EPR Spin-Trapping for Monitoring Temporal Dynamics of Singlet Oxygen during Photoprotection in Photosynthesis.

A central goal of photoprotective energy dissipation processes is the regulation of singlet oxygen (1O2*) and reactive oxygen species in the photosynthetic apparatus. Despite the involvement of 1O2* in photodamage and cell signaling, few studies directly correlate 1O2* formation to nonphotochemical quenching (NPQ) or lack thereof. Here, we combine spin-trapping electron paramagnetic resonance (EPR) and time-resolved fluorescence spectroscopies to track in real time the involvement of 1O2* during photoprotection in plant thylakoid membranes. The EPR spin-trapping method for detection of 1O2* was first optimized for photosensitization in dye-based chemical systems and then used to establish methods for monitoring the temporal dynamics of 1O2* in chlorophyll-containing photosynthetic membranes. We find that the apparent 1O2* concentration in membranes changes throughout a 1 h period of continuous illumination. During an initial response to high light intensity, the concentration of 1O2* decreased in parallel with a decrease in the chlorophyll fluorescence lifetime via NPQ. Treatment of membranes with nigericin, an uncoupler of the transmembrane proton gradient, delayed the activation of NPQ and the associated quenching of 1O2* during high light. Upon saturation of NPQ, the concentration of 1O2* increased in both untreated and nigericin-treated membranes, reflecting the utility of excess energy dissipation in mitigating photooxidative stress in the short term (i.e., the initial ∼10 min of high light).

EPR studies of ferredoxin in spinach and cyanobacterial thylakoids related to photosystem I-driven NADP+ reduction.

Photosynthetic light-dependent reactions occur in thylakoid membranes where embedded proteins capture light energy and convert it to chemical energy in the form of ATP and NADPH for use in carbon fixation. One of these integral membrane proteins is Photosystem I (PSI). PSI catalyzes light-driven transmembrane electron transfer from plastocyanin (Pc) to oxidized ferredoxin (Fd). Electrons from reduced Fd are used by the enzyme ferredoxin-NADP+ reductase (FNR) for the reduction of NADP+ to NADPH. Fd and Pc are both small soluble proteins whereas the larger FNR enzyme is associated with the membrane. To investigate electron shuttling between these diffusible and embedded proteins, thylakoid photoreduction of NADP+ was studied. As isolated, both spinach and cyanobacterial thylakoids generate NADPH upon illumination without extraneous addition of Fd. These findings indicate that isolated thylakoids either (i) retain a "pool" of Fd which diffuses between PSI and membrane bound FNR or (ii) that a fraction of PSI is associated with Fd, with the membrane environment facilitating PSI-Fd-FNR interactions which enable multiple turnovers of the complex with a single Fd. To explore the functional association of Fd with PSI in thylakoids, electron paramagnetic resonance (EPR) spectroscopic methodologies were developed to distinguish the signals for the reduced Fe-S clusters of PSI and Fd. Temperature-dependent EPR studies show that the EPR signals of the terminal [4Fe-4S] cluster of PSI can be distinguished from the [2Fe-2S] cluster of Fd at > 30 K. At 50 K, the cw X-band EPR spectra of cyanobacterial and spinach thylakoids reduced with dithionite exhibit EPR signals of a [2Fe-2S] cluster with g-values gx = 2.05, gy = 1.96, and gz = 1.89, confirming that Fd is present in thylakoid preparations capable of NADP+ photoreduction. Quantitation of the EPR signals of P700+ and dithionite reduced Fd reveal that Fd is present at a ratio of ~ 1 Fd per PSI monomer in both spinach and cyanobacterial thylakoids. Light-driven electron transfer from PSI to Fd in thylakoids confirms Fd is functionally associated (< 0.4 Fd/PSI) with the acceptor end of PSI in isolated cyanobacterial thylakoids. These EPR experiments provide a benchmark for future spectroscopic characterization of Fd interactions involved in multistep relay of electrons following PSI charge separation in the context of photosynthetic thylakoid microenvironments.

Quantum Dot-Organic Molecule Conjugates as Hosts for Photogenerated Spin Qubit Pairs.

