Photophysical properties and OLED performance of light-emitting platinum(ii) complexes

Article abstract of DOI:10.1039/c3dt50364j

The synthesis, photophysical properties and application as emitters in solution-processed multi-layer organic light-emitting diodes (OLEDs) of a series of blue-green to red light-emitting phosphorescent platinum(ii) complexes are reported. These complexes consist of phenylisoquinoline, substituted phenylpyridines or tetrahydroquinolines as C^N cyclometalating ligands and dipivaloylmethane as an ancillary ligand. Depending on both the structure of the C^N cyclometalating ligands and the dopant concentration in the matrix, these platinum(ii) complexes exhibit different aggregation tendencies. This property affects the photoluminescence spectra of the investigated compounds and colour-stability of the fabricated OLEDs. Using the blue-green to yellow-green emitting complexes, the best results were obtained with the 2-(4- trifluoromethylphenyl)-5,6,7,8-tetrahydroquinoline based platinum(ii) complex. A maximum luminous efficiency of 4.88 cd A^-1 and a power efficiency of 4.65 lm W^-1, respectively, were achieved. Employing the red emitting phenylisoquinoline based complex as an emitter, colour-stable and efficient (4.71 cd A^-1, 5.12 lm W^-1) devices were obtained.

Full text of DOI:10.1039/c3dt50364j

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Photophysical properties and OLED performance of
light-emitting platinum(II) complexes†
Cite this: Dalton Trans., 2013, 42, 13612
Dimitrios Kourkoulos,a Cüneyt Karakus,b Dirk Hertel,a Ronald Alle,a
Sebastian Schmeding,c Johanna Hummel,c Nikolaus Risch,c Elisabeth Holder*b and
Klaus Meerholz*a
The synthesis, photophysical properties and application as emitters in solution-processed multi-layer
organic light-emitting diodes (OLEDs) of a series of blue-green to red light-emitting phosphorescent
platinum(II) complexes are reported. These complexes consist of phenylisoquinoline, substituted phenyl-
pyridines or tetrahydroquinolines as C^N cyclometalating ligands and dipivaloylmethane as an ancillary
ligand. Depending on both the structure of the C^N cyclometalating ligands and the dopant concen-
tration in the matrix, these platinum(II) complexes exhibit different aggregation tendencies. This property
affects the photoluminescence spectra of the investigated compounds and colour-stability of the fabri-
cated OLEDs. Using the blue-green to yellow-green emitting complexes, the best results were obtained
Received 5th February 2013,
Accepted 9th July 2013
with the 2-(4-trifluoromethylphenyl)-5,6,7,8-tetrahydroquinoline based platinum(II) complex.
A
maximum luminous efficiency of 4.88 cd A−1 and a power efficiency of 4.65 lm W−1, respectively, were
achieved. Employing the red emitting phenylisoquinoline based complex as an emitter, colour-stable and
efficient (4.71 cd A−1, 5.12 lm W−1) devices were obtained.
DOI: 10.1039/c3dt50364j
www.rsc.org/dalton
behaviour is ascribed to platinum–platinum or π–π inter-
Introduction
actions.12 Although this property can be used to fabricate
Phosphorescent platinum(II) complexes have gained increasing white light-emitting OLEDs13 with just one phosphorescent
attention over the last few decades owing to their employment complex, it is not suitable for applications where colour purity
in chemosensors,1 colorimetric oxygen sensors,2 photo- is needed.
voltaics3 and electrophosphorescent devices.4 Phosphorescent
In the present investigation we discuss besides the photo-
transition-metal complexes5 are suitable for application in physical and electro-optical characterisation of the platinum(II)
light-emitting devices6 due to the strong spin–orbit coupling compounds the synthesis of phenylisoquinoline14,15 and tetra-
induced by the heavy metal ion.7 This effect allows harvest- hydroquinoline-type16 C^N ligands, leading to the blue-green
ing of both triplet and singlet excitons that are electrogene- through yellow-green to red light-emitting cyclometalated com-
rated in a ratio of 3 : 1 according to simple spin statistics, plexes, using dipivaloylmethane (dpmH) as an ancillary
leading to potentially achievable internal quantum efficiencies ligand. Contrary to the phenylisoquinoline ligand, which is
up to 100%.8 Most of the common triplet emitters are based received by a Suzuki coupling reaction,14,15,17 the tetrahydro-
on octahedral iridium(III)9 or square planar platinum(II)10 com- quinoline ligands were obtained via a Mannich-reaction.16 The
plexes. The latter often tends to form aggregates or excimers, platinum(II) complexes were embedded in a matrix to study the
causing shifts in the emission spectra.11 Conventionally, this optical properties in the solid state and were employed in solu-
tion-processed multi-layer phosphorescent OLEDs. Depending
on both the structure of the ligands and the complex concen-
tration, fabricated devices displayed dissimilar colour stability.
aDepartment of Chemistry, Universität zu Köln, Luxemburger Strasse 116,
50939 Köln, Germany. E-mail: klaus.meerholz@uni-koeln.de;
Fax: +49 (0) 221 470-5144; Tel: +49 (0) 221 470-3275
bFunctional Polymers Group and Institute of Polymer Technology, University of
Wuppertal, Gaußstr. 20, 42097 Wuppertal, Germany.
