Fluorescent conjugated polymer nanoparticles by polymerization in miniemulsion

Article abstract of DOI:10.1021/ja905077c

Highly fluorescent conjugated polymer nanoparticles were prepared directly by polymerization in aqueous miniemulsion, employing Glaser coupling polymerization as a suitable step-growth reaction. A 4,4′-dinonyl-2, 2′-bipyridine-modified catalyst was found to be suited for the polymerization in the aqueous heterophase system. Nanoparticles of poly(arylene diethynylenes) (arylene = 2,5-dialkyoxy phenylenes and 9,9′-dihexyl fluorene) with molecular weights in the range of M_n 10⁴ to 10⁵ g mol^-1 and with sizes of ≤30 nm, as observed by TEM, result. N,N′-Di(4-ethynylphenyl)-1,7-di[4-(1,1,3,3-tetramethylbutyl) phenoxy]-perylene-3,4:9,10-tetracarboxdiimide or 2,7-diethynylfluorenone was converted completely during the heterophase polymerization to afford colloidally stable nanoparticles of poly(arylene diethynylenes) with 0.1-2 mol % covalently incorporated perylene dye and 2-9 mol % of covalently incorporated fluorenone dye, respectively. Fluorescence spectroscopy of the aqueous dispersions reveals an efficient energy transfer to the dye in the nanoparticles, which enables a variation of the luminescence emission color between red (λ_em (max.) ca. 650 nm) and the green emission of the nanoparticles without dye.

Full text of DOI:10.1021/ja905077c

Published on Web 09/18/2009
Fluorescent Conjugated Polymer Nanoparticles by
Polymerization in Miniemulsion
Moritz C. Baier, Johannes Huber, and Stefan Mecking*
Chemical Materials Science, Department of Chemistry, UniVersity of Konstanz,
UniVersita¨tsstrasse 10, D-78457 Konstanz, Germany
Received June 20, 2009; E-mail: Stefan.Mecking@uni-konstanz.de
Abstract: Highly fluorescent conjugated polymer nanoparticles were prepared directly by polymerization
in aqueous miniemulsion, employing Glaser coupling polymerization as a suitable step-growth reaction. A
4,4′-dinonyl-2,2′-bipyridine-modified catalyst was found to be suited for the polymerization in the aqueous
heterophase system. Nanoparticles of poly(arylene diethynylenes) (arylene ) 2,5-dialkyoxy phenylenes
and 9,9′-dihexyl fluorene) with molecular weights in the range of Mn 104 to 105 g mol-1 and with sizes of
e30 nm, as observed by TEM, result. N,N′-Di(4-ethynylphenyl)-1,7-di[4-(1,1,3,3-tetramethylbutyl)phenoxy]-
perylene-3,4:9,10-tetracarboxdiimide or 2,7-diethynylfluorenone was converted completely during the
heterophase polymerization to afford colloidally stable nanoparticles of poly(arylene diethynylenes) with
0.1-2 mol % covalently incorporated perylene dye and 2-9 mol % of covalently incorporated fluorenone
dye, respectively. Fluorescence spectroscopy of the aqueous dispersions reveals an efficient energy transfer
to the dye in the nanoparticles, which enables a variation of the luminescence emission color between red
(λem (max.) ca. 650 nm) and the green emission of the nanoparticles without dye.
realm of free-radical polymerization,4 while the preparation of
luminescent conjugated polymers of interest requires catalytic
Introduction
step-growth polymerizations.5 To date, this has been circum-
vented by employing secondary dispersion techniques (i.e.,
dispersing solutions of preformed polymers in organic solvents
in water).1-3 While this approach can be advantageous in terms
of applying commercially available polymers, on the longer term
it is desirable to also be able to prepare luminescent conjugated
polymer nanoparticles by polymerization in disperse systems
as this in principle provides access to a much broader scope of
materials, for example, colloidally stable particles with sizes
reasonably small for cell imaging3,6 or the preparation of
ultrathin films5,7 (<ca. 30 nm),8 or structured particles, and is
not limited to polymers with a high solubility in organic solvents.
Nanoparticles of luminescent conjugated polymers, dispersed
in water as a continuous phase, are finding increasing interest
in materials science and biological systems. For example, they
may contribute to resolve some key challenges in the processing
and structuring of conjugated polymers, particularly to multilayer
devices.1 In this context, their compatibility with standard
printing techniques is attractive. Because of their high degree
of dispersion, nanoparticle dispersions are useful for the
preparation of nanocomposites.2 Most recently, the potential of
nanoparticles of conjugated polymers for cell imaging has been
recognized.3
Aqueous dispersions of submicrometer polymer particles are
conveniently prepared by emulsion polymerization processes,
with mini- and microemulsion polymerizations as well-known
variations of classical emulsion polymerization. However,
polymerizations in disperse aqueous systems are traditionally a
(4) (a) Urban, D.; Takamura, K. Polymer Dispersions and Their Industrial
Applications; Wiley-VCH: Weinheim, Germany, 2002. (b) van Herk,
A. M. Chemistry and Technology of Emulsion Polymerisation;
Blackwell Publishing: Oxford, 2005. (c) Fitch, R. M. Polymer Colloids;
Academic Press: San Diego, CA, 1997. (d) Lovell, P. A.; El-Aasser,
M. S. Emulsion Polymerization and Emulsion Polymers; Wiley:
Chichester, 1997. (e) Landfester, K. Angew. Chem., Int. Ed. 2009,
48, 4488-4507.
