Amorphous iridium complexes for electrophosphorescent light emitting devices

Article abstract of DOI:10.1039/b200957a

Iridium complexes with fluorene-modified phenylpyridine ligands are resistant to crystallization and can be used in the fabrication of single layer light emitting diodes.

Full text of DOI:10.1039/b200957a

Amorphous iridium complexes for electrophosphorescent light emitting
devices†
Jacek C. Ostrowski, Matthew R. Robinson,bd Alan J. Heegerbcd and Guillermo C. Bazan*abd
ad
a
Department of Chemisty, UC Santa Barbara, Santa Barbara CA 93106, USA.
E-mail: bazan@chem.ucsb.edu
Department of Materials Engineering, UC Santa Barbara, Santa Barbara CA 93106, USA
Department of Physics, UC Santa Barbara, Santa Barbara CA 93106, USA
Institute for Polymers and Organic Solids, UC Santa Barbara, Santa Barbara CA 93106, USA
b
c
d
Received (in Purdue, IN, USA) 29th January 2002, Accepted 20th February 2002
First published as an Advance Article on the web 14th March 2002
Iridium complexes with fluorene-modified phenylpyridine
ligands are resistant to crystallization and can be used in the
fabrication of single layer light emitting diodes.
Phosphorescent metal complexes that can be incorporated into
1,2
organic light emitting diodes (OLEDs) are under intense
investigation in academic and industrial laboratories.3 This
interest arises from the potential to increase device efficiency,
,4
5
relative to devices containing all-organic emissive materials. It
is generally accepted that charge recombination statistics in
organic LEDs lead to a theoretical maximum efficiency of
6
2
5%. It should be noted that the 25% limitation for maximum
efficiency in organic materials, while widely quoted in
literature, does not necessarily hold under all circumstances.7
Nevertheless, phosphorescent emitters with heavy metal ions
allow for circumvention of this limitation if the excitons
generated by hole–electron recombination reside at a site where
efficient spin-orbit coupling leads to efficient singlet–triplet
mixing.9
,8
Highly efficient LEDs with multilayer structure have been
treating 2,5-dibromopyridine with two equivalents of 9,9-di-
hexyl-2-pinacolatoboranefluorene19 in the presence of
10
constructed with tris(2-phenylpyridine)Ir(III
)
(1) as a dopant
5a
in 4,4A-N,NA-dicarbazolebiphenyl host as the emissive layer.
2
Pd(dppf)Cl (dppf = 1,1A-bis(diphenylphosphino)ferrocene)
Recent work shows that it is possible to tune the frequency of
emission from the complex by modifying the phenylpyridine
ligand.11 Additional efforts show that it is possible to reduce
self-quenching by the introduction of bulky groups on the basic
phenylpyridine ligand.12 In the cases discussed above, devices
were prepared by vapor-depositing techniques.
Ideally, one would like to fabricate efficient LEDs by casting
the electroemissive layer directly from solution and simultane-
ously limiting the number of layers between electrodes. Indeed,
devices that use a blend of 1 with poly(vinylcarbazole) have
been demonstrated.13 The use of conjugated polymers doped
with lanthanide emitters has also been reported.14
affords 2,5-bis(2A-(9A,9A-dihexylfluorene)pyridine in 50% yield.
Compound 4 is obtained by refluxing a glycerol solution
containing six equivalents of 2,5-bis(2A-(9A,9A-dihexylfluor-
3
ene)pyridine and Ir(acac) . Similar protocols are used to
generate compounds 2 and 3. Materials used in the optical and
morphological studies, and device fabrication, were purified by
multiple silica gel chromatographic separations and tested for
purity by HPLC.
The solution absorption and emission of compounds 2–4 in
the visible region are summarised in Table 1. The bands
1
20
corresponding to the ligand (p?p*) are the most intense.
These become increasingly red-shifted as the conjugation
It occurred to us that single component iridium-containing
devices could be fabricated if the complex was designed to
allow for hole and electron transport. Additionally, molecular
attributes15 would have to be incorporated such that the bulk
length of the ligand increases, from 330 nm, for 2, to 375 nm for
4
. For each compound, one observes weaker transitions red-
1
shifted from the (p?p*) band, corresponding to excitation to
1
3
3
20a,e
MLCT, MLCT and (p?p*) states (Table 1).
Observa-
16
material would be resistant to crystallization and able to form
thin films directly from solution, despite modest dimensions.1
In this communication, we report the synthesis, character-
ization, and preliminary device performance of complexes 2–4.
Oligofluorene-type moieties were included as part of the ligand
framework to facilitate charge transport across the bulk. Of
additional interest is the effect of ligand structure on the solid-
state quantum efficiency. Compounds 2–4 provide a progres-
sion of steric protection around the metal center and make it
possible to examine the effect of ligand substitution patterns on
the bulk morphology and solid state phosphorescence.17
The ligand frameworks were prepared by taking advantage of
Suzuki coupling protocols.18 As an example, we note that
3
3
tion of the MLCT and (p?p*) bands confirms strong spin
orbital coupling. Emission maxima are observed at 545, 550,
and 595 nm for 2, 3, and 4, respectively.
5c
The solution photoluminescence efficiencies were deter-
mined using quinoline bisulfate in 0.1 N H SO as a standard.
2 4
2
1
Quantum efficiencies of all the complexes were determined in a
degassed solution of toluene, and the complexes were excited at
Table 1 Physical properties of 2, 3, and 4
Solution
QE (%)
Emission/
nm
a
b
b
FSSPL (%)
Absorbance/nm
2
3
4
a
59
61
63
1.3 ± 1
1.7 ± 0.5
11.2 ± 1
330, 412, 449, 470
345, 401, 449, 470
375, 441, 513
545
550
595
†
Electronic supplementary information (ESI) available: experimental
details, synthetic and spectroscopic data. See http://www.rsc.org/suppdata/
cc/b2/b200957a/
All films were spun from toluene. b In toluene.
7
84
CHEM. COMMUN., 2002, 784–785
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1
the (p?p*) transition. Table 1 contains the values for the three
casting these complexes directly from solution and that these
iridium complexes.
DSC analysis shows that complexes 2–4 are amorphous
solids upon removal of the solvent (Fig. 1). All DSC data were
films operate as the emissive layer within an LED configura-
tion. Further optimization of LED performance is expected
upon blending conjugated polymers with the iridium complexes
and by better engineering of the device structure.
The authors are grateful to the NSF and to the ONR for
financial support of this work. J. C. O. is a National Kodak
Fellow.
2
1
taken at a scanning rate of 10 °C min . The ligand framework
influences the glass transition temperature and the stability of
the amorphous state. Complexes 2 and 3 show only glass
transitions at 98 and 110 °C, respectively. The heating trace of
complex 4 shows a glass transition at 83 °C, followed by
crystallization at 130 °C, a solid–solid state transition and
melting at 203 °C. For this set of compounds, the molecules
with smaller ligands give rise to higher glass transition
temperatures and are more resistant to crystallization. AFM
studies show that, for all cases, casting from toluene results in
smooth films, with an average mean roughness of ca. 0.3 nm.
Solid state fluorescence efficiencies (FSSPL) were deter-
mined using an integrating sphere according to published
protocols (Table 1).22 All films were spun cast onto a quartz
substrate from 1% toluene solutions. For these measurements
the complexes were excited at 365 nm. An efficiency of 1.3 ±
Notes and references
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Fig. 1 DSC scans of 2–4: (a) crystallization; (b) melting.
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