(CC*) Cyclometalated binuclear N-heterocyclic biscarbene platinum(ii) complexes-highly emissive phosphorescent emitters

Article abstract of DOI:10.1039/c3dt50841b

A series of bimetallic N-heterocyclic carbene (NHC) platinum(ii) complexes with the general formula [Pt(NHC)(L)]₂Ph were synthesized, which are composed of two [Pt(NHC)(L)] (L = acetylacetone, dipivaloylmethane or dimesitoylmethane; NHC = 3-met

Full text of DOI:10.1039/c3dt50841b

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(
C^C*) Cyclometalated binuclear N-heterocyclic
biscarbene platinum(II) complexes – highly emissive
phosphorescent emitters†
Cite this: Dalton Trans., 2013, 42, 9847
Alexander Tronnier and Thomas Strassner*
A series of bimetallic N-heterocyclic carbene (NHC) platinum(II) complexes with the general formula
[
2
Pt(NHC)(L)] Ph were synthesized, which are composed of two [Pt(NHC)(L)] (L = acetylacetone, dipivaloyl-
methane or dimesitoylmethane; NHC = 3-methylimidazole or 3-benzylimidazole) fragments. Both frag-
ments are cyclometalated to the same central phenyl ring in the para- or meta-position. All complexes
have been fully characterized by standard techniques, two of them additionally by solid-state structures.
Photoluminescence properties were investigated and very high quantum yields of 76–93% at room
temperature have been observed. For a single-matrix device with an emitter concentration of 30%, a
current efficiency of 25 cd A− , an external quantum efficiency of 8.5%, and a luminance efficiency of 10
lm W−1 were achieved at 300 cd m−2. Density functional theory (DFT) calculations were performed to
support experimental data and gain further insight into the photoluminescence behaviour.
Received 28th March 2013,
Accepted 28th April 2013
1
DOI: 10.1039/c3dt50841b
www.rsc.org/dalton
4
Pt–Pt distance. Other approaches introduced a metal inter-
action via the π-system of an adequate ligand spacer.
Introduction
5
Transition metal complexes are well known for their promising
Recently our group has introduced cyclometalated mono-
phosphorescence properties arising from heavy-metal induced nuclear platinum(II) NHC complexes of the type [Pt(NHC)-
spin–orbit coupling and their potential application in phos- (acac)] where NHC is a monoanionic bidentate aryl–alkyl func-
1
6
phorescent organic light-emitting devices (PhOLEDs). Despite tionalized imidazole ligand. These complexes, with the acetyl-
the broad range of metals investigated in organometallic com- acetonate auxiliary ligand, show moderate to excellent
2
3
plexes, neutral iridium and platinum compounds carrying quantum yields and good thermal stability.
ligands with an extended π-system have been found to be
So far bimetallic cyclometalated platinum(II) complexes
especially interesting because of their exceptional lumine- where both metal atoms are bound to the same phenyl ring
scence properties. While iridium complexes in most cases carry are very rare and of the general type A or B while the com-
7
three cyclometalated bidentate ligands (either as homo- or pounds can be neutral or charged (Chart 1). Additionally
8
heteroleptic complexes), platinum(II) complexes often bear one some complexes bearing phosphor donors were presented, as
9
bidentate or tridentate cyclometalated ligand.
well as complexes with the two metals in the meta-position.
Over the last decade efforts were made to tune the photo- Compound C is the first bimetallic cyclometalated example for
physical properties of platinum complexes not only by a [M(C^C*)] type complex with an imidazole based carbene
1
0
different substitution patterns at the ligands but also by donor.
metal–metal interaction. A number of systems have since been
developed, where a direct d–d orbital interaction of the metal
atoms leads to significant colour shifts in the emission wave-
length and alternate absorption behaviour depending on the
Physical Organic Chemistry, Technische Universität Dresden, Bergstrasse 66,
0
1069 Dresden, Germany. E-mail: thomas.strassner@chemie.tu-dresden.de;
Fax: (+49) 351-463-39679; Tel: +0351-46338571
Electronic supplementary information (ESI) available: Experimental details and
†
characterization data. The CIF files, details of the photophysical measurements,
NMR spectra as well as Cartesian coordinates for the optimized structures.
CCDC 930701–930703. For ESI and crystallographic data in CIF or other
electronic format see DOI: 10.1039/c3dt50841b
Chart 1 Examples of bimetallic cyclometalated complexes.
