White-light emitting hydrogen-bonded supramolecular copolymers based on #-conjugated oligomers

Article abstract of DOI:10.1021/ja807996y

Three different ?-conjugated oligomers (a blue-emitting oligofluorene, a green-emitting oligo(phenylene vinylene), and a red-emitting perylene bisimide) have been functionalized with self-complementary quadruple hydrogen bonding ureidopyrimidinone (UPy) units at both ends. The molecules self-assemble in solution and in the bulk, forming supramolecular polymers. When mixed together in solution, random noncovalent copolymers are formed that contain all three types of chromophores, resulting in energy transfer upon excitation of the oligofluorene energy donor. At a certain mixing ratio, a white emissive supramolecular polymer can be created in solution. In contrast to their unfunctionalized counterparts, bis-UPy-chromophores can easily be deposited as smooth thin films on surfaces by spin coating. No phase separation is observed in these films, and energy transfer is much more efficient than in solution, giving rise to white fluorescence at much lower ratios of energy acceptor to donor. Light emitting diodes based on these supramolecular polymers have been prepared from all three types of pure materials, yielding blue, green, and red devices, respectively. At appropriate mixing ratios of these three compounds, white electroluminescence is observed. This approach yields a toolbox of molecules that can be easily used to construct ?-conjugated supramolecular polymers with a variety of compositions, high solution viscosities, and tuneable emission colors.

Full text of DOI:10.1021/ja807996y

Published on Web 12/18/2008
White-Light Emitting Hydrogen-Bonded Supramolecular
Copolymers Based on π-Conjugated Oligomers
Robert Abbel, Christophe Grenier, Maarten J. Pouderoijen, Jan W. Stouwdam,
Philippe E. L. G. Lecle`re, Rint P. Sijbesma, E. W. Meijer,* and
Albertus P. H. J. Schenning*
Laboratory of Macromolecular and Organic Chemistry, EindhoVen UniVersity of Technology,
P.O. Box 513, 5600 MB EindhoVen, The Netherlands
Received October 10, 2008; E-mail: e.w.meijer@tue.nl; a.p.h.j.schenning@tue.nl
Abstract: Three different π-conjugated oligomers (a blue-emitting oligofluorene, a green-emitting oligo(phe-
nylene vinylene), and a red-emitting perylene bisimide) have been functionalized with self-complementary
quadruple hydrogen bonding ureidopyrimidinone (UPy) units at both ends. The molecules self-assemble
in solution and in the bulk, forming supramolecular polymers. When mixed together in solution, random
noncovalent copolymers are formed that contain all three types of chromophores, resulting in energy transfer
upon excitation of the oligofluorene energy donor. At a certain mixing ratio, a white emissive supramolecular
polymer can be created in solution. In contrast to their unfunctionalized counterparts, bis-UPy-chromophores
can easily be deposited as smooth thin films on surfaces by spin coating. No phase separation is observed
in these films, and energy transfer is much more efficient than in solution, giving rise to white fluorescence
at much lower ratios of energy acceptor to donor. Light emitting diodes based on these supramolecular
polymers have been prepared from all three types of pure materials, yielding blue, green, and red devices,
respectively. At appropriate mixing ratios of these three compounds, white electroluminescence is observed.
This approach yields a toolbox of molecules that can be easily used to construct π-conjugated
supramolecular polymers with a variety of compositions, high solution viscosities, and tuneable emission
colors.
Introduction
polymer resulted in white photoluminescence. However, white
light emitting thin films and electroluminescent devices based
on supramolecular interactions such as hydrogen bonding, π-π-
stacking, or solvophobic effects have never been reported.
The creation of multicomponent π-conjugated systems18-20
by a supramolecular approach combines the advantages of small
White emitting plastic materials1,2 are attractive candidates
for applications in flexible full-color light-emitting diodes
(LEDs) and in backlights for liquid-crystalline displays.3-5
A
common approach to obtain these materials is based on polymers
blended with luminescent dyes that simultaneously emit over
the whole visible range due to partial energy transfer from
energy donor polymers to acceptor dyes.6-11 Chen et al. have
expanded this principle to create linearly polarized white light
based on aligned fluorene oligomers.12 Alternatively, copolymers
containing moieties emitting at different wavelengths have been
employed to avoid phase separation of the polymer and the
dye.13-16 Recently, a supramolecular concept has been reported
by Ajayaghosh and co-workers in which a π-conjugated polymer
was encapsulated in a π-organogel.17,18 At a specific composi-
tion and temperature, partial energy transfer from the gel to the
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J. AM. CHEM. SOC. 2009, 131, 833–843 833

A R T I C L E S
Abbel et al.
