In situ gel-to-crystal transition and synthesis of metal nanoparticles obtained by fluorination of a cyclic β-aminoalcohol gelator

Article abstract of DOI:10.1002/chem.201202615

A new fluorinated version of a cyclic β-aminoalcohol gelator derived from 1,2,3,4-tetrahydroisoquinoline is presented. The gelator is able to gel various nonprotic solvents through OH×××N hydrogen bonds and additional CH×××F interactions due to the introduction of fluorine. A bimolecular lamellar structure is formed in the gel phase, which partly preserves the pattern of molecular organization in the single crystal. The racemate of the chiral gelator shows lower gelation ability than its enantiomer because of a higher tendency to form microcrystals, as shown by X-ray diffraction analysis. The influence of fluorination on the self-assembly of the gelator and the properties of the gel was investigated in comparison to the original fluorine-free gel system. The introduction of fluorine brings two new features. The first is good recognition of o-xylene by the gelator, which induces an in situ transition from gels of o-xylene and of an o-xylene/toluene mixture to identical single crystals with unique tubular architecture. The second is the enhanced stability of the toluene gel towards ions, including quaternary ammonium salts, which enables the preparation of a stable toluene gel in the presence of chloroaurate or chloroplatinate. The gel system can be used as a template for the synthesis of spherical gold nanoparticles with a diameter of 5 to 9 nm and wormlike platinum nanostructures with a diameter of 2 to 3 nm and a length of 5 to 12 nm. This is the first example of a synthesis of platinum nanoparticles in an organogel medium. Therefore, the appropriate introduction of a fluorine atom and corresponding nonbonding interactions into a known gelator to tune the properties and functions of a gel is a simple and effective tactic for design of a gel system with specific targets. Just add fluorine: Fluorination of a gelator brings new features to the resulting gel, that is, the in situ gel-to-crystal transition for selective recognition of o-xylene by the gelator, which indicates the structural evolution of the gel architecture, and also the synthesis of Au and Pt nanoparticles by using a toluene gel as a template that provides enhanced stability towards the ions (see figure; TOA-Au=(C₈H₁₇)₄N⁺[AuCl ₄]^-; TOA-Pt=(C₈H₁₇) ₄N⁺[PtCl₆]^-). Copyright

Full text of DOI:10.1002/chem.201202615

FULL PAPER
DOI: 10.1002/chem.201202615
In Situ Gel-to-Crystal Transition and Synthesis of Metal Nanoparticles
Obtained by Fluorination of a Cyclic b-Aminoalcohol Gelator
Yang Xu,[a, b] Chuanqing Kang,*[a] Yu Chen,[a] Zheng Bian,[a] Xuepeng Qiu,[a]
Lianxun Gao,*[a] and Qingxin Meng[b]
Abstract: A new fluorinated version of
a cyclic b-aminoalcohol gelator derived
from 1,2,3,4-tetrahydroisoquinoline is
presented. The gelator is able to gel
various nonprotic solvents through
OH···N hydrogen bonds and additional
CH···F interactions due to the introduc-
tion of fluorine. A bimolecular lamellar
structure is formed in the gel phase,
which partly preserves the pattern of
molecular organization in the single
crystal. The racemate of the chiral ge-
lator shows lower gelation ability than
its enantiomer because of a higher ten-
dency to form microcrystals, as shown
by X-ray diffraction analysis. The influ-
ence of fluorination on the self-assem-
bly of the gelator and the properties of
the gel was investigated in comparison
to the original fluorine-free gel system.
The introduction of fluorine brings two
new features. The first is good recogni-
tion of o-xylene by the gelator, which
induces an in situ transition from gels
of o-xylene and of an o-xylene/toluene
mixture to identical single crystals with
unique tubular architecture. The
second is the enhanced stability of the
toluene gel towards ions, including qua-
ternary ammonium salts, which enables
the preparation of a stable toluene gel
in the presence of chloroaurate or
chloroplatinate. The gel system can be
used as a template for the synthesis of
spherical gold nanoparticles with a di-
ameter of 5 to 9 nm and wormlike plat-
inum nanostructures with a diameter of
2 to 3 nm and a length of 5 to 12 nm.
This is the first example of a synthesis
of platinum nanoparticles in an organo-
gel medium. Therefore, the appropriate
introduction of a fluorine atom and
corresponding nonbonding interactions
into a known gelator to tune the prop-
erties and functions of a gel is a simple
and effective tactic for design of a gel
system with specific targets.
