The structures of two types of guanidine-quinoline copper complexes have been investigated by single-crystal X-ray crystallography, K-edge X-ray absorption spectroscopy (XAS), resonance Raman and UV/Vis spectroscopy, cyclic voltammetry, and density functional theory (DFT). Independent of the oxidation state, the two structures, which are virtually identical for solids and complexes in solution, resemble each other strongly and are connected by a reversible electron transfer at 0.33 V. By resonant excitation of the two entatic copper complexes, the transition state of the electron transfer is accessible through vibrational modes, which are coupled to metal-ligand charge transfer (MLCT) and ligand-metal charge transfer (LMCT) states.

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 guanidine–quinoline ligand dimethylethyleneguanidinoquinoline (DMEGqu) is able to stabilise bis(chelate) copper complexes in an intermediate geometry between tetrahedral and square‐planar environments. The structures of the obtained complexes model the entatic state and have been investigated in solid state by single‐crystal X‐ray diffraction and in the solid state and in solution by X‐ray absorption spectroscopy. The dimethylethyleneguanidine (DMEG) unit of the DMEGqu ligand displays a smaller steric encumbrance than the tetramethylguanidine (TMG) counterpart; this allows slightly larger structural changes upon oxidation than those for the TMG counterparts. Moreover, triflate coordination was possible for the CuII DMEG complexes. DFT analyses revealed that good structural and optical descriptions are possible through the use of the hybrid functionals B3LYP and TPSSh in combination with the triple‐zeta basis set def2‐TZVP and the inclusion of empirical dispersion with Becke–Johnson damping and a suitable solvent model. The orbital analysis gives insights into the electronic structure of the complexes and their charge‐transfer behaviour.

Die Struktur von zwei Typen von Guanidin-Chinolin-Kupfer-Komplexen wurde mittels Einkristallrçntgen-strukturanalyse, K-Kanten-Rçntgenabsorptionsspektroskopie (XAS), Resonanz-Raman-und UV/Vis-Spektroskopie, Cyclovoltammetrie und Dichtefunktionaltheorie (DFT) untersucht. Unabhängig vom Oxidationszustand weisen beide Strukturen sowohl im Festkçrper als auch in Lçsung nahezu die gleiche Struktur auf und kçnnen durch einen reversiblen Elektronentransfer bei 0.33 V ineinander überführt werden. Durch eine resonante Anregung der beiden entatischen Kupferkomplexe kann der Übergangszustand des Elektronentransfers über die Schwingungsmoden, die entweder den Metall-Ligand-ChargeTransfer-Zustand (MLCT) oder den Ligand-Metall-ChargeTransfer-Zustand (LMCT) verbinden, untersucht werden.

We present a micro-jet sample delivery system for Raman measurements. Compared to cuvette measurements, the observed Raman signal is enhanced by more than one order of magnitude and does not contain signal distortions from the liquid-glass interface. Furthermore, the signal stability of repeated measurements is enhanced due to reduced sample damage effects by constantly replenishing the sample. This allows the study of sensitive samples that can only be produced in low concentrations. Our setup consists of a controlled sample environment that can be either under vacuum or an exchange gas, which allows the study of samples that are unstable in air. Finally, by matching the effective source point of the Raman instrument with the diameter of the jet, controlled experiments using laser beams of different wavelengths are possible. We see future applications of our setup for resonance Raman and time-resolved Raman measurements of bioinorganic samples.

The cover picture shows a guanidine–quinoline–copper complex as a new entatic‐state model. Small changes in the ligand strongly affect the molecular structure, which demonstrates the subtle balance of steric and electronic effects at a metal center. The unique combination of solid‐state and solution structural investigations by classical single‐crystal X‐ray diffraction and X‐ray absorption spectroscopy together with density functional theory reveals deep insights into this entatic‐state model. Details are discussed in the article by S. Herres‐Pawlis et al. on http://onlinelibrary.wiley.com/doi/10.1002/ejic.201600655/abstract. For more on the story behind the cover research, see the http://onlinelibrary.wiley.com/doi/10.1002/ejic.201601165/full.

Invited for the cover of this issue is the group of Sonja Herres‐Pawlis from RWTH Aachen University, Germany. The cover image shows our entatic‐state model. The research described in the article is the result of strongly collaborative work of four groups within the interdisciplinary DFG Research Unit FOR1405 (“Dynamics of Electron Transfer Processes within Transition Metal Sites in Biological and Bioinorganic Systems”).