Tetradentate Gold(III) Complexes as Thermally Activated Delayed Fluorescence (TADF) Emitters: Microwave-Assisted Synthesis and High-Performance OLEDs with Long Operational Lifetime

Article abstract of DOI:10.1002/anie.201914661

Structurally robust tetradentate gold(III)-emitters have potent material applications but are rare and unprecedented for those displaying thermally activated delayed fluorescence (TADF). Herein, a novel synthetic route leading to the preparation of highly

Full text of DOI:10.1002/anie.201914661

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Angewandte
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Accepted Article
Title: Luminescent Tetradentate Gold(III)-TADF Emitters: Microwave-
Assisted Synthesis and High Performance OLEDs With EQEs Up
To 25 % and Long Operational Lifetime
Authors: Dongling Zhou, Wai-Pong To, Glenna So Ming Tong, Gang
Cheng, Lili Du, David Lee Phillips, and Chi-Ming Che
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To be cited as: Angew. Chem. Int. Ed. 10.1002/anie.201914661
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10.1002/anie.201914661
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RESEARCH ARTICLE
Luminescent Tetradentate Gold(III)-TADF Emitters: Microwave-
Assisted Synthesis and High Performance OLEDs With EQEs Up
To 25 % and Long Operational Lifetime
Dongling Zhou,[a] Wai-Pong To,[a] Glenna So Ming Tong,[a] Gang Cheng,[a,b] Lili Du,[a] David Lee
Phillips,[a] and Chi-Ming Che*[a,b]
Abstract: Structurally robust tetradentate gold(III)-emitters have
potent material applications but are rare and unprecedented for
those displaying thermally activated delayed fluorescence (TADF). In
this work, a novel synthetic route leading to the preparation of highly
emissive, charge-neutral tetradentate [C^C^N^C] gold(III) complexes
with 5-5-6 membered chelate rings has been developed through
microwave-assisted C−H bond activation. These complexes show
high thermal stability and with emission origin (3IL, 3ILCT and TADF)
tuned by varying the substituents of the C^C^N^C ligand. With
phenoxazine/diphenylamine substituent, we prepared the first
tetradentate gold(III)-TADF emitters with emission quantum yields of
up to 94 % and emission lifetimes of down to 0.62 s in
deoxygenated toluene. These tetradentate AuIII-TADF emitters
showed good performance in vacuum-deposited OLEDs with
maximum EQEs of up to 25 % and LT95 of up to 5280 h at 100 cd m-
TADF mechanism.[1] The mainstream design of luminescent AuIII
complexes is the combined use of two or more ligands. As the
stability of metal complexes could be improved by increasing the
denticity of chelating ligands and metal-ligand bond strength, the
use of rigid tetradentate ligands having C-donor atom(s) is
conceived to endow high thermal stability to the complex and to
restrict excited state structural distortion. While strongly
luminescent tetradentate PtII and PdII emitters are well
documented,[2] there is one literature report on non-porphyrin type
charge-neutral tetradentate AuIII complexes prepared by an
intramolecular cyclization between two ligands with tridentate
alkynylgold(III) complexes as precursors, reported by Yam and co-
workers.[3d] Nonetheless, it would be a formidable challenge to use
this post-synthetic modification strategy for preparing structurally
diverse tetradentate AuIII core structures. On the other hand, the
radiative decay rate constants of phosphorescent AuIII complexes
are usually small as the emissive excited states being mainly
ligand localized, involve only minor metal parentage, which is
detrimental in OLED applications due to large efficiency roll-off.[3]
In this regard, OLEDs fabricated with AuI and AuIII-TADF emitters
(τ < 2 s) have achieved small roll-off (< 10 %).[1a,b,e] A recent
study suggested that improving the rigidity of emitters could
suppress the structural deformation upon S1−T1 transformation
thereby reducing the activation energy barrier for reverse
intersystem crossing (RISC) and leading to efficient TADF.[4] It is
thus conceived that AuIII complexes supported by trianionic
tetradentate ligands and exhibiting efficient TADF at room
temperature would be a way forward.
2
.
Introduction
Gold metal is appealing in the development of new generation
of luminescent molecular materials due to strong gold-ligand bond
that confers high thermal stability, large spin-orbit coupling (SOC)
constant for harvesting triplet excitons, and much higher earth
abundance than the other precious noble metals. Nonetheless,
the practical applications of luminescent gold complexes remain in
the “proof-of-concept” stage. Despite the extensive works on
luminescent gold complexes over the past decade, examples of
gold OLEDs with exemplary performance data close to that of the
best iridium-OLEDs are very rare, and these rare examples were
reported only in the past several years (EQEs of AuI and AuIII
OLEDs up to 27.5 % and 23.8 % respectively) by exploiting the
[a]
D. Zhou, Dr. W.-P. To, Dr. G. S. M. Tong, Dr. G. Cheng, Dr. L. Du,
Prof. Dr. D. L. Phillips, Prof. Dr. C.-M. Che
State Key Laboratory of Synthetic Chemistry, HKU-CAS Joint
Laboratory on New Materials, Department of Chemistry, The
University of Hong Kong
Pokfulam Road, Hong Kong SAR (China)
E-mail: cmche@hku.hk
[b]
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.
Figure 1. Chemical structures of 1−9.
