Bent-core liquid crystals in a route to efficient organic nonlinear optical materials

Article abstract of DOI:10.1039/b923616c

The design, synthesis and nonlinear optical (NLO) response of some bent compounds, at both the molecular and macroscopic level, are reported. Based on a bent-core NLO-phore with a μβ₀ in the range of 390 × 10^-48 esu (1907 nm), the second order susceptibility tensor for second harmonic generation is evaluated at excitation wavelengths of 1064 and 1600 nm in a mesogenic material with a SmCP mesophase. The NLO coefficients are in the range 10-100 pm V^-1. Thus it is deduced that a molecular design leading to bent-core liquid crystals with large β values is a correct strategy to achieve NLO-materials for bulk second order effects. The Royal Society of Chemistry.

Full text of DOI:10.1039/b923616c

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PAPER
www.rsc.org/materials | Journal of Materials Chemistry
Bent-core liquid crystals in a route to efficient organic nonlinear optical
materials†
a
Inmaculada C. Pintre, Jose Luis Serrano, M. Blanca Ros, Josu Martınez-Perdiguero, Ibon Alonso,
b
a
c
c
*
ꢀ
Josu Ortega, Cesar L. Folcia, Jesus Etxebarria, Raquel Alicante and Belen Villacampa
´
d
c
c
e
e
ꢀ
ꢀ
ꢀ
Received 11th November 2009, Accepted 4th January 2010
First published as an Advance Article on the web 17th February 2010
DOI: 10.1039/b923616c
The design, synthesis and nonlinear optical (NLO) response of some bent compounds, at both the
molecular and macroscopic level, are reported. Based on a bent-core NLO-phore with a mb0 in the
range of 390 ꢀ 10ꢁ48 esu (1907 nm), the second order susceptibility tensor for second harmonic
generation is evaluated at excitation wavelengths of 1064 and 1600 nm in a mesogenic material with
a SmCP mesophase. The NLO coefficients are in the range 10–100 pm Vꢁ1. Thus it is deduced that
a molecular design leading to bent-core liquid crystals with large b values is a correct strategy to achieve
NLO-materials for bulk second order effects.
effort has been made to provide materials where the molecules
are self-assembled in a suitable way for applications. In the case
of second order effects, a serious problem arises at the macro-
scopic level, as molecules with large hyperpolarizability values (b)
alone are not sufficient since the molecular disposition within the
material must be non-centrosymmetric. With this aim in mind an
outstanding material engineering has been developed.2b,d,g,h,j,3
Among the strategies followed to improve NLO-materials
for second order effects, liquid crystals should be mentioned.1,2g,4
These are materials with good processability and easily inte-
grable with semiconductor devices. In addition, among the liquid
crystals, the so-called ferroelectric liquid crystals (FLCs) are
fluids possessing thermodynamically stable polar order. In these
phases the NLO active units (NLO-phores) can be ordered in
a non-centrosymmetric polar fashion. FLCs can be considered as
an attractive alternative to NLO nematic polymers because the
latter need very strong electric fields to achieve the polar order
required. However, from the structural point of view, FLCs
suffer from severe design limitations since the introduction of
bulky 1D NLO-phores5 along the polar axis is incompatible with
the large aspect ratio required for mesogeneity (Fig. 1a). In
Introduction
The ability to manipulate light has enormous scope for techno-
logical applications such as optical processing, development of
new lasers, optical filters and optical recording. For these
applications to be achieved key issues to be addressed include the
development of suitable materials and advances in processing
and devices. Nonlinear optical (NLO) materials are good
candidates to achieve some of these targets. The ideal material
for applications in NLO devices should have a combination of
physicochemical properties which are difficult to achieve:
appropriate values for given parameters, high optical damage
threshold, high mechanical strength and thermal stability, fast
response, processability, ease of fabrication and optical trans-
parency.1 Compared with inorganic NLO materials, organic
materials can fulfil many of these requirements and also over-
come some drawbacks associated with other NLO alternatives,
such as low mechanical strength, environmental stability or
variable performance at low or high temperature. Consequently,
in the last decades much attention has been focused on organic
NLO materials opening up new possibilities and challenges in
the field.2
Applications based on NLO effects require materials rather
than molecules. Thus, even though progress at the molecular
level always offers a very good starting point, a great deal of
a
´
ꢀ
ꢀ
Quımica Organica, Instituto de Ciencia de Materiales de Aragon, Facultad
de Ciencias, Universidad de Zaragoza – C.S.I.C, 50009-Zaragoza, Spain.
