Luminescent neutral platinum complexes bearing an asymmetric N^N^N ligand for high-performance solution-processed OLEDs

Article abstract of DOI:10.1002/adma.201202123

The synthesis and full characterization of new platinum complexes bearing a bulky asymmetric dianionic tridentate ligand is reported. The hindrance of the ligand prevents detrimental intermolecular interactions yielding to highly emitting species in both crystalline state and thin-film. Such properties prompted their successful use in solution-processed OLEDs, showing remarkable external quantum efficiency up to 5.6%. Copyright

Full text of DOI:10.1002/adma.201202123

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Luminescent Neutral Platinum Complexes Bearing an
Asymmetric N^N^N Ligand for High-Performance Solution-
Processed OLEDs
Cristina Cebrián, Matteo Mauro, Dimitrios Kourkoulos, Pierluigi Mercandelli, Dirk Hertel,
Klaus Meerholz, Cristian A. Strassert,* and Luisa De Cola*
have been reported. Herein we present a new family of neutral
In the past few decades, the search for highly emitting mate-
platinum(II) complexes comprising this N^N^N-type chromo-
rials for electroluminescent devices has attracted a great deal
phoric ligand that displays remarkable photophysical proper-
of attention because of their potential applications in energy-
ties in solid state (Figure 1). The introduction of bulky and alkyl
saving organic light-emitting diodes (OLEDs) for full-color dis-
plays and solid-state lighting.[1] Phosphorescent transition metal
complexes are particularly appropriate for this purpose due to
the strong spin-orbit coupling exerted by the heavy metal.[2]
This effect allows the harvest of both singlet and triplet elec-
trogenerated excitons in devices containing such compounds,
leading to a potentially achievable 100% internal quantum
efficiency.[2a,3] In this respect, a leading position is held by d6
octahedral iridium(III)[4] and d8 square-planar platinum(II)[5]
complexes since they possess wide and tunable emission color
range as well as high photoluminescence quantum yields
(PLQYs).[2a] However, the latter often have low solubility and
strong tendency towards aggregation through platinum-plat-
inum and π−π interactions,[6] which hamper handling and
purification, and in many cases have detrimental effects on
photoluminescence properties.
Interestingly, while there are several reports on highly effi-
cient iridium(III)-based light-emitting devices,[7] the number
of the platinum(II)-based counterparts is rather low, and
only few showed comparable performances with the former
ones.[8] Amongst the possible ligands coordinated to the plat-
inum ion, relevant role is played by the symmetric and asym-
metric tridentate systems.[9] In most cases the cyclometalated
chelates have been used and to the best of our knowledge no
complexes bearing an asymmetric dianionic N^N^N ligand
substituents into the structure enhanced solubility and process-
ability of the final complexes, and prevented axial interactions
between neighbor molecules.[10] Even though these compounds
showed poor photoluminescence quantum yield in fluid solu-
tion, their emission switched-on in solid state reaching the
remarkable value of 0.61 in poly(methyl methacrylate) (PMMA)
thin-film. By using these bulky asymmetric platinum complexes