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Figure 2. Synthetic scheme of AuIII complexes 1−9 and ORTEP drawing for 9 with omission of hydrogen atoms (thermal ellipsoids probability level: 30%).
Herein are described new synthetic routes for preparing
tetradentate AuIII complexes supported by O-bridged/spiro-
arranged C^C^N^C ligand scaffolds through microwave-induced
C−H activation. This synthetic strategy has previously been used
for the preparation of bidentate/tridentate AuIII complexes.[5] With
judicious choice of substituent(s) on the ligand scaffold, the
emission origin of these complexes could be tuned from long-
yields (roughly 10 %). The desired product was not formed when
cyclometalated AuIII-OCOCF3 was refluxed in ACN/H2O (1:1 v/v)
for 24 hours, thus establishing the crucial role of microwave
irradiation in the synthesis of tetradentate cyclometalated AuIII
complexes. Cyclometalated AuIII chlorides supported by ligands
with
amino-substituted
moieties
(diphenylamine
or
phenoxazine)/spiro-fluorene are difficult to be obtained by
traditional methods such as transmetalation from organomercury
compounds or direct auration from L-AuCl3. Therefore,
complexes 3, 4, 7 and 8 were prepared by Pd-catalyzed
reactions between 2 or 6 and corresponding amines in moderate
yields (40−47 %). Complex 9 was obtained in 36 % yield via
mercury-free direct auration by using Au(OAc)3 in two
consecutive microwave-assisted reactions. Microwave-assisted
direct auration with KAuCl4 or Au(OAc)3 failed to give 1, 2, 5 and
6.
3
lived IL excited state to TADF from ILCT excited state. Stable,
highly efficient tetradentate AuIII-TADF emitters with emission
quantum yields () of up to a record-high 94 % and lifetimes ()
down to 0.62 s have been realized and applications of these
emitters in OLEDs have also been explored.
Results
The crystal structure of 9 is depicted in Figure 2 and that of 1
and 5 are depicted in Figure S3-S4. The AuIII atoms adopt distort
square-planar geometries with C−Au−C and C−Au−N angles of
157.98−161.31o and 170.47−173.40o, respectively. The Au−C
and Au−N bond distances are 1.950−2.028 Å and 2.028−2.115
Å, respectively. Except for the moieties that are not coordinated
to the metal ion (4-methoxyphenyl and phenoxyphenyl), the
Au(C^C^N^C) unit of 1 is nearly coplanar. Unlike 1, the
phenoxyphenyl moiety of 5 is slightly out-of-plane, making a
torsion angle with the Au(C^N^C) plane of 15.13o. For 9, spiro-
fluorene moiety exhibits out-of-plane bending conformation
Synthesis and Characterization
In these complexes, 3,5-diphenoxyphenyl, 2-methyl-5-
phenoxyphenyl and 3,5-diphenyl moieties were used to ensure
the presence of a pendent phenyl/phenoxy ring which can
coordinate to AuIII trans to the pyridine to form the C^C moiety.
As shown in Figure 2, complexes 1, 2, 5 and 6 were prepared by
using cyclometalated AuIII trifluoroacetate (OCOCF3 ) as
precursors in a 1:1 (v/v) mixture of ACN/H2O in high yields
(~90 %) under microwave heating at 110−130 °C.
Cyclometalated AuIII chlorides could also be used, albeit with low
-
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Table 1. Photophysical and electrochemical data of 1−9.
Absorption
Complex
Emission[b]
E (V vs. SCE)[e]

abs [nm]
In toluene
In thin films
Oxidation 1st Reduction
(ε [×103 mol-1 dm3 cm-1])[a]

em [nm] (;  [μs]; kr [103 s-1])

em [nm] (;  [μs]; kr [103 s-1])
(Epa
)
(E1/2)
492, 523, 560[c]
__ __
495, 526, 565
(0.54; 93.1; 5.8)
498, 526, 567
(0.40; 77.1; 5.2)
524, 550
(0.77; 94.3; 8.2)
−1.45
−1.38[h]
−1.43
−1.36
−1.50
−1.47
−1.51
1
2
3
4
5
6
7
317(15.16), 331(14.20), 381(5.21)
315(12.75), 381(5.60)
(0.20; 43.8; 4.57)
490, 522, 562[c]
__ __
(0.04; 90.1; 0.44)
550[c]
1.01[g]
302(38.73), 382(29.02), 456(br,
(0.47; 56.8; 8.27)
12.29)
311(37.04), 380(13.35), 465(br,
2.07)
568[d]
0.75[f]
612
(0.47; 0.62; 758)
493, 521
(0.28; 225; 1.2)
485, 518, 552
(0.26; 152; 1.7)
(0.89; 1.69; 527)
489, 519, 555[c]
(0.06; 147; 0.41)
__ __
304(8.09), 379(5.89), 394(5.00)
300(8.53), 378(5.70), 390(4.75)
486, 518, 555[c]
__ __
(0.06; 90.4; 0.66)
520[d]
301(37.06), 380(13.26),
393(12.32), 424(br, 6.68)
303(19.28), 325(13.88),
378(10.98), 391(9.21), 422(br,
1.16)
533
1.04
(0.94; 1.61; 584)
(0.82; 2.08; 394)
0.77[f]
−1.48
−1.59
580
568[d]
(0.71; 2.54; 280)
8
9
(0.74; 0.79; 937)
__ __
480, 513, 557
(0.003; 1.04; 2.88)
482, 515, 552[c]
(0.06, 36.01, 1.67)
305(22.07), 337(9.35), 395(2.61)
[a] In degassed toluene (2 × 10-5 mol dm-3) at room temperature. “br” stands for broad. [b] Emission quantum yields were measured with Hamamatsu C11347
Quantaurus-QY Absolute PL quantum yields measurement system. [c] PMMA thin film samples (with 4 wt% of AuIII complex). [d] TCTA thin film samples (with 16
wt% of 4, 8 wt% of 7 and 4 wt% of 8). [e] Epa refers to anodic peak potential for the irreversible oxidation waves. Epc refers to cathodic peak potential for the
irreversible reduction waves. E1/2 = (Epa + Epc)/2. [f] Reversible oxidation couples. [g] Quasi-reversible oxidation couples. [h] Irreversible reduction waves.
