Attaching perylene dyes to polyfluorene: Three simple, efficient methods for facile color tuning of light-emitting polymers

Article abstract of DOI:10.1021/ja0205784

The emission color of fluorene-based polymers can be facilely tuned across the whole visible spectrum by copolymerization with perylene dyes. Methods are demonstrated for incorporation of the dyes in the polymer mainchain, at the chain termini, or as side

Full text of DOI:10.1021/ja0205784

Published on Web 12/17/2002
Attaching Perylene Dyes to Polyfluorene: Three Simple,
Efficient Methods for Facile Color Tuning of Light-Emitting
Polymers
Christophe Ego, Dirk Marsitzky, Stefan Becker, Jingying Zhang,
Andrew C. Grimsdale, Klaus Mu¨llen,* J. Devin MacKenzie, Carlos Silva, and
Richard H. Friend
Contribution from the Max-Planck-Institut fu¨r Polymerforschung, Ackermannweg 10,
55128 Mainz, Germany and CaVendish Laboratory, UniVersity of Cambridge, Madingley Road,
Cambridge CB3 0HE, United Kingdom
Received April 22, 2002; E-mail: muellen@mpip-mainz.mpg.de
Abstract: The emission color of fluorene-based polymers can be facilely tuned across the whole visible
spectrum by copolymerization with perylene dyes. Methods are demonstrated for incorporation of the dyes
in the polymer mainchain, at the chain termini, or as side chains. Efficient energy transfer causes the
emission to come solely from the dye units. Efficient LEDs have been made from the copolymers with
dyes in the mainchain.
Introduction
blending of red-emitting tetraphenylporphyrin into a blue-
emitting polyfluorene has been used to obtain a red-emitting
Polymer light-emitting diodes (PLEDs) have been the subject
of intense academic and industrial research interest since their
discovery in 19891 and are now emerging as commercial
products. The main advantages of electroluminescent conjugated
polymers as compared with inorganic or molecular organic
materials for LEDs are lower production costs, high flexibility,
readier processability, the possibility of uniformly covering large
areas by inexpensive solution processing techniques such as spin
coating, and the many ways to fine-tune their optical and
electrical properties by varying the structure.2 In this paper, we
present three simple new methods for tuning the emission color
of light-emitting polymers across the entire visible spectrum.
To achieve a full color flat-panel display, red, green, and blue
light-emitting polymers with narrow emitting spectra are
required. Further material requirements are good processability,
high luminescence efficiencies with low turn on and operating
voltages, and good chemical and electrical stability as well as
photostability. Color tuning conjugated polymers may be
achieved by substitution (e.g., alkyl or alkoxy substitution of
PPV changes the emission color from green to red-orange2) by
controlling the effective conjugation length3 or by blending with
another emissive material. If two materials with different band
gaps (and hence different emission colors) are mixed together,
then energy transfer from the higher to the lower band gap
material may occur leading to emission solely or predominantly
from the latter. One method for color tuning is thus to blend a
dye chromophore into a polymer matrix; for example, the
LED.4
Though such blending has the advantage of being easy to do
and enables one to take advantage of the wide range of
commercially available dyes, blends of polymers and dyes have
a tendency to show phase separation over time, which leads to
instability in the device’s performance. Our new solution to this
problem is to covalently attach the dye to the polymer. The
significant advantages that this approach has compared with
blending are first, that migration of the dye chromophore inside
the polymer matrix, which may lead to aggregation and poor
device performance, is prevented; and second, that efficient
energy transfer to the dye chromophore and the confinement
of the singlet excitons in the chromophore therein is facilitated.
We have now achieved effective tuning of the emission color
over the whole visible region, by attachment of perylene dyes
to polyfluorene chains either as (i) comonomers in the main
chain, (ii) as endcapping groups at the chain termini, or (iii) as
pendant side groups, by simple high yielding syntheses. These
polymers are suitable individually to act as efficient red, green,
or blue emitters for full color displays or might be blended to
produce other emission colors including pure white light.