as triplet emitters in solution-processed polymer-based light-
emitting diodes (PLEDs), power efficiency (PE) and external
quantum efficiency (EQE) as high as 16.4 lm W−1 and 5.6%,
respectively, have been achieved. The attained results outper-
form the already reported PLED devices based on platinum(II)
dopants and are approaching the more sophisticated, yet gener-
ally more efficient, vacuum-processed OLEDs.[8a−c]
Since both 1,2,4-triazole and tetrazole rings bind to the plat-
inum ion as anionic ligands by N-deprotonation,[11] we have
designed the asymmetric ligand 2-((3-adamantan-1-yl)-1H-1,2,4-
triazol-5-yl)-6-(1H-tetrazol-5-yl)pyridine (H2trzpyttz). It com-
bines the dianionic nature, leading to neutral complexes upon
Pt(II) coordination, with the possibility of easily substituting
the tridentate (chromophoric) ligand for tuning the HOMO–
LUMO gap. In order to obtain neutral platinum(II) complexes,
ancillary ligands L such as triphenylphosphine and 4-amylpy-
ridine were selected, yielding to the complexes Pt(trzpyttz)
phos and Pt(trzpyttz)py, respectively. The synthesis is a one-pot
reaction employing the asymmetric H2trzpyttz, the desired L,
PtCl2(DMSO)2 (DMSO = dimethylsulfoxide) as platinum(II)
precursor, triethylamine as non-coordinating and non-nucle-
ophilic base and acetonitrile as solvent.[8d] The final compounds
were easily purified by column chromatography and obtained
Prof. L. De Cola, Dr. C. A. Strassert, Dr. C. Cebrián,
Dr. M. Mauro
Physikalisches Institut and Center for
Nanotechnology (CeNTech)
Universität Münster
Heisenbergstrasse 11, 48149 Münster, Germany
Tel.: +49-251-53406957 or +49-251-53406906
Fax: +49-251-53406958
E-mail: ca.s@uni-muenster.de; decola@uni-muenster.de
D. Kourkoulos, Dr. D. Hertel, Prof. Dr. K. Meerholz
Department of Chemistry
Universität zu Köln
Luxemburger Strasse 116, 50939 Köln, Germany
Dr. P. Mercandelli
Department of Chemistry
Università degli Studi di Milano
Via Giacomo Venezian 21, 20133 Milan, Italy
DOI: 10.1002/adma.201202123
Figure 1. General molecular structure of complexes Pt(trzpyttz)L.
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T a b le 1 . Photophysical properties of the complexes Pt(trzpyttz)phos and Pt(trzpyttz)py in solution in degassed THF, thin-film and 77 K 2-MeTHF rigid
matrix.
a,b)
Comp.
Medium (T)
[K]
PLQY
0.003
kr
knr
λ
max,abs (ε)
λmax,em
τobs
[104 s−1
]
[104 s−1
]
[nm, 104 M−1 cm−1
]
[nm]
Pt(trzpyttz) phos
THF (298)
268 (8.1), 304 (1.5),
416 (0.07)
504, 532, 572(sh)
1 ns (75%),
94 ns (15%)c)
3.7
1228.7
2-MeTHF (77)
PMMA 10 wt% (298)
Neat film (298)
-
-
-
483, 515, 552, 592(sh)
504, 530
25.0 μs
19.2 μs
0.29
0.13
1.5
1.2
3.7
7.8
508(sh), 541
3.0 μs (10%),
11.1 μs (90%)
Crystal (298)
THF (298)
-
483, 515, 552, 592(sh)
501, 528, 570(sh)
0.40
0.05
2.1
6.7
3.1
19.4 μs
Pt(trzpyttz) py
262 (2.6), 307 (1.7),
413 (0.07)
18 ns (15%),
751 ns (85%)c)
126.5
2-MeTHF (77)
-
-
460, 493, 524, 575(sh)
568
16.0 μs
PMMA 10 wt% (298)
0.55
0.60
5.5
6.0
4.5
4.0
1.0 μs (73%),
9.9 μs (27%)
Neat film (298)
-
573
0.8 μs (84%),
10.0 μs (16%)
a)‘sh’ denotes a shoulder; b) exc: 300 nm; c)
λ λexc: 405 nm.
as microcrystalline yellow-green powders. They were fully char-
acterized by using 1H and 31P NMR, high-resolution mass spec-
trometry and elemental analysis (see Schemes S1 and S2 of the
Supporting Information).