(5) Mu¨llen, K.; Scherf, U. Organic Light-Emitting DeVices; Wiley-VCH:
Weinheim, Germany, 2005.
(6) (a) Dubertret, B.; Skourides, P.; Norris, D. J.; Noireaux, V.; Brivanlou,
A. H.; Libchaber, A. Science 2002, 298, 1759–1762. (b) Wallace, P. K.;
Mulrhead, K. A. Immunol. InVest. 2007, 36, 527–561.
(1) (a) Handbook of Luminescence, Display Materials, and DeVices;
Nalwa, H. S., Rohwer, L. S., Eds.; American Scientific Publishers:
Stevenson Ranch, CA, 2003. (b) Landfester, K.; Montenegro, R.;
Scherf, U.; Gu¨ntner, R.; Asawapirom, U.; Patil, S.; Neher, D.; Kietzke,
T. AdV. Mater. 2002, 14, 651–655.
(2) (a) Kietzke, T.; Neher, D.; Landfester, K.; Montenegro, R.; Güntner,
R.; Scherf, U. Nat. Mater. 2003, 2, 408–412. (b) Bourgeat-Lami, E.
In Encyclopedia of Nanoscience and Nanotechnology; Nalwa, H. S.,
Ed.; American Scientific Publishers: Stevenson Ranch, CA, 2004; Vol.
8, p 305.
(3) (a) Wu, C.; Szymanski, C.; Cain, Z.; McNeill, J. J. Am. Chem. Soc.
2007, 129, 12904–12905. (b) Wu, C.; Bull, B.; Szymanski, C.;
Christensen, K.; McNeill, J. ACS Nano 2008, 2, 2415–2423. (c) Moon,
J. H.; McDaniel, W.; MacLean, P.; Hancock, L. F. Angew. Chem.,
Int. Ed. 2007, 46, 8223–8225. (d) Howes, P.; Thorogate, R.; Green,
M.; Jickells, S.; Daniel, B. Chem. Commun. 2009, 2490–2492. (e)
Hu, L.; Mao, Z.; Gao, C. J. Mater. Chem. 2009, 19, 3108–3115.
(7) (a) Burroughes, J. H.; Bradley, D. D. C.; Brown, A. R.; Marks, R. N.;
Mackay, K.; Friend, R. H.; Burns, P. L.; Holmes, A. B. Nature 1990,
347, 539–541. (b) Geffroy, B.; le Roy, P.; Prat, C. Polym. Int. 2006,
55, 572–582. (c) Tong, Q.; Krumova, M.; Mecking, S. Angew. Chem.,
Int. Ed. 2008, 47, 4509–4511.
(8) Such small particles have been obtained by phase inversion by injecting
THF solutions of preformed polymer into water. However, this required
high dilution affording particle dispersion with mass concentrations
in the ppm range. The colloidal stabilization mechanism remains
unclear. Wu, C.; Szymanski, C.; McNeill, J. Langmuir 2006, 22, 2956–
2960.
9
10.1021/ja905077c CCC: $40.75  2009 American Chemical Society
J. AM. CHEM. SOC. 2009, 131, 14267–14273 14267

A R T I C L E S
Baier et al.
Scheme 1. Synthesis of Diethynyl Perylenediimide (4)
To date, catalytic polymerizations in emulsion have been
restricted to chain growth polymerizations.9-11 By compari-
son, in step-growth reactions the active metal center is
released from the polymer chain after each incorporation of
a monomer unit. This can enhance deactivation, if the
intermediates are water sensitive. This could be detrimental,
as in step-growth polymerizations high conversions are a
requirement for achieving reasonable molecular weights. The
latter also requires strict stoichiometric balance for reactions
based on A-B functional group coupling, which can be
difficult to achieve in disperse systems in which all compo-
nents have variable partition coefficients between the different
phases present. Therefore, a high-yielding reaction, which
is not sensitive to water, is required for the polymerization
to achieve high molecular weights.
by Sonogashira coupling of the dibromo compounds with
trimethylsilylacetylene (TMS-acetylene), followed by the
cleavage of the TMS group.
To control the emission color of the conjugated polymer
nanoparticles by covalent incorporation of a fluorescent dye,
a diethynyl-substituted perylenediimide derivative (Scheme
1) and 2,7-diethynylfluorenone (5)17 were prepared. Perylene
dyes are known to be very photostable and emit with high
quantum yields.14 Bromination of perylenetetracarboxylic
acid dianhydride in concentrated sulfuric acid afforded the
For electroluminescence12 as well as photoluminescence,
ability to adjust the emission wavelengths is very desirable.
