Cyclometalated red iridium(iii) complexes containing carbazolyl- acetylacetonate ligands: Efficiency enhancement in polymer LED devices

Article abstract of DOI:10.1039/c0dt00728e

The design, synthesis, photophysical and significantly improved electrooptical properties of a series of red emitting cyclometalated iridium(iii) complexes containing carbazolyl-acetylacetonate ligands are described.

Full text of DOI:10.1039/c0dt00728e

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Cyclometalated red iridium(III) complexes containing carbazolyl-
acetylacetonate ligands: efficiency enhancement in polymer LED devices†
Nan Tian,a,c Yaroslav V. Aulin,b Daniel Lenkeit,a Simon Pelz,a Oleksandr V. Mikhnenko,b,c Paul W. M. Blom,b,c
Maria Antonietta Loi*b,c and Elisabeth Holder*a,c
Received 24th June 2010, Accepted 30th July 2010
DOI: 10.1039/c0dt00728e
The design, synthesis, photophysical and significantly im-
proved electrooptical properties of a series of red emitting
cyclometalated iridium(III) complexes containing carbazolyl-
acetylacetonate ligands are described.
emission color of the iridium(III) complexes unchanged. In order
to keep a deep red emission color the use of 1-phenylisoquinoline
cyclometalating ligands should be beneficial.
In this contribution we describe the synthesis of deep red
emitting cyclometalated iridium(III) complexes 1 and 2 (Chart 1)
by using this design strategy based on carbazolyl-based ancillary
ligands and 1-phenylisoquinoline cyclometalating ligands.
These deep red emitting Ir(III) complexes are compared to a very
well-known and commonly used iridium dye [(btp)2Ir(III)(acac)]5
(Chart 1) in order to benchmark their properties with respect to ap-
plication in light-emitting devices. For this, the Ir(III) compounds
were mixed in specific ratios into blends of PVK:PBD polymer
matrixes. Such active layer blends were utilized determining their
favorable photoluminescence and electroluminescence properties,
their significantly reduced phosphorescence lifetimes as well as
their enhanced luminous efficiencies and power efficiencies.
Neutral, cyclometalated iridium(III) complexes of a het-
eroleptic architecture were formed using 1-(9H-carbazol-9-yl)-
5,5-dimethylhexane-2,4-dione (3) or 1-(3,6-bis(4-hexylphenyl)-
9H-carbazol-9-yl)-5,5-dimethylhexane-2,4-dione (4) as ancillary
ligands1e,2c and 1-phenylisoquinoline cyclomatalating ligands.
Prior to this, a dimeric m-chloro-bridged iridium(III) complex
carrying 1-phenylisoquinoline ligands was prepared in 78% yield
according to previously introduced procedures.1e,2c Ancillary lig-
and 3 (yield: 62%) or ancillary ligand 4 (yield: 80%) were synthe-
sized according to modified procedures introduced previously1e,2c
(Scheme 1) and were subsequently coordinated to the precursor
complex. The resulting red light emitting iridium(III) complexes 1
and 2 were obtained in yields of 67% and 72%, respectively.
The intense absorption bands of 1 and 2 below 300 nm were at-
tributed to spin-allowed p→p* transitions of the cyclometalating
ligand 1-phenylisoquinoline (Table 1).
Cyclometalated iridium(III) complexes are of major interest in a
series of optical applications like organic light-emitting diodes,1
sensors,2 or biomedical imaging applications.3 In all of these
applications the favorable and general properties such as high
quantum yields, short lifetimes, or color-tunability of the irid-
ium(III) complexes4 are in the focus. In red light-emitting diodes
one particular interest is the enhancement of luminous and power
efficiencies of the heteroleptic iridium(III) complexes while keeping
the phosphorescence lifetimes short. One successful strategy is
the variation of the iridium(III) triplet emitters by introducing
functional groups to potentially enhance the charge recombination
at the emitter site and to increase the miscibility with the polymer
matrix.