Figure S2 and SI for computational details). In the case of
Pt(trzpyttz)phos, the differences between calculated and experi-
mental geometrical parameters were systematically below 1.5%
(Table S1), confirming the suitability of the selected theoretical
model. Moreover, for the most relevant frontier orbitals, partial
molecular orbital diagram along with their isodensity surface
plots and corresponding energies are shown in Figures S3 and
S4 and Table S2, respectively. Despite the fact that the two com-
plexes bear ancillary ligands with different electronic proper-
ties, they both show rather similar HOMO and LUMO, which
are mainly localized on the tridentate ligand. In particular,
the HOMO (–6.34 eV and –6.39 eV, for Pt(trzpyttz)phos and
Pt(trzpyttz)py, respectively) appears to be the combination of π
orbitals of the triazole (trz) and the dxz orbitals of the platinum,
with minor contribution of the π system of the pyridine (py)
and tetrazole (ttz). On the other hand, for both derivatives the
LUMO lies at –2.28 eV and –2.36 eV, for Pt(trzpyttz)phos and
Pt(trzpyttz)py, respectively (–3.51 eV and –3.48 eV as experi-
mentally obtained by means of cyclic voltammetry, see Table
S3 and Figure S5). Such virtual orbital displays a π∗ character
and is mostly centered on the pyridine moiety of the triden-
tate ligand, being the contribution of the metal and ancillary
ligand orbitals negligible. Thus, the triazole and the pyridine of
the tridentate ligand are expected to be strongly involved in the
description of the low-lying excited states (see below).
The most meaningful photophysical data are listed in Table 1
(extended version in Table S4), and the absorption and emis-
sion spectra displayed in Figure 2. In diluted tetrahydrofuran
(THF) fluid solution, the complexes exhibit strong absorption
bands (ε = 1.5–2.6 × 104 M−1 cm−1 ) at high energy (λ < 350 nm).
These bands mainly correspond to convolution of singlet-man-
ifold ligand-centered (1LC) transitions involving the triden-
tate chromophoric ligand (π–π∗) with a minor contribution of
metal-centered d orbitals, as evidenced by TD-DFT calculations
(Figures 2a and S6a for Pt(trzpyttz)phos, Figures 2c and S6b for
Pt(trzpyttz)py and Table S5). The low energy (λ = 350–500 nm),
Single crystals of the phosphine derivative Pt(trzpyttz)phos,
suitable for X-ray diffractometric analysis, were obtained by
slow diffusion of n-hexane into a concentrated dichloromethane
solution at room temperature. In the complexes the platinum
atom adopts a square-planar coordination geometry, distorted
in order to comply with the constraints imposed by the triden-
tate ligand (Figure S1a). In particular, the N(trz)–Pt–N(py) and
the N(ttz)–Pt–N(py) bond angles [78.50(11)° and 78.21(10)°,
respectively] are narrower than expected for a square-planar
complex. Steric hindrance between the adamantyl substituent
on the triazole ring and the triphenylphosphine ancillary ligand
results in a sensible deviation from planarity for the complex,
as can be measured by the out-of-plane bending of the phos-
phorus atom of 8.92(17)°. The coordination mode of the tria-
zole ring through its N(1) atom was confirmed.[12] This result is
in line with the known structures of platinum complexes with
1,2,4-triazolato ligands, which invariably adopts a κN1 coor-
dination mode.[13] As far as the crystal packing is concerned,
the complexes are oriented in a parallel head-to-tail staggered
fashion, with alternating shorter and longer Pt···Pt distances
(above 5 Å), ruling out the presence of any significant metal–
metal interaction (Figure S1c). In addition, despite of the face-
to-face arrangement of the tridentate ligands in nearby com-
plexes (with an interplanar distance of ca. 3.2 Å), their interac-
tions also appears to be negligible. Indeed, due to the presence
of the bulky adamantyl and triphenylphosphine moieties, the
ligands adopt a parallel-displaced arrangement and the shortest
distances between the centroids of the heteroaromatic rings are
larger than ca. 4 Å (Figure S1b).
For both complexes, geometrical optimization of their
electronic ground state (S0) was carried out by means of den-
sity functional theory (DFT) calculations at PBE0 level (see
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Figure 2. a) Absorption spectrum for complex Pt(trzpyttz)phos in fluid degassed THF solution. Calculated absorption peaks in vacuum are shown
for comparison; b) emission spectra for complex Pt(trzpyttz)phos in fluid degassed THF (solid line), glassy 2-MeTHF (circles) and 10 wt% PMMA
thin-film (triangles). λexc = 300 nm; c) absorption spectrum for complex Pt(trzpyttz)py in fluid degassed THF solution. Calculated absorption peaks in
vacuum are shown for comparison; d) emission spectra for complex Pt(trzpyttz)py in fluid degassed THF (solid line), glassy 2-MeTHF (circles) and
10 wt% PMMA thin-film (triangles). λexc = 300 nm.
broader, featureless and less intense band (ε ≈ 104 M−1 cm−1 )
represents the S0→S1 transition (HOMO→LUMO), and mainly
involves the platinum d orbitals (partially mixed with the π
orbitals of the tridentate ligand) and π∗ orbitals of the chelate.
