Highly Luminescent Pincer Gold(III) Aryl Emitters: Thermally Activated Delayed Fluorescence and Solution-Processed OLEDs

Article abstract of DOI:10.1002/anie.201707193

Herein are described the synthesis, photophysical properties and applications of a series of luminescent cyclometalated Au^III complexes having an auxiliary aryl ligand. These complexes show photoluminescence with emission quantum yields of up to 0.79 in solution and 0.84 in thin films (4 wt % in PMMA) at room temperature, both of which are the highest reported values among Au^III complexes. Thermally activated delayed fluorescence (TADF) is the emission origin for some of these complexes. Solution-processed OLEDs made with these complexes showed sky-blue to green electroluminescence with external quantum efficiencies (EQEs) of up to 23.8 %, current efficiencies of up to 70.4 cd A^?1, and roll-off of down to 1 %, highlighting the bright prospect of Au^III-TADF emitters in OLEDs.

Full text of DOI:10.1002/anie.201707193

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Accepted Article
Title: Highly Luminescent Pincer Gold(III) Aryl Emitters. Thermally
Activated Delayed Fluorescence and Solution-Processed OLEDs
with EQE and Luminance of up to 23.8 % and 57340 cd m-2
Authors: Chi-Ming Che
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To be cited as: Angew. Chem. Int. Ed. 10.1002/anie.201707193
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10.1002/anie.201707193
Angewandte Chemie International Edition
COMMUNICATION
Highly Luminescent Pincer Gold(III) Aryl Emitters. Thermally
Activated Delayed Fluorescence and Solution-Processed OLEDs
with EQE and Luminance of up to 23.8 % and 57340 cd m-2
Wai-Pong To, Dongling Zhou, Glenna So Ming Tong, Gang Cheng, Chen Yang, and Chi-Ming Che*
Abstract: Herein are described the synthesis, photophysical properties
and applications of a series of luminescent cyclometalated Au(III)
complexes having an auxiliary aryl ligand. These complexes show
photoluminescence with emission quantum yields of up to 0.79 in
solution and 0.84 in thin films (4 wt% in PMMA) at room temperature,
both of which are the highest reported values among Au(III) complexes.
Thermally activated delayed fluorescence (TADF) is the emission origin
for some of these complexes. Solution-processed OLEDs fabricated with
these complexes showed sky-blue to green electroluminescence with
external quantum efficiencies (EQEs) of up to 23.8 %, current
efficiencies of up to 70.4 cd A-1, and roll-off of down to 1 %, highlighting
the bright prospect of Au(III)-TADF emitters in OLEDs.
quantum yields of up to 0.79 in solutions / 0.84 in thin films and
radiative decay rates (kr) of up to 1.17 × 106 s-1. These solution
emission quantum yields are > 500-fold higher than that of the
phenylacetylide analogue.[6] Notably, solution-processed OLEDs
based on these complexes showed EQEs, current efficiencies and
luminance of up to 23.8 %, 70.4 cd A-1 and 57340 cd m-2
respectively, suggesting that Au(III)-TADF emitters, as far as EQE
and luminance are concerned, are as competitive as the best
platinum(II) and iridium(III) emitters in OLED application. We note
that Bochmann and co-workers mentioned the possibility of gold(III)
complexes showing TADF property based on the emission shift
from being red at 77 K to blue at 298 K.[7]
Figure 1 depicts the structures of the Au(III) complexes 1–8.
