New poly(phenylenevinylene)-methyl methacrylate-based photonic crystals

Article abstract of DOI:10.1002/pola.24049

Monodisperse colloids have been prepared efficiently by (copolymerization of methyl methacrylate and fluorescent first- and second-generation poly(phenylenevinylene) dendrons under surfactant-free emulsion polymerization conditions. The copolymers were ch

Full text of DOI:10.1002/pola.24049

New Poly(phenylenevinylene)-Methyl Methacrylate-Based Photonic
Crystals
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SYLVAIN ACHELLE,1 ALVARO BLANCO, MARTIN LOPEZ-GARCIA, RICCARDO SAPIENZA,2 MARTA IBISATE,2
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CEFE LOPEZ, JULIAN RODRIGUEZ-LOPEZ
1Facultad de Qu´ımica, Universidad de Castilla-La Mancha, Avda. Camilo Jose´ Cela, 10, Ciudad Real 13071, Spain
2Instituto de Ciencia de Materiales de Madrid (CSIC), C/ Sor Juana Ine´s de la Cruz, 3, Madrid 28049, Spain
Received 4 February 2010; accepted 29 March 2010
DOI: 10.1002/pola.24049
Published online in Wiley InterScience (www.interscience.wiley.com).
ABSTRACT: Monodisperse colloids have been prepared effi-
ciently by copolymerization of methyl methacrylate and fluo-
rescent first- and second-generation poly(phenylenevinylene)
dendrons under surfactant-free emulsion polymerization condi-
tions. The copolymers were characterized by UV–vis and fluo-
rescence spectroscopy and size exclusion chromatography.
Transmission electron microscopy revealed that the copoly-
mers were microspheres with smooth surfaces and narrow dis-
persity. The bead diameter could be varied by changing the
monomer/water ratio. The materials could be crystallized to
give polymer opal photonic crystals. The emission was not
affected by the periodic structure because of the large spectral
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distance between the emission and the pseudogap position.
2010 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem
48: 2659–2665, 2010
KEYWORDS: colloids; emulsion polymerization; fluorescence;
photonic crystals; PPV dendrimers
INTRODUCTION Over the past decade, there has been con-
siderable interest in the synthesis and study of functional
and patterned artificial opals.1 Such materials, which are
also called photonic crystals, consist of self-assembled colloi-
dal crystals with diameters ranging from 200 to 900 nm. As
a result of their periodic nanostructure, the assemblies are
able to reflect light that matches their periodicity [ultraviolet
(UV) to infrared (IR) radiation depending on the size of the
colloids]. The range of reflected wavelengths is known as a
photonic band gap, an energy interval in which photonic
states are not present. Artificial opals are commonly formed
by silica2 or polymer3 beads, with the latter systems having
superior optical quality.
threshold lasers.7 The incorporation of such units can be car-
ried out by infiltrating the opal with fluorescent material
from solution8 or by coating of the preformed spheres.9
However, self-aggregation of dye molecules at high concen-
trations often results in almost complete suppression of fluo-
rescence. Another possibility consists of incorporating the
fluorophore directly into the matrix, but generally the dye is
not covalently bonded to the photonic crystal elements.10
Monodisperse dendritic materials have emerged as attractive
candidates for photonic applications. Dendritic poly(phenylene-
vinylene)s (PPVdend), also called stilbenoid dendrimers, repre-
sent an important group within this class of molecules. Numer-
ous different studies have been published to date concerning
the synthesis and properties of these materials.11 For example,
such compounds have been used successfully as light-emitting
materials.12 Therefore, PPVdend are good candidates to be
incorporated into artificial opals as fluorescent dyes.
Polymer-based opals can be prepared from various types of
polymer colloids (mainly polystyrene or polymethacrylates).4
To obtain a high-quality polymer opal, it is important to pre-
pare a stable polymer colloid with a spherical geometry and
a high monodispersity (polydispersity less than 5%). This
process can be carried out by the surfactant-free emulsion
polymerization (SFEP) technique.5
Only a few examples of artificial opals that incorporate den-
drimers can be found in the literature, but they are not used
as fluorescent dyes.13 We describe here the preparation of
monodisperse colloids from first- and second-generation
PPVdend-methyl methacrylate (MMA) copolymers, which can
be used to form patterned opals. The optical properties and,
in particular, the influence of the photonic band gap on the
spontaneous-emission lifetimes of the resulting materials
were studied.
