High Efficiency Sky-Blue Gold(III)-TADF Emitters**

Article abstract of DOI:10.1002/chem.202003855

Highly efficient sky-blue luminescent gold(III) complexes with emission quantum yields up to 82 %, lifetimes down to 0.67 μs and emission peak maxima at 470–484 nm were prepared through a consideration of pincer gold(III) donor–acceptor complexes. Photophysical studies and time-dependent density functional theory (TDDFT) calculations revealed that the emission nature of these gold(III) complexes is most consistent with TADF. Solution-processed OLEDs with these gold(III) complexes as dopants afforded electroluminescence maxima at 465–473 nm with FWHM of 64–67 nm and maximum external quantum efficiencies (EQEs) of up to 15.25 %. This research demonstrates the first example of gold(III)-OLEDs showing electroluminescence maxima at smaller than 470 nm, and highlights the potential of using gold(III)-TADF emitters in the development of high efficiency blue OLEDs and blue emissive dopant in WOLEDs.

Full text of DOI:10.1002/chem.202003855

Accepted Article
Accepted Article
Title: High Efficiency Sky-Blue Gold(III)-TADF Emitters
Authors: Dongling Zhou, Gang Cheng, Glenna So Ming Tong, and Chi-
Ming Che
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To be cited as: Chem. Eur. J. 10.1002/chem.202003855
Link to VoR: https://doi.org/10.1002/chem.202003855
01/2020

10.1002/chem.202003855
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High Efficiency Sky-Blue Gold(III)-TADF Emitters
Dongling Zhou,[a] Gang Cheng,*[a,b,c] Glenna So Ming Tong,*[a] and Chi-Ming Che*[a,b,c]
[a]
Dr. D. Zhou, Dr. G. Cheng, Dr. G. S. M. Tong, 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: ggcheng@hku.hk; tongsm@hku.hk; cmche@hku.hk
Dr. G. Cheng, Prof. Dr. C.-M. Che
Centre of Machine Learning for Energy Materials and
Devices, The University of Hong Kong
[b]
[c]
Pokfulam Road, Hong Kong SAR (China)
Dr. G. Cheng, Prof. C.-M. Che
HKU Shenzhen Institute of Research and Innovation
Shenzhen (China)
Supporting information for this article is given via a link at the end of the document.
Abstract: Highly efficient sky-blue luminescent gold(III) complexes
with emission quantum yields up to 82%, lifetimes down to 0.67 s
and emission peak maxima at 470484 nm were prepared through a
consideration of pincer gold(III) donor-acceptor complexes. Photo-
physical studies and time-dependent density functional theory
(TDDFT) calculations revealed that the emission nature of these
gold(III) complexes is most consistent with TADF. Solution-
processed OLEDs with these gold(III) complexes as dopants afforded
electroluminescence maxima at 465473 nm with FWHM of 6467
nm and maximum external quantum efficiencies (EQEs) of up to
15.25%. This research demonstrates the first example of gold(III)-
OLEDs showing electroluminescence maxima at smaller than 470
nm, and highlights the potential of using gold(III)-TADF emitters in the
development of high efficiency blue OLEDs and blue emissive dopant
in WOLEDs.