Details of their synthesis and characterization are given in the
Supporting Information (SI) (Scheme S1). The incorporation of aryl
ligand to the cyclometalated Au(III) moiety was achieved by the
Thermally activated delayed fluorescence (TADF) has recently
emerged as a promising strategy for efficient light emission at room
temperature (rt).[1] The key process involved is reverse intersystem
crossing (RISC) from the lowest triplet excited state (T1) to the
lowest singlet excited state (S1), followed by a spin-allowed,
radiative transition to the singlet ground state (S0). To achieve efficient
TADF, a fine balance between E(S1T1) and oscillator strength is of
paramount importance. At present, a great variety of organic
molecules showing TADF comprise a twisted donor-acceptor pair
within the molecule.[2] In contrast, examples of metal complexes
showing TADF are largely limited to Cu(I) complexes[3] and a handful
of Pd(II), Ag(I) and Au(I) complexes reported recently.[4] In particular,
Au(I) complexes having donor-acceptor pair at an intermediate
dihedral angle were reported to exhibit E(S1T1) close to zero.[4d]
In the course of our design on luminescent Au(III) complexes,
we found that arylacetylide ligands have a relatively high-lying
HOMO which may produce a non-radiative triplet ligand-to-ligand
charge transfer (3LLCT) (3[π(C≡CAr)–π*(C^N^C)]) excited state that
dissipates the emissive triplet intraligand (3IL) excited state, leading
to low emission quantum yields at rt.[5] Accordingly, we conceive that
their luminescence can be improved by removing the C≡C bond of
the arylacetylide (i.e. changing from arylacetylide to aryl) so as to
lower the HOMO of the auxiliary ligand and increase the 3LLCT
energy. In this study, a series of cyclometalated Au(III) aryl
complexes have been synthesized. Strikingly, several Au(III)
complexes having amino-substituted aryl ligand exhibit TADF with
Figure 1. Chemical structures of 1–8.
reaction between aryl boronic acid and [Au(C^N^C)OH] in toluene at
o
60 C for 12 hours.[8] These complexes were purified by column
chromatography on SiO2, and characterized by 1H NMR, 13C NMR,
mass spectrometry and elemental analysis. They appear pale
yellow (1, 4, 5, 6) and yellow (2, 3, 7 and 8) in the solid state. They
are stable in air and moisture, and possess moderate to good
thermal stability with decomposition temperature (Td, defined as the
temperature at which the complex shows a 2 % weight loss)
ranging 228–333 C under a nitrogen atmosphere (Figure S1).
The structures of 1–5, 7 and 8 were determined by X-ray
crystallography (see SI for details). Their Au atoms adopt a distorted
square-planar geometry with C-Au-C angles of 161.44°162.74° and
nearly linear N-Au-C(aryl ligand) arrangement (175.5–180.0°). The
AuC and AuN distances are 2.010–2.115 and 2.022–2.058 Å,
respectively. The aryl ligand is not coplanar with the C^N^C ligand
[*]
Dr. W.-P. To, D. Zhou, Dr. G. S. M. Tong, Dr. G. Cheng, Dr. C.
Yang, Prof. Dr. C.-M. Che
State Key Laboratory of Synthetic Chemistry
Department of Chemistry, The University of Hong Kong
Pokfulam Road, Hong Kong SAR, China.
E-mail: cmche@hku.hk
Dr. G. Cheng, Prof. Dr. C.-M. Che
HKU Shenzhen Institute of Research and Innovation
Shenzhen, Guangdong, 518053, China
Supporting information for this article is given via a link at the end of
the document.
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10.1002/anie.201707193
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COMMUNICATION
Table 1: Photophysical data of Au(III) complexes at rt.
Absorption
plane, forming a dihedral angle of 49.80-68.08°. This is quite
different from arylacetylide analogues, the corresponding dihedral
angle of which varies from 16.3 to 88.9°, suggestive of insignificant
energy barrier for rotation.[6] Except for 8, in these crystal structures,
π–π stacking interactions exist between the C^N^C ligand planes
with interplanar distances of 3.3593.509 Å. Complex 8 shows
complex pairs in a head-to-tail fashion without intermolecular π–π or
Au...Au interactions, presumably due to the presence of 2,6-
dimethylphenyl ring orthogonal to the C^N^C ligand plane.
