Metallic nanoparticle-based photocatalysts have gained a lot of interest in catalyzing oxidation–reduction reactions. In previous studies, the poor performance of these catalysts is partly due to their operation that relies on picosecond-lifetime hot carriers. In this work, electrons that accumulate at a photostationary state, generated by photocharging the catalysts, have a much longer lifetime for catalysis. This approach makes it possible to determine and tune the photoredox potentials of the catalysts. As demonstrated in a model reaction, the photostationary state of the photocatalyzed oxidative etching of colloidal gold nanoparticles using FeCl3 was established under continuous irradiation of different wavelengths. The photoredox potentials of the nanoparticles were then calculated using the Nernst equation. The potentials can be tuned to a range of 1.28 to 1.40 V (vs SHE) under irradiation of different wavelengths in the range of 450 to 517 nm. The effects of particle size or optical power on the photoredox potentials are small compared to the wavelength effect. Control over the photoredox potential of the particles using different excitation wavelengths can potentially be used to tune the activities and selectivities of metallic nanoparticle photocatalysts.

Harnessing hot carriers from photoexcited metallic nanoparticles for catalysis is very challenging because these carriers have extremely short lifetimes. Here, we demonstrate that smaller particles have higher surface-to-volume ratios that allow hot carriers to diffuse to particle surfaces with a higher probability and thereby exhibit higher photocatalytic activities as quantified by quantum yields. The measured photocatalytic activities for photoinduced etching of gold nanospheres by FeCl3, and the previously unreported aqueous hydrogenation of styrene using sodium borohydride under interband excitation show perfect dependence on the reciprocal of particle size. The size-dependent photocatalytic activity for photoinduced etching of gold nanospheres by FeCl3 under plasmon excitation, however, slightly deviates from this scaling law and may be influenced by other factors such as the surface field enhancement effect. This scaling law is expected to apply to other nanomaterial-based photocatalysts that rely on hot carrier diffusion to a surface for catalysis. Future design of nanomaterials for the harnessing of hot carriers for catalysis should take this scaling law into account.

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Iron-sulfur proteins play essential roles in various biological processes. Their electronic structure and vibrational dynamics are key to their rich chemistry but nontrivial to unravel. Here, the first ultrafast transient absorption and impulsive coherent vibrational spectroscopic (ICVS) studies on 2Fe-2S clusters in Rhodobacter capsulatus ferreodoxin VI are characterized. Photoexcitation initiated populations on multiple excited electronic states that evolve into each other in a long-lived charge-transfer state. This suggests a potential light-induced electron-transfer pathway as well as the possibility of using iron-sulfur proteins as photosensitizers for light-dependent enzymes. A tyrosine chain near the active site suggests potential hole-transfer pathways and affirms this electron-transfer pathway. The ICVS data revealed vibrational bands at 417 and 484 cm, with the latter attributed to an excited-state mode. The temperature dependence of the ICVS modes suggests that the temperature effect on protein structure or conformational heterogeneities needs to be considered during cryogenic temperature studies.

Rationale cardiac myocyte contraction is caused by Ca2+ binding to troponin C, which triggers the cross-bridge power stroke and myofilament sliding in sarcomeres. Synchronized Ca2+ release causes whole cell contraction and is readily observable with current microscopy techniques. However, it is unknown whether localized Ca2+ release, such as Ca2+ sparks and waves, can cause local sarcomere contraction. Contemporary imaging methods fall short of measuring microdomain Ca2+-contraction coupling in live cardiac myocytes. Objective To develop a method for imaging sarcomere-level Ca2+-contraction coupling in healthy and disease-model cardiac myocytes. Methods and Results Freshly isolated cardiac myocytes were loaded with the Ca2+-indicator Fluo-4. A confocal microscope equipped with a femtosecond-pulsed near-infrared laser was used to simultaneously excite second harmonic generation (SHG) from A-bands of myofibrils and two-photon fluorescence (2PF) from Fluo-4. Ca2+ signals and sarcomere strain correlated in space and time with short delays. Furthermore, Ca2+ sparks and waves caused contractions in subcellular microdomains, revealing a previously underappreciated role for these events in generating subcellular strain during diastole. Ca2+ activity and sarcomere strain were also imaged in paced cardiac myocytes under mechanical load, revealing spontaneous Ca2+ waves and correlated local contraction in pressure overload-induced cardiomyopathy. Conclusions Multi-modal SHG-2PF microscopy enables the simultaneous observation of Ca2+ release and mechanical strain at the sub-sarcomere level in living cardiac myocytes. The method benefits from the label-free nature of SHG, which allows A-bands to be imaged independently of T-tubule morphology and simultaneously with Ca2+ indicators. SHG-2PF imaging is widely applicable to the study of Ca2+-contraction coupling and mechano-chemo-transduction in both health and disease.