the stringent requirements on designing complexes with high
emission energy that avoids deactivating ligand-to-metal charge
transfer (LMCT) or d-d ligand field excited states, high emission
quantum yields with short exciton lifetimes are also critical
parameters of the metal phosphors used in fabricating blue
OLEDs with long operational lifespan. Although purely organic
blue TADF emitters have been reported, most of the as-
fabricated devices showed high efficiency roll-off with the device
lifetime far from satisfactory owing to the singlet-triplet and triplet-
triplet annihilation caused by the relatively long triplet excited
state lifetime of organic compounds (milliseconds to seconds). In
this regard, the design of blue gold(III)-TADF emitters is an
appealing approach. Organogold(III) complexes with emission
color towards the blue spectral region have been reported in the
literature; for instance, in 2017, Venkatesan and co-workers
reported gold(III) complexes constructed with NHC-containing
C^C chelates that show vibronic-structured emission with
maxima locating at ~380480 nm.[4] Yam and co-workers also
reported blue gold(III) phosphors with photoluminescence
maxima at ~470500 nm and electroluminescence maxima at
Introduction
Due to the low-lying d orbitals of gold(III) ion, the emissive excited
states of phosphorescent gold(III) complexes are usually
dominated by long-lived ligand-centered triplet excited states
(3IL), with emission lifetimes being hundreds of microseconds[1]
which accounts for large efficiency roll-offs in organic light-
emitting diodes (OLEDs) based on phosphorescent Au(III)
emitters.[1d,j-l] The year 2017 marks a great leap in the
development of gold(III) OLED emitters, in which luminescent
gold(III) complexes showing efficient thermally activated delayed
fluorescence (TADF) with high quantum yields approaching unity
and short lifetimes of < 2 s have been realized; these
luminescent parameters are comparable to those of 3MLCT-
(MLCT = metal-to-ligand charge transfer) or mixed 3IL/3MLCT-
emitting Ir(III)/Pt(II) phosphors.[2,3] More importantly, recent works
on OLEDs fabricated with tetradentate gold(III)-TADF emitters
showed maximum external quantum efficiencies (EQEs) of up to
3
~480500 nm.[1k-l, 5] However, vibronically structured IL excited
states of gold(III) complexes have long emission lifetimes (tens
to hundreds of microseconds), thus rendering these complexes
unsuitable for the fabrication of OLEDs with practical interest. In
our previous work in 2017,[2] a green-emitting gold(III)-TADF
complex (a5 in Figure 1) with emission maximum at 524 nm in
toluene solution at room temperature was used to fabricate
OLEDs that showed sky-blue emission peak at 486 nm with
Commission Internationale de L’Eclairage (CIE) coordinates of
(0.21, 0.42) and maximum EQEs of 15.7%. This encouraging
result prompted us to prepare blue gold(III)-TADF emitters.
As a TADF emitter, the dominant emission comes from the
S1 excited state. From our previous work, the S1 excited state of
complex a5 is predominantly derived from the HOMO  LUMO
transition where the HOMO is localized on the aryl donor
triphenylamine (TPA) and the LUMO is localized mainly on the
C^N^C ligand (Figure 1), and hence, of ligand-to-ligand charge
transfer (LLCT) character. Thus, to blue-shift the TADF emission
of complex a5 to realize blue gold(III)-TADF emitter, either
lowering the HOMO energy of the aryl donor or raising the LUMO
energy of the C^N^C ligand could increase the S1 energy to the
blue region. Two sky-blue gold(III) complexes (1 and 2 in Figure
1) were thus prepared by replacing the OEt group in complex a5
25% and long operational lifetimes (LT95) of 5280 h at 100 cd m-
2 [3b]
.
These recent findings underpin the competitiveness of
gold(III)-TADF emitters among the most efficient metal
phosphors of IrIII and PtII.
While phosphorescent green- and red-emitting materials for
use in OLED industry are mature, the development of efficient
and stable blue phosphors remains a challenging task. Besides
1
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with a more electron-donating dimethylamine (NMe2) group to
raise the LUMO energy; for 2, the phenyl rings of the TPA donor
were further substituted with electron-withdrawing fluoride to
lower the HOMO energy. Complex 3 with no substituents at the
pyridine moiety of the C^N^C ligand was also prepared for
comparison. It was found that 1 and 2 show good performance
when used as an emissive dopant for fabrication of blue OLED
with maximum EQE as high as 15.25% as well as WOLED device
showing maximum EQE of 13.35%, suggesting that Au(III)-TADF
complexes could emerge to become a new class of luminescent
metal dopants for high efficiency blue OLEDs with potential
practical interest.
