This report details spectroscopic characterizations of rare-earth, core-shell nanoparticles decorated with the f-element chelator 3,4,3-LI(1,2-HOPO). Evidence of photon downconversion is corroborated through detailed power dependence measurements, which suggest two-photon decay paths are active in these materials, albeit only representing a minority contribution of the sum luminescence, with emission being dominated by normal, Stokes' shifted fluorescence. Specifically, ultraviolet ligand photosensitization of Nd³⁺ ions in a NaGdF₄ host shell results in energy transfer to a Nd³⁺/Yb³⁺-doped NaGdF₄ nanoparticle core. The population and subsequent decay of core, Yb^{3+ 2} F _5/2 states result in a spectral shift of 620 nm, manifested in a NIR emission displaying luminescence profiles diagnostic of Yb³⁺ and Nd³⁺ excited state decays. Emphasis is placed on the generality of this material architecture for realizing ligand-pumped, multi-photon downconversion, with the Nd³⁺/Yb³⁺ system presented here functioning as a working prototype for a design principle that may be readily extended to other lanthanide pairs.

Photosystem II supports four manganese centers through nine oxidation states from manganese(II) during assembly through to the most oxidized state before O₂ formation and release. The protein-based carboxylate and imidazole ligands allow for significant changes of the coordination environment during the incorporation of hydroxido and oxido ligands upon oxidation of the metal centers. We report the synthesis and characterization of a series of tetramanganese complexes in four of the six oxidation states from Mn^II₃Mn^III to Mn^III₂ Mn^IV₂ with the same ligand framework (L) by incorporating four oxido ligands. A 1,3,5-triarylbenzene framework appended with six pyridyl and three alkoxy groups was utilized along with three acetate anions to access tetramanganese complexes, Mn₄O _x , with x = 1, 2, 3, and 4. Alongside two previously reported complexes, four new clusters in various states were isolated and characterized by crystallography, and four were observed electrochemically, thus accessing the eight oxidation states from Mn^II₄ to Mn^IIIMn^IV₃. This structurally related series of compounds was characterized by EXAFS, XANES, EPR, magnetism, and cyclic voltammetry. Similar to the ligands in the active site of the protein, the ancillary ligand (L) is preserved throughout the series and changes its binding mode between the low and high oxido-content clusters. Implications for the rational assembly and properties of high oxidation state metal-oxido clusters are presented.

Early-late intermetallic phases have garnered increased attention recently for their catalytic properties. To achieve the high surface areas needed for industrially relevant applications, these phases must be synthesized as nanoparticles in a scalable fashion. Herein, Pt₃Y-targeted as a prototypical example of an early-late intermetallic-has been synthesized as nanoparticles approximately 5-20 nm in diameter via a solution process and characterized by XRD, TEM, EDS, and XPS. The key development is the use of a molten borohydride (MEt₃BH, M = Na, K) as both the reducing agent and reaction medium. Readily available halide precursors of the two metals are used. Accordingly, no organic ligands are necessary, as the resulting halide salt byproduct prevents sintering, which further permits dispersion of the nanoscale intermetallic onto a support. The versatility of this approach was validated by the synthesis of other intermetallic phases such as Pt₃Sc, Pt₃Lu, Pt₂Na, and Au₂Y.

Herein, Ca K-edge X-ray absorption spectroscopy (XAS) is developed as a means to characterize the local environment of calcium centers. The spectra for six, seven, and eight coordinate inorganic and molecular calcium complexes were analyzed and determined to be primarily influenced by the coordination environment and site symmetry at the calcium center. The experimental results are closely correlated to time-dependent density functional theory (TD-DFT) calculations of the XAS spectra. The applicability of this methodology to complex systems was investigated using structural mimics of the oxygen-evolving complex (OEC) of PSII. It was found that Ca K-edge XAS is a sensitive probe for structural changes occurring in the cubane heterometallic cluster due to Mn oxidation. Future applications to the OEC are discussed.

X-ray absorption spectroscopy at the L-edge of 3d transition metals provides unique information on the local metal charge and spin states by directly probing 3d-derived molecular orbitals through 2p-3d transitions. However, this soft x-ray technique has been rarely used at synchrotron facilities for mechanistic studies of metalloenzymes due to the difficulties of x-ray-induced sample damage and strong background signals from light elements that can dominate the low metal signal. Here, we combine femtosecond soft x-ray pulses from a free-electron laser with a novel x-ray fluorescence-yield spectrometer to overcome these difficulties. We present L-edge absorption spectra of inorganic high-valent Mn complexes (Mn ∼ 6-15 mmol/l) with no visible effects of radiation damage. We also present the first L-edge absorption spectra of the oxygen evolving complex (Mn₄CaO₅) in Photosystem II (Mn < 1 mmol/l) at room temperature, measured under similar conditions. Our approach opens new ways to study metalloenzymes under functional conditions.