Polyfluorenes were chosen as the polymer backbone, owing
to their large band gaps, high photoluminescence quantum
yields, excellent chemical and thermal stability as well as
photostability, good solubility and film-forming properties, and
the ready availability from high yielding synthetic routes of well-
defined high molar-mass polymers.5 Perylene dyes were chosen
as the low band gap chromophores because of their excellent
light fastness, high chemical stability, high photoluminescence
(1) Burroughes, J. H.; Bradley, D. D. C.; Brown, A. R.; Marks, R. N.; Mackay,
K.; Friend, R. H.; Burn, P. L.; Holmes, A. B. Nature 1990, 347, 539-541.
(2) Kraft, A.; Grimsdale, A. C.; Holmes, A. B. Angew. Chem., Int. Ed. 1998,
37, 402-428.
(3) Burn, P. L.; Holmes, A. B.; Kraft, A.; Bradley, D. D. C.; Brown, A. R.;
Friend, R. H.; Gymer, R. W. Nature 1992, 356, 47-49.
(4) Virgili, T.; Lidzey, D. G.; Bradley, D. D. C. AdV. Mater. 2000, 10, 58-
62.
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J. AM. CHEM. SOC. 2003, 125, 437-443
437

A R T I C L E S
Ego et al.
Scheme 1 . Synthesis of Dyes 4 and 5
Scheme 2 . Copolymerization of Perylene Dyes with
Dibromodioctylfluorene
comonomers in the chain, is readily realized by the statistical
copolymerization of dibromodialkylfluorene 1 with suitable
bisbrominated perylenes (Scheme 2) by nickel(0) mediated
polycoupling (Yamamoto polymerization7). The comonomers
chosen were the dibromoperylene dyes 2-5 (Figure 1) with
emission maxima at 525 (green), 540 (yellow), 584 (orange),
and 626 nm (red-orange). The syntheses of 28 and 39 have
previously been published. It should be noted that dyes 2 and
3 consist of inseparable 1:1 mixtures of the 3,9- and 3,10-
dibromo isomers, but this has no effect on their optical
properties. The dyes 4 and 5 were prepared in high yield, as
shown in Scheme 1. Condensations of 1,6,7,12-tetrachloro- and
1,7-dibromo-perylene tetracarboxylic acid anhydride, respec-
tively, with 4-bromoaniline in refluxing butyric or propionic
acid gave the diimides, which then underwent nucleophilic
susbtitution of the halides in the bay positions with 4-tert-
octylphenoxide to produce 4 and 5 in overall yields of 60%
and 86%, respectively. The copolymers with 2 and 3 retain
unbroken conjugation, while the imide units in 4 and 5 interrupt
conjugation in their copolymers which might be expected to
affect their electrical and optical properties.
quantum yield, and the large range of colors accessible by
variation of the substitution pattern on the perylene core through
high yielding synthetic routes.6
We thus assembled a “molecular toolbox” of fluorene and
perylene monomers which enabled us to prepare copolymers
with emission colors covering the visible spectrum in high
yielding onepot syntheses.
Polymerization of a mixture of the appropriate dibromo-
perylene dye (1-5 wt %) and 2,7-dibromo-9,9-dioctylfluorene
1a with nickel(0) in DMF-toluene for 24 h, followed by
reaction with an excess of bromobenzene to remove any residual
terminal bromine functionality (endcapping), was then
performed to give the copolymers 6-9 (Figure 2) in 62-90%
yield, as illustrated in Scheme 2 for the preparation of 7. The
removal of the bromine endgroups is important, as it has been
shown that such groups are detrimental to LED device perfor-
mance.10
Results and Discussion
The preparation of the first type of the perylene-fluorene
copolymer described previously, that is, with the perylenes as
(5) (a) Bernius, M. T.; Inbasekaran, M.; O′Brien, J.; Wu, W. AdV. Mater. 2000,
12, 1737. (b) Leclerc, M. J. Polym. Sci., Part A: Polym. Chem. 2001, 39,
2867. (c) Neher, D. Macromol. Rapid Commun. 2001, 22, 1365. (d) Scherf,
U.; List, E. W. J. AdV. Mater. 2002, 14, 477.
