Hierarchically structured, blue-emitting polymer hybrids through surface-initiated nitroxide-mediated polymerization and water templated assembly

Article abstract of DOI:10.1039/c3tc31122h

A facile and robust approach for fabricating structured, blue-emitting polymer hybrids is explored by grafting poly(styrene) incorporating π-conjugated oligo(fluorene) side-chains, to fluoromica silicate layers through surface-initiated nitroxide-mediated polymerization (SI-NMP). It is expected that the polymer intercalation can effectively reduce π-stacking, chain-chain interactions, twists and bends, and interfacial effects, leading to significant difference in the electronic/optoelectronic properties, and improved optical, thermal and chemical stability of the materials. The experimental results indicate that the bottom-up strategy is rational and efficacious. The hybrids exhibit a blue photoluminescence quantum yield (PL-QY) as high as 0.90, even in the solid, which makes the materials appealing for polymer light-emitting devices (PLEDs). The materials also show significantly enhanced thermal, and chemical stabilities with respect to the organic precursors. If processed under specific controlled conditions, the hybrids spontaneously assemble into highly ordered microporous films, where an organization of matter at different length scales is obtained. Since the introduction of surface patterning in the active layer could enhance the extraction of light generated in the device, this hierarchical organization is a promising tool for the further development of optimized hybrid PLEDs.

Full text of DOI:10.1039/c3tc31122h

Journal of
Materials Chemistry C
View Article Online
View Journal
PAPER
Hierarchically structured, blue-emitting polymer hybrids
through surface-initiated nitroxide-mediated
Cite this: DOI: 10.1039/c3tc31122h
polymerization and water templated assembly†
Giuseppe Leone,* Umberto Giovanella, Fabio Bertini, Sajjad Hoseinkhani,
William Porzio, Giovanni Ricci, Chiara Botta and Francesco Galeotti*
A facile and robust approach for fabricating structured, blue-emitting polymer hybrids is explored by
grafting poly(styrene) incorporating p-conjugated oligo(fluorene) side-chains, to fluoromica silicate
layers through surface-initiated nitroxide-mediated polymerization (SI-NMP). It is expected that the
polymer intercalation can effectively reduce p-stacking, chain–chain interactions, twists and bends, and
interfacial effects, leading to significant difference in the electronic/optoelectronic properties, and
improved optical, thermal and chemical stability of the materials. The experimental results indicate that
the bottom-up strategy is rational and efficacious. The hybrids exhibit a blue photoluminescence
quantum yield (PL-QY) as high as 0.90, even in the solid, which makes the materials appealing for
polymer light-emitting devices (PLEDs). The materials also show significantly enhanced thermal, and
chemical stabilities with respect to the organic precursors. If processed under specific controlled
conditions, the hybrids spontaneously assemble into highly ordered microporous films, where an
organization of matter at different length scales is obtained. Since the introduction of surface
patterning in the active layer could enhance the extraction of light generated in the device, this
hierarchical organization is a promising tool for the further development of optimized hybrid PLEDs.
Received 11th June 2013
Accepted 7th August 2013
DOI: 10.1039/c3tc31122h
www.rsc.org/MaterialsC
non-pure emission (green or greenish-blue rather than pure
blue) and poor stability against oxygen and moisture. In this
1
Introduction
Hierarchical hybrid composites are widely observed in nature, respect, the polymer intercalation turns out to be an effective
where specic functionalities are oen obtained by structuring
the matter across a range of length scales. The supramolecular
organization of chlorophyll molecules in poly(peptide) cages,
method: (i) to impose constraints on the mesoscopic and
molecular structures of the polymer (i.e., reduced p–p stacking,
chain–chain interactions, twists and bends, and interfacial
involved in photosynthesis, explains, for instance, why plants effects), (ii) to prevent oxygen and moisture from penetrating
are such efficient optoelectronic systems. This peculiarity of into the emissive layer, preserving the emitter against degra-
nature has inspired scientists to design and process organic– dation, while retaining a high degree of optical transparency,
inorganic hybrids, in which both phases are intimately and (iii) to improve the bulk mechanical properties (i.e.,
2
d,4
combined, optimizing complementary properties and functions strength, modulus and toughness).
1
(e.g., density, permeability, mechanics). A key example in this
Layered silicates are particularly appealing for fabricating
such functional hybrids due to their adsorptive properties, ion-
exchange ability and high specic surface area, which permit
them to easily tune the interaction between the emitting centers
context is given by the intercalation of conjugated polymers
such as poly(uorene)s (PFs), poly(aniline), poly(thiophene),
2
poly(p-phenylene) in between layered solids, i.e., silicates, V
2
O
5
2
3
6
and MoO
3
4
metal oxide, and chalcogenide compounds MoS
by surface chemistry and a sandwich-type intercalation. The
and SnS
2
, with the aim of enhancing their light emissive materials can be prepared by two strategies. The rst one
properties. In particular, PFs have emerged as semiconducting involves the direct assembly of a pre-formed polymer from the
2
,4
materials for applications in PLEDs because of the high PL-QY, solution method or the exfoliation–adsorption process.
wide band gaps, and good charge-transport properties.
5
The second strategy, i.e., bottom-up approach, implies the
However, PLEDs fabricated from PF-type materials suffer from
material's construction using precursors that become organized
mainly through chemical processes. In this context, SI-NMP
7
represents a valuable expedient for the fabrication of polymer
brushes and hybrid nanoassemblies. An advantage of SI-NMP
CNR Istituto per lo Studio delle Macromolecole (ISMAC), via E. Bassini 15, 20133
8
Milano, Italy. E-mail: giuseppe.leone@ismac.cnr.it; francesco.galeotti@ismac.cnr.it
1
is that no metal catalysts are required. This obviates the need
†
Electronic supplementary information (ESI) available: H NMR of St-Fl
3
. DSC
and TGA/DTG curves. Normalized UV-Vis spectra. See DOI: 10.1039/c3tc31122h
for additional purication steps and reduces the chance of
This journal is ª The Royal Society of Chemistry 2013
J. Mater. Chem. C

