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nanoclusters from a single, easily prepared precursor, Au
11
(PPh
3
)
8
Cl
3
. By con-
trolled ligand exchange, organic-soluble nanoclusters can be produced through the
reaction with octadecanethiol (ODT), while water-soluble ones can be obtained by
ligand exchange with
N
,
N
-dimethylaminoethanethiol (DMAT) or mercaptoethane-
sulfonic acid (MESA). From the UV-Vis absorption spectra shown in Fig.
2B
, the
obtained nanoclusters showed similar absorption features but with more fine
structures and broadened absorption peaks compared to the Au
11
(PPh
3
)
8
Cl
3
precur-
sor. The TEM characterizations showed that all of the thiol-protected Au
11
nanoclusters displayed the same core size as the initial Au
11
(PPh
3
)
8
Cl
3
clusters
(~0.8 nm). More importantly, after ligand exchange, the nanoclusters exhibited
remarkably enhanced thermal stability compared with the precursor clusters. With
a slightly different procedure from that described by Bartlett et al., Yang and Chen
[
59
] successfully synthesized stable and monodispersed Au
11
C
l3
(PPh
3
)
7
nanoclusters upon reduction of the precursor chloro(triphenylphosphine)gold
(I) (AuPPh
3
Cl) by NaBH
4
in ethanol. It was also found that in contrast to the
Au
11
C
l3
(PPh
3
)
7
clusters with no fluorescence, photoluminescence at 840 nm was
observed after the ligand exchange with alkanethiols. This study strongly suggests
that the surface functionalization can dramatically change the electronic energy
structure of metal clusters.
3.5 Au
13
Nanoclusters
Recently, M
13
-type clusters have attracted special attention as they can take the
smallest geometrically stable shell-closed structure (“magic number”) [
52
]. As for
Au
13
nanoclusters, the previous structure study on single crystal has shown that they
have icosahedral core geometry [
2
] which has been proposed to be the subunit in the
structures of larger gold clusters such as Au
25
and Au
38
[
23
,
24
,
49
]. Although
the Au
13
nanoclusters have been expected to be thermodynamically stable due to the
closed shell geometry, the synthesis of monodispersed clusters is still a great
challenge. In the earlier syntheses, nonconventional reducing agents were used in
the preparation of Au
13
nanoclusters [
2
,
60
]. Fortunately, by using the unique
capability of HCl to induce the nuclearity convergence in polydispersed Au clusters,
Konishi and coworkers [
61
,
62
] successfully synthesized highly pure Au
13
nanoclusters protected by C2-bridged diphosphine ligand (Ph
2
P-(CH
2
)
2
-PPh
2
,
dppe). As shown in the Scheme
1
, a polydispersed mixture of ultrasmall gold
nanoclusters (Au
N
;9
15) was first synthesized by reducing Au
2
(dppe)Cl
2
(dppe: 1,2-bis(diphenylphosphino) ethane) with NaBH
4
in dichloromethane/ethanol
(96:4 v/v). In the second step, aqueous hydrochloric acid was added into the
polydispersed sample dissolved in ethanol. From the ESI-MS spectra shown in
Fig.
3A
, after the polydispersed clusters were treated with HCl for 3 h, the product
showed a very simple MS spectrum that was mostly dominated by a set of signals at
m/z
N
1,541, which could be assigned to [Au
13
(dppe)
5
Cl
2
]
3+
. Accordingly, with
treatment time increasing, two new absorption bands centered at 360 and 490 nm