owing to steric strain, showing a dihedral angle with the
C^C^N^C ligand plane of ~74.5o. Intermolecular π-π stacking
between C^C^N^C ligands exists in 1 and 5 with interplanar
distances of ~3.4 Å.
intraligand charge transfer transitions (1ILCT) from π(PXZ/DPA)
to π*(C^C^N^C). More intense CT absorption is observed in the
complexes having DPA [ε = (7–12) ×103 dm3 mol-1 cm-1] than the
ones with PXZ [ε = (1–2) ×103 dm3 mol-1 cm-1]. DFT calculations
revealed that this lowest energy absorption band is derived from
the HOMO  LUMO transition with the HOMO mainly localized
on the PXZ / TPA (TPA = triphenylamine) moieties whereas the
LUMO is dominantly localized on the C^N^C part of the
C^C^N^C ligand (Table S10). As the PXZ moiety in both
complexes 4 and 8 is disposed nearly perpendicular to the
C^N^C plane, the overlap between the HOMO and LUMO
orbitals is small, leading to a small oscillator strength for the
PXZ-bearing complexes (f = 0.00020.0003). On the other hand,
one of the phenyl substituents in the TPA moiety of complexes 3
and 7 is nearly coplanar with the C^N^C plane, resulting in better
overlap between the HOMO and LUMO and thus larger
oscillator strength (f = 0.14910.2383).
The electrochemical properties of 1−9 in DMF were
investigated (Table 1). Except for
2 which displays an
irreversible reduction wave at -1.38 V, these complexes show
one reversible reduction couple at -1.36 to -1.59 V. These
reduction peaks are assigned to the reduction of C^C^N^C
ligands. Complexes 4 and 8 having a phenoxazine (PXZ) as a
donor unit display a reversible oxidation couple at 0.75–0.77 V,
while complexes 3 and 7 bearing a diphenylamine (DPA) show a
quasi-reversible couple / irreversible wave at 1.01–1.04 V.
These oxidation peaks are attributable to the oxidation of the
amino-substituted moieties.
Spectroscopic and Photophysical Properties
In deoxygenated toluene, complexes 1−9 display blue-green
to red photoluminescence at room temperature with three
different types of emission profiles. Complexes 1, 2, 5, 6 and 9
display blue-green to green emission featuring vibronically
structured emission bands with  and  of up to 0.54 and 225 s,
respectively; their radiative decay rate constants (kr) range from
1.2 × 103 to 5.8 × 103 s-1. Given the large Stokes shifts
(4500−6000 cm-1), vibronically structured emission bands and
similar kr of moderate magnitudes (~103 s-1), the emissions of 1,
The UV-visible absorption and emission spectral data of 1−9 are
given in Table 1 and S8. In toluene, complexes 1−9 show
intense absorption bands [ε = (1−4) ×104 dm3 mol-1 cm-1] at ca.
300−330 nm and moderately intense absorption bands [ε = (3–
29) ×103 dm3 mol-1 cm-1] at ca. 340−400 nm, both of which are
attributed to metal-perturbed intraligand π to π* transitions (1IL)
of the [C^C^N^C] ligand. Lower-energy absorption bands/tails
are observed in amino-substituted complexes, assignable to
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2, 5, 6 and 9 are assigned to 3π-π*[C^C^N^C] phosphorescence. AuIII-TADF complexes.[1a,e] Thus, the emission of 4, 7, and 8
Complexes 3, 4, 7 and 8 which possess amino-substituent on
the ligand display broad emission bands with emission color
ranging from yellowish green to red; in particular, the  reaches
a record-high value of 0.94 among Au(III) metal complexes. In
aerated toluene, the emissions of 3, 4, 7 and 8 are strongly
quenched by more than >90 %, suggesting that their triplet
excited states contribute to the observed emission.
likely involves TADF.
To further elucidate the nature of the emissive excited states
of 4, 7 and 8, their emission in oxygen-free toluene was
measured at different temperatures (Figure S9 and Table S9). It
is observed that when the temperature was lowered from 20 to
70 C, kr of all three complexes decreased with temperature,
giving support that the emission mechanism is thermally
activated and is thus consistent with the assignment of emission
origins involving TADF. When the temperature was raised from
o
20 to 60 C, the kr of 7 and 8 dropped while that of 3 increased
slightly; this indicates that the energy barrier in the thermally
activated process is not a constant and may vary with
temperature. It should be noted that the charge separation in a
charge transfer excited state depends on the polarity of the
solvent, which in turn affects the size of exchange interaction
and hence, the EST.[6] Since the polarity of toluene decreases
with increasing temperature,[7] it is possible that such increase in
temperature would reduce the charge separation in the 1,3CT
excited states[6] and hence, EST increases. That is, as the
temperature increases from 20 to 60 C, EST increases, thus
decreasing the contributions from the TADF emission
mechanism, and hence, kr decreases.