View Article Online
Journal of Materials Chemistry C
Paper
introducing impurities, which is advantageous, especially for nitrogen line and standard Schlenk techniques. Fluoromica
sensitive applications. A further advantage provided by this silicate, i.e., Somasif ME100 (SME) (CO-OP Chemicals), was
ꢀ
ꢁ3
approach is that incompatibility between the hydrophilic inor- dried at 130 C under reduced pressure (10 bar) for two weeks
ganic component and the hydrophobic polymer is strongly and then it was stored under nitrogen. Rhodamine 6G (R6G,
reduced, and hence, the segregation of single-phase micro- Acros Organics, dye content 99%), 4-acetamido-2,2,6,6-tetra-
domains is overcome. In addition, control over the electronic methylpiperidine 1-oxyl radical (Sigma-Aldrich, $98% pure),
3 4
structure and the interlayer geometrical conformation of the Pd(PPh ) (Sigma-Aldrich, $99% pure), 9,9-dioctyluorene-2,7-
polymer can be achieved by a judicious choice of (co)monomers bis(trimethyleneborate) (Sigma-Aldrich, $97% pure), 2,7-
9
and by tuning the initiator graing density. As a consequence, dibromo-9,9-dioctyluorene (Sigma-Aldrich, $96% pure), and
not only the primary structure but also the higher-order struc- 4-vinylphenylboronic acid (Sigma-Aldrich, $98% pure) were
tures of light-emitting polymers can be tailored to achieve used as received. 2-Bromo-9,9-dioctyluorene was synthesized
13
advanced functions. The fabrication of hierarchical structures according to a literature procedure. Ethyl alcohol (Carlo Erba,
is a key example in this context, as a way of developing materials 96% pure) and ortho-dichlorobenzene (o-DCB) (Sigma-Aldrich,
with high level of sophistication and miniaturization, which are anhydrous grade) were degassed under vacuum then by
at the same time robust and adaptable for high value-added bubbling nitrogen, kept over molecular sieves and used without
applications (i.e., smart membranes and solid support for any further purication. Diethyl ether (Fluka, $99.5% pure) and
10
sensors). Self-assembly is one of the most reliable ways to THF (Sigma-Aldrich, $99.0% pure) were reuxed over Na/K
construct higher-order and even hierarchical structures. The benzophenone for ca. 8 h, then distilled and stored over
stepwise fabrication of such integrated hybrids with the help of molecular sieves under nitrogen. Carbon disulde, methanol,
self-organization processes would therefore have the capability hexane, and toluene (Sigma-Aldrich) were used without further
of producing patterned lm surfaces in a simple and cost- purication. Styrene (Sigma-Aldrich, 99% pure) was reuxed for
effective manner, thus eliminating the need for external-eld 4 h over calcium hydride, then distilled trap-to-trap and stored
and lithography techniques, i.e., top-down strategies. In under nitrogen. Distilled deionized water was used for all
particular in PLEDs, since most of the light generated in the cation-exchange experiments.
active layer of a device with conventional architecture remains
entrapped as waveguide and substrate modes, the organization
into patterned lms is functional in introducing a perturbation
in the waveguide modes, which may enhance the light
2
.2 Synthesis of the TEMPO-like nitroxide cation
4
2
-Acetamido-2,2,6,6-tetramethylpiperidine 1-oxyl radical (0.5 g,
.92 mmol) was placed into a 50 mL three-neck round bottomed
11
outcoupling.

ask charged with diethyl ether (25 mL), allowed to stir for 1 h at
ꢀ
In this work, we describe a two-step method for fabricating
blue-emitting poly(styrene) with oligo(uorene) side-chains/
silicate hybrids, in the form of lms. The strategy involves the
direct addition of the silicate, pre-intercalated by 2,2,6,6-tetra-
methyl-1-piperidyloxyl (TEMPO) bearing an ammonium func-
tional moiety, to the (co)monomers mixture, i.e., styrene and
styrene derivatives para-substituted with (oligo)uorene moie-
ties, followed by SI-NMP. Successively, the polymer hybrids were
allowed to self-assemble into microporous lms, by applying
2
4 C and then washed with HCl (aq., 1 M, 3 ꢂ 5 mL). The
aqueous layers were combined and the solvent was removed
under vacuum for several days. The product was obtained as red
14
powder. FTIR (nujol): 2926 (br, s), 2365 (m), 2339 (m), 1622 (m),
611 (m), 1522 (m), 1479 (m), 1391 (s), 1352 (w), 1243 (m), 1223
1
ꢁ1
(w), 1178 (w), 1095 (m), 1018 (w), 953 (w), 886 (w), 744 (m) cm .
2.3 (Co)intercalation of R6G and the TEMPO-like cation
12
the breath-gure (BF) templating conditions.
G7 was synthesized according to the procedure previously
14
The experimental results indicate that the approach is reported. Typically, to a 250 mL three-neck round bottomed
rational and efficacious: the hybrid composites exhibit high ask, equipped with a stirring bar, were added SME (402 mg)
solution-processability, improved thermal stability, and the and water (150 mL). The mixture was stirred for 4 days at room
blue PL-QY as high as 0.90, even in the solid state, which makes temperature. A solution of the TEMPO salt (52.4 mg, 0.34 mmol)
the materials appealing for PLEDs. Moreover, the adjustment of in ethanol is prepared. A solution of R6G (72.7 mg, 0.16 mmol)
overall hydrophilicity of the material, through a proper in ethanol is prepared. The aimed loading for the sample G7 is
balancing of the amounts of the components, led to hierarchi- 70% and 34% of the total SME cation exchange capacity (CEC)
cally structured hybrids in the form of patterned lms orga- for TEMPO and R6G cations, respectively. Both the R6G and
nized at two different length scales, i.e., intercalated polymer TEMPO-like cation solutions were added to the SME suspension
chains and micrometric periodic structures, which offer in water. The cation-exchange reaction proceeds for an addi-
signicant promise as a platform for the further development of tional 48 h at room temperature. To collect the loaded SME, the
non-planar PLEDs and other technological applications.
suspension was ltered and washed with a water–ethanol
mixture (1 : 1). Once the solvent was removed under reduced
pressure, the product was extracted with ethanol through a
Soxhlet method for 8 h. The residual fraction was dried in a
vacuum and then ground in the agate mortar. FTIR (KBr):
2
Experimental
2.1 Materials
ꢁ1
ꢁ1
ꢁ1
Manipulations of air- and/or moisture-sensitive materials were (nC–H, 2852 cm ; dC–H, 1343 cm ), (nN–O, 1391 cm ), (nC–
carried out under an inert atmosphere using a dual vacuum/ N, 1261 cm ), (nN–H, 1519 cm ), (nC–H, 1320 cm ), 1611,
ꢁ
1
ꢁ1
ꢁ1
J. Mater. Chem. C
This journal is ª The Royal Society of Chemistry 2013