The inherent spin polarization present in photogenerated spin-correlated radical pairs makes them promising candidates for quantum computing and quantum sensing applications. The spin states of these systems can be probed and manipulated with microwave pulses using electron paramagnetic resonance spectrometers. However, to date, there are no reports on magnetic resonance-based spin measurements of photogenerated spin-correlated radical pairs hosted on quantum dots. In the current work, we prepare dye molecule-inorganic quantum dot conjugates and show that they can produce photogenerated spin-polarized states. The dye molecule, D131, is chosen for its ability to undergo efficient charge separation, and the nanoparticle materials, ZnO quantum dots, are chosen for their promising spin properties. Transient and steady state optical spectroscopy performed on ZnO quantum dot-D131 conjugates shows that reversible photogenerated charge separation is occurring. Transient and pulsed electron paramagnetic resonance experiments are then performed on the photogenerated radical pair, which demonstrate that (1) the radical pair is polarized at moderate temperatures and well modeled by existing theories and (2) the spin states can be accessed and manipulated with microwave pulses. This work opens the door to a new class of promising qubit materials that can be photogenerated in polarized states and hosted by highly tailorable inorganic nanoparticles.

Unraveling Triplet Formation Mechanisms in Acenothiophene Chromophores.

The evolution of molecular platforms for singlet fission (SF) chromophores has fueled the quest for new compounds capable of generating triplets quantitatively at fast time scales. As the exploration of molecular motifs for SF has diversified, a key challenge has emerged in identifying when the criteria for SF have been satisfied. Here, we show how covalently bound molecular dimers uniquely provide a set of characteristic optical markers that can be used to distinguish triplet pair formation from processes that generate an individual triplet. These markers are contained within (i) triplet charge-transfer excited state absorption features, (ii) kinetic signatures of triplet-triplet annihilation processes, and (iii) the modulation of triplet formation rates using bridging moieties between chromophores. Our assignments are verified by time-resolved electron paramagnetic resonance (EPR) measurements, which directly identify triplet pairs by their electron spin and polarization patterns. We apply these diagnostic criteria to dimers of acenothiophene derivatives in solution that were recently reported to undergo efficient intermolecular SF in condensed media. While the electronic structure of these heteroatom-containing chromophores can be broadly tuned, the effect of their enhanced spin-orbit coupling and low-energy nonbonding orbitals on their SF dynamics has not been fully determined. We find that SF is fast and efficient in tetracenothiophene but that anthradithiophene exhibits fast intersystem crossing due to modifications of the singlet and triplet excited state energies upon functionalization of the heterocycle. We conclude that it is not sufficient to assign SF based on comparisons of the triplet formation kinetics between monomer and multichromophore systems.

Primary donor triplet states of Photosystem I and II studied by Q-band pulse ENDOR spectroscopy.

The photoexcited triplet state of the "primary donors" in the two photosystems of oxygenic photosynthesis has been investigated by means of electron-nuclear double resonance (ENDOR) at Q-band (34 GHz). The data obtained represent the first set of 1H hyperfine coupling tensors of the 3P700 triplet state in PSI and expand the existing data set for 3P680. We achieved an extensive assignment of the observed electron-nuclear hyperfine coupling constants (hfcs) corresponding to the methine α-protons and the methyl group ß-protons of the chlorophyll (Chl) macrocycle. The data clearly confirm that in both photosystems the primary donor triplet is located on one specific monomeric Chl at cryogenic temperature. In comparison to previous transient ENDOR and pulse ENDOR experiments at standard X-band (9-10 GHz), the pulse Q-band ENDOR spectra demonstrate both improved signal-to-noise ratio and increased resolution. The observed ENDOR spectra for 3P700 and 3P680 differ in terms of the intensity loss of lines from specific methyl group protons, which is explained by hindered methyl group rotation produced by binding site effects. Contact analysis of the methyl groups in the PSI crystal structure in combination with the ENDOR analysis of 3P700 suggests that the triplet is located on the Chl a' (PA) in PSI. The results also provide additional evidence for the localization of 3P680 on the accessory ChlD1 in PSII.

Nitrogen-Defective Polymeric Carbon Nitride Nanolayer Enabled Efficient Electrocatalytic Nitrogen Reduction with High Faradaic Efficiency.

Identifying highly selective catalysts and accurately measuring NH3 yield without false-positives from contaminations remain two challenges in electrochemical nitrogen reduction reaction (NRR). Here, we report N-defective carbon nitride grown on carbon paper (CN/C) as a highly selective electrocatalyst. The NH3 yield was determined reliably by the slope of mNH3-time plot rather than averaging the accumulated amount over time. Results showed the as-synthesized CN/C600 (synthesized at 600 °C) with a higher density of C=N-C N2C vacancies achieved an NH3 production of 2.9 µg mgcat.-1 h-1 at -0.3 V (versus RHE), ∼5.7-fold higher than CN/C500. The Faradaic efficiency for CN/C600 is among the highest of 62.1%, 33.9%, and 16.8% at -0.1 V, -0.2 V, and -0.3 V, respectively. The NH3 production was verified by isotope 15N2 experiments. Further increase of N-defects on CN/C600 using plasma etching led to higher NH3 yield than comparably larger current, pointing to N-defects sites for promoting NRR.

