.
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(
azobenzene derivatives), and differ only in the chain length
secondary ligands are collected mainly above and below the
clusters. For the material AuL4, the X-ray diffraction pattern
is characteristic of a columnar structure (see Figure S8), which
is formed by chains of metal spheres surrounded by meso-
genic ligands collected mainly at the equatorial plane of each
metal sphere. When heated, the lamellar and columnar phases
reversibly melted to the isotropic liquid; in the isotropic
phase, a single diffused signal related to the average distance
between particles was observed in the X-ray diffraction
pattern. The transition between the isotropic liquid and the
positionally ordered phases is triggered by changes in the
distribution of mesogenic ligands around the metal cluster.
The lamellar phase was also observed for AuL2L5,
a hybrid material with a low density of photoactive ligands
in the organic corona. AuL2L5 is composed of gold nano-
particles grafted with two promesogenic ligands: the azo
derivative L2 and the biphenyl derivative L5, which was
described previously as a promesogenic agent that promotes
of the substituents on this mesogenic unit and the way in
which these terminal chains are connected to the core
structure. Covalent bonding of the ligand molecules to the
nanoparticle surface was assured by a thiol group at the end of
one of the terminal chains. No mesomorphic behavior was
observed for these ligands. However, synthetic intermediates
with a bromine atom instead of the thiol group formed
smectic phases (see the Supporting Information).
To determine the exact composition of organic coronas of
the studied nanoparticles, we used various analytical tech-
niques, including thermogravimetric analysis (TGA), TEM,
1
H NMR spectroscopy, and XPS (see the Supporting Infor-
mation). According to the results obtained and on the basis of
theoretical predictions reported by Murray and co-workers,
[16]
we estimated that on average each gold nanoparticle is
covered with 92 ligands and each silver nanoparticle is
covered with 410 ligands. The ratio of mesogenic to n-alkyl
thiols is approximately 1:1 (see the Supporting Information
for the composition of organic coronas of the hybrid systems
studied).
The liquid-crystalline properties of the hybrid nanopar-
ticles were investigated by XRD and TEM. Homogenously
aligned samples were obtained by shearing a small amount of
the material at a slightly elevated temperature (ca. 708C) on
a Kapton tape. All hybrid materials composed of gold
nanoparticles covered with n-hexanethiol and promesogenic
ligands exhibited long-range positionally ordered structures,
of either the lamellar or the columnar type (Table 1).
[17]
liquid-crystalline polymorphism of gold nanoparticles.
Silver-based hybrid materials with promesogenic ligands
L1–L3, which have 8 or 10 methylene groups between the
thiol functionality and the mesogenic core, exhibited only
short-range order. Apparently, in the case of larger metallic
clusters, such a linkage is too short to assure sufficient
flexibility of the grafting layer for the formation of an ordered
structure. However, when the spacer between the metal
surface and the ligand core was extended to 16 methylene
groups, as in AgL4, the lamellar phase was observed. In
contrast to gold-based hybrids, the lamellar phase of AgL4
showed weak optical birefringence (see Figure S5) as evi-
dence of some degree of orientational order of the mesogenic
cores in the organic sublayers.
Table 1: Phase sequence of the hybrid nanoparticles.
Compound
Phase sequence
All mesophases of hybrid nanoparticles were photores-
ponsive owing to the presence of photoactive azo units in
their organic coronas. The liquid-crystalline ordered struc-
tures formed by NPs could be reversibly melted by irradiation
of the sample with UV light (wavelength 365 nm), which led
to the isomerization of azo units from the trans to the cis
conformation. Apparently, ligand isomerization to the kinked
cis form disturbed the reshaping of hybrid NPs into aniso-
tropic objects as required for their self-assembly into the LC
phase. The process could be followed by X-ray diffraction:
Under UV irradiation, the XRD signals related to the LC
phase of the hybrid NPs disappeared, and a diffused signal
corresponding to the average distance between NPs in the
isotropic liquid appeared that indicated a slightly smaller
distance than in the temperature-induced isotropic phase. The
difference can be attributed to the different length of ligand
molecules in the cis and trans conformations. When the UV
light was turned off, the LC phase was restored immediately,
with layer periodicity the same as before irradiation
(Figure 1). No absorption of visible light is even necessary
to restore the trans conformation of the azo groups;
apparently, in the condensed state, the linear, trans confor-
mation of ligand molecules is strongly favored over the
kinked, cis conformation.
AuL1
AuL2
AuL3
AuL4
AuL2L5
AgL1
AgL2
AgL3
AgL4
Lam 1608C Iso
Lam 1608C Iso
Lam 1658C Iso
Col 1558C Iso
Lam 1658C Iso
no LC phase
no LC phase
no LC phase
Lam 1578C Iso
Lam=lamellar phase, Col=columnar phase, Iso=isotropic liquid.
Gold particles coated with ligands L1, L2, and L3 formed
lamellar phases, in which layers of metallic clusters were
separated by organic layers formed mainly by mesogenic
ligands. Their X-ray diffraction patterns showed commensu-
rate, sharp Bragg reflections corresponding to layer perio-
dicity along the direction perpendicular to the shearing, and
a weak, diffused signal at the equatorial position owing to the
short-range order of particles within the metal-rich layer. The
distance corresponding to the diffused signal is similar to the
interparticle distance measured for nanoparticles before the
ligand-exchange reaction; apparently, promesogenic ligands
only weakly contribute to the in-plane separation of nano-
particles. Thus, it can be assumed that the lamellar phase is
obtained by redistribution of the secondary grafting mole-
cules around the metallic cluster, the mesogenic cores of
For comparison, we also studied the hybrid material
AuL2L5, in which the density of photoactive units in the
secondary grafting layer was lowered by the use of coligands
2
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Angew. Chem. Int. Ed. 2014, 53, 1 – 5
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