E-mail: bros@unizar.es
bInstituto de Nanociencia de Aragoꢀn, Facultad de Ciencias, Universidad de
Zaragoza, 50009-Zaragoza, Spain
c
´
Fısica de la Materia Condensada, Facultad de Ciencia y Tecnologıa, 48080
Bilbao, Spain
´
d
´ ´
Fısica Aplicada II, Facultad de Ciencia y Tecnologıa, 48080 Bilbao, Spain
e
´
Fısica de la Materia Condensada, Instituto de Ciencia de Materiales de
Aragoꢀn, Facultad de Ciencias, Universidad de Zaragoza – C.S.I.C,
50009-Zaragoza, Spain
† Electronic supplementary information (ESI) available: Experimental
techniques, synthetic procedures, experimental data for compounds A,
B, and a 50% mol mixture A + C, and bulk second harmonic
generation measurements. See DOI: 10.1039/b923616c
Fig. 1 NLO-phore designs for calamitic liquid crystalline materials
(calamitic FLCs) (a) and bent-core liquid crystals (b).
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recent years, a new alternative has been considered, the so-called
bent-core liquid crystals.6,7 Different approaches concerning
the use of bent molecules to obtain materials with a large NLO
response have been proposed by several authors.1a,2d,2g,8 In
particular 2D NLO bent-core liquid crystals as studied in this
work will allow for a significant component of the hyper-
polarizability along the polarization direction. (Fig. 1b). The
compact packing of the kinked molecules leads to strong polar
order within the layers that often can be switched by external
stimuli (Fig. 2).The high degree of polar order results in large
macroscopic polarizations (Ps >5 ꢀ 10ꢁ3 C mꢁ2) and can lead to
large bulk NLO parameters.
In a previous paper we carried out an analysis of the NLO
activity, namely second harmonic generation (SHG) perfor-
mance, of a representative series of bent-core liquid crystals.9 We
concluded that the bent molecular shape is appropriate for
a correct design of NLO materials. Furthermore, the degree of
polar stereocontrol achievable with this kind of materials is also
very good, much better than those of poled polymers or
calamitic FLC.4,5 But we also acknowledged that in order to
improve the NLO efficiency, p-bridged systems linked to
enhanced electron donating and withdrawing groups must be
incorporated.
As a further step in this direction we present herein new
results. Two new bent-core molecules A and B (Chart 1) suitable
for NLO-responses have been designed, synthesized and
characterized.
Chart 1 Chemical structure of compounds A, B, C, D and E.
crystalline order the azobenzene moiety was selected as the p-
system. Strong electron-withdrawing groups such as a-cyano-
cinnamate (–CH]CH(CN)COO–) (A) or nitro (–NO2) (B)
structures, and a piperazine moiety as the electron-donating
group, have been chemically connected all along the lateral
structures of the molecule. The longitudinal b along the lateral
core of the molecule (bl) does not average out to zero in the
appropriate mesophase (SmCP) since there is always a b compo-
nent along the macroscopic polar axis.
In order to induce the bent-core liquid crystalline character to
the materials, 3,40-biphenylene (A) and 1,3-phenylene (B) deriv-
atives were chosen. To harmonize both NLO-activity and liquid
Synthesis and characterization of the liquid crystal
properties
Schemes 1 and 2 show the routes we established to synthesize
compounds A and B. In the case of the intermediate compounds
510 and 8,11 the synthetic procedures were adapted from the
methods described in the literature. The synthesis of compounds
4 and 7 was carried out starting from the commercially available
phenylpiperazine. In order to prepare compound 4 and accord-
ing to different retrosynthetic routes, several synthetic
approaches were checked. The best results were afforded for the
synthetic route shown in Scheme 1. This was not the case for
compound 7 and the route shown in Scheme 2 was the only one
used. Work-up with azo-compounds were carried out under light
filters and red light to prevent their photoisomerization in solu-
tion. Further details of the synthesis are given in the supple-
mentary information.