For a given polymer system, this can be achieved by blending
with fluorescent dyes.13 An elegant approach to prevent
microphase separation, which among others can detoriate
energy transfer to the dye, is its covalent incorporation during
polymerization.14
We now report on the direct preparation of luminescent high
molecular weight conjugated polymer in the form of nanopar-
ticles, with adjustable emission wavelength.
(9) Huber, J.; Mecking, S. Angew. Chem., Int. Ed. 2006, 45, 6314–6317.
(10) (a) Mecking, S.; Held, A.; Bauers, F. M. Angew. Chem., Int. Ed. 2002,
41, 544–561. (b) Claverie, J. P.; Soula, R. Prog. Polym. Sci. 2003,
28, 619–662. (c) Mecking, S. Colloid Polym. Sci. 2007, 285, 605–
619.
(11) ADMET in aqueous emulsion affording poly(phenylene vinylene) of
Mn 103 g mol-1: Pecher, J.; Mecking, S. Macromolecules 2007, 40,
7733–7735.
Results and Discussion
(12) Burroughes, J. H.; Bradley, D. D. C.; Brown, A. R.; Marks, R. N.;
Mackay, K.; Friend, R. H.; Burns, P. L.; Holmes, A. B. Nature 1990,
347, 539–541.
The Glaser reaction is a high-yielding oxidative coupling
reaction of monosubstituted alkynes to the diethynyls under
very mild conditions. Air can serve as the oxidant.15 The
preparation of conjugated polymers by Glaser coupling has
been demonstrated in organic solution.16 In terms of classical
polycondensation, the reaction is akin to polymerization of
an AB monomer (with the A and B functional group
identical). Unlike A-A + B-B polycondensation, exact
adjustment of stoichiometry, which could be particularly
challenging in multiphase systems such as aqueous emulsions,
is not an issue. These features prompted us to study
nanoparticle synthesis by Glaser coupling.
(13) (a) Virgili, T.; Lidzey, D. G.; Bradley, D. D. C. AdV. Mater. 2000,
12, 58–62. (b) Lyons, B. P.; Wong, K. S.; Monkman, A. P. J. Chem.
Phys. 2003, 118, 4707–4711. (c) Wu, C.; Zheng, Y.; Szymanski, C.;
McNeill, J. J. Phys. Chem. C 2008, 112, 1772–1781.
(14) Ego, C.; Marsitzky, D.; Becker, S.; Zhang, J.; Grimsdale, A. C.;
Mu¨llen, K.; MacKenzie, J. D.; Silva, C.; Friend, R. H. J. Am. Chem.
Soc. 2003, 125, 437–443.
(15) Siemsen, P.; Livingston, R. C.; Diederich, F. Angew. Chem., Int. Ed.
2000, 39, 2632–2657.
(16) (a) Hay, A. J. Org. Chem. 1960, 25, 1275–1276. (b) Hay, A. S. J.
Polym. Sci., Part A: Polym. Chem. 1969, 7, 1625–1634. (c) Wu, X.;
Dirlikov, S. K. Polym. Mater. Sci. Eng. 1989, 60, 762–766. (d) Kwock,
E. W.; Baird, T.; Miller, T. M. Macromolecules 1993, 26, 2935–2940.
(e) Williams, V. E.; Swager, T. M. J. Polym. Sci., Part A: Polym.
Chem. 2000, 38, 4669–4676. (f) Lo, P. K.; Sleiman, H. F. Macro-
molecules 2008, 41, 5590–5603.
Monomer Synthesis. 1,4-Diethynyl-2,5-bis(2-ethylhexyloxy)-
benzene (1), 1,4-diethynyl-2,5-dihexyloxybenzene (2), and 2,7-
diethynyl-9,9-dihexylfluorene (3) were prepared similarly to
published procedures. The acetylene moieties were introduced
(17) Lewis, J.; Raithby, P. R.; Wong, W.-Y. J. Organomet. Chem. 1998,
556, 219–228.
9
14268 J. AM. CHEM. SOC. VOL. 131, NO. 40, 2009

Fluorescent Conjugated Polymer Nanoparticles
A R T I C L E S
Scheme 2. Polymers Prepared in the Form of Nanoparticle Dispersions
Table 1. Polymerizations in Aqueous Miniemulsiona
b
c
entry
monomer
monomer/catalyst
t [h]
Mnb [g mol-1
]
Mw [g mol-1
]
Mn,NMR [g mol-1
]
DPnc
particle sized [nm]
1-1
1-2
1-3
1-4
1-5
1-6
1
1
1
2
3
3
6.9
6.8
22.3
5.1
7.0
21.9
48
48
96
96
72
2.3 × 104
4.1 × 104
2.2 × 104
7.6 × 104
2.5 × 105
5.3 × 104
1.8 × 104
5.7 × 104
2.6 × 104
9.4 × 104
1.0 × 104
1.3 × 104
47
150
68
290
26
22 ( 11
27 ( 12
n. d.
