Semiconducting polyfluorenes with electrophosphorescent on-chain platinum-salen chromophores

Article abstract of DOI:10.1039/b501297j

The synthesis of statistical fluorene-type copolymers with on-chain Pt-salen phosphorescent units and their use in electrophosphorescent OLEDs is reported. The Royal Society of Chemistry 2005.

Full text of DOI:10.1039/b501297j

View Article Online / Journal Homepage / Table of Contents for this issue
COMMUNICATION
www.rsc.org/chemcomm | ChemComm
Semiconducting polyfluorenes with electrophosphorescent on-chain
platinum–salen chromophores{
Frank Galbrecht,a Xiao Hui Yang,b Benjamin S. Nehls,a Dieter Neher,b Tony Farrella and Ullrich Scherf*a
Received (in Cambridge, UK) 27th January 2005, Accepted 4th March 2005
First published as an Advance Article on the web 18th March 2005
DOI: 10.1039/b501297j
The synthesis of statistical fluorene-type copolymers with on-
Table 1 Reaction conditions and polymer analysis for the copoly-
merisation of 2,7-dibromo-9,9-dialkylfluorene 2 and Pt–salen mono-
mer 1
chain Pt–salen phosphorescent units and their use in electro-
phosphorescent OLEDs is reported.
a
a
Mn
Sample Solvent
Time
Mw
Mw/Mn
Recently, there has been much progress in the application of
cyclometallated complexes as emitting components in electrolumi-
nescent (EL) devices.1–3 The phosphorescent metal complexes
allow for the utilization of both singlet and triplet excitons which
are generated upon charge injection from the electrodes.
Consequently, internal quantum efficiencies approaching 100%
may be achieved. Additionally, the possibility to tune the emission
color by manipulating the ligand sphere of the metal atom is a very
attractive goal. Many examples of electrophosphorescent metal
complexes based on iridium, osmium, platinum and rare-earth
metal complexes are known.1–3 Nonetheless, there are still
relatively few examples of electrophosphorescent (co)polymers as
single-component OLED materials. Examples include semicon-
ducting polyfluorenes with side-chain or main-chain iridium
complexes,4,5 ladder poly(para-phenylene)s with electrophosphor-
escent palladium centers,6 self-assembled Schiff-base polymers7
and platinum-based side-chain copolymers.8 Transition metal (Co,
Ni, Zn) Schiff-base polymers have been prepared by oxidative
polymerisation, transesterification and condensation of salen-type
monomers but they have not been applied in OLEDs.9,10
Remarkably, whether it is due to design or serendipidity, only a
trace amount of the heavy metal atoms is necessary for efficient
triplet emission.6
3ab
3bb
PF2/6c
a
THF
12 min
357 400 169 500 2.11
65 400 24 300 2.70
3 days 22 2200 125 700 1.77
DMF–toluene 12 min
THF
b
After Soxhlet extraction with ethyl acetate. Microwave as heat
c
source (3a: T 5 115 uC, 3b: T 5 220 uC). Conventional heating at
80 uC.
A
cyclometallated Pt(II)–salen complex suitable for the
Yamamoto-type coupling procedure was prepared from the
reaction of K2PtCl4 with bis(5-chlorosalicylidene)ethylenediamine.
The chloro derivative was easily accessible from this protocol
unlike the corresponding dibromo monomer. Fortunately, chloro-
aryl monomers readily undergo homocoupling via a Yamamoto-
type procedure.13 The copolymers were synthesised using feed
ratios of ca. 0.2 mol equivalents of the Pt–salen monomer to 2,7-
dibromo-9,9-bis(alkyl)fluorene. The reaction was carried out in
THF using microwaves as the heat source within short reaction
times of ca. 12 min.13,14 A maximum temperature of 115 uC was
employed which was reached in less than 30 s. The crude polymer
was Soxhlet-extracted using ethyl acetate for two days to remove
oligomeric by-products and yielded the yellow polymers 3a and 3b
(Table 1). The actual percentage of the Pt(II) phosphor
incorporated into the main chain of 3a and 3b was calculated
from the relative 1H NMR intensities of the ethylene bridge
protons of the salen ligand at d y 3.8 ppm relative to the sum of
the aryl protons (Scheme 1).
A recent paper by Che et al. described the utilisation of vapour-
deposited Pt–salen complexes as efficient electrophosphorescent
dyes in multilayer OLED devices with a maximum luminous
efficiency of 31 Cd A21 11
. Recent work has intimated that higher
The monomeric Schiff-base Pt–salen complex 1 (Fig. 1) displays
a broad absorption maximum peaking at 436 nm and a PL
maximum at 563 nm in chloroform solution.{ The solution UV-vis
spectrum of the fluorene-based copolymer 3a shows an intense
absorption band at 383 nm due to the polyfluorene chromophore
and a low intensity absorption shoulder at ca. 450 nm due to the
incorporated Pt–salen phosphors (Fig. 2). The solution PL
spectrum is dominated by the PF emission with a maximum
at 413 nm.
OLED efficiencies can be achieved when the phosphorescent
chromophore is molecularly dispersed within the composite
material. Thus, our strategy towards main-chain electrophos-
phorescent copolymers involves the covalent incorporation of Pt–
salen phosphors into the backbone of a solution-processable
semiconducting copolymer. As proof of concept we chose to
incorporate such Pt–salen phosphors into a polyfluorene (PF)
backbone. We anticipated that a Ni(0)-mediated, aryl–aryl
coupling protocol would best fulfil the challenge of preparing
high molecular weight fluorene copolymers containing randomly
incorporated platinum complexes (Table 1).12
However, the PL spectrum of copolymer 3a in the solid state
(film) displays a somewhat red-shifted PF emission (428 nm) and
an additional lower energy emission with appreciable intensity
peaking at 575 nm (excitation lex 5 380 nm) (Fig. 2). The lower
energy peak is attributed to the phosphorescence of the Pt–salen
complex and indicates considerable energy transfer from the
excited PF chromophore to the green emitting Pt–salen phosphor.
When the copolymer 3a is excited directly at 450 nm i.e. at the
{ Electronic supplementary information (ESI) available: Instrumental,
synthesis and device fabrication. See http://www.rsc.org/suppdata/cc/b5/
b501297j/
*scherf@uni-wuppertal.de
2378 | Chem. Commun., 2005, 2378–2380
This journal is ß The Royal Society of Chemistry 2005

