Bergman cyclopolymerization within the channels of functional hybrid nanocomposites formed by co-assembly of silica and polymerizable surfactant monomer

Article abstract of DOI:10.1039/b603195a

The Bergman cyclopolymerization of polymerizable surfactant monomer was carried out within the hexagonal channels of functional hybrid nanocomposite formed by co-assembly with silica. The Royal Society of Chemistry 2006.

Full text of DOI:10.1039/b603195a

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Bergman cyclopolymerization within the channels of functional hybrid
nanocomposites formed by co-assembly of silica and polymerizable
surfactant monomer{
Chetan Jagdish Bhongale, Chung-He Yang and Chain-Shu Hsu*
Received (in Cambridge, UK) 3rd March 2006, Accepted 12th April 2006
First published as an Advance Article on the web 26th April 2006
DOI: 10.1039/b603195a
The Bergman cyclopolymerization of polymerizable surfactant
monomer was carried out within the hexagonal channels of
functional hybrid nanocomposite formed by co-assembly with
silica.
Chart 1 Chemical structure of amphiphilic surfactant monomer.
Ordered periodic mesoscopic materials allow the construction of
composites with many guest types like organic molecules or
materials synthesis view-point. Amphiphilic polymerizable surfac-
polymers. Inclusion of dye molecules such as Coumarin 40,
tant monomer was synthesized (Chart 1).
Rhodamine BE50, Oxazine 1 inside the nanopores has been
demonstrated.1–3 Nanocomposites that contain conjugated poly-
It has two long alkyl chains with hydrophilic hydroxyl head
groups at one end and the phenyl ring with two terminal acetylene
mers confined within a silica matrix show enhanced conductivity,
groups ortho to each other at the tail end. (see Supplementary
mechanical strength, processability, environmental stability, and
Information{ for synthetic details).
other unique properties4 that allow for potential use in light
emitting diodes, information storage devices, optical signal
processors, and sensors. To name a few, nanocomposite formation
of polymers such as poly(phenylene vinylene),5 polyaniline,6
polydiacetylene,7 poly(2,5-thienylene ethynylene),4 polythiophene,
polypyrrole, and polyacetylene8,9 have been reported. Several
synthetic efforts to obtain such nanocomposites used mainly slow
procedures like monomer or polymer infiltration of inorganic
nanostructures5,10–13 or sequential deposition.14,15 Such nanocom-
posites are heterogeneous, exhibiting two distinct conjugated
polymer environments, that is, polymers inside and outside the
hexagonally arranged pore channels of the silica particles.
However, self-assembly, one of the few practical strategies for
making ensembles of nanostructures provides one solution to the
fabrication of ordered aggregates from components with sizes
from nanometers to micrometers.16 It typically employs asym-
metric molecules that are pre-programmed to organize into well-
defined supramolecular assemblies.
Beginning with a homogeneous solution of soluble silica—
tetraethoxy orthosilicate (TEOS), acid catalyst, and surfactant
monomer in ethanol–water solvent, thin films were drawn by dip-
coating or spin-coating. Solvent evaporation during the coating
process enriches nonvolatile components and induces their
co-assembly into liquid crystalline mesophases.4 Polymerizaion of
silica during the coating process freezes the mesophases and
spatially organizes the monomer surfactant into mesostructures.
These films were vacuum-dried overnight and kept immersed in
benzene in thick-walled screw cap glass tubes which were capped in
the glove box under nitrogen atmosphere prior to heating (see
Supplementary Information for experimental details{).
Fig. 1 A and B show scanning electron microscopy (SEM) and
transmission electron microscopy (TEM) images of the nanocom-
posite formed after the evaporative self-assembly from the gel
solution mounted on the substrate, respectively. The highly
˚
ordered nanocomposite shows a center-to-center spacing of 53 A.
X-ray diffraction (XRD) patterns of the nanocomposites before
(solid lines) and after (dashed lines) the Bergman cyclopolymeriza-
tion are shown in Fig. 1D. Along with an intense diffraction peak
at around 2h = 1.93, additional higher order diffraction peaks are
‘clearly’ observed at 2h = 3.3, 3.8, and 5. The intense one is
attributed to the [100] orientation of the hexagonal mesophase.
There was no change in the XRD peak position for the heat-
treated (polymerized) sample, except a decrease in the intensity of
the spectrum which indicates that the nanocomposite is intact even
after heat-treatment (polymerization). Energy dispersive spectro-
scopy (EDS) elemental analysis identified carbon, silicon and
oxygen as expected for the nanocomposite prepared and is shown
in Fig. 1C.
The use of polymerizable surfactants as both structure-directing