Time-Resolved Absorption Spectroscopy
The excited state nature and dynamics of the complexes have
been investigated by nano/femtosecond time resolved
spectroscopy. Nanosecond time-resolved absorption difference
(ns-TA) spectra of 1−9 were obtained with 355 nm laser pulse
excitation (Figure S10). Complexes 1 and 2 (5 and 6), which are
supported by similar C^C^N^C ligand scaffold and exhibit 3IL
phosphorescence, show similar ns-TA spectra, revealing that
their emissive excited states share similar electronic nature. For
3, 4, 7, and 8, their TA absorption profiles are unique to each
complex, indicating that they are not derived from the same
excited state. This is consistent with their steady-state emission
characteristics where the emissions of 3, 4, 7, and 8 are charge
transfer in nature, which depends on the donor and acceptor
identities. Although complex 9 also displays 3IL emission, the
ligand scaffold is different from that of 1, 2, 5, and 6; hence the
ns-TA profiles of 9 are also different from those of 1, 2, 5, and 6.
Figure 3. a) and b) Absorption and emission spectra of 5−8 in degassed
toluene (conc. 210-5 mol dm-3) at room temperature; c) emission spectra of 3
in different solvents; d) emission spectra of 4 in degassed toluene at different
temperatures.
Solvent Effect and TADF Emission
To further elucidate the nature of the emissive excited state of
complexes 3, 4, 7, and 8, the solvent effect on their emission
properties was examined. The emissions of 3, 4, 7 and 8 display
significant positive solvatochromism with the emission λmax red-
shifted by 638−2113 cm-1. The Lippert–Mataga plots are linear
with large positive slopes of 8490–12630 cm-1 (Figure S8),
indicative of a large change in dipole moment upon attaining the
emissive excited state and revealing the strong charge-transfer
character of these emissive excited states. Although the
emission profiles of these complexes vary from vibronically-
structured in hexane to broad and featureless in polar solvents,
the linear Lippert-Mataga plot with no inflection suggests a
simple evolution from one state to another. As the emission of
complex 3 is relatively long-lived ( = 94 s) with moderate kr
Femtosecond time-resolved absorption (fs-TA) spectra of
these complexes have been recorded except for 2, 6 and 9
which show poor solubility in toluene at a high concentration
(Figure S11). Complexes 1, 3−5, 7 and 8 show striking
differences between their fs-TA spectra, revealing different
electronic configurations of their excited states. Clear spectral
transformation with sets of isosbestic points can be observed in
the early absorption spectra from the initial precursor state to the
developed successor state. Since the successor state observed
in fs-TA is consistent with the corresponding ns-TA spectra
(Figure S10), the spectral transformation can be reasonably
assigned as ISC from Sn to T1. In the late-time fs-TA spectra of
3
(8.16×103 s-1), the emission of 3 is probably derived from ILCT
[π(diphenylamine) to π*(C^C^N^C)] mixed with 3IL π-
[3d]
π*[C^C^N^C] excited state.
Complexes 4, 7 and 8, on the
other hand, being short-lived ( < 2 s) with large kr (~105−106 s-
1) display emission behaviors that are similar to the reported
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these complexes, an ultrafast process is found in the triplet
excited state, attributable to solvent reorientation or vibrational
reflected from the charge density difference (CDD) maps and
the associated natural transition orbitals (NTO) depicted in
Figure S12 and Figure 5 respectively. The NTO pair, highest
occupied NTO (HONTO) and the lowest unoccupied NTO
(LUNTO), contributes dominantly to the S1 and T1 excited states
(eigenvalue  > 0.97). For both complexes, the HONTO is
localized mainly on the DPA moiety and the phenyl ring the DPA
directly attached to while the LUNTO is mainly on the C^N^C
part of the C^C^N^C ligand. Of particular note is that, in complex
3, the DPA moiety is directly attached to the phenyl ring of the
C^N^C part of the C^C^N^C ligand; as such, the HONTO-
LUNTO overlap is larger in 3 than in 7 (Table S12) and the
energy gap between the S1 and T1 excited states (EST
=
E(S1)E(T1)) of 3 is much larger than that of 7 (EST = 2466 and
Figure 4. a) The early spectral conversion and b) the late-time process of fs-
TA spectrum of 3 in toluene; c) kinetic study of fs-TA of 3; d) ns-TA spectrum
of 3 in toluene.
cooling. Kinetic study at selected wavelengths of these fs-TA
spectra reveals that there are two components in the spectral
evolution, defined as growth and decay, with the fast component
growth correlating with the process of ISC. Except for 8 with
relatively long growth of 56 ps, the intersystem crossing time
constants (ISC
= 0.4−10 ps) of these compounds are
comparable to those of the reported AuIII complexes with ISC of
~0.2−13 ps.[8] The kISC of these tetradentate AuIII complexes can
be estimated according to the formula of kISC = 1/growth and also
given herein (Table 2). The relatively long decay of 45−2436 ps in
Figure S11 are thus ascribed to the aforementioned ultrafast
process in the triplet excited state.