E-mail: holder@uni-wuppertal.de; Fax: +49 (0) 202 439-3880;
Results and discussion
Tel: +49 (0) 202 439-3879
cOrganic Chemistry, University of Paderborn, Warburger Str. 100, 33098 Paderborn,
Synthesis
Germany
The phenylisoquinoline ligand L1 was synthesized from
1-chloroisoquinoline and phenylboronic acid using
†Electronic supplementary information (ESI) available. See DOI: 10.1039/
c3dt50364j
a
13612 | Dalton Trans., 2013, 42, 13612–13621
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Scheme 1 Synthetic route of phenylisoquinoline ligand L1 obtained in 69%
yield.
Scheme 3 Synthetic route of the ligands 1–7 via µ-chloro-bridged precursor
dimers 1–7 to the platinum(II) complexes 1–7.
conventional palladium-catalyzed Suzuki cross-coupling reac-
tion (Scheme 1).14,15,17
The ligands 2-phenylpyridine (L2) and 2,4-difluorophenyl-
pyridine (L3) were purchased. To prepare the tetrahydroquino-
line based C^N ligands L4–L7, 1-morpholinocyclohexene and
the corresponding Mannich-bases that were synthesized
through aminomethylation of the respective phenylketone
derivatives with N,N-dimethylmethylene-iminium-chloride
were used to form the 1,5-diketones. These compounds were
subsequently cyclised using NH2OH·HCl in order to obtain
the target C^N ligands L4–L7 (Scheme 2). The ligands L1–L7
were readily soluble in solvents of medium polarity.
Upon receipt of the ligands L1–L7 the platinum(II) com-
plexes 1–7 were prepared in a two-step synthesis.13,16,18 The
μ-chloro-bridged precursor dimers were synthesized by heating
K2PtCl4 and the corresponding C^N ligand in a mixture of
2-ethoxyethanol and water (3/1) at 80 °C for 16 h and were
obtained in yields between 66% and 84%. Subsequently, these
dimers were heated with dpmH and Na2CO3 in 2-ethoxyetha-
nol at 100 °C for 3–6 h to prepare the heteroleptic platinum(II)
complexes 1–7 in yields of 55% to 84% (Scheme 3). The crude
platinum(II) compounds 1–7 were chromatographically puri-
fied over silica gel. The structures of the synthesized emitters
1–7 are illustrated in Fig. 1.
Fig. 1 Molecular structures of the platinum(II) complexes 1–7.
The complexes 1–7 were well soluble in solvents of medium
polarity. The molecular ions [M + H]+ (M = neutral complexes)
were observed in the ESI mass spectra at m/z 583.2 (1), 533.2 (2),
569.2 (3), 587.2 (4), 655.2 (5), 605.2 (6) and 666.1 (7). The
1H- and 13C-NMR spectra show the olefinic proton at
5.83–5.90 ppm and the characteristic splitting of the signals
–CF3 groups with the expected resonances. The complexes 1–7
were additionally characterized by FT-IR spectroscopy. The
purity of the synthesized emitters 1–7 was confirmed by
elemental analysis.
t
Optical properties in solution and electrochemical properties
for Bu from the dpmH at 1.20–1.40 ppm. For 3, 5 and 6
19F-NMR confirmed the aromatic fluorine and the aromatic
In the following, we classify the investigated complexes in
different categories: complex 1 differs from the greenish com-
plexes 2–7 through its red emission colour and excellent
colour-stability. The greenish phenylpyridine and tetrahydro-
quinoline based complexes 2–7 can be divided into two
groups: the compounds 3 and 6 that were fluorinated at the
aromatic system of the C^N cyclometalating ligand and show a
strong tendency to form aggregates and the non-fluorinated
complexes 2, 4, 5 and 7 that show a less pronounced aggregate
formation.
The UV-vis absorption and emission data of the complexes
in CH2Cl2 are summarized in Table 1, Fig. 2 and 3. The extinc-
tion coefficients of 6500–52 500 are in the same range as the
values of similar compounds previously reported in the
literature.16,19–22 The intense absorption for all complexes at
240–340 nm is assigned to spin-allowed intraligand transitions
Scheme 2 Synthesis of 5,6,7,8-tetrahydroquinoline derivatives L4–L7. Yield of
L4 98%, L5 54%, L6 50%, and L7 46%. R1 = –H (L4), –CF3 (L5), –F (L6), –Br (L7).
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Table 1 UV-vis absorption, emission maxima of 1–7 in CH2Cl2 (c = 1 × 10−5
mol L−1), PLQY (φ) in n-hexane or in CHCl3 (c = 1 × 10−5 mol L−1) at room temp-
erature and redox properties (scan rate: 100 V s−1, Pt-disk-electrode, CH2Cl2,
TBAPF6)
UV-vis [nm]
Complex {ε (dm3 mol−1 cm−1)}
λmax-em. [nm]
φ
ER1/e2d [V]
1
2
3
4
5
255 (43 000), 274 (38 300), 597, 639
345 (17 800), 408 (13 900)
253 (52 600), 280 (37 300), 486, 518
315 (17 700), 367 (11 000)
249 (46 900), 309 (15 900), 468, 498
323 (16 600), 364 (10 600)
250 (27 600), 278 (25 100), 498, 528
317 (11 700), 367 (6500)
0.15d −2.18a
0.19e −2.52b
0.09d −2.42b
0.08e −2.75c
0.07d −2.37a
251 (36 600), 280 (34 700), 501, 537
317 (15 800), 332 (15 500),
381 (8800)
251 (26 000), 274 (25 900), 489, 523
330 (9500), 361 (7400)
254 (39 000), 280 (40 600), 501, 535
335 (16 100), 372 (10 200)
6
7
0.05d −2.58c
0.10d −2.46c
a Roughly reversible. b Partially reversible. c Irreversible. d In n-hexane.
e In CHCl3.