View Article Online
Paper
Journal of Materials Chemistry C
ꢁ
1
1
529, and 1501 cm associated with in-plane vibration of the a glass substrate. Honeycomb structured lms were obtained by
ꢁ
1
R6G xanthene ring, and nC]O, 1721 cm . The contents of casting 150 mL of a carbon disulde solution of the polymer
TEMPO and R6G were 12.0 and 4.1 wt%, respectively.
14
2
hybrid on a 1.5 ꢂ 1.5 cm glass cover slip under a ow of moist
ꢀ
ꢁ1
air (60% R.H. at 25 C), obtained by owing air at 2 L min
2
9
.4 9,9-Dioctyl-2-(9,9-dioctyl-2-(9,9-dioctyl-2-(4-vinylphenyl)-
H-uoren-7-yl)-9H-uoren-7-yl)-9H-uorene (St-Fl
through a two-necked ask lled with water, at ambient
3
)
temperature. Polymer concentration was varied from 5 to 10 mg
ꢁ1
mL in order to set the optimal conditions for BF formation.
It was synthesized through a stepwise Suzuki coupling approach,
as depicted in Scheme S1.† A mixture of 2-bromo-9,9-dioctyl-
uorene (500 mg, 1.07 mmol), 9,9-dioctyluorene-2,7-bis-
2.7 Characterization techniques
(
trimethyleneborate) (1.492 g, 2.67 mmol), K CO (2.0 M aqueous
2
3
FTIR spectra were recorded by a Bruker IFS 48 spectrometer.
Size exclusion chromatography (SEC) measurements were
carried out on a Waters SECV2000 system equipped with two
PLGel Mixed C columns, a 2414 RI detector and a 490 UV diode-
array detector. THF was used as solvent and poly(styrene) with
solution, 4.0 mL), Pd(PPh
)
3 4
(61 mg, 0.05 mmol) and THF (9 mL)
ꢀ
were placed in a Schlenk ask and stirred at 55 C for 48 h. Aer
coolingtoroomtemperature, thereactionmixturewas pouredinto
a large excess of petroleum ether. The organic layer separated was
washed with brine and dried over anhydrous Na SO . Aer ltra-
tion and evaporation of the solvent, thecrude product was puried
2
4
6
ꢁ1
molar masses ranging from 162 to 3.28 ꢂ 10 g mol were used
as standards. TGA was performed on a Perkin-Elmer TGA-7
by column chromatography on silica gel with hexane to afford the
ꢁ
1
instrument under a nitrogen ow rate of 35 mL min . Before
1
borolane terminated dimeras white crystals (Fl
NMR (600 MHz, CDCl
H, –OCH –), 2.02 (m, 8H, C(CH
OCH CH –), 1.17–1.06 and 0.79–0.67 (m, 60H, C(CH C H ) ). In
2
, 370 mg, 40%). H
ꢀ
performing the TGA run, the sample (2–3 mg) was held at 100 C
3
): (d, ppm) 7.36–7.28 (m, 13H, Ar), 4.21 (t,
), 1.53 (m, 2H,
ꢀ
for 1 h; the scan was carried out from 100 to 750 C at a heating
4
–
2
2 7 15 2
C H )
ꢀ
ꢁ1
rate of 10 C min . The residue of the TGA run was oxidized at
7
2
2
2
7
15 2
ꢀ
ꢁ1
50 C for 10 min under an air atmosphere (35 mL min ).
the following step, Fl (325 mg, 0.38 mmol) and 2,7-dibromo-9,9-
2
Differential scanning calorimetry (DSC) measurements were
carried out on a Perkin-Elmer Pyris 1 instrument. The sample,
about 5 mg, was placed in a sealed aluminum pan and the
dioctyluorene (330 mg, 0.565 mmol) were allowed to react as
mentioned in the previous step. Aer 20 h, 4-vinylphenylboronic
acid (118 mg, 0.80 mmol) was added to the reaction mixture and
allowed to react at 45 C for another 96 h. The same workup and
column separation as that mentioned above gave St-Fl
yellow semisolid (280 mg, 58%). H NMR (600 MHz, CDCl ): (d,
ppm) 7.76–7.50 (m, 23H, Ar), 6.78 (m, 1H, –CH]), 5.79 (m, 1H, ]
CH), 5.27 (m, 1H, ]CH), 2.02 (m, 12H, C(CH C H ) ), 1.24–1.04
and 0.79–0.70 (m, 90H, C(CH C H ) ). Anal. calcd for C H128: C,
8
ꢀ
measurements were carried out from 40 to 200 C under a
ꢀ
ꢀ
nitrogen atmosphere using heating and cooling rates of 20 C
3
as a light
ꢁ
1
ꢀ
min . X-ray diffraction (XRD) data were obtained at 25 C using
an Anton&Parr Camera under nitrogen ux and a Siemens
1
3
D-500 diffractometer (Cu K
operating voltage and current were 40 kV and 40 mA, respec-
a
radiation, l ¼ 0.154 nm). The
2
7
15 2
2
7
15 2
95
ꢀ
ꢀ
ꢀ
tively. Data were collected from 3 to 33 2q at 0.05 2q intervals.
UV-Visible absorption measurements were carried out via Per-
kinElmer Lambda900. The continuous wave PL spectra were
recorded by a liquid nitrogen cooled charge-coupled device
combined with a monochromator (Spex 270M), and excited
using a monochromated xenon lamp. The spectra were cor-
rected for the instrument's response. PL-QYs of the lms have
been measured with a home-made integrating sphere. Fluo-
rescence microscopy images were obtained by using a Nikon
Eclipse TE2000-U inverted confocal microscope. SEM images
were recorded using the Zeiss Leo 982 apparatus at 3 kV tension.
9.84; H, 10.16%. Found: C, 89.67; H, 10.22%.
2
.5 SI-NM(co)polymerization of styrene with St-Fl
3
The polymerizations were carried out in a 25 mL round
bottomed Schlenk ask, which had been dried on the high
vacuum line by heating at 80 C, and equipped with a magnetic
stirring bar. The reactor vessel was charged with G7, o-DCB,
styrene, and St-Fl in that order. The reaction mixture was
3
degassed by nitrogen bubbling for 20 min. The polymerization
was carried out at 125 C for 17 h. When the system was cooled
ꢀ
16
ꢀ
to room temperature, polymerization was stopped by addition
of methanol (20-fold excess). The precipitated polymer was
collected by ltration, repeatedly washed with fresh methanol
3
Results and discussion
and then dried in a vacuum to constant weight. The product was 3.1 Synthesis and characterization
then extracted with methanol through a Soxhlet method for 10 h
to completely remove the unreacted St-Fl comonomer. The
3
residual fraction was dried in a vacuum. The amount of G7,
Somasif (SME), described by the structural formula
Na [Si8ꢁnAl ]Mg , where n represents the unit cell charge,
n
n
6 20 4
O F
is a semi-synthetic crystalline uoromica with an aspect-ratio of
styrene, and St-Fl in the polymer hybrids was determined from
2
3
about 6000, an average charge unit density of 0.77 nm , and a
thermogravimetric analysis (TGA) data, according to Smitha
ꢁ1 17
CEC of 120 mequiv. 100 g
. To apply the SI-NMP technique to
15
et al. TGA data useful for evaluating the composition of the
polymer hybrid (PF1–3) are reported in Table S1.†
SME, rstly, we synthesized a TEMPO alkoxyamine bearing an
ammonium functional moiety for anchoring to the layer surface
by sodium-cation exchange. SME was then organo-modied by
2.6 Films preparation
(
co)intercalation of the TEMPO cation (70% vs. CEC) and R6G
14
Films for optical characterization were obtained by casting or (34% vs. CEC) (sample G7), as we recently reported. R6G
ꢁ
1
spin-coating 17 mg mL toluene solution of the compound on ensured the increase of polar functionality of the material,
This journal is ª The Royal Society of Chemistry 2013
J. Mater. Chem. C

