Synthesis and characterisation of π-conjugated polymer/silica hybrids containing regioregular ionic polythiophenes

Article abstract of DOI:10.1039/c0jm03598j

Highly regioregular poly{3-[n-(1-methylimidazolium-3-yl)alkyl]thiophene-2, 5-diyl bromide} (n = 4, 6, 8) (10-12) has been prepared by reaction of poly[3-(n-bromoalkyl)thiophene-2,5-diyl] (n = 4, 6, 8) (7-9) with 1-methylimidazole. Acid-catalyzed hydrolysi

Full text of DOI:10.1039/c0jm03598j

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PAPER
Synthesis and characterisation of p-conjugated polymer/silica hybrids
containing regioregular ionic polythiophenes†
a
a
b
a
Sebastien Clement, Akim Tizit, Simon Desbief, Ahmad Mehdi, Julien De Winter,c Pascal Gerbaux,c
*
ꢀ
ꢀ
Roberto Lazzaronib and Bruno Bourya
Received 22nd October 2010, Accepted 1st December 2010
DOI: 10.1039/c0jm03598j
Highly regioregular poly{3-[n-(1-methylimidazolium-3-yl)alkyl]thiophene-2,5-diyl bromide} (n ¼ 4, 6,
8) (10–12) has been prepared by reaction of poly[3-(n-bromoalkyl)thiophene-2,5-diyl] (n ¼ 4, 6, 8) (7–9)
with 1-methylimidazole. Acid-catalyzed hydrolysis and polycondensation of tetraethoxysilane (TEOS)
were carried out in the presence of these polymers. The strong ionic interactions between the
imidazolium moieties of the polythiophene and the charged silanol groups enabled the homogeneous
dispersion of the polythiophene within silica. The resulting composite materials were characterised by
SEM, TEM and tapping mode atomic force microscopy (TM-AFM). The optical and thermal
properties were investigated by UV and TGA measurements.
characteristics. A major key point in the preparation of sol–gel
Introduction
hybrid materials lies in the nature and strength of the interactions
Organic–inorganic hybrid materials have rapidly emerged as
between the inorganic and the organic building blocks. In fact, in
a new class of materials for optical applications such as elec-
the absence of specific interactions between the two components,
troluminescent devices,1 thin-film transistors,2 photochromic,
aggregation of the organic moieties in the inorganic matrix often
photovoltaic devices3 and optical waveguides.4 In this regard, the
occurs during the sol–gel process.5 In this work, we aim at the
development of synthesis methods affording an effective control
elaboration of p-conjugated polymers (especially poly-
at the nanometre scale is a permanent requirement for the
thiophene)–silica hybrid materials, where polythiophenes (PTs)
construction of advanced materials with enhanced properties.
are well dispersed in the inorganic matrix. Since PTs are the most
The sol–gel method is probably the utmost ‘‘soft’’ chemistry
versatile, important class of p-conjugated polymers and organic
process and the best experimental approach. Indeed, this process
electronic materials,6 their hybrids with silica can provide new
exhibits two very important characteristics: first, high compati-
materials with interesting properties. For instance, the incorpo-
bility with all types of organic units (molecules, polymer,
ration of the polymer into the silica matrix is expected to reduce
biochemical and biological units), which open very wide possi-
the aggregation of the conjugated chains, a phenomenon often
bilities of applications. Second, because hydrolysis and poly-
detrimental to the optical properties.
condensation reactions occurring during the sol–gel process are
To prevent phase separation during the sol–gel process, several
under kinetic control, the final materials are highly dependent on
groups have reported about the incorporation of p-conjugated
polymers into silica using physical interactions such as hydrogen
bonding,7 aromatic8 and ionic interactions.9 Nanostructured
the experimental conditions: temperature, pH, concentrations,
nature of reagents, solvent, etc. Therefore, it is possible to tune
all these parameters to design materials with specific
polyacetylene/silica hybrids have been successfully prepared by
the sol–gel method by using ionic interactions between cationic
pyridinium side groups and anionic silanol moieties.10 Kubo
aInstitut Charles Gerhardt, Chimie Moleꢀculaire et Organisation du Solide,
et al. reported on polythiophene/silica composites using a poly
Universiteꢀ Montpellier 2, cc 1701. Place E. Bataillon, 34095 Montpellier
(3-hexylthiophene) containing hydroxy groups.11 The strong
Cedex 5, France. E-mail: ahmad.mehdi@univ-montp2.fr; Fax: +33 4
6714 38 52; Tel: +33 4 67 14 38 32
hydrogen bonding between hydroxy groups and silanol groups
resulted in a homogeneous composite. However, if too many
hydroxy groups are introduced on the alkyl side chains,
a dramatic decrease of the conjugation length is observed.
