UV-Driven Oxygen Surface Exchange and Stoichiometry Changes in a Thin-Film, Nondilute Mixed Ionic Electronic Conductor, Sr(Ti,Fe)O3-d.

Enabling light-controlled ionic devices requires insight into photoionic responses in technologically relevant materials. Mixed-conducting perovskites containing nondilute Fe─serving as electrodes, catalysts, and sensors─can support large, electronically accommodated excursions in oxygen content, typically controlled by temperature, bias, and gas atmosphere. Instead, we investigated the ability of low-fluence, above-bandgap illumination to adjust oxygen stoichiometry and drive oxygen fluxes in nondilute Sr(Ti1-xFex)O3-x/2+δ (x = 0.07, 0.35) thin films with high baseline hole concentrations. Films' optical transmission at 2.8 eV was used as a probe of oxygen stoichiometry in the range ∼100-500 °C. We compared pO2-step-driven and UV (3.4 eV)-step-driven visible optical transmission relaxations in films, finding that the time constants and activation energies of the relaxations were consistent with each other and thus with oxygen-surface-exchange-limited kinetics. Blocking oxygen exchange at the solid-gas interface with a UV-transparent capping layer resulted in no UV-induced optical relaxations. These results demonstrate that above-bandgap illumination can increase oxygen content in nondilute compositions through oxygen flux into the solid from the gas. First-principles simulations of defect formation enthalpies indicate that oxygen vacancies are energetically less favorable under steady-state illumination owing to shifts in quasi-Fermi levels. A larger 2.8 eV-optical response to UV illumination in x = 0.07 vs x = 0.35 samples was further investigated through ultrafast transient spectroscopy, where it was found that the x = 0.07 sample exhibits a slower carrier recombination. Together, these results suggest potential design principles for materials supporting large stoichiometry changes under above-gap illumination: (1) long excited carrier lifetimes and (2) highly charged, rather than neutral, defects/associates.

High-Spin State of a Ferrocene Electron Donor Revealed by Optical and X-ray Transient Absorption Spectroscopy.

Ferrocene is one of the most common electron donors, and mapping its ligand-field excited states is critical to designing donor-acceptor (D-A) molecules with long-lived charge transfer states. Although 3(d-d) states are commonly invoked in the photophysics of ferrocene complexes, mention of the high-spin 5(d-d) state is scarce. Here, we provide clear evidence of 5(d-d) formation in a bimetallic D-A molecule, ferrocenyl cobaltocenium hexafluorophosphate ([FcCc]PF6). Femtosecond optical transient absorption (OTA) spectroscopy reveals two distinct electronic excited states with 30 and 500 ps lifetimes. Using a combination of ultraviolet, visible, near-infrared, and short-wave infrared probe pulses, we capture the spectral features of these states over an ultrabroadband range spanning 320 to 2200 nm. Time-dependent density functional theory (DFT) calculations of the lowest triplet and quintet states, both primarily Fe(II) (d-d) in character, qualitatively agree with the experimental OTA spectra, allowing us to assign the 30 ps state as the 3(d-d) state and the 500 ps state as the high-spin 5(d-d) state. To confirm the ferrocene-centered high-spin character of the 500 ps state, we performed X-ray transient absorption (XTA) spectroscopy at the Fe and Co K edges. The Fe K-edge XTA spectrum at 150 ps shows a red shift of the absorption edge that is consistent with an Fe(II) high-spin state, as supported by ab initio calculations. The transient signal detected at the Co K-edge is 50× weaker, confirming the ferrocene-centered character of the excited state. Fitting of the transient extended X-ray absorption fine structure region yields an Fe-C bond length increase of 0.25 ± 0.1 Å in the excited state, as expected for the high-spin state based on DFT. Altogether, these results demonstrate that the high-spin state of ferrocene should be considered when designing donor-acceptor assemblies for photocatalysis and photovoltaics.

Excited-State Identification of a Nickel-Bipyridine Photocatalyst by Time-Resolved X-ray Absorption Spectroscopy.

Photoassisted catalysis using Ni complexes is an emerging field for cross-coupling reactions in organic synthesis. However, the mechanism by which light enables and enhances the reactivity of these complexes often remains elusive. Although optical techniques have been widely used to study the ground and excited states of photocatalysts, they lack the specificity to interrogate the electronic and structural changes at specific atoms. Herein, we report metal-specific studies using transient Ni L- and K-edge X-ray absorption spectroscopy of a prototypical Ni photocatalyst, (dtbbpy)Ni(o-tol)Cl (dtb = 4,4'-di-tert-butyl, bpy = bipyridine, o-tol = ortho-tolyl), in solution. We unambiguously confirm via direct experimental evidence that the long-lived (∼5 ns) excited state is a tetrahedral metal-centered triplet state. These results demonstrate the power of ultrafast X-ray spectroscopies to unambiguously elucidate the nature of excited states in important transition-metal-based photocatalytic systems.

Palladium K-edge X-ray Absorption Spectroscopy Studies on Controlled Ligand Systems.

X-ray absorption spectroscopy (XAS) is widely used across the life and physical sciences to identify the electronic properties and structure surrounding a specific element. XAS is less often used for the characterization of organometallic compounds, especially for sensitive and highly reactive species. In this study, we used solid- and solution-phase XAS to compare a series of 25 palladium complexes in controlled ligand environments. The compounds include palladium centers in the formal I, II, III, and IV oxidation states, supported by tridentate and tetradentate macrocyclic ligands, with different halide and methyl ligand combinations. The Pd K-edge energies increased not only upon oxidizing the metal center but also upon increasing the denticity of the ligand framework, substituting sigma-donating methyl groups with chlorides, and increasing the charge of the overall metal complex by replacing charged ligands with neutral ligands. These trends were then applied to characterize compounds whose oxidation states were otherwise unconfirmed.

Intraligand Charge Transfer Enables Visible-Light-Mediated Nickel-Catalyzed Cross-Coupling Reactions.

We demonstrate that several visible-light-mediated carbon-heteroatom cross-coupling reactions can be carried out using a photoactive NiII precatalyst that forms in situ from a nickel salt and a bipyridine ligand decorated with two carbazole groups (Ni(Czbpy)Cl2 ). The activation of this precatalyst towards cross-coupling reactions follows a hitherto undisclosed mechanism that is different from previously reported light-responsive nickel complexes that undergo metal-to-ligand charge transfer. Theoretical and spectroscopic investigations revealed that irradiation of Ni(Czbpy)Cl2 with visible light causes an initial intraligand charge transfer event that triggers productive catalysis. Ligand polymerization affords a porous, recyclable organic polymer for heterogeneous nickel catalysis of cross-coupling reactions. The heterogeneous catalyst shows stable performance in a packed-bed flow reactor during a week of continuous operation.