Ryan G. Hadt

Paramagnetic metal complexes are powerful tools in chemical catalysis and, increasingly, quantum information science, positioning them as ideal tools for quantum sensing within reactions themselves. However, this application requires an interface between spins and light in chemically relevant platforms. Here, we demonstrate that circularly polarized light can generate electron spin coherence in low-symmetry, catalytically relevant Pd(I) complexes. By measuring ultrafast changes in optical polarization with time-resolved Faraday rotation, we optically detect coherent superhyperfine interaction between the Pd(I)-centered electron spin and adjacent 31P nuclei from the ligand at ambient temperature in solution. The electronic origins of this behavior are attributed to excited-state orbital angular momentum through combined spectroscopic and computational analysis, and the optically measured coherence is benchmarked against pulsed electron paramagnetic resonance. Spin polarization is coherently exchanged between the electron spin and initially incoherent nuclear spins, constituting a rare example of optical access to a nuclear spin register in an ambient-temperature, solution-phase molecular system. Both reactivity and quantum spin function are mediated by the bisphosphine ligand; taken together with in situ detection of coherent Pd(I) spin dynamics directly in reaction mixtures, this work moves molecular qubits toward quantum sensing in chemical reactions.

Optically addressable, biocompatible spin qubits represent promising platforms for quantum sensing. However, current efforts have focused on genetic encodability and microwave-based approaches thus far limited to organic radicals. Utilizing time-resolved Faraday rotation, we measured electron spin coherence in plastocyanin, a protein in the photosynthetic electron transport chain. Persisting hundreds of picoseconds, this is the first measurement of spin coherence in a metalloprotein under ambient conditions. We show how the protein’s active-site geometric and electronic structure responsible for biological function also enabled a spin–photon interface. A related blue copper protein—azurin—also demonstrated all-optical spin coherence. The ability to measure bioinorganic spin coherence under functional conditions foreshadows metalloproteins as native quantum sensors and opens questions about the potential role of spin coherences in electron transfer.

Mechanistic elucidation of a photochemically driven CO2 reduction reaction (CO2RR) using an IrIII photosensitizer and an iron metalloporphyrin reduction catalyst, which has been proposed to transform CO2 into CH4, has been conducted using various steady-state and time-resolved absorption and emission spectroscopic methods, cyclic voltammetry, reaction kinetics, quantum yield measurements, spectroelectrochemistry, and isotope-labeling experiments. A dynamic reactivity landscape was subsequently uncovered where the key photochemical mechanistic steps are altered as a function of reaction progress from an initial photoinduced single electron transfer to an energy transfer mechanism, which is supported by the in situ generation of a ferrous-carbonyl intermediate that was subjected to Stern–Volmer analysis and transient absorption spectroscopy to determine the overall photochemical mechanism using a select IrIII photosensitizer. This alteration of the photochemical mechanistic steps stymies further photoinduced reduction of the ferrous-carbonyl intermediate, which is a proposed on-cycle step toward the photoevolution of CH4. Instead, cyclic voltammetry experiments implicated a plausible CH4 evolution mechanism involving degradation of the porphyrin structure through attack of a second equivalent of the reduced metalloporphyrin catalyst. This mechanistic picture is consistent for the observed formation of ethane under steady-state irradiation conditions, which also does not arise from the CO2RR but instead represents a degradation product of the sacrificial electron donor, triethylamine, following single electron oxidation.

Homogeneous molecular catalysts for electrochemical CO2 reduction are generally limited to two-electron-reduced C1 products, primarily CO. Here we propose and demonstrate a fundamentally distinct role for molecular catalysts: as reversible CO shuttles that enhance the local CO activity at the electrical double layer. Using a water-soluble iron porphyrin as a CO shuttle, we observe the formation of isotopically verified C1–C4 hydrocarbons from CO2 in aqueous electrolyte. These products are not observed even in CO-saturated electrolyte in the absence of the iron porphyrin, indicating that the shuttle enhances local CO activity at the electrode surface beyond what bulk saturation can provide. Electrochemical and spectroscopic data support the formation of a stable FeII–CO species outside the compact layer, which releases CO upon further reduction at the electrode surface, consistent with an electrochemically gated shuttle mechanism. These findings suggest a potential design principle in which homogeneous catalysts function as molecular intermediate shuttles at the electrode surface and provide an alternative framework for interpreting beyond-CO products observed in heterogenized molecular catalyst systems.