View Article Online
Journal of Materials Chemistry C
Paper
a
which was fundamental to form a stable interface with water Table 1 SI-NM (co)polymerization of styrene with St-Fl
3
droplets during BF formation, and allowed us to tag the SME
tactoids, thus following the dispersion of the inorganic coun-
terpart in the polymer matrix (see Section 3.3). Subsequently,
b
b
b
c
ꢁ4
St/St-Fl
Run (mole ratio) (g)
3
Yield G7
St
St-Fl
3
M
w
[ꢂ10
]
ꢁ1
(wt%) (wt%) (wt%)
(g mol
)
the macromolecular chains were ionically graed to the SME PF1
layers through the radical copolymerization of styrene with PF2
9700
2600
133
1.25
1.15
0.88
0.3
0.7
6.6
99.1
98.1
76.7
0.6
1.2
16.7
19.7
18.7
6.7
PF3
styrene para-substituted with the p-conjugated tri(9,9-dioctyl-
a

uorene) fragment (St-Fl
layer poly(St-co-St-Fl ) stripes (Scheme 1). The polymerization
experiments were carried out in a round bottomed Schlenk
ask, charged with G7, o-DCB, styrene and St-Fl . Different
3
), forming conned extended mono-
Polymerization conditions G7, 13 mg (PF3: G7, 65 mg); St, 2.74 g;
b
ꢀ
temperature, 125 C; time, 17 h and total volume, 10 mL. G7, St and
3
c
St-Fl
3 w
contents in the polymer hybrids were determined by TGA. M ,
molecular weight determined by SEC.

3
feedstock comonomers composition ([St]/[St-Fl ] from 133 : 1 to
3
9700 : 1) and inorganic/organic ratios were investigated
(
Table 1). Solvent polymerization was preferred to bulk because controls the polymer chain growth, affording a living polymer-
we wanted to maintain a high dispersion of the inorganic ization. The random copolymers are amorphous by DSC with
8,18
particles in the reaction medium; o-DCB is chosen to facilitate a narrow range of glass transition temperature values (T
g
, 85–
ꢀ
the diffusion of the highly hindered oligo(uorene) comonomer 87 C, Fig. S1†). In addition, the synthesized polymer hybrids
ꢀ
within the SME layers as a result of the electron withdrawing were thermally stable over 430 C (Tdeg, Table S1 and Fig. S2†),
effect of the benzene ring.
which makes them appealing for practical applications.
We examined the extent of intercalation that takes place as a
Poly(St-co-St-Fl ) hybrids (PF1–PF3, Table 1) were obtained in
3
high yield, somewhat independent for the initial comonomer result of the growth of the macromolecular chains within the
mole ratio from 2600 : 1 to 9700 : 1, and then decreased for a SME layers by XRD, determining the (001) basal spacing
St/St-Fl
3 3
ratio of 133 : 1. Increasing the charged St-Fl
, the between the uoromica platelets, and hence, the calculation of
comonomer incorporation in the copolymer raised. Specically, the clearance space between the inorganic layers, namely
ꢁ
4
ꢁ3
at a St-Fl
(
3
feed content as low as 5.62 ꢂ 10 and 1.36 ꢂ 10
M
d(free), aer the subtraction of the layer thickness.
PF1 and PF2, respectively) the St-Fl incorporation does not
In Fig. 1, the XRD patterns of the polymer hybrids are shown,
3
exceed 1 wt%. St-Fl3 incorporation as high as 16.7 wt% was together with those of dehydrated SME and G7 precursors. The
ꢀ
reached increasing the St-Fl feed concentration up to 2.19 ꢂ XRD pattern of SME shows only one diffraction peak at 9.40 2q,
3
ꢁ2
10
M (run PF3). SEC analysis yielded a molecular weight of corresponding to the interlayer spacing of 0.94 nm of sodium
4
ꢁ1
4
ꢁ1
19
6
.7 ꢂ 10 g mol for PF3 and ca. 19 ꢂ 10 g mol for PF1 and exchangeable cation layers. A shi of the (001) diffraction peak
PF2, suggesting a certain extent of chain transfer reactions over toward low angle, and thus an increase of the layer periodicity,
the increase of the charged oligo(uorene) monomer. In was registered for G7, thus providing evidence for the interca-
14
contrast to what one might expect for a controlled SI-NMP, lation of organic moieties. Basically, G7 shows almost two
broader than expected molecular weight distribution (M /M z diffraction peaks, partially overlapped. By applying peak prole
w
n
2
.4) may be due to trace interlayer water impurities and inade- analysis through a pseudo-Voigt function, we found that the
quate kinetic control. Indeed, the polymerizations were carried broad single peak can be deconvoluted and quantied to t
ꢀ
ꢀ
out in the absence of the free TEMPO agent, while it is well peaks at 4.65 and 5.85 2q, corresponding to the interlayer
known how the addition of the latter creates an overall spacing of 1.90 [d(free) ¼ 0.94 nm] and 1.53 nm [d(free)
concentration of nitroxide in the reaction mixture which 0.57 nm], respectively.
Scheme 1 Schematic and simplified illustration of the polymer hybrids (PF1–3) synthesis.
J. Mater. Chem. C
This journal is ª The Royal Society of Chemistry 2013