View Article Online
attributed to the phosphorescence of the Pt–salen chromophores
(Fig. 2). The additional emission peak at 640 nm can be attributed
to an emission from excimers of the Pt–salen phosphors. The
formation of phosphorescent excimers has also been observed in
OLED devices constructed from (2-(49,69-difluorophenyl)-
pyridinato-N,C29)(2,4-pentanedionato)platinum(II).15
The EL efficiencies of the devices based on the pure copolymer
3a are quite low (ca. 0.1–0.3 cd A21), depending somewhat on the
amount of Pt–salen phosphor in the copolymer and most probably
influenced by the presence of aggregate quenching. In order to
reduce or suppress such concentration effects, we blended the
copolymer 3b into a matrix based on poly(vinyl carbazole) (PVK)
and
(2-(4-biphenylyl)-5-(4-tert-butylphenyl)-1,3,4-oxadiazole)
(PBD) with a weight ratio PVK:PBD of 4:1.{ In this case, the
640 nm contribution to the EL spectrum is largely suppressed and
the phosphorescence closely resembles the PL of the Pt–salen
chromophore.{ Interestingly, the EL spectrum of the copolymer 3b
blended into the PVK:PBD matrix is slightly red-shifted by about
0.1 eV compared to that of a device containing the monomeric Pt–
salen complex 4 in the same matrix. However, the energy transfer
between PVK and 3b is rather inefficient and the EL contribution
of the PVK matrix at 438 nm is dominant. Therefore we switched
to a poly[9,9-bis(2-ethylhexyl)fluorene] (PF2/6) matrix. Devices
based on a blend of the copolymer 3b in PF2/6 (5 and 10 wt%)
show a much more efficient energy transfer (Fig. 3). Based on
recent findings for polyfluorenes with covalently attached Ir-
complexes, we propose the occurrence of a mixed triplet state in
our novel copolymers.4 The efficiency of the devices were be
substantially improved to ca. 3–6 cd A21 for the copolymer 3b–
PF2/6 blends (Fig. 4). This improvement is attributed to the
reduction of the electronic interaction between Pt–salen phosphors
(excimer formation) in the latter device.
Scheme 1 Copolymer synthesis via microwave-assisted Yamamoto-type
coupling. Molar ratio of incorporated Pt–salen units was determined from
1
the integrated H NMR spectra.
Fig. 1 Structure of Pt–salen complexes 1 and 4.
In summary, OLED-devices with promising performances
based on novel fluorene-type copolymers with on-chain Pt–salen
units have been designed with maximum luminescence efficiencies
of up to 6 Cd A21. However, the EL spectra of the devices still
show a minor EL contribution from the matrix polymer PF2/6.
Further optimizations of our approach towards high efficiency
Fig. 2 Absorption and emission (lex 5 380 nm) spectra of 3a in
chloroform and of a thin spin-coated film at 298 K. The inset shows the
luminescence of the film excited at 450 nm.
lmax of the Pt–salen complex, the emission at 575 nm is dominant
confirming the origin of the green emission since the absorption of
the PF is negligible at this wavelength.
Organic light-emitting diodes were fabricated with
a
ITO|PEDOT:PSS anode and an evaporated Ca|Al cathode. The
EL spectrum of the device based on the pure Pt–salen copolymer
3a is rather broad. The most prominent peak is at ca. 640 nm, with
a shoulder at ca. 580 nm.{ The 580 nm-band is also observed in
the solid-state PL spectrum of a film of the copolymer 3a and is
Fig. 3 EL spectra of copolymer 3b doped into poly[9,9-bis(2-ethylhex-
yl)fluorene] (PF2/6) (10 wt% 3b in PF2/6) at different current densities
(device configuration: ITO|PEDT:PSS|PF2/6–3b|Ca|Al).
This journal is ß The Royal Society of Chemistry 2005
Chem. Commun., 2005, 2378–2380 | 2379