This transition is computed at 410 (f = 0.01) and 411 nm (f =
0.01) for Pt(trzpyttz)phos and Pt(trzpyttz)py, respectively, and
can be ascribed to the spin-allowed singlet-manifold metal-
ligand-to-ligand charge-transfer (1MLLCT). Such findings are
in nice agreement with similar already reported compounds.[14]
Upon excitation at 300 nm, in deareated fluid THF solution at
room temperature, both complexes showed structured emission
in the green region of the visible spectrum, suggesting an emit-
ting state with sizeable LC character (Figure 2b for Pt(trzpyttz)
phos and Figure 2d for Pt(trzpyttz)py). The PLQYs are rather
low in solution and the excited-state decays with multiexponen-
tial kinetics. Interestingly, the photophysics depend on the ancil-
lary ligand since for Pt(trzpyttz)phos the emission quantum
yield and excited state lifetimes are lower than for Pt(trzpyttz)
py. This can be explained considering the distortion of the com-
plex (see above) and therefore, a higher non-radiative constant
(Table 1). Moreover, this intricate behavior could be further
explained assuming thermally accessible dark states close to the
emitting one (see below).[15] The good solubility of these com-
plexes allowed concentration studies in order to investigate the
possible formation of aggregates, that were carried out on the
flatter Pt(trzpyttz)py (Figure S7). Similarly to Pt(trzpyttz)phos
in the crystalline state, Pt(trzpyttz)py did not seem to aggregate
in solution within the 10−5 –10−4 M concentration range, which
strongly suggests that only one adamantyl group is enough for
avoiding axial interactions in this kind of platinum complexes.
Furthermore, for Pt(trzpyttz)py upon decreasing solvent polarity
from DMF to THF to toluene, a slight blue shift (163 cm−1 ) was
observed. Concomitantly, the emission displayed noticeable
increase of the vibronic structure, followed by enhancement
of PLQY and elongation of the excited state lifetime, that is
monoexponential in the case of toluene solution (PLQY = 0.35,
τ = 3.91 μs) (Table S4, Figure S8). Going from fluid solution
to 2-methyl-THF (2-MeTHF) glassy matrix at 77 K, the emis-
sion spectra showed hypsochromic shifts and even more pro-
nounced vibronic progression attributable to intraligand modes
(range 1230–1450 cm−1 ),[16] with monoexponential lifetimes of
25 and 16 μs, for complex Pt(trzpyttz)phos and Pt(trzpyttz)py,
respectively, which is typical of phosphorescent platinum(II)
complexes (Figures 2b and 2d). All these findings are indicative
of the admixed metal-perturbed triplet-manifold ligand centered
character of the radiative transition.[14] Moreover, they support
the presence of a thermally-accessible and strongly quenching
state with intraligand charge-transfer (ILCT) nature, close in
energy to the lowest-lying emitting one as depicted in Figure
S9. For both complexes at their optimized T1 state (see Table
S6), the Lowest Singly Occupied Molecular Orbital (LSOMO)
is located throughout the chromophoric ligand and the metal,
whilst the Highest Singly Occupied Molecular Orbital (HSOMO)
comprises only the triazole and the pyridine π systems, and it
is slightly perturbed by the metal orbitals (Figure S10). Com-
paring Pt(trzpyttz)phos and Pt(trzpyttz)py, the calculated emis-
sion energies by means of ΔSCF technique (2.19 and 2.23 eV,
respectively) nicely agree with the different emission energies
observed at 77 K in 2-MeTHF (2.53 and 2.70 eV, respectively).