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To date, only a very limited number of transition metal
complexes of the general C type have been published. Herein
we present new bimetallic cyclometalated platinum(II) com-
plexes of type C as well as the very first platinum(II) NHC com-
pound with two transition metals cyclometalated to the same
ring in a meta fashion.
Dalton Transactions
11
The geometry of these novel platinum(II) complexes is dis-
cussed on the basis of two solid-state structures, supported by
DFT calculations on the B3LYP/6-31G(d) level. Since investi-
gations of bimetallic complexes with two platinum(II) centres
as phosphorescent emitters are scarce, we conducted a
thorough examination of their photoluminescent behaviour.
The performance in a light-emitting diode has been tested for
one of the complexes. DFT calculations were used to examine
electronic and emitting states in more detail.
Chart 2 Synthesized complexes 3a–e.
1
195
characteristic long-range H- Pt coupling (7.44 ppm, 27.5
Hz), resulting in the formation of a pseudo-triplet, was found.
Results and discussion
Synthesis
1
95
For four of the five complexes
be performed. The chemical shifts range from −3354 ppm to
3402 ppm in chloroform and up to −3416 ppm for 3e in
Pt-NMR spectroscopy could
2
The synthesis of [Pt(C^C*)(acac)] Ph complexes follows the
established route: reaction of imidazolium salts (Scheme 1)
with silver(I) oxide to the silver(I) complexes, subsequent trans-
−
DMSO indicating only small differences in the electron density
at the platinum atom.
metalation
with
dichloro(1,5-cyclooctadiene)platinum(II)
To verify the three-dimensional structure, single crystals of
without further purification, followed by treatment with excess
3b and 3c suitable for X-ray diffraction were obtained by slow
auxiliary ligand and potassium tert-butanolate after solvent
evaporation of the solvent from a saturated dichloromethane–
acetonitrile solution under ambient conditions (Fig. 1).
6
exchange. Complexes 3a–e could be synthesized in yields
ranging from 6 to 21% (Chart 2). Initially we synthesized com-
pounds 3a and 3e with the acac ligand, but due to their poor
solubility in common organic solvents they could only be
characterized in concentrated DMSO solutions. To achieve
better solubility and to probe their effects on the photo-
physical properties auxiliary ligands with bulky substituents
were chosen for 3b–d. This strategy was unfortunately not
applicable to 3e due to steric hindrance of the substituents
between the two auxiliary ligands.
In both compounds each metal atom is square-planar co-
ordinated and as expected the Pt–C bond lengths differ
slightly. The distance between the metal and the cyclometa-
lated carbon atom C5 is larger than the carbene carbon plati-
num bond. Very small dihedral angles including the central
five-membered ring with the platinum(II) centre are proof of a
Characterization
All complexes were investigated by 1D- and 2D-NMR (COSY,
HSQC, HMBC, NOESY) spectroscopy to unambiguously assign
the NMR signals (see ESI†). Interestingly only for 3b a
Fig. 1 Solid-state structures of 3b and 3c. Thermal ellipsoids are drawn at 50%
probability. Hydrogen atoms are omitted for clarity. Selected bond lengths [Å]
and angles [°]: 3b: Pt1–C1 1.943(3); Pt1–C5 1.987(3); Pt1–O1 2.086(2); Pt1–O2
2.051(2); C1–Pt1–C5 80.40(12); O1–Pt1–O2 89.82(8); C4–N1–C1–Pt1 −4.1(4);
N1–C1–Pt1–O1 −174.1(2); 3c: Pt1–C1 1.955(4); Pt1–C5 1.980(4); Pt1–O1 2.075(3);
Pt1–O2 2.046(3); C1–Pt1–C5 80.34(16); O1–Pt1–O2 90.10(11); C4–N1–C1–
Pt1 4.9(5); N1–C1–Pt1–O1 175.9(3).
Scheme 1 Synthesis of imidazolium salts 2a–c.