Emission
4 wt% in PMMA thin
In toluene
film

abs [nm]
(ε [×103 mol-1 dm3 cm-1])[a]

em [nm]
(;  [μs]; kr [105 s-1])[b]

em [nm]
(;  [μs]; kr [105 s-1])[c]
1
2
3
4
5
6
7
8
311(10.7), 321(10.6), 360(3.1),
375(3.6), 394(2.9)
468, 503, 534
(0.14; 61.5; 0.02)
534
471, 504, 534
(0.14; 51; 0.03)
523
310(27.2), 357(2.9), 375(3.6),
394(3.6), 410(br, 1.4)
(0.74; 1.03; 7.2)
596
(0.66; 1.35; 4.9)
564
312(10.3), 319(9.9), 357(2.4),
374(3.1), 393(2.9), 424(br, 1.0)
The UV-visible absorption and emission spectral data of 1–8 are
summarized in Table 1 and Table S2; the absorption and emission
spectra of 1, 2, 5, and 7 are shown in Figure 2. In toluene, 16 and 8
exhibit intense absorption bands (ε = (1–3) × 104 dm3 mol-1 cm-1)
ranging from 295 nm to 345 nm and moderately intense, vibronically
structured bands (ε = (2–7) × 103 dm3 mol-1 cm-1) from 334 nm to 402
nm at rt. Additional broad absorption bands/tails were observed for
complexes having p-NAr2- or p-NMe2-substituted aryl ligand (2, 3, 5,
6, 8), but not for 4 bearing a p-carbazolyl-substituted aryl ligand.
Compared to 16, the absorption of 7 was significantly red-shifted,
with lower-energy vibronically structured bands (ε = (2–4) × 103 dm3
mol-1 cm-1) at 396–440 nm and an absorption tail ending at ~500 nm.
For 1, 2, 5, there were only minor shifts in the low-energy vibronically
structured absorption peak maxima in solvents of different polarity (3-5
nm; Table S2). These absorption bands are assigned to singlet metal-
perturbed IL π–π* transitions of the C^N^C ligand. For 2, 3, 5–8, the
additional absorption tails were assigned as singlet LLCT transitions
from the π orbital of the electron-rich, amino-substituted aryl ligand to
the π* orbital of the C^N^C ligand.[6,9] All these complexes display
photoluminescence in solutions at rt. In degassed toluene, 1–4 with
the same C^N^C ligand display two different kinds of emission profiles:
2 and 3 show broad, structureless emission bands with short lifetimes
(λmax = 534 and 596 nm, and τ = 1.03 and 0.32 μs respectively) while
1 and 4 exhibit vibronically structured emission bands with long
lifetimes (λmax = 468-534 nm; τ = 61.5 and 43.5 μs respectively),
which strongly suggests that their emissions are of different origins.
Solvatochromic studies revealed that the emission peak maxima
of 1 and 4 are insensitive to solvent polarity, but that of 2 shows a
significant positive solvatochromism (max 534 nm in toluene, 582 nm
in C6H5Cl, 619 nm in CH2Cl2; Table S2, Figure S11). The Lippert–
Mataga plot (νabs-νem against orientation polarizability of solvent) of 2
shows a positive slope of 9549.5 cm-1, suggesting a large increase in
dipole moment upon attaining the emissive excited state (Figure S12).
The kr values of 1 and 4 are on the order of (2–3) × 103 s-1, whereas
those of 2 (7.2 × 105 s-1) and 3 (5.6×105 s-1) are ~200 times larger.
The emissions of 1 and 4 are thus assigned to 3IL transitions of the
C^N^C ligand,[10] whereas the emissions of 2 and 3 are assigned to
originate from LLCT excited state arising from the electronic transition
from the π orbital of the amino-substituted aryl ligand to the π* orbital
of the C^N^C ligand.[9,11] This is in line with the lower emission energy
of 3 (596 nm) compared to 2 (534 nm) due to the stronger donor
strength of NMe2 group of 3 than that of NPh2 group of 2. Importantly,
among these complexes, 5 in degassed toluene exhibits a high
emission quantum yield of 0.79 and kr of 11.1×105 s-1, both of which
are unprecedented in reported luminescent Au(III) complexes.[12]
The emissions of 18 in PMMA thin film (4 wt%) were also
measured (Figure 3, Figure S13). They exhibit features similar to
those recorded in toluene: 1, 4, and 7 show vibronically structured
emission bands with kr of (2–3) × 103 s-1, suggestive of 3IL emission; 2,
3, 5, 6 and 8 show broad, structureless emission bands with large kr of
(0.18; 0.32; 5.6)
469, 503, 532
(0.13; 43.5; 0.03)
524
(0.42; 0.74; 5.7)
471, 504, 534
(0.13; 34; 0.04)
517
312(11.3), 321(11.4), 344(6.4),
359(3.4), 376(3.9), 395(2.9)
304(25.4), 349(4.2), 368(3.4),
398(br, 1.7)
(0.79; 0.71; 11.1)
511
(0.84; 0.72; 11.7)
513
295(31.5), 334(6.4), 351(6.2),
368(4.8), 394(br, 2.3)
(0.68; 0.75; 9.1)
591, 641, 702
(0.065; 29.4; 0.02)
527
(0.78; 0.85; 9.2)
593, 642, 701
(0.031; 16.5; 0.02)
521
327(21.8), 345(20.5), 396(2.7),
417(3.9), 440(3.5)
312(48.4), 323(39.7), 365(5.4),
383(7.2), 402(7.4), 426(br, 2.0)
(0.75; 1.10; 6.8)
(0.67; 1.10; 6.1)
[a] In toluene at 2 × 10-5 M. “br” stands for broad. [b] Emission quantum yields
() were measured with BPEA (9,10-bis(phenylethynyl)-anthracene) in
benzene as the standard ( = 0.85). [c] Absolute emission quantum yields of
thin film samples were measured with Hamamatsu C11347 Quantaurus-QY
Absolute PL quantum yields measurement system.