Liquid crystalline properties of compounds A and B were
studied by polarized optical microscopy (POM), calorimetry
(DSC) and X-ray diffraction (XRD) at various temperatures and
the results are shown in Table 1. The XRD for A shows
a temperature range where a diffuse halo is observed at wide
angles (Fig. 3). This is characteristic of a liquid crystalline state.
Fig. 2 Schematic representations used for bent molecules (up). Models
proposed for the arrangements of the molecules in different columnar
mesophases (middle) and models proposed for the antiferroelectric
SmCAPA mesophase and the ferroelectric SmCSPF mesophase which
appears on applying electric fields (down).
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Table 1 Thermal properties of compounds A–E
a b
,
Compound
Phase transition T/ꢃC and enthalpy/kJ molꢁ1
A
C 147 (35.8) Col 164 (13.7) Is
Is 159 (10.7) Col 104 (39.4) C
C 158 (47.7) Is
C 74 (21.4) SmCAPA 157 (27.9) Is
Is 157 (27.1) SmCAPA 68 (17.5) C
C 130 (32.7) SmA 173 decomp.c
C 128 (22.7) SmA 179 (3.6) Is polymeriz.c
B
C
D
E
a
C: crystalline phase, Col: columnar mesophase, Is: isotropic liquid,
SmCAPA: anticlinic and antiferroelectric smectic C polar mesophase,
SmA: smectic A mesophase. Onset values for transitions observed for
b
c
the second scans at 10 ꢃC minꢁ1
.
Onset values for transitions
.
observed for the first scans at 10 ꢃC minꢁ1
behavior was observed for compound B. An unidentified phase
was detected on cooling, which exhibits birefringent textures
(Fig. 4b). XRD diagrams showed features of a crystalline
structure (sharp peaks at wide angles, see Fig. 5) with a high
degree of lamellar order. In spite of the bent-core shape of B and
the long tri-ester lateral structure incorporated, the presence of
a 1,3-phenylene central core seems to prevent the stabilization of
any liquid crystal order for this compound.
Scheme 1 Synthetic route to compound A.
Nonlinear optical properties
The ability of these compounds to generate nonlinear optical
responses was explored at both the molecular and macroscopic
level. The NLO activity of these bent-core compounds at
molecular level was studied by using the Electric Field Induced
Second Harmonic (EFISH) generation technique. Thus mb0
values of 390 (ꢂ20) ꢀ 10ꢁ48 esu and 440 (ꢂ50) ꢀ 10ꢁ48 esu were
extrapolated for A and B respectively, from data measured at
a wavelength of 1907 nm (see Table 2). These values are in the
range of those reported for classic NLO-phores frequently
incorporated in NLO-materials2,3,12 and, interestingly, they are
one order of magnitude larger than the values measured for
traditional bent-core liquid crystal molecules9 such as compound
C7g (Chart 1 and Table 2). Furthermore, if we compare these mb0
values with those measured for compounds D13a and E,13b with
a donor–p-acceptor system structurally close to the ones incor-
porated in the lateral structures of A and B respectively, we
observe that on going from 1D to 2D-systems very similar mb0
values can be obtained.
Neither A nor B exhibit the appropriate mesophase leading to
supramolecular non-centrosymmetric arrangements. To
promote the suitable mesophase in order to evaluate their NLO
activity in bulk, blends of A or B with transparent SmCP
compounds showing small SHG efficiency were prepared. This
approach was unsuccessful for compound B, as every mixture
prepared was non-homogenous. However, a suitable 50% molar
mixture of compounds A and C could be achieved and charac-
terized. This blend shows a SmCAPA mesophase (Fig. 2) within
a significant range of temperature (C 115 ꢃC SmCAPA 156 ꢃC Is)
as was confirmed by POM (see Fig. 4c and 4d) and DSC.
Furthermore XRD studies at variable temperature showed that
along with the diffuse halo at wide angles two peaks at small
angles, corresponding to (001) and (002) reflections, could be
observed (see Table S1) that leads us to propose that there
Scheme 2 Synthetic route to compound B.
Apart from this, since a unique peak appears at small angles, no
precise information could be further extracted. The optical
textures, on the other hand, seem to indicate a columnar type of
arrangement (see Fig. 2 and Fig. 4a). The material is not
switchable by an electric field. Alternatively, a different thermal
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Fig. 3 DSC traces corresponding to the first scans (10 ꢃC minꢁ1) of compound A and X-ray diagram at 138 ꢃC. The broad maximum at wide angles is
indicative of a liquid crystal phase.