View Article Online
Paper
Journal of Materials Chemistry C
ꢀ
and 5.60 2q, corresponding to the interlayer spacing of 1.90
and 1.59 nm, respectively. Basically, the former, marked with an
asterisk, stems from the layers with R6G exchangeable cations,
the d(free) being close to 0.94 nm as for the most intense peak of
the G7 precursor, indicating that R6G–SME stacks are still
present without the intercalated polymer. The second one at
ꢀ
5
.60 2q, that stems from the intercalated TEMPO-like cations,
was shied to lower 2q with respect to the peak of the starting
G7. Specically, the height spacing was 0.06 nm larger than that
of G7, meaning that only a slight expansion occurred over the
TEMPO-mediated polymer chains growth. By subtracting
the thickness of SME lamellae from the (001) basal spacing, the
maximum thickness of the polymer brushes was calculated to
be 0.63 nm [d(free) ¼ 1.59 ꢁ 0.96 in nm], meaning that the
graed polymer chains adopted an almost extended planar
conformation rather than a coiled or a fully extended brush one.
This result is in agreement with that obtained for poly(aniline)
and poly(9,9-dioctyluorene) intercalated within layered MoS2
0
and SnS2 chalcogenides, and for poly[2-methoxy-5-(2 -ethyl-
2,4
hexyloxy)-1,4-phenylenevinylene] in between layered silicates.
Fig. 1 XRD spectra of (from the bottom to the top) native dehydrated SME, NT/
Even more, the polymer monolayer with at conformation is
consistent with the fact that the SME interlayer spacing did not
increase with the polymer molecular weight. Indeed, a similar
trend was inferred by XRD for PF1 and PF2 which have a
R6G-(G7) intercalated SME, and bulk poly(St-co-St-Fl
profiles were properly scaled to ease the reading.
3
) hybrids (PF1–3). The
The competitive (co)intercalation of R6G and TEMPO-like molecular weight three times greater than PF3, although in the
cations led to mixed-layer structures, in which mainly two kinds rst two samples the layer stacking was hardly to be detected
of unit layers are arranged along the c-axis direction, as sche- likely due to the low inorganic content.
matically shown in Scheme 1. We found that R6G and TEMPO-
In addition, in the case of PF3, the tightness of the reection
like cations were intercalated in separate interlayer spacing: the indicates that the layers have restacked to form plates about 25
stratication of the (co)intercalated molecules, that is, the layers thick (extracted from the full width at half maximum).
segregation of them, was likely due to the fact that the two
cations organize within the inorganic layers in distinct
arrangements, the TEMPO molecule being small enough to t
well into a single unit charge in a at monolayer arrangement,
3
.2 Optical properties
The polymer intercalation with immobilization of pendant St-
Fl oligo(uorene)s in the restricted geometries prevents their
aggregation and leads to a planarization of adjacent oligo(-
ꢀ
14
3
and R6G folded up to 43 with respect to the SME layer surface.
This phenomenon has been fully described in our previous

uorene) moieties, causing the extension of the oligomers
14
work, and will be only briey outlined here. Taking into
account the SME layer charge density and the TEMPO molecule
size (the thickness is ca. 0.4 nm and its long molecular axis ca.
2d,4,21
conjugation length,
hence affecting the optical properties.
Indeed, a bathochromic shi of PL spectra was reported for
oligo(uorene)s with uorenes linearly fused in a ladder-type
conformation together with the appearance of better resolved
vibronic features. The UV-Vis absorption and PL properties of
the polymer hybrid lms are summarized in Table 2 together
0.6 nm), a at monolayer disposition is consistent with the
calculated d(free) of 0.57 nm; in such an arrangement there is a
22
reliable match between the area required by an intercalated
2
2
molecule (0.26 nm ) and that available (0.77 nm ). On the other
hand, a larger increase of the interlayer space was registered for
the R6G intercalated layers. This layers spacing can be
explained if a tilted distribution of the R6G long-molecular axis
with respect to the SME layer surface, is considered. Taking into
account the R6G molecular dimensions (1.38 ꢂ 1.15 ꢂ 0.9 nm,
Scheme 1) and that the d(free) is given by the dye long-molec-
ular axis multiplied by the sin of the angle (a) formed between
this axis and the surface plane, a was determined by means of
3
with those of the St-Fl lm and solution.
The absorption spectra of PF1, PF2 and PF3 lms and St-Fl3
solution exhibited a maximum at about 364 nm related to p /
p* transition of the St-Fl comonomer, that is red-shied by
3
about 10–15 nm with respect to tri(9,9-dioctyluorene) due to
2
3
Table 2 Optical properties of polymer hybrid films
abs
max
PL
max
Sample
l
l
PL-QY
ꢀ
sin a ¼ d(free)/1.38 in nm, to be 43 . Such a perpendicular
a
a
a
orientation of the dyes, usually observed for high dye loading, is St-Fl3
366, 364
364
434, 410
405
0.62, 0.81
0.71
0.82
adopted to reduce the inter-electronic p-system dye–dye PF1
20
PF2
PF3
364
364
412
427
interactions.
0.90
Aer the polymerization, the resulting PF3 hybrid clearly
ꢀ
a
exhibited two distinct diffraction peaks having maxima at 4.65
Toluene solution. The PL-QY values are estimated to be ꢃ0.07.
This journal is ª The Royal Society of Chemistry 2013
J. Mater. Chem. C

View Article Online
Journal of Materials Chemistry C
Paper
the presence of an additional phenyl ring that extends the neat St-Fl
3
(Table 2) as well as for other uorene-based systems,
conjugation length (Fig. S3†). The uorescence spectra of PF1 but is retained, and even enhanced, for the polymer hybrid
and PF2 showed maxima at 405 and 412 nm, respectively, lms. In fact, an exceptionally efficient blue-emitting lm, with
similar to neat St-Fl in solution, while the PF3 emission spec- a PL-QY value of about 0.9, can be easily achieved by spin-
3
trum was red-shied with the main peak at 427 nm (Fig. 2). coating PF3 from toluene solution.
Vibronic side bands typical of uorene based-oligomers were
3
The high dilution of the non-intercalated St-Fl side-chains
evident in the PL spectra of the lms, better resolved than in the along the poly(styrene) backbone in PF1 and PF2 lms led to
absorption spectra and sharper as the content of St-Fl
increases (Table 1).
3
optical properties typical of St-Fl
Table 2). The higher concentration of the oligo(uorene) side-
3
oligomers in solution (see
The strong emission efficiency (PL-QY ¼ 0.7–0.8) recorded chains and the uoromica counterpart in PF3 (Table 1), allows
for St-Fl and the hybrid materials PF1–3 in solution was only for an efficient energy transfer from the higher energy non-
3
slightly lower than that of pristine tri(9,9-dioctyluorene) intercalated oligo(uorene)s to the more planar intercalated
22,24
reported in the literature.
More interestingly, this high PL ones, leaving only a weak high energy band (around 410 nm) in
efficiency was not reduced in the solid-state, as it occurs for the the PL spectrum of the PF3 lm (Fig. 2). As a result, PF3 lms
showed an enhanced PL-QY due to the emission from highly
stable and planar oligo(uorene) polymer side-chains interca-
22a
lated within the uoromica layers.
This superior optical
performance in the solid-state as compared to other efficient
25

