Chemistry Reference
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Fig. 1 X-ray crystal structures of the cationic moieties of the Au
6
and Au
8
clusters and the growth
from Au
6
to Au
8
through the reaction with [Au(PPh
3
)Cl]. Gold:
gray
spheres. Reprinted from [
52
]
with permission by Wiley-VCH
as-synthesized Au
8
nanoclusters show strong size-dependent emission, with a
quantum yield of ~41% in aqueous solution, which makes them novel fluorophores
due to the more than 100-fold enhancement in quantum yield.
Konishi and coworkers [
52
] reported the synthesis of two novel cluster cations
[Au
8
(dppp)
4
Cl
2
]
2+
and [Au
8
(dppp)
4
]
2+
1,3-bis(diphenylphosphino) pro-
pane) through a growth/etching process (as shown in Fig.
1
). The synthesis starts
from well-defined Au
6
nanoclusters ([Au
6
(dppp)
4
]I(NO
3
)
2
), whose core contains a
tetrahedral Au
4
unit plus two gold atoms bridged at opposite edges of the tetrahedron.
After the Au
6
nanoclusters were mixed with [Au(PPh
3
)Cl] in methanol/chloroform at
room temperature, a gradual color change of the solution from intense blue to optic
pink was observed, indicating the formation of a new nanocluster. The reaction
process was monitored by UV-Vis absorption spectroscopy. With the reaction time
increasing, the characteristic absorption of Au
6
nanoclusters decreased, while a new
band at 510 nm appeared and gradually grew up. From the ESI-MS of the product, a
set of signals at approximately
m/z
1,648 appeared, which was unambiguously
assigned to the divalent [Au
8
(dppp)
4
Cl
2
]
2+
cluster cation. After crystallization from
dichloromethane/ether, only a sole cluster species (
m/z
(dppp
¼
1613), assigned to
[Au
8
(dppp)
4
]
2+
, was isolated. X-ray crystallographic analysis revealed that the core
of such cluster adopts edge-shared tri-tetrahedral geometry and thus has a prolate
shape, which is clearly different from that of the [Au
8
(dppp)
4
Cl
2
]
2+
. Interestingly, the
authors found that [Au
8
(dppp)
4
]
2+
could be instantly oxidized to [Au
8
(dppp)
4
Cl
2
]
2+
under aerobic conditions upon addition of tetraethylammoniumchloride (Cl
and air).
In the reverse reaction, [Au
8
(dppp)
4
Cl
2
]
2+
may be reduced to [Au
8
(dppp)
4
]
2+
using
NaBH
4
. This work emphasizes that the electronic properties of Au nanoclusters
depend on not only the Au number in the core but also the core geometry and oxidation
states. Moreover, this study also displayed the preparation of novel small nanoclusters
with unique geometries through post-synthetic methods utilizing growth/etching
processes.
Inspired by the etching route for the synthesis of fluorescent gold nanoclusters,
Guo et al. [
53
] demonstrated a unique heterophase ligand-exchange-induced etch-
ing process to synthesize water-soluble fluorescent Au
8
nanoclusters. In the
¼