Paramagnetic metal complexes are powerful tools in chemical catalysis and, increasingly, quantum information science, positioning them ideally for quantum sensing within reactions themselves. However, this application requires an interface between spins and light in chemically relevant platforms. Here we demonstrate that circularly polarized light can generate electron spin coherence in low-symmetry, catalytically relevant Pd(I) complexes. Measuring ultrafast changes in optical polarization with time-resolved Faraday rotation, we optically detect coherent superhyperfine interaction between the Pd(I)-centered electron spin and adjacent ³¹ P nuclei from the ligand at ambient temperature in solution. The electronic origins of this behavior are attributed to excited-state orbital angular momentum through combined spectroscopic and computational analysis, and the optically measured coherence is benchmarked against pulse electron paramagnetic resonance. Spin polarization is coherently exchanged between the electron spin and initially incoherent nuclear spins, constituting a rare example of optical access to a nuclear-spin register in an ambient-temperature, solution-phase molecular system. Both reactivity and quantum spin function are mediated by the bisphosphine ligand; taken together with in situ detection of coherent Pd(I) spin dynamics directly in reaction mixtures, this work moves molecular qubits toward quantum sensing in chemical reactions.

The electrochemical characterization and application of silicon phthalocyanine µ-oxo oligomers to symmetric, slurry redox flow batteries are reported. Electrolytes comprising (C₆H₁₃)₃-Si-O-Si(Pc)-O-Si(C₆H₁₃)₃, (C₆H₁₃)₃-Si-O-Si(Pc)-O-Si(Pc)-O-Si(C₆H₁₃)₃, (C₆H₁₃)₃Si-O-Si(Pc)-O-Si(Pc)-O-Si(Pc)-O-Si(C₆H₁₃)₃, Ketjenblack EC-600JD carbon, and TBAPF₆ in propylene carbonate exhibit near-unity Coulombic efficiencies and near to greater than 60% energy efficiencies. Notably, (C₆H₁₃)₃Si-O-Si(Pc)-O-Si(Pc)-O-Si(Pc)-O-Si(C₆H₁₃)₃ exhibits negligible capacity fade over two weeks of charge-discharge cycling at 50% state of charge. The unique multi-electron oxidative stability of higher-order oligomers is rationalized through a combination of time-dependent density functional theory (TDDFT) and ab initiocalculations. These materials provide a promising class of multi-electron transfer reagents for use in semi-solid, non-aqueous redox flow batteries.

Previously, we reported a new macrocyclic ligand, ^Mes N ₆ , which enables the room‐temperature coherence of Cu( ^Mes N ₆ )(OTf) ₂ . In this study, we characterize a new series of complexes based on the square planar [M( ^Mes N ₆ )] ²⁺ motif with different anions, X, (M = Co, Cu; X = SiF, OTf, and Cl) to tune the electron spin relaxation properties of Cu(II) and Co(II) through changes to the axial ligand. For Cu(II), a weakly‐coordinating SiF counterion minimizes orbital angular momentum (OAM) and prolongs longitudinal relaxation. An inverted trend is observed for Co(II) where a more donating Cl axial ligand decreases OAM and prolongs spin relaxation. The most pronounced effect occurs when the singly‐occupied molecular orbital (SOMO) is and its energy is most significantly impacted by anion binding. Using changes in coordination of Co(II) we detect chloride anions at 125 µM (0.25 equivalents) through relaxometry as measurable differences in longitudinal spin relaxation (T ₁ ). Additionally, chloride coordination enables T _m ‐based detection by Hahn echo of a Co(II)‐chloride species. Overall, we demonstrate predictable changes to electron spin relaxation through systematic variation of the axial ligand field, providing a strategy for molecular quantum sensing of anionic ligands.