Electrochemistry
The cyclic voltammograms of 1−3 in N,N-dimethylformamide
(DMF) were recorded with 0.1 mol dm-3 [nBu4N]PF6 as supporting
electrolyte and saturated calomel electrode (SCE) as the
reference electrode (Figure S2). A reversible reduction couple
was observed at E1/2 of 1.71 to 1.70 V for 1 and 2 but at a much
less negative E1/2 of 1.37 V for 3 (Table 1); this reduction couple
is thus attributed to the reduction of C^N^C ligands,[2,3a] with
LUMO energy (ELUMO) being –2.67 eV for 1 and 2 and –3.02 eV
for 3 (Table 1). When compared with complex a5 (ELUMO = 2.85
eV) which bears an OEt substituent on the pyridine moiety of the
C^N^C ligand, it is evident that changing the OEt substituent to
more electron-donating NMe2 to form 1 and 2 substantially
increases the LUMO energy, and the LUMO of 1 and 2 is 0.35 eV
higher-lying than that of the unsubstituted complex 3. A quasi-
reversible oxidation couple was observed at Epa = 0.94 V for 1
and 3 with HOMO energy (EHOMO) being ca. –5.16 eV and at Epa
= 0.97 V for 2 with EHOMO being –5.19 eV (Table 1). These
oxidation couples are assigned to the oxidation of the TPA
donor;[2,3a] the fluorine substituents at the TPA ligand thus slightly
lower the HOMO energy by ca. 0.03 eV.
Photophysical properties
The UV-vis absorption and emission spectra of 13 in toluene
solutions are shown in Figure 2a, and the corresponding photo-
physical data are tabulated in Table 1. Complexes 13 showed
intense absorption bands at ~300 nm (ε = (35)104 dm3 mol-1 cm-
1), and moderately intense absorption bands at 330390 nm (ε =
(427)103 dm3 mol-1 cm-1); these intense absorption bands are
1
assigned to metal-perturbed π-π* transitions (1IL) of the C^N^C
ligand.[2] Additional weak absorption bands/tails at ca. 380430 nm
with ε = (14)103 dm3 mol-1 cm-1 were observed, which are
assigned to be derived from metal-perturbed 1(π(TPA)

Figure 1. Top: Previously reported arylgold(III)-TADF complex a5[2] and its
HOMO and LUMO surfaces. Bottom: donor-acceptor type C^N^C gold(III)
complexes 13 in this work.
π*(C^N^C)) LLCT transition.[2] Upon photo-excitation, all three
complexes display structureless emission bands (Figure 2a) with kr
on the order of 105106 s-1 and emission lifetimes of < 1 s.
Complexes 1 and 2 display sky-blue emission at 495 and 483 nm
with high emission quantum yields of 0.81 and 0.60, respectively.
As a reference, complex 3, which does not have electron-donating
substituent at the pyridine moiety, has the emission peaked at 566
nm with a high emission quantum yield of 0.93 and a short lifetime
of 0.84 s. Thus, introducing an NMe2 substituent on 3 to give 1
leads to a blue shift in emission maximum by ~2500 cm1 (~0.31
eV), close to the LUMO energy shift of 0.35 eV as estimated from
the electrochemical data. The incorporation of F substituents at the
TPA ligand in 2 leads to a further blue shift of ~500 cm1 (~0.06 eV),
which is also comparable to the HOMO energy change of ~0.03 eV
estimated from CV measurements. Thus, our present approach in
blue-shifting the emission band from the green to the blue region is
valid and it gives support to the proposition that the emission is
derived from a HOMO  LUMO LLCT excited state. The charge
transfer nature of the emissive excited state is further
corroborated by emission measurements in different polarity
solvents. Complex 1 has been chosen as a representative
example. As depicted in Figure 2b, an increase in solvent polarity
leads to a conspicuous red shift of 45 nm (~1680 cm1) in emission
maxima. The Lippert-Mataga plot of 1 indicates a large positive
slope of 11555 cm-1 (Figure 2c), supporting that the emissive
excited state of 1 has significant charge transfer character.