This contribution describes a similar approach based on ionic
interactions. Polythiophenes are essentially insoluble in water;
the introduction of an ionic group enables the extension of their
chemistry to aqueous media, a requirement for potential bio-
logical applications as well as sol–gel chemistry. The resulting
bLaboratory for Chemistry of Novel Materials, CIRMAP, University of
Mons UMONS, Place du Parc 20, 7000 Mons, Belgium
cMass Spectrometry Center, University of Mons UMONS, Place du Parc
20, 7000 Mons, Belgium
† Electronic supplementary information (ESI) available: 1H NMR
spectra of polymers 7–9; MALDI-ToF spectra of compounds 8 and 9;
UV-Vis absorption spectra of polymers 7–9 in CHCl3; TGA diagrams
1
for polymers 7–9; H and 13C{1H} NMR spectra of polymer 11; SEM
images of 10@silica and 12@silica hybrid materials; and TGA curve of
polymer 11 and hybrid material (run 2). See DOI: 10.1039/c0jm03598j
This journal is ª The Royal Society of Chemistry 2011
J. Mater. Chem., 2011, 21, 2733–2739 | 2733

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strong ionic interaction between the cationic polythiophene
polyelectrolyte and the charged silanol moieties is expected to
lead to an efficient dispersion of the polymer in silica gel matrix.
In recent years, many efforts have been devoted to the synthesis
of new water-soluble cationic conjugated polyelectrolytes with
polythiophene main chains for biological applications.12,13
Leclerc and Ho have reported cationic polythiophenes with
imidazolium-capped short alkoxy side chains for detection of
nucleic acids and human thrombin. These polythiophene poly-
electrolytes were obtained from an oxidative polymerization by
using FeCl3 leading to regiorandom polythiophenes.12 It is
recognized that the electronic and optical functionalities of poly-
thiophene strongly depend on the regioregularity in the main
chain.14 Therefore, we decided to use a synthetic strategy based
on the post-functionalization of regioregular head-to-tail poly
(3-bromoalkylthiophene) (alkyl ¼ butyl, hexyl, octyl) with
1-methylimidazolium. By this way, the regioregularity of the
resulting cationic polythiophene polyelectrolyte is predetermined
by the regioregularity of the precursor polymer. In parallel to this
work, this post-functionalization approach has been very recently
exploited by Bondarev et al., who designed structures containing
a regioregular polythiophene with imidazolium side chains.15
Previous research in our group has been focused on design and
functionalisation of hybrids materials by using the self-assembly
strategy during the sol–gel process.16
was dried over anhydrous MgSO4 and the removal of solvent
gave a crude product. The residue was purified on silica gel using
petroleum ether (40–60 ꢀC) as eluent, affording compounds 4–6
as yellow oil.
Compound 4. Yield: 85%. Rf (petroleum ether): 0.58, 1H NMR
(CDCl3): d ¼ 1.76 (m, 4H, CH2), 2.59 (t, 2H, CH2–Th, 3JH–H ¼ 7
3
Hz), 3.46 (t, 2H, Br–CH2, JH–H ¼ 7 Hz), 6.88 (s, 1H, Th).
13C{1H} NMR (CDCl3): d ¼ 28.1, 29.5, 33.8, 39.6 (CH2), 108.9,
111.2, 131.2, 142.4 (Th) ppm. Anal. Calcd for C8H9Br3S: C
25.49, H 2.41, S 8.51. Found: C 25.38, H 2.29, S 8.42%.
Compound 5. Yield: 90%. Rf (petroleum ether): 0.52, 1H NMR
(CDCl3): d ¼ 1.48 (m, 6H, CH2), 1.86 (m, 2H, CH2), 2.56 (t, 2H,
3
3
CH2–Th, JH–H ¼ 7 Hz), 3.44 (t, 2H, Br–CH2, JH–H ¼ 7 Hz),
6.81 (s, 1H, Th). 13C{1H} NMR (CDCl3): d ¼ 28.3, 28.6, 29.7,
29.8, 33.1, 34.3 (CH2), 108.9, 110.5, 131.3, 143.0 (Th) ppm. Anal.
Calcd for C10H13Br3S: C 29.66, H 3.24, S 7.92. Found: C 29.54,
H 3.16, S 7.73%.
