Multicolour self-assembled particles of fluorene-based bolaamphiphiles

Article abstract of DOI:10.1039/b822943k

Bolaamphiphilic fluorene-based oligomers self-assemble in water to form fluorescent nanoparticles with tuneable emission colours covering the entire visible range, even including white. The Royal Society of Chemistry.

Full text of DOI:10.1039/b822943k

View Article Online / Journal Homepage / Table of Contents for this issue
COMMUNICATION
www.rsc.org/chemcomm | ChemComm
Multicolour self-assembled particles of fluorene-based bolaamphiphileswz
Robert Abbel, Rob van der Weegen, E. W. Meijer* and Albertus P. H. J. Schenning*
Received (in Cambridge, UK) 19th December 2008, Accepted 27th January 2009
First published as an Advance Article on the web 17th February 2009
DOI: 10.1039/b822943k
Bolaamphiphilic fluorene-based oligomers self-assemble in water
to form fluorescent nanoparticles with tuneable emission colours
covering the entire visible range, even including white.
Over the last few years, fluorescent nanoparticles have received
considerable attention in biological imaging applications.1,2
Nanocrystals of inorganic semiconductors (so-called quantum
dots)3–7 are frequently used, in which the emission colour can
be adjusted by their size.7,8 p-Conjugated polymer nano-
particles have been prepared as well.9–16 Here, the fluorescence
characteristics are controlled by the chemical structures.17–19
Fig. 1 Chemical structures of the fluorene-based bolaamphiphiles 1–4.
Recently, monodisperse amphiphilic p-conjugated oligomers
qualitatively similar to those of the corresponding polymers
(Table 1).34,35 In agreement with the variations in electron-
accepting character of the central aromatic units, the photo-
emission spectra varied from blue (1) via green (2) and yellow
(3) to red (4), thus covering the entire range of visible light.
The optical properties (including the lack of circular dichroic
signals)33 present an argument for the presence of molecularly
dissolved species and the absence of aggregates.
have been reported as a new class of surfactants capable of
forming spherical assemblies in water.20–25 Most of these
assemblies consist of only one type of oligomer, while the
possibility to tune the fluorescence wavelength by simply
mixing different types of molecules due to (partial) energy
transfer is virtually unexplored.25,26
Here, we report a set of four bolaamphiphilic fluorene-based
oligomers (1–4; Fig. 1)27 that form self-assembled particles in
water with fluorescence colours spanning the entire visible
spectrum. Mixed assemblies containing two or more different
bolaamphiphiles could be prepared, which showed white
emission at appropriate compositions. By contrast, no
exchange of components in time or upon heating was observed
when pure particles were present.
Self-assembly in water was observed after injection of
concentrated THF stock solutions into water (typically
between 100- and 1000-fold excess by volume). The spectral
characteristics of samples prepared in this way changed
compared to solutions in good solvents and suggested the
formation of aggregates (vide infra), albeit with a low internal
helical order, as was obvious from the low values of the
degrees of circular polarisation in absorption.33 Despite some
shifts, the emission colours still spanned the entire visible
spectrum (Fig. 2) and fluorescence quantum yields up to
70% were observed in aqueous solution (Table 1).36
The bolaamphiphilic fluorene co-oligomers 1–4 consist of a
conjugated chromophore of two 9,90-di((S)-3,7-dimethyloctyl)-
fluorene units connected by an aromatic linker unit with a
varying electron-accepting capacity. Since only slight structural
adaptations were carried out, no significant impact of these
differences on the self-assembly behaviour was expected. The
synthesis included a Suzuki coupling of aromatic dibromo
compounds28–30 with an aminofluorene boronic ester,31
followed by reaction of the resulting diamino chromophores
with acid chlorides of hydrophilic tetra(ethylene glycol) (EG)
wedges,32 ensuring solubility in water. All compounds were
purified by recycling GPC and fully characterised.33
Visualisation of the nanoparticles formed by 3 and 4 was
possible using fluorescence microscopy (Fig. 3).37 The images
obtained from these samples showed spherical objects with a
size distribution between ca. 400 nm and 2 mm. Because the
size of the particles was approaching the diffraction limit, it
was impossible to determine the diameter of the smallest
particles.38
Other than by variation of the chemical structures, the
emission colours could also be tuned by partial energy transfer
when mixed particles were used. Such aggregates containing
two different chromophores (1 and 3) were prepared by
injection of a premixed THF stock solution into water. With
increasing energy acceptor (3) concentration, a strong decrease
in the blue donor emission was found which was more
pronounced than expected purely based on the ratio of 1 to
3 (Fig. 4a–c). This observation indicates the presence of energy
transfer similar as earlier reported for a system of mixed
OPV bolaamphiphiles.25,39 A red shift in the acceptor emission
was observed with increasing concentration of 3, which is
The bolaamphiphiles are soluble in common apolar organic
solvents such as toluene, chloroform or THF, and all char-
acteristics of the absorption and fluorescence spectra are
Eindhoven University of Technology, P.O. Box 513, 5600 MB
Eindhoven, The Netherlands. E-mail: a.p.h.j.schenning@tue.nl.