Keywords: fluorine · gels · gel-to-
crystal transition · nanoparticles ·
self-assembly
Introduction
is to integrate an original gelator structure with new groups
that can provide new driving forces or specific functions. A
small structural change could have a great influence on the
self-assembly of a gelator and the organization of 3D nano-
fibrous networks, which subsequently changes the macro-
scopic properties.[5] Understanding certain fundamental as-
pects, such as the self-assembly mechanisms of LMOGs,
control of molecular packing in the fibers of a gel, and the
relationship between the molecular packing in gelation and
crystallization, are primary requirements for effective ma-
nipulation of the gel formation and for designing new
LOMGs.[6] A few examples of in situ gel to single crystal
transitions have opened up investigation of the organization
and mechanisms of molecule aggregates in gel state,[7,8]
which are also interesting topics from a physical view-
point.[9]
In particular, few fluorinated LMOGs with unique proper-
ties and functions, such as the gelation of perfluorocarbons
or supercritical carbon dioxide and templating of fluorinated
silica nanomaterials, have been reported.[10] R. G. Weiss
et al. described the design of fluorine-containing three-com-
ponent gelators to tune molecular aggregation and gel sta-
bility.[11] K. L. Caran et al. made a comparison between the
gelation properties of fluorinated and nonfluorinated prop-
argylic alcohols, in which weak CH···F interactions were pro-
posed to play a role in the self-assembly of the gelators.[12]
Low-molecular-mass organogelators (LMOGs) have attract-
ed much attention from the chemistry community owing to
their applications in various interdisciplinary fields.[1] Driven
by multiple weak nonbonding interactions,[2] the LMOGs
self-aggregate into entangled 3D nanofibrous networks that
immobilize liquids through surface tension and capillary
forces.[3] Control and exploitation of supramolecular gelation
are important aspects of the grand challenge of directed as-
sembly of extended structures with targeted properties.[4]
One facile tactic to tune the properties or functions of a gel
[a] Y. Xu, Dr. C. Kang, Y. Chen, Dr. Z. Bian, Prof. Dr. X. Qiu,
Prof. Dr. L. Gao
State Key Laboratory of Polymer Physics and Chemistry
Changchun Institute of Applied Chemistry
Chinese Academy of Sciences
5625 Renmin Street, Changchun 130022 (China)
Fax : (+86) 431-85262926
E-mail: kangcq@ciac.jl.cn
lxgao@ciac.jl.cn
[b] Y. Xu, Dr. Q. Meng
Department of Polymer Materials
Changchun University of Technology
2055 Yan’an Street, Changchun 130022 (China)
Supporting information for this article is available on the WWW
under http://dx.doi.org/10.1002/chem.201202615.
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Fluorine seems to be an indispensable structural factor for
gelation in these cases. The strongly electronegative fluorine
atom usually forms strong interactions with electrophilic
atoms, such as the CH···F bonding observed in crystallogra-
phy.[13] However, no report has ever clarified the generic
role of fluorine in the formation of gel phase.
Gel test: The gelation ability of molecules 2–5 in various
common solvents was tested by using the inverted-tube
method. Compounds 3–5 showed good solubility but no ge-
lation ability in all tested solvents. Compound 2 formed an
organogel in most of the tested solvents, as summarized in
Table 1. The minimum gelation concentration (MGC) of 2 is
0.8, 0.5, 0.6, 0.9, 2.5, and 4.0%gmLÀ1 in benzene, toluene, o-
xylene, chlorobenzene, acetonitrile, and ethyl acetate at
208C, respectively. Chloroform, dichloromethane, and tetra-
hydrofuran could be gelled by 2 below 08C at a concentra-
tion of 5.0%gmLÀ1, but the gels quickly convert to solu-
tions at room temperature.
An exciting application of gels is as a template in the syn-
thesis of metal nanoparticles.[14] There have been a number
of reports on the growth of metal nanoparticles, such as
silver nanoparticles and gold nanoparticles (AuNPs), within
the gel matrix.[15] A. K. Das reported the only example to
date of the synthesis of platinum nanoparticles (PtNPs) in
bolaamphiphile hydrogels.[16] The formation and size of
nanoparticles within the gel phase are controlled by tunable
cages that effectively separate metal ions to prevent the ag-
gregation of metal particles on reduction.[17] The challenge is
to form the desired small particles with narrow size distribu-
tion. The common requirements for gels to template the
synthesis of metal nanoparticles are sufficient stability to-
wards metal ions, compatibility with surfactants and stabiliz-
ers, and stabilization of the nanoparticles. We are also inter-
ested in the synthesis of metal nanoparticles with a gel
system that fulfills these requirements.