(6) (a) Mu¨llen, K.; Quante, H.; Benfaremo, N. Polymeric Materials Encyclo-
pedia; Salamone, J. C., Ed.; CRC Press: Boca Raton, FL, 1996; p 4999.
(b) Quante, H.; Geerts, Y.; Mu¨llen, K. Chem. Mater. 1997, 9, 495.
(7) Yamamoto, T. Prog. Polym. Sci. 1992, 17, 1153-1205.
(8) Quante, H.; Schlichting, P.; Rohr, U.; Geerts, Y.; Mu¨llen, K. Macromol.
Chem. Phys. 1996, 197, 4029-4044.
(9) Schlichting, P.; Rohr, U.; Mu¨llen, K. Liebigs Ann./Recl. 1997, 395-407.
9
438 J. AM. CHEM. SOC. VOL. 125, NO. 2, 2003

Facile Color Tuning of Light-Emitting Polymers
A R T I C L E S
Figure 3. Photoluminescence spectra for 9 in thin film (solid cyan line)
and solution (dashed cyan line) and the solid-state absorption spectrum of
the perylene 5 (red line).
pronounced as in the case of the copolymers 6 or 7, which have
blue-shifted absorptions. In the case of 7, the perylene absorption
near 470 nm overlaps well with the PF emission and the
optically stimulated color tuning effect is nearly complete as
the integrated perylene emission intensity is more than 2 orders
of magnitude higher than the remaining PF emission. This is
consistent with the most general description of resonance energy
transfer, where the rate constant of exciton transfer from the
donor fluorophore to the acceptor chromophore, kDA, is given
in units of inverse picoseconds by11
Figure 1. Molecular toolbox for color tuning.
kDA ) 1.18VDA ∞ fD(Vj)AA(Vj)dVj
(1)
2
∫
0
within a Fermi golden rule approximation. Here, fD and AA are
the fluorescence and absorption spectra of the donor and
acceptor, respectively, normalized such that the integral over
all photon energies is unity. VDA2 is the energy transfer electronic
coupling matrix element between donor and acceptor. Com-
parison of the degree of spectral overlap between the polyfluo-
rene donor and dyes 2 and 4 showed that the degree of spectral
overlap is higher for 2 than for 4. The spectral overlap derived
from eq 1 is 2.1 × 10-4 cm-1 for dye 2 but is only 6.1 × 10-5
cm-1 for dye 4. If the electronic couplings between PF and the
two dyes are assumed to be similar, the rate constant for energy
transfer to dye 2 is thus an order of magnitude higher than that
to dye 4. To explore this issue further, we also made a
copolymer 10 containing both 5 and the green-emitting 2 (3%
of each) to see if transfer via the green-emitting segments would
improve the overall energy transfer to the red-emitting chro-
mophores.
Figure 2. Perylene dye-fluorene mainchain copolymers.
The physical characteristics of the copolymers 6-10 and of
a sample of the polyfluorene homopolymer 11 prepared using
the same reaction conditions are given in Table 1. As can be
seen, the copolymers all show high thermal stability with
decomposition temperatures above 400 °C by TGA, comparable
with the case of the homopolymer. Their molar masses as
determined by GPC against polystyrene standards are generally
slightly lower than those for the homopolymer with number
averaged masses (Mn) of 30-140 000 corresponding to degrees
To test the effect of varying the amount of dye on the optical
and other properties, copolymer 9 was prepared using 1% and
5% of dye. In the case of the longer wavelength emitting
perylenes such as 5, the overlap of the PF emission and the
perylene absorption spectra, as shown in Figure 3, is relatively
poor with the absorption of the perylene reaching a maximum
at 590 nm, while the emission of the PF segments is centered
near 465 nm. The solid-state optical color tuning, though still
much stronger than that observed in solution, is not as
(11) May, V.; Ku¨hn, O. Charge and Energy Transfer Dynamics in Molecular
Systems; Wiley: Berlin, 2000.
