Double helical silica fibrils by sol-gel transcription of chiral aggregates of gemini surfactants

Article abstract of DOI:10.1039/b202799m

Silica fibrils with a novel double stranded helical structure are prepared by sol-gel transcription of twisted bilayer ribbons formed by cationic gemini surfactants.

Full text of DOI:10.1039/b202799m

Double helical silica fibrils by sol–gel transcription of chiral aggregates
of gemini surfactants†
Kazunori Sugiyasu,a Shun-ichi Tamaru,a Masayuki Takeuchi,a Damien Berthier,b Ivan Huc,b Reiko
Odab and Seiji Shinkai*a
a Department of Chemistry and Biochemistry, Graduate School of Engineering, Kyushu University, Fukuoka
812-8581, Japan. E-mail: seijitcm@mbox.nc.kyushu-u.ac.jp
b Institut Européen de Chimie et Biologie, 16 Av. Pey Berland, 33607 Pessac Cedex, France
Received (in Cambridge, UK) 19th March 2002, Accepted 23rd April 2002
First published as an Advance Article on the web 3rd May 2002
Silica fibrils with a novel double stranded helical structure
are prepared by sol–gel transcription of twisted bilayer
ribbons formed by cationic gemini surfactants.
Both in natural and artificial systems, the self-assembly of
organic building blocks give rise to supramolecular structures
of various sizes, shapes, chemical composition and function. In
order to develop novel inorganic materials closely correspond-
Fig. 1 TEM image of a right handed twisted ribbon in a xerogel prepared
from a
v/v).
L-1/D-1 mixture (3+1 mol/mol, 50% ee) in water/pyridine (1+1
ing to these organic assemblies, one approach is to transcribe
them to produce inorganic replicas, thus mimicking bio-
mineralization processes. Amphiphilic molecules for example,
exhibit one of the richest polymorphism of structures and
mesophases.1 Among these, vesicles,2 ultra-thin membranes,3
and others,4 have been utilized as templates to create mesopor-
ous inorganic materials.5 In order to ‘design’ the sizes and
shapes of such materials (as organic compounds and assemblies
can be ‘designed’), it is essential to explore and elucidate the
mechanisms of transcription of various organic precursors.
Recently, increasing attention has been paid to low molec-
ular-weight compounds that can efficiently gelate various
organic solvents upon forming networks of fibrillar aggregates.6
In studies devoted to cholesterol-based gelators, we found that
even liquid silanol derivatives can be gelated.7 It thus occurred
to us that if the sol–gel polycondensation of silanol proceeds in
the organogel phase, the fibrillar aggregate should act as a
template that, after calcination, would leave a void in the
resulting silica. After trial-and-error, we found that the fibers of
certain ‘cationic’ cholesterol derivatives gelate a tetraethoxy-
silane (TEOS) solution and are efficiently transcribed to give a
novel mesoporous silica with a tubular, macaroni-like struc-
ture.8 Condensation of anionic silica occurs preferentially at the
surface of the cationic aggregate. Under some limited condi-
tions, a unique, right-handed ‘helical’ silica was obtained.8
Meanwhile, the French group of this paper reported that
‘cationic’ gemini surfactants having chiral tartrate counter ions
assemble into twisted ribbons both in chlorinated and aromatic
organic solvents, and in water.9 The handedness of the ribbons
and their helical pitch can be tuned continuously upon varying
a feature previously observed in pure water10 but not in organic
solvents. The dimensionless parameter T/W characterises the
aspect ratio of the twisted structures.10 In water/pyridine (1+1
v/v) T/W = 7.1 for 100% ee, 12.0 for 50% ee (Fig. 1) and 18.7
for 33% ee. These values are comparable to those reported in
water.
Sol–gel polycondensation of silica was carried out as follows:
L
-1 and
-1 (total 2.0 mg, 2.6 3 1026 mol) were added to water
D
(100 ml)/pyridine (100 ml)/TEOS (10 ml)/benzylamine (5 ml) and
the mixture was warmed until a transparent solution was
obtained. The reaction mixture was placed at room temperature
under static condition for 3 days. The product was dried under
vacuum at room temperature. Finally, the gelator was removed
by calcination at 100 °C for 1 h, 200 °C for 1 h, 500 °C for 2 h
under a nitrogen atmosphere, and 500 °C for 4 h under aerobic
conditions.
A typical TEM image of a fiber before calcination (e.g.
composed of both silica and the dimeric amphiphiles) obtained
by sol–gel transcription is shown in Fig. 2. One can recognize
two intertwined fibrils. The handedness could not clearly be
determined. The helical pitch (300–500 nm) is a little shorter
than that of the corresponding purely organic ribbon (about
400–800 nm). These observations suggest that the condensation
of silica does not alter very significantly the concomitant
aggregation of amphiphiles.
the ratio between
L
and
D
tartrate (e.g. the pitch increases for
decreasing % ee).10 This kind of ‘fine tuning’ is nearly
impossible in other gelators. Here, we report on the sol–gel
transcritpion of this system and the observation of novel double
helical silica fibers with a tunable pitch.
Fig. 2 TEM image of the fibers obtained by sol–gel transcription of a
1 mixture (3+1 mol/mol, 50% ee) before calcination.
L-1/D-
We first seeked for a transcription medium capable of
dissolving TEOS, containing water, and compatible with the
formation of the twisted ribbons of
water and pyridine (1+1 v/v) satisfies these requirements.
Indeed -1, -1 and -1/ -1 mixtures assemble in this mixture as
they do in pure water or pyridine.9 A typical TEM image of the
xerogel prepared from gels containing -1 and -1 is shown in
L
-1 and
D-1. A mixture of
Fig. 3 shows a SEM image of the silica after calcination
obtained from a gel of pure -1 (100% ee). One can observe a
network of rope-like right-handed helical fibrils. This is the
expected handedness from the enantiomer of 1. Judging from
L
L
D
L
D
L
L
D
the inclination of the ropes with respect to the axis of the fibers,
one may presume that they consist of a double stranded
structure.‡ When the ee is decreased from 100 to 20%, double
helices with increasing pitch are observed (Fig. 4). The aspect
ratio of these inorganic fibers was found to be T/W = 1.6 for
100% ee, 4.3 for 50% ee, 5.1 for 33% ee and 9.8 for 20% ee
These values are somewhat smaller than those of the precursor
Fig. 1. Additionaly, the twist pitch (T) and the width (W) of
these ribbons were shown to vary with the ee in water/pyridine,
† Electronic supplementary information (ESI) available: Fig. S1: TEM
image of double stranded silica obtained by sol–gel transcription of
L
-1/
D-1
gel (2+1 mol/mol, 33% ee
b2/b202799m/
L-1 excess). See http://www.rsc.org/suppdata/cc/
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CHEM. COMMUN., 2002, 1212–1213
This journal is © The Royal Society of Chemistry 2002

