The entatic state denotes a distorted coordination geometry of a complex from its typical arrangement that generates an improvement to its function. The entatic-state principle has been observed to apply to copper electron-transfer proteins and it results in a lowering of the reorganization energy of the electron-transfer process. It is thus crucial for a multitude of biochemical processes, but its importance to photoactive complexes is unexplored. Here we study a copper complex-with a specifically designed constraining ligand geometry-that exhibits metal-to-ligand charge-transfer state lifetimes that are very short. The guanidine-quinoline ligand used here acts on the bis(chelated) copper(I) centre, allowing only small structural changes after photoexcitation that result in very fast structural dynamics. The data were collected using a multimethod approach that featured time-resolved ultraviolet-visible, infrared and X-ray absorption and optical emission spectroscopy. Through supporting density functional calculations, we deliver a detailed picture of the structural dynamics in the picosecond-to-nanosecond time range.

The structure–function relationship is at the heart of biology, and major protein deformations are correlated to specific functions. For ferrous heme proteins, doming is associated with the respiratory function in hemoglobin and myoglobins. Cytochrome c (Cyt c) has evolved to become an important electron-transfer protein in humans. In its ferrous form, it undergoes ligand release and doming upon photoexcitation, but its ferric form does not release the distal ligand, while the return to the ground state has been attributed to thermal relaxation. Here, by combining femtosecond Fe Kα and Kβ X-ray emission spectroscopy (XES) with Fe K-edge X-ray absorption near-edge structure (XANES), we demonstrate that the photocycle of ferric Cyt c is entirely due to a cascade among excited spin states of the iron ion, causing the ferric heme to undergo doming, which we identify. We also argue that this pattern is common to a wide diversity of ferric heme proteins, raising the question of the biological relevance of doming in such proteins.

Time-resolved X-ray methods are widely used for monitoring transient intermediates over the course of photochemical reactions. Ultrafast X-ray absorption and emission spectroscopies as well as elastic X-ray scattering deliver detailed electronic and structural information on chemical dynamics in the solution phase. In this work, we describe the opportunities at the Femtosecond X-ray Experiments (FXE) instrument of European XFEL. Guided by the idea of combining spectroscopic and scattering techniques in one experiment, the FXE instrument has completed the initial commissioning phase for most of its components and performed first successful experiments within the baseline capabilities. This is demonstrated by its currently 115 fs (FWHM) temporal resolution to acquire ultrafast X-ray emission spectra by simultaneously recording iron Kα and Kβ lines, next to wide angle X-ray scattering patterns on a photoexcited aqueous solution of [Fe(bpy)3]2+, a transition metal model compound.

The European X-ray Free-Electron Laser (EuXFEL) delivers extremely intense (>1012 photons pulse−1 and up to 27000 pulses s−1), ultrashort (<100 fs) and transversely coherent X-ray radiation, at a repetition rate of up to 4.5 MHz. Its unique X-ray beam parameters enable novel and groundbreaking experiments in ultrafast photochemistry and material sciences at the Femtosecond X-ray Experiments (FXE) scientific instrument. This paper provides an overview of the currently implemented experimental baseline instrumentation and its performance during the commissioning phase, and a preview of planned improvements. FXE's versatile instrumentation combines the simultaneous application of forward X-ray scattering and X-ray spectroscopy techniques with femtosecond time resolution. These methods will eventually permit exploitation of wide-angle X-ray scattering studies and X-ray emission spectroscopy, along with X-ray absorption spectroscopy, including resonant inelastic X-ray scattering and X-ray Raman scattering. A suite of ultrafast optical lasers throughout the UV–visible and near-IR ranges (extending up to mid-IR in the near future) with pulse length down to 15 fs, synchronized to the X-ray source, serve to initiate dynamic changes in the sample. Time-delayed hard X-ray pulses in the 5–20 keV range are used to probe the ensuing dynamic processes using the suite of X-ray probe tools. FXE is equipped with a primary monochromator, a primary and secondary single-shot spectrometer, and a timing tool to correct the residual timing jitter between laser and X-ray pulses.

Combining the new aromatic bis(guanidine) ligand bis(tetramethylguanidino)toluene (TMG2tol) and different copper salts led to four new complexes. The molecular structures were analysed by X‐ray diffraction. Three of the complexes are active in biomimetic activation of oxygen. It is the first time a bis‐µ‐oxido species of an aromatic guanidine copper complex is analysed by Raman and UV/Vis spectroscopy. The formation and decay of the bis(µ‐oxido)dicopper species were investigated at low temperatures up to 273 K by UV/Vis spectroscopy with a stopped‐flow setup. The results showed that the Cu2O2 intermediate of an aromatic guanidine–copper complex decays much faster than most known bis(guanidine) complexes. Moreover, natural bond orbital (NBO) analyses revealed that the aromatic guanidine is the stronger donor compared to the aliphatic one.

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An atomistic understanding of the photoinduced spin-state switching (PSS) within polynuclear systems of d 4 -d 7 transition metal ion complexes is required for their rational integration into light-driven reactions of chemical and biological interests. However, in contrast to mononuclear systems, the multidimensional dynamics of the PSS in solvated molecular arrayshave not yet been elucidated, due to the expected complications associated to the connectivity between the metal centers and the strong interactions with the surroundings. In this work, the PSS in a solvated triiron(II) metallogrid complex is characterized using transient optical absorption and X-ray emission spectroscopies on the femtosecond timescale. The complementary measurements reveal the photoinduced creation of energy-rich (hot) and longlived quintet states, whose dynamics differ critically from their mononuclear congeners. This finding opens major prospects for developing novel schemes in solution-phase spin chemistry that are driven by the dynamic PSS process in compact oligometallic arrays. TOC GRAPHICSKEYWORDS molecular squares, photophysics, time-resolved X-ray techniques, light-induced processes, ultrafast phenomena, spin-state switching.

Single-ion magnets (SIMs) constitute the ultimate size limit in the quest for miniaturizing magnetic materials. Several bottlenecks currently hindering breakthroughs in quantum information and communication technologies could be alleviated by new generations of SIMs displaying multifunctionality. Here, ultrafast optical absorption spectroscopy and X-ray emission spectroscopy are employed to track the photoinduced spin-state switching of the prototypical complex [Co(terpy) 2 ] 2+ (terpy = 2,2′:6′,2″-terpyridine) in solution phase. The combined measurements and their analysis supported by density functional theory (DFT), time-dependent-DFT (TD-DFT) and multireference quantum chemistry calculations reveal that the complex undergoes a spin-state transition from a tetragonally elongated doublet state to a tetragonally compressed quartet state on the femtosecond timescale, i.e., it sustains ultrafast Jahn-Teller (JT) photoswitching between two different spin multiplicities. Adding new Co-based complexes as possible contenders in the search for JT photoswitching SIMs will greatly widen the possibilities for implementing magnetic multifunctionality and eventually controlling ultrafast magnetization with optical photons.