31 ( 10
29 ( 11
n. d.
2.1 × 104
6.4 × 104
120
3.4 × 104
1.1 × 105
34
a Reaction conditions: 25 °C (entry 1-2: 50 °C); 60 µmol of CuCl/dnbpy (1:1); 1 mL of toluene; 30 mL of 1% aqueous TTAB solution.
b Determined by GPC in THF at 40 °C vs polystyrene standards. c Determined by 1H NMR in CDCl3 at 25 °C (entry 1-4: 100 °C in C2D2Cl4).
d Number average particle sizes determined by TEM.
disubstituted product. Imidation with 4-bromoaniline in
propionic acid, followed by selective substitution of the
perylene-bound bromide groups with 4-(1,1,3,3-tetrameth-
ylbutyl)phenolate, afforded the soluble dibromo-substituted
perylenediimide. Catalytic coupling with TMS-acetylene and
subsequent cleavage of the trimethylsilyl moiety with tet-
rabutyl ammonium fluoride afforded the diethynyl perylene-
diimide 4.
the aqueous phase rather than in the organic droplets. In
addition, the poor solubility of catalysts with these ligands
in apolar organic solvents is problematic in its own right.15
From studies of various bidentate nitrogen ligands, we found
4,4′-dinonyl-2,2′-bipyridine (dnbpy)21 to afford a catalyst
suitable for Glaser coupling polymerization in miniemulsion,
in combination with CuCl. This lipophilic biypridine deriva-
tive is capable of solubilizing the catalyst in the organic
droplets.
For polymerization, a miniemulsion obtained by ultrasoni-
cation of an aqueous surfactant solution (tetradecyltrimethyl-
ammonium bromide, TTAB) with an organic phase containing
the monomer and catalyst (toluene solution of CuCl/dnbpy 1:1)
was stirred in air at room temperature. Note that during this
procedure the small portion of toluene introduced with the
catalyst is removed by evaporation as the azeotrope. Intensely
colored dispersions were obtained (cf. Figure 2). Over a period
of observation of more than one year, the dispersions were found
to be colloidally stable. No precipitate was observed in the
dispersions, and no aggregation or other changes of the particles
were observed in TEM images after one year of storage. For
example, for a poly-1 dispersion a particle size of 22 ( 11 nm
was determined after polymerization and 23 ( 8 nm after one
year of storage.
As reported by Wu¨rthner et al.,18 bromination of the
perylene bisanhydride in concentrated H2SO4 is not regiose-
lective, rather, a mixture of the desired 1,7- and the 1,6-
substituted regioisomer was obtained. These isomers were
reported to not be separable by column chromatography in
the case of dibromoperylene bisimides.18 Analysis of the
perylene dibromides (anhyride or imide) is hampered by their
low solubility. In the final product (4), however, no undesired
1
regioisomers were detected by 600 MHz H NMR spectros-
copy. Apparently, any regioisomers likely formed in the
bromination step were removed by the column chromato-
graphic purifications after the introduction of the bulky
alkylphenoxy side groups.
Nanoparticle Preparation. A classical emulsion polymeri-
zation mechanism requires diffusion of monomer through the
aqueous phase. This hampers its application to very hydro-
phobic monomers and can result in undesired composition
gradients in the case of copolymers. Therefore, a miniemul-
sion technique was employed. Glaser coupling polymerization
in miniemulsion requires a catalyst specifically suited for
these conditions. Under standard Hay conditions19 (CuCl/
tmeda as catalyst; tmeda ) N,N,N′,N′-tetramethyl ethylene
diamine), the Glaser coupling reaction does not occur in the
presence of substantial amounts of water.20 All ligands
commonly used in the Glaser reaction have a high water
solubility, which would result in location of the catalyst in
All three monomers studied were converted quantitatively
to the corresponding poly(arylene diethynylene) nanopar-
ticles, with functional group conversions >98% (Scheme 2).
Polymer molecular weights are in the range 104 to 105 g
mol-1, as determined by both gel permeation chromatography
(GPC) and 1H NMR spectroscopic determination of the
HCt Cs end groups (Table 1). Results of both methods are
in reasonable agreement, considering that data from GPC are
apparent molecular weights (referenced to polystyrene stan-
(21) ATRP with dnbpy/Cu(I)/Cu(II) in bulk, miniemulsion, and emulsion: (a)
Matyjaszewski, K.; Patten, T. E.; Xia, J. J. Am. Chem. Soc. 1997,
119, 674–680. (b) Matyjaszewski, K.; Qiu, J.; Tsarevsky, N. V.;
Charleux, B. J. Polym. Sci., Part A: Polym. Chem. 2000, 4724–4734.
(c) Peng, H.; Cheng, S.; Feng, L.; Fan, Z. J. Appl. Polym. Sci. 2003,
89, 3175–3179.
(18) Wu¨rthner, F.; Stepanenko, V.; Chen, Z.; Saha-Moller, C. R.; Kocher,
N.; Stalke, D. J. Org. Chem. 2004, 69, 7933–7939.