of the organic template but do not completely surround it. Two
possible rationales come to our mind to explain double helix
formation. TEOS polycondensation may proceed preferentially
along the gutter-shaped faces of the twisted ribbons where the
density of positive charge is larger. Alternatively, it may
proceed preferentially along the edges of the ribbons, where the
density of positive charge is smaller but which are more
exposed to the solvent. Since the ribbons have two edges and
two faces, two intertwined silica fibrils would be expected in
both rationales. However the distance between the fibrils would
be expected to be of the order of a ribbon thickness when
condensation occurs on the faces, and of the order of a ribbon
width when condensation occurs on the edges. If the fibrils
observed before calcination (Fig. 2) are indeed the final silica
fibrils, the distance between them (50 nm) corresponds to the
width of a twisted ribbon, suggesting condensation along the
edges. On the other hand, the fibrils in the double helices
observed after calcination are not separated by any gap. If
condensation occurs preferentially at the edges, considerable
shrinkage of the structure during calcination must be invoked to
bring the fibrils in close contact. Further studies on the
mechanism are currently continued in these laboratories. In
particular, concentration, stoichiometry and reaction times are
expected to significantly affect the transcription process.
As a final remark, chiral silica phases generally refer to
organic moieties bearing asymmetric centers tethered to achiral
silica particles. In contrast, chirality in the helical fibers
described here belongs to the silica itself. It will be worth
exploring how this chiral silica performs in applications such as
chiral separation or enantioselective catalysis.
Fig. 3 SEM image of the silica obtained by sol–gel transcription of a L-1 gel
(2.0 wt%, after calcination).
Fig. 4 Influence of the ee (L-1 in excess) on the helical pitch of double
stranded silica: A, 100% ee; B, 50% ee; C, 33% ee; D, 20% ee.
Notes and references
‡ Fig. 4 shows a perspective view; the inclination of the ropes with respect
to the axis of the fibers is measured on those fibers which lay flat in the plane
of the micrograph.
twisted ribbons, but one may rationalize this discrepancy from
a possible shrinkage of silica fibrils during the calcination
process. The double stranded helical structure appears partic-
ularly clearly for silica fibrils obtained at 50% ee and 33% ee
(Fig. 4B and C).
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Thus, the helical shape, the handedness and even the tunable
chirality of the organic twisted ribbons are efficiently tran-
scribed to the inorganic replicas. But the question remains of
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L
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L
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