(10) Lee, J.-I.; Klaerner, G.; Miller, R. D. Chem. Mater. 1999, 11, 1083.
9
J. AM. CHEM. SOC. VOL. 125, NO. 2, 2003 439

A R T I C L E S
Ego et al.
Table 1. Physical Data for the Polymers 6-11, 14, and 20
a
b
polymer (% dye)
M a (g/mol)
D
TGA (°C)
PL efficiencyc (%)
n
6 (3%)
7 (5%)
8 (5%)
9 (1%)
9 (5%)
10 (3% + 3%)
11
47 930
32 300
63 510
142 500
75 210
46 650
84 500
20 730
10 430
4.1
4.9
3.6
3.8
6.3
7.7
2.8
2.1
2.6
448
443
447
445
445
451
463
453
454
51
40
33
42
38
7
56
>60
14
20
aMolar mass (Mn) and polydispersity (D) were determined by GPC in
THF against polystyrene standards with UV detection set between 360 and
390 nm. b TGA data show the temperature for onset of the primary mass
loss. c PL efficiencies were determined for thin films spun upon quartz by
the method of de Mello et al.13
Figure 4. Fluorescence of films of 9 (left) and 11 (right) under UV
irradiation.
of polymerization of about 90-400 units and polydispersities
(D) of usually around 4. The polydispersities of the copolymers
are higher than those for the homopolymer (2.8), which is most
readily explained by the reactivity of the bromodyes being lower
than that for the bromofluorene, leading to the slower addition
of fluorene to a terminal dye unit and greater probability of a
chain terminating debromination occurring at such a site.
Slightly different Mn and D values were determined, depending
upon whether the UV detector in the GPC was set in the range
for maximum absorption of the fluorene homopolymer (360-
390 nm) or of the attached dye. The values in Table 1 are for
detection at the former. It should be noted that the molar masses
obtained for rigid rod polymers from GPC against polystyrene
standards are known to be frequently too high, with an
overestimation factor of 2.7, having been determined for
polyfluorenes by comparison of GPC data with more accurate
masses determined by light scattering methods.12 The copolymer
9 had a higher Mn and lower polydispersity when made with
1% rather than with 5% of the dye 5, suggesting that incorpora-
tion of larger amounts of dye might be detrimental to the
polymer properties. The copolymers all show excellent solubility
in common organic solvents and good film-forming properties.
The absorption and emission spectra of these copolymers in
solution show predominantly absorption by and emission from
the fluorene units, as might be expected for materials containing
only low amounts of the dye chromophores in the absence of
energy transfer from the fluorenes to the dyes. However, in the
solid state, efficient energy transfer to the chromophores occurs
so that the emission is predominantly from the latter, though
significant emission is still seen from the fluorene backbone,
as illustrated for copolymer 9 in Figure 3.
The efficient intermolecular solid-state energy transfer process
in the copolymer can be seen in the dramatic enhancement of
the 620 nm perylene emission in thin film (solid cyan line) as
compared with the solution spectrum (dashed cyan line). The
red line shows the solid-state absorption of the perylene which
only partially overlaps the PF emission. Figure 4 shows the
fluorescence of films of the copolymer 9 and the homopolymer
11, illustrating how dramatic is the difference in emission color.
The absolute photoluminescence efficiencies (Table 1) were
measured from thin films spun on quartz substrates following
the method of de Mello et al.13 The PL efficiencies of the
copolymers are lower than those for the homopolymer 11, with
the efficiencies being lowest for the copolymers 8-10 where
Table 2. LED Performance of Polymers 6-9 in Devices
ITO/PEDOT-PSS/Polymer/Ca-Al
external EL
efficiency [%]
luminescence
[Cd/A]
copolymer
onset voltage [V]
6
7
8
9
0.6
0.2
0.5
0.3
12
11
8
0.9
0.4
1.6
1.4
15
the conjugation in the main chain is interrupted. That the PL
efficiency is lowest (33%) for the copolymer 8 may be due to
the particularly strong tendency of the chromophore 4 to
aggregate. The energy transfer in copolymer 9 appears to be as
efficient with 1% of dye as with 5%, showing that large amounts
of chromophore are unnecessary for efficient color tuning. The
higher PL quantum efficiency in the copolymer with 1% of dye
may be due to a lower degree of aggregation of the perylene
units in this polymer. The very low (7%) PL efficiency for the
copolymer 10 is probably due to aggregation or a similar
fluorescence quenching process, as a stepwise energy transfer
from fluorene to 2 to 5 should not be less efficient than a direct
transfer to 5.