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848 | Dalton Trans., 2013, 42, 9847–9851
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Table 1 Photoluminescence data (2 wt% in PMMA, room temperature) of
cyclometalated complexes 3b–3d
Complexes 3b–d emit in the blue-green region of the visible
spectrum. The emission spectra given in Fig. 2 reveal a distinct
band structure arising from vibronic coupling and a predomi-
nant NHC ligand participation in the emission process. Three
strong bands at 487, 523 and 560 nm with a weak fourth are
discernible.
a
c
e
f
g
λ
[
exc
λ
em
τ
o
k
r
k
nr
3
b
d
[μs] [10 s−1] [10 s−1]
3
nm] CIE x;y
[nm]
ϕ
3
3
3
b
c
d
355
355
340
0.233; 0.537 486, 521 0.86 10.9
0.255; 0.544 489, 523 0.76 8.8 113.8
0.240; 0.552 487, 524 0.93 10.4 96.8
91.9
14.9
35.9
7.3
For 3a and 3e no PMMA films could be fabricated because
of their poor solubility.
a
d
b
c
Excitation wavelength. CIE coordinates at rt. Emission wavelength.
e
Quantum yield at λexc, N atmosphere. Decay lifetimes (excited by
2
f g
o v r v v
laser pulses (355 nm, 1 ns)) given as τ = τ /ϕ. k = ϕ/τ . knr = (1 − ϕ)/τ .
Device performance
Complex 3b was selected for tests in a device because of its
promising photoluminescence properties and especially its
synthetic accessibility. The layout of the device is given in
Fig. 3. The device was produced by thermal deposition under
high vacuum conditions onto a glass substrate coated with
indium-tin-oxide (ITO), where different concentrations
planar complex with only the substituents of the acac sticking
out (C1–N1–C4–C5 −0.9° for 3b and −3.1° for 3c). While the
C1–Pt1–C5 angle (around 80°) deviates significantly from the
perfect 90° angle, the O1–Pt1–O2 angle of the auxiliary acac
ligand (around 90°) is ideal for a square-planar coordination
geometry. Both complexes show higher symmetry in the solid-
state structure with an inversion centre in the middle of the
central phenyl ring. For the NHC precursor 2b single crystals
suitable for X-ray diffraction could be obtained as well (see
ESI, Fig. S17†).
(
5–30%) of complex 3b were doped into the matrix material 4.
The devices were fabricated using 2,9-dimethyl-4,7-diphenyl-
1,10-phenanthroline as the electron-transport layer, Ir(dpbic)
3
5
as hole-transport material and 2,8-bis(triphenylsilyl)dibenzo-
[b,d]furan 4 as a host to facilitate an efficient trapping of both
holes and electrons in the emissive layer. Measurements were
undertaken either in single-matrix or mixed-matrix experi-
ments. In the mixed-matrix experiments the iridium(III)
complex 5 was added in different concentrations to improve
hole transport. Indeed the operating voltage could thus be
significantly lowered.
Photophysical properties
To investigate the photophysical properties, the absorption
and emission spectra were measured in amorphous poly
6
(methyl methacrylate) films (PMMA) at room temperature with
2
wt% emitter concentration (Table 1). A maximum absorption
Independent from the emitter concentration the emission
spectra for the single-matrix as well as the mixed-matrix com-
position showed the same maxima at 480 nm and 532 nm and
CIE coordinates of 0.27/0.55 (Table 2).
for all compounds is found around 220 nm (Fig. 2) which can
be mainly attributed to the matrix material.
A number of discernible absorption bands are found
between 240 nm and 360 nm. While the first band can be
attributed to high energy π–π* transitions from the NHC
ligand, the minor transitions at higher wavelengths have d–π*
MLCT character. The characteristics of these bands are very
similar for all three complexes albeit all the signals for 3d are
red-shifted by about 20 nm.
Measurements in 100% emitter films show only little dis-
cernible fine structure and no bands for 3d due to crystalliza-
tion (ESI, Fig. S2†).
A maximum luminance efficiency of 12.9 lm W− could be
achieved (30% 3b, 30% 5). The maximum external quantum
efficiency (EQE) is reached at the lowest current density and
the highest doping concentration of 30% emitter with 8.5%
for the single-matrix device (Fig. 4) and 7.4% for the mixed-
matrix device (30% 3b, 30% 5). The single-matrix device shows
1
−
1
peak current and power efficiencies of 24.9 cd A and 10 lm
−1
−2
−2
W
at 300 cd m . A maximum luminance of 37 093 cd m
was achieved.
3
Fig. 3 Device layout. 4 = 2,8-bis(triphenylsilyl)dibenzo[b,d]furan, 5 = [Ir(dpbic) ].