E-mail: e.w.meijer@tue.nl; Fax: +31 40 245 1036;
Tel: +31 40 247 3101
w Dedicated to Prof. Roeland Nolte on the occasion of his 65th
birthday.
z Electronic supplementary information (ESI) available: Synthetic
procedures and characterisation as well as additional experimental
data. See DOI: 10.1039/b822943k
ꢀc
This journal is The Royal Society of Chemistry 2009
Chem. Commun., 2009, 1697–1699 | 1697

View Article Online
Table 1 Optical characteristics of compounds 1–4
UV-vis absorption
Compound
Photoluminescence
max/nm (fPL(%))a
l
max/nm (log(e)/Mꢁ1cmꢁ1
)
l
CHCl3
Waterb
CHCl3
Waterb
1
2
3
4
343 (5.00)
332 (4.72)
421 (7)
431 (6)
333 (4.86), 386 (4.45)
339 (4.94), 428 (4.53)
368 (4.88), 509 (4.24)
329 (4.65), 390 (4.31)
332 (4.74), 430 (4.25)
354 (4.67), 514 (3.78)
519 (50)
569 (80)
679 (10)
510 (33)
567 (70)
660 (10)
a
Reference compounds: quinine bisulfate in 1 N H2SO4 (1, 2), N,N0-bis(pentylhexyl)perylene bisimide in CH2Cl2 (3), Rhodamine 101 in EtOH (4).
Value depends on the preparation method (cf. ref. 36 and ESIz).
b
Fig. 2 Normalised photoluminescence spectra of 1–4 in aqueous
solution (10ꢁ6 M).
Fig. 4 (a) PL spectra of mixed nanoparticles of 1 and 3 in aqueous
solution. Arrows indicate spectral changes with decreasing donor to
acceptor ratio from 99.5 : 0.5 to 50 : 50. Excitation wavelength lexc
=
342 nm. (b) Dependence of the emission intensity of 1 on the nature of
the particles (mixed or separate). (c) Photograph of two aqueous
dispersions (mixed or separate) containing 1 and 3 in a ratio of
95 : 5 under UV illumination (365 nm). (d) White emission from
mixed nanoparticles. Excitation wavelength lexc = 360 nm. Total
chromophore concentration 1.5 ꢂ 10ꢁ6 M in all cases.
showed a linear dependence on the concentrations, pointing to
the absence of energy transfer.33 At room temperature, the PL
spectra of these mixtures did not change within a time frame of
several hours, giving evidence for the absence of exchange of
molecules between the particles. Fluorescence microscopy
revealed a similar situation for assemblies of pure 3 and 4.
Two types of particles were observed having all the optical
characteristics of either 3 or 4 (Fig. 5).
Fig. 3 Fluorescence microscopy image of 3 (a) and 4 (b) in a
water–gelatine mixture. The scale bars correspond to 10 mm. Concen-
tration 30 mM. Excitation wavelengths 458 nm (a) and 543 nm (b).
likely to be caused by aggregation of 3 at higher acceptor
percentages.35 The fluorescence spectra did not change signi-
ficantly upon heating up to 70 1C or dilution to 10ꢁ9 M,
revealing a high thermal stability and a low critical aggrega-
tion concentration.33 To demonstrate the strength of this
modular tuning of the emitted light, we also prepared white-
emitting particles. To this end, the composition of mixed
aggregates containing 1 (blue), 2 (green), 3 (yellow) and 4
(red) was optimised, with the best result at a molar ratio of
93.5 : 0.13 : 0.13 : 6.24 (Fig. 4d). The low percentages of 2 and
3 reflect their high fluorescence quantum yields as well as
efficient energy transfer due to good spectral overlap with the
emission of 1. Both values were clearly lower for 4, which is
why it had to be added in a much higher amount.40
Fig. 5 Fluorescence microscopy image of a mixture of aggregates of
pure 3 and 4 in a water–gelatine mixture. The scale bar corresponds to
10 mm. Total chromophore concentration 30 mM, molar ratio 50 : 50.