(19) Hay, A. S. J. Org. Chem. 1962, 27, 3320–3321.
(20) Knol, K. E.; Horssen, L. W. v.; Challa, G.; Havinga, E. E. Polym.
Commun. 1985, 26, 71–73.
9
J. AM. CHEM. SOC. VOL. 131, NO. 40, 2009 14269

A R T I C L E S
Baier et al.
Figure 2. Dilute polymer dispersions of poly(1-co-4) with (from left to
right) 0, 0.5, 1, and 2.1% incorporated dye; poly(3-co-4) with 0, 0.1, 0.2,
and 0.8% incorporated dye; poly(2-co-4) with 0 and 0.4% incorporated dye
under UV light (λ ) 366 nm).
Figure 1. TEM image (left) and cryo-TEM images (right)23 of poly-3
nanoparticles.
dards).22 Molecular weight determination of poly-2 was
hampered by its very low solubility in organic solvents. A
molecular weight of ca. Mn 105 g mol-1 was estimated by
high-temperature NMR (in C2D2Cl4 at 100 °C). Polymeri-
zation at an elevated temperature of 50 °C resulted in
increased molecular weights as studied for poly-1 (entry
1-2). Following the reaction over time revealed that the
polymerization is essentially completed after 24 h.
Particle sizes of the dispersions were studied by transmission
electron microscopy (TEM; Figure 1 and Figure S1 in the
Supporting Information). Number average particle sizes amount
to 22-31 nm (Table 1). While the samples feature a certain
particle size distribution, no larger particles (>100 nm) or
significant aggregates were observed. Cryo-TEM on shock-
vitrified samples, which reflects the structure of the aqueous
dispersions, demonstrates more clearly the nonaggregated nature
of the particles in the dispersions (Figure 1). The observed
nonspherical shape of the poly-3 particles can be tentatively
associated with the relatively high persistence length of con-
jugated polymers, which is on the same order of magnitude as
the particle sizes. This may result in a nonspherical shape being
energetically favored compared to a sphere, as it can better
accommodate the conjugated segments of the polymer chains.
Figure 3. Diluted polymer dispersions (from left to right) of poly-1, poly(1-
co-5) (9.4%), and poly(1-co-4) (2.1%) under UV light (λ ) 366 nm).
Incorporation of the dye (4) into the polymer chain was
studied by 1H NMR spectra (Figure S2 in the Supporting
Information). For poly(1-co-4) (entry 2-3), the aromatic
perylene protons resonate at δ 9.69, 8.67, and 8.38 ppm. As
expected, incorporation of the perylene dye into the polymer
does not affect the shifts of these signals by comparison to
monomer 4, but they are significantly broadened. In addition
to 1-ethynyl-2,5-bis(2-ethylhexyloxy)benzene end groups orig-
inating from the main monomer 1 (δ 3.34), perylene imide-
substituted HCt Cs end groups (δ 3.14) are observed. The
portion of the latter end groups is slightly higher than the molar
incorporation of 4, which indicates a slightly lower reactivity
of end groups based on 4 by comparison to end groups from 1.
Polymerization of 1 at an elevated temperature of 50 °C (entry
1-2) had no significant effect on the particle shape or particle
size.
Copolymerizations with the diethynyl perylenediimide dye
4 and diethynylfluorenone 5 proceeded analogously to the
aforementioned homopolymerizations (Table 2) to afford col-
loidally stable dispersions (Figures 2 and 3). No significant
undesired reduction of the high functional group conversion and
polymer molecular weights was observed, which indicates that
the reactivities of the ethynyl moieties of 4 and 5 are not
unfavorably diminished compared to the main monomers, 1, 2,
or 3, respectively (vide infra), and that the catalyst and coupling
reaction is compatible with the perylene moiety and vice versa.
Particle sizes are also not affected by the presence of dye in
the polymerization reaction (Table 2).
1
The incorporated amount of 4 as determined by H NMR is
identical to the composition of the polymerization reaction
mixture within experimental error of the NMR technique; that
is, the dye appears to be completely incorporated covalently
into the polymer. Incorporation of dye was also demonstrated
by chromatography, which is more sensitive than NMR for this
purpose. Chromatography of poly(1-co-4) and for comparison
a physical mixture of poly-1 and 0.01 mol % perylene (with
respect to repeat units in 1) on a TLC plate resulted in a clear
separation of the perylene dye from the homopolymer, while
no residual dye was observed in the copolymer (Figure S3 in
the Supporting Information).
Copolymerizations with the diethynyl-substituted fluorenone
(5) also afforded complete incorporation into the polymer chain,
1
as concluded from H NMR spectroscopy (Figure S4 in the
Supporting Information). In this case, no ethynyl resonances
of dye end groups were detected (monomer 5: δ 3.17 for
HCt Cs). Molecular weights and particle sizes of the diethy-
nylfluorenone copolymers also do not differ significantly from
the corresponding poly-1 homopolymers.