Table 2. Estimated kISC of tetradentate AuIII complexes.
Complex
1
3
4
5
7
8

growth [ps]
0.4
25
10
1.0
1.7
5.9
0.4
25
9.3
1.1
56
kISC [1011 s-1]
0.18
DFT/TDDFT Calculations
It is observed from 3 and 7 that the position of the DPA in the
complex has a strong impact on their kr (kr of 7 is of the order
105 s1 while that of 3 is ~8  103 s1 in both solution and film
states). To understand the factor(s) that lead to the very different
kr in 3 and 7, DFT/TDDFT calculations have been performed on
these two complexes. Both the first singlet (S1) and triplet (T1)
excited states display charge transfer (CT) character, as
Figure 5. NTO pairs for the S1 (top) and T1 (bottom) excited states of
complexes 3 (left) and 7 (right). Isovalue = 0.02 a.u.
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Table 3. Key performances of OLEDs based on 4 with different doping concentrations.
Current efficiency [cd A-1]
Power efficiency [Im W-1]
EQE [%][b]
at 1000 cd m-2
Conc.
[wt%]
Luminance
[cd m-2][a]
CIE [(x, y)][c]
Max.
65.59
70.82
77.78
at 1000 cd m-2
53.23
Max.
91.98
81.73
94.00
at 1000 cd m-2
49.18
Max.
20.84
22.72
25.03
4
8
7300
16500
22700
16.79
0.38, 0.56
0.40, 0.55
0.43, 0.54
64.44
62.38
21.21
22.01
16
67.48
63.72
[a] At a driving voltage of 7 V. [b] External quantum efficiency. [c] CIE coordinates at 1000 cd m-2.
686 cm1 for complexes 3 and 7 respectively). The much smaller
EST in 7 suggests more favorable ISC and RISC; in turn, the
much faster kr in 7 is thus likely to be due to more efficient TADF.
To give further theoretical support that the much faster kr in
complex 7, as compared with complex 3, is due to more efficient
TADF in the former, their kr values have also been computed
based on optimized excited state geometries. The character of
the optimized S1 and T1 excited states are also CT in nature and
are similar to those at their respective optimized S0 geometries
(Figure S13S14). At the optimized 3CT geometries, EST
remains large for 3 (EST ~ 2800 cm1). The corresponding
calculated krP(3CT  S0) is 1.06  103 s1 and the average kr
Figure 6. a) Normalized EL spectra and b) EQE-luminance characteristics of
OLEDs based on 4 (16 wt%), 7 (8 wt%) and 8 (4 wt%).
(kr,avg), which takes into account both phosphorescence and
TADF assuming Boltzmann statistics, slightly increases to 1.10 
103 s1 (assuming that the 1,3CT excited states are in fast thermal
equilibrium). As the experimental kr is also of the order 103 s1,
.
External quantum efficiency (EQE) versus luminance
characteristics of the optimized 4, 7 and 8 devices are depicted
in Figure 6b and key EL performances of 4 are summarized in
Table 3. These AuIII-TADF emitters realize maximum EQEs of
over 20 % (23 % and 20 % of the device with 4 wt% 7 and 4 wt%
8 respectively). Both maximum EQE and efficiency roll-off of the
device with 16 wt% 4 were superior to those of the other devices,
owing to its higher Φ and shorter  in thin films as described in
Table 1. Maximum EQE of 25.03 %, CE of 77.78 cd A-1 and PE
of 94.00 lm W-1 were achieved in the device with 16 wt% 4.
Notably, a high EQE of 22.01 % was maintained at a high
luminance of 1000 cd m-2. Such performance is among the best
of reported AuIII-OLEDs and comparable to that of AuI-OLEDs
(Table S17).[1a,b,d,e] The operational lifetime of OLED prepared
with 4 has been measured under our laboratory conditions and
compared with that of the best pincer Au(III)-acetylide TADF
emitter from our previous report[1e] under the same device
configuration (Table S18). The OLED of 4 exhibited operational
lifetime LT95 of 1.97 h at initial luminance of 10390 cd m-2,
corresponding to 105 h at 1000 cd m-2 and 5280 h at 100 cd m-2,
which are the best among all reported Au(III)-OLEDs.
the emission of
3
is consistent with dominant 3CTS0
phosphorescence. On the other hand, for 7, at the optimized 3CT
geometry, EST increases to 1700 cm1, possibly due to the
shortened AuN(pyridine) and C(phenyl)N(DPA) distances,
leading to stronger HONTO-LUNTO overlap (Table S13). The
corresponding calculated krP(3CT  S0) is 2.48  103 s1 and
kr,avg increases by an order of magnitude to 1.24  104 s1.
Interestingly, there is a low frequency mode (16 ~ 168 cm1) that
involves
the elongations
of
AuC(phenyl-DPA)
and
C(phenyl)N(DPA) distances; thus we have also computed the
kr and EST with this thermally accessible 16 distortion (7-
distorted). The EST decreases to ~1200 cm1 with krP(3CT 
S0) increased to 4.95  103 s1. More importantly, the kr,avg
becomes 1.15  105 s1. Hence, TADF is much more efficient in
7. Compared with the experimental kr of 7, which is also of the
order 105 s1, the much faster kr of 7, as compared with 3, is due
to the much enhanced TADF efficiency in the former complex
because the placement of the DPA at the phenyl ring trans to
the pyridine ring allows better separation of the HOMO and
LUMO and hence a smaller EST.