Fig. 3 Photoluminescence spectra of (a) 1–3 and (b) 4–7 dissolved in CH2Cl2
(c = 1 × 10−5 mol L−1) recorded at room temperature.
can directly be seen in the emission spectra (Fig. 3) that
exhibit a characteristic pattern, which reveals two maxima and
ranges from the green-blue to the red spectral region. The
higher the electronegativity of the substituent, the larger is the
resulting blue-shift of emission. In contrast, the higher the
aromatic conjugation of the C^N ligand, the larger is the
observed red-shift in the photoluminescence (PL) spectra.
The photoluminescence quantum yields (PLQY) of 1–7 in
CHCl3 or n-hexane vary between 0.05 and 0.19 (Table 1) and
are in the same range as that for structurally related complexes
reported in the literature.18
The electrochemical properties of the platinum(II) com-
plexes were investigated by cyclic voltammetry (CV) using the
redox couple ferrocene/ferrocenium as a reference. The CV-
measurements showed that the oxidation of 1–7 is completely
irreversible. It is reported that this is due to a nucleophilic
attack of the solvent on the platinum centre of the complex,
since the oxidation takes place at the metal centre, whereas
the reduction occurs at the C^N ligand.18 The best results con-
cerning reversibility of the reduction of 1–7 were obtained
using a fast scan rate of 100 V s−1. The reduction was roughly
Fig. 2 UV-vis spectra of (a) 1–3 and (b) 4–7 dissolved in CH2Cl2 (c = 1 × 10−5
mol L−1) recorded at room temperature.
(π → π*) that show similar absorption bands in this region.23,24 reversible for 1 and 5, partially reversible for 2 and 3 and irre-
The metal-to-ligand charge transfer (MLCT) and ligand-to- versible for 4, 6 and 7. The position of the reduction wave
ligand charge transfer (LLCT) transitions are located between ranges from −2.18 V for 1 to −2.75 V for 4. The estimation of
350 nm and 450 nm.18
the LUMO of the electron transporting material OXD-7 is
The emission of 1–7 is ascribed to a mixed LC-MLCT state based on a measurement with a scan rate of 100 mV s−1
and the specific effects of the substituent at the C^N ligands Regardless of the reduced reversibility of the values of 1–7,
3
.
13614 | Dalton Trans., 2013, 42, 13612–13621
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which were obtained under identical measurement conditions,
we use them to discuss the capability of these compounds as
traps for electrons. The reduction potential for both scan rates
and the estimated LUMOs are summarized in Table S1 in the
ESI.†
Optical properties in the solid state
Due to the significant influence of the emitter concentration
on the electroluminescence spectra of the devices based on
2–6, the effect of the dopant concentration on the photo-
luminescence of films containing various amounts of these
complexes was investigated. To be consistent with OLED
measurements, a mixture of poly(vinyl)carbazole (PVK) and
1,3-bis(N,N-t-butyl-phenyl)-1,3,4-oxadiazole (OXD-7) served as
the host and equimolar amounts of 1–6 were embedded into
the matrix. For comparison, all photoluminescence data of the
films are summarized in Table S2† and the PL spectra of the
samples containing equimolar amounts of 2–6 are shown in
Fig. S1 in the ESI.†
Unlike octahedral six-fold coordinated osmium(II), ruthe-
nium(II) or iridium(III) complexes,6,25 four-fold coordinated
platinum(II) complexes16 exhibit a square planar coordination
geometry.26 For this reason, platinum(II) compounds with
sterically undemanding substituents are basically flat. This
allows intermolecular interactions that are mandatory for
aggregates like dimers27 in the ground state or excimers27,28 in
the excited state. The tendency to aggregate depends on
various parameters such as the steric demand of the substitu-
ents13,29,30 and the concentration of the compound in the
matrix or solution.27
Fig. 4 Photoluminescence spectra of films consisting of PVK/OXD-7 doped
with various equimolar amounts of complexes (a) 2 and (b) 3 recorded 720 ns
after excitation with a laser pulse and normalized to the highest energy emis-
sion peak. Arrows indicate the effect of increasing dopant concentration.
shift of the emission colour, down to 80 K, this red-shift could
For low dopant concentrations, the PL spectra of the films be significantly reduced for both compounds (Fig. S2†).
are comparable to the emission spectra of the complexes in
Based on the literature12,13,26–28 we assume that the
solution. However, as the concentration of the dopants 2–6 in increase in intensity of the low-energy emission band of the
the matrix is raised, an increase of the intensity of the energe- greenish complexes 2–6 arises from aggregates and is caused
tically low-lying emission band is observed. In contrast, the PL by dimer and/or excimer emission.