9
14270 J. AM. CHEM. SOC. VOL. 131, NO. 40, 2009

Fluorescent Conjugated Polymer Nanoparticles
A R T I C L E S
Table 2. Copolymerization of Perylene Dye (4) and Diethynylfluorenone (5) in Aqueous Miniemulsiona
b
c
entry
monomer (mol % 4 or 5)
monomer/catalyst
t [h]
Mnb [g mol-1
]
Mw [g mol-1
]
Mn,NMR [g mol-1
]
DPnc
particle sized [nm]
2-1
2-2
2-3
2-4
2-5
2-6
2-7
2-8
2-9
2-10
1 (0.5% 4)
1 (1.0% 4)
1 (2.1% 4)
2 (0.4% 4)
3 (0.1% 4)
3 (0.2% 4)
3 (0.8% 4)
1 (2.2% 5)
1 (5.3% 5)
1 (9.4% 5)
6.7
8.8
7.0
7.7
7.2
7.0
6.3
6.7
6.9
7.8
120
120
72
72
48
72
96
72
72
2.4 × 104
3.7 × 104
3.3 × 104
1.7 × 104
2.2 × 104
2.0 × 104
4.1 × 104
5.0 × 104
2.5 × 104
8.5 × 104
1.1 × 105
9.5 × 104
6.0 × 104
6.0 × 104
6.4 × 104
1.3 × 105
1.6 × 105
8.8 × 104
1.9 × 104
2.4 × 104
1.8 × 104
1.8 × 104
9.9 × 103
1.2 × 104
9.0 × 103
2.4 × 104
2.4 × 104
2.8 × 104
50
63
47
55
26
32
24
63
73
63
n. d.
n. d.
40 ( 16
n. d.
n. d.
n. d.
32 ( 11
n. d.
n. d.
22 ( 7
120
a Reaction conditions: 25 °C; 60 µmol of CuCl/dnbpy (1:1); 1 mL of toluene; 30 mL of 1% aqueous TTAB solution. b Determined by GPC in THF
at 40 °C vs polystyrene standards. c Determined by 1H NMR in CDCl3 at 25 °C (entry 2-4: 100 °C in C2D2Cl4). d Number average particle sizes
determined by TEM.
Table 3. Optical Properties of Polymer Nanoparticles, Chloroform
Solutions, and Filmsa
polymer (% dye)
λabs [nm]
λem [nm]
Φf [%]
aqueous dispersion poly-1
434, 472 496
445, 488 501, 563
385, 413 502
5
4
11
5
poly-2
poly-3
poly(1-co-4) (2.1%) 469
641
poly(2-co-4) (0.4%) 446, 490 503, 621
poly(3-co-4) (0.8%) 386, 413 634
4
6
6
77
65
53
65
10
13
poly(1-co-5) (9.3%) 467
565
484
428
483
434
520
507
solution
film
poly-1
poly-3
457
422
poly(1-co-4) (2.1%) 457
poly(3-co-4) (0.8%) 422
poly-1
poly-3
a λabs
:
wavelength of absorption maximum, λem: wavelength of
emission maximum, Φf: fluorescence quantum yield. Polymer films were
prepared from 10 g L-1 chloroform solutions by spincoating.
Fluorescence Properties. UV/vis absorption and emission
spectra of the nanoparticle dispersions and of polymer films
and polymer solutions in chloroform for comparison were
studied. Most notably, emission of the nanoparticles is red-
shifted by comparison to polymer solutions (Table 3). This is
particularly pronounced for the polyfluorene poly-3 (Figure 4),
with a ∆λ ) 74 nm red-shift of the fluorescence maximum of
the nanoparticles (λ ) 502 nm) vs a corresponding polymer
solution (λ ) 428 nm). Such an effect is well-known from
studies of bulk conjugated polymers. By comparison to the
emission maximum observed in solution (0-0 transition), energy
transfer from the excited state to lower band gap chromophores
in the solid state results in longer wavelength fluorescence
emission. Interchain interactions (e.g., aggregate formation by
π-stacking) in the solid have also been reported to promote
radiationless decay, at the expense of fluorescence quantum
yields.24
Figure 4. Absorption (dashed) and fluorescence (solid line) spectra of
aqueous dispersions (green), chloroform solutions (black), and thin films
(red) of poly-3 (top image, λexc ) 410 nm) and poly-1 (bottom image, λexc
) 450 nm). Insets: photographs of polymer solution (top) and nanoparticle
dispersion (bottom).
is observed for the latter (Figure S5 in the Supporting
Information). This agrees with the assumption that interchain
interactions, which are less hindered by the less bulky
alkyloxy side groups in poly-2, promote energy transfer to
lower energy chromophores. “Isolation” of individual poly-
mer chains by appropriate substituents is a concept for
decreasing interchain interactions, which can result in solu-
tion-like properties in the solid.25
An increased polymerization temperature of 50 °C for the
preparation of poly-1 nanoparticles had no significant effect on
the optical properties. Fluorescence spectra and quantum yields
(6% for polymerization at 50 °C) are virtually identical to the
material prepared at room temperature.