Table 1. Gelation by 2 and its racemate.[a]
Solvent
2
rac-2
benzene
toluene
o-xylene
chlorobenzene
acetonitrile
ethyl acetate
chloroform
dichloromethane
tetrahydrofuran
CG (0.8)
CG (0.5)
CG (0.6)
CG (0.9)
CG (2.5)
CG (4.0)
CG (5.0)[b]
CG (5.0)[b]
CG (5.0)[b]
CG (2.0)
CG (0.9)
CG (2.4)
CG (2.0)
OG (4.8)
PG[c]
[d]
–
–
[d]
[d]
–
With this knowledge in hand, we investigated whether
and how fluorine provides nonbonding intermolecular inter-
actions to determine the self-aggregation of molecules into
[a] Gelation was measured with inverted-tube method at 208C; only sol-
vents gelled by 2 are listed; CG=clear gel, OG=opaque gel, PG=parti-
al gel; numbers in parenthesis are minimum gelation concentrations
[%gmLÀ1]. [b] Gels were formed below 08C. [c] Concentration >5.0%.
[d] Not measured.
organogels.
A cyclic b-aminoalcohol organogelator (1)
based on the 1,2,3,4-tetrahydroisoquinoline (THIQ) scaffold
(Scheme 1) was synthesized and characterized in our previ-
ous study.[18] Herein, we incorporate fluorine into the cyclic
b-aminoalcohol to evaluate the influence of a fluorine atom
on the self-assembly and gelation properties. Molecules 2–5,
in which fluorine or trifluoromethyl replace the correspond-
ing proton on 1, are considered (Scheme 1). We present the
results of the influence of the fluorination on gelation, in
situ phase transition, and stability towards ions. The en-
hanced stability enables the synthesis of metal nanoparticles
by using this gel system.
The range of solvents gelled by 2 is narrower than that of
its nonfluorinated analogue 1, given that alcohols and ke-
tones cannot be gelled by 2. The reason for this may be the
high potential of protic solvents (alcohols) or solvents with
active a-protons (ketones) to form hydrogen bonds with flu-
orine atoms, which would interrupt the gelator–gelator inter-
actions responsible for gelation.[21] The poor gelation ability
of 4 compared with its epimer 2 suggests the importance of
the cis arrangement of 3-hydroxymethyl to 1-aryl for gelati-
on. Molecules 3 and 5, which contain a trifluoromethyl
group, may afford better compatibility with solvent mole-
cules to give better solvation of the gelators. Gelation arises
from a balance between solvent–gelator interactions and ge-
lator–gelator interactions. Therefore, good solvation would
damage aggregation of gelators and result in the poor gelati-
on ability of molecules 3–5, which is very similar to the em-
pirical understanding of gelation manipulated by different
peripheral groups or solvents.[21]
Scheme 1. THIQ-based cyclic b-aminoalcohols
Molecule rac-2 displays gelation ability with higher MGC
values of 2.0, 0.9, 2.4, 2.0, and 4.8%gmLÀ1 in benzene, tol-
uene, o-xylene, chlorobenzene, and acetonitrile, respectively.
A partial gel is formed in ethyl acetate at 5%gmLÀ1. The
gelation ability of rac-2 is not as good as its enantiomer 2.
Results and Discussion
Synthesis: Enantiomeric aminoalcohols 2–5 were prepared
from l-DOPA by using known procedures;[18,19] the race-
mate of 2 was synthesized by methods we previously report-
ed.[20] All structures were confirmed by using 1D and
2D NMR spectroscopy and mass spectroscopy (the synthesis
and characterizations are given in detail in the Supporting
Information).
Micromorphology: Micromorphological investigations of the
xerogels of 2 were carried out by using scanning electronic
microscopy (SEM) techniques. The images in Figure 1 show
the subtle influence of the solvent on the formation of the
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Fluorination of a Cyclic b-Aminoalcohol Gelator
FULL PAPER
Figure 2. Crystals formed by an in situ gel-to-crystal transition in o-
xylene gel.
