A new family of tunable iron bipyridine coordination complexes has been synthesized and tested in a non-aqueous flow battery.

We report the syntheses, structures, and luminescence properties of a series of copper-iodo cuboids supported by L-type antimony ligands. The cuboids are of general formula [(SbR3)4Cu4(I)4] (1–4, 8), where SbR3 is a series of homoleptic and heteroleptic stibines containing both phenyl and a variety of alkyl substituents (R = Cy, i Pr, t Bu, Ph); triphenyl, i Pr2Ph, and Me2Ph stibines resulted in the formation of dimers of type [(SbR3)4(Cu)2(I)2] (5–7). While similar luminescent copper-halide cubes have been studied, the corresponding “heavy-atom” congeners have not been studied, and ligation of such heavy-atom moieties is often associated with long-lived triplet states and low-energy absorption and emission profiles. Overall, two obligate parameters are found to imbue NIR emission: (i) short Cu–Cu bonds and (ii) high crystallographic symmetry; both of these properties are found only in [(Sb i Pr3)4Cu4(I)4] (1, in I23; λem = 711 nm). The correlation between NIR emission and high crystallographic symmetry (which intrinsically includes high molecular symmetry)–versus only molecular symmetry–is confirmed by the counterexample of the molecularly symmetric t Bu-substituted cuboid [(Sb t Bu3)4Cu4(I)4] (3, λem = 588 nm, in R-3), which crystallizes in the lower symmetry trigonal space group. Despite the indication that the stronger donor strength of the Sb t Bu3 ligand should red-shift emission beyond that of the Sb i Pr3-supported cuboid, the emission of 3 is limited to the visible region. To further demonstrate the connection between structural parameters and emission intensity, X-ray structures for 1 and 3 were collected between 100 and 300 K. Lastly, DFT calculations for 1 on its singlet (S0) and excited triplet state (T1) demonstrate two key factors necessary for low-energy NIR emission: (i) a significant contraction of the interconnected Cu4 intermetallic contacts [∼2.45 → 2.35 Å] and (ii) highly delocalized (and therefore low-energy) A 1 symmetry HOMO/LUMO orbitals from which the emission occurs. Thus, any molecular or crystallographic distortion of the Cu4 core precludes the formation of highly symmetric (and low-energy) HOMO/LUMO orbitals in T1, thereby inhibiting low-energy NIR emission.

Energy storage is becoming the chief barrier to the utilization of more renewable energy sources on the grid. With independent service operators aiming to acquire gigawatts in the next 10–20 years, there is a large need to develop a suite of new storage technologies. Redox flow batteries (RFB) may be part of the solution if certain key barriers are overcome. This Review focuses on a particular kind of RFB based on nonaqueous media that promises to meet the challenge through higher voltages than the organic and aqueous variants. This class of RFB is divided into three groups: molecular, macromolecular, and redox‐targeted systems. The growing field of theoretical modeling is also reviewed and discussed.

Invited for this month's cover is the joint redox flow battery team from Sandia and Los Alamos National Laboratories. The cover image shows the stylized components of a redox flow battery (RFB) in the foreground, with renewable sources of energy generation in the background. The Review itself is available at 10.1002/cssc.202002354.

The Front Cover shows a chemist modifying the anolyte side of the flow cell and renewable energy sources in the background. More information can be found in the Research Article by C. X. Kolesnichenko et al.