One of the most important aspects in the study of functional
artificial opals concerns the incorporation of fluorescent
materials to investigate the influence of the photonic band
structure on photoluminescence properties.6 In particular,
opals with fluorescent dyes can show an enhanced stimu-
lated emission that can lead to applications such as in low-
Additional Supporting Information may be found in the online version of this article. Correspondence to: J. Rodrı´guez-Lo´ pez (E-mail: julian.
rodriguez@uclm.es)
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Journal of Polymer Science: Part A: Polymer Chemistry, Vol. 48, 2659–2665 (2010) 2010 Wiley Periodicals, Inc.
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JOURNAL OF POLYMER SCIENCE: PART A: POLYMER CHEMISTRY DOI 10.1002/POLA
SCHEME 1 Synthesis of first-generation PPV dendrons by HWE and Suzuki reactions.
EXPERIMENTAL
RESULTS AND DISCUSSION
Chemicals
Synthesis of Dendritic Monomers
MMA stabilized with tert-butylcatechol was purified by
washing with aqueous 2M NaOH and water, dried over mag-
nesium sulfate, and used immediately. All other commercially
available products were obtained from Aldrich or Acros and
were used without further purification. Deionized water was
obtained from a Milli-Q system.
The starting first-generation PPV dendrons required for the
copolymerization with MMA were easily synthesized by two
different approaches used previously by some of us in the
preparation of PPV dendritic architectures.11
The first reaction sequence involved the Horner-Wadsworth-
Emmons reaction of two molecules of the appropriate para-
substituted benzaldehyde derivative with the readily avail-
able diphosphonate 115 to form the first-generation aryl
iodides 2a–c (Scheme 1). This step was followed by a Suzuki
cross coupling reaction with potassium vinyltrifluoroborate16
to give the desired products 3a–c in yields ranging from 69
to 81%.
Synthesis of Dendritic Monomers
Experimental details for the synthesis of dendritic monomers
are given in the Supporting Information.
Synthesis of Copolymer Microspheres
The colloid particles were synthesized in a 250-mL flask fit-
ted with an argon inlet, a condenser, and a mechanical stir-
rer by SFEP in batches from 100 to 150 mL reaction volume.
The flask was charged with water and potassium peroxodi-
sulfate (0.01M) and was immersed in an oil bath preheated
at 90 ꢀC and flushed with argon for 30 min. The internal
temperature reached 75 ꢀC. A solution of the dendritic
monomer (0.5 mol %) in MMA was added rapidly, and the
polymerization started. During the polymerization, the mix-
ture was stirred at 1200 rpm. After 5 h, the flask was
opened (oxygen), and the colloids were purified from large
agglomerations by filtration through glass wool, followed by
repeated centrifugation, wash, and redispersion cycles. The
resulting suspensions were almost free of salt and low mo-
lecular weight impurities. The suspensions were stored at a
concentration of 5–20 wt %.
The second method started with the Heck cross coupling
reaction17 of two molecules of the easily accessible para-sub-
stituted styrene derivatives 4 with 3,5-dibromobenzaldehyde.
In this way, the corresponding first-generation PPV dendritic
aldehydes 5a–c were obtained in moderate yield. A Wittig
reaction with methyltriphenylphosphonium iodide afforded
the first-generation dendritic compounds 3a–c in good yield
(Scheme 2). Starting from 3a, and following the aforemen-
tioned methodology, the second-generation PPV dendron 7
was also obtained (Scheme 3).
All the dendrons bear a polymerizable styryl group at the
focal point and have an all-trans stereochemistry for the
double bonds, a structure that was unequivocally estab-
lished on the basis of the coupling constant of the vinylic
protons in the 1H NMR spectra (J % 16 Hz). Further-
more, the exact molecular weights were measured by ma-
trix-assisted laser desorption ionization (MALDI) time-of-
flight mass spectrometry, with all the experimental results
in good agreement with the expected molecular weights
for the structures. The optical properties are listed in
Table 1.