The crystals of 2 from o-xylene gel are suitable for X-ray
single-crystal analysis, and display a monoclinic system with
P21 space group. As shown in Figure 3a, each unit cell is
composed of two gelator molecules and two o-xylene mole-
cules. The molecules of 2 are arranged parallel to the ac
plane through OH···N hydrogen bonds (1.906 ꢁ) to form a
linear aggregate along the b axis, in which two adjacent mol-
ecules are assembled in a staggered head-to-head manner
(Figure 3b). Additional CH···F interactions (2.552 ꢁ) be-
tween the fluorine atoms in one linear aggregate and the
methoxy protons in another linear aggregate drive the for-
mation of a tubular architecture along the b axis, with a di-
ameter of about 8 ꢁ, which acts as a host for o-xylene mole-
Figure 1. SEM images of xerogels from a) toluene, b) o-xylene, c) ben-
zene, d) chlorobenzene, e) ethyl acetate, and f) acetonitrile. Scale bars: 2
(a), 5 (b, e, f), and 20 mm (c, d).
fibrous networks. Well-developed fibrous networks that
range in size from the nano- to micrometer scale are formed
in all solvents except benzene, in which a crinkled film is
formed. This film may be derived from the entanglement of
thin fibrous networks. The xerogel of the chlorobenzene gel
is composed of recognizable fibers that are woven into a
compact network like the film in benzene. Polar solvents,
such as ethyl acetate and acetonitrile, seem to favor the for-
mation of rigid nanofibers. SEM images of xerogels from
toluene and o-xylene display a remarkable difference in that
toluene induces bundles of nanofibers in a parallel arrange-
ment whereas o-xylene produces distinctive nanofiber mi-
cromorphology. The gel fibers in o-xylene extend randomly,
which indicates that the solvent separates the nanofibers
more effectively than toluene. The fiber–fiber interactions in
o-xylene are disrupted to a similar degree as in polar sol-
vents, which implies subtle difference in the properties of
the toluene and o-xylene gels.
Gel-to-crystal transitions and single crystal analysis: Most of
the gels of 2 are stable enough to survive long-term stand-
ing. However, an exception is the o-xylene gel, which is
spontaneously converted into a single crystal on standing for
around 2 d at room temperature. As shown in Figure 2, a
few crystals appear in the gel matrix, followed by collapse of
the gel into a solution with more crystal particles. The gela-
tor tends to be precipitated directly from o-xylene if the
concentration is greater than 3%gmLÀ1. The in situ transi-
tion from organogel to single crystals affords an opportunity
for deep insights into the self-assembly of the gelator.
Figure 3. Single-crystal analysis of 2, showing a) the unit cell; b) the
linear aggregation of the gelator through OH···N hydrogen bonds; c) the
cyclic tetramer formed by OH···N and CH···F interactions; d) multi-tubu-
lar stacking forms host sites for o-xylene molecules; e) parallel stacking
of o-xylene along the b axis. For clarity, only hydrogen atoms on hetero-
AHCTUNGERTGaNNUN toms or involved in nonbonding interactions in a), b), and c) are shown.
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C. Kang, L. Gao et al.
cules (Figure 3c). The tubular structure extends along the a
and c axes to form a multitubular aggregate through van der
Waals and CH···F interactions, respectively (Figure 3d). The
solvent molecules are encapsulated in the multitubular
structures and constrained in antiparallel stacks by weak
CH···p interactions between the methyl and aryl moieties of
adjoining o-xylene molecules (Figure 3e).
Further experiments were carried out to understand the
role of toluene and o-xylene in the self-assembly of the gela-
tors. A gel of a o-xylene/toluene mixture (1:1) can convert
into a single crystal with exclusive inclusion of o-xylene in
the same stacking pattern as that of crystals from the o-
xylene gel. A single crystal with same architecture was also
obtained on concentration of a dilute solution of 2 in a mix-
ture of o-xylene and toluene (1:1) under a weak nitrogen
flow. The ratio of solvents was changed to about 95:5 when
the crystals had formed. This solvent selectivity suggests
high recognition of the spatial size and shape of o-xylene
molecules during assembly of the gelators.[22] A toluene gel
collapses and converted to crystals if it is covered with o-
xylene for two weeks. It is likely that dissolution of fibrous
network of the toluene gel and crystal formation take place
in combination with diffusion of o-xylene into the gel phase.
