Simple Air-Stable [3]Radialene Anion Radicals as Environmentally Switchable Catholytes in Water

The hexacyano[3]radialene radical anion (1) is an attractive catholyte material for use in redox flow battery (RFB) applications. The substitution of cyano groups with ester moieties enhances solubility while maintaining redox reversibility and favorable redox potentials. Here we show that these ester-functionalized, hexasubstituted [3]radialene radical anions dimerize reversibly in water. The dimerization mode is dependent on the substitution pattern and can be switched in solution. Stimuli-responsive behavior is achieved by exploiting an unprecedented tristate switching mechanism, wherein the radical can be toggled between the free radical, a π-dimer, and a σ-dimer-each with dramatically different optical, magnetic, and redox properties-by changing the solvent environment, temperature, or salinity. The symmetric, triester-tricyano[3]radialene (3) forms a solvent-responsive, σ-dimer in water that converts to the radical anion with the addition of organic solvents or to a π-dimer in brine solutions. Diester-tetracyano[3]radialene (2) exists primarily as a π-dimer in aqueous solutions and a radical anion in organic solvents. The dimerization behavior of both 2 and 3 is temperature dependent in methanol solutions. Dimerization equilibrium has a direct impact on catholyte stability during galvanostatic charge-discharge cycling in static H-cells. Specifically, conditions that favor the free radical anion or π-dimer exhibit significantly enhanced cycling profiles.

Synthesis and disassembly of an organometallic polymer comprising redox-active Co4S4 clusters and Janus biscarbene linkers

Here, we show for the first time that main-chain organometallic polymers (MCOPs) can be prepared from Janus N-heterocyclic carbene (NHC) linkers and polynuclear cluster nodes. The crosslinked framework Co₄S₄-MCOP is synthesized via ligand displacement reactions and undergoes reversible electron transfer in the solid state. Discrete molecular cluster species can be excised from the framework by digesting the solid in solutions of excess monocarbene. Finally, we demonstrate a synthetic route to monodisperse framework particles via coordination modulation.

Self-Assembly-Directed Exciton Diffusion in Solution-Processable Metalloporphyrin Thin Films

The study of excited-state energy diffusion has had an important impact in the development and optimization of organic electronics. For instance, optimizing excited-state energy migration in the photoactive layer in an organic solar cell device has been shown to yield efficient solar energy conversion. Despite the crucial role that energy migration plays in molecular electronic device physics, there is still a great deal to be explored to establish how molecular orientation impacts energy diffusion mechanisms. In this work, we have synthesized a new library of solution-processable, Zn (alkoxycarbonyl)phenylporphyrins containing butyl (ZnTCB4PP), hexyl (ZnTCH4PP), 2-ethylhexyl (ZnTCEH4PP), and octyl (ZnTCO4PP) alkoxycarbonyl groups. We establish that, by varying the length of the peripheral alkyl chains on the metalloporphyrin macrocycle, preferential orientation and molecular self-assembly is observed in solution-processed thin films. The resultant arrangement of molecules consequently affects the electronic and photophysical characteristics of the metalloporphyrin thin films. The various molecular arrangements in the porphyrin thin films and their resultant impact were determined using UV-Vis absorption spectroscopy, steady-state and time-resolved fluorescence emission lifetimes, and X-ray diffraction in thin films. The films were doped with C60 quencher molecules and the change in fluorescence was measured to derive a relative quenching efficiency. Using emission decay, relative quenching efficiency, and dopant volume fraction as input, insights on exciton diffusion coefficient and exciton diffusion lengths were obtained from a Monte Carlo simulation. The octyl derivative (ZnTCO4PP) showed the strongest relative fluorescence quenching and, therefore, the highest exciton diffusion coefficient (5.29 × 10-3 cm2 s-1) and longest exciton diffusion length (~81 nm). The octyl derivative also showed the strongest out-of-plane stacking among the metalloporphyrins studied. This work demonstrates how molecular self-assembly can be used to modulate and direct exciton diffusion in solution-processable metalloporphyrin thin films engineered for optoelectronic and photonic applications.

Stepwise Assembly of an Electroactive Framework from a Co6 S8 Superatomic Metalloligand and Cuprous Iodide Building Units

The design of metal-organic frameworks (MOFs) that incorporate more than one metal cluster constituent is a challenging task. Conventional one-pot reaction protocols require judicious selection of ligand and metal ion precursors, yet remain unpredictable. Stable, preformed nanoclusters, with ligand shells that can undergo additional coordination-driven reactions, provide a platform for assembling multi-cluster solids with precision. Herein, a discrete Co₆ S₈ (PTA)₆ (PTA=1,3,5-triaza-7-phosphaadamantane) superatomic-metalloligand is assembled into a three-dimensional (3D) coordination polymer comprising Cu₄ I₄ secondary building units (SBUs). The resulting heterobimetallic framework (1) contains two distinct cluster constituents and bifunctional PTA linkers. Solid-state diffuse reflectance studies reveal that 1 is an optical semiconductor with a band-gap of 1.59 eV. Framework-modified electrodes exhibit reversible redox behavior in the solid state arising from the Co₆ S₈ superatoms, which remain intact during framework synthesis.

Stepwise Assembly of an Electroactive Framework from a Co6 S8 Superatomic Metalloligand and Cuprous Iodide Building Units

Invited for the cover of this issue is Christopher Bejger and co-workers at UNC Charlotte, Columbia University, and Donghua University. The image depicts a pair of star clusters in the constellation Perseus as the structure of two metal clusters in the reported framework. Read the full text of the article at 10.1002/chem.20201215.

Photothermally and magnetically controlled reconfiguration of polymer composites for soft robotics

New materials are advancing the field of soft robotics. Composite films of magnetic iron microparticles dispersed in a shape memory polymer matrix are demonstrated for reconfigurable, remotely actuated soft robots. The composite films simultaneously respond to magnetic fields and light. Temporary shapes obtained through combined magnetic actuation and photothermal heating can be locked by switching off the light and magnetic field. Subsequent illumination in the absence of the magnetic field drives recovery of the permanent shape. In cantilevers and flowers, multiple cycles of locking and unlocking are demonstrated. Scrolls show that the permanent shape of the film can be programmed, and they can be frozen in intermediate configurations. Bistable snappers can be magnetically and optically actuated, as well as biased, by controlling the permanent shape. Grabbers can pick up and release objects repeatedly. Simulations of combined photothermal heating and magnetic actuation are useful for guiding the design of new devices.