemiquinone (BQH^•) by UV illumination of BQ dissolved in 2-propanol at cryogenic temperatures and investigated the electronic and geometric structures of BQH^• in the solid state (80 K) using EPR and ENDOR techniques at 34 GHz. The <i>g</i>-tensor of BQH^• was found to be similar to that of the anionic semiquinone species (BQ^•–) in frozen solution. The peaks present in the ENDOR spectrum of BQH^• were identified and assigned by ¹H/²H substitutions. The experiments reconfirmed that the hydroxyl proton (O–H) on BQH^•, which is abstracted from a solvent molecule, mainly originates from the central CH group of 2-propanol. They also showed that the protonation has a strong impact on the electron spin distribution over the quinone. This is reflected in the hyperfine couplings (hfc’s) of the ring protons, which dramatically changed with respect to those typically observed for BQ^•–. The hfc tensor of the O–H proton was determined by a detailed orientation-selection ENDOR study and found to be rhombic, resembling those of protons covalently bound to carbon atoms in a π-system (i.e., α-protons). It was found that the O–H bond lies in the quinone plane and is oriented along the direction of the quinone oxygen lone pair orbital. DFT calculations were performed on different structures of BQH^• coordinated by four, three, or zero 2-propanol molecules. The O–H bond length was found to be around 1.0 Å, typical for a single covalent O–H bond. Good agreement between experimental and DFT results were found. This study provides a detailed picture of the electronic and geometric structures of BQH^• and should be applicable to other naturally occurring quinones

Electronic Structure of Fullerene Acceptors in Organic Bulk-Heterojunctions: A Combined EPR and DFT Study

Organic photovoltaic (OPV) devices are a promising alternative energy source. Attempts to improve their performance have focused on the optimization of electron-donating polymers, while electron-accepting fullerenes have received less attention. Here, we report an electronic structure study of the widely used soluble fullerene derivatives PC₆₁BM and PC₇₁BM in their singly reduced state, that are generated in the polymer:fullerene blends upon light-induced charge separation. Density functional theory (DFT) calculations characterize the electronic structures of the fullerene radical anions through spin density distributions and magnetic resonance parameters. The good agreement of the calculated magnetic resonance parameters with those determined experimentally by advanced electron paramagnetic resonance (EPR) allows the validation of the DFT calculations. Thus, for the first time, the complete set of magnetic resonance parameters including directions of the principal <i>g</i>-tensor axes were determined. For both molecules, no spin density is present on the PCBM side chain, and the axis of the largest <i>g</i>-value lies along the PCBM molecular axis. While the spin density distribution is largely uniform for PC₆₁BM, it is not evenly distributed for PC₇₁BM

Charge Separation in P3HT:SWCNT Blends Studied by EPR: Spin Signature of the Photoinduced Charged State in SWCNT

Single-wall carbon nanotubes (SWCNTs) could be employed in organic photovoltaic (OPV) devices as a replacement or additive for currently used fullerene derivatives, but significant research remains to explain fundamental aspects of charge generation. Electron paramagnetic resonance (EPR) spectroscopy, which is sensitive only to unpaired electrons, was applied to explore charge separation in P3HT:SWCNT blends. The EPR signal of the P3HT positive polaron increases as the concentration of SWCNT acceptors in a photoexcited P3HT:SWCNT blend is increased, demonstrating long-lived charge separation induced by electron transfer from P3HT to SWCNTs. An EPR signal from reduced SWCNTs was not identified in blends due to the free and fast-relaxing nature of unpaired SWCNT electrons as well as spectral overlap of this EPR signal with the signal from positive P3HT polarons. However, a weak EPR signal was observed in chemically reduced SWNTs, and the <i>g</i> values of this signal are close to those of C₇₀-PCBM anion radical. The anisotropic line shape indicates that these unpaired electrons are not free but instead localized

In the Bottlebrush Garden: The Structural Aspects of Coordination Polymer Phases formed in Lanthanide Extraction with Alkyl Phosphoric Acids