The gel phase is an important transition state for the self-
assembly of the fluorinated gelator. The gel-to-crystal transi-
tion is considered to be a stage in the route from gelation
through dissociation to crystallization.[8,23] In the case of the
fluorinated gel system, competition between the two phases
favors crystallization in the presence of o-xylene, which
helps break the metastable gelation structure and promote
stable crystal aggregation. It is likely that o-xylene repairs
defects in the molecular self-assembly in the gel phase and
stabilizes the tubular structure of the gelators in the crystal
phase. The in situ transition from gel to single crystal reveals
an evolution of the self-assembly of the fluorinated gelators
from low orderliness to high orderliness, in which both addi-
tional CH···F interactions and the appropriate spatial size
and architectural adjustment of solvent molecules are impor-
tant factors.
Figure 4. a) The simulated XRD of a single crystal of 2 from o-xylene
and b) the powder XRD of xerogel 2 from toluene.
The strongest intermolecular interactions in the gel system
are the hydrogen bonds between hydroxy and amine groups,
which spontaneously initiate the self-assembly of free mole-
cules into head-to-head bimolecular units and subsequently
to linear aggregates. The lateral fluorine atoms drive the
side-by-side arrangement of the linear aggregates through
CH···F interactions that extend to form a bimolecular layer
with a space of around 18.1 ꢁ, equivalent to the (100) plane
of the single crystal (Figure S1 in the Supporting Informa-
tion). The packing pattern of molecules in the single crystal
is partially preserved in the gel phase.
A few studies have reported differences in gelation ability
between the enantiomer and the racemate of an organogela-
tor;[24] in one of our studies we revealed the reason for this
difference at the molecular level.[7] For this fluorinated b-
aminoalcohol gelator, the gelation ability of rac-2 is lower
than its enantiomeric counterpart. The powder XRD pattern
of xerogel rac-2 from toluene (Figure 5) provides clues for
this difference. The diffractions at d-spacings of 17.2, 8.6,
5.8, 4.4, and 3.4 ꢁ are attributed to a lamellar structure in
the xerogel. However, all the diffractions are far sharper
and narrower than those of the enantiomer. In addition,
there are also a few other sharp diffractions that do not
appear in the XRD pattern of the enantiomer. These results
strongly suggest that the molecules are aggregated in a more
regular and orderly arrangement. It is likely that the gel of
rac-2 is comprised of a lot of microcrystals that are responsi-
ble for the higher orderliness.
Powder XRD analysis of xerogel 2: As shown in Figure 4a,
the simulated XRD pattern of a single crystal of 2 demon-
strates a significant diffraction at (001) with a d-spacing of
13.26 ꢁ that corresponds to the space of the tubular struc-
ture along the c axis. The diffraction at (100) with a d-spac-
ing of 12.16 ꢁ corresponds to the hydrogen-bond-driven
staggered head-to-head arrangement, the space of the tubu-
lar structure along the a axis.
Elucidation of the aggregation of molecules in the gel
fibers was carried out by using powder XRD analysis for xe-
rogel 2 from toluene. Attempts to prepare xerogel from o-
xylene failed to crystallize during solvent evaporation. As
shown in Figure 4b, the broad peaks at d-spacings of 18.1,
8.7, 6.2, and 4.5 ꢁ correspond to a lamellar structure. The
diffractions are attributable to the (100) plane given the hi-
erarchical self-assembly of the gelators and the high similari-
ty between the XRD patterns of xerogel 2 and xerogel 1.[18]
Figure 5. The powder XRD pattern of xerogel of rac-2 from toluene.
Stability towards ions: The stability of the toluene gel to-
wards ions was evaluated by contact with an aqueous solu-
tion of metal ions, and the durability of fluorinated gelator 2
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Fluorination of a Cyclic b-Aminoalcohol Gelator
FULL PAPER
was compared with that of nonfluorinated gelator 1.[18] The
experiments were carried out by covering a toluene gel with
aqueous solutions of metal ions. The gels of fluorinated ge-
lator 2 displayed enhanced stability. The toluene gel of 2 re-
mains stable in the presence of many metal ions, such as LiI,
NaI, KI, MgII, CaII, ZnII, MnII, CoII, and NiII ions, which is in
agreement with the results for the toluene gel of 1. In the
presence of FeIII and CuII ions, it takes 4 to 5 d for the tol-
uene gel of 2 to be completely destroyed. In contrast, the
collapse of the toluene gel of 1 takes less than 10 h. In the
presence of AgI ions, the gel of 2 is completely destroyed
within 30 min, which is far faster than the nonfluorinated gel
system. The reason for this is the strong interactions be-
tween AgI and the fluorine groups. Significantly, the toluene
gel of 2 is stable in the presence of quaternary ammoniums,
such as tetraoctylammonium bromide (TOAB), chloroau-
rate (TOA-Au), or chloroplatinate (TOA-Pt), whereas the
toluene gel of 1 is quickly converted into loose gel blocks
that disperse into clear solvent. The enhanced stability sheds
light on the effects of the introduction of fluorine and relat-
ed interactions that tune the gel system to stronger stability
towards specific ions.