The polymers 6-9 have been tested as emissive materials in
LEDs and were found to exhibit good device performance. Data
from the nonoptimized LEDs using a poly(styrene sulfonate)
doped poly(3,4-ethylenedioxythiophene) (PEDOT/PSS) hole-
transporting buffer layer and a calcium-aluminum cathode are
shown in Table 2.
The EL spectra show emission only from the dye chro-
mophores (Figure 5). This is presumably because the smaller
band gaps of the perylene units cause them to act as traps for
the injected charges, leading to effective exciton confinement,
which acts to significantly increase the dye to fluorene emission
ratios compared with those seen for optical excitation. This is
supported by the high electron affinities that have been observed
for similar perylenes containing electron-withdrawing diimide
bridges.14 The EL spectra of 6, 7, and 9 closely resemble their
PL spectra with the omission of the fluorene emission, but the
EL emission from 8 is red shifted and broader. Thus, 6 showed
bright green EL with a maximum at 520 nm, (CIE x,y
coordinates of 0.362,0.555), polymer 7 produced yellow emis-
sion with a maximum at 558 nm (x,y ) 0.414,0.519), and 9
gave red-orange EL peaking at 600 nm (x,y ) 0.590,0.365).
The emission from devices using the copolymers 9 and 7 and
(12) Grell, M.; Bradley, D. D. C.; Long, X.; Chamberlain, T.; Inbasekaran, M.;
Woo, E. P.; Soliman, M. Acta Polym. 1998, 49, 439-444.
(13) de Mello, J. C.; Wittman, H. F.; Friend, R. H. AdV. Mater. 1997, 9, 230.
(14) Schmidt-Mende, L.; Fechtenko¨tter, A.; Mu¨llen, K.; Moons, E.; Friend, R.
H.; MacKenzie, J. D. Science 2001, 293, 1119.
9
440 J. AM. CHEM. SOC. VOL. 125, NO. 2, 2003

Facile Color Tuning of Light-Emitting Polymers
A R T I C L E S
Figure 5. EL spectra of the copolymers 6-9 in devices ITO/PEDOT-
PSS/polymer/Ca-Al.
Figure 7. Time-resolved normalized EL spectra of a blend of dye 4 (3 wt
%) in polymer 11.
devices show stable emission with no significant change in the
emission spectra over lifetimes of >103 s. By contrast, similar
devices using a blend of perylene dyes and polyfluorenes
showed rapid degradation with the emission intensity dropping
to 50% of the original luminance after only 60 s, accompanied
by a decrease of an order of magnitude in luminescence
efficiency, a change in emission spectra from the perylene
toward the fluorene, and a rapid increase in voltage demand,
which is explicable in terms of the dye aggregating and forming
nonradiative trap sites. As shown in Figure 7, within 15 min
the emission from a blend of dye 4 in poly(dioctylfluorene)
becomes dominated by the polyfluorene, closely resembling the
solution spectrum of the copolymer 8.
Given that because of the low concentration of dyes these
copolymers should be highly compatible with each other and
the homopolymer for blending purposes, it should be possible
by blending suitable combinations of these materials to obtain
efficient emission with colors covering much of the visible
spectrum and even potentially white emission. White emission
has been previously achieved by us by blending 12 with a blue-
emitting polymer.18
The second type of polymer is made by the same synthetic
procedure as that described previously but using a mono-
brominated perylene derivative (Scheme 3). As addition of this
to the growing polymer chain acts as a terminating (endcapping)
step, the polymers so obtained contain perylene units only at
the end of the polymer. As the dyes are only found at the end
of the chains, these polymers are more suitable for studying
the physics of energy transfer between the fluorene and the dye
units than the polymers described in the previous section, in
which the dyes are randomly distributed throughout the material.