Compound 6. Yield: 83%. Rf (petroleum ether): 0.56, 1H NMR
(CDCl3) d ¼ 1.35 (m, 10H, CH2), 1.88 (m, 2H, CH2), 2.53 (t, 2H,
3
3
CH2–Th, JH–H ¼ 7 Hz), 3.43 (t, 2H, Br–CH2, JH–H ¼ 7 Hz),
6.80 (s, 1H, Th). 13C{1H} NMR (CDCl3): d ¼ 28.6, 28.8, 29.3,
29.5, 29.7, 29.8, 33.1, 34.3 (CH2), 108.5, 110.9, 131.4, 143.3 (Th)
ppm. Anal. Calcd for C12H17Br3S: C 33.28, H 3.96, S 7.40.
Found: C 33.22, H 3.83, S 7.31%.
Herein, we report the synthesis and properties of highly
regioregular cationic polythiophenes containing ionic liquid-like
side chains (1-alkyl-4-methyl-1-imidazolium, alkyl ¼ butyl, hexyl
and octyl). The nature of the alkyl chains substituent is expected
to play an important role in determining the structuration of the
polythiophene that in turn governs the electronic and physical
properties. These conjugated polyelectrolytes were then intro-
duced into a silica gel matrix via the sol–gel method, and the
optical properties of these ionic polythiophenes/silica nano-
composites were assessed.
Regioregular poly[3-(n-bromoalkyl)thiophene-2,5-diyl] (7–9,
respectively n ¼ 4, 6, 8)
Into a round bottom flask containing 2,5-dibromo-3-(n-bro-
moalkyl)thiophene (4–6, n ¼ 4, 6, 8, respectively) (10 mmol)
previously dried by three successive azeotropic distillation with
toluene, dry THF (15 mL) was added. iPrMgCl (2.0 M solution
in THF, one equivalent) was then added via a syringe and the
reaction mixture was stirred at 0 ꢀC for 30 min. This solution was
thenꢀtransferred to the right amount of catalyst in THF (15 mL)
at 0 C and the reaction mixture was stirred for 1 d at RT. The
solution was quenched quickly with HCl 5 N (5 mL) and stirred
for 30 min. Then, the mixture was extracted with chloroform.
The organic layer was washed two times with water, dried over
anhydrous MgSO4 and the solvent was evaporated under
vacuum to obtain the crude polymer. Cold methanol was added
to the residue and the suspension was filtered. The solid polymer
was further purified with refluxing methanol and hexane by using
a Soxhlet apparatus and finally dried under vacuum. Polymers
7–9 were obtained in yields ranging from 52 to 90%.
Experimental
Materials
All reactions were carried out under argon by using standard
high vacuum and Schlenk techniques. Solvents were dried and
distilled just before use. [1,3-Bis(diphenylphosphino)propane]-
nickel(II) chloride (Ni(dppp)Cl2) (99%) was stored in a glovebox
and used as received. 3-(4-Bromobutyl)thiophene, 3-(6-bromo-
hexyl)thiophene, 3-(8-bromooctyl)thiophene were prepared
according to a literature procedure.17 N-Bromosuccinimide
(NBS) was freshly recrystallized from water.
General method for bromination of precursors 1–3
Compound 7. 1H NMR (CDCl3): d ¼ 1.94 (m, 4H, CH2), 2.85
3
Compounds 4–6 were obtained according to a slightly modified
literature procedure.18 3-(n-Bromoalkyl)thiophene (1–3, respec-
tively n ¼ 4, 6, 8) (20 mmol) was dissolved in THF and acetic acid
(20 : 20 mL, v/v) in a 100 mL two necked flask equipped for
stirring. NBS (7.08 g, 40 mmol) was added to the solution in one
(m, 2H, CH2–Th), 3.48 (t, 2H, CH2–Br, JH–H ¼ 7 Hz), 7.00
(s, 1H, Th) ppm. 13C{1H} NMR (CDCl3): d ¼ 28.7, 29.2, 32.7,
33.8 (CH2), 128.9, 131.2, 134.0, 139.3 (Th) ppm. UV-visible
(CHCl3): lmax ¼ 438 nm, SEC (THF, polystyrene standard)
Mn ¼ 3900 g molꢂ1; polydispersity ¼ 1.18.