Figure 6. TEM images of a) AuNPs in xerogel matrix; b) enlargement of
the region highlighted by a rectangle in a); and c) PtNPs in the xerogel
matrix.
coupled with the gelators on reduction from Au3+. The ag-
gregation of AuNPs is also inhibited by their absorbance on
the surface of the nanofibers.[28] The compositions of the
AuNPs was confirmed by using energy-dispersive X-ray
analysis (EDX, Figure S2 in the Supporting Information).
The TEM images of PtNPs-containing xerogels display a
different micromorphology and dispersion. A small amount
of spherical nanoparticles and a large amount of wormlike
nanostructures are observed and are scattered across the
field of view (Figure 6c). The diameter of the spherical
nanoparticles and wormlike nanostructures is 2 to 3 nm and
the length of the wormlike structures ranges from 5 to
12 nm. The wormlike PtNPs predominate over the spherical
structures. The composition of the PtNPs was confirmed by
using EDX (Figure S3 in the Supporting Information). Ex-
tensive scattering of the PtNPs that is not limited to the gel
fibers suggests poor interactions between the chloroplatinate
ions and the gel fibers or gelators. The chloroplatinate ions
are probably shielded by excess TOABs. Tetraalkylammoni-
um salts were used as a soft template for the synthesis of
metal nanoparticles by mediating shape and dispersion of
metal ions.[27,29] In this fluorinated gel system, the chloropla-
tinate ions are considered to be encapsulated within the sur-
factant tetraoctylammonium. The gel serves as a supporting
system to cage the TOA-Au and control the distribution and
shape within a stable fibrous network. The PtNPs should be
scattered in the spaces between the gel fibers.
Synthesis of gold and platinum nanoparticles: The synthesis
of small AuNPs and PtNPs was carried out on the merits of
the stability of the toluene gel of 2 towards TOA-Au and
TOA-Pt. The metal nanoparticles were prepared by mixing
an aqueous solution of hydrogen chloroaurate or hydrogen
chloroplatinate (0.06m, 0.8 mL) with toluene (5.4 mL) that
contained TOAB (0.048 and 0.12 mmol for chloroaurate and
chloroplatinate, respectively). The biphasic mixture was stir-
red vigorously before the color of aqueous layer turned col-
orless. The higher concentration of TOAB in toluene for
chloroplatinate ions guarantees uniformity and stability of
the TOA-Pt in toluene. The red organic layer was separated
to give a TOA-Au or TOA-Pt solution with a concentration
of 0.009m, and stored at ambient temperature in the absence
of light. Gelator 2 (30 mg) was dissolved in the toluene solu-
tion of TOA-Au or TOA-Pt (1.0 mL) by heating. A gel con-
taining TOA-Au or TOA-Pt was formed on cooling and
standing at room temperature. The toluene solution of
TOA-Au and the gelator turned from red to colorless before
gelation, which indicated reduction of AuIII to AuI by the ge-
lators.[25] The TOA-Pt-containing solution did not show this
phenomenon. Both metal ions in gels can be reduced to
zero valency by being covered with aqueous sodium borohy-
dride (0.2m, 0.8 mL).[16,26,27] The AuNPs-containing gel
changed color to dark purple whereas the PtNPs-containing
gel is off-white.
The gel system of 2 can be used as a template and con-
tainer for AuNPs and PtNPs due their stability towards
TOA-Au and TOA-Pt and consequently their ability to sta-
bilize the nanoparticles. This advancement relies on the in-
troduction of fluorine into the original cyclic b-aminoalco-
hol, 1. The properties of the metallic ions and the use of the
surfactant result in different distributions and shapes of
AuNPs and PtNPs.
The gold and platinum nanoparticles in the xerogels were
observed by using transmission electron microscopy (TEM).