This method also permits control of the Mn and hence the
physical properties, including film-forming abilities, of the
resulting polymers by varying the ratio of monomer to endcap-
per, as has been demonstrated by Scherf et al. for polyfluorenes
with triarylamine endcappers.19 As the chain can also be
Figure 6. Electroluminescence from devices using polymers 9 (left), 7
(center), and 11 (right).
the homopolymer 11 is shown in Figure 6. The emission from
8 was deep red (CIE x,y ) 0.636,0.338) with a maximum at
675 nm, which is over 30 nm red shifted from the PL maximum
(643 nm). Somewhat surprisingly given its relatively low PL
efficiency, the copolymer 8 showed relatively high EL efficiency
and luminance and the lowest onset potential of the copolymers.
Presumably because the difference in electron affinities between
the dye and the fluorene is greatest for the red emitting perylene
(∼0.4 eV), the charge trapping is particularly efficient in this
case.
The enhancement in color tuning is shown clearly by the
comparison of the emissions from 9 under optical (Figure 4)
and electrical stimulation (Figure 6). The external EL efficien-
cies of 0.2-0.6% for nonoptimized devices show that these
copolymers have considerable potential as emissive materials
for full-color displays. The maximum EL efficiencies for the
LEDs using 6 (0.9 Cd/A) and 8 (1.6 Cd/A) are higher than those
seen for green-emitting PPV derivatives under similar condi-
tions15,16 and are not greatly inferior to the efficiencies (2-7
Cd/A) of optimized blue- or green-emitting polyfluorene devices
using similar electrode materials.17 The efficiencies for the red-
orange-emitting polymers 8 and 9 are an order of magnitude
higher than those for the red-orange-emitting copolymer 12
made by the Hagihara polycoupling of 3 with a diethynylben-
zene8 which has a maximum efficiency of only 0.02%.18a The
(15) Kim, J. S.; Granstrom, M.; Johansson, N.; Salaneck, W. R.; Friend, R. H.;
Daik, R.; Feast, W. J.; Cacialli, F. J. Appl. Phys. 1998, 84, 6859-6870.
(16) Brown, T. M.; Kim, J. S.; Friend, R. H.; Cacialli, F.; Daik, R.; Feast, W.
J. Appl. Phys. Lett. 1999, 75, 1679-1681.
(17) (a) Grice, A. W.; Bradley, D. D. C.; Bernius, M. T.; Inbasekaran, M.; Wu,
W. W.; Woo, E. P. Appl. Phys. Lett. 1998, 73, 629. (b) Kim, J. S.; Friend,
R. H.; Cacialli, F. Appl. Phys. Lett. 1999, 74, 3084-3086. (c) Ho, P. K.
H.; Thomas, D. S.; Friend, R. H.; Tessler, N. Science 1999, 285, 233-
236. (d) Gross, M.; Mu¨ller, D. C.; Nothofer, H.-G.; Scherf, U.; Neher, D.;
Meerholz, K. Nature 2000, 405, 661.
(18) (a) Tasch, S.; List, E. W. J.; Hochfilzer, C.; Leising, G.; Schlichting, P.;
Rohr, U.; Geerts, Y.; Scherf, U.; Mu¨llen, K. Phys. ReV. B: Condens. Matter
1997, 56, 4479. (b) List, E. W. J.; Tasch, S.; Hochfilzer, C.; Leising, G.;
Schlichting, P.; Rohr, U.; Geerts, Y.; Scherf, U.; Mu¨llen, K. Opt. Mater.
1998, 9, 183.
(19) Miteva, T.; Meisel, A.; Knoll, W.; Nothofer, H. G.; Scherf, U.; Mu¨ller, D.
