Facile modulation of optical properties of octagold clusters through the control of ligand-mediated interactions

Abstract

In the recent development of structurally defined ligand-stabilized gold clusters, it has been revealed that not only the inorganic units but also the surrounding organic ligands substantially affect their electronic/optical properties. In this work, a series of core + exo type Au8 clusters decorated by dppp (Ph2P(CH2)3PPh2) and arylthiolate ligands ([Au8(dppp)4(SR)2](2+), 1-5) were synthesized, and their optical properties were studied in order to gain insights into the perturbation effects of the organic ligands. 1-5 showed visible absorption and photoluminescence emission bands at longer wavelengths compared to their chloro- and acetylide-modified analogues, suggesting the contribution of weak non-bonding interactions of the Au framework with the ligand heteroatoms. Upon acid treatment, 2- and 4-pyridinethiolate clusters (R = Py, 2 and 4) showed larger red shifts of the absorption and emission bands than the 3-pyridyl isomer (3), implying the involvement of the resonance structures of the SPy units. On the other hand, all regioisomers (2-4) showed large photoluminescence enhancements upon pyridine protonation. X-ray crystallographic and NMR analyses of 4 and its protonated form (4') showed that the electron-deficient pyridinium rings of 4' form π-stacks with neighbouring phenyl groups of dppp, suggesting that the orientation of the surface aromatics is a plausible factor governing the emission efficiency. These observations provide examples of successful modulation of optical properties of small gold clusters through the electronic and/or steric perturbation by the proximal organic ligands, highlighting the importance of the ligand design in the fine tuning of cluster properties directed for optical chemosensors and luminescent materials.

Toward Controlling the Electronic Structures of Chemically Modified Superatoms of Gold and Silver

“…[ 124 ] As a result, the oblate superatoms with an electron configuration of (1S) 2 (1P) 4 become stable due to the closure of the electronic shell, [ 124 ] as can be found in Au 7 (6e), [ 125,126 ] Au 8 (6e), [ 34 ] and the crown‐shaped Au 9 (6e) [ 29,127–131 ] ( Table 6 ). In contrast, the prolate superatoms with an electron configuration of (1S) 2 (1P) 2 become stable, as can be found in Au 6 (4e) [ 132–140 ] and Au 7 (4e) [ 141–143 ] (Table 6). The magic stability of the prolate superatoms with n * = 14 such as Au 20 (14e), [ 144,145 ] Au 23 (14e), [ 146,147 ] Ag 23 (14e), [ 75 ] and Au 25 (14e) [ 148 ] can be explained by the electronic shell closure with an electron configuration of (1S) 2 (1P) 6 (1D) 6 .…”

“…[ 98–100,103 ] These examples convince us that a unique class of quasi‐molecules can be developed by bonding various superatomic units via various bonding schemes. [ 75,98–120,135–154 ]…”

Terahertz Raman Spectroscopy of Ligand-Protected Au8 Clusters

“…These properties are also known to be quite sensitive to secondary core-ligand interactions such as Au•••π interactions. [12][13][14] Such weak interactions can cause local strain of the gold core unit, which can be visualized by vibrational and nuclear resonance spectroscopic techniques, [13][14] rather than the single crystal X-ray analysis.…”

“…The Au8 clusters of [Au8X2(dppp)4] 2+ and [Au8(PPh3)8] 2+ have different gold frameworks: the former has a [core + exo]-type Au8 framework (Figure 1a) 13,33 whereas the latter has a capped centered chair framework (Figure 1b). [34][35] Particularly for [Au8X2(dppp)4] 2+ , various monodentate ligands can be used as the terminal ligand X, keeping the same framework but changing the presence/absence of core-ligand interactions, [12][13]33 providing an ideal platform to investigate structural differences of gold frameworks and weak secondary core-ligand interactions in THz Raman spectroscopy. [Au8(PPh3)8] 2+ .…”

Red-Emitting Copper Nanoclusters: From Bulk-Scale Synthesis to Catalytic Reduction

“…Metal nanoclusters with fascinating fluorescent properties and usefulness have drawn a considerable research interest among nanomaterial scientists. − Nanoclusters show fascinating fluorescent behavior due to their quantum size effects unlike their bulk metallic counterparts . They hold interesting applications as bioimaging probes, , semiconductor materials, , catalysts, sensing probes, , and many others.…”