uorene derivative emitters suggests the application of PF3 as
the electroluminescent layer in a PLED.
3.3 Breath gures formation
To go one step further towards a hierarchically structured
material, the polymer hybrids were dissolved in carbon disul-

de and allowed to self-assemble into microporous lms, by
applying the BF templating conditions (Scheme 2). This tech-
nique in fact allows for the realization of microporous lms
with cavities arranged in a packed hexagonal fashion. The
phenomenon has already been fully described, and will be only
26
briey outlined here.
BF formation is driven by the condensation of water droplets
on the surface of a polymer solution, cast in a humid
atmosphere.
3
Fig. 2 PL spectra of St-Fl solution and the PF1–3 film excited at 370 nm.
During solvent evaporation the water droplets, stabilized by
the polymer, arrange themselves, forming a template for the
honeycomb lm. When the process is complete, a honeycomb-
like imprint of the droplets is le on the polymer lm surface.
The formation of micropores was observed for all the materials
under study (Fig. 3). The presence of both St-Fl
3
comonomer
and R6G, allowed us to extensively use the uorescence
27
Scheme 2 Schematic step-by-step of the BF fabrication technique.
microscopy to characterize the lms obtained. In fact, the blue
Fig. 3 Fluorescence microscopy images of honeycomb films prepared with different polymer hybrid materials: PF1 (a), PF2 (b) and PF3 (c). Pictures obtained by UV
excitation. Insets: reduced views of the same areas by green excitation. Scale bars: 30 mm.
J. Mater. Chem. C
This journal is ª The Royal Society of Chemistry 2013

View Article Online
Paper
Journal of Materials Chemistry C
3
emission of St-Fl highlights the morphology of the polymer

lm, while the red emission of R6G is useful for simultaneously
localize the distribution of the R6G-doped SME stacks (Fig. 3
insets).
Films prepared with PF1 and PF2 both revealed a non-
homogeneous and randomly distributed porous morphology.
Areas with micrometric pores (1–6 mm) arranged in a more
packed way (Fig. 3a and b) and with giant (20–50 mm) irregular
cavities were visible within the same lm (images not shown).
The modication of the operational conditions, including
polymer concentration and the evaporation rate, did not lead to
any improvement of the quality of porous lms. In contrast, and
intriguingly enough, highly ordered honeycomb lms were
obtained with PF3 (Fig. 3c). Films prepared with this specic
polymer hybrid material showed in fact extremely regular cavi-
ties having an external diameter of 1.5 mm and a center-to-
center distance of 2.3 mm (Fig. 4a and b). The hexagonal
arrangement of cavities is homogeneous on the whole lm, and
the ordered areas without defects, except for the local discon-
tinuities due to the presence of SME tactoids, are as wide as
2
1
mm (Fig. 4c).
To a closer view (Fig. 4d and e), it is possible to notice that
these big aggregates, that exceed the thickness of the lm, are
coated by a thin layer of polymer, and that this layer is patterned
as well. BFs on particles and small objects have been previously
observed by using elastomeric core cross-linked star polymers
28
with low Tg, since the so-owing nature of these materials
allows the honeycomb to contour to non-at surfaces. Surpris-
ingly, in our case, we achieved a conformal coating of the R6G–
3
SME particles by the patterned lm using a poly(St-co-St-Fl )
linear polymer, hence a poorly elastic material. This can be
interpreted as further evidence that the inorganic nanoparticles
are wrapped by the polymer, as we previously observed by AFM
29
in similar composite materials.
In Fig. 4f, the simultaneous emission of the St-Fl pendants
3
(green matrix) and the R6G (red spots) intercalated within the
SME layers is evidenced by uorescence microscopy with blue
excitation light. The green colour of the honeycomb lm is due
to the lter used, which cuts the blue part of the St-Fl emission
3
band. Once again, the image attests the presence of different
levels of organization in the same material: nano-conned St-
Fig. 4 Microscopy images of PF3 honeycomb films. (a) Close view by fluores-
cence microscopy (UV excitation, scale bar 4 mm). (b) SEM micrograph (scale bar
Fl3 and R6G in between the SME lamellae and the micro- 2 mm). (c) Wider fluorescence view showing long-range order (scale bar 15 mm).
(
d) Zoomed detail of the area of (c) in the white square. (e) Same view by green
structured lm. This multilevel organization is certainly
appealing for an optimization of polymer hybrid-based LEDs.
Indeed, the introduction of surface patterning in the active layer
excitation. (f) Fluorescence image by blue excitation (scale bar 5 mm). (g) Fluo-
rescence image of the bottom of the film by UV excitation (scale bar 5 mm).
5
could enhance the extraction of light generated in the device.
The reason why PF3 turned out to be so effective in
producing honeycomb structures is yet to be claried. Previ- particles. We did not observe any regular BF arrays by using
ously, two different studies have reported on BF lms with pure poly(styrene) or pure St-Fl solutions, which suggests that
polymer/silicate hybrid systems, but those lms were well far the presence of uoromica plays a role in the stabilization of
3
30
30a
away from the degree of order reached here.
water droplets.
On the other hand, PF1 and PF2, which
An all polymeric material containing both styrene and oli- contain all the components of PF3, but in different ratios,
go(uorene) moieties, but with a star-polymer nature, was produced only disordered BFs. We can therefore infer that in
31
reported to give BF arrays. However, star polymers are known PF3, apparently, the ratio between SME, R6G and St-Fl3
to be particularly suitable for BFs, in fact they were actually amounts are optimized so as to achieve maximal stabilization of
32
observed for the rst time with this class of materials. Herein, water droplets during their formation and arrangement on the
we have a linear copolymer surrounding nano/micrometric SME solution surface; hence, ordered patterns are obtained.
This journal is ª The Royal Society of Chemistry 2013
J. Mater. Chem. C