An effective energy transfer to polymer-incorporated dye is
already evident from the appearance of the nanoparticle disper-
sions to the eye under UV light (Figures 2 and 3). The effect is
more pronouncedly observed for films, obtained by simple
drying of a drop of dispersion on a glass slide, under UV
excitation (Figure 5).
Overall, the fluorescence properties in terms of emission
energy and quantum yields of the nanoparticles were found
to resemble the bulk solid, as represented by the solution-
cast film samples (Figure 4). In detail, comparing emission
of nanoparticle dispersions of poly-1 and its less bulky-
substituted analogue poly-2, a more intense red-shifted band
(22) For a discussion of GPC of rigid rod polymers, see: Grell, M.; Bradley,
D. D. C.; Long, X.; Chamberlain, T.; Inbasekaran, M.; Woo, E. P.;
Soliman, M. Acta Polym. 1998, 49, 439–444.
(23) The occasional small light spots are due to beginning melting of the
vitrified ice matrix under the electron beam.
(24) (a) Nguyen, T.-Q.; Martini, I. B.; Liu, J.; Schwartz, B. J. J. Phys.
Chem. B 2000, 104, 237–255. (b) Schwartz, B. J. Annu. ReV. Phys.
Chem. 2003, 54, 141–172.
(25) Swager, T. Acc. Chem. Res. 2008, 41, 1181–1189.
9
J. AM. CHEM. SOC. VOL. 131, NO. 40, 2009 14271

A R T I C L E S
Baier et al.
The incorporation of copolymerizable dyes results in an
adjustability of the emission maximum of the nanoparticles, as
demonstrated here by poly-1: 496 nm; poly(1-co-5): 565 nm,
and poly(1-co-4): 641 nm (Figure 7; also cf. Figure 3).
The fluorescence properties of the dispersions were found to
be essentially unaltered by storage for a period of more than a
year (Figure S9 in the Supporting Information).
Figure 5. Films obtained by drop-coating of dispersions of (from left to
right) poly-1, poly(1-co-4) (2.1%), poly-3 and poly(3-co-4) (0.8%) under
UV light (λ ) 366 nm).
Summary and Conclusions
Nanoparticles of high molecular weight poly(arylene
diethynylene)s can be accessed by Glaser polymerization in
highly disperse aqueous systems, a catalytic step-growth
reaction that does not require stoichiometric balance of
reagents as in common A2 + B2 condensation. The catalyst
system CuCl/dnbpy was found to be suited for promoting
polymerization in the organic phase in the miniemulsion
procedure employed, with air as the oxidant at room
temperature. Particles of only ca. 30 nm size are obtained.
A copolymerizable difunctional perylenediimide is covalently
incorporated, without any undesired effect on the progress
of the polymerization reaction or particle sizes. The nano-
particle dispersions are colloidally stable, and their fluores-
cence properties are unaltered by prolonged storage. In
nanoparticles of dye copolymers, an efficient energy transfer
from the conjugated polymer backbone to the dye can occur,
resulting in emission from the dye virtually exclusively for
appropiate copolymer nanoparticles. This enables increasing
fluorescence wavelength of the particles by dye incorporation.
The preparation of nanoparticles of fluorescent conjugated
polymers directly by polymerization, in a disperse aqueous
system, is a general and versatile approach to these types of
materials, which are finding rapidly increasing attention.
Among others, it is of interest for the generation of
core-shell particles, with covalently bound hydrophilic
moieties on their surface affording colloidal stabilization or
particle recognition.
Figure 6. Fluorescence spectra of poly-1 and poly(1-co-4) nanoparticles
(top, λexc ) 450 nm) and of poly-3 and poly(3-co-4) nanoparticles (bottom,
λexc ) 420 nm) with various amounts of incorporated perylene dye.
Fluorescence spectroscopy of copolymer nanoparticle disper-
sions reveals the efficiency of energy transfer to the perylene-
diimide repeat units (Figure 6 and Figure S6 in the Supporting
Information). Emission from the latter increases with increasing
dye incorporation. At a molar incorporation of 0.8 mol %
(poly(3-co-4)) or 2 mol % (poly(1-co-4)), emission occurs nearly
exclusively from the dye. In detail, a slight bathocromic shift
of the dye emission is observed with increasing dye incorpora-
tion. By clear contrast to the solid nanoparticles, no energy
transfer to the perylenediimide repeat units is observed for
copolymer solutions (Figure S7 in the Supporting Information).
This is in agreement with theoretical considerations that energy
transfer in conjugated polymers proceeds via a two-step mech-
anism, the initial exciton hopping along the polymer chain being
limiting in solution.26
Incorporation of the fluorenone derivative 5 into poly-1 results
in a broadening of the fluorescence spectra at low concentrations
(Figure S8 in the Supporting Information). At a concentration
of around 9%, an orange emission with a maximum at 565 nm
originating from the dye comonomer almost exclusively occurs
(Figure 7). At the same time, the absorption spectra of these
copolymers are largely identical to the homopolymer.