The TEM images of the AuNPs-containing xerogel indicate
the presence of spherical nanoparticles with diameters of 5
to 9 nm (Figure 6a and b). The fact that the AuNPs are
clearly arranged along the gel nanofibers suggests effective
separation of gold ions (Au3+ and Au+) by the aggregates
of the gelators. The low-valence gold ions (Au+) may have
Conclusion
In summary, we have developed a new fluorinated version
of a cyclic b-aminoalcohol gelator and evaluated the influ-
ence of the introduced fluorine on the gel properties and ap-
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C. Kang, L. Gao et al.
Crystallization of 2: The single crystals for X-ray crystallography analysis
were prepared by three methods. The first two methods were in situ tran-
sitions from gels: a gel of 2 prepared in o-xylene (0.8%gmLÀ1) or from a
mixture of toluene and o-xylene (1:1, 1%gmLÀ1) was allowed to stand at
RT and single crystals formed in the gel within 2 or 3 d. The third
method involved formation from a solution: a solution of 2 in a mixture
of toluene and o-xylene (1:1, 0.5%gmLÀ1) was placed under a weak ni-
trogen flow, and the o-xylene/toluene ratio increased to more than 95:5,
as determined by gas chromatography, as the single crystal formed. The
structural information and crystal data are listed in Table 2. CCDC-
874186 (2) contains the supplementary crystallographic data for this
paper. These data can be obtained free of charge from The Cambridge
Crystallographic Data Centre via www.ccdc.cam.ac.uk/data_request/cif.
plications. Fluorinated cyclic b-aminoalcohol 2 and its race-
mate are able to gel several common nonprotic solvents. A
single crystal obtained from the o-xylene gel of 2 by an in
situ gel-to-crystal transition shows a tubular structure com-
posed of gelators taking part in unique OH···N hydrogen
bonds and CH···F interactions and filled with o-xylene mole-
cules. Single crystals grown from the gel of a mixture of tol-
uene and o-xylene have identical crystal architecture, which
indicates high selectivity for and recognition of o-xylene
during the self-assembly of the gelators. A lamellar structure
with bimolecular layers is formed in the gel phase of 2 and
rac-2 according to the XRD patterns, and was compared to
the structure of the single crystal. The introduction of a fluo-
rine and subsequently additional CH···F interactions enhan-
ces the stability of the toluene gel towards ions, including
TOA-Au and TOA-Pt. AuNPs and PtNPs were prepared by
using the toluene gel as a template. Spherical AuNPs with a
diameter of 5 to 9 nm were arranged along the gel fibers,
whereas wormlike PtNPs with a diameter of 2 to 3 nm and a
length of 5 to 12 nm predominated in number over spherical
PtNPs with a diameter of 2 to 3 nm. This is the first synthe-
sis of PtNPs by using an organogel.[16] Therefore, the intro-
duction of fluorine atoms and CH···F interactions signifi-
cantly change the properties of the gel system and offer ap-
plications in nanoparticles synthesis. These results demon-
strate simple and effective tactics to tune the properties and
functions of gelators, which we believe to be useful for the
design of gel system. Further studies of applications of the
gold and platinum nanocomposites in catalysis are in prog-
ress.
Gel stability towards ions: Gel stability towards cations was tested by
covering a toluene gel of 2 (1 mL, 0.8%gmLÀ1) with a fresh solution of
cation (0.1 mmol metal ion in 1 mL deionized water). The bilayer sample
(aqueous upper layer) was left to stand for at least one week to deter-
mine the time for gels to be completely broken down.
Synthesis of gold nanoparticles: Hydrogen tetrachloroaurateACHTUNGTRENNUNG(III) trihy-
drate (18.6 mg, 0.047 mmol) was dissolved in deionized water (0.8 mL).
Table 2. Single-crystal data for 2 formed from o-xylene gel (method 1),
xylene/toluene gel (method 2), and xylene/toluene solution (method 3).