View Article Online
Journal of Materials Chemistry C
Paper
Fig. 4g shows the uorescence microscope image of a PF3
patterned lm viewed from its bottom face, i.e. through the
glass cover slip used as the substrate. Remarkably, underneath
the regularly arranged micrometric network, a second level of
interconnected and disordered voids is visible. Such a foam-like
structure suggests that these lms are, at least partially, opened
at the bottom. This observation, quite uncommon for BFs,
opens the way to the possible application of these lms as
multifunctional membranes.
4 E. Aharon, A. Albo, M. Kalina and G. Frey, Adv. Funct. Mater.,
2006, 16, 980.
5 (a) L. H. Xie, C. R. Yin, W. Y. Lai, Q. L. Fan and W. Huang,
Prog. Polym. Sci., 2012, 37, 1192; (b) S. Kappaun, C. Slugovc
and E. J. W. List, Adv. Polym. Sci., 2008, 212, 273; (c)
A. Monkman, C. Rothe, S. King and F. Dias, Adv. Polym.
Sci., 2008, 212, 187; (d) S. A. Chen, H. H. Lu and
C. W. Huang, Adv. Polym. Sci., 2008, 212, 49.
6 (a) N. Bitinis, M. Hernandez, R. Verdejo, J. M. Kenny and
M. A. Lopez-Manchado, Adv. Mater., 2011, 23, 5229; (b)
M. Ogawa and K. Kuroda, Chem. Rev., 1995, 95, 399; (c)
F. L ´o pez Arbeloa, V. Mart ´ı nez Mart ´ı nez, T. Arbeloa and
I. L ´o pez Arbeloa, J. Photochem. Photobiol., C, 2007, 8, 85; (d)
G. Leone, A. Boglia, F. Bertini, M. Canetti and G. Ricci,
J. Polym. Sci., Part A: Polym. Chem., 2010, 48, 4473.
4
Conclusions
In summary, we have demonstrated a convenient strategy to
fabricate hybrid materials comprising a blue-emitting polymer
graed to the uoromica interlayers surface. The hybrids have
7
(a) E. Ruiz-Hitzky, P. Aranda, M. Darder and M. Ogawa,
Chem. Soc. Rev., 2011, 40, 801; (b) J. Pyun and
K. Matyjaszewski, Chem. Mater., 2001, 13, 3436.
been fabricated through
a simple, controllable, SI-NMP
synthesis. In this way, an intimate mixing of the ‘so’ polymer
with the ‘hard’ inorganic component has been promoted, thus
preventing nanoparticle aggregation, and making the materials
easy to process by solution. Hence, uniform thin-lms have
been fabricated by spin-coating, showing remarkably high PL
efficiency (PL-QY z 0.90), which makes the materials appealing
for PLEDs. Notably, under optimized conditions, the hybrids
spontaneously assemble into highly ordered microporous lms,
where an organization of the matter at different length scales is
obtained. The hierarchically structured hybrids, besides having
a good potential in optoelectronics, may be valuable for novel
technological applications.
8
(a) C. J. Hawker, A. W. Bosman and E. Harth, Chem. Rev.,
2
001, 101, 3661; (b) J. Nicolas, Y. Guillaneuf, C. Lefay,
D. Bertin, D. Gigmes and B. Charleux, Prog. Polym. Sci.,
013, 38, 63; (c) M. A. Tasdelen, J. Kreutzer and Y. Yagci,
2
Macromol. Chem. Phys., 2010, 211, 279; (d) M. K. Brinks
and A. Studer, Macromol. Rapid Commun., 2009, 30, 1043;
(
e) R. Barbey, L. Lavanant, D. Paripovic, N. Sch u¨ wer,
C. Sugnaux, S. Tugulu and H.-A. Klok, Chem. Rev., 2009,
109, 5437; (f) X. Fan, C. Xia and R. C. Advincula, Langmuir,
2003, 19, 4381.
9
(a) S.-M. Shau, T.-Y. Juang, W.-H. Ting, M.-Y. Wu, S. A. Dai
and R.-J. Jeng, Polym. Chem., 2011, 2, 2341; (b) B. Lego,
M. Francois, W. G. Skene and S. Giasson, Langmuir, 2009,
Acknowledgements
2
5, 5313.
0 (a) K. Watanabe, K. Suda and K. Akagi, J. Mater. Chem. C,
013, 1, 2797; (b) M.-Q. Zhao, Q. Zhang, J.-Q. Huang and
The authors thank Mr A. Giacometti Schieroni for SEC charac-
terization and Dr A. Boglia for technical assistance.
1
2
F. Wei, Adv. Funct. Mater., 2012, 22, 675; (c) S. F ¨o rster and
T. Plantenberg, Angew. Chem., Int. Ed., 2002, 41, 688; (d)
S. Cao, L. Fang, Z. Zhao, Y. Ge, S. Piletsky and
A. P. F. Turner, Adv. Funct. Mater., 2013, 23, 2162.
Notes and references
1
(a) C. Sanchez, H. Arribart and M. M. Giraud-Guille, Nat.
Mater., 2005, 4, 277; (b) B. Descalzo, R. Mart ´ı nez-M ´a n~ ez, 11 (a) Y. Bai, J. Feng, Y. F. Liu, J. F. Song, J. Simonen, Y. Jin,
F. Sancen ´o n, K. Hoffmann and K. Rurack, Angew. Chem.,
Int. Ed., 2006, 45, 5924; (c) D. Yan, J. Lu, W. Mei,
D. G. Evans and X. Duan, J. Mater. Chem., 2011, 21, 13128;
Q. D. Chen, J. Zi and H. B. Sun, Org. Electron., 2011, 12,
1927; (b) W. H. Koo, S. M. Jeong, F. Araoka, K. Ishikawa,
S. Nishimura, T. Toyooka and H. Takezoe, Nat. Photonics,
2010, 4, 222; (c) T. Bocksrocker, J. Hoffmann,
C. Eschenbaum, A. Pargner, J. Preinfalk, F. Maier-Flaig and
U. Lemmer, Org. Electron., 2013, 14, 396.
(
d) C. Sanchez, B. Lebeau, F. Chaput and J.-P. Boilot, Adv.
Mater., 2003, 15, 1969; (e) C. Sanchez, B. Juli ´a n,
P. Belleville and M. Popall, J. Mater. Chem., 2005, 15, 3559.
2
3
(a) T.-W. Lee, O. O. Park, J. Yoon and J. J. Kim, Adv. Mater., 12 (a) P. Escal ´e , L. Rubatat, L. Billon and M. Save, Eur. Polym. J.,
2
001, 13, 211; (b) J. H. Park, Y. T. Lim, O. O. Park,
2012, 48, 1001; (b) M. Hernandez-Guerrero and
M. H. Stenzel, Polym. Chem., 2012, 3, 563.
J. K. Kim, J.-W. Yu and Y. C. Kim, Adv. Funct. Mater., 2004,
1
4, 377; (c) C. Chakraborty, K. Dana and S. Malik, J. Colloid 13 J. Ding, M. Day, G. Robertson and J. Roovers,
Interface Sci., 2012, 368, 172; (d) C. Chakraborty,
Macromolecules, 2002, 35, 3474.
P. K. Sukul, K. Dana and S. Malik, Ind. Eng. Chem. Res., 14 G. Leone, U. Giovanella, F. Bertini, W. Porzio, F. Meinardi,
013, 52, 6722.
C. Botta and G. Ricci, J. Mater. Chem. C, 2013, 1, 1450.
(a) C.-G. Wu, D. C. DeGroot, H. O. Marcy, J. L. Schindler, 15 V. S. Smitha, K. A. Manjumol, S. Ghosh, M. Brahmakumar,
2
C. R. Kannewurf, Y.-J. Liu, W. Hirpo and M. G. Kanatzidis,
Chem. Mater., 1996, 8, 1992; (b) R. Bissessur,
C. Pavithran, P. Perumal and K. G. Warrier, J. Am. Ceram.
Soc., 2011, 94, 1731.
D. C. DeGroot, M. G. Kanatzidis, J. L. Schindler and 16 J. Moreau, U. Giovanella, J.-P. Bombenger, W. Porzio,
C. R. Kannewurf, J. Chem. Soc., Chem. Commun., 1993, 687.
V. Vohra, L. Spadacini, G. Di Silvestro, L. Barba,
J. Mater. Chem. C
This journal is ª The Royal Society of Chemistry 2013