One thriving approach is the use of hole-accepting moieties
like carbazolyl functions. In such systems, the ligands have a
significant impact on the charge acceptor properties. Directed
charge recombination shall avoid recombination at non-emitting
sites within the light-emitting device. We anticipated that the
presence of carbazolyl moieties in the ancillary ligand will provide
a measurable enhancement of the light-emitting efficiency of the
complexes while moreover reducing the lifetime but keeping the
aFunctional Polymers Group and Institute of Polymer Technology, Univer-
sity of Wuppertal, Gaußstr. 20, D-42097, Wuppertal, Germany. E-mail:
holder@uni-wuppertal.de; Fax: +49-202-439-3880; Tel: +49-202-439-3879
bUniversity of Groningen, Zernike Institute for Advanced Materials, Gronin-
gen, The Netherlands. E-mail: M.A.Loi@rug.nl; Fax: +31-50-363-8751;
Tel: +31-50-363-4119
The spin-allowed singlet 1MLCT transitions were located in the
visible part of the spectrum at around 400 nm, these were visible
as a shoulder overlapping with the ligand p→p* transitions. The
slight absorptions typically at wavelengths above 400 nm were
cDutch Polymer Institute (DPI), P.O. Box 513, NL-5600 AX, Eindhoven,
The Netherlands
† Electronic supplementary information (ESI) available: Experimental
details. See DOI: 10.1039/c0dt00728e
Chart 1
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Table 1 Spectroscopic properties of 1 and 2 (see also Fig. S1 and Fig. S2,
ESI)
Absorbance/nm (log
Maximum of
emission/nm
Quantum
yielda(%)
e/L mol-1 cm-1)
Tdb/◦C
356
1
2
247 (5.33), 291 (5.20)
341 (3.93), 478 (3.29)
264 (3.97), 299 (3.98)
359 (3.40), 481 (2.79)
624
625
31
30
340
a Determined according to the method of Demas and Crosby.6 b @ 5%
weight loss.
Fig. 1 Luminous and power efficiencies of 1, 2 and [(btp)2Ir(III)(acac)] as
a function of the relative dopant amount in PVK:PBD blends of a 70 : 30
wt.% ratio (see also Fig. S3, ESI†).
reasonable to arise as spin-forbidden triplet 3MLCT excited states.
However, the absorptions in the far-UV, at about 270 nm, were
found to be most likely due to b-diketonate-based transitions,
analogous to the absorptions of the free b-diketone. The phospho-
rescence of complexes 1 and 2 was obtained at maximum emission
wavelengths of 624 nm and 625 nm, correspondingly. Iridium(III)
complexes 1 and 2 reveal favorably high quantum yields of 31%
and 30%, respectively. Note these high quantum yields were even
obtained in aerated chloroform solutions. In addition, 1 and 2
were remarkably temperature stable revealing 5% weight loss at
340 ◦C and 356 ◦C, correspondingly.
In order to benchmark the carbazolyl-containing iridium(III)
emitters 1 and 2 for their suitability in polymer light-emitting
diodes (PLEDs), one needs to compare the new compounds to the
commonly used red emitter [(btp)2Ir(III)(acac)].5 Thus, all of these
complexes were doped in a polymer matrix of hole conducting
poly(vinylcarbazole) (PVK) and electron conducting oligomer
2-(biphenyl-4-yl)-5-(4-tert-butylphenyl)-1,3,4-oxadiazole (PBD).