Fig. 4 Microphotographs of the textures observed for: (a) compound A at 138 ꢃC on cooling (columnar mesophase) (3 mm wide) (b) compound B at
127 ꢃC (3 mm wide) and (c) 50% molar mixture of A and C at 153 ꢃC on cooling (SmCAPA mesophase) (250 mm wide). The darkness in the (c) texture is
(apart from the absorption) because of the small birefringence of the mesophase, which presumably is close to an orthoconic state. The same sample
region is also shown in Fig. 4d upon field application (20 V mmꢁ1). The texture becomes more brilliant because the field gives rise to a SmCsPF structure
with higher birefringence.
exists a liquid crystalline lamellar packing with a layer spacing
ꢁ
of 42.3 A.
were obtained between the electrodes (see Fig. 6). A good
alignment is a fundamental requirement for quality SHG
measurements and the strategy used is the only effective method
for the alignment of bent-core liquid crystals.
In order to compare the SHG performance of compound A
with previous results in other bent-core compounds7,9 we
measured the second order susceptibility coefficients at the same
fundamental wavelength (1064 nm) as in previous works.
Therefore we used a Nd:YAG laser, so the SHG signal is
generated at a wavelength of 532 nm, deep into the absorption
band of these compounds (see Table 2 and supporting infor-
mation). This fact greatly complicates the measurement strategy
because the absorption must be taken into account in the data
process14 (see Section 5 in the supporting information).
The SHG efficiency is expected to be mainly driven by the
chromophore, which is set along the lateral structure of the
molecule. Following Araoka’s approach,15 this implies that only
the contribution of the longitudinal hyperpolarizability bl along
the active lateral structure must be taken into account. There-
fore, under this scheme only two hyperpolarizability components
bzzz and bzxx must be considered. These coefficients are referred
to a coordinate frame in which, considering the bent-molecule as
a bow, z is along the arrow of the bow, x along the string and y is
perpendicular to both directions. The connection between these
b components and the macroscopic second order susceptibility
tensor can be made using only two characteristic parameters D
and d given by expressions15:
SHG measurements were carried out in home-made cells with
in-plane electrodes. Five cells of different thicknesses were
measured to deduce the optical parameters of the material (see
Figure S3 in the supporting information). Applying an electric
field parallel to the substrate well-aligned homeotropic domains
Fig. 5 DSC traces corresponding to the second scans (10 ꢃC minꢁ1) of compound B and X-ray diagram below 122 ꢃC. Sharp peaks at wide angles
indicate a crystalline structure. The narrow maximum at small angles is representative of a rather high degree of lamellar order.
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Table 2 Molecular nonlinearities and wavelength of maximum
absorption (lmax) of compounds A–E
Compound
mb ꢀ 10ꢁ48/esu a
mb0 ꢀ 10ꢁ48/esu
lmax/nm b
A
B
C
D
E
560 (ꢂ30)
630 (ꢂ70)
40 (ꢂ15)
390 (ꢂ20)
440 (ꢂ50)
35 (ꢂ14)
485
480
265
472
460
470 (ꢂ20)
460 (ꢂ40)
330 (ꢂ15)
330 (ꢂ30)
Fig. 7 Nonlinear response E2u of the bent-core molecules to the
fundamental excitation Eu. The whole d tensor can be fully characterized
by two parameters (d and D) whose physical meaning is depicted in (a)
and (b) respectively. The highest expected value is D. P is the macroscopic
polarization.
a
EFISH measurements carried out in CH2Cl2 at 1907 nm. mb0values
extrapolated at zero frequency using a simple two level dispersion
b
model. Absorption band maximum. UV-vis spectra measured in
CH2Cl2 solution.
deduce the D parameter in eqn (2) (see eqn (S2) of the supporting
information). The calculated value is D ¼ 140 pm Vꢁ1 and
therefore, the whole second order susceptibility tensor of the
SmCP phase is determined.
To put in context these values, Table 3 gives an idea of the
progress achieved in the field of NLO by comparing some SHG
parameters of several significant compounds: previous bent-core
compounds, SmC* calamitic materials and commercially avail-
able solid-state NLO materials (to transform the D and
d parameters of the present bent-core compounds into dij coef-
ficients see eqn (2)). Although, this result is by far the highest
SHG efficiency ever reported in a liquid crystal, this result must
be interpreted carefully since the obtained NLO coefficients are
resonance enhanced and therefore for practical purposes the
important effect of the absorption cannot be ignored.