Colloidal Crystal Fabrication
Copolymer opal photonic crystals were fabricated by the ver-
tical deposition method.14 Glass substrates previously
cleaned with piranha were fixed vertically into the copoly-
mer suspension with a concentration of 0.1 wt % anꢀd were
maintained for 3–4 days at constant temperature (45 C).
SCHEME 2 Synthesis of first-generation PPV dendrons by Heck and Wittig reactions.
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SCHEME 3 Preparation of the sec-
ond-generation PPV dendron 7.
Because of the meta arrangement of the dendrons, the
absorption spectra consisted of a superposition of the
absorptions due to the different stilbene chromophores. The
introduction of different groups has a negligible influence on
the extinction coefficients, although a red shift of the absorp-
tion band was observed on increasing the electron-donating
strength of the peripheral group. Comparison of the first-
generation monomer 3a and second-generation monomer 7
shows, as one would expect, that the molar extinction coeffi-
cient increases with increasing generation, whereas the fluo-
rescence quantum yield remains constant. The dimethyl-
amino derivative 3b presents a lower quantum yield in
comparison with the hexyloxy derivative 3a. The diphenyl-
amino derivative 3c exhibits the highest extinction coefficient
and quantum yield.
Synthesis and Characterization of Copolymers
Although the homopolymerization of various type of dendrons
with either styryl or acrylate groups has been reported
widely,19 only a few examples involving copolymerization have
been described.20 The copolymerization of PPV dendrons with
a styryl focal point (3a–c, 7) and MMA (0.5 mol % of dendron)
ꢀ
was carried out in water at 75 C under conventional radical,
surfactant-free emulsion conditions (Scheme 4).21 This method
led to monodisperse colloidal suspensions with submicron
scale beads (see later) of copolymers CP1a–c and CP2.
TABLE 1 UV-Vis and Photoluminescence Data for First- and
Second-Generation PPV Dendrons and Copolymers in
Dichloromethane
It proved difficult to determine the chemical structures of
the copolymers obtained because of the low concentrations
of dendritic monomer used. NMR is not a sufficiently sensi-
tive technique, and signals corresponding to the dendron
were not easily observed. IR spectra did not provide any in-
formation about the structure, and the only characteristic
peak observed was that corresponding to the ester function
of the MMA moiety (1724 cmꢁ1). However, the UV–vis and
fluorescence spectra of CH2Cl2 solutions of the copolymers
showed characteristic bands for PPV dendrons, indicating
that the fluorescent dendritic monomer had been incorpo-
rated into the chain (Table 1). The photophysical properties
of copolymers CP1a–c and CP2 are similar to those of their
corresponding monomers, although the fluorescence quan-
tum yields are usually a little lower. The colloid obtained by
polymerization under the same conditions of MMA in the
presence of a PPV dendron without a polymerizable styryl
UV–Vis, kmax (nm)
(e, M–1 cm–1
PL, kmax,
b
Compounda
)
(nm)
UF
3a
3b
3c
327 (36,900)
366 (52,400)
300 (41,100)
378 (67,900)
328 (134,000)
326
408
459
460
0.33
0.24
0.70
7
421
409
455
458
422
0.32
0.27
0.15
0.74
0.21
CP1a
CP1b
CP1c
CP2
a
370
300, 376
330
Spectra for PPV dendron 3a–c, 7 were recorded at c ffi 1Á10–5 to 5Á10–6 M.
CP1a–c and CP2 were recorded at c ¼ 10 g/L.
b
Fluorescence quantum yield (610%) obtained using quinine sulfate in
0.1 M H2SO4 (Uf ¼ 0.58) as standard.18
PPV-METHYL METHACRYLATE PHOTONIC CRYSTALS, ACHELLE ET AL.
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JOURNAL OF POLYMER SCIENCE: PART A: POLYMER CHEMISTRY DOI 10.1002/POLA
The sphere diameter, which can be varied by changing the
monomer/water ratio, was determined by transmission elec-
tron microscopy (TEM) (Table 2). All colloids described have
a smooth surface and a narrow size distribution (less than
5%) and should, therefore, have the potential to crystallize
in an opal structure. Some examples of TEM images are
given in Figure 1.