An Active-Site Sulfonate Group Creates a Fast Water Oxidation Electrocatalyst That Exhibits High Activity in Acid.

The storage of solar energy in chemical bonds will depend on pH-universal catalysts that are not only impervious to acid, but actually thrive in it. Whereas other homogeneous water oxidation catalysts are less active in acid, we report a catalyst that maintained high electrocatalytic turnover frequency at pH values as low as 1.1 and 0.43 (kcat =1501±608 s-1 and 831±254 s-1 , respectively). Moreover, current densities, related to catalytic reaction rates, ranged from 15 to 50 mA cm-2 mM-1 comparable to those reported for state-of-the-art heterogeneous catalysts and 30 to 100 times greater than those measured for two prominent literature homogeneous catalysts at pH 1.1 and 0.43. The catalyst also exhibited excellent durability when a chemical oxidant was used (CeIV , 7400 turnovers, TOF 0.88 s-1 ). Preliminary computational studies suggest that the unusual active-site sulfonate group acts a proton relay even in strong acid, as intended.

Polaron and Exciton Delocalization in Oligomers of High-Performance Polymer PTB7.

A key characteristic of organic photovoltaic cells is the efficient charge separation in the active layer. Sufficient delocalization of the positive polaron in organic photovoltaics is considered essential for the effective separation of the opposite charges and the suppression of recombination. We use light-induced EPR and ENDOR spectroscopy combined with DFT calculations to determine the electronic structure of the positive polaron in PTB7-type oligomers. Utilizing the superior spectral resolution of high-frequency (130 GHz) D-band EPR, the principal components of the g tensors were determined. Pulsed ENDOR spectroscopy at X-band allowed the measurement of 1H hyperfine coupling constants. A comparison of g tensors and 1H hyperfine coupling constants of the PTB7-type oligomers with the high-performance PTB7 polymer revealed a delocalization of the positive polaron in the polymer over about four monomeric units, corresponding to about 45 Å in length. Our current study thus not only determines the polaron delocalization length in PTB7 but also validates the approach combining EPR/ENDOR spectroscopy with DFT-calculated magnetic resonance parameters. This is of importance in those cases where oligomers of defined length are not easily obtained. In addition, the delocalization of the neutral triplet exciton was also determined in the oligomers and compared with polymer PTB7. The analysis revealed that the neutral triplet exciton is substantially more delocalized than the positive polaron, exceeding 10 monomeric units.

Interprotein electron transfer biohybrid system for photocatalytic H2 production.

Worldwide there is a large research investment in developing solar fuel systems as clean and sustainable sources of energy. The fundamental mechanisms of natural photosynthesis can provide a source of inspiration for these studies. Photosynthetic reaction center (RC) proteins capture and convert light energy into chemical energy that is ultimately used to drive oxygenic water-splitting and carbon fixation. For the light energy to be used, the RC communicates with other donor/acceptor components via a sophisticated electron transfer scheme that includes electron transfer reactions between soluble and membrane bound proteins. Herein, we reengineer an inherent interprotein electron transfer pathway in a natural photosynthetic system to make it photocatalytic for aqueous H2 production. The native electron shuttle protein ferredoxin (Fd) is used as a scaffold for binding of a ruthenium photosensitizer and H2 catalytic function is imparted to its partner protein, ferredoxin-NADP+-reductase (FNR), by attachment of cobaloxime molecules. We find that this 2-protein biohybrid system produces H2 in aqueous solutions via light-induced interprotein electron transfer reactions (TON > 2500 H2/FNR), providing insight about using native protein-protein interactions as a method for fuel generation.

Triplet-triplet energy transfer in artificial and natural photosynthetic antennas.