Fig. 6 Pictures of the cell gap (70 mm width) under electric field between
crossed polarizers (white arrows): parallel to the polarizer (a) and rotated
45ꢃ (b), at T below 154 ꢃC. Although the phase retardation is low because
of the small thickness of the cell (1.3 mm), the extinction is clearly
observed, proving the quality of the molecular alignment. The reddish
color is due to the sample absorption. Both pictures are 200 mm wide.
ꢀ ꢁ
ꢀ ꢁ
ꢀ ꢁ
In order to account for the transparency-efficiency trade-off it
is interesting to perform an experimental study of the dispersion
behavior of the second order susceptibility tensor. With this
purpose we carried out a new SHG measurement at a wavelength
deep into the transparent region of the material for the frequency
doubled signal (1600 nm for the fundamental light, see
Figure S1). In this case we used a Ti:sapphire oscillator-regen-
erative amplifier laser system that can provide a tunable output
signal in the 300–2600 nm range.
a
a
a
bzxx ¼ cos
sin2
bl; bzzz ¼ cos3
bl
(1)
2
2
2
D ¼ Nf 3hcos4ibzxx; d ¼ Nf 3hcos34ibzzz
where N is the density of molecules in the mesophase, f a local
field factor, a is the molecular bending angle, and 4 the angle
between z and the spontaneous polarization direction. The
physical meaning of these NLO parameters is explained in Fig. 7.
Under high enough electric field the material exhibits
a
SmCSPF phase. Therefore, according to the previous
The studied mixture, sample cells, and experimental geometry
were the same as in the previous SHG measurement. Three cells
of different thicknesses were used together with two different
polarization configurations, i.e., fundamental light polarized
parallel (p) or perpendicular (s) to the electric field direction,
while the second harmonic light was always parallel (p) to the
electric field (see supporting information). Figures S4a and S4b
show the experimental results for pp and sp configurations
respectively. The resulting coefficients were d ¼ 4 pm Vꢁ1 and
Dsin2q ¼ 2.5 pm Vꢁ1. By extrapolating to the pure A compound
d ꢄ8 pm Vꢁ1 and D ꢄ17 pm Vꢁ1 values were estimated. These
new results, although much lower than those obtained in the
absorptive case, are however, very remarkable and represent an
important improvement in the SHG performance of liquid
crystals.
approaches, the whole second order susceptibility tensor in
a contracted representation can be expressed, in a reference
frame XYZ in which X is perpendicular to the smectic layers, Z is
parallel to the polar axis and q is the tilt angle, as follows:7c
2
3
D
0
0
0
0
0
0
d
sin2q Dcos2q
0
6
6
6
6
4
7
7
7
7
5
2
D
Dsin2q
sin2q
0
d ¼
(2)
2
D
Dcos2q Dsin2q
0
0
sin2q
2
Measurements were carried out at normal incidence with the
fundamental and second harmonic lights polarized along the
electric field direction. In this configuration (Fig. 7a), according
to eqn (2), the component of the susceptibility tensor that
contributes to the SHG signal is d. Extrapolating the obtained
value in the mixture to the pure compound, d ¼ 48 pm Vꢁ1 is
obtained.
In this respect, it must be mentioned that due to the inherent
experimental difficulties of SHG measurements in bent-core
compounds, it is important to check the reliability of the
obtained results. This is an important point in order to state
properly the NLO efficiency of liquid crystals. In fact, large
disagreements, even in the order of magnitude, have been
On the other hand, following the previously mentioned model
presented by Araoka et al.,15 assuming a perfect molecular order
in the bulk and a molecular bending angle of 120ꢃ, it is possible to
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Table 3 SHG coefficients (fundamental wavelength 1064 nm) for several compounds of relevant NLO material families
Compound
SHG coefficients (pm Vꢁ1
)
Main drawbacks
Comments
This work.