Synthesis and Characterization of Opals
Thin opal films were grown on glass substrates by means
of the vertical deposition method from aqueous suspen-
sions of copolymer spheres having different diameters (Ta-
ble 2). The final thickness can be easily controlled by vary-
ing experimental parameters such as concentration,
temperature, or humidity.22 Details on the fabrication and
previous optical characterization by means of reflection and
transmission spectroscopy can be found elsewhere.14 Artifi-
cial opals self-assemble in an fcc lattice with (111) planes
parallel to the substrate, an arrangement that is very con-
venient for further optical characterization. A scanning elec-
tron microscopy (SEM) image of a film obtained from sam-
ple S3 (sphere size: 180 nm) is shown in Figure 2. This
picture summarizes the average quality of the samples pre-
pared. Polycrystalline structures with a domain size of
approximately 5–10l are caused by polydispersity. Image
processing yields a lattice parameter on the surface of 250
nm—a value slightly higher than expected, which indicates
a close-packed structure that is not perfect. This situation
was also observed in the optical characterization, as dis-
cussed later.
SCHEME 4 Copolymerization of PPV dendrons and MMA.
group (i.e., an iodo group at the focal point) did not show
any emission band. This finding provides evidence that the
dendrons are effectively integrated into the macromolecules
and are covalently bonded. The level of incorporation of the
dendrons was estimated by UV–vis on the assumption of a
similar molar absorptivity for the free dendron and for the
linked dendron. The level of functionalization was approxi-
mately 0.05% for the first-generation compounds 3a–c,
although in the case of the second-generation dendron 7 this
percentage was lower—probably due to the low solubility of
this compound in MMA. A decrease in the incorporation ratio
observed on increasing the generation number in copolymer-
izations has been observed previously.20
Optical characterization was performed using a Fourier
transform infrared spectrophotometer attached to an optical
microscope. Normal incidence reflectance measurements
explore directions perpendicular to (111) planes and are
sensitive to sphere stacking (monolayers) and, therefore,
gaps occurring in that particular direction (CL direction
within the first Brillouin zone). The specular reflectance of
the opal film obtained from S3 in the visible range is shown
in Figure 3. The observed band corresponds to a photonic
pseudogap (dependent on the propagation direction within
the structure) and its spectral position, kB (at normal inci-
dence), can be approximately estimated using the following
relation:
Size exclusion chromatography and MALDI analyses also
indicated the absence of unreacted dendrons in the purified
colloidal beads. The synthesized copolymers showed molecu-
lar weights ranging between 39,499 and 74,368 g molꢁ1 and
polydispersities ranging between 1.10 and 1.29 (Table 2).
TABLE 2 Data for Synthesized Colloidal Beads
Sphere
Diameterb (nm)
Sample
Copolymer
Batch (mL)
Monomer (mL)
Mw (g mol–1
)
Polydispersitya
a
S1
S2
S3
S4
S5
S6
a
CP1a
CP1a
CP1b
CP1b
CP1c
CP2
150
100
100
100
100
100
4
74,368
39,499
47,096
50,490
42,614
43,992
1.29
1.17
1.10
1.14
1.24
1.24
118
200
180
150
234
208
8
8
6
7.5
8
b
Molecular weight (Mw) and polydispersity were determined by SEC
using polystyrene as standard.
Determined by TEM.
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ARTICLE
FIGURE 1 TEM images of colloidal samples S2 (left) and S4
(right).
rffiffiffi
2
kB ¼ 2 Á
Á D Á neff
(1)
3
FIGURE 2 SEM image of a freshly prepared film of S3 (sphere
diameter: 180 nm). Scale bar is 5l.
where D is the sphere diameter, and neff is the average re-
fractive index of the structure. The latter value was calcu-
lated taking into account the components of the structure
and their filling fraction as follows:
increase in the emission lifetime in nanostructured den-
drimers (from 300 to 500–600 ps) is probably due to a
barely change in the chemical environment rather than any
photonic effect, which in any case should take place within
the photonic band gap region, approximately 500–600 nm,
where the decay time curve is almost flat (approximately
575 ps).