The blend ratio of PVK to PBD was kept constant at 70 : 30 wt.%,
respectively, in order to serve as active layers in PLEDs. The
dopant ratio was varied from 4 to 16 molecules of 1, 2 and
[(btp)2Ir(III)(acac)]5 per 1000 monomer units of PVK translating to
a range of about 1–4 wt.%. The simple benchmarking test device
architecture7 for each series of dopants consisted of the follow-
ing layout: ITO/PEDOT:PSS (60 nm)/active layer (80 nm)/Ba
(5 nm)/Al (100 nm). Fig. 1 shows the luminous and power
efficiencies as a function of the respective dopant concentration
for each series of test devices operating at 10 V. For the whole
range of concentrations the novel red emitting and carbazolyl-
containing Ir(III) complexes 1 and 2 revealed better luminous
efficiencies then the standard red iridium(III) phosphorescent
emitter [(btp)2Ir(III)(acac)].5 As depicted in Fig. 1 the best PLED
devices were comprised of 8–12 molecules of 1 or 2 per 1000
monomer units of PVK. In these molar concentration ranges both
newly designed iridium(III) complexes 1 and 2 revealed a superior
luminous efficiency enhancement of about 30% compared to the
commonly used triplet emitter [(btp)2Ir(III)(acac)].5 Subsequently,
the photophysical properties of the devices active layers were
studied. Time resolved photoluminescence measurements were
performed exciting the samples with 150 fs pulses at 380 nm, and
revealing the decays with a Hamamatsu Streak Camera working
in single sweep mode.
The steady state photoluminescence and electroluminescence
spectra of compounds 1, 2 and [(btp)2Ir(III)(acac)]5 in the active
layers of the devices are shown in Fig. 2. The emission intensity of
PVK (not shown) is negligible compared to the emission intensity
Fig. 2 Normalized steady state photoluminescence and electrolumines-
cence spectra of the PLEDs active layers at various dopant compositions.
Scheme 1 Synthetic route to ligand 4 (ligand 32c is obtained in a similar fashion when starting from 9H-carbazole). (i) K2CO3, ethyl-2-bromoacetate,
DMF, 70 ◦C, (62%). (ii) KHMDS, 3,3-dimethylbutan-2-one, THF, RT, (80%).
8614 | Dalton Trans., 2010, 39, 8613–8615
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of the triplet emitting compounds 1, 2 and [(btp)2Ir(III)(acac)]5
indicating efficient energy transfer from the PVK matrix to all
red triplet emitters. The photoluminescence reveals two maxima
at 590 nm and 650 nm, respectively, for both of the compounds
1 and 2. These emission maxima are 10 nm and 25 nm blue-
shifted compared to [(btp)2Ir(III)(acac)].5 The electroluminescence
of the triplet emitter series compounds 1, 2 and [(btp)2Ir(III)(acac)]5
are typically about 40 nm and 20 nm red-shifted for each of the
emission maxima compared to the observed photoluminescence
spectra. This is a typical finding for systems with a charge
trapping mechanism of exciton formation at dopant sites, which
are characterized by inhomogeneous broadening of the density of
states.8 Preferably, the electroluminescence emission spectra of 1,
2 and [(btp)2Ir(III)(acac)]5 would be almost congruent in shape.
The observed decay curves were identical for every dopant
concentration in the studied range, indicating the absence of
concentration quenching. The monoexponential fit curves shown
in Fig. 3 reveal decay times of 1.2 ms for complexes 1 and 2
and 5.1 ms for [(btp)2Ir(III)(acac)].5 Thus, the phosphorescence
decay times of the newly designed triplet emitters 1 and 2 are
significantly shorter making them thus attractive candidates in
advanced light-emitting diode applications.
short phosphorescence lifetimes making them promising light-
emitting candidates for a series of applications in advanced organic
and polymer light-emitting devices.
Acknowledgements
E.H. acknowledges the Deutsche Forschungsgemeinschaft (DFG)
for financial support. This research forms part of the research pro-
gram of the Dutch Polymer Institute (DPI), within collaborating
projects #518 and #629.
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Fig. 3 Normalized phosphorescence decays of 1, 2 and [(btp)2Ir(III)(acac)]
in PVK:PBD blends with a 70 : 30 wt.% ratio.
In summary, we prepared newly designed, red emitting,
carbazolyl-containing cyclometalated iridium(III) emitters that
were compared to a commonly used reference emitter. The in-
troduced triplet emitters revealed excellent performance in bench-
marking devices and are additionally furnished with preferably
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