C.; Merrholz, K.; Yasuda, A.; Neher, D. AdV. Mater. 2001, 13, 565-570.
9
J. AM. CHEM. SOC. VOL. 125, NO. 2, 2003 441

A R T I C L E S
Ego et al.
Scheme 3 . Synthesis of a Perylene-Endcapped Polyfluorene
Scheme 4 . Synthesis of Copolymer 20 with Pendant Perylene Dye
Groups
terminated by reaction of the intermediate aryl-nickel species
with a proton leading to endcapping with a hydrogen, the
concentration of the endcapping groups is slightly less than the
theoretical value from the proportion of endcapper in the reaction
mixture.19 We used a fluorene/endcapper ratio of 18:1 (5.3%
endcapper) to obtain a polymer with a Mn of 21 000 g/mol and
a polydispersity of 2.1. By comparison, Scherf19 obtained Mn
values of 48 000 (D ) 1.6) for 4% endcapper and 12 000 (D )
2.6) for 9% endcapper. The perylene derivative 1320 used
maintains the conjugation with the polyfluorene chains, and so,
the copolymer 14 exhibits an excellent PL quantum efficiency
(>60%) in the solid state. As with the copolymers described
previously, energy transfer is not seen in solution but is almost
complete in the solid state leading to a narrow red emission
with a maximum at 613 nm. Because of their possessing only
one or two chromophores in a well-defined spatial relationship,
these materials are suitable for studying the process of energy
transfer between the chromophores by single molecular spec-
troscopy techniques. Preliminary investigations21 have shown
that it is possible to distinguish between individual polymer
chains bearing one or two chromophores, from which we
estimate that approximately 40% of the chains in the sample
which bear chromophores bear two dye units, with the remainder
having only one chromophore, which is consistent with the
observed MALDI-TOF mass spectrum. Studies are also under-
way into the process of energy transfer between the fluorene
and the dye units.
monooctylfluorenes, which were separated by column chroma-
tography to give the monooctylfluorene 17 in 24% yield. To
account for this product mixture, we postulate that alkylation
of the fluorenyl anion and deprotonation of 17 by it to give a
monooctylfluorenyl anion, which is then alkylated to give the
dioctyl 1a, occur equally readily. The desired perylene contain-
ing monomer 18 was then obtained in 38% yield by alkylation
of 17 with 16 using LDA in THF as base. The low yield is due
to the alkylating agent 15 undergoing competing side reactions
to give a mixture of byproducts, the main component of which
was identified by the peak at m/z 988.5 in the mass spectrum
as the 1,2-bisperylenylethene 19, which can be explained as
being formed by the deprotonation of 16 followed by alkylation
with a second molecule of 16. Copolymerization of 17 with
2,7-dibromo-9,9-dioctylfluorene 1a under Yamamoto conditions
gave the desired polymer 20, containing 33 mol % of fluorene
units with pendant dyes. Whereas the previous types of polymer
with the perylenes attached to the mainchain, either internally
or at the ends, show energy transfer only in the solid state,
efficient energy transfer to the pendant groups is clearly apparent
in the solution PL spectrum of 20 (Figure 8). Even when the
excitation is performed at 370 nm (where essentially only the
fluorene absorbs), the emission is almost entirely from the
perylene. The emission spectra is slightly different in solution
and in film. In solution, the emission maximum is at 561 nm
The third type of perylene containing polymer we have
synthesized has the perylenes as pendant groups attached to the
9-position of some of the fluorene units. This polymer design
has the advantage of permitting the incorporation of a high
concentration of perylene dye without affecting the electronic
properties of the polyfluorene backbone. The synthesis is shown
in Scheme 4. Chloromethylation of a perylene monoimide 15
with chloromethyl methyl ether and tin(IV) chloride at 0 °C
proceeded smoothly to give the monochloromethylated deriva-
tive 16 (61%). Alkylation of dibromofluorene with 1 equiv each
of sterically hindered lithium diisopropylamide (LDA) as base
and octyl bromide was found to give an approximately equimo-
lar mixture of starting material and the dioctyl- (1a) and
(20) Schlichting, P.; Duchscherer, B.; Seisenberger, G.; Basche, T.; Bra¨uchle,
C.; Mu¨llen, K. Chem.sEur. J. 1999, 5, 2388-2395.