Discussion
Organic-Light Emitting Devices
With a view to developing robust and strongly emissive AuIII
complexes for practical material applications, we conceive that
the research on AuIII complexes supported by tetradentate
ligands having C-donor atoms is critical yet far less explored.
Vacuum-deposited OLEDs based on 4, 7 and 8 were fabricated
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10.1002/anie.201914661
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Tetradentate AuIII complexes supported by confused porphyrin
and corrole have been reported to be emissive, but they exhibit
NIR phosphorescence with low quantum yields and long
emission lifetimes.[9] The work on N-bridged tetradentate AuIII
complexes by Yam in 2017 is a great leap in such direction.[3d]
By connecting the bound tridentate C^N^C and monodentate
acetylide ligand via an amino linkage, tetradentate AuIII
complexes having a 12-membered metallocycle have been
furnished. These complexes exhibit phosphorescence with  of
0.41-0.49 and τ of 5.3-10.8 μs in deaerated CH2Cl2 at room
temperature. However, due to the presence of amino linker in
the ligand, their emissive excited states are confined to 3ILCT
ones, which results in a narrow emission maxima range of 584-
615 nm, posing a constraint on the tuning of photophysical and
excited state properties.
complexes is highly tunable and spans from blue-green (485 nm)
to red (612 nm). It is interesting to note that although the amino-
substituted aryl moiety in complexes 7 and 8 is linked by an O-
bridge which may limit the rotation and dihedral angle of the aryl
moiety, these two complexes still exhibit efficient TADF similar to
the tridentate counterparts.[1a]
Emitters having high Ф and short τ are ideal for realizing
OLEDs with high EQE and small efficiency roll-off. Vacuum-
deposited OLEDs based on AuIII-TADF emitters 4, 7 and 8 all
showed excellent performance with maximum EQE over 20 %,
comparable to the best ones based on tridentate AuIII
emitters[1a,d,e] and significantly higher than OLEDs with N-bridged
tetradentate AuIII emitters (maximum EQE = 11.1 %).[3d] The
OLED fabricated with 16 wt% of 4 is the best performing one,
with EQE of 22.01 % at a practical luminance of 1000 cd m-2.[1e]
The high Ф (0.89) and short τ (1.69 μs) of 4 in thin film are the
critical factors accounting for the high efficiency at high
brightness. In our previous study, the operational lifetime of the
device fabricated with tridentate gold(III) TADF emitter[1e], as
evaluated by Samsung, was the best for Au(III)-OLEDs[1d]. In this
work, the device data evaluated under our laboratory conditions
revealed that the device with 4 showed a LT95 of 5280 hours at
100 cd m-2, which is more than 10 times better than those
achieved with tridentate emitters. This finding signifies the
importance of employing tetradentate ligand scaffold in the
design of stable and structurally robust Au(III)-TADF emitters for
practical applications.
In this work, an aryloxy-substituted C^N^C ligand serves as
the core tetradentate C^C^N^C ligand scaffold for complexes 1-
8. A major hurdle in the preparation of these complexes is the
difficulty in forming the third metallocycle. Traditional synthesis
via conventional heating has been attempted but failed to give
the desired product. In light of the successful preparation of
[Au(C^C^N)] complexes via the use of microwave irradiation,[5c,e]
we then sought to achieve the last cyclometalation step with this
method which was found to give the expected tetradentate AuIII
complexes in high yields (90 %). Regarding the design of the
ligand, the placement of the aryloxy-substituent on the 3-position
of the phenyl ring of C^N^C moiety allows the formation of 5-5-6
membered chelate rings which may confer higher structural
rigidity for minimizing knr. Furthermore, since the aryloxy-related
excited state would be higher-lying than that arising from amino-
substituents, it allows post-synthetic installation of different
amino-substituents on the C^C^N^C scaffold for tuning the
emission color over a wide range. For 9, instead of an O-bridge,
a spiro-fluorene has been employed to link pyridine and phenyl
for forming a 6-membered metallocycle. Unlike the O-bridged
complexes, this spiro-fluorene-bridged complex could only be
prepared via successive microwave-assisted C-H activation.
Although 9 is also supported by a C^C^N^C ligand, it is
unexpectedly very weakly emissive (Ф = 0.003, in toluene). One
possible reason is the facile motion of its spiro-fluorene unit in
Conclusion
In summary, charge-neutral, cyclometalated tetradentate AuIII
complexes with trianionic O-bridged/spiro-arranged C^C^N^C
ligands have been successfully synthesized through microwave-
induced reactions. By modifying the nature and positions of
substituents, these tetradentate AuIII complexes display strong
photoluminescence originating from three different kinds of
3
emission origins (3IL, ILCT and TADF) with  of up to 0.94 in
degassed toluene. Temperature-dependent emission studies
and DFT/TDDFT calculations both support that the emission
mechanism of complexes with large kr and short  involves
TADF. Ultrafast time-resolved spectroscopic measurements
conducted on these tetradentate AuIII complexes revealed ISC of
0.4−56 ps and estimated kISC of 0.2−25  1011 s-1. Vacuum-
deposited OLEDs fabricated with these tetradentate AuIII-TADF
emitters showed excellent performance with maximum EQEs of
up to 25 %, and EQEs of up to 22 % at a luminance of 1000 cd
m-2. The findings in this work underpin the competitiveness of
AuIII-TADF emitters among phosphorescent IrIII and PtII emitters
for OLED application. It is envisaged that this type of AuIII
complexes would serve as a robust and versatile class of
material which will excel in diverse applications.