Comp. A
D ¼ 140, d ¼ 48
Resonance-enhanced
coefficients
Difficult alignment
Previous bent-core LC
SmC* rod-like LC
D ¼ 6, d ¼ 3.8
Data for the classical bent-core LC 8-O-PIMB in its
SmCPF phase.7g
Data for W314, a SmC* LC with one of the highest
polar order reported.
along P) in bent-core LC.17
d21 ¼ 0.06, d22 ¼ 0.3, d23 ¼ 0.12,
d25 ¼ ꢁ0.02
Small coefficients
16
d
22
corresponds to d (value
KDP
LiNbO3
Solid organic NLO materials
d14 ¼ 0.5, d36 ¼ 0.46
d22 ¼ 3, d31 ¼ ꢁ6, d33 ¼ 30
d11 ¼ 290, d12 ¼ 15, d15 ¼ 10,
d31 ¼ 8
KH2PO4, commercial inorganic NLO material
Commercial inorganic NLO material
Data for DAPSH at 1907 nm. Presently the best NLO
material18
Typical data for poled NLO polymers or
dendrimers19
Poled polymers
d33 z 60
Thermodynamically
unstable phase
reported for the second order susceptibility tensor in some bent
core compounds.7 This point has been deeply studied in ref. 7g
and 9, and the conclusion drawn is that up to now the highest
NLO efficiency in bent-core compounds is D ꢄ8 pm Vꢁ1 far from
absorption bands.
shown that a simple method to estimate the dij coefficients from
the b values works reasonably well, offering a way to design and
select the most suitable bent-core molecules prior to their
synthesis.
A final remark can be made regarding the applicability of these
materials in technological devices. One of their main advantages
is that they can be easily integrated with semiconductors to
produce optical quality devices and, therefore, offer good pros-
pects for use as ultrafast electrooptic modulators. Of the various
electrooptic phenomena, Pockels effect is one of the most widely
used mechanisms in high-speed optical modulation and sensing
devices,1a,2j for this purpose high Pockels coefficients are required
and it is well known that this condition always implies a good
SHG performance.
The reliability of our results can be checked by comparing
them with the efficiency of the material at a microscopic level.
This study, as previously mentioned, has been carried out by
means of EFISH measurements. A theoretical estimation of the
second order susceptibility parameters can be made in terms of
molecular and structural parameters. Starting from the value
obtained by EFISH at l ¼ 1907 nm in the active lateral structure
of the molecule (mb ¼ 470 ꢀ 10ꢁ48 esu in compound D) and using
the AM1 molecular model to calculate the dipole moment of this
structure (m ¼ 4.5 Debyes), we can estimate the main hyper-
polarizability component of the bent-core compound A to be
bl ¼ 100 ꢀ 10ꢁ30 esu. Then, from expressions (1), assuming
a distribution of director azimuths similar to that of the classical
P-8-OPIMB compound9 (hcos4i ¼ 0.61, hcos34i ¼ 0.43), a mass
density of 1 g cmꢁ3, and the local field factor f3 ¼ 3 (in the Lorentz
approximation f ¼ (n2 + 2)/3, being n the refractive index), we
obtain D ¼ 14.9 pm Vꢁ1 and d ¼ 3.4 pm Vꢁ1 at l ¼ 1907 nm.
Using a two level model with lo ¼ 472 nm (obtained from
Fig. S1) the new NLO parameters at l ¼ 1600 nm result to be
D ¼ 17.5 pm Vꢁ1 and d ¼ 4 pm Vꢁ1. As can be seen the agreement
between bulk measurements and calculations starting from
EFISH experiments is rather satisfactory which supports the
goodness of our experimental method. Unfortunately it is not
possible to carry out the same calculation for the SHG results in
the highly absorbent region since close to absorption bands the
two level model is not valid to account for parameter dispersion.
The obtained results are thus very promising in the field of
new materials for electronic electrooptic modulators, and
imply an important improvement in the design of NLO liquid
crystals which can be used either as soft materials or alternatively
as a tool towards the achievement of new polar functional
materials.
Acknowledgements
Research supported by CICYT-FEDER of Spain-UE
ꢀ
MAT2006-13571-CO2 and MAT2008-06522-CO2, the Aragon
Government (E04), and the Basque Country Government
(Project No. IT-484-07). I.A. is grateful to UPV for a grant, and
ꢀ
I. C. P, J. M-P., and R. A. thank the Aragon, the Basque and the
Spain Governments respectively for grants.
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In summary the results from this work confirm that through an
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This journal is ª The Royal Society of Chemistry 2010
J. Mater. Chem., 2010, 20, 2965–2971 | 2971