spectrum of the red emitting complex 1 is not affected by the
Investigations on platinum(II) complexes with pyridylazolate
dopant concentration. The degree of this concentration- based chelates showed that cyclometalated complexes with
induced red-shift of the emission colour depends highly on very asymmetric electronic properties tend to display a strong
the substituent at the C^N cyclometalating ligands and is most tendency for the formation of aggregates.31 Considering that
distinct for the fluorinated compounds 3 and 6, whereas it is the complexes 2–7 consist of an electron poor pyridine frag-
much less pronounced for the non-fluorinated greenish emit- ment and a comparable electron rich substituted phenyl frag-
ting compounds 2, 4 and 5.
ment, this effect might also contribute to the observed dimer
To illustrate the dissimilar concentration dependence of or excimer emission.
the PL spectra of films containing the fluorinated and non-
Time-resolved PL measurements of a film doped with
fluorinated greenish complexes, exemplary PL spectra of films 39.1 wt% 3 showed that at least for this compound, the red-
containing various concentrations of the non-fluorinated com- shift can be assigned to excimer emission. The intensity of the
pound 2 and fluorinated compound 3 are depicted in Fig. 4.
monomer signal (λmax = 470 nm) decays after the excitation
The comparison of the concentration-dependent PL spectra pulse, whereas the intensity of the excimer emission (λmax
=
of films doped with the fluorinated compounds 3 and 6 592 nm) increases (Fig. 5). These data are in good agreement
(Fig. S1†) shows that the concentration-induced red-shift is with the literature, since the excimer emission of a structurally
less pronounced for 6. The higher steric demand of ligand L6 related complex with a sterically less demanding ancillary
compared to L3 might be one possible reason for this ligand is reported to be located at 598 nm.27
behaviour.
According to these results, the aggregate emission, which
By cooling films containing various amounts of 3 and 6, can result from dimers in the ground state or excimers in the
which show the most pronounced concentration-dependent excited state, cannot be absolutely prevented by introducing a
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traps for electrons, due to the energy difference to the lowest
unoccupied molecule orbital (LUMO) of the electron transport-
ing component OXD-7. The LUMOs of 4 and 6 correspond,
within the limits of accuracy of the CV measurement, nearly to
the LUMO of OXD-7.
The two hole transporting layers consisting of cross-linked
QUPD and OTPD were used to enhance the hole injection and
for their electron blocking capabilities. To improve the charge
balance and hence for better positioning of the recombination
zone, an additional 1,3,5-tris(1-phenyl-1H-benzo[d]imidazol-2-
yl)benzene (TPBi) layer was introduced, acting as an exciton/
hole blocking and electron transport layer. The OLED contain-
ing 8.9 wt% of the red-emitting dopant 1 achieved a high
maximum luminous efficiency and power efficiency of 4.71 cd A−1
and 5.12 lm W−1, respectively.
Fig. 5 Photoluminescence spectra of a film containing 39.1 wt% 3 in a PVK/
OXD-7 matrix 20 ns and 708 ns after excitation, recorded at 295 K.
For OLEDs employing a structurally related emitter based
on phenylisoquinoline, efficiencies up to 1.88 cd A−1 are
reported.32 Possibly our more complex device architecture
might contribute to the enhanced performance. However,
more efficient vacuum-processed OLEDs based on red emitting
platinum(II) complexes with efficiencies up to 24.6 cd A−1 have
been reported.33
dpmH as an ancillary ligand. Nevertheless, the concentration
induced red-shift is less pronounced compared to platinum(II)
complexes with identical or structurally related C^N cyclometa-
lating ligands and sterically less demanding ancillary
ligands.13 Moreover, the introduction of dpmH enhances the
solubility of the complexes in non-polar solvents compared to
compounds with simple acetylacetones as ancillary ligands.
The photoluminescence decay of the monomer emission of
1–6 was observed at the highest energy emission wavelength
and exhibits a mono-exponential kinetic for the red light-emit-
ting complex 1 and a bi-exponential kinetic for 2–6. Due to
self-quenching an increase of the dopant concentration causes
a significant decrease of the (averaged) photoluminescence
lifetime.
Depending on the concentration, the photoluminescence
lifetime of 1 varies between 4.49 and 3.94 µs and the average
lifetime of the 2–6 monomers between 1.49 and 5.56 µs
(Table S2†).
The PLQY of 1–6 in a PVK/OXD-7 matrix varies depending
on the dopant and its concentration between 0.69 for 2 and
0.07 for 6 (Table S2†). Due to concentration quenching, high
amounts of the greenish platinum(II) complexes cause a
decrease in PLQY. For the red platinum dopant 1 only a slight
concentration-induced decrease in PLQY was observed in the
investigated concentration range.