View Article Online
E-mail: scherf@uni-wuppertal.de; Fax: 49 202 439 3880;
Tel: 49 202 439 2493
bUniversita¨t Potsdam, Institut fu¨r Physik, Am Neuen Palais 10, D-14471
Potsdam, Germany. E-mail: neher@rz.uni-potsdam.de;
Fax: 49 331 977 1290; Tel: 49 331 977 1265
Notes and references
1 (a) M. A. Baldo, M. E. Thompson and S. R. Forrest, Pure Appl. Chem.,
1999, 71, 2098; (b) X. Gong, J. C. Ostrowski, G. C. Bazan, D. Moses,
A. J. Heeger, M. S. Liu and A. K. Y. Jen, Adv. Mater., 2003, 15, 45.
2 (a) S. A. Bagnich, C. Im, H. Ba¨ssler, D. Neher and U. Scherf, Chem.
Phys., 2004, 299, 11; (b) M. D. McGehee, T. Bergstedt, C. Zhang,
A. P. Saab, M. B. O’Regan, G. C. Bazan, V. I. Srdanov and
A. J. Heeger, Adv. Mater., 1999, 11, 1349.
3 (a) X. H. Yang and D. Neher, Appl. Phys. Lett., 2004, 84, 2476; (b)
C.-L. Lee, K. B. Lee and J. J. Kim, Appl. Phys. Lett, 2000, 77, 2280; (c)
Y.-L. Tung, S.-W. Lee, Y. Chi, Y.-T- Tao, C.-H. Chien, Y.-M. Cheng,
P.-T. Chou, S.-M. Peng and C.-S. Liu, J. Mater. Chem., 2005, 4, 460.
4 A. J. Sandee, C. K. Williams, N. R. Evans, J. E. Davies, C. E. Boothby,
A. Ko¨hler, R. H. Friend and A. B. Holmes, J. Am. Chem. Soc., 2004,
126, 7041.
Fig. 4 Efficiency of OLED devices based on PF2/6–3b blends as a
function of current density (device configuration: ITO|PEDT:PSS|PF2/6–
3b|Ca|Al).
5 X. Chen, J.-L. Liao, Y. Liang, M. O. Ahmed, H.-E. Tseng and
S.-A. Chen, J. Am. Chem. Soc., 2003, 125, 636.
6 J. M. Lupton, A. Pogantsch, T. Piok, E. J. W. List, S. Patil and
U. Scherf, Phys. Rev. Lett., 2002, 89, 167401.
7 C.-C. Kwok, S.-C. Yu, I. H. T. Sham and C.-M. Che, Chem. Commun.,
2004, 2758.
8 P. T. Furuta, L. Deng, S. Garon, M. E. Thompson and J. M. Fre´chet,
J. Am. Chem. Soc., 2004, 126, 15388.
9 R. P. Kingsborough and T. M. Swager, Adv. Mater., 1998, 10, 1100.
10 D. Wo¨hrle and A. D. Pomogailo, Metal Complexes and Metals in
Macrocycles, Wiley-VCH, Weinheim, 2003, p. 230.
phosphorescent OLEDs will be directed towards optimising the
concentration of the Pt–salen phosphor in the copolymer and of
the copolymer in a suitable host polymer. Additional improve-
ments are expected from the synthesis of related carbazole-based
copolymers containing Pt–salen phosphors. The use of carbazole-
based oligomers and polymers as hosts for phosphorescent guests
has been described by Brunner et al., wherein the fine tuning of the
energy levels leads to improved energy transfer and results in
higher OLED efficiencies.16
11 C.-M. Che, S.-C. Chan, H. F. Xiang, M. C. W. Chan, Y. Liu and
Y. Wang, Chem. Commun., 2004, 1484.
12 K. R. Carter, Macromolecules, 2002, 35, 6757.
13 E. C. Hagberg, D. A. Olson and V. V. Sheares, Macromolecules, 2004,
37, 4748.
14 B. S. Nehls, U. Asawapirom, S. Fu¨ldner, E. Preis, T. Farrell and
U. Scherf, Adv. Funct. Mater., 2004, 14, 352.
We thank the German Ministry of Science and Education for
supporting this work (contract no. 13N8213). B. S. N. thanks the
VCI foundation for financial support.
15 W. D’Andrade, J. Brooks, V. Adamovich, M. E. Thompson and
S. R. Forrest, Adv. Mater., 2002, 14, 1032.
16 K. Brunner, A. van Dijken, H. Bo¨rner, J. J. A. M. Bastiaansen, N. M.
M. Kiggen and B. M. W. Langeveld, J. Am. Chem. Soc., 2004, 126,
6035.
Frank Galbrecht,a Xiao Hui Yang,b Benjamin S. Nehls,a Dieter Neher,b
Tony Farrella and Ullrich Scherf*a
aBergische Universita¨t Wuppertal, Makromolekulare Chemie,
Gauss-Str. 20, D-42097 Wuppertal, Germany.
2380 | Chem. Commun., 2005, 2378–2380
This journal is ß The Royal Society of Chemistry 2005