Coordination polymers (CPs) of metal ions are central to a large variety of applications, such as catalysis and separations. These polymers frequently occur as amorphous solids that segregate from solution. The structural aspects of this segregation remain elusive due to the dearth of the spectroscopic techniques and computational approaches suitable for probing such systems. Therefore, there is a lacking of understanding of how the molecular building blocks give rise to the mesoscale architectures that characterize CP materials. In this study we revisit a CP phase formed in the extraction of trivalent lanthanide ions by diesters of the phosphoric acid, such as the bis­(2-ethylhexyl)­phosphoric acid (HDEHP). This is a well-known system with practical importance in strategic metals refining and nuclear fuel reprocessing. A CP phase, referred to as a “third phase”, has been known to form in these systems for half a century, yet the structure of the amorphous solid is still a point of contention, illustrating the difficulties faced in characterizing such materials. In this study, we follow a deductive approach to solving the molecular structure of amorphous CP phases, using semiempirical calculations to set up an array of physically plausible models and then deploying a suite of experimental techniques, including optical, magnetic resonance, and X-ray spectroscopies, to consecutively eliminate all but one model. We demonstrate that the “third phase” consists of hexagonally packed linear chains in which the lanthanide ions are connected by three O–P–O bridges, with the modifying groups protruding outward, as in a bottlebrush. The tendency to yield linear polynuclear oligomers that is apparent in this system may also be present in other systems yielding the “third phase”, demonstrating how molecular geometry directs polymeric assembly in hybrid materials. We show that the packing of bridging molecules is central to directing the structure of CP phases and that by manipulating the steric requirements of ancillary groups one can control the structure of the assembly

Protein Delivery of a Ni Catalyst to Photosystem I for Light-Driven Hydrogen Production

The direct conversion of sunlight into fuel is a promising means for the production of storable renewable energy. Herein, we use Nature’s specialized photosynthetic machinery found in the Photosystem I (PSI) protein to drive solar fuel production from a nickel diphosphine molecular catalyst. Upon exposure to visible light, a self-assembled PSI-[Ni­(P₂^PhN₂^Ph)₂]­(BF₄)₂ hybrid generates H₂ at a rate 2 orders of magnitude greater than rates reported for photosensitizer/[Ni­(P₂^PhN₂^Ph)₂]­(BF₄)₂ systems. The protein environment enables photocatalysis at pH 6.3 in completely aqueous conditions. In addition, we have developed a strategy for incorporating the Ni molecular catalyst with the native acceptor protein of PSI, flavodoxin. Photocatalysis experiments with this modified flavodoxin demonstrate a new mechanism for biohybrid creation that involves protein-directed delivery of a molecular catalyst to the reducing side of Photosystem I for light-driven catalysis. This work further establishes strategies for constructing functional, inexpensive, earth-abundant solar fuel-producing PSI hybrids that use light to rapidly produce hydrogen directly from water

Charge Separation Related to Photocatalytic H₂ Production from a Ru–Apoflavodoxin–Ni Biohybrid

The direct creation of a fuel from sunlight and water via photochemical energy conversion provides a sustainable method for producing a clean source of energy. Here we report the preparation of a solar fuel biohybrid that embeds a nickel diphosphine hydrogen evolution catalyst into the cofactor binding pocket of the electron shuttle protein, flavodoxin (Fld). The system is made photocatalytic by linking a cysteine residue in Fld to a ruthenium photosensitizer. Importantly, the protein environment enables the otherwise insoluble Ni catalyst to perform photocatalysis in aqueous solution over a pH range of 3.5–12.0, with optimal turnover frequency 410 ± 30 h^–1 and turnover number 620 ± 80 mol H₂/mol hybrid observed at pH 6.2. For the first time, a reversible light-induced charge-separated state involving a Ni­(I) intermediate was directly monitored by electron paramagnetic resonance spectroscopy. Transient optical measurements reflect two conformational states, with a Ni­(I) state formed in ∼1.6 or ∼185 μs that persists for several milliseconds as a long-lived charge-separated state facilitated by the protein matrix