ꢀ
portion, and the solution was stirred for 1 h at 40 C. Then the
solution was extracted with Et2O (3 ꢁ 20 mL); the organic
extracts were washed with an aqueous saturated NaHCO3
solution (3 ꢁ 20 mL) and water (3 ꢁ 20 mL). The organic layer
Compound 8. 1H NMR (CDCl3): d ¼ 1.48 (m, 8H, CH2), 1.73
(m, 2H, CH2), 1.90 (m, 2H, CH2), 2.82 (m, 2H, CH2–Th, 3JH–H
¼
6 Hz), 3.43 (t, 2H, CH2–Br, 3JH–H ¼ 7 Hz), 7.02 (s, 1H, Th) ppm.
2734 | J. Mater. Chem., 2011, 21, 2733–2739
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13C{1H} NMR (CDCl3): d ¼ 28.2, 28.8, 29.5, 30.5, 32.9, 34.2
(CH2), 128.8, 130.8, 133.9, 139.8 (Th) ppm. UV-visible (CHCl3):
lmax ¼ 443 nm, SEC (THF, polystyrene standard) Mn ¼ 6800 g
molꢂ1; polydispersity ¼ 1.29.
for 300 s. After this spin coating procedure, the sol–gel coatings
were left to age for two days at room temperature.
Techniques
1H, 13C{1H} NMR spectra were recorded at RT in CDCl3 (50 mg
mLꢂ1). All the chemical shifts and coupling constants are
reported in ppm and Hz, respectively. Average molecular weights
and molecular weight distributions of the different poly-
thiophenes were measured using size exclusion chromatography
(SEC) on a system equipped with a guard column and PLgel 500,
103, 104 columns (Polymer Laboratories) and a differential
refractive-index detector (Waters). The eluent used was THF at
a flow rate of 1 mL minꢂ1 at 30 ꢀC. Polystyrene standards
(Polymer Laboratories) ranging from 1400 to 1300 ꢁ 103 g molꢂ1
were used to calibrate the SEC. UV-Vis absorption spectra were
recorded from 300 to 600 nm. Matrix-assisted laser desorption
ionization time-of-flight (MALDI-ToF) mass spectra were
recorded using a spectrometer equipped with a nitrogen laser,
operating at 337 nm with a maximum output of 500 J mꢂ2
delivered to the sample in 4 ns pulses at 20 Hz repeating rate.
Time-of-flight mass analyses were performed in the reflection
mode at a resolution of about 10 000. All the samples were
analyzed using (DCTB) trans-2-[3-(4-tert-butylphenyl)-2-methyl-
prop-2-enylidene]malonitrile matrix that was prepared as 20 mg
mLꢂ1 solution in CHCl3. The matrix solution (1 mL) was applied
to a stainless steel target and air dried. Polymer samples were
dissolved in CH2Cl2 to obtain 1 mg mLꢂ1 solutions. 1 mL aliquots
of those solutions were applied onto the target area already
bearing the matrix crystals, and air dried. For the recording of
the single-stage MS spectra, the quadrupole (rf-only mode) was
set to pass ions from 500 to 10 000 Th, and all ions were trans-
mitted into the pusher region of the time-of-flight analyzer where
they were mass analyzed with a 1 s integration time. Trans-
mission Electron Microscopy (TEM) observations were carried
out at 100 kV (JEOL 1200 EXII). Samples for TEM measure-
ments were prepared by embedding the hybrid material in
AGAR 100 resin, followed by ultramicrotomy techniques and
deposition on copper grids. Scanning Electron Microscopy
(SEM) images were obtained with a Hitachi S2600N microscope.
Atomic Force Microscopy (AFM) images were obtained in
ambient conditions with an ICON microscope (Veeco Instru-
ments Inc.) operating in tapping mode. Si cantilevers with
a resonance frequency in the 150–300 kHz range were used. The
images were displayed and analyzed with the WSxM software,19
with topography coded from dark to bright. Elemental analyses
were carried out by the ‘‘Service Central de Micro-Analyse du
CNRS, Vernaison, France’’. Thermogravimetric analyses were
carried out at 10 ꢀC minꢂ1 under N2 and using ca. 10 mg samples
in Pt pans.
Compound 9. 1H NMR (CDCl3): d ¼ 1.38 (m, 8H, CH2), 1.70
(m, 2H, CH2), 1.84 (m, 2H, CH2), 2.80 (m, 2H, CH2–Th), 3.40
3
(t, 2H, CH2–Br, JH–H ¼ 7 Hz), 7.02 (s, 1H, Th) ppm. 13C{1H}
NMR (CDCl3): d ¼ 28.4, 28.9, 29.5, 29.6, 30.5, 30.7, 33.0, 34.2
(CH2), 128.9, 130.7, 133.9, 140.0 (Th) ppm. UV-visible (CHCl3):
lmax ¼ 443 nm, SEC (THF, polystyrene standard) Mn ¼ 5100 g
molꢂ1; polydispersity ¼ 1.28.
