Quantifying the Effect of Guest Binding on Host Environment.

The environment around a host-guest complex is defined by intermolecular interactions between the complex, solvent molecules, and counterions. These interactions govern both the solubility of these complexes and the rates of reactions occurring within the host molecules and can be critical to catalytic and separation applications of host-guest systems. However, these interactions are challenging to detect using standard analytical chemistry techniques. Here, we quantify the hydration and ion pairing of a FeII4L4 coordination cage with a set of guest molecules having widely varying physicochemical properties. The impact of guest properties on host ion pairing and hydration was determined through microwave microfluidic measurements paired with principal component analysis (PCA). This analysis showed that introducing guest molecules into solution displaced counterions that were bound to the cage, and that the solvent solubility of the guest has the greatest impact on the solvent and ion-pairing dynamics surrounding the host. Specifically, we found that when we performed PCA of the measured equivalent circuit parameters and the solubility and dipole moment, we observed a high (>90%) explained variance for the first two principal components for each circuit parameter. We also observed that cage-counterion pairing is well-described by a single ion-pairing type, with a one-step reaction model independent of the type of cargo, and that the ion-pairing association constant is reduced for cargo with higher water solubility. Quantifying hydration and cage-counterion interactions is a critical step to building the next generation of design criteria for host-guest chemistries.

Sc2C, a 2D Semiconducting Electride.

Electrides are exotic materials that typically have electrons present in well-defined lattice sites rather than within atoms. Although all known electrides have an electropositive metal cation adjacent to the electride site, the effect of cation electronegativity on the properties of electrides is not yet known. Here, we examine trivalent metal carbides with varying degrees of electronegativity and experimentally synthesize Sc2C. Our studies identify the material as a two-dimensional (2D) electride, even though Sc is more electronegative than any metal previously found adjacent to an electride site. Further, by exploring Sc2C and Al2C computationally, we find that higher electronegativity of the cation drives greater hybridization between metal and electride orbitals, which opens a band gap in these materials. Sc2C is the first 2D electride semiconductor, and we propose a design rule that cation electronegativity drives the change in its band structure.

Bonding in 2D Donor-Acceptor Heterostructures.

The ability to alter distances between atoms is among the most important tools in materials design. Despite this importance, controlling the interlayer distance in stacks of 2D materials remains a challenge. Here we show from first-principles that stacking electrenes-a new class of electron-donating 2D materials-with other 2D materials provides this control. The resulting donor-acceptor heterostructures have interlayer distances 1 Å less than van der Waals layered materials but 1 Å more than covalent or ionic bonds. This yields a class of quasi-bonds that exhibit characteristics of both ordinary chemical bonds and van der Waals interactions. We show how quasi-bonds have tunable polarities and strengths and that these bonds can be understood by drawing on familiar concepts from molecular orbital theory. We also demonstrate several useful properties of 2D donor-acceptor heterostructures, including superlubricity, ultralow work functions, and greatly improved voltages for lithium-ion batteries.

Rubber-elasticity and electrochemical activity of iron(ii) tris(bipyridine) crosslinked poly(dimethylsiloxane) networks.

2,2'-Bipyridine-terminated poly(dimethylsiloxane)s (bpyPDMS) with number average molecular weights, MN, of 3300, 6100, 26 200, and 50 000 g mol-1 were synthesized. When mixed with Fe(BF4)2 at low concentrations, red solutions formed with UV-vis spectra that match those of iron(ii) tris(2,2'-bipyridine) (Fe(bpy)32+). Upon solvent evaporation, Fe(bpy)32+ crosslinked PDMS networks (bpyPDMS/Fe(ii)) formed, and were studied using oscillating shear rheometry. The shear storage moduli (0.084 to 2.6 MPa) were found to be inversely proportional to the MN of the PDMS, though the storage moduli at low molecular weights greatly exceeded the storage moduli of comparable covalently crosslinked PDMS networks. The shear storage moduli exhibited the characteristic rubbery plateau up to ∼135 °C. Films of bpyPDMS/Fe(ii) coated onto electrodes were found to be electrochemically active, especially so when the PDMS MN is low. The Fe(bpy)32+ crosslinks can be reversibly oxidized over ∼500 nm away from the electrode surface in the presence of a suitable electrolyte.

First-Principles Prediction of Electrochemical Electron-Anion Exchange: Ion Insertion without Redox.

It is widely assumed that the gain or loss of electrons in a material must be accompanied by its reduction or oxidation. Here, we report a system in which the insertion/deinsertion of an electron occurs without any reduction or oxidation. Using first-principles methods, we demonstrate this effect in the Y2CF2-[Y2C]2+(e-)2 material system, where (e-) indicates a lattice site containing a bare electron. We present a model in which Y2CF2 is in contact with a fluoride-containing electrolyte and the application of a positive voltage drives fluorination while a negative voltage reverses the process. We show that this chemistry does not change the oxidation states of the host lattice, causes no significant volume expansion, and occurs rapidly at room temperature. Finally, we demonstrate that this mechanism of ion insertion may enable a broad class of anion shuttle battery electrodes, some with gravimetric capacities nearly double those employed in intercalation-type Li-ion batteries.