Chart 1. Chemical Structures of the di-UPy Functionalized Chromophores Used in This Study and a Schematic Illustration of the Creation of
White Photoluminescence
molecules with those of polymers and allows us to influence
the morphology and phase separation through noncovalent
interactions.21 Self-assembled electroactive multicomponent
systems in which a close spatial proximity and controlled
orientation between the different species facilitates electronic
processes such as electron transfer,22 singlet-singlet,23-26 or
triplet-triplet27 energy transfer, have been reported. In the past,
we have synthesized blue emissive oligofluorenes (OF)25
equipped with quadruple hydrogen bonding 2-ureido-4[1H]-
pyrimidinone28,29 (UPy) end groups. These molecules self-
assembled into supramolecular polymers and could be dynami-
cally end-capped with energy acceptor molecules by adding
mono-UPy green- and red-emissive end cappers. Due to the self-
complementarity of the UPy units, these systems inevitably
contained a fraction of homodimers of hydrogen bonded
acceptors, which reduced the energy transfer efficiency. Ad-
ditionally, the virtual molecular weight was significantly
decreased with increasing end capper concentration. This
problem should in principle be solved by using exclusively
difunctionalised UPy chromophores, avoiding acceptor ho-
modimers and leaving the virtual molecular weight unchanged.
Complications to this approach might, however, arise from ring-
chain equilibria because at low concentrations supramolecular
polymers are known to exist preferably as cycles, whereas at
higher concentrations linear polymers are formed.30,31
(20) Hoeben, F. J. M.; Jonkheijm, P.; Meijer, E. W.; Schenning, A. P. H. J.
Chem. ReV. 2005, 105, 1491.
(21) (a) El-ghayoury, A.; Schenning, A. P. H. J.; van Hal, P. A.; van Duren,
J. K. J.; Janssen, R. A. J.; Meijer, E. W. Angew. Chem., Int. Ed. 2001,
40, 3660. (b) Jonkheijm, P.; van Duren, J. K. J.; Kemerink, M.;
Janssen, R. A. J.; Schenning, A. P. H. J.; Meijer, E. W. Macromol-
ecules 2006, 39, 784.
(22) Schenning, A. P. H. J.; van Herrikhuyzen, J.; Jonkheijm, P.; Chen,
Z.; Wu¨rthner, F.; Meijer, E. W. J. Am. Chem. Soc. 2002, 124, 10252.
(23) Hoeben, F. J. M.; Schenning, A. P. H. J.; Meijer, E. W. ChemPhy-
sChem 2005, 6, 2337.
Here, we report our results of this novel concept by using
three difunctionalized UPy chromophores: a blue emitting OF
(UPy-OF3-UPy), a green emitting OPV (UPy-OPV5-UPy), and
a red emitting PB (UPy-Pery-UPy) to create red-green-blue
(RGB) supramolecular materials (Chart 1). In a mixed system
of these hydrogen bonded copolymers the formation of acceptor
(24) Neuteboom, E. E.; Beckers, E. H. A.; Meskers, S. C. J.; Meijer, E. W.;
Janssen, R. A. J. Org. Biomol. Chem. 2003, 1, 198.
(25) Dudek, S. P.; Pouderoijen, M.; Abbel, R.; Schenning, A. P. H. J.;
Meijer, E. W. J. Am. Chem. Soc. 2005, 127, 11763.
(26) Zhao, C.-V.; Tung, Q.-X.; Li, Z.-X.; Wu, L.-Z.; Zhang, L.-P.; Tung,
C.-H. Tetrahedron. Lett. 2004, 45, 6807.
(27) Wang, S.-M.; Yu, M.-L.; Ding, J.; Tung, C.-H.; Wu, L.-Z. J. Phys.
Chem. A 2008, 112, 3865.
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E. W. J. Am. Chem. Soc. 1998, 120, 6761.
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A R T I C L E S
Scheme 1. Synthesis of UPy-OF3-UPya
a (1) (S)-3,7-Dimethyloctylbromide, DMSO/aq. NaOH, benzyltriethyl ammonium bromide, overnight, RT, 74 - 95%. (2) THF, 1. nBuLi, 2.
isopropoxypinacolatoborolane, 1 h, -78 °C, 64%. (3) (S)-3,7-Dimethyloctylbromide, DMSO/KOH, KI, overnight, RT, 58%. (4) SnCl2, EtOH/EtOAc, Ar,
3d, reflux, 93%. (5): Pd(PPh3)4, Na2CO3, dioxane/water (2:1), Ar, 3d, 90 °C, 55%. (6) CDI activated tridecylisocytosine, CHCl3, Ar, overnight, reflux, 56%.
Figure 1. (a) Recycling GPC trace of UPy-OF3-UPy in THF + 1% TFA. (b) Analytical GPC trace of UPy-OF3-UPy in THF after recycling GPC. (c)
MALDI-TOF mass spectrum of UPy-OF3-UPy. The small peaks at lower m/z values are due to fragmentation of the N-C bonds of the urea moieties.
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1
Figure 2. Full H NMR spectra and magnification of the hydrogen bonding region (inset) of UPy-OF3-UPy in THF-d8 (a) and in THF-d8 containing ca.