Results and Discussion
X-ray crystallography
A diffraction-quality crystal of 3 was obtained by slow evaporation
of a fluorobenzene solution of this complex. Its structure
determined by X-ray crystal analysis is depicted in Figure S1 and
selected bond lengths/angles are tabulated in Table S1. The gold
atom of this complex adopts a slightly distorted square-planar
geometry with the C−Au−C angle of Au(C^N^C) and the
N−Au−C(ancillary ligand) angle being 161.40(15)o and
176.32(12)o, respectively. The Au−C bond distances within the
Au(C^N^C) unit [2.051(4)−2.069(4) Å] are slightly longer than the
Au−C(ancillary ligand) bond distance [2.016(3) Å] and the Au−N
bond distance is 2.033(3) Å. The Au-bound phenyl ring of the N-
substituted ancillary ligand (TPA) is not coplanar with the
Au(C^N^C) plane, making a dihedral angle of 61.36o. All of these
structural features are akin to those of the reported (C^N^C)-AuIII
aryl complexes.[2] The molecules of 3 are packed in a head-to-tail
fashion throughout the crystal structure with inter-planar
distances of approximately 3.23.5 Å (Figure S1).
2
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Table 1. Photophysical and electrochemical properties of complexes 13.
Epa [V];[c]
퐸1r/e2d [V];
Emission[b]
Absorption

abs [nm] (ε×103 [dm3 mol-1 cm-1])[a]
Complex
EHOMO [eV][d]
ELUMO [eV][d]
In toluene
In PMMA thin films

em [nm] ( [%];  [μs]; kr×105 [s-1])

em [nm] ( [%];  [μs]; kr×105 [s-1])
495
484
0.94;
−1.71;
−2.67
301(44.98), 333(25.25), 347(14.79),
361(6.49), 386(br, 3.87)
1
2
3
(0.81; 0.68; 11.9)
(0.82; 0.97; 8.5)
−5.16
483
470
0.97;
−1.70;
−2.67
300(53.59), 332(27.61), 346(20.28),
361(10.12), 386(br, 4.56)
(0.60; 0.67; 9.0)
(0.34; 0.95; 3.6)
−5.19
566
550
0.94;
−1.37;
−3.02
302(33.70), 354(3.83), 372(5.27),
390(5.17), 430(br, 1.45)
(0.93; 0.84; 11.1)
(0.81; 0.69; 11.7)
−5.15
[a] In degassed toluene (2×10-5 mol dm-3) at room temperature. “br” stands for broad. [b] Emission quantum yields in toluene and PMMA were measured under the
excitation wavelength at 386 nm (for 1 and 2) and 430 nm (for 3) with Hamamatsu C11347 Quantaurus-QY Absolute PL quantum yields measurement system.
Samples for thin films were made with 4 wt% of gold(III) complex doped in PMMA. [c] Potentials versus SCE; Epa refers to anodic peak potential for quasi-reversible
) potentials based on EHOMO/LUMO = [4.8 + (퐸ox/red versus ferrocene E1/2)] eV; the potentials
ox
onset
red
oxidation wave. [d] Estimated from oxidation (퐸
) and reduction (퐸
onset
onset
E1/2 of ferrocene lie in the range of 0.490.50 V.
Figure 2. a) Absorption and emission spectra of 13 in degassed toluene; inset: the photo of 2 in degassed toluene under 365 nm excitation. b) Emission spectra
of 1 in different deoxygenated solvents. c) The Lippert-Mataga plot of 1. d) Emission spectra of 13 in PMMA thin films; inset: the photo of 2 in PMMA thin film under
365 nm excitation. e) Emission spectra of 3 doped in PMMA thin films with different concentrations and in neat film at room temperature. f) Emission lifetimes of 1
plotted versus temperature.