Excitation wavelength 458 nm (3) and 543 nm (4).
The dynamics of the particles were studied by mixing
solutions containing only pure nanoparticles of 1 or 3. In this
case, the PL intensity of the donor and acceptor emission
ꢀc
This journal is The Royal Society of Chemistry 2009
1698 | Chem. Commun., 2009, 1697–1699

View Article Online
In conclusion, we have prepared a set of bolaamphiphilic
p-conjugated co-oligomers that self-assemble in water into
fluorescent particles. Due to their differences in band gap
energies, these compounds provide a toolbox for the easy
creation of red–green–blue and white fluorescent assemblies
in water. Their resistance towards the exchange of molecules
and thus the stability of their emission colours make them
promising candidates for multitarget labelling in biological
imaging protocols.
20 J. H. Yao, K. Y. Mya, X. Li, M. Parameswaran, Q.-H. Xu,
K. P. Loh and Z.-K. Chen, J. Phys. Chem. B, 2008, 112, 749.
21 Y. Koizumi, S. Seki, S. Tsukuda, S. Sakamoto and S. Tagawa,
J. Am. Chem. Soc., 2006, 128, 9036.
22 J.-H. Ryu, D.-J. Hong and M. Lee, Chem. Commun., 2008, 1043.
23 S. Chen, B. Song, Z. Wang and X. Zhang, J. Phys. Chem. C, 2008,
112, 3308.
24 L. Zhu, C. Yang and J. Qin, Chem. Commun., 2008, 6303.
25 F. J. M. Hoeben, I. O. Shklyarevskiy, M. J. Pouderoijen,
H. Engelkamp, A. P. H. J. Schenning, P. C. M. Christianen,
J. C. Maan and E. W. Meijer, Angew. Chem., Int. Ed., 2006, 45,
1232.
26 F. J. M. Hoeben, M. Wolffs, J. Zhang, S. De Feyter, P. E. L.
G. Leclere, A. P. H. J. Schenning and E. W. Meijer, J. Am. Chem.
Soc., 2007, 129, 9819.
The authors gratefully acknowledge the Dutch Organisation
for Scientific Research, Chemical Sciences (NWO-CW)
for financial support, ir. Marcel Wijlaars for carrying out
fluorescence microscopy and Dr Xianwen Lou and Henk
Eding for analytical measurements.
27 For completeness, it is mentioned that a trifluorene bolaamphi-
phile has also been prepared that has very similar emission proper-
ties to 1. The synthesis and characterisation of all five compounds
is described in the ESIz.
28 M. J. Edelmann, J.-M. Raimundo, N. F. Utesch, F. Diederich,
C. Boudon, J.-P. Gisselbrecht and M. Gross, Helv. Chim. Acta,
2002, 85, 2195.
Notes and references
1 J. Rao, A. Dragulescu-Andrasi and H. Yao, Curr. Opin. Biotechnol.,
2007, 18, 17.
2 P. Sharma, S. Brown, G. Walter, S. Santra and B. Moudgil, Adv.
Colloid Interface Sci., 2006, 123–126, 471.
29 B. A. DaSilveira Neto, A. Sant’Ana Lopes, G. Ebeling,
R. S. Gonc¸ alves, V. E. U. Costa, F. H. Quina and J. Dupont,
Tetrahedron, 2005, 61, 10975.
3 I. L. Medintz, H. T. Uyeda, E. R. Goldman and H. Mattoussi,
Nat. Mater., 2005, 4, 435.
30 D. D. Kenning, K. A. Mitchell, T. R. Calhoun, M. R. Funfar,
D. J. Sattler and S. C. Rasmussen, J. Org. Chem., 2002, 67, 9073.
31 R. Abbel, C. Grenier, M. J. Pouderoijen, J. W. Stouwdam,
P. E. L. G. Leclere, R. P. Sijbesma, E. W. Meijer and A. P. H. J.
Schenning, J. Am. Chem. Soc., 2009, 131, 833.