30 vol% TFA-d1 (b). The peak at ca. 13.2 ppm in Figure 2b is due to TFA.
dimers could be successfully avoided, whereas the virtual
molecular weight was affected by the formation of cyclic
structures. Nevertheless, the degree of polymerization was
sufficiently high to create white photoluminescent supramo-
lecular polymers both in solution and in spin coated films.
Finally, we succeeded in applying these materials in white light
emitting diodes (LEDs).
(Figure 1). Using this technique, the compound was obtained
in high purity (>99%), as was confirmed by MALDI-TOF, 1H
NMR, and analytical GPC.
After recycling GPC, UPy-OF3-UPy was obtained as a light
orange solid that showed a rather limited solubility in common
organic solvents (chloroform, THF, toluene) under ambient
conditions. Concentrated solutions (in the mM regime and
above) could only be reached by heating, by addition of strong
hydrogen bond breaking additives, such as TFA,25 or by using
o-dichlorobenzene (ODCB) as the solvent.
1H NMR experiments revealed association of the UPy groups
in THF-d8 (Figure 2). The solutions were very viscous and peak
broadening in the whole spectrum was observed, pointing to
the presence of aggregates of high virtual molecular weight.
Three weak signals that were strongly shifted upfield (δ 12.5,
12.8, and 13.3 ppm) indicated the presence of quadruple
hydrogen bonds, and in agreement with the high association
constant,28 no unbound UPy groups could be detected. Also,
the only NMR peaks observed corresponded to the pyrimidinone
tautomer, and no enol form28 was found. Upon addition of TFA-
d1, the viscosity of the solution strongly decreased, the NH
Results and Discussion
Synthesis, Purification, and Characterization. Based on the
expertise in our group,22-25 we chose a perylene bisimide (PB),
an oligo(phenylene-vinylene) (OPV), and an oligofluorene (OF)
as the RGB emissive bis-UPy chromophores. In the case of the
OFs, a series of di- and monosubstituted UPy compounds of
various conjugation lengths (up to five) was prepared based on
our earlier reported synthetic procedure25 for 9,9′-dihexylfluo-
rene derivatives (Scheme 1 and Supporting Information). In the
present study we will focus on the trimer UPy-OF3-UPy as
the blue emissive species.
After conventional workup (silica column chromatography),
it was evident from MALDI-TOF, 1H NMR, and GPC analysis32
that the final product UPy-OF3-UPy still contained some
impurities. Therefore it was subjected to recycling GPC33
(32) It should be mentioned that the resolution of the analytical GPC
chromatograms very strongly depended on the solvent used. In
chloroform, which is a good solvent for unfunctionalised oligofluo-
renes, strong tailing of the peaks prevented efficient separation,
whereas, in THF, separation was possible on an analytical scale without
further additives.
(33) Abbel, R.; Wolffs, M.; Bovee, R. A. A.; van Dongen, J. L. J.; Lou,
X.; Henze, O.; Feast, W. J.; Meijer, E. W.; Schenning, A. P. H. J.
AdV. Mater., DOI: 10.1002/adma.200802416.
(29) Sijbesma, R. P.; Beijer, F. H.; Brunsveld, L.; Folmer, B. J. B.;
Hirschberg, J. H. K. K.; Lange, R. F. M.; Lowe, J. K. L.; Meijer,
E. W. Science 1997, 278, 1601.
(30) Ten Cate, A. T.; Sijbesma, R. P. Macromol. Rapid Commun. 2002,
23, 1094.
(31) Ten Cate, A. T.; Kooijman, H.; Spek, A. L.; Sijbesma, R. P.; Meijer,
E. W. J. Am. Chem. Soc. 2004, 126, 3801.
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Scheme 2. Synthesis of UPy-OPV5-UPya
a (1) KOtBu, 4-cyanobenzyldiethylphosphonate, DMF/THF, overnight, RT, 85%. (2) LiAlH4, Et2O, overnight, RT, 90%. (3) CDI activated tridecylisocytosine,
CHCl3, reflux, overnight, 25%.
protons disappeared from the spectrum due to exchange with
deuterium, and the residual peaks sharpened, indicating that the
compound was not hydrogen bonded any more.
2,6-di-tert-butylbenzene and subsequent coupling with acti-
vated isocytosine analogous to the OPV and OF derivatives37
(Scheme 3 and Supporting Information). We found that after
column chromatography the only impurity present was the
1,6-diphenoxy isomer38 (ca. 10%). As it was very difficult
to remove and not expected to influence the electronic
properties of the material significantly, no further purification
was performed. The final product is a dark red solid and
dissolves well in a variety of organic solvents. In contrast to
UPy-OF3-UPy, its NMR spectrum in THF-d8 shows rather
sharp peaks already without the addition of TFA-d1 (Sup-
porting Information).