(21) Ja¨ckel, F.; De Feyter, S.; Hofkens, J.; Ko¨hn, F.; Rousseau, E.; De Schryver,
F. C.; Ego, C.; Mu¨llen, K., unpublished results.
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442 J. AM. CHEM. SOC. VOL. 125, NO. 2, 2003

Facile Color Tuning of Light-Emitting Polymers
A R T I C L E S
Conclusion
We have synthesized polymers in which perylene dye
chromophores are covalently linked to polyfluorenes as statisti-
cally distributed comonomers in the polymer backbone, as
endcapping units at the chain termini, or as pendant side chains,
thus permitting tuning of the emission color through efficient
energy transfer to the chromophore units. The statistical
copolymers have been used to make efficient LEDs with
emission colors covering the whole visible spectra. The end-
capped polymers also show color tuning, with control over the
molar mass, and are suitable materials for probing the funda-
mental processes of energy transfer via single molecule spec-
troscopy. While these two types of polymer show complete
energy transfer only in the solid state, the polyfluorene contain-
ing perylene as a side chain shows complete energy transfer in
solution.
Figure 8. Absorption (blue line) and emission spectra (red line) of
copolymer 20 in solution.
Experimental Section
Thin-film LED structures comprised an O2 plasma-treated ITO-
coated glass anode, a poly(styrene sulfonate)-doped poly(3,4-ethylene-
dioxythiophene) (PEDOT/PSS) hole injection layer, a poly(fluorene-
co-perylene) emitter layer, and a Ca-Al cathode. The PEDOT/PSS
films (70 nm) were spun from a filtered aqueous solution and then
heated to 100 °C under nitrogen for 30 min. The emissive layers (∼100
nm) were spin coated from xylene solutions in a glovebox. The cathode
consisting of a 50 nm Ca electron injecting and a 150 nm Al protective
layer were deposited in a vacuum (∼5 × 10-6 mbar) with patterning
by a shadow mask. The EL testing was performed in a vacuum (∼10-1
mbar) at room temperature under slow-ramped drive conditions.
Figure 9. Absorption (blue line) and emission (red line) spectra of a spin-
cast film of copolymer 20.
Acknowledgment. Funding for this research was provided
by the European Commission (Brite-Euram BE-97-4438 Project
OSCA) and the Bundesministerium fu¨r Bildung und Forschung
(Projects 13N8165 Du¨nnsicht-Laser and 13N8215 OLED). C.E.
thanks the Fondation Universitaire David et Alice Van Buuren
for financial assistance.
with a shoulder at 599 nm, while, in the film, the emission
maximum is at 599 nm (Figure 9).
That energy transfer can also occur in solution in 20 is
unsurprising, given the high concentration of the perylene units
in the polymer. This high perylene content also means that there
exists a potential percolation pathway for charges to hop from
perylene to perylene in the solid state. We have recently shown
that perylene dyes make excellent electron acceptors for use in
photovoltaic devices.12 The copolymer 20 is thus a potentially
promising candidate for the electron accepting and transporting
component in an organic solar cell. A solar cell with an external
quantum efficiency of 7% has been made using a blend of 20
and a soluble poly(phenylene vinylene) derivative as an electron
donating polymer.22
Supporting Information Available: Full experimental details
for the synthesis of all new materials. Figure illustrating spectral
overlap of dyes 2 and 4 with polymer 11. This material is
available free of charge via the Internet at http://pubs.acs.org.
JA0205784
(22) Russell, D. M.; Arias, A. C.; Friend, R. H.; Silva, C.; Ego, C.; Grimsdale,
A. C.; Mu¨llen, K. Appl. Phys. Lett. 2002, 80, 22.
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