The emissions of these complexes doped in poly(methyl
methacrylate) (PMMA) thin films with 4 wt% doping concentration
(Figure 2d) show emission profiles similar to those in toluene
solutions and maintain high  of up to 0.82 and short  of less than
1 s; only 2 showed an obvious decrease in emission quantum
yield from 0.60 in toluene solution to 0.34. It is particularly
noteworthy that the full width at half maximum (FWHM) of the
emission band for 1 and 2 is only 82 and 74 nm, respectively. The
emission properties of 3 in PMMA thin films with different
concentrations and in neat film have also been studied since this
complex shows appealing intermolecular interaction in its crystal
packing (Figure 3e, vide infra). As depicted in Figure 2e, the
emission maxima are progressively red-shifted from 550 nm (4
wt%) to 580 nm (100 wt%; neat film). Such red-shifted emission
maxima might be presumably due to the aggregation arising from
- stacking between adjacent (C^N^C)-AuIII planes. Similar
aggregation-induced red-shifted emission in PMMA thin films has
also been reported for a pyrazine-containing C^N^C gold(III)
complex.[1h] However, different from the reported gold(III) complex
featuring aggregation-induced enhancement in emission
quantum yields, complex 3 shows concentration-dependent
photoluminescence quenching from 81% (in 4wt% thin film) to 10%
(in neat film). For a comparison, the emission of 1 and 2 in thin
films with different doping concentrations was also examined
(Figure S7 and Table S3). With increased doping concentration in
PMMA matrix, both 1 and 2 showed red-shifted emission maxima
3
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by 1726 nm and obvious aggregation-induced emission
quenching, similar to the behavior of 3.
emission mechanism most consistent with the experimental
observations.
Since the photo-physical behaviors of 13 in solutions are
3]
similar to the previously reported Au(III) complexes,[2,
it is
Aggregation of pincer AuIII-TADF complexes
speculated that the emission of the complexes studied herein is
also derived from TADF. In order to further elucidate the emission
mechanism of these complexes, we have performed emission
measurements in aerated toluene solution at room temperature
(Table S3). Compared to the emission in oxygen-free toluene, the
emission quantum yields of 13 in aerated toluene are found to
decrease substantially (by 9697%), indicating that triplet excited
states are involved in the emission process. The study of variable-
temperature emission lifetimes was carried out for 1 in toluene over
the temperature range of 31377 K. As depicted in Figure 2f, the
emission lifetimes slowly increase from 313 to 150 K and below
150 K, there is a dramatic increase in radiative lifetime which
reaches 65.65 s at 77 K, together with a more than 2 orders of
magnitude decrease in kr (8.23×103 s-1 at 77 K as compared to
1.27×106 s-1 at 313 K). The significant change in kr from ~106 s1
at RT to ~103 s1 thus implies that the emission mechanism
involves thermal activation with the higher-lying excited state
having kr at least two orders of magnitude faster than the lower-
lying excited state. Typical vibronic-structured emission bands with
vibrational spacings of 10501380 cm-1 and the order of kr being
103 s-1, recorded at 77 K, together suggest that the emission at 77
K predominantly stems from the 3ILC^N^C excited state.[1b, 1f]
The study of the emission of complex 3 doped in thin films with
different
concentrations suggests the feasibility
that
intermolecular interactions lead to red-shift in emission energy
upon aggregation. To further investigate the aggregation
properties of the complex, emission measurement was conducted
in a mixed solvent system (acetone/water). Complex 3 is non-
emissive in pure aerated acetone at room temperature. Upon
addition of water into acetone with increasing water fraction (fw)
from 60% to 90%, the emission intensity of 3 showed conspicuous
enhancement (from 7% to 26%) with a concomitant emission
color change from yellow to orange-red (Figure 3a inset and 3b).