Previously, we reported a new macrocyclic ligand, ^Mes N ₆ , which enables the room‐temperature coherence of Cu( ^Mes N ₆ )(OTf) ₂ . In this study, we characterize a new series of complexes based on the square planar [M( ^Mes N ₆ )] ²⁺ motif with different anions, X, (M = Co, Cu; X = SiF, OTf, and Cl) to tune the electron spin relaxation properties of Cu(II) and Co(II) through changes to the axial ligand. For Cu(II), a weakly‐coordinating SiF counterion minimizes orbital angular momentum (OAM) and prolongs longitudinal relaxation. An inverted trend is observed for Co(II) where a more donating Cl axial ligand decreases OAM and prolongs spin relaxation. The most pronounced effect occurs when the singly‐occupied molecular orbital (SOMO) is and its energy is most significantly impacted by anion binding. Using changes in coordination of Co(II) we detect chloride anions at 125 µM (0.25 equivalents) through relaxometry as measurable differences in longitudinal spin relaxation (T ₁ ). Additionally, chloride coordination enables T _m ‐based detection by Hahn echo of a Co(II)‐chloride species. Overall, we demonstrate predictable changes to electron spin relaxation through systematic variation of the axial ligand field, providing a strategy for molecular quantum sensing of anionic ligands.

Molecular orientation plays a critical role in determining the optoelectronic and mechanical properties of organic thin films and functional interfaces, yet the orientations are often heterogeneous, leading to nanometer-scale domain structures. Characterizing three-dimensional molecular orientations with nanoscale spatial resolution remains a significant challenge for conventional ensemble-averaged spectroscopic techniques. Here, we report a quantitative method to resolve local molecular orientation by combining scattering-type scanning near-field optical microscopy (s-SNOM) with dipole reorientation mapping. By analyzing hyperspectral nano-FTIR data of Cu(Open-^MesN₆)(OTf)₂ films, we developed an experiment-to-simulation workflow that extracts molecular orientation from relative vibrational peak intensities through transition dipole moment (TDM) projections, with a spatial resolution of ∼40 nm. Our analysis reveals that while molecules maintain a uniform alignment within domains, they undergo a sharp reorientation at grain boundaries, structural details that are invisible to topographic imaging alone. Further transformation allows us to extract the orientation of the nanocrystals that host the molecules. We found that the crystal structure exhibits an ∼55° tilt difference relative to the surface normal at the grain boundary, while maintaining a constant in-plane rotation angle. This methodology provides a robust, generalizable framework for decoding complex molecular/nanocrystal anisotropic organization in organic semiconductors, catalytic surfaces, and biological assemblies beyond the diffraction limits.

The activation of C–F bonds remains a central challenge in organometallic chemistry due to their exceptional bond strength and the limited availability of practical synthetic methods for converting fluorinated feedstocks into well-defined organometallic intermediates. Here, we report a general and practical strategy for dual C–F activation of polyfluoroarenes using a commercially available Ni(0) precursor, ligands, and aryl fluorides, providing direct access to a family of air- and moisture-stable Ni(II) diaryl complexes. This platform accesses a diverse substrate scope and exhibits functional group tolerance, including electron-rich/poor, reactive groups, and π-functionalized/sterically bulky arenes. Moreover, it exhibits high chemo- and regioselectivity, favoring C–F activation over competing C–H activation, enabling para-selective functionalization. Extensive structural validation is available, including 15 single-crystal X-ray diffraction structures across the series. Mechanistically, we demonstrate that Ni(cod) ₂ alone is competent for C–F activation, even in coordinating solvents such as acetonitrile, providing new insight into Ni-mediated C–halogen bond activation pathways. This reactivity enables the formation of a cis–(MeCN) ₂ Ni(PyF ₄ ) ₂ synthon directly from pentafluoropyridine, bypassing traditional salt metathesis approaches that rely on prefunctionalized and often hazardous organometallic reagents. This represents a conceptually distinct and synthetically enabling entry point into Ni(II) diaryl chemistry. Finally, these well-defined complexes exhibit stimulus-dependent chemodivergent reactivity: thermal activation promotes hydrodefluorination, whereas photochemical excitation drives C(sp ² )–C(sp ² ) homocoupling for the same complex. This ability to switch reaction pathways through external stimuli, rather than ligand or reagent changes, introduces a new dimension of control in Ni-mediated bond activation chemistry.