Method 1
Method 2
Method 3
formula
Mr
C18H20FNO3·C8H10 C18H20FNO3·C8H10 C18H20FNO3·C8H10
423.51
423.51
423.51
crystal
system
monoclinic
monoclinic
monoclinic
space group P21
P21
P21
a [ꢁ]
b [ꢁ]
c [ꢁ]
a [8]
b [8]
12.403(2)
12.324(2)
7.1557(13)
13.305(2)
90.00
99.788
90.00
12.341(2)
7.1652(11)
13.369(2)
90.00
99.926(3)
90.00
7.2868(13)
13.523(2)
90.00
101.350
90.008
g [8]
V [ꢁ3]
Z
1198.3(4)
2
1156.2(4)
2
1164.5(3)
2
1calcd
[gcmÀ3
1.174
1.216
1.208
Experimental Section
]
m [mmÀ1
T [K]
]
0.081
293(2)
0.084
293(2)
0.084
295(2)
Materials and methods: All reagents and solvents were commercially
available and used without further purification. Molecules 2–5 were syn-
thesized according to literature procedures and are detailed in the Sup-
porting Information. SEM images were recorded by using an XL 30
ESEM FEG field emission scanning electron microscope. Powder XRD
patterns were recorded by using a Rigaku D/max 2000 X-ray diffractom-
eter equipped with a CuKa1 radiation source. X-ray single crystallographic
analysis was performed by using a Bruker SMART APEX II CCD dif-
fractometer with graphite-monochromatic MoKa radiation (l=
0.71073 ꢁ). The crystal structures were solved by direct methods using
the SHELXL-97 program and refined with anisotropic thermal parame-
ters by full matrix least squares on F2 values with SHELXL-97.[30] TEM
images were recorded by using a TECNAI G2 high-resolution transmis-
sion electron microscope operated at an acceleration voltage of 200 kV.
EDX was carried out to confirm the nanoparticles compositions by using
an EDAX detector coupled with TEM. Samples were prepared by
wiping a carbon-coated copper grid on nanoparticles-contained gel fol-
lowed by natural evaporation of the solvent.
crystal form block
block
0.21ꢂ0.15ꢂ0.10
block
0.31ꢂ0.22ꢂ0.11
crystal size
0.32ꢂ0.21ꢂ0.10
[mm]
crystal color colorless
colorless
multi-scan
colorless
multi-scan
absorption
correction
total data
collected
multi-scan
7161
5677
6338
unique data 4192
2830
1995
3368
2833
observed
data
3429
[I>2s(I)]
Rint
qmax [8]
0.0162
25.02
0.0443
25.02
À14ꢀhꢀ14
À8ꢀkꢀ5
À15ꢀlꢀ15
0.1118
0.0303
26.00
À15ꢀhꢀ15
À4ꢀkꢀ8
À16ꢀlꢀ16
0.0590
index range À12ꢀh ꢀ14
À8ꢀkꢀ8
À16ꢀlꢀ14
R1
0.0580
0.1693
0.0698
Gel preparation: An accurately weighed amount of gelator (2–3 mg) was
added to a glass vial that contained solvent (1 mL) and sealed. The sus-
pension was then heated to give a homogeneous solution that was then
left to cool to RT. After 30 min at RT, if no gel was formed, more gelator
(1–2 mg) was added and dissolved in the solution by heating. The proce-
dure was repeated until a gel formed, at which point the concentration of
gelator in the solvent was recorded to give the MGC value. For racemic
gelator rac-2, several hours of standing at RT was sometimes required to
form a gel.
[I>2s(I)]
wR(F2)
0.2738
0.1524
[I>2s(I)]
R1, all data
0.1376
0.3031
0.0693
0.1634
wR2 (F2), all 0.01875
data
S
0.924
1.143
284
1.013
284
parameters 272
16960
www.chemeurj.org
ꢀ 2012 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
Chem. Eur. J. 2012, 18, 16955 – 16961

Fluorination of a Cyclic b-Aminoalcohol Gelator
FULL PAPER
The aqueous solution was then added to a solution of TOAB (48.0 mg,
0.149 mmol) in toluene (5.4 mL) and stirred vigorously until all the red
color had transferred from the aqueous layer into the organic layer. The
organic layer was separated by using a pipette. The calculated concentra-
tion of chloroaurate in the toluene was 0.009m. Gelator 2 (30.0 mg) was
suspended in the TOA-Au solution (1.0 mL) and heated to form a solu-
tion. The red solution turned colorless before the gel was formed on
cooling to RT. The gel was then covered with aqueous lithium borohy-
dride (0.2m, 0.8 mL). The color of the gel changed to dark purple, which
indicated the formation of an AuNPs-containing gel.
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Received: July 24, 2012
Published online: November 4, 2012
Chem. Eur. J. 2012, 18, 16955 – 16961
ꢀ 2012 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
www.chemeurj.org
16961