As shown in Table S5, the kr values of the complex in aerated
acetone-water mixtures are approximately 105 s-1, comparable to
that in deoxygenated toluene solution. A slight increase in  from
26% to 30% was observed upon degassing the solution (fw: 90%)
of 3. Obvious red-shifted vibronic absorption bands were
observed in solutions with fw of 6090% compared to the
absorption spectrum recorded in pure acetone solution (Figure
3c). The complex in a mixed acetone-water system (fw: 6090%)
was subjected to nanoparticle tracking analysis (NTA) revealing
the formation of nanoparticles as depicted in Figure S8.
Computational study
In the literature, there are two proposed thermally activated
emission mechanism that involves triplet excited state in the
emission process: (1) TADF, where the higher-lying states with
faster kr is the singlet excited state and the low-lying state is the
triplet
excited
state;
and
(2)
thermally
stimulated
phosphorescence,[6] where the higher-lying excited state is also a
triplet excited state but has a much faster kr and is thermally
accessible. To help differentiate the two mechanisms,
DFT/TDDFT computations were performed on 1 as an illustrative
example. From TDDFT optimizations of the triplet excited states,
there are two close-lying triplet excited states that are thermally
accessible, the 3IL(C^N^C) and the 3LLCT excited states with the
adiabatic triplet excited state energy being 2.81 and 2.78 eV,
respectively. For both triplet excited states, owing to their little
metal
character
involved
(<12%),
the
computed
phosphorescence radiative decay rate constants are of the order
103 s1 (Table S7), which are almost three orders of magnitude
smaller than the observed kr (Table 1); thus, it is unlikely that the
emission
mechanism
involves
thermally
stimulated
phosphorescence as there is no low-lying triplet excited state that
has kr > 106 s1. On the other hand, the 1LLCT, i.e., the S1 excited
state, is only ~500 cm1 above the 3LLCT excited state at the
optimized 3LLCT geometry and the radiative decay rate constant
for 1LLCT  S0 is of the order 107 s1; when TADF is invoked as
the emission mechanism (assuming Boltzmann statistics, see eq.
(S1) in the Supporting Information), the computed kr is 1.71  106
s1. Given that the computed vertical emission energies of the
1LLCT and 3LLCT excited states at their respective optimized
excited state geometry are ~491 and 482 nm, respectively, both
the computed emission energy and the radiative decay rate
constant are in good agreement with the experimental value of
495 nm in toluene solution at RT, thus showing that TADF is the
Figure 3. a) Plots of emission quantum yields versus fw of 3; inset: Photos of 3
in pure aerated acetone and acetone-water mixtures under 365 nm excitation.
b) PL spectra of 3 in acetone-water mixtures with different fw. c) Absorption
spectra of 3 in acetone with different water contents. d) SEM image of
nanostructures obtained from 3 in acetone/H2O (1:9, v/v). e) Crystal packing of
3 with intermolecular contacts of 3.23.5 Å.
4
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The SEM image of 3 in acetone-water (1:9, v/v) also showed the
formation of self-assembled particles in the nanoscale regime
(Figure 3d). Aggregation-induced red-shifted vibronic absorption
bands were also reported in other (C^N^C)-AuIII complexes.[7]
Since 3 is insoluble in water, molecules would aggregate upon
addition of water, presumably driven by intermolecular
interactions between the Au(C^N^C) planes as revealed in the
crystal packing (Figure 3e). The enhancement of emission
quantum yield upon increasing water content is possibly due to
the increased population of aggregated species (Figure S8).
Since the absorption energies of the complex in acetone-water
mixtures with fw of 6090% are similar (Figure 3c), the observation
of red-shifted emission maximum (from 577 to 609 nm; by ~910
cm-1) may be due to the stabilization of the charge-transfer excited
state in a more polar medium when the water content increases.