Regioregular poly{3-[N-(1-methylimidazolium-3-yl)alkyl]-
thiophene-2,5-diyl bromide} (10–12, respectively n ¼ 4, 6, 8)
Poly[3-(n-bromoalkyl)thiophene] (7–9, n ¼ 4, 6, 8, respectively)
(0.8 mmol of monomer units) was allowed to react with
1-methylimidazole (8 mmol) in refluxing CHCl3 (10 mL) for 3 d.
After cooling to room temperature, the main part of the solvent
mixture was evaporated and the concentrated solution (ca. 2 mL)
was poured to THF to precipitate polymers 10–12. The crude
polymers obtained were repeatedly washed with THF and
diethyl ether to remove residues of 1-methylimidazole and finally
dried under vacuum at 40 ꢀC. The solid polymers 10–12 were
further purified with refluxing diethyl ether by using a Soxhlet
ꢀ
apparatus and finally dried under vacuum at 40 C.
1
Compound 10. Yield: 72%. H NMR (CD3OD): d ¼ 1.80 (m,
2H, CH2), 2.06 (m, 2H, CH2), 2.97 (m, 2H, CH2), 3.97 (s, 3H,
N–CH3), 4.38 (m, 2H, CH2), 7.15 (s, 1H, Th), 7.62 (s, 1H, Har),
7.76 (s, 1H, Har), 9.13 (s, 1H, NCHN) ppm. 13C{1H} NMR
(CD3OD): d ¼ 27.5, 30.3, 31.5, 36.7 (CH2), 50.9 (CH3), 123.8,
124.9, 130.4, 131.8, 135.0, 137.9, 141.5 ppm. UV-visible (EtOH):
lmax ¼ 436 nm.
1
Compound 11. Yield: 79%. H NMR (CD3OD): d ¼ 1.50 (m,
4H, CH2), 1.77 (m, 2H, CH2), 1.97 (m, 2H, CH2), 2.91 (m, 2H,
CH2), 3.97 (s, 3H, N–CH3), 4.25 (m, 2H, CH2), 7.15 (s, 1H, Th),
7.60 (s, 1H, Har), 7.70 (s, 1H, Har), 9.10 (s, 1H, NCHN) ppm.
13C{1H} NMR (CD3OD): d ¼ 27.3, 30.1, 30.4, 31.3, 31.6, 36.8
(CH2), 51.0 (CH3), 123.9, 125.1, 130.4, 131.9, 135.1, 138.0, 141.6
ppm. UV-visible (EtOH): lmax ¼ 440 nm.
1
Compound 12. Yield: 81%. H NMR (CD3OD): d ¼ 1.44 (m,
8H, CH2), 1.76 (m, 2H, CH2), 1.93 (m, 2H, CH2), 2.87 (m, 2H,
CH2), 3.96 (s, 3H, N–CH3), 4.25 (m, 2H, CH2), 7.12 (s, 1H, Th),
7.61 (s, 1H, Har), 7.68 (s, 1H, Har), 9.06 (s, 1H, NCHN) ppm.
13C{1H} NMR (CD3OD): d ¼ 27.6, 30.4, 30.5, 30.6, 30.7, 31.5,
31.9, 36.9 (CH2), 51.1 (CH3), 123.9, 125.2, 130.1, 131.8, 135.1,
138.1, 141.6 ppm. UV-visible (EtOH): lmax ¼ 443 nm.
Results and discussion
Nanocomposite thin films. Given amounts of polymer, tetrae-
thoxysilane (TEOS), EtOH, hydrochloric acid (see Table 1) were
stirred to produce a transparent solution. The solution was
filtered. Thin films were then prepared by spin-coating on glass,
quartz and ITO substrates. The substrate surface was first
completely covered with the solution, and the sample rotation
was turned on immediately at 50 rpm for 8 s and then, 500 rpm
Synthesis of the monomers
Our synthetic strategy relies on the post-polymerization func-
tionalization of bromide-bearing polythiophenes to prepare
regioregular head-to-tail cationic polythiophene polyelectrolyte
containing n-(1-methylimidazolium-3-yl)alkyl (n ¼ 4, 6, 8) side
groups with bromide counterions. The regioregularity of the
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J. Mater. Chem., 2011, 21, 2733–2739 | 2735

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Table 1 Preparation of the polymer/silica hybrids
Run
Polymer/mg
TEOS/g
EtOH/mL
HCl/mL
Appearance
Si/Na (molar)
1
2
3
10 (15)
11 (15)
12 (15)
0.4
0.4
0.4
4
4
4
0.3
0.3
0.3
Transparent
Transparent
Transparent
27 (40)
16 (40)
16 (40)
a
Calculated from the elemental analysis of Si and N atoms (theoretical values).