Green-emitting UPy-OPV5-UPy was prepared by stepwise
Wittig couplings of benzyl triphenylphosphonium salts and
aromatic aldehydes34 and subsequent reduction of the peripheral
cyano groups to benzyl amines, which finally were reacted with
an activated isocytosine derivative35 to yield the di-UPy
compound (Scheme 2 and Supporting Information). Similar to
the case described for UPy-OF3-UPy, conventional purification
procedures did not suffice to obtain the product in a pure form,
but recycling GPC enabled us to reach a high purity level
(<99%). The yellow-orange solid is well soluble in a variety
of organic solvents.
Chloroform solutions (µM regime) of UPy-OPV5-UPy and
UPy-Pery-UPy showed absorption spectra that were typical for
these types of chromophore39-43 (Table 1 and Figure 3).
UPy-Pery-UPy was synthesized in analogy to the corre-
sponding mono-UPy derivative25 by reaction of an diaryloxy
substituted perylene anhydride36 with excess 1,4-diamino-
(37) It should be noted that, due to solubility reasons, the UPy groups in
UPy-Pery-UPy were substituted with a racemic ethylpentyl chain,
whereas straight tridecyl chains were used for UPy-OF3-UPy and
UPy-OPV5-UPy. No racemic substituents were used in these latter
two cases to exclude the formation of diastereomers.
(38) Wu¨rthner, F.; Stepanenko, V.; Chen, Z.; Saha-Mo¨ller, C. R.; Kocher,
N.; Stalke, D. J. Org. Chem. 2004, 69, 7933.
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Scheme 3. Synthesis of UPy-Pery-UPya
a (1) 1,4-Diamino-2,6-di-tert-butylbenzene, Zn acetate, DMA, 190 °C, 3 h, 60%. (2) CDI activated ethylhexylisocytosine, CHCl3, reflux, 18 h, 97%.
Table 1. Optical Properties of UPy-OF3-UPy, UPy-OPV5-UPy,
and UPy-Pery-UPy in Diluted Chloroform Solution
beneficial fluorescence properties of the chromophores were
preserved upon functionalization with the hydrogen bonding end
groups.
a
Abs
PL
λmax
(nm)
log(ꢀ)
(M-1 cm-1
λmax
(nm)
φPL
(%)
)
Energy Transfer Studies in Solution. Before we started with
energy transfer between the di-UPy chromophores, we first
determined the UPy dimerization constant for our type of
systems in an energy transfer dilution experiment. To simplify
the calculations, the monofunctional UPy analogues UPy-OF3
and UPy-OPV425 (Figure 4a) were used instead of UPy-OF3-
UPy and UPy-OPV5-UPy, because they formed exclusively
dimers. As expected, quenching of the blue oligofluorene
emission was observed already after addition of small amounts
of acceptor, indicating the close spatial proximity of UPy-OPV4
and UPy-OF3 due to the formation of heterodimers23 (Figure
UPy-OF3-UPy
UPy-OPV5-UPy
UPy-Pery-UPy
364
445
513, 550
4.91
4.90
4.57, 4.70
407, 429
520, 554
584, 620
50
45
55
a Reference compounds: 9,10-diphenylanthracene in cyclohexane (for
UPy-OF3-UPy), N,N′-bis(pentylhexyl) perylene bisimide in dichloro-
methane (for UPy-OPV5-UPy) and Rhodamine 101 in ethanol (for
UPy-Pery-UPy).
Compared to unfunctionalized trifluorenes,44,45 the spectrum of
UPy-OF3-UPy was red-shifted by ca. 15 nm, presumably due
to the electron-donating influence of the UPy group, which is
directly conjugated to the aromatic core.25 Upon excitation at
their maximum absorption energies all three compounds emitted
visible light with fluorescence quantum yields of ∼50% (Table
1). All photoluminescence spectra consisted of two maxima and
a shoulder at the low energy end (Figure 3). In accordance with
the changes in the optical band gaps, the emission colors shifted
from blue for UPy-OF3-UPy to green for UPy-OPV5-UPy and
to red for UPy-Pery-UPy. As in the case of the absorption
spectra, the observed photoluminescence features were typical
for the respective conjugated systems,39-43 with a 15 nm red
shift for UPy-OF3-UPy.44,45 These results indicated that the
(39) Stalmach, U.; Kolshorn, H.; Brehm, I.; Meier, H. Liebigs Ann. Org.
Bioorg. Chem. 1996, 9, 1449.
(40) Peeters, E.; Marcos Ramos, A.; Meskers, S. C. J.; Janssen, R. A. J.
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Zhu, D. J. Org. Chem. 2004, 69, 9049.
(44) Geng, Y.; Trajkovska, A.; Katsis, D.; Ou, J. J.; Culligan, S. W.; Chen,
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2004, 10, 2681.
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Figure 3. Normalized absorption (a) and fluorescence (b) spectra of diluted (µM regime) solutions of UPy-OF3-UPy (blue), UPy-OPV5-UPy (green), and
UPy-Pery-UPy (red) in chloroform.