Figure 2. a) UV/vis absorption and b) emission spectra of 1, 2, 5 and 7 in
toluene at rt.
Figure 3. Emission spectra of 1, 2, 5 and 7 in PMMA thin film (4 wt%).
(4.9–11.7) × 105 s-1 (Table 1), which suggests that the emission
originated from LLCT transitions similar to those in solution.
Despite support of the LLCT assignment for these complexes
by solvatochromic studies, the unusually large kr of their emissions,
particularly the kr 7.2  105 s1 for 2 bearing a p-NPh2-substituted
phenyl ligand, prompted us to employ DFT/TDDFT calculations to
elucidate the emission origin. DFT calculations revealed that T1 of 2
is derived from a HOMO (localized on p-NPh2-C6H4 ligand) 
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LUMO (localized on C^N^C ligand) transition (Figure 4) and could
thus be described as a LLCT excited state. However, for this 3LLCT
 S0 transition, the calculated kr of 1.75  103 s1 is two orders of
magnitude smaller than the experimental value. From TDDFT
which was proposed by Yersin and co-workers to account for
delayed fluorescence due to thermally activated conversion from T1
to the close-lying S1.[1a] The E(S1T1) is 393 cm-1 for 3 (Figure S15)
and 318 cm-1 for 5, the latter is consistent with the calculated value
of 500 cm-1. Although we are unable to observe the estimated triplet
emission lifetime due to instrumental limitation, the above data, and
their good agreement with results from DFT calculations, altogether
suggest that the broad, structureless LLCT emissions of 5, and also
those of 2, 3, 6 and 8, originate from TADF. This is accordant with
the proposed structural prerequisite for efficient TADF, which is a
spatial separation and a twisted angle between the electron donor
(amino-substituted aryl ligand) and the electron acceptor (C^N^C
ligand) of the emitting molecule.[1b-d] One possible reason for the
observation of 3IL emission instead of TADF in 7 is that 3IL excited
state of 7 is much lower-lying than 1LLCT and 3LLCT excited states
due to the large exchange energy for localized IL excited state,
resulting in emission from 3IL excited state by Kasha’s rule.
3
calculations at the optimized LLCT geometry, S1 is also derived
mainly from a HOMO  LUMO transition and is only ~200 cm1
above the T1, with calculated kr of the 1LLCT  S0 transition being
1.58  107 s1. To assess the possibility that room temperature
emissions of 2 could arise from TADF mechanism, Boltzmann
equilibrium of the S1 and T1 excited states is assumed and the
average kr was estimated to be 1.69  106 s1, comparable with the
experimental kr of 2 (7.2  105 s1). Hence, 2 is probably a TADF
emitter.
Figure 4. MO surfaces of 2.