4 W. J. Parak, T. Pellegrino and C. Plank, Nanotechnology, 2005, 16, R9.
5 A. Hoshino, N. Manabe, K. Fujioka, K. Suzuki, M. Yasuhara and
K. Yamamoto, J. Artif. Organs, 2007, 10, 149.
6 J. K. Jaiswal and S. M. Simon, Trends Cell Biol., 2004, 14, 497.
7 A. M. Smith, G. Ruan, M. N. Rhyner and S. Nie, Ann. Biomed.
Eng., 2006, 34, 3.
8 W. W. Yu, L. Qu, W. Guo and X. Peng, Chem. Mater., 2003, 15,
2854.
9 G. Tu, H. Li, M. Forster, R. Heiderhoff, L. J. Balk, R. Sigel and
U. Scherf, Small, 2007, 3, 1001.
10 T. Kietzke, D. Neher, K. Landfester, R. Montenegro, R. Guntner
¨
and U. Scherf, Nat. Mater., 2003, 2, 408.
11 C. Wu, C. Szymanski, Z. Cain and J. McNeill, J. Am. Chem. Soc.,
2007, 129, 12904.
12 U. Asawapirom, F. Bulut, T. Farrell, C. Gadermaier, S. Gamerith,
32 L. Brunsveld, H. Zhang, M. Glasbeek, J. A. J. M. Vekemans and
E. W. Meijer, J. Am. Chem. Soc., 2000, 122, 6175.
33 See ESIz.
34 W.-C. Wu, C.-L. Liu and W.-C. Chen, Polymer, 2006, 47, 527.
35 B. Liu, W.-L. Yu, Y.-H. Lai and W. Huang, Chem. Mater., 2001,
13, 1984.
36 It should be noted that a remarkable dependence of the optical
properties on the preparation method was observed for compound
1: two types of aggregates with distinctly differing optical proper-
ties could be prepared when the concentration of the injected THF
stock solution was varied (cf. ESIz). No such differences were
found in any of the other compounds. This particularity of 1 is
currently being investigated in more detail.
37 Since 1 and 2 only absorb light in the UV region, excitation was
technically different with the available fluorescence microscope.
38 Despite the distribution in size, our results are reproducible.
Currently, we are investigating the possibility to separate the
different sized assemblies and characterise them by cryoTEM, light
scattering and atomic force microscopy.
39 In control experiments using chloroform as the solvent, no energy
transfer was found regardless of the preparation method used,
which is the expected result for molecularly dissolved species.
40 White emission in solution could also be achieved by preparing a
mixture of separate aggregates, but in this case, the concentrations
of the acceptors had to be increased while that of the donor had to
be decreased (cf. ESIz). This different composition was due to the
almost complete lack of energy transfer, meaning that there was
hardly any quenching of 1 and that 2, 3 and 4 almost exclusively
emitted by direct excitation.
R. Guntner, T. Kietzke, S. Patil, T. Piok, R. Montenegro, B. Stiller,
¨
B. Tiersch, K. Landfester, E. J. W. List, D. Neher, C. Sotomayor
Torres and U. Scherf, Macromol. Symp., 2004, 212, 83.
13 K. Landfester, R. Montenegro, U. Scherf, R. Guntner,
¨
U. Asawapirom, S. Patil, D. Neher and T. Kietzke, Adv. Mater.,
2002, 14, 651.
14 C. Wu, H. Peng, Y. Jiang and J. McNeill, J. Phys. Chem. B, 2006,
110, 14148.
15 C. Wu, C. Szymanski and J. McNeill, Langmuir, 2006, 22, 2956.
16 J. Y. Park, N. Koenen, M. Forster, R. Ponnapati, U. Scherf and
R. Advincula, Macromolecules, 2008, 41, 6169.
17 M. T. Bernius, M. Inbasekaran, J. O’Brien and W. Wu, Adv.
Mater., 2000, 12, 1737.
18 M. Inbasekaran, E. Woo, W. Wu, M. Bernius and L. Wujkowski,
Synth. Met., 2000, 111–112, 397.
19 S.-A. Chen, H.-H. Lu and C.-W. Huang, Adv. Polym. Sci., 2008,
212, 49.
ꢀc
This journal is The Royal Society of Chemistry 2009
Chem. Commun., 2009, 1697–1699 | 1699