Considering that 1 and 2, which contain pincer type Au(C^N^C)
plane similar to that of 3 and also display red-shifted emission
induced by the increased concentration in thin films, the
aggregation properties of 1 and 2 in acetone-water mixtures (fw =
90%) were investigated. The observation of red-shifted
absorption and the NTA measurement (Figure S9) for 1 and 2
lend support to the formation of nanoparticles upon adding water
into their acetone solutions, akin to the case of 3.
emission peak maximum of 1 is at 490 nm when PVK:OXD-7 was
used as the host (Figure S14b), being red-shifted by 20 nm when
compared with that in PYD2 host, leading to a poor color purity
with CIE coordinates of (0.19, 0.39). Such red-shifted EL emission
could be the result of less confinement of excitons in PVK:OXD-7
host and/or the different polarity of different hosts.
In view of the high  achieved by 3, EL properties of 3 were
also investigated in a solution-processed OLED with a structure
of ITO/PEDOT:PSS/PVK:OXD-7:3 (60 nm)/TPBi (40 nm)/LiF (1.2
nm)/Al (100 nm). Compared to the configuration of blue-emitting
devices, the mixture of PVK and OXD-7 was utilized as the host
instead of high-triplet-energy PYD2 with an aim to match the
narrower bandgap of 3; OTPD and DPEPO layers were also
removed.[2] High EQEs of up to 24.32% was found in the green-
emitting 3-based device with EL emission maximum at 534 nm,
attributable to the optimized device structure for Au(III)-TADF
complexes[2] and the high  of 3 in thin film.
Compared to TADF emitters, traditional fluorescent molecules
are more attractive for OLEDs in terms of long-term stability and
color purity owing to their much shorter emission lifetimes (~ns)
and narrower emission spectra. High-efficiency fluorescent
OLEDs based on energy down-conversion from phosphorescent
or TADF sensitizers to fluorescent emitters have been
demonstrated in previous reports.[11] Nonetheless, most
fluorescent OLEDs based on energy down-conversion were
fabricated by vacuum deposition, and those fabricated by less
expensive solution-processed technique were seldom reported.
In this work, we used emitter 1 as the sensitizer and widely used
fluorescent molecule rubrene as the emitter to testify the
possibility of energy-down-conversion fluorescent OLEDs using a
solution-processing technique. The device had a similar structure
with the one applied in the 1-based device described above
except that rubrene was introduced in the EML as the fluorescent
dopant; ITO/PEDOT:PSS/OTPD (4 nm)/PYD2:1:rubrene (60
nm)/DPEPO (10 nm)/TPBi (40 nm)/LiF (1.2 nm)/Al (100 nm). In
the EML of this device, the doping concentrations of 1 and
rubrene were 10 and 1 wt%, respectively. For comparison, a
regular fluorescent OLED was also fabricated by removing 1 from
the EML. As depicted in Figure 4c, the intense host emission in
the regular device is suggestive of an inefficient energy transfer
from the host to rubrene, leading to poor performance of the
device, as shown in Figure 4d. With the addition of sensitizer 1,
the host emission vanished because of the efficient energy
transfer from the host to 1, and the device performance was
strongly boosted; maximum luminance and maximum EQEs were
respectively increased by 30 and 4.59 times in the energy down-
conversion device. Although the residual emission from 1 existed
in the down-conversion device, the emission from rubrene
dominates the EL spectrum. The enhanced luminance and EQEs
could be ascribed to the efficient energy transfer from both singlet
and triplet excited states of 1 to rubrene.[11b] The residual emission
from 1, ascribed to the short emission lifetime of 1, was
detrimental to the color purity of the rubrene-based devices.[11a,c]
Nonetheless, the idea of fabricating a white OLED by combining
the blue emission of 1 and orange emission of rubrene may
thereby be realized via reducing the concentration of rubrene.[12]
Electroluminescent properties
Solution-processed blue-emitting OLEDs were fabricated by
utilizing 1 and 2 as emissive dopants with the device configuration
of ITO/PEDOT:PSS/OTPD (4 nm)/PYD2: Au-emitter (60
nm)/DPEPO (10 nm)/TPBi (40 nm)/LiF (1.2 nm)/Al (100 nm).