resulting polymer is predetermined by the regioregularity of the
poly(3-(n-bromoalkyl)thiophene) (n ¼ 4, 6, 8) precursor since
the modification of the bromo end groups does not modify the
main-chain regioregularity. The preparation of poly(3-(n-bro-
moalkyl)thiophene) (n ¼ 4, 6, 8) can be achieved by GRIM-type
polymerization. This synthetic method represents an efficient
strategy for the construction of regioregular head-to-tail poly-
thiophenes combining soft and safe operating conditions.20 For
this polycondensation, dibromide monomers 4–6 were synthe-
Polymers 7–9 are readily soluble in common organic solvents
such as tetrahydrofuran (THF) and chloroform. The regior-
egularity of these polymers was more than 97%, as determined
from the peak integrations of H NMR.23 1H NMR spectra of
1
these polymers show that the methylene groups adjacent to the
bromine atoms display a signal around 3.40 ppm (Fig. S1–S3,
ESI†).
MALDI-ToF mass spectrometry analyses of 7–9 exhibit a set
of peaks corresponding to Br/H chain ends. The MALDI-ToF
spectra of polymers 8 and 9 are shown in Fig. S4 and S5, ESI†,
respectively. Maximum molecular weights of 6400 and 7400 were
measured for 8 and 9, respectively. Such values are in perfect
agreement with those expected from the calculated monomer-to-
initiator ratio.
sized according to Scheme 1 by following the method reported by
€
17
Bauerle et al. 1-[(n-Bromoalkyl)oxy]-4-methoxybenzene was
first prepared from 4-methoxyphenol by Williamson ether-
ification.
In a second step, that compound was coupled with 3-bromo-
thiophene by using the Kumada–Tamao–Corriu coupling reac-
tion leading to 3-(n-4-methoxyphenoxy)alkyl]thiophene. Then,
3-(n-bromoalkyl)thiophenes (1–3, n ¼ 4, 6, 8, respectively) were
obtained by reacting 3-(n-4-methoxyphenoxy)alkyl]thiophene
with hydrogen bromide. Finally, dibromination of precursors
1–3 with NBS afforded 2,5-dibromo-3-(n-bromoalkyl)thiophene
(4–6, n ¼ 4, 6, 8).
The UV-Vis absorption spectra of polymers 7–9 in CHCl3
show an absorption maximum around 440 nm (Fig. S6, ESI†),
characteristic of the p–p* interband transition of the poly-
thiophene backbone.24 The thermal properties of these poly-
thiophene containing bromide-terminated alkyl side chains were
also studied by TGA (Fig. S7, ESI†). Compounds 7–9 show an
identicalꢀbehavior characterized by a weight loss between 200
and 350 C due to the loss of the bromide end group. A second
weight loss is observed above 400 ꢀC which is probably related to
the decomposition of the poly(3-alkylthiophene) backbone, as
previously described in the literature.25
Polymerization
The poly(3-(n-bromoalkyl)thiophene) (7–9, n ¼ 4, 6, 8, respec-
tively) was synthesized by following the method reported by us
for poly(3-hexylthiophene) (Scheme 2).21 After the polymeriza-
tion, the reaction mixture was quenched with 5 M HCl, following
the procedure by Yokozawa et al.22 This quenching procedure
prevents the polymer chains from undergoing dimerization, in
order to maintain the narrow polydispersity while efficiently
removing the inorganic impurities from the product. Number-
averaged molecular weight (Mn) going from 3900 to 6800 and
polydispersity index (Mw/Mn) below 1.3 were obtained from the
monomodal traces of the polymers 7–9 in SEC analyses.