agents and monomers in various evaporation-driven self-assembly
schemes represents a general, efficient route to the formation of
robust and functional nanocomposites.7 In this research we utilize
this self-assembly4 route to form mesostructured polynaphthalene/
silica nanocomposites. One of the many approaches to form the
polynaphthalenes (PN) is through Bergman cycloaromatiza-
tion,17,18 a remarkable isomerization in which an endiyne forms
an arene 1,4-diradical. Here, we make use of this approach with a
Department of Chemistry, National Chiao Tung University, Hsinchu,
Taiwan 30050. E-mail: chetan.ac90g@nctu.edu.tw;
cshsu@mail.nctu.edu.tw; Tel: +86-5712121
{ Electronic supplementary information (ESI) available: Scheme, synthetic
and experimental details. 1H NMR and EI-MS data of surfactant
monomer (S1–S7). See DOI: 10.1039/b603195a
The topochemicity generated during the co-assembly favors the
Bergman cyclopolymerization. Formation of polymer within the
hexagonal channels of the nanocomposite was verified by UV-vis
2274 | Chem. Commun., 2006, 2274–2276
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Fig. 3 FTIR spectra of (A) monomer and (B) polymer formed by
heating monomer in benzene.
Fig. 1 (A) SEM (B) TEM (C) EDS and (D) XRD of the monomer/silica
nanocomposite formed.
polymer were characterized by infra-red (IR) spectra for
comparison, as shown in Fig. 3. The characteristic alkyne triple
and photoluminescence spectroscopy. UV-vis spectroscopy has
been used previously to relate the degree of polymerization and
absorbance.4 Fig. 2A shows the absorption spectra of the
nanocomposite before and after the polymerization. Monomer
nanocomposite showed maximum absorbance at about 285 nm,
whereas polymer nanocomposite showed absorbance between 345
to 390 nm tailing up to 450 nm, indicating extensive conjugation of
the polymer backbone.19 Direct molecular weight determination of
such type of polymer/nanocomposite films is difficult.4 Monomer
nanocomposite showed photoluminescence (PL) in UV-blue
region with maximum peak at 353 nm. After polymerization, the
photoluminescence was red-shifted with a maximum peak at
415 nm and a shoulder at around 483 nm.
bond C–H and CMC stretching bands at 3280 and 2107 cm21
respectively, present in the monomer disappeared after the
polymerization.21 A new broad and strong band at 1625 cm21
,
,
due to CLC stretching was observed, indicating formation of
polymer with extensive conjugation.19 The degree of polymeriza-
tion in thin film and in bulk might be different as these are two
different environments and should affect the extent of conjugation.
There are controversies regarding the structure of the polymer
formed by Bergman cyclization of monomer with terminal triple
bonds, as presence of five-membered ring and terminal triple
bonds has been reported recently.22 However, from the IR spectra
shown above, the possibility of presence of terminal triple bonds in
the final product is ruled out in this case. Enhanced topochemistry
due to the self-assembly and orientation of the surfactant
monomers within the inorganic framework limits the formation
of unidentified polymeric by-products, thereby giving small
chain polynaphthalenes or their oligomers. We envisage
formation of low molecular weight PN obtained by Bergman
cyclization of polymerizable surfactant monomer within the
hybrid nanocomposites formed by evaporative self-assembly.
The polymers thus obtained could be soluble and be applied in
opto-electronic devices. This route to polynaphthalenes (or
polyphenylenes) and their derivatives is most attractive since it
requires no exogenous chemical catalysts or reagents for the
polymerization.
Bergman cyclopolymerization of the monomers with terminal
acetylene groups, as in the present case, generally gives insoluble
polymers,20 making their characterization difficult. We carried out
the polymerization of surfactant monomer dissolved in benzene as
described in literature.20 Heating the monomer at about 140 uC for
one day indeed afforded insoluble red brown solid. Monomer and
In summary, we have demonstrated Bergman cyclopolymeriza-
tion of polymerizable surfactant monomer within the hexagonal
channels of functional hybrid nanocomposite formed by evapora-
tive self-assembly. Such
a technique allows the patterned
polymerization of the polymer precursor material directly onto a
surface, thus avoiding solubility related problems that may be
encountered when attempting to directly coat the pristine
polymeric material, which is often insoluble,20 thereby facilitating
its use in device fabrication.
We thank National Science Council of the Republic of China
for financial support from the grant (NSC 94-2120-M-009-009).
Fig. 2 (A) UV-vis absorbance and (B) normalized PL spectra of the
nanocomposites formed.
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