Figure 4. (a) Structural formulas of UPy-OF3 and UPy-OPV4. (b) Fluorescence titration experiment between UPy-OF3 and UPy-OPV4 with constant
donor concentration (8.8 × 10-6 M) in CHCl3. Arrows indicate the spectral changes upon increasing the concentration of UPy-OPV4. Excitation wavelength
λexc ) 359 nm. The inset shows the corresponding Stern-Volmer plot. (c) Concentration dependence of acceptor emission in an equimolar mixture of
UPy-OF3 and UPy-OPV4 (9) and UPy-OF3 and an unfunctionalised OPV4 (0), see Supporting Information. Excitation wavelength λexc ) 359 nm.
Figure 5. (a) Titration experiment in chloroform solution (blue: pure UPy-OF3-UPy, green: successive addition of UPy-OPV5-UPy, red: further addition
of UPy-Pery-UPy). The solid arrows indicate spectral changes upon addition of UPy-OPV5-UPy to UPy-OF3-UPy, the dotted arrow upon addition of
UPy-Pery-UPy to a mixture of UPy-OF3-UPy and UPy-OPV5-UPy. The inset shows the spectrum corresponding to a ratio of 59:33:8. c(UPy-OF3-UPy)
) 1.6 × 10-6 M. Excitation wavelength λexc ) 364 nm. (b) Solutions of pure di-UPy chromophores and a white emitting mixture in chloroform under UV
irradiation (λexc ) 365 nm).
4b). The dimerization constant was determined by dilution of
an equimolar mixture of UPy-OF3 and UPy-OPV4 and
recording its fluorescence spectra at various concentrations
(Figure 4c). A decrease in acceptor emission and thus in energy
transfer efficiency was observed upon dilution below 10-7 M.
The contribution of direct excitation could be neglected, since
a control experiment with an unsubstituted OPV showed hardly
any OPV fluorescence. Assuming equal probability for the
formation of UPy dimers between identical and different
chromophores (corresponding to 50% of heterodimers and 25%
of each type of homodimer), we could extract a dimerization
constant of 2 × 107 M-1 from the data. This value is in good
agreement with results reported for other UPy systems in
chloroform28 and high enough to ensure that, at the concentra-
tions used in the following experiments (10-6-10-5 M), the
bisUPy compounds were present exclusively as hydrogen
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investigate if the virtual molecular weight of UPy-OF3-UPy
was affected by the incorporation of the energy acceptors. These
measurements were carried out in the concentration range later
used for spin coating (mM regime; Vide infra) because a certain
solution viscosity was crucial to yield thin films of a typical
thickness (ca. 50 nm) required for LED applications. Due to
the limited solubility of UPy-OF3-UPy in chloroform we chose
ODCB instead, since it is a good solvent for UPy-OF3-UPy
and frequently used for spin coating (Figure 6).47 For the
viscosimetry experiments, the specific viscosities of solutions
were measured as a function of concentration. In the case of
UPy-OF3-UPy, a double logarithmic plot gave a slope of 1.34
at low and of 2.48 at high concentrations with an inflection point
at 2 mM. By contrast, UPy-Pery-UPy gave a linear dependence
with a slope of only 1.06 over the whole concentration range
investigated. Since all UPy groups were hydrogen bonded and
a slope close to 1 is indicative of noninteraction particles of
constant size,31 we concluded that UPy-Pery-UPy must be
primarily present as hydrogen bonded cycles. Based on its rigid
molecular structure in which the UPy groups are arranged in a
fixed angle of 180°, this behavior is surprising. Instead, the
formation of linear polymers seems more likely. A possible
explanation might be that the UPy groups do not dimerize in a
planar but slightly bent fashion. In the case of UPy-OF3-UPy,
in which the UPy groups are arranged at an angle smaller than
180°, most likely rings also were present below 2 mM. The
slope 1.34 at low concentrations might be explained either by
rings that aggregate further by secondary interactions other than
hydrogen bonding48 or by an increase in the cycle size with
concentration. At higher concentrations the strong increase in
viscosities is probably due to a transition to linear polymers, as
is frequently found in other di-UPy systems.30,31,46 Our results
demonstrate that the virtual molecular weights for both systems
were remarkably different, although both consisted entirely of
difunctionalized molecules. A mixture of UPy-OF3-UPy and
UPy-Pery-UPy in a 4:1 ratio exhibited viscosities intermediate
between the two pure compounds. It still changed the slope at
ca. 2 mM, but both the slopes at concentrations below and above
this threshold were lower than in the case of pure UPy-OF3-
UPy.