In photosynthetic organisms, protection against photooxidative stress due to singlet oxygen is provided by carotenoid molecules, which quench chlorophyll triplet species before they can sensitize singlet oxygen formation. In anoxygenic photosynthetic organisms, in which exposure to oxygen is low, chlorophyll-to-carotenoid triplet-triplet energy transfer (T-TET) is slow, in the tens of nanoseconds range, whereas it is ultrafast in the oxygen-rich chloroplasts of oxygen-evolving photosynthetic organisms. To better understand the structural features and resulting electronic coupling that leads to T-TET dynamics adapted to ambient oxygen activity, we have carried out experimental and theoretical studies of two isomeric carotenoporphyrin molecular dyads having different conformations and therefore different interchromophore electronic interactions. This pair of dyads reproduces the characteristics of fast and slow T-TET, including a resonance Raman-based spectroscopic marker of strong electronic coupling and fast T-TET that has been observed in photosynthesis. As identified by density functional theory (DFT) calculations, the spectroscopic marker associated with fast T-TET is due primarily to a geometrical perturbation of the carotenoid backbone in the triplet state induced by the interchromophore interaction. This is also the case for the natural systems, as demonstrated by the hybrid quantum mechanics/molecular mechanics (QM/MM) simulations of light-harvesting proteins from oxygenic (LHCII) and anoxygenic organisms (LH2). Both DFT and electron paramagnetic resonance (EPR) analyses further indicate that, upon T-TET, the triplet wave function is localized on the carotenoid in both dyads.

One Electron Multiple Proton Transfer in Model Organic Donor-Acceptor Systems: Implications for High Frequency EPR.

EPR spectroscopy is an important spectroscopic method for identification and characterization of radical species involved in many biological reactions. The tyrosyl radical is one of the most studied amino acid radical intermediates in biology. Often in conjunction with histidine residues, it is involved in many fundamental biological electron and proton transfer processes, such as in the water oxidation in photosystem II. As biological processes are typically extremely complicated and hard to control, molecular bio-mimetic model complexes are often used to clarify the mechanisms of the biological reactions. Here we present theoretical calculations to investigate the sensitivity of magnetic resonance parameters to proton-coupled electron transfer events, as well as conformational substates of the molecular constructs which mimic the tyrosine-histidine (Tyr-His) pairs found in a large variety of proteins. Upon oxidation of the phenol, the Tyr analogue, these complexes can perform not only one-electron one-proton transfer (EPT), but also one-electron two-proton transfers (E2PT). It is shown that in aprotic environment the gX-components of the electronic g-tensor are extremely sensitive to the first proton transfer from the phenoxyl oxygen to the imidazole nitrogen (EPT product), leading to a significant increase of the gX-value of up to 0.003, but are not sensitive to the second proton transfer (E2PT product). In the latter case the change of the gX-value is much smaller (ca. 0.0001), which is too small to be distinguished even by high frequency EPR. The 14N hyperfine values are also too similar to allow differentiation between the different protonation states in EPT and E2PT. The magnetic resonance parameters were also calculated as a function of the rotation angles around single bonds. It was demonstrated that rotation of the phenoxyl group results in large positive changes (>0.001) in the gX-values. Analysis of the data reveals that the main source of these changes is related to the strength of the H-bond between phenoxyl oxygen and the proton(s) on N1 and N2 positions of the imidazole.

Semi-artificial Photosynthetic CO2 Reduction through Purple Membrane Re-engineering with Semiconductor.

The engineering of biological pathways with man-made materials provides inspiring blueprints for sustainable fuel production. Here, we leverage a top-down cellular engineering strategy to develop a new semi-artificial photosynthetic paradigm for carbon dioxide reduction via enveloping Halobacterium purple membrane-derived vesicles over Pd-deposited hollow porous TiO2 nanoparticles. In this biohybrid, the membrane protein, bacteriorhodopsin, not only retains its native biological function of pumping protons but also acts as a photosensitizer that injects light-excited electrons into the conduction band of TiO2. As such, the electrons trapped on Pd cocatalysts and the protons accumulated inside the cytomimetic architecture act in concert to reduce CO2 via proton-coupled multielectron transfer processes. This study provides an alternative toolkit for developing robust semi-artificial photosynthetic systems for solar energy conversion.

Molecular Cobalt Catalysts for H2 Generation with Redox Activity and Proton Relays in the Second Coordination Sphere.