1
solution, as reflected by the H NMR spectra of 9 in deuterated
toluene at room temperature and at -60 oC both showing that the
two tBu groups on the spiro moiety give identical signal (a
singlet; note the different environments of the two tBu groups in
the crystal structure of 9 depicted in Figure 2).
The O-bridged tetradentate ligand system described herein is
highly versatile, affording AuIII complexes exhibiting long-lived
3IL emissive excited states with Ф of up to 0.54 and τ of up to
225 μs in degassed solution at room temperature, and also
complexes showing TADF with Ф of up to 0.94 and τ < 2 μs via
the addition of amino-substituent on the ligand. The Ф of 0.94
shown by 7 is the highest value among luminescent AuIII
complexes.[10] In contrast to N-bridged tetradentate AuIII
complexes, the emission color of these O-bridged AuIII
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Lai, W.-L. Cheung, M.-Y. Chan, V. W.-W. Yam, Chem. Sci. 2019, 10,
Acknowledgements
594-605.
This work was supported by the Basic Research Program of
[4]
[5]
M. Saigo, K. Miyata, S. Tanaka, H. Nakanotani, C. Adachi, K. Onda, J.
Phys. Chem. Lett. 2019, 10, 2475-2480.
Shenzhen
(JCYJ20160229123546997,
JCYJ20170818141858021 and JCYJ20180508162429786),
Hong Kong Research Grants Council (HKU 17330416), CAS-
Croucher Funding Scheme for Joint Laboratories, the Major
Program of Guangdong Basic and Applied Research
(2019B030302009) and the National Key Basic Research
Program of China (2013CB834802). 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 1, 5 and 9.
a) A. P. Shaw, M. Tilset, R. H. Heyn, S. Jakobsen, J. Coord. Chem.
2011, 64, 38-47; b) E. Langseth, C. H. Gorbitz, R. H. Heyn, M. Tilset,
Organometallics 2012, 31, 6567-6571; c) R. Kumar, A. Linden, C.
Nevado, Angew. Chem. Int. Ed. 2015, 54, 14287-14290; Angew. Chem.
2015, 127, 14495-14498; d) T. von Arx, A. Szentkuti, T. N. Zehnder, O.
Blacque, K. Venkatesan, J. Mater. Chem. C 2017, 5, 3765-3769; e) M.
S. M. Holmsen, A. Nova, K. Hylland, D. S. Wragg, S. Oien-Odegaard, R.
H. Heyn, M. Tilset, Chem. Commun. 2018, 54, 11104-11107.
[6]
[7]
[8]
A. P. Monkman in H. Yersin, Ed. Highly Efficient OLEDs-Materials
Based on Thermally Activated Delayed Fluorescence. Wiley-VCH:
Weinheim, Germany, 2019, p. 425-463.
Keywords: gold • TADF • tetradentate • microwave • OLED
a) F. I. Mopsik, J. Chem. Phys. 1969, 50, 2559-2569; b) M. Aydemir, S.
Xu, C. Chen, M. R. Bryce, Z. Chi, A. P. Monkman, J. Phys. Chem. C
2017, 121, 17764-17772.
[1]
a) W.-P. To, D. Zhou, G. S. M. Tong, G. Cheng, C. Yang, C.-M. Che,
Angew. Chem. Int. Ed. 2017, 56, 14036-14041; Angew. Chem. 2017,
129, 14224-14229; b) D. Di, A. S. Romanov, L. Yang, J. M. Richter, J.
P. H. Rivett, S. Jones, T. H. Thomas, M. A. Jalebi, R. H. Friend, M.
Linnolahti, M. Bochmann, D. Credgington, Science 2017, 356, 159-163;
c) P. J. Conaghan, S. M. Menke, A. S. Romanov, S. T. E. Jones, A. J.
Pearson, E. W. Evans, M. Bochmann, N. C. Greenham, D. Credgington,
Adv. Mater. 2018, 30, 1802285; d) L.-K. Li, M.-C. Tang, S.-L. Lai, M. Ng,
W.-K. Kwok, M.-Y. Chan, V. W.-W. Yam, Nat. Photonics 2019, 13, 185-
191; e) D. Zhou, W.-P. To, Y. Kwak, Y. Kwak, Y. Cho, G. Cheng, G. S.
M. Tong, C.-M. Che, Adv. Sci. 2019, 6, 1802297.
a) W.-P. To, K. T. Chan, G. S. M. Tong, C. Ma, W.-M. Kwok, X. Guan,
K.-H. Low, C.-M. Che, Angew. Chem. Int. Ed. 2013, 52, 6648-6652;
Angew. Chem. 2013, 125, 6780-6784; b) K. T. Chan, G. S. M. Tong,
W.-P. To, C. Yang, L. Du, D. L. Phillips, C.-M. Che, Chem. Sci. 2017, 8,
2352-2364.