Using the non-fluorinated greenish light-emitting dopants
2, 4, 5 and 7, the highest maximum luminous efficiency of
4.88 cd A−1 and power efficiency of 4.65 lm W−1 were achieved
with devices containing 10.0 wt% 5. However, the devices
based on complex 2 showed a slightly better performance at
higher device brightness. In the literature, OLEDs using struc-
turally related emitters based on non-fluorinated 2-phenylpyri-
dines are described that achieved efficiencies up to 1.5 cd
A−1 29
It is possible that also our more complicated device
.
architecture might result in higher efficiency. For vacuum-pro-
cessed OLEDs employing a complex structurally related to 2
chelating a 2-pyridylhexafluoropropoxide as ancillary ligand
efficiencies up to 13.5 cd A−1 were achieved.34 For structurally
non-related greenish platinum(II) complexes efficiencies up to
50.0 cd A−1 have been reported.35 However, the comparison of
solution- and vacuum-processed devices is difficult, since we
cannot exclude aggregate formation of the Pt(II) complexes
during the spin coating process.
The performance data (Table S3†) show that for all investi-
gated platinum(II) emitters, apart from 7, the optimum dopant
concentration corresponds to an equimolar amount of 10.0
OLED performance
To investigate the electroluminescence properties of the plati- wt% 5. The luminance-, current-, and voltage-characteristics of
num(II) emitters 1–7 various equimolar amounts were blended the devices with emitter concentrations that correspond to an
in a PVK/OXD-7 matrix (70 wt% PVK, 30 wt% OXD-7). The elec- equimolar amount of 10.0 wt% of complex 5 are depicted and
tron transporting material OXD-7 was used in order to improve summarized in Fig. S6† and Table 2, whereas the external
the bipolar charge-transport properties of the host. The chemi- quantum efficiencies (EQE) of these devices are summarized
cal structure of the materials used for device fabrication is in Table S4.† The most efficient OLED using dopant 7 as an
given in Fig. S3.† The devices comprise the following layout:
emitter contained only 5.1 wt% of this complex. Indeed,
ITO//PEDOT:PSS (32 nm)//QUPD (18 nm)//OTPD (10 nm)// complex 7 is not suitable as an emitter in OLEDs since only a
PVK+OXD-7+ 1–7 (30 nm)//TPBi (30 nm)//CsF (3 nm)//Al (120 nm) very low maximum device brightness of 248 cd m−2 was
(Fig. S4†).
achieved. One possible explanation for the poor performance
The HOMO and LUMO levels of the materials used in the of the devices that employed 7 as an emitter could be the
devices are schematically depicted in Fig. S5 in the ESI.† As quenching of luminance due to heat generation during device
can be seen in Fig. S5,† all dopants apart from 4 and 6 act as operation, since previous investigations showed that 7 displays
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Table 2 Performance of the investigated OLEDs containing equimolar amounts of 1–7
Luminous efficiency
CIE-coordinates
@ 500 cd m−2
[X/Y]
Max. luminous
Max. power
@ 500 cd m−2
efficiency
efficiency
[cd A−1
@ [V]
]
Complex [wt%]
PL emission colour
[cd A−1
]
[lm W−1
]
1
2
3
4
5
6
7
8.9
8.1
8.7
9.0
10.0
9.2
Red
Green
Blue-green
Green
Yellow-green
Green
0.63/0.36
0.27/0.51
0.31/0.42
0.32/0.47
0.34/0.51
0.34/0.44
—
4.71
4.52
1.71
3.09
4.88
2.08
1.66
5.12
3.73
1.57
3.02
4.65
2.25
1.31
1.98 @ 6.6
3.52 @ 5.8
0.95 @ 7.5
1.86 @ 6.7
2.39 @ 6.6
0.95 @ 7.0
—
10.2
Yellow-green
a strong temperature quenching effect.16,36 Furthermore, the
irreversibility of the reduction of 7 that was observed during
the CV measurement indicates that this complex might
degrade during device operation.
Employing the fluorinated compounds 3 and 6 as emitters,
comparable efficiencies for both emitters were obtained: 1.71
cd A−1 for 3 and 2.08 cd A−1 for 6.
Similar to the PL spectra of the dopants 2–6, the Electro-
luminescence (EL) spectra are also sensitive to the emitter con-
centration and, moreover, to the driving voltage. An increase
in dopant concentration as well as an increase in driving
voltage contributes to the growth of the low-energy emission.
In analogy to the PL, this red-shift is most pronounced for the
fluorinated compounds 3 and 6, whereas this effect is less dis-
tinct for the devices based on the non-fluorinated complexes
2, 4 and 5. The concentration-dependent shift in the EL
spectra is ascribed to an increase in aggregate emission. We
assume that the current dependence might be ascribed to the
formation of excimers, since an increase in driving voltage
causes an increase in the number of excited platinum(II) com-
plexes that can potentially form excimers. It is also possible
that other effects like field dependent shifts of the recombina-
tion zone contribute to the growth of the low-energy emission
band. To illustrate the dissimilar current- and concentration-
dependent EL spectra of devices based on the fluorinated and
non-fluorinated greenish emitters, the corresponding EL
spectra of devices based on 2 and 3 are shown in Fig. 6 and 7,
exemplarily. The detailed spectra of all dopants are shown in
Fig. S7 and S8 in the ESI.†
Fig. 6 Electroluminescence spectra (normalized to the highest energy emission
peak) of devices with various concentrations of (a) 2 and (b) 3 recorded at a
current density of 20 mA cm−2. Arrows indicate the effect of increasing dopant
concentration.