With OEt and F substituents on the pyridyl and phenyl moieties
of the C^N^C ligand, respectively, 5 displays the highest kr (~106 s1)
among 18. DFT calculations revealed that the T1 excited state is
derived from a HOMO  LUMO transition and is also best
described as a 3LLCT (see SI). The calculated kr for the 3LLCT  S0
transition of 5 is only 7.57  102 s1, hence the emission is also
3
unlikely to be purely derived from the decay of the LLCT excited
state. The corresponding S1 excited state (1LLCT) is ~500 cm1
above the 3LLCT excited state at the optimized 3LLCT excited state
geometry. Assuming Boltzmann equilibrium between the S1 and T1
excited states, the average kr computed is 1.45  106 s1 and is in
agreement with that derived from experiments (1.11  106 s1).
Therefore, complex 5 should also be a TADF emitter.
Figure 5. Emission spectra and lifetime data of 5. Left) Emission spectra of
the complex (4 wt%) in PMMA thin film at rt and 77 K, and in toluene at rt.
Right) Plot of emission lifetime monitored at 517 nm against temperature and
fitting of the data (red line) to the equation for TADF.
In view of the high emission efficiency, these complexes have
been employed as emissive dopant in OLEDs.[13] Solution-
processed OLEDs based on 2 and 5 as emitters with different
dopant concentrations were fabricated and characterized. The
details of the device structure are described in SI. Normalized EL
spectra and EQE-luminance characteristics of selected devices with
2 and 5 are depicted in Figure 6a,b, respectively; the device
performances are summarized in Table 2. The emission maxima of
OLEDs with 2 and 5 showed a gradual red-shift with increasing
dopant concentration (Figure S16). EL maxima of the devices with 4
wt% 2 and 5 locate at 509 and 500 nm, respectively (Figure 6a),
and are blue-shifted by 14-17 nm compared to the emission
maxima of 2 and 5 in PMMA film. The maximum EQEs are
respectively 14.8 % and 23.8 % for the OLEDs based on 2 and 5 at
optimized dopant concentration as depicted in Figure 6b; the EQEs
at high luminance of 1000 cd m-2 are respectively 14.7 % and 16.5
%, which reveals a small efficiency roll-off of down to 1 % for the
device with 2. More importantly, the emission color of the devices
based on 5 is sky blue, whose high efficiency of 23.8 % is
comparable with the best sky-blue OLEDs fabricated by solution-
process technique.[4d,14] The emission energy of 5 can be tuned by
using host materials with different band-gaps. By using a higher
band-gap material PYD2 (2,6-dicarbazolo-1,5-pyridine)[3b,e] to
replace PVK: OXD-7 as the host, the emission peak maxima of the
device with 5 further blue-shifts to 486 nm with CIE coordinates of
(0.21, 0.42) (Figure S17). The maximum EQE of this
To examine the TADF properties, the emission spectra and
lifetimes of 2, 3, and 5 in PMMA thin films at various temperatures
were recorded (Figure S14, S15 and 5, respectively). The emission
of 2 changed from a broad, structureless one to a vibronically
structured one upon decreasing temperature from rt to 77 K (Figure
S14), implying a change of emissive excited state from LLCT at rt to
mainly 3IL (C^N^C) at 77 K. The emission intensity decay gradually
changed from being monoexponential at rt to biexponential at low
temperature. Due to potential complications arising from the change
in emissive excited state, we examined 5, which has a larger energy
3
difference between the LLCT and IL excited states. As shown in
Figure 5, the emission profile remained broad and structureless
upon decreasing temperature except for the emergence of a small
peak/shoulder at 442 nm which could be ascribed to the first
emission peak of the 3IL emission of the C^N^C ligand.[10] The decay
of the emission intensity monitored at the emission maxima can be
well fitted monoexponentially, and the plots of their emission lifetime
against temperature showed an excellent fit (R2 = 0.9976; Figure 5)
with the equation relating the observed emission lifetime and
temperature with E(S1T1), and (S1) and (T1):
(
)
ΔE S1 − T1
3 + exp [−
1
]
푘B푇
( )
휏 푇 =
(
)
3
ΔE S1 − T1
푘B푇
+
exp [−
]
휏(T1) 휏(S1)
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10.1002/anie.201707193
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COMMUNICATION
Table 2: Key performances of OLEDs based on 2 and 5 as emitters.