The subsequent quaternization reaction between 1-methyl-
imidazole and the polymer bromide groups afforded polymers
10–12 in 70–80% yield (Scheme 2). NMR spectra of conjugated
polyelectrolytes 10–12 are well resolved and consistent with the
1
structures described in Scheme 2 (the H and 13C{1H} NMR
spectra of polymer 11 are described as examples, in Fig. S8 and
S9, ESI†, respectively). The complete substitution of bromide
groups with N-methylimidazole is proved by the shift of the
triplet representing the methylene side-chain end group from 3.40
ppm in the polymers 7–9 (measured in CDCl3) to around
4.20–4.40 ppm (CD3OD) for compounds 10–12. The presence of
signals typical of 3-methylimidazolium-1-yl group is also
observed between 7 and 9.5 ppm. The signal at about 9 ppm is
Scheme
2 Reaction pathway to poly{3-[n-(1-methylimidazolium-3-
Scheme 1 Preparation of compounds 4–6.
2736 | J. Mater. Chem., 2011, 21, 2733–2739
yl)alkyl]-thiophene-2,5-diyl bromide} (10–12).
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unambiguously assigned to the electron-deficient imidazolium
proton NC(H)N.
especially for polymer 10, which has the shortest alkyl group,
hence the strongest steric perturbation by the imidazolium
group.
The solubility of the regioregular head-to-tail polyelectrolyte is
different from the precursor polymers. Contrary to poly(3-
(n-bromoalkyl)thiophene) 7–9, the polymers 10–12 are insoluble
in THF, chloroform, methylene chloride and toluene but readily
soluble in water, ethanol and methanol as expected. Water
solubility of conjugated polyelectrolytes 10–12 arises from the
hydrophilic ionic liquid side chain (imidazolium salt) which
overcomes the hydrophobic p–p stacking interactions between
the polythiophene backbones. The solubility of these polymers is
influenced by the length of the alkyl chains, the hydrophilic
character decreasing in the series butyl, hexyl, octyl.26 The
UV-Vis spectra of the conjugated polyelectrolytes 10–12 film and
in solution in EtOH (10ꢂ3 M with respect to the monomeric units)
are compared in Fig. 1.
Preparation of conjugated polyelectrolyte/silica nanocomposite
films
All materials were prepared under the exact same conditions by
acid-catalyzed hydrolysis and polycondensation of TEOS in the
presence of polymers (10, 11 and 12) with different length of
alkylene chains by spin coating (Scheme 3).
Highly transparent and homogeneous films were obtained and
the results were summarized in Table 1 (run 1–3). Elemental
analyses were performed to estimate the organic group contents.
It was found that for all polymers 10, 11 and 12 Si/N ratio values
obtained from elemental analysis are lower than the expected
values calculated on the basis of the composition of the initial
mixture (Table 1). This difference can be explained by the partial
polymerisation of silica precursor during the sol aging time
before deposition (see Experimental section). From these results,
the wt% polymer content can be estimated to around 20% which
is still high.
The spectra of the conjugated polyelectrolytes 10–12 in the
solid state show a broad, structureless absorption band with
maximum around 520 nm and a weak shoulder at about 600 nm
for 11 and 12. This band attributed to the p–p* transition in the
polymer main chains is very sensitive to the effective conjugation
length.27 The maxima wavelength of the absorption spectra of the
polymer films dropcast underwent red shifts of about 60–90 nm
in comparison to those in the solutions. These changes indicate
the extension of the conjugation length (chain planarization) in
the solid state resulting from the interchain interactions in the
films.28 However, this red shift in absorption from solution to
film is smaller than that previously obtained for poly(3-hexy-
lthiophene) (P3HT) (100 nm), indicating a lower increase in the
planarity of the polymer chains when going to the solid state.
This is attributed to the steric hindrance induced by the imida-
zolium moeities.15 As the alkyl side group gets longer, the
imidazolium ring is located further away from the conjugated
backbone, the steric hindrance decreases, the p-conjugated is
improved and the absorption gradually shifts to longer wave-
lengths. The presence of a vibronic structure in the absorption
profile can also be used to estimate the degree of order in poly-
thiophene films.29 Here no vibronic structure is present in the
spectrum of compound 10 and it is very faint for polymers 11 and
12. This globally indicates a low degree of order in those films,
In order to determine the morphology of the hybrid materials
on the microscopic scale, SEM measurements were performed on
nanocomposite films containing polymers 10–12. The SEM
images reveal that all materials have similar morphologies with
thin and regular hybrid and homogenous films with 1 mm of
thickness (see Fig. S10, ESI†).
As an example, Fig. 2 shows the morphology of a polymer
11@silica thin film.
Transmission Electron Microscopy measurements were per-
formed using ultramicrotomy techniques. A general view of
polymer 11@silica hybrid material is presented in Fig. 3. A
magnification of the same material (Fig. 3) reveals the presence
of homogeneous vermicular structure.