Figure 6. Double logarithmic plot of specific viscosity vs concentration
of UPy-OF3-UPy, UPy-Pery-UPy, and a 4: 1 mixture of both compounds
in ODCB. Numbers indicate the slopes of the linear fits.
bonded species. Based on previous data for UPy supramolecular
polymers,30,31,46 it could be anticipated that under these condi-
tions the rigid UPy-OF3-UPy and UPy-OPV5-UPy, in which
the UPy groups are arranged at an angle of less than 180°, were
present as cycles rather than linear polymers. By contrast, such
ring formation should be excluded for UPy-Pery-UPy, owing
to the strictly linear array of its UPy functionalities.
In fluorescence titration experiments of UPy-OF3-UPy with
either UPy-OPV5-UPy or UPy-Pery-UPy (Supporting Infor-
mation), quenching of the donor emission was observed when
the samples were excited at the maximum absorption wavelength
of UPy-OF3-UPy (λexc ) 364 nm). Simultaneously, the
emission of the acceptor molecules increased. Quenching of the
donor fluorescence, as expressed by the Stern-Volmer constants
(KSV), was higher when the spectral overlap (JF) between the
donor emission and the acceptor absorption was better (JF ) 2
× 1015 nm4 M-1 cm-1 and KSV ) 7.2 × 106 M-1 for UPy-
OPV5-UPy vs JF ) 5 × 1014 nm4 M-1 cm-1 and KSV ) 3.5 ×
106 M-1 for UPy-Pery-UPy as the acceptor). Both KSV values
were more than doubled compared to the case of our earlier
reported titrations of di-UPy-trifluorene with mono-UPy OPV
or PB acceptor molecules (KSV ) 2.9 × 106 M-1 and KSV
)
1.3 × 106 M-1, respectively).25 Since the chromophores in both
experiments were similar, the spectral overlap was comparable
for the mono- and di-UPy derivatives. Therefore, the higher
KSV found in the present study was due to the increased
probability for an energy acceptor to be bound to a donor as a
result of the absence of acceptor or donor homodimers.
Spin Coated Films. When spin coated from concentrated
ODCB solution (4.5-9 mM) all di-UPy compounds were found
to form smooth homogeneous thin films49 with surface rough-
nesses of 5 nm or below, as was shown by atomic force
microscopy (Supporting Information). This result proved that
the hydrogen bonded chromophores could be successfully spin
coated even at concentrations where the ring-chain equilibria
were shifted toward cycles, which might indicate that the rings
were large enough to be easily deposited in the same fashion
as a polymer. Alternatively, the rather low volatility of the
ODCB might have allowed additional polymerization of the cy-
clic oligomers during the deposition process. By contrast, the
mono-UPy compounds used to determine the UPy dimerization
Simultaneous incorporation of UPy-OPV5-UPy and UPy-
Pery-UPy into the UPy-OF3-UPy chains led to a proposed
supramolecular copolymer containing all three species even at
very low concentrations (µM regime). When UPy-Pery-UPy
was titrated to a mixture containing UPy-OF3-UPy and UPy-
OPV5-UPy at a constant ratio (64:36), both green (OPV) and
remaining blue (OF) emissions were quenched. At a certain ratio
(UPy-OF3-UPy/UPy-OPV5-UPy/UPy-Pery-UPy ) 59:33:8),
simultaneous photoluminescence of comparable intensity over
the whole visible spectrum could be achieved, leading to white
emission (Figure 5) due to partial energy transfer. The relatively
high ratio of UPy-OPV5-UPy to UPy-OF3-UPy suggested that
cascade energy transfer took place in which UPy-OPV5-UPy
also acted as an energy donor for UPy-Pery-UPy. By contrast,
white emission could not be achieved using mixtures of bare
OF, OPV, and PB chromophores that lack the UPy units.25
(47) Fluorescence energy transfer titration studies in diluted ODCB solutions
(mM regime) gave a similar Stern-Volmer constant as determined
in chloroform solution for energy transfer from UPy-OF3-UPy to
UPy-Pery-UPy (KSV ) 3.2 × 106 M-1 vs 3.5 × 106 M-1; see
Supporting Information). Since KSV reflects the dimerization constant
between the UPy units, we expected comparable values in both
solvents.
Viscosimetry. Before spin coating solutions of the di-UPy
(48) A specification of the type of secondary interaction that accounts for
the additional aggregation is not possible at the moment, but π-stacking
interactions are rather unlikely to be the case, as they should be
accompanied by characteristic shifts in both NMR and optical spectra,
which were absent in our systems.
chromophores, we conducted viscosimetric experiments to
(46) So¨ntjes, S. H. M.; Sijbesma, R. P.; van Genderen, M. H. P.; Meijer,
E. W. Macromolecules 2001, 34, 3815.
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840 J. AM. CHEM. SOC. VOL. 131, NO. 2, 2009

White-Light Emitting Supramolecular Copolymers
A R T I C L E S
Figure 7. Normalized absorption (a) and fluorescence (b) spectra of thin films of UPy-OF3-UPy (blue), UPy-OPV5-UPy (green), and UPy-Pery-UPy
(red) spin coated from ODCB solutions.