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Mater. 2017, 29, 1605444; c) F. B. Dias, T. J. Penfold, A. P. Monkman,
Methods Appl. Fluoresc. 2017, 5, 012001; d) Z. Yang, Z. Mao, Z. Xie, Y.
Zhang, S. Liu, J. Zhao, J. Xu, Z. Chi, M. P. Aldred, Chem. Soc. Rev. 2017, 46,
915–1016; e) Y. Im, M. Kim, Y. J. Cho, J.-A Seo, K. S. Yook, J. Y. Lee, Chem.
Mater. 2017, 29, 1946–1963.
Com- Dopant
plex
Max.
Max.
Max.
PE[d]
EQE [%]
CIE
coordinates
(x, y)
lumin.[b] CE[c]
conc.[a]
[wt%]
Max.
At 1000
[cd m-2] [cd A-1] [lm W-1]
cd m-2
8.3
2
4
27400
34200
53840
57340
11450
22750
27900
33740
24.0
36.9
43.9
44.9
34.9
60.3
60.2
70.4
12.7
20.9
22.5
23.6
22.5
43.0
41.1
47.3
8.3
0.29, 0.52
0.30, 0.53
0.31, 0.54
0.32, 0.55
0.25, 0.47
0.26, 0.49
0.26, 0.50
0.27, 0.51
8
12.5
14.6
14.8
12.7
20.6
20.9
23.8
12.3
14.6
14.7
8.7
12
16
4
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8
14.5
15.2
16.5
12
16
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Figure 6. a) Normalized EL spectra of 2 and 5 (4 wt%) in solution-processed
OLEDs with a mixture of PVK and OXD-7 as host. b) EQE-luminescence
characteristics of solution-processed OLEDs of 2 (16 wt%) and 5 (16 wt%).
device is 15.7 % which slightly decreased to 14.4 % at 1000 cd m-2
(Figure S18).
In summary, a series of pincer Au(III) aryl complexes displaying
intense photoluminescence with emission quantum yields of up to
0.79 in solution and 0.84 in thin films at rt have been studied. The
presence of amino substituent on the auxiliary aryl ligand, which is
twisted with respect to the C^N^C ligand plane, leads to TADF in
some of these complexes. DFT calculations and variable
temperature-emission lifetime measurements altogether revealed
small E(S1T1) of 200-500 cm-1 for some of the complexes. The
successful fabrication of OLEDs showing record-high maximum
EQE and luminance of up to 23.8 % and 57340 cd m-2, and EQEs of
up to 16.5 % at 1000 cd m-2 clearly established the practical
usefulness of Au(III)-TADF emitters in OLED technology.
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Acknowledgements
This work was supported by the National Key Basic Research
Program of China (No. 2013CB834802), the University Grants
Committee Areas of Excellence Scheme (AoE/P-03/08), and Basic
Research Program of Shenzhen (JCYJ20160229123546997,
JCYJ20160530184056496). This work was also conducted in part
using the research computing facilities and/or advisory services
offered by Information Technology Services, The University of Hong
Kong. We thank Dr. K.-H. Low for assistance in solving the X-ray
crystal structures of 4, 5 and 8.
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Keywords: gold • thermally activated delayed fluorescence •
ligand-to-ligand charge transfer • organic light-emitting device
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Angewandte Chemie International Edition
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Angewandte Chemie International Edition
COMMUNICATION
Table of Contents
COMMUNICATION
Pincer Au(III) aryl complexes
display intense
Wai-Pong To, Dongling Zhou,
Glenna So Ming Tong, Gang
Cheng, Chen Yang, and Chi-Ming
Che*
photoluminescence with emission
quantum yields of up to 0.79 in
solution and 0.84 in thin films.
Some of them with N-substituents
based on donor-acceptor
structures display stronger
photoluminescence originated
from thermally activated delayed
fluorescence (TADF). The OLEDs
fabricated with these TADF
emitters exhibit high EQE over 23
%.
Page No. – Page No.
Highly
Luminescent
Pincer
Gold(III) Aryl Emitters. Thermally
Activated Delayed Fluorescence
and Solution-Processed OLEDs
with EQE and Luminance of up
to 23.8 % and 57340 cd m-2
This article is protected by copyright. All rights reserved.