Electronic Properties of [Core+<i>exo</i>]‑type Gold Clusters: Factors Affecting the Unique Optical Transitions

Unusual visible absorption properties of [core+<i>exo</i>]-type Au₆ (<b>1</b>), Au₈ (<b>2</b>), and Au₁₁ (<b>3</b>) clusters were studied from experimental and theoretical aspects, based on previously determined crystal structures. Unlike conventional core-only clusters having no <i>exo</i> gold atoms, these nonspherical clusters all showed an isolated visible absorption band in solution. Density functional theory (DFT) studies on corresponding nonphenyl models (<b>1</b>′–<b>3</b>′) revealed that they had similar electronic structures with discrete highest occupied molecular orbital (HOMO) and lowest unoccupied molecular orbital (LUMO) bands. The theoretical spectra generated by time-dependent DFT (TD-DFT) calculations agreed well with the experimentally measured properties of <b>1</b>–<b>3</b>, allowing assignment of the characteristic visible bands to HOMO–LUMO transitions. The calculated HOMO–LUMO transition energies increased in the order Au₁₁ < Au₆ < Au₈, as was found experimentally. Frontier orbital analyses indicated that the HOMO and LUMO were both found in proximity to the terminal Au₃ triangles containing the <i>exo</i> gold atom, with the HOMO → LUMO transition occurring in the core → <i>exo</i> direction. The HOMO/LUMO distribution patterns of <b>1</b>′ and <b>3</b>′ were similar to each other but were markedly different from that of <b>2</b>′, which has longer core-to-<i>exo</i> distances. These findings showed that not only nuclearity (size) but also geometric structures have profound effects on electronic properties and optical transitions of the [core+<i>exo</i>]-type clusters

Hexanuclear Platinum(II) Thiolate Macrocyclic Host: Charge-Transfer-Driven Inclusion of a Ag^I Ion Guest

The inclusion of a Ag^I ion by a hexanuclear platinum­(II) thiolate macrocycle in solution was demonstrated, and the inclusion structure was determined by X-ray crystallography. Unique host–guest intermetallic interactions driven by charge transfer were elucidated by optical absorption spectroscopy and theoretical calculations

[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

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