According to L-edge sum rules, the number of 3d vacancies at a transition metal site is directly proportional to the integrated intensity of the L-edge X-ray absorption spectrum (XAS) for the corresponding metal complex. In this study, the numbers of 3d holes are characterized quantitatively or semi-quantitatively for a series of manganese (Mn) and nickel (Ni) complexes, including the electron configurations 3d→ 3d. In addition, extremely dilute (<0.1% wt/wt) Ni enzymes were examined by two different approaches: (1) by using a high resolution superconducting tunnel junction X-ray detector to obtain XAS spectra with a very high signal-to-noise ratio, especially in the non-variant edge jump region; and (2) by adding an inert tracer to the sample that provides a prominent spectral feature to replace the weak edge jump for intensity normalization. In this publication, we present for the first time: (1) L-edge sum rule analysis for a series of Mn and Ni complexes that include electron configurations from an open shell 3d to a closed shell 3d; (2) a systematic analysis on the uncertainties, especially on that from the edge jump, which was missing in all previous reports; (3) a clearly-resolved edge jump between pre-L and post-L regions from an extremely dilute sample; (4) an evaluation of an alternative normalization standard for L-edge sum rule analysis. XAS from two copper (Cu) proteins measured using a conventional semiconductor X-ray detector are also repeated as bridges between Ni complexes and dilute Ni enzymes. The differences between measuring 1% Cu enzymes and measuring <0.1% Ni enzymes are compared and discussed. This study extends L-edge sum rule analysis to virtually any 3d metal complex and any dilute biological samples that contain 3d metals.

Photoinduced charge-transfer dynamics and the influence of cluster size on the dynamics were investigated using five iron-sulfur clusters: the 1Fe-4S cluster in Pyrococcus furiosus rubredoxin, the 2Fe-2S cluster in Pseudomonas putida putidaredoxin, the 4Fe-4S cluster in nitrogenase iron protein, and the 8Fe-7S P-cluster and the 7Fe-9S-1Mo FeMo cofactor in nitrogenase MoFe protein. Laser excitation promotes the iron-sulfur clusters to excited electronic states that relax to lower states. The electronic relaxation lifetimes of the 1Fe-4S, 8Fe-7S, and 7Fe-9S-1Mo clusters are on the picosecond time scale, although the dynamics of the MoFe protein is a mixture of the dynamics of the latter two clusters. The lifetimes of the 2Fe-2S and 4Fe-4S clusters, however, extend to several nanoseconds. A competition between reorganization energies and the density of electronic states (thus electronic coupling between states) mediates the charge-transfer lifetimes, with the 2Fe-2S cluster of Pdx and the 4Fe-4S cluster of Fe protein lying at the optimum leading to them having significantly longer lifetimes. Their long lifetimes make them the optimal candidates for long-range electron transfer and as external photosensitizers for other photoactivated chemical reactions like solar hydrogen production. Potential electron-transfer and hole-transfer pathways that possibly facilitate these charge transfers are proposed.

The optical control of spin state is of interest in the development of spintronic materials for data processing and storage technologies. Photomagnetic effects at the single-molecule level have recently been observed in the thin film state at 300 K in photochromic cobalt dioxolenes. Visible light excitation leads to ring-closure of a photochromic spirooxazine bound to a cobalt dioxolene, which leads to generation of a high magnetization state. Formation of the photomagnetic state occurs through a photoisomerization-induced spin-charge excited-state process and is dictated by the spirooxazine ligand dynamics. Here, we report a mechanistic investigation by ultrafast spectroscopy in the UV-vis region of the photochemical ring-closing process in the parent spirooxazine, azahomoadamantylphenanthroline spirooxazine, and the photomagnetic spirooxazine cobalt-dioxolene complex. The cobalt appears to stabilize a photomerocycanine transient intermediate, presumably the TCC isomer, formed along the ground-state potential energy surface (PES). Structural changes associated with the TCC isomer induces formation of the high-spin Co(II) form, suggesting that magnetization dymanics can occur along the excited-state PES, leading to ultrafast switching on the ps time scale. We demonstrate the full ring closure of the spiro-oxazine ligand is not required to switch magnetization states which can be induced with a higher yielding isomerization reaction. The ability of this system to undergo optically induced spin state switching on the ps time scale in the solid state makes it a promising canididate for resistive nonvolatile memory technologies.