[9]
a) M. Toganoh, T. Niino, H. Furuta, Chem. Commun. 2008, 4070-4072;
b) M. Soll, K. Sudhakar, N. Fridman, A. Müller, B. Röder, Z. Gross, Org.
Lett. 2016, 18, 5840-5843; c) A. B. Alemayehu, N. U. Day, T. Mani, A.
B. Rudine, K. E. Thomas, O. A. Gederaas, S. A. Vinogradov, C. C.
Wamser, A. Ghosh, ACS Appl. Mater. Interfaces 2016, 8, 18935-18942;
d) C. M. Lemon, D. C. Powers, P. J. Brothers, D. G. Nocera, Inorg.
Chem. 2017, 56, 10991-10997.
[2]
a) D. A. K. Vezzu, J. C. Deaton, J. S. Jones, L. Bartolotti, C. F. Harris,
A. P. Marchetti, M. Kondakova, R. D. Pike, S. Huo, Inorg. Chem. 2010,
49, 5107-5119; b) S. C. F. Kui, P. K. Chow, G. S. M. Tong, S.-L. Lai, G.
Cheng, C.-C. Kwok, K.-H. Low, M. Y. Ko, C.-M. Che, Chem. Eur. J.
2013, 19, 69-73; c) G. Cheng, P.-K. Chow, S. C. F. Kui, C.-C. Kwok, C.-
M. Che, Adv. Mater. 2013, 25, 6765-6770; d) S. C. F. Kui, P. K. Chow,
G. Cheng, C.-C. Kwok, C. L. Kwong, K.-H. Low, C.-M. Che, Chem.
Commun. 2013, 49, 1497-1499; e) G. Cheng, S. C. F. Kui, W.-H. Ang,
M.-Y. Ko, P.-K. Chow, C.-L. Kwong, C.-C. Kwok, C. Ma, X. Guan, K.-H.
Low, S.-J. Su, C.-M. Che, Chem. Sci. 2014, 5, 4819-4830; f) T.
Fleetham, G. Li, L. Wen, J. Li, Adv. Mater. 2014, 26, 7116-7121; g) K.-
Y. Liao, C.-W. Hsu, Y. Chi, M.-K. Hsu, S.-W. Wu, C.-H. Chang, S.-H.
Liu, G.-H. Lee, P.-T. Chou, Y. Hu, N. Robertson, Inorg. Chem. 2015, 54,
4029-4038; h) P.-K. Chow, G. Cheng, G. S. M. Tong, C. Ma, W.-M.
Kwok, W.-H. Ang, C. Y.-S. Chung, C. Yang, F. Wang, C.-M. Che, Chem.
Sci. 2016, 7, 6083-6098; i) T. Fleetham, Y. Ji, L. Huang, T. S. Fleetham,
J. Li, Chem. Sci. 2017, 8, 7983–7990; j) G. Li, A. Wolfe, J. Brooks, Z.-Q.
Zhu, J. Li, Inorg. Chem. 2017, 56, 8244-8256; k) L. Liu, X. Wang, N.
Wang, T. Peng, S. Wang, Angew. Chem. Int. Ed. 2017, 56, 9160-9164;
Angew. Chem. 2017, 129, 9288-9292; l) Z.-Q. Zhu, C.-D. Park, K.
Klimes, J. Li, Adv. Opt. Mater. 2019, 7, 1801518.
[10] a) J. M. López-de-Luzuriaga, M. Monge, M. E. Olmos, Dalton Trans.
2017, 46, 2046-2067; b) R. Kumar, C. Nevado, Angew. Chem. Int. Ed.
2017, 56, 1994-2015; Angew. Chem. 2017, 129, 2024-2046.
[3]
a) G. Cheng, K. T. Chan, W.-P. To, C.-M. Che, Adv. Mater. 2014, 26,
2540-2546; b) K. T. Chan, G. S. M. Tong, Q. Wan, G. Cheng, C. Yang,
C.-M. Che, Chem. Asian J. 2017, 12, 2104-2120; c) M.-C. Tang, C.-H.
Lee, S.-L. Lai, M. Ng, M.-Y. Chan, V. W.-W. Yam, J. Am. Chem. Soc.
2017, 139, 9341-9349; d) B. Y.-W. Wong, H.-L. Wong, Y.-C. Wong, M.-
Y. Chan, V. W.-W. Yam, Angew. Chem. Int. Ed. 2017, 56, 302-305;
Angew. Chem. 2017, 129, 308-311; e) M.-C. Tang, W.-K. Kwok, S.-L.
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Dongling Zhou, Wai-Pong To, Glenna
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Luminescent Tetradentate Gold(III)-
TADF Emitters: Microwave-Assisted
Synthesis and High Performance
OLEDs With EQEs Up To 25 % and
Long Operational Lifetime
Gold(III) complexes supported by tetradentate ligands have been prepared under
microwave conditions. Depending on the ligand substituents, these complexes
exhibit long-lived 3IL, 3ILCT emissive excited states and short-lived TADF with
quantum yields of up to 0.94. OLEDs fabricated with gold(III)-TADF emitters show
maximum EQE of up to 25 % and LT95 of up to 5280 h at 100 cd m-2.
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