Figure 8. (a) Simultaneous photoluminescence of all three di-UPy chromophores (ratio 84:10:6) in a thin spin coated film. Excitation wavelength λexc
365 nm. (b) Thin spin coated films of pure di-UPy chromophores and a mixture under UV irradiation (365 nm). (c) AFM height (left) and phase (right)
images of thin film spin coated from a 80:10:10 mixture of the three chromophores. The z-scale corresponds to a height of 10 nm.
)
constant (Figure 4a) formed very inhomogeneous films, indicat-
ing that it is necessary to have bis-UPy chromophores.
The UV absorption and photoluminescence spectra of the
UPy-OF3-UPy spin coated films strongly resembled those in
solution (see Figure 3), and the film was blue emissive (Figure
7). Compared to their solution spectra, the fluorescence maxima
of UPy-OPV5-UPy and UPy-Pery-UPy in the films were
shifted to lower energies and exhibited a rather broad emission
band, but their emission colors were still in the appropriate color
regime (Figure 7). This set of di-UPy chromophores enabled
us to prepare RGB emissive supramolecular materials.
Energy Transfer Studies in Thin Films. To create white
emissive materials by partial energy transfer, various mixtures
of the three components in ODCB were spin coated onto quartz
substrates. AFM images of the obtained films showed very
smooth, featureless surfaces that closely resembled those of the
pure components. No indications of phase separation could be
found at various compositions, suggesting that supramolecular
statistical copolymers were present in these mixed films (Figure
8 and Supporting Information). Fluorescence quenching of the
OF emission was observed at much lower acceptor-to-donor
ratios than in dilute solution, indicating more pronounced energy
transfer in the films due to the narrower packing of the
chromophores (Supporting Information). Compared to the
photoluminescence spectra of the pure materials, the emission
maxima of both energy acceptors were shifted significantly to
the blue in the mixed films (∆λ ) 80-90 nm). The sensitized
acceptor fluorescence was reminiscent of that of hydrogen
bonded species in solution (Table 1), implying the presence of
isolated acceptors inside the donor assemblies.50 This is further
evidence for random copolymers being present in the films
instead of phase separated domains. As the photoexcitation took
place at the absorption maximum of the energy donor (375 nm),
where also both acceptor molecules absorbed, direct excitation
contributed to some extent to the observed fluorescence spectra,
as was confirmed by excitation spectroscopy (Supporting
Information). A white photoluminescent film (CIE coordinates
of 0.33 and 0.31; Figure 8) was achieved with a mixture of the
chromophores in a ratio of UPy-OF3-UPy/UPy-OPV5-UPy/
UPy-Pery-UPy ) 84:10:6.
(49) The film thickness was too small to allow infrared spectra to be
recorded, but as powder samples showed a characteristic band at ca.
1700 cm-1, the assumption was supported that in the solid state and
thus also in spin coated films hydrogen bonded UPy keto tautomers
were the predominant species.
(50) Hoeben, F. J. M.; Herz, L. M.; Daniel, C.; Jonkheijm, P.; Schenning,
A. P. H. J.; Silva, C.; Meskers, S. C. J.; Beljonne, D.; Phillips, R. T.;
Friend, R. H.; Meijer, E. W. Angew. Chem., Int. Ed. 2004, 43, 1976.
9
J. AM. CHEM. SOC. VOL. 131, NO. 2, 2009 841

A R T I C L E S
Abbel et al.
Figure 9. Device architecture of the OLEDs (a) and normalized electroluminescence spectra of UPy-OF3-UPy (b), UPy-OPV5-UPy (c), and UPy-Pery-
UPy (d).
Table 2. Summary of the Device Characteristics
low performance, although at high voltages red emission
could be detected. Current-voltage curves measured under
turn-on
voltage
(V)
max. intensity
(cd/m2)
efficiency
(cd/A)
compound
λ
max(nm)
reverse and forward bias, however, revealed that the measured
currents were almost completely due to leakage and hardly
any charge injection took place.55
UPy-OF3-UPy
UPy-OPV5-UPy
UPy-Pery-UPy
410, 435, 485
534, 553
600
32
210
-
0.025
0.051
-
7.4
4.5
-
In the electroluminescence spectra of mixed systems, even
lower acceptor concentrations than in the photoluminescence
of mixed films were sufficient to obtain simultaneous emission
(Figure 10 and Table 3). This effect could be caused by direct
injection of charges into the UPy-OPV5-UPy at the voltages
used during operation (up to 24 V). As in the case of the
fluorescence spectra, the luminescence of UPy-OPV5-UPy
embedded in a matrix of UPy-OF3-UPy was reminiscent of
nonaggregated molecules. At appropriate mixing ratios, it was
possible to obtain an electroluminescence spectrum close to the
CIE coordinates of white light. A small decrease in blue and
an increase in green emission were observed when the percent-
age of UPy-OPV5-UPy was increased, with the CIE values
changing correspondingly. This shows that partial energy
transfer can be controlled by variation of the donor to acceptor
ratios without changing film quality.