The OLEDs based on the red light-emitting complex 1
exhibit excellent colour-stability even at various emitter con-
centrations and driving voltages.
doped in PVK/OXD-7 showed that, at least for this complex,
the growth of the low-energy emission band can be assigned to
the formation of excimers. The degree of the observed red-
shifts depends on the substituent at the C^N ligands and the
concentration of the complex in the matrix, whereas this effect
is most distinct for the fluorinated compounds 3 and 6.
Conclusions
The synthesis of the platinum(II) complexes 1–7 and their
Using the non-fluorinated greenish complexes the most
photo- and electroluminescence properties are reported. The efficient and colour-stable device was obtained using com-
red-shifts in the PL and EL spectra of the greenish phenylpyri- pound 5 as an emitter. A maximum luminous efficiency of
dine and tetrahydroquinoline based compounds 2–6 are 4.88 cd A−1 and a power efficiency of 4.65 lm W−1 were
ascribed to aggregate formation in the ground or excited state. achieved. The most efficient devices using the fluorinated com-
Time-resolved measurements of films consisting of 39.1 wt% 3 pounds as emitters were based on 6, whereas a maximum
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BRUKER III AVANCE 600; 19F-NMR spectra were recorded using
a BRUKER AVANCE 400. Chemical shifts were referenced relative
to the internal standard tetramethylsilane (Me4Si) in CDCl3
solutions. For 1H (s = singlet, d = doublet, t = triplet, q = quartet
and m = multiplet), 13C-NMR data, J values are given in hertz
(Hz). Mass spectrometry was carried out using electron impact
(EI, 70 eV) and electrospray ionization (ESI) techniques on a Fin-
nigan MAT 8230 and a Bruker Daltonics MICROTOF instru-
ment, respectively. Fourier transform infrared (FT-IR) spectra
were recorded using a JASCO FT/IR-4200 as well as a Nicolet 510
P FT-IR spectrometer. Elemental analysis (EA) was measured
using a Perkin Elmer 240 B and a HEKAtech EuroEA 3000
(CHNS) setup, correspondingly. UV-vis spectra were measured
with a JASCO V-550 UV-vis spectrometer (1 cm cuvettes) at con-
centrations of 1 × 10−5 mol L−1. The emission spectra were per-
formed using a CARY Eclipse fluorescence spectrophotometer
at a concentration of 1 × 10−5 mol L−1. Chromatography: separ-
ations were carried out using Geduran Si 60 (0.063–0.200 mm).
Thin layer chromatography (TLC) was done on Analtech uni-
plate silica gel GF plates (500 micron, 20 × 20 cm) and develo-
ped with (n-hexane–ethyl acetate: 10/3).
Methyleneiminiumchloride,37,38 the enamine39 and 1-mor-
pholinocyclohexene,40 phenylisoquinoline,14,15 as well as the
dimers13,16,41 D1 and D2 and complexes13,16,41 1 and 2 were
synthesized conforming to partly modified literature pro-
cedures. Besides phenylisoquinoline, D1 and complex 1, all
synthetic procedures can be found in the ESI.†
Photoluminescence quantum yields were measured with a
Hamamatsu Photonics Absolute PL quantum yield measure-
ment system (C9920-02) equipped with a L9799-01 CW Xenon
light source (150 W), monochromator, C10027 photonic multi-
channel analyzer, integrating sphere and employing U6039-05
PLQY measurement software (Hamamatsu Photonics Ltd.,
Shizuoka, Japan).
Fig. 7 Electroluminescence spectra (normalized to the highest energy emission
peak) of devices containing (a) 8.1 wt% 2 and (b) 8.7 wt% 3 driving at various
voltages. Arrows indicate the effect of increasing voltage.
luminous efficiency of 2.08 cd A−1 and a power efficiency of
2.25 lm W−1 were achieved.
As opposed to the phenylpyridine and tetrahydroquinoline
based complexes 2–6, the red-emitting complex 1 did not show
any concentration induced red-shift of the emission colour
and the application of this dopant in OLEDs results in colour
stable and efficient (4.71 cd A−1, 5.12 lm W−1) devices.
We showed that it was possible to enhance the device per-
formance compared to structurally related red and green emit-
ting platinum(II) complexes with sterically less demanding
ancillary ligands previously used in OLEDs with a simpler
device architecture.29,32 However, it is possible to fabricate sig-
nificantly more efficient green and red light-emitting OLEDs
through vapour deposition that employs other classes of
platinum(II) complexes as emitters.33,35
Fabrication of OLEDs
The OLEDs were prepared on pre-cleaned, UV-ozone-treated
ITO-coated glass substrates. A layer of 32 nm PEDOT:PSS
(Clevios P, AI4083, Heraeus) was spin coated onto the substrates
under clean-room conditions and baked at 120 °C for 3 minutes
to remove residual water. To deposit the hole-transporting layers
QUPD42
(N,N′-bis(4-[6-[(3-ethyloxetane-3-yl)methoxy]-hexyl-
oxyphenyl]-N,N′-bis(4-methoxyphenyl)biphenyl-4,4′-diamine))
and OTPD42 (N,N′-bis(4-[6-[(3-ethyloxetane-3-yl)methoxy]-hexyl-
phenyl]-N,N′-diphenyl-4,4′-diamine)) were dissolved in toluene
and 1.5 wt% of the photoinitiator OPPI (4-octyloxydiphenyl-
iodonium-hexafluoroantimonate, Organica Feinchemie) was
added. After spin coating, the films were exposed to UV-light
(360 nm) for 10 s, annealed at 110 °C for 90 s to promote cross-
linking and non-cross-linked residues were removed with THF.