Two new Co(II) complexes have been synthesized and investigated as catalysts for H2 generation. These catalysts were designed to incorporate redox-active bipyridine components and nitrogen groups, which can participate in electron and proton transfer steps in the catalytic cycle. The two catalysts differ by only one amino group, yielding a completely closed macrocycle and an open "macrocycle" complex. Removing just one nitrogen linker between the Co(II)-binding bipyridine groups has a profound impact on the molecular geometry observed by single crystal analysis. Photocatalysis experiments show that both catalysts are highly active for aqueous proton reduction at moderate pH levels, with the closed macrocycle reaching almost 2 × 104 turnovers of H2 when photodriven by [Ru(2,2'-bipyridine)3]2+ using ascorbate as an electron relay and a phosphine compound as the terminal electron donor. Measurements of the electrocatalytic activity were used to investigate key steps in the mechanism of proton reduction by the molecular catalysts. The formation of a new reversible peak on addition of moderately strong acids in organic solvents suggests that protonation of the macrocycle plays an important role in H2 generation. Onset of the catalytic current occurs near the reduction potential of the bipyridine components, suggesting that catalysis is mediated by electron transfer from the macrocycle to the cobalt center. From these observations, we propose a mechanism for catalytic proton reduction to H2, which involves both intramolecular proton and electron transfer steps from the macrocycle ligand to the cobalt center. The vital role of the second coordination sphere in the catalytic cycle places these relatively simple complexes on the pathway toward molecular catalysts that mimic the valuable features of enzymatic catalysis.

Electron Paramagnetic Resonance Characterization of the Triheme Cytochrome from Geobacter sulfurreducens.

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 Geobacter sulfurreducens. 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 g-values. The observed g-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.

Changing the site energy of per-614 in the Peridinin-chlorophyll a-protein does not alter its capability of chlorophyll triplet quenching.

The peridinin-chlorophyll-a protein (PCP) is a water-soluble light harvesting protein of the dinoflagellate Amphidinium carterae, employing peridinin (Per) as the main carotenoid to fulfil light harvesting and photo-protective functions. Per molecules bound to the protein experience specific molecular surroundings which lead to different electronic and spectral properties. In the refolded N89 L variant PCP (N89 L-RFPCP) a significant part of the intensity on the long wavelength side of the absorption spectrum is shifted to shorter wavelengths due to a significant change in the Per-614 site energy. Since Per-614 has been shown to be the main chlorophyll (Chl) triplet quencher in the protein, and the relative geometry of pigments is not affected by the mutation as verified by X-ray crystallography, this variant is ideally suited to study the dependence of the triplet-triplet energy transfer (TTET) mechanism on the pigment site energy. By using a combination of Optically Detected Magnetic Resonance (ODMR), pulse Electron Paramagnetic Resonance (EPR) and Electron Nuclear DOuble Resonance (ENDOR) we found that PCP maintains the efficient Per-614-to-Chl-a TTET despite the change of Per-614 local energy. This shows the robustness of the photoprotective site, which is very important for the protection of the system.

Immobilization of an Amphiphilic Molecular Cobalt Catalyst on Carbon Black for Ligand-Assisted Water Oxidation.

We have prepared the amphiphilic molecular catalyst [CoIII(LOC18)(pyrr)2]ClO4 (1), where LOC18 is the deprotonated form of N, N'-[4,5-bis(octadecyloxy)-1,2-phenylene]dipicolinamide. Species 1 can be anchored onto a carbon black support to yield the assembly 1@CB, which can catalyze water oxidation at an affordable onset overpotential of 0.32 V, with a current density of 10 mA/cm2 at 0.37 V. Moreover, 1@CB displays TOF = 3850 h-1. A mechanism is proposed based on the experimental and density-functional-theory-calculated data.

Structural and Functional Characterization of a Short-Chain Flavodoxin Associated with a Noncanonical 1,2-Propanediol Utilization Bacterial Microcompartment.

Bacterial microcompartments (BMCs) are proteinaceous organelles that encapsulate enzymes involved in CO2 fixation (carboxysomes) or carbon catabolism (metabolosomes). Metabolosomes share a common core of enzymes and a distinct signature enzyme for substrate degradation that defines the function of the BMC (e.g., propanediol or ethanolamine utilization BMCs, or glycyl-radical enzyme microcompartments). Loci encoding metabolosomes also typically contain genes for proteins that support organelle function, such as regulation, transport of substrate, and cofactor (e.g., vitamin B12) synthesis and recycling. Flavoproteins are frequently among these ancillary gene products, suggesting that these redox active proteins play an undetermined function in many metabolosomes. Here, we report the first characterization of a BMC-associated flavodoxin (Fld1C), a small flavoprotein, derived from the noncanonical 1,2-propanediol utilization BMC locus (PDU1C) of Lactobacillus reuteri. The 2.0 Å X-ray structure of Fld1C displays the α/ß flavodoxin fold, which noncovalently binds a single flavin mononucleotide molecule. Fld1C is a short-chain flavodoxin with redox potentials of -240 ± 3 mV oxidized/semiquinone and -344 ± 1 mV semiquinone/hydroquinone versus the standard hydrogen electrode at pH 7.5. It can participate in an electron transfer reaction with a photoreductant to form a stable semiquinone species. Collectively, our structural and functional results suggest that PDU1C BMCs encapsulate Fld1C to store and transfer electrons for the reactivation and/or recycling of the B12 cofactor utilized by the signature enzyme.