The conformation of these ionic polymers (10–12) is depen-
dent on the nature of the environment. Under hydrophilic
conditions, the hydrophobicity of the alkyl chain as well as the
aromatic polythiophene backbone induces the formation of
micelle-like structures consisting of a poly(alkylthiophene) core
and a 1-methylimidazolium bromide shell.15 This organization of
the polymer chains in a hydrophobic solution, as is the case here,
may support the arrangement of the hybrid material in
a vermicular structure. Such behaviour was previously noticed by
Chujo and Ogoshi with an ionic polyacetylene in a silica gel
matrix.10
Fig. 1 Optical absorption spectra of conjugated polyelectrolyte solu-
tions 10–12 in EtOH and spectra of the polymer films dropcast on
a quartz substrate.
Scheme 3 Synthesis of the hybrid materials.
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Fig. 2 SEM image of polymer 11@silica hybrid material.
Fig. 5 Optical absorption spectra of the ionic polythiophene (10–12)/
silica films deposited on a quartz substrate.
morphology most often observed in thin deposits of poly-
alkylthiophenes. This indicates that the polymer chains at the
hybrid surface are not as ordered as in ‘classical’ poly-
alkylthiophenes, probably as a result of the bulkiness of the
imidazolium groups and the constraints imposed during the
sol–gel process.
To evaluate the optical properties of these polythiophene/silica
hybrid films, UV measurements were carried out (Fig. 5). Films
of polythiophene/silica show a UV absorption maximum around
430, 435 and 460 nm for polymers 10–12, respectively. These
values are significantly blue-shifted in comparison to the poly-
thiophene polyelectrolyte thin films described above. The spectra
are more similar to those obtained in solution (albeit slightly
broader), indicating a twisting of the polythiophene backbone,
probably as a result of the interactions between imidazolium and
charged silanol groups in the silica matrix. This finding is fully
consistent with the nanoscale organization of the hybrid mate-
rial, where the ionic polythiophene chains seem to form micelle-
like structures and no sign of ordering is present at the surface.
Thermal properties of cationic polythiophene/silica hybrids
were also studied by TGA measurements. The TGA curves of the
cationic polymer 11 and the hybrid material (run 2) are respec-
tively shown as an example, in Fig. S11 and S12, ESI†. The
thermal stability of the polymer in the hybrid material is
markedly improved about 100 ꢀC in comparison with the
polythiophene alone.
Fig. 3 TEM micrographs of hybrid material (run 2). Scale bar ¼ 2 mm
(a). Scale bar ¼ 100 nm (b).
The surface morphology of the hybrid materials was analysed
with AFM (Fig. 4). The surface of the films clearly shows large
flat grains, corresponding to the platelets observed in SEM.
Those grains are partly covered by layers that have a rather
constant height. The thickness of those layers is dependent on the
length of the alkyl side chain: the steps measured by AFM
˚
˚
between the layer and the surface of the grain are 6.5 A, 9.5 A,
˚
and 11.5 A for polymers 10 to 12, which have side groups with
n ¼ 4, 6 and 8, respectively. This clearly indicates that the
overlayers are made of the ionic polythiophenes. The difference
in height suggests that the chains are adsorbed ‘edge-on’ with
respect to the substrate; with the side groups oriented roughly
perpendicular to the surface. The overlayers appear to have
a granular morphology, rather than the highly regular fibrillar
Conclusion
In this work, we described the synthesis and characterization of
new organic–inorganic hybrid materials as highly transparent
films containing regioregular ionic polythiophene units. We
evidenced that the strong ionic interaction between the cationic
imidazolium side groups and protonated silanol moieties gener-
ated in the sol–gel reaction resulted in the formation of
homogenous composites without destroying the p-conjugated
structure of the polymer. The almost preserved conjugation
length despite this strong interaction as well as the excellent
solubility of these ionic polymers in water and some water-
miscible solvent will permit the preparation of a wide variety of
conducting polymer - inorganic oxide composite as light-emit-
ting, photoactive and conductive materials. The conducting and
optical properties depending on the nanostructure of the ionic
polythiophene and the composition of the sol–gel precursor
Fig. 4 AFM images of the hybrids with polymers 11 (A) and 12 (B). The
vertical grayscale is 26 nm (A) and 20 nm (B).
2738 | J. Mater. Chem., 2011, 21, 2733–2739
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Acknowledgements
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The authors thank the CNRS and the Universite Montpellier II
1718.
for financial support. Research at CIRMAP is supported by the
ꢀ
European Commission and Region Wallonne FEDER program,
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