Experimental section
General remarks
All reagents were used as purchased from commercial suppliers The platinum(II) emitters 1–7 were mixed with PVK (poly(vinyl)-
without further purification. All reactions were carried out by carbazole, Sigma Aldrich) and OXD-7 (1,3-bis(N,N-t-butyl-
using standard Schlenk techniques under an atmosphere of dry phenyl)-1,3,4-oxadiazole, Sensient) in a THF solution, whereas
argon. Solvents were used as purchased without further purifi- the PVK : OXD-7-ratio was 70 wt% PVK and 30 wt% OXD-7. TPBi
cation. 1H-NMR and 13C-NMR spectra were recorded using a (1,3,5-tris(1-phenyl-1H-benzo[d]imidazol-2-yl)benzene, Sensient)
BRUKER ARX 125, ARX 200 and ARX 500 as well as using a was evaporated in a K. J. Lesker spectros evaporator at a rate of
13618 | Dalton Trans., 2013, 42, 13612–13621
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0.2 Å s−1 at a base pressure of 2 × 10−7 mbar. The cathode, con- powder. νmax/cm−1 3047, 1678, 1578, 1562, 1398, 1117, 863,
sisting of 3 nm CsF (Alfa Aesar) and 120 nm of Al (Alfa Aesar), 834, 807, 783, 648. 1H-NMR (600 MHz; CDCl3) δ = 7.51–7.58h,i,j,d
was deposited by thermal evaporation in a Leybold high- (m, 4H), 7.67–7.68b (d, 3J = 5.7 Hz, 1H), 7.70–7.74e,g,k (m,
3
3
vacuum chamber at a base pressure of 2 × 10−6 mbar. The struc- 3H), 7.90–7.91f (d, J = 8.3 Hz, 1H), 8.13–8.14c (d, J = 8.3 Hz,
tures of all the used materials are shown in Fig. S3.†
The current density–voltage–luminescence (J–V–L) charac- CDCl3): δ = 119.9, 126.7, 127.0, 127.2, 127.6, 128.4, 128.6,
teristics were measured with a source-measure unit (Keithley 129.9, 130.2, 136.8, 139.6, 142.3. m/z (ESI): 206.1 (M+
1H), 8.64–8.65a (d, 3J = 5.7 Hz, 1H). 13C-NMR (100 MHz;
−
2400) and a calibrated photodiode under an argon atmos- C15H11N requires 205.09). C15H11N (205.09): calcd C 87.77, H
phere. The EL spectra were measured with a calibrated CCD 5.40, N 6.82; found C 87.76, H 5.38, N 6.88.
spectrometer (Ocean Optics).
General procedure for the platinum(II)-µ-dichloro-bridged
dimers13,16,41
Time-resolved spectroscopy
A detailed description of the experimental setup for the time-
resolved photoluminescence measurements can be found else-
where.43 In short, the photoluminescence (PL) was excited
with the third harmonic (355 nm) of a Nd:YAG laser operated
at a repetition rate of 10 Hz. The PL was collected and focused
onto the entrance slit of the monochromator and detected by
an intensified gateable CCD camera. The spectral resolution
was set to 2 nm. The instrument response function is about
1.7 ns. Time resolved measurements have been done at 295 K
under a continuous flow of nitrogen.
K2PtCl4 and the corresponding ligand HC^N (2.2 equiv.) were
stirred in a Schlenk flask for 16 h at 80 °C in a mixture of 2-
ethoxyethanol and water (3 : 1). After cooling down to room
temperature, the respective dimeric complexes were precipi-
tated after the addition of water (20 mL), filtered, and washed
with water and ethanol. The precursor complexes were dried
under reduced pressure.
Dimer D1.14,15 Phenylisoquinoline (205 mg, 1 mmol) and
K2PtCl4 (189 mg, 0.455 mmol). Yield: 130 mg (66%) ochre powder.
General procedure for the platinum(II) complexes13,16,41
Electrochemistry
The respective dimer complexes were stirred in a Schlenk flask
with dipivaloylmethane (3 equiv.) and Na2CO3 (10 equiv.) in 2-
ethoxyethanol at 80 °C for 3–6 h. The crude complexes were
collected by filtration and purified using thin layer chromato-
graphy (n-hexane–ethyl acetate) or column chromatography
(CH2Cl2–n-hexane) and re-crystallized, correspondingly.
Complex 1.
The cyclic voltammetry was carried out in a one-compartment
cell fitted with a Luggin capillary under an argon atmosphere at
room temperature using an Ecochemie Autolab PGSTAT302N as
a potentiostat/galvanostat. For all measurements a Pt-disk elec-
trode was used as a working electrode and an Ag-wire, finally
calibrated with the redox-system ferrocene/ferrocenium, as a
counter electrode. CH2Cl2 was used as a solvent and tetrabutyl-
ammonium hexafluorophosphate (TBAPF6) as a supporting elec-
trolyte (0.1 M). The denoted potentials are the arithmetic
average of the peak potential of corresponding waves of the
semi-derivatives of the current-density-curves.