Through-Space Ultrafast Photoinduced Electron Transfer Dynamics of a C70-Encapsulated Bisporphyrin Covalent Organic Polyhedron in a Low-Dielectric Medium.

Ultrafast photoinduced electron transfer (PIET) dynamics of a C70-encapsulated bisporphyrin covalent organic polyhedron hybrid (C70@COP-5) is studied in a nonpolar toluene medium with fluorescence and transient absorption spectroscopies. This structurally rigid donor (D)-acceptor (A) molecular hybrid offers a new platform featuring conformationally predetermined cofacial D-A orientation with a fixed edge-to-edge separation, REE (2.8 Å), without the aid of covalent bonds. Sub-picosecond PIET (τET ≤ 0.4 ps) and very slow charge recombination (τCR ≈ 600 ps) dynamics are observed. The origin of these dynamics is discussed in terms of enhanced D-A coupling (V = 675 cm-1) and extremely small reorganization energy (λ ≈ 0.18 eV), induced by the intrinsic structural rigidity of the C70@COP-5 complex.

Intramolecular Hydrogen Bonding Restricts Gd-Aqua-Ligand Dynamics.

Aqua ligands can undergo rapid internal rotation about the M-O bond. For magnetic resonance contrast agents, this rotation results in diminished relaxivity. Herein, we show that an intramolecular hydrogen bond to the aqua ligand can reduce this internal rotation and increase relaxivity. Molecular modeling was used to design a series of four Gd complexes capable of forming an intramolecular H-bond to the coordinated water ligand, and these complexes had anomalously high relaxivities compared to similar complexes lacking a H-bond acceptor. Molecular dynamics simulations supported the formation of a stable intramolecular H-bond, while alternative hypotheses that could explain the higher relaxivity were systematically ruled out. Intramolecular H-bonding represents a useful strategy to limit internal water rotational motion and increase relaxivity of Gd complexes.

Long Coherence Times in Nuclear Spin-Free Vanadyl Qubits.

Quantum information processing (QIP) offers the potential to create new frontiers in fields ranging from quantum biology to cryptography. Two key figures of merit for electronic spin qubits, the smallest units of QIP, are the coherence time (T2), the lifetime of the qubit, and the spin-lattice relaxation time (T1), the thermally defined upper limit of T2. To achieve QIP, processable qubits with long coherence times are required. Recent studies on (Ph4P-d20)2[V(C8S8)3], a vanadium-based qubit, demonstrate that millisecond T2 times are achievable in transition metal complexes with nuclear spin-free environments. Applying these principles to vanadyl complexes offers a route to combine the previously established surface compatibility of the flatter vanadyl structures with a long T2. Toward those ends, we investigated a series of four qubits, (Ph4P)2[VO(C8S8)2] (1), (Ph4P)2[VO(ß-C3S5)2] (2), (Ph4P)2[VO(α-C3S5)2] (3), and (Ph4P)2[VO(C3S4O)2] (4), by pulsed electron paramagnetic resonance (EPR) spectroscopy and compared the performance of these species with our recently reported set of vanadium tris(dithiolene) complexes. Crucially we demonstrate that solutions of 1-4 in SO2, a uniquely polar nuclear spin-free solvent, reveal T2 values of up to 152(6) µs, comparable to the best molecular qubit candidates. Upon transitioning to vanadyl species from the tris(dithiolene) analogues, we observe a remarkable order of magnitude increase in T1, attributed to stronger solute-solvent interactions with the polar vanadium-oxo moiety. Simultaneously, we detect a small decrease in T2 for the vanadyl analogues relative to the tris(dithiolene) complexes. We attribute this decrease to the absence of one nuclear spin-free ligand, which served to shield the vanadium centers against solvent nuclear spins. Our results highlight new design principles for long T1 and T2 times by demonstrating the efficacy of ligand-based tuning of solute-solvent interactions.