Furthermore, complementary information are provided by ana-
lyzing plots of electron spin density and the difference in elec-
tron density between the excited state T1 and the ground state S0
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Figure 3. Electroluminescence (EL) characteristics of the device A and B, incorporating complex Pt(trzpyttz)phos and Pt(trzpyttz)py, respectively. The
devices have the following architecture: ITO/PEDOT:PSS (32 nm)/QUPD (10 nm)/OTPD (8 nm)/PVK:OXD-7:platinum complex (30 nm)/TPBi (25 nm)/
CsF (3 nm)/Al (120 nm). a) Schematic representation of the energy levels in the optimized devices; b) optimized device architecture; c) EL spectra
of the device A (circles) and B (triangles) recorded at current density of 20 mA cm−2 ; d) power efficiency (PE) vs. current density plots for device A
(circles) and device B (triangles).
using different dopant/polymer ratios. The best devices were
obtained when the platinum complexes were blended in a
PVK/OXD-7 matrix, where PVK is poly(vinyl)carbazole and
OXD-7 is 1,3-bis(N,N-t-butyl-phenyl)-1,3,4-oxadiazole. In such
devices, the electron transporting material, namely OXD-7, was
employed in order to improve the charge-transport properties
of the matrix. The chemical structure of all the materials used
for the device fabrication is given in Figure S13. The simpli-
fied architecture of the devices can be described as follows:
ITO/PEDOT:PSS (32 nm)/QUPD (15 nm)/OTPD (13 nm)/
PVK:OXD-7:platinum complex (40 nm)/CsF (3 nm)/Al (120 nm)
(Figure S14). The HOMO and LUMO levels of the materials
used in the devices are schematically depicted in Figure S15.
The two cross-linkable hole transporting layers, namely N,N′-
bis(4-[6-[(3-ethyloxetane-3-yl)methoxy]-hexyloxyphenyl]-N,N′-
computed at the T1 optimized geometry (Figure S11) (S0←T1).
Noteworthy, the triplet spin density is shared between triazole
and pyridine rings, as well as the platinum atom, where also
reorganization of the electron density takes place, confirming
3
the admixed metal-perturbed LC/3ILCT character of the radia-
tive deactivation process.
The luminescence efficiency of both derivatives was enhanced
upon dispersion into PMMA rigid matrix. In particular, in the
case of the bulkier Pt(trzpyttz)phos, no visible Pt···Pt inter-
action was observed going from solution to solid state, with
PLQYs of 0.29 in 10 wt% PMMA thin-film and 0.40 in crystal-
line state, which is amongst the highest for a phosphorescent
crystal at ambient temperature (see Figures 2b and S12a).[17]
Nonetheless, the behavior of the flatter Pt(trzpyttz)py respect
to the former was remarkably different, displaying broad and
featureless emission spectra for samples in thin films at any
concentration level, with a concomitant bathochromic shift of
ca. 46 nm (1518 cm−1 ) (Figures 2d and S12b). The formation of
a lower-lying excited-state upon establishment of π−π interac-
tions in the solid state could account for such behavior. How-
ever, the rigid media contribute to the decrease of radiationless
deactivation processes in comparison to fluid solution (Table 1),
thus promoting the increase of the phosphorescence quantum
yield, which reached values as high as 0.61 at 25 wt% doping
concentration of Pt(trzpyttz)py in PMMA.
bis(4-methoxyphenyl)biphenyl-4,4′-diamine)
(QUPD)
and
N,N′-bis(4-[6-[(3-ethyloxetane-3-yl)methoxy]-hexylphenyl]-N,N′-
diphenyl-4,4′-diamine) (OTPD), were used to enhance the hole
injection as well as for their electron blocking capabilities.[18]
The best electroluminescence (EL) performances were obtained
when equimolar doping concentration of Pt(trzpyttz)phos and
Pt(trzpyttz)py, 15.6 and 13.5 wt% respectively, were used into
the host, resulting in devices with maximum PE of 3.9 and
4.7 lm W−1 , respectively.