ETL = electron-transport layer, HBL = hole-blocking layer, EML = emission layer,
HTL = hole-transport layer, HIL = hole-injection layer. BCP = 2,9-dimethyl-4,7-
diphenyl-1,10-phenanthroline.
Fig. 2 Absorption (left) and emission (right) spectra for 2 wt% emitter in
PMMA at room temperature.
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Table 2 Device performance of 3b at 300 cd m− (doping ratio for 3b : 5)
2
a
Doping
%]
Cur. density
Voltage
[V]
Cur. eff.
Lum. eff.
[lm W ]
EQE
[%]
−
2
−1
−1
[
[mA cm
]
[cd A
]
5
1
3
3
3
3
: 0
5.1
2.8
1.2
1.9
1.7
1.4
12.10
10.80
7.90
5.30
5.25
5.31
5.87
10.70
24.87
15.77
17.44
21.80
1.53
3.10
9.88
9.35
10.44
12.89
2.06
3.71
8.47
5.43
5.95
7.37
0 : 0
0 : 0
0 : 5
0 : 10
0 : 30
Fig. 5 Spin densities computed on the first excited triplet state of emissive
complexes 3b–3d (B3LYP/6-31G(d), isovalue = 0.02).
a
External quantum efficiency.
geometry changes were observed for the change of the multi-
plicity from the singlet to the triplet for either functional. We
recently developed a simple method to reliably predict the
emission wavelength of cyclometalated transition metal com-
1
2
plexes using BP86. Predicted wavelengths and measured data
differ only by a few nanometres (Table S3†). Frontier molecular
orbitals computed on the optimized singlet ground state
(
Fig. S20†) reveal a strong admixture of platinum d-orbitals to
the HOMO, which is mainly located at the NHC ligand.
The spin density for the three emitting complexes was
plotted as well (Fig. 5). For all three emissive complexes the
electrons of the two SOMOs are mainly located at the NHC
3
ligand and the metal centre. This suggests an MLCT/ILCT
character of the emitting state, which is in good agreement
with the observed experimental results and the common
understanding of the excited state in platinum(II) triplet
emitters.
Conclusion
We were able to synthesize a new class of bimetallic platinum(II)
NHC complexes with bisimidazole ligands, where both
metal atoms are cyclometalated (C^C*) to the same central
phenyl ring in either trans or even meta fashion. The com-
plexes show high thermal stability, no air or moisture sensibi-
lity and interesting photophysical properties. These were
investigated for 3b–3d in the light of their potential appli-
cation as emitters for OLEDs. The bimetallic complexes emit
in the blue-green region of the visible spectrum and show
quantum yields as high as 93% together with significantly
Fig. 4 Top: external quantum efficiency (EQE), bottom: current density of the
single-matrix device at different emitter concentrations.
DFT calculations
Singlet and triplet states have been calculated by DFT methods shorter decay lifetimes compared to the corresponding mono-
B3LYP/6-31G(d) level with a Hay–Wadt-ECP for platinum) for nuclear complexes. Devices fabricated for 3b produced peak
(
−
1
−1
all complexes. Geometries from the optimization in the singlet current and power efficiencies of 24.9 cd A and 10 lm W at
−
2
ground state agree well with experimental data from the solid- 300 cd m , 8.5% EQE and a maximum luminance of 37 093
state structure determination (Table S2†). A possible isomer of cd m− . DFT calculations revealed an MLCT/IL transition
2
3
e with the second cyclometalation taking place at the position centred on the NHC ligand for the main emissive process. In
between the two imidazole fragments (Fig. S19†) was calcu- this regard an additive effect from two heavy metal atoms and
lated to be thermodynamically less favoured by 21.9 kcal thus enhanced SOC would explain the excellent photophysical
−
1
mol and could indeed not be observed experimentally.
properties. Two solid-state structures were presented confirm-
To probe electronic properties and predict the emission ing the general square-planar structure of these platinum(II)
wavelength, calculations on the lowest triplet state were carried compounds and the double cyclometalation at the central
out additionally utilizing the BP86 functional. No significant phenyl ring.
9
850 | Dalton Trans., 2013, 42, 9847–9851
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Y.-L. Chang, Z.-H. Lu and S. Wang, J. Am. Chem. Soc.,
Acknowledgements
2012, 134, 13930.
We are grateful to Dr Fuchs (BASF), Dr Metz (BASF), Dr Wagenblast
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helpful discussions, photophysical measurements and device
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(FKZ: 13N10477) for funding.
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