Chemistry Reference
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electron-withdrawing character of the anionic ligands attached to the
exo
gold
atoms, which may weaken the Au-Au interaction between the
exo
Au atoms and
the central Au
6
unit [
101
]. This is noteworthy in comparison with the short
Au-AuCl distances in Au
13
clusters [
34
].
As a different core+
exo
type geometry, Sharp et al. reported Au
8
cluster coor-
dinated by sterically hindered monophosphines ([Au
8
(PMes
3
)
6
]
2+
(10)[
86
]. This
cluster has a tetrahedral core with edge distances of 2.697-2.711 Å, where two
triangles are attached to the adjacent tetrahedral vertexes. The
exo
-to-core distances
(2.616 and 2.623 Å) are slightly shorter than the tetrahedron edges.
5 Optical Properties
5.1 Electronic Absorption Spectra
Generally PGCs are highly colored, so UV-visible electronic absorption spectra
have been widely utilized for characterization. Unlike well-known red colors of
large gold colloids (sizes of ~2 nm or above) offered by localized surface plasmon,
the colors of ultrasmall gold clusters including PGCs essentially originate from the
discrete electronic transitions, which is supported theoretically [
102
]. Therefore,
the absorption spectra are useful not only for characterization but also to obtain
insights into the electronic structures of cluster compounds. On the other hand, in
the nano-size regime, it is well accepted that the lowest optical transition energy of
metal atom assemblies increases as the size is reduced, which is known as quantum-
size effects. In this relation, studies on the relationship between the nuclearity and
the absorption spectra would provide useful knowledge about the intermediate
between nano-objects and molecules.
5.1.1 Centered Polyhedral Clusters
Solution absorption spectra of Au
8
P
8
(8·BF
4
), Au
9
P
8
(14·NO
3
), Au
11
P
8
Cl
2
(29·Cl)
Au
11
P
10
(30·NO
3
), Au
13
P
10
Cl
2
(34·Cl), and Au
13
P
10
Cl
2
(41·Cl) clusters, which
adopt centered polyhedral geometries, are shown in Fig.
6a
. The spectra except
8·BF
4
were measured in our laboratory for crystallographically characterized or
equivalently pure samples. The spectrum of 8·BF
4
is from the literature [
34
]. The
spectral data together with those in literatures are summarized in Table
5
.Asa
common feature, these centered polyhedral clusters exhibit tail-and-humps spectral
characteristics, in which relatively weak visible absorption bands are overlapped
with the main band in the UV region tailing to the red. For example, Au
9
P
8
cluster
(14) shows the lowest energy visible band at ~450 nm and more intense UV band at
~315 nm. Au
11
clusters ([Au
11
P
8
Cl
2
]
+
(29) and [Au
11
P
10
]
3+
(41)) show sets of a
visible band at ~420 nm and more intense UV bands at ~300 nm. The spectral