To improve the charge balance and, hence, better posi-
tioning the recombination zone, an additional 1,3,5-tris(1-
phenyl-1H-benzo[d]imidazol-2-yl)benzene (TPBi) layer was
introduced acting as exciton/hole blocking and electron trans-
port layer, while keeping similar the total thickness of the
device. The energy level diagram is depicted in Figure 3a. The
The appealing properties exhibited by Pt(trzpyttz)phos and
Pt(trzpyttz)py such as good solubility in solvents like THF, low
tendency towards aggregation and remarkably high PLQYs in
solid state prompted us to investigate their application in elec-
troluminescent devices. We constructed polymer-based devices
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Mancha, DAAD and BMBF for financial support. MM gratefully
thanks the Alexander von Humboldt Foundation for financial support.
Alessandro Aliprandi is gratefully acknowledged for the electrochemical
measurements.
simplified architecture of the devices can be described as fol-
lows: ITO/PEDOT:PSS (32 nm)/QUPD (10 nm)/OTPD (8 nm)/
PVK:OXD-7:platinum complex (30 nm)/TPBi (25 nm)/CsF
(3 nm)/Al (120 nm) (Figure 3b). As expected, the TPBi layer
led to a significant improvement of the device performances.
For Pt(trzpyttz)phos, maximum luminance efficiency (LE) of
13.4 cd A−1 and PE of 13.8 lm W−1 were obtained at the optimal
15.6 wt% doping concentration (device A), with maximum
EQE of 4.9%. Moreover, it is noticeable that devices fabricated
with Pt(trzpyttz)py as triplet emitter outperformed those with
Pt(trzpyttz)phos when the best architecture device was used.
In particular, the employment of 13.5 wt% Pt(trzpyttz)py
allowed the achievement of high performances with peak LE
of 15.5 cd A−1 , PE of 16.4 lm W−1 and EQE as high as 5.6%
(device B). Noteworthy, the here reported efficiencies, yet for
simple solution-processed devices, approach the highest values
ever reported for green emitting vacuum-processed OLEDs
incorporating platinum(II)-based emitters.[8a] The comparison
of the EL spectra and PE vs. current density plots for devices A
and B are shown in Figures 3c and 3d, respectively. The other
current-voltage-luminance (J–V–L), EQE–J and EQE–L charac-
teristics are depicted in Figure S16 and EL performances listed
in Table S7. In such devices, the comparison of the EL spectra
of Pt(trzpyttz)py with respect to Pt(trzpyttz)phos displays the
expected slight hypsochromic shift of the emission (Figure 3c).
Furthermore, both EL spectra are very similar to the corre-
sponding photoluminescence (PL) spectra of the complexes in
solution, as exemplary depicted for Pt(trzpyttz)py in Figure S17,
supporting the idea that in such condition and for both com-
pounds the same radiative transition is involved in both optical
and electrically generated excited states. It is worth to notice
that such findings rule out any significant Pt···Pt or π–π inter-
actions between the dopant molecules in device condition, in
spite of the relatively high dopant concentration, showing excel-
lent color purity,[19] with very weak voltage dependence of the
emission spectra (Figure S18) and great stability (Figure S19).
In conclusion, we have shown that asymmetric dianionic
tridentate ligands can be used to synthesize highly emitting
neutral platinum complexes with improved solubility and sup-
pressed aggregation tendency. The polymer-based devices, incor-
porating such compounds as triplet emitters, displayed notice-
ably good performances and high color purity, approaching
those attained by the vacuum processed analogs.
Received: May 27, 2012
Revised: July 23, 2012
Published online:
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Experimental Section
Details
regarding
the
synthetic
procedures,
photophysical
characterization, computational studies and device fabrication are
described in the Supporting Information, and are available online
from Wiley InterScience or from the authors. CCDC-879817 contains
the crystallographic data for Pt(trzpyttz)phos·CH2Cl2. These data can
be obtained free of charge from the Cambridge Crystallographic Data
Centre.
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Acknowledgements
The authors would like to thank the German Federal Ministry for
Education and Research (BMBF) for funding with the project no.
13N10529 (So-light). CC acknowledges the University of Castilla-La
©
wileyonlinelibrary.com
Adv. Mater. 2012,
DOI: 10.1002/adma.201202123
2012 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim
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©
6
wileyonlinelibrary.com
2012 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim
Adv. Mater. 2012,
DOI: 10.1002/adma.201202123