Electroluminescent Devices. The three chromophores were
employed as the active materials in layered electrolumines-
cent diodes constructed of a glass substrate, transparent ITO
electrodes, PEDOT/PSS, spin coated films of the di-UPy
compounds, and electrodes of barium (5 nm) and aluminum
(100 nm). In all cases light emission could be observed when
a sufficiently high voltage was applied (Figure 9, Table 2
and Supporting Information), which proves that the presence
of UPy groups in these LEDs is not detrimental for their
performance. With the highest intensity and efficiency, the
lowest turn-on voltage, and yellow-green electroluminescence
closely resembling those in its PL spectrum, UPy-OPV5-
UPy exceeded the device characteristics of the other two
compounds. UPy-OF3-UPy gave somewhat less propitious
values, and aside from its typical blue emission, also a
significant contribution in the green region was found. This
low energy emission is diagnostic for fluorenone defects.51-53
As the material had been purified extensively by recycling
GPC, any chemical contamination should have been re-
moved,33 which was also supported by the absence of green
emission in the PL spectrum of UPy-OF3-UPy. Therefore
we suppose that the degradation took place during device
operation.54 The devices containing UPy-Pery-UPy had a
Conclusion
Three chromophores with emission spectra in the RGB
wavelength region have been functionalized with self-comple-
mentary hydrogen bonding UPy groups at both ends, providing
us with a novel toolbox of compounds for an easy modular
approach toward supramolecular white emissive materials.
Under appropriate conditions, these materials could be purified
to a high extent by recycling GPC like small organic molecules.
Depending on the concentration, hydrogen bonded cycles and
linear polymers were observed in solution. Mixtures of the three
chromophores gave white emissive supramolecular random
copolymers without the formation of unwanted acceptor dimers.
(51) Gamerith, S.; Gadermaier, C.; Scherf, U.; List, E. J. W. Phys. Status
Solidi A 2004, 201, 1132.
(52) Montilla, F.; Mallavia, R. AdV. Funct. Mater. 2007, 17, 71.
(53) List, E. J. W.; Guentner, R.; Scanducci de Freitas, P.; Scherf, U. AdV.
Mater. 2002, 14, 374.
(55) Cyclic voltammetry in ODCB gave an HOMO energy of-6.2 eV (see
Supporting Information), which is well below the work function of
the ITO electrode (-5.2 eV) and therefore did not allow the injection
of holes into the organic layer.
(54) Janssen, F. J. J.; van Ijzendoorn, L. J.; Schoo, H. F. M.; Sturm, J. M.;
Andersson, G. G.; Dernier van der Gon, A. W.; Brongersma, H. H.;
de Voigt, M. J. A. Synth. Met. 2002, 131, 167.
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842 J. AM. CHEM. SOC. VOL. 131, NO. 2, 2009

White-Light Emitting Supramolecular Copolymers
A R T I C L E S
Figure 10. (a) Normalized electroluminescence spectra of mixtures of UPy-OF3-UPy, UPy-OPV5-UPy, and UPy-Pery-UPy at 24 V. The arrows indicate
spectral changes upon decreasing percentrage of UPy-OF3-UPy. (b) Position of the EL and PL spectra in the CIE color space.
Table 3. CIE Values of the Device Emission Spectra for Mixed
OLEDs of Various Compositions
mixtures. We have demonstrated the proof of principle that
supramolecular polymers based on hydrogen bonded monodis-
perse chromophores can be applied successfully in organic light
emitting diodes. In general, our modular approach is a promising
strategy to create multicomponent systems that can be used as
functional optoelectronic materials.
UPy-OF3-UPy (mol %)
UPy-OPV5-UPy (mol %)
UPy-Pery-UPy (mol %)
CIE coordinates (x, y)
98.5
1.0
0.5
0.31, 0.45
97.0
2.0
1.0
0.28, 0.44
93.0
5.0
2.0
0.26, 0.41
Acknowledgment. The authors are grateful to Joost van Dongen
and Ralf Bovee for their support with recycling and analytical GPC,
to Dr. Xianwen Lou and Henk Eding for analytical measurements,
and to Dr. Tom de Greef for fruitful discussions.
Partial energy transfer was observed even in diluted solutions
(µM regime) due to the high strength of the quadruple hydrogen
bonding motif. Our modular approach allowed emission color
tuning by simple mixing instead of labor-intensive synthesis of
new materials. Binary and ternary mixtures exhibited high
solution viscosities, which enabled us to apply spin coating to
obtain homogeneous smooth films without phase separation. At
appropriate ratios, partial energy transfer was observed, yielding
white emissive films. Finally, these materials could be applied
in solution processed OLEDs both as pure compounds and as
Supporting Information Available: Synthetic procedures,
experimental details and supporting figures that complement the
article. This material is available free of charge via the Internet
at http://pubs.acs.org.
JA807996Y
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