1=2
neff ¼ ðn2p Á 0:74 þ 0:26Þ
(2)
np being the polymer refractive index (taken as 1.59) and
taking into account the fcc filling fraction of 0.74. From this
formula, the estimated sphere diameter corresponding to a
peak spectral value of 578 nm is 242 nm, which is consistent
with the value obtained by SEM analysis. Rapid oscillations
observed in the low energy range of the spectrum are Fabry-
Perot modes due to sample thickness. These oscillations can
also be used to calculate the total thickness of the structure,
which in this particular case is 50 monolayers (12l).23
It is well known that photonic crystals can produce changes
in the local density of states that might inhibit or enhance
emission for any emitter inside the lattice such as quantum
dots25 or organic emitters. In this case, it would be expected
that some change in emission would occur for the opals fab-
ricated with the samples in Table 2 because of the periodic-
ity of the media. However, it can be observed that the emis-
sion maxima of the opals (approximately 450 nm) and the
pseudogap of the photonic crystals (around 590 nm) are
Emission measurements were carried out on the bulk mate-
rial, and opals fabricated with the different spheres are
shown in Table 2. The equipment used for the measure-
ments was a Hamamatsu streak camera coupled to an
80 MHz repetition rate UV laser (k ¼ 355 nm) pulsed at 12 ps.
The spectral range available with this equipment covers the
visible and ultraviolet spectrum (355–800 nm). At the same
time, decay time measurements were performed. The tempo-
ral range between 70 ps and 2 ns is available. The first
measurements were made without any photonic structure
over the emitter, and the results show that lifetimes of the
molecule are approximately 200–300 picoseconds depending
on the sample. It is important to note that decay times are
multiexponential, as one would expect for organic emitters.24
As a result, it is necessary to ignore the first 50 ps decays
because of fast electronic transitions to obtain the real decay
time. The decay time was obtained by adjusting the value in
the first approximation to a monoexponential decay. This fit-
ting seems to work well despite the fact that different pro-
cesses are involved in the emission of the dendrimer. The
measurements for a freshly prepared opal from sample S4
(copolymer CP1b) are shown in Figure 4. Notice that the
FIGURE 3 Optical reflectance of the opal film obtained from
sample S3. The observed peak corresponds to the first pho-
tonic pseudogap (Bragg Peak). Fabry-Perot oscillations from
0.65 to 0.9l can also be observed.
PPV-METHYL METHACRYLATE PHOTONIC CRYSTALS, ACHELLE ET AL.
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JOURNAL OF POLYMER SCIENCE: PART A: POLYMER CHEMISTRY DOI 10.1002/POLA
FIGURE 4 (a) Emission spectrum of a freshly prepared opal from sample S4 (copolymer CP1b). (b) Wavelength dependence of the
emission lifetime for the same sample.
13, 421–425; (b) Lange, B.; Metz, N.; Tahir, M. N.; Fleischhaker,
separated by more than 140 nm. Therefore, because emis-
F.; Theato, P.; Schroder, H.-C.; Muller, W. E. G.; Tremel, W.;
¨
¨
sion does not take place in the pseudogap spectral region,
the crystal does not seem to have any effect on the photonic
crystal emission intensity or lifetime.
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In this article, we have described the copolymerization of
highly fluorescent first- and second-generation PPV dendrons
with MMA by SFEP. The copolymers are obtained as mono-
disperse colloidal suspensions of microspheres with smooth
surfaces. The fluorescent dendrons are chemically bonded to
the polymer main chain, and their emission properties are
preserved. Artificial photonic crystals were synthesized using
these copolymers; however, the emission is not affected by
the opal structure because of the difference between the
pseudogaps of the opals and the emission wavelength. Inves-
tigations are currently underway aimed at obtaining pho-
tonic crystals with a periodicity that matches the emission
band of the dendrons.
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gaag, K.; Vanmaekelbergh, D.; Vos, W. L. Nature 2004, 430,
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This work was funded by the Ministerio de Ciencia e In-
novacio´n, Spain—projects CSD2006-0019 (Nanolight.es),
MAT2006-09062, and CTQ2006-08871 (co-funded by FEDER,
European Union) and the Junta de Comunidades de Castilla-La
Mancha—project PCI08-0033.
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