Protonation-Induced Chromism of Pyridylethynyl-Appended [core+<i>exo</i>]‑Type Au₈ Clusters. Resonance-Coupled Electronic Perturbation through π‑Conjugated Group

A series of [core+<i>exo</i>]-type Au₈ clusters bearing two alkynyl ligands on the <i>exo</i> gold atoms ([Au₈(dppp)₄(CCR)₂]²⁺, <b>2</b>–<b>6</b>) were synthesized by the reaction of [Au₈(dppp)₄]²⁺ (<b>1</b>) with alkynyl anions. Although the CC moieties directly attached to the Au₈ units did not affect the optical properties arising from intracluster transitions, the pyridylethynyl-bearing clusters (<b>4</b>–<b>6</b>) exhibited reversible visible absorption and photoluminescence responses to protonation/deprotonation events of the terminal pyridyl moieties. The chromism behaviors and proton-binding constants of these clusters were highly dependent on the relative position of the pyridine nitrogen atom, such that the 2-pyridyl (<b>4</b>) and 4-pyridyl (<b>6</b>) isomers showed more pronounced responses than the 3-pyridyl isomer (<b>5</b>). These results suggest that the resonance-coupled movement of the positive charge upon protonation is involved in the optical responses, where the formation of extended charged resonance structures causes significant perturbation effects on the electronic properties of the Au₈ unit and also contributes to the high binding affinities

[Au₇]³⁺: A Missing Link in the Four-Electron Gold Cluster Family

Ligand-stabilized ultrasmall gold clusters offer a library of diverse geometrical and electronic structures. Among them, clusters with four valence electrons form an exceptional but interesting family because of their unique geometrical structures and optical properties. Here, we report a novel diphosphine-ligated four-electron Au₇ cluster (<b>2</b>). In good agreement with previous theoretical predictions, <b>2</b> has a “core+<i>one</i>” structure to exhibit a prolate shape. The absorption spectrum showed an isolated band, similar to the spectra of Au₆ and Au₈ clusters with “core+<i>two</i>” structures. TD-DFT studies demonstrated that the attachment of only one gold atom to a polyhedral core is sufficient to generate unique electronic structures and characteristic absorptions. The present result fills the missing link between Au₆ and Au₈ in the four-electron cluster family, showing that the HOMO–LUMO gap increases with increasing nuclearity in the case of the tetrahedron-based “core+<i>exo</i>” clusters

Protonation-Induced Chromism of Pyridylethynyl-Appended [core+<i>exo</i>]‑Type Au₈ Clusters. Resonance-Coupled Electronic Perturbation through π‑Conjugated Group

A series of [core+<i>exo</i>]-type Au₈ clusters bearing two alkynyl ligands on the <i>exo</i> gold atoms ([Au₈(dppp)₄(CCR)₂]²⁺, <b>2</b>–<b>6</b>) were synthesized by the reaction of [Au₈(dppp)₄]²⁺ (<b>1</b>) with alkynyl anions. Although the CC moieties directly attached to the Au₈ units did not affect the optical properties arising from intracluster transitions, the pyridylethynyl-bearing clusters (<b>4</b>–<b>6</b>) exhibited reversible visible absorption and photoluminescence responses to protonation/deprotonation events of the terminal pyridyl moieties. The chromism behaviors and proton-binding constants of these clusters were highly dependent on the relative position of the pyridine nitrogen atom, such that the 2-pyridyl (<b>4</b>) and 4-pyridyl (<b>6</b>) isomers showed more pronounced responses than the 3-pyridyl isomer (<b>5</b>). These results suggest that the resonance-coupled movement of the positive charge upon protonation is involved in the optical responses, where the formation of extended charged resonance structures causes significant perturbation effects on the electronic properties of the Au₈ unit and also contributes to the high binding affinities

Thermal and Electrochemical Stability of Tetraglyme–Magnesium Bis(trifluoro­methane­sulfonyl)amide Complex: Electric Field Effect of Divalent Cation on Solvate Stability

Phase behavior of binary mixtures of tetraglyme (G4) and Mg­[TFSA]₂ (TFSA: bis­(trifluoro­methane­sulfonyl)­amide) was investigated. In a 1:1 molar ratio, G4 and Mg­[TFSA]₂ formed a stable complex with a melting point of 137 °C. X-ray crystallography of a single crystal of the complex grown from a G4-Mg­[TFSA]₂ binary mixture revealed that the G4 molecule wraps around Mg²⁺ to form a complex [Mg­(G4)]²⁺ cation, and the two [TFSA]⁻ anions also participate in the Mg²⁺ coordination in the crystal. The thermal stability of [Mg­(G4)]­[TFSA]₂ was examined by thermogravimetry, and it was found that the complex is stable up to 250 °C. Above 250 °C, desolvation of the Mg²⁺ ion takes place and G4 evaporates. On the other hand, the weight loss starts at around 140 °C in solutions containing excess G4 (<i>n</i> > 1 in Mg­[TFSA]₂:G4 = 1:<i>n</i>) due to the evaporation of free (uncoordinated) G4. The suppression of G4 volatility in the [Mg­(G4)]­[TFSA]₂ complex is attributed to strong electrostatic and induction interactions between divalent Mg²⁺ and G4. In addition, complexation of G4 with Mg²⁺ is effective in enhancing the oxidative stability of G4. Linear sweep voltammetry revealed that the oxidative decomposition of [Mg­(G4)]­[TFSA]₂ occurs at electrode potentials >5 V vs Li/Li⁺, while the oxidation of uncoordinated G4 occurs at around 4.0 V. This oxidative stability enhancement occurs because the HOMO energy level of G4 is reduced by complexation with Mg²⁺, which is supported by the <i>ab initio</i> calculations

Promising Cell Configuration for Next-Generation Energy Storage: Li₂S/Graphite Battery Enabled by a Solvate Ionic Liquid Electrolyte

Lithium-ion sulfur batteries with a [graphite|solvate ionic liquid electrolyte|lithium sulfide (Li₂S)] structure are developed to realize high performance batteries without the issue of lithium anode. Li₂S has recently emerged as a promising cathode material, due to its high theoretical specific capacity of 1166 mAh/g and its great potential in the development of lithium-ion sulfur batteries with a lithium-free anode such as graphite. Unfortunately, the electrochemical Li⁺ intercalation/deintercalation in graphite is highly electrolyte-selective: whereas the process works well in the carbonate electrolytes inherited from Li-ion batteries, it cannot take place in the ether electrolytes commonly used for Li–S batteries, because the cointercalation of the solvent destroys the crystalline structure of graphite. Thus, only very few studies have focused on graphite-based Li–S full cells. In this work, simple graphite-based Li–S full cells were fabricated employing electrolytes beyond the conventional carbonates, in combination with highly loaded Li₂S/graphene composite cathodes (Li₂S loading: 2.2 mg/cm²). In particular, solvate ionic liquids can act as a single-phase electrolyte simultaneously compatible with both the Li₂S cathode and the graphite anode and can further improve the battery performance by suppressing the shuttle effect. Consequently, these lithium-ion sulfur batteries show a stable and reversible charge–discharge behavior, along with a very high Coulombic efficiency