Experimental Section
Materials and General Considerations. 4,4′-Dinonyl-2,2′-
bipyridine (97%) (Aldrich), tetradecyltrimethylammonium bromide
(>98%) (Fluka), and CuCl (ACS grade) (Merck) were used as
received. Nylon filters (0.45 µm pore size) were purchased from
VWR.
NMR spectra were recorded on a Varian Unity INOVA 400,
on a Bruker AC 250, or on a Bruker Avance DRX 600
1
spectrometer. H and 13C chemical shifts were referred to the
solvent signal. Mass spectrometry was performed on a Finnigan
MAT8200 EI-MS instrument. GPC was carried out on a Polymer
Laboratories PL-GPC 50 with two PLgel 5 µm MIXED-C
columns in THF at 40 °C against polystyrene standards. TEM
images were obtained on a Zeiss Libra 120 instrument (accelera-
tion voltage 120 kV). For cryo-TEM, a Gatan cryo-transfer
attachment CT3500 was used. Dispersions were dialyzed for
TEM analysis to remove any free surfactant and applied to a
copper grid. Samples were not contrasted. Specimens for the
cryo-TEM investigations were prepared in a Leica CPC instru-
ment by freezing a thin film of the dispersion by plunging in
liquid ethane. The thin film was created by dipping a small
amount of the dispersion on a holey carbon film. A meniscus,
thin enough for use in TEM, formed over the holes and was
rapidly frozen to afford a vitrified sample. The sample was cryo
transferred into the TEM and examined at a temperature around
90 K with minimal electron dose. Absorption spectra were
recorded on a Varian Cary 50 spectrometer. Fluorescence spectra
were obtained on a custom-made setup consisting of a xenon
(26) (a) Beljonne, B.; Pourtois, G.; Silva, C.; Hennebicq, E.; Herz, L. M.;
Friend, R. H.; Scholes, G. D.; Setayesh, S.; Mu¨llen, K.; Bre´das, J. L.
Proc. Natl. Acad. Sci. U.S.A. 2002, 99, 10982–10987. (b) Hennebicq,
E.; Pourtois, G.; Scholes, G. D.; Herz, L. M.; Russell, D. M.; Silva,
C.; Setayesh, S.; Grimsdale, A. C.; Mu¨llen, K.; Bre´das, J. L.; Beljonne,
D. J. Am. Chem. Soc. 2005, 127, 4744–4762. (c) Becker, K.; Lupton,
J. M. J. Am. Chem. Soc. 2006, 128, 6468–6479.
9
14272 J. AM. CHEM. SOC. VOL. 131, NO. 40, 2009

Fluorescent Conjugated Polymer Nanoparticles
A R T I C L E S
dissolved completely. The green catalyst solution obtained was
mixed with a solution of the monomers in 0.5 mL of toluene and
immediately added to 30 mL of a 1% aqueous TTAB solution with
a syringe. The mixture was ultrasonicated for 2 min (Bandelin HD
2200 with a KE76 tip operated at 120 W). The miniemulsion was
stirred at room temperature in a Schlenk tube open to air for several
days (for specific reaction times, cf. Tables 1 and 2). The polymer
dispersions were dialyzed for 4 days to remove residual surfactant
and catalyst, followed by filtering through a paper filter to remove
any agglomerates.
Acknowledgment. Financial support by the DFG (Me1388/7-
1) is gratefully acknowledged. We thank Andreas Zumbusch and
his group for access to the fluorescence and absorption spectrometer.
TEM was carried out by Marina Krumova. GPC analysis were
conducted by Lars Bolk. S.M. is indebted to the Fonds der
Chemischen Industrie.
Figure 7. Fluorescence spectra (λexc ) 450 nm) of poly-1, poly(1-co-5),
and poly(1-co-4) nanoparticles.
flash lamp, a monochromator (Oriel 77250 1/8M), a spec-
trograph, and a nitrogen-cooled CCD camera enabling photo-
luminescence detection from 310 to 940 nm.27 Polymer film
fluorescence spectra and fluorescence quantum yields were
obtained on a Hamamatsu Absolute PL Quantum Yield Mea-
surement System C9920-02. For measurements in solution,
spectroscopic-grade chloroform (Uvasol, Merck) was used.
Polymer films for fluorescence measurements were prepared by
spincoating of a 10 g L-1 chloroform solution at 4000 rpm on
a glass coverslip.
Supporting Information Available: Additional synthetic
procedures, NMR spectrum of copolymer, additional fluores-
cence spectra and photograph of TLC of copolymer, and TEM
image of polymer dispersion. This material is available free of
charge via the Internet at http://pubs.acs.org.
JA905077C
General Polymerization Procedure. CuCl and dnbpy were
mixed with 0.5 mL of toluene and stirred until the solids had
(27) Menges, F. Diploma Thesis, University of Konstanz, Konstanz,
Germany, 1999.
9
J. AM. CHEM. SOC. VOL. 131, NO. 40, 2009 14273