Synthesis of phenylisoquinoline14,15
Dimer D1 (128 mg, 0.147 mmol), 2,2,6,6-tetramethyl-3,5-heptane-
dione (81 mg, 0.441 mmol) and Na2CO3 (156 mg,
1-Chloroisoquinoline (3 g, 18.303 mmol) was stirred with tetra- 1.470 mmol). Purification by column chromatography on silica
kis(triphenylphosphine)palladium(0) (1.057 g, 0.915 mmol) in gel (CH2Cl2–n-hexane 1/3 as an eluent) yielded red crystals
toluene (90 mL) at room temperature. Phenylboronic acid (95 mg, 55%). (Found C 53.56, H 5.13, N 2.56; C26H29NO2Pt
(2.232 g, 18.303 mmol) in ethanol (60 mL) and aqueous satu- requires C 53.60, H 5.02, N 2.40); νmax/cm−1 3049, 2953, 2925,
rated Na2CO3 solution (60 mL) were added. The mixture was 2899, 2858, 2368, 2346, 2314, 1579, 1544, 1541, 1523, 1496,
heated under reflux for 2 h, cooled down to room temperature 1477, 1452, 1436, 1420, 1391, 1382, 1353, 1312, 1277, 1245,
and poured into 2 N HCl (150 mL). The crude product was col- 12223, 1188, 1175, 1153, 1140, 1099, 1057, 1038; 1H-NMR
lected with dichloromethane (3 × 100 mL). The combined (600 MHz; acetone-d6; Me4Si): δ = 1.33m (s, 9H, CH3), 1.34k (s,
organic layers were washed with a saturated NaHCO3 solution 9H, CH3), 5.96l (s, 1H, CH), 7.16–7.21h (m, 1H, ArH), 7.24i (td,
(75 mL) and water (15 mL), dried over MgSO4 and the solvent J = 7.3, 1.3 Hz, 1H, ArH), 7.78b (d, J = 6.5 Hz, 1H, ArH), 7.84d,j
was removed under reduced pressure. The crude product was (ddd, J = 15.2, 7.7, 1.3 Hz, 2H, ArH), 7.92e (ddd, J = 8.1, 7.0, 0.9
purified by column chromatography on silica gel (n-hexane– Hz, 1H, ArH), 8.08f (d, J = 8.1 Hz, 1H, ArH), 8.22g (d, J = 7.3 Hz,
ethylacetate 10/3 as an eluent). Yield: 2.606 g (69%) colourless 1H, ArH), 9.04c (d, J = 8.7 Hz, 1H, ArH), 9.05–9.12a (m, 1H,
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Dalton Transactions
ArH); 13C-NMR (100 MHz; acetone-d6; Me4Si): δ = 29.3, 29.6,
42.4, 42.9, 94.6, 121.7, 124.6, 127.4, 127.5, 129.4, 130.1, 130.5,
130.6, 132.4 133.2, 139.1, 140.5, 144.8, 147.8, 170.0, 193.9,
9 L. Flamigni, A. Barbieri, C. Sabatini, B. Ventura and
F. Barigelletti, Top. Curr. Chem., 2007, 281, 143; E. Holder,
B. M. W. Langeveld and U. S. Schubert, Adv. Mater., 2005,
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195.4 ppm; C 53.60, H 5.02, N 2.40; m/z (ESI): 583.2 (M+
−
C26H29NO2Pt requires 582.18); λmax-absorption(CH2Cl2)/nm 255 10 J. Kalinowski, V. Fattori, M. Cocchi and J. A. G. Williams,
(ε/dm3 mol−1 cm−1 43 000), 274 (38 300), 345 (17 800), 408
(13 900); λmax-emission(CH2Cl2)/nm 597, 639.
Coord. Chem. Rev., 2011, 255, 2401; K. Wong and V. Yam,
Coord. Chem. Rev., 2007, 251, 2477.
11 S. Develay and J. A. G. Williams, Dalton Trans., 2008, 4562.
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Acknowledgements
The authors acknowledge the DFG (Deutsche Forschungsge-
meinschaft) for financially supporting this work with grant 13 V. Adamovich, J. Brooks, A. Tamayo, A. M. Alexander,
applications ME1246/18-1, HE5577/2-1, HO3911/3-1, HO3911/
6-1 and RI449/9-1. K.M., D.H. and D.K. acknowledge funding
P. I. Djurovich, B. W. D’Andrade, C. Adachi, S. R. Forrest
and M. E. Thompson, New J. Chem., 2002, 26, 1171.
by the State of Northrhine-Westfalia and the Europäischer 14 N. Tian, Y. V. Aulin, D. Lenkeit, S. Pelz, O. V. Mikhnenko,
Fonds für Regionale Entwicklung (EFRE) through the
PROTECT project, which is part of the Centre of Organic Pro-
P. W. M. Blom, M. A. Loi and E. Holder, Dalton Trans.,
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Ullrich Scherf for granting access to the tools of the Macro-
molecular Chemistry at the University of Wuppertal (BUW).
R. Schiewek, D. Klink, O. J. Schmitz, L. González, M. Schäferling
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