View Article Online
Paper
Journal of Materials Chemistry C
G. Arrighetti, S. Destri, M. Pasini, M. Saba, F. Quochi, 23 W. C. Tsoi, A. Charas, A. J. Calby, G. Khalil, A. M. Adawi,
A. Mura, G. Bongiovanni, M. Fiorini, M. Uslenghi and
C. Botta, ChemPhysChem, 2009, 10, 647.
A. Iraqi, B. Hunt, J. Morgado and D. G. Lidzey, Adv. Funct.
Mater., 2008, 18, 600.
1
1
7 (a) S. M. Auerbach, K. A. Carrado and S. M. Dutta, Handbook 24 F. Jaramillo-Isazaab and M. L. Turner, J. Mater. Chem., 2006,
of Layered Materials, Marcel Dekker Inc., New York, 2004; (b) 16, 83.
L. A. Utracki, B. Broughton, N. Gonz ´a lez-Rojano, L. Hecker 25 Y. Zou, J. Zou, T. Ye, C. Yang, H. Wu, D. Ma, J. Qin and
de Carvalho and C. A. Achete, Polym. Eng. Sci., 2011,
69.
8 (a) L. Tebben and A. Studer, Angew. Chem., Int. Ed., 2011, 50,
034; (b) U. Velten, R. A. Shelden, W. R. Caseri and
Y. Cao, Adv. Funct. Mater., 2013, 23, 1781.
5
26 (a) F. Galeotti, V. Calabrese, M. Cavazzini, S. Quici,
C. Poleunis, S. Yunus and A. Bolognesi, Chem. Mater.,
2010, 22, 2764; (b) M. Srinivasarao, D. Collings, A. Philips
and S. Patel, Science, 2001, 292, 79; (c) H. Yabu and
M. Shimomura, Chem. Mater., 2005, 17, 5231.
5
U. W. Suter, Macromolecules, 1999, 32, 3590.
19 H. Hata, Y. Kobayashi and T. Mallouk, Chem. Mater., 2007,
1
9, 79.
27 Fluorescence microscopy images were taken with three
different excitation lters. UV: 356 nm excitation bandpass
and 420 nm lter cut. Blue: 470 nm bandpass and 515 nm
lter cut. Green: 353 nm bandpass and 515 nm lter cut.
2
0 (a) U. Giovanella, G. Leone, G. Ricci, T. Virgili, I. Suarez
Lopez, S. K. Rajendran and C. Botta, Phys. Chem. Chem.
Phys., 2012, 14, 13646; (b) J. Bujdak, N. Iyi, Y. Kaneko,
A. Cz ´ı merov ´a and R. Sasai, Phys. Chem. Chem. Phys., 2003, 28 (a) L. A. Connal and G. G. Qiao, Adv. Mater., 2006, 18, 3024;
5
2
, 4680; (c) J. Bujd ´a k and N. Iyi, Colloid Polym. Sci., 2009,
87, 157.
1 J. R. Tozoni, F. E. G. Guimar ~a es, T. D. Z. Atvars, B. Nowacki,
(b) L. A. Connal and G. G. Qiao, So Matter, 2007, 3, 837.
29 G. Leone, U. Giovanella, W. Porzio, C. Botta and G. Ricci,
J. Mater. Chem., 2011, 21, 12901.
2
2
A. Marlleta, L. Akcelrud and T. J. Bonagamba, Eur. Polym. J., 30 (a) B. P. Nair and C. Pavithran, Langmuir, 2010, 26, 12948; (b)
2011, 47, 2259.
X. Xu, L. P. Heng, X. J. Zhao, J. Ma, L. Lin and L. Jiang, J.
Mater. Chem., 2012, 22, 10883.
2 (a) H. H. Fan, L. Guo, K. F. Li, M. S. Wong and K. W. Cheah,
J. Am. Chem. Soc., 2012, 134, 7297; (b) P. L. Wu, P. F. Xia, 31 J. C. Hsu, K. Sugiyama, Y. C. Chiu, A. Hirao and W. C. Chen,
Z. H. Li, X. J. Feng, H. L. Tam, K. F. Li, Y. Jiao, M. S. Wong
Macromolecules, 2010, 43, 7151.
and K. W. Cheah, Chem. Commun., 2009, 5421.
32 G. Widawski, M. Rawiso and B. Francois, Nature, 1994, 369, 387.
This journal is ª The Royal Society of Chemistry 2013
J. Mater. Chem. C