Herein, OTPD [N,N′-bis(4-(6-((3-ethyloxetan-3-yl)methoxy))-
hexylphenyl)-N,N′-diphenyl-4,4′-diamine]
underwent
cross-
linking upon annealing after being spin-coated onto the
PEDOT:PSS layer from toluene solution.[8] The ultra-thin OTPD
layer facilitated the hole injection to the emissive layer (EML) and
prevented the quenching of excitons from the PEDOT:PSS layer;
PYD2 [di(9H-carbazol-9-yl)pyridine] served as the host material
for
[di(phenyl)phosphino]phenyl}ether oxide] and TPBi [2,2',2"-
(1,3,5-benzinetriyl)tris(1-phenyl-1-H-benzimidazole)] as
1
or
2
in the EML while DPEPO [bis{2-
hole/exciton blocking and electron transporting materials,
respectively. Figure 4a shows normalized EL spectra of 1 and 2
at their optimized dopant concentrations (see Figure S13 for the
details). The 1- and 2-based devices show electroluminescence
maxima at 473 and 465 nm, respectively, with FWHM of 6467
nm and CIE coordinates of (0.16, 0.25) for the former and (0.16,
0.23) for the latter. The color purity of these blue OLEDs is among
the best of reported Au(III) complexes and comparable to the best
blue gold(I)-TADF emitters, as compared in Table S8,[1k-l,5,9] and
even better than the most widely used phosphorescent Ir(III) and
Pt(II) complexes.[10] Although the color purity of the 1-based
device is not as high as that of the device made with 2, the
maximum EQE of 15.25% shown by the 1-based device is much
higher than that of the 2-based device (6.76%), because of the
higher  of emitter 1 in thin films. In the 1- and 2-based blue-
emitting OLEDs, the host mixture of PVK (polyvinylcarbazole) and
OXD-7 [(1,3-bis[(4-tert-butylphenyl)-1,3,4-oxadiazolyl]phenylene)] With a 10:0.25 wt% combination of 1:rubrene, white emission with
employed in our previously reported AuIII-OLEDs was replaced by
PYD2 to avoid the quenching of high-energy-emitting 1 and 2 by
PVK. In fact, the maximum EQE of 1 (15.25%) in PYD2 is much
higher than that in PVK:OXD-7 (7.53%, Figure S14a). The
CIE coordinates of (0.32, 0.36) and color rendering index (CRI) of
69.44 was realized with maximum EQE of 13.35% and maximum
power efficiency of 21.61 lm W-1.
5
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10.1002/chem.202003855
Chemistry - A European Journal
FULL PAPER
Table 2. Key performance of solution-processed OLEDs based on 1, 2 and 3.
Current efficiency [cd A-1]
Power efficiency [lm W-1
]
External quantum efficiency [%]
CIE (x, y)[a]
Complex
(Conc.)
Max
Luminance
Max
at 1000 cd m-2
Max
at 1000 cd m-2
Max
at 1000 cd m-2
[cd m-2]
1 (6 wt%)
2 (6 wt%)
3 (20 wt%)
8850
28.36
12.42
78.16
18.55
4.72
18.46
8.25
8.97
1.85
15.25
6.76
9.98
2.51
0.16, 0.25
0.16, 0.23
0.35, 0.56
2800
48000
58.35
93.31
48.95
24.32
18.40
[a] Commission internationale de l’éclairage (CIE) coordinates at 1000 cd m-2.
development of efficient blue OLEDs and the blue dopants in
WOLEDs.
Experimental Section
General
All chemicals, unless otherwise noted, are commercially available and
used without further purification. All solvents for the reactions were of
HPLC grade. 2,6-Bis(2,4-difluorophenyl)pyridine[13] and 2,6-bis(2,4-
difluorophenyl)-N,N-dimethylpyridin-4-amine were prepared by Suzuki
coupling reactions between 2,4-difluorophenylboronic acid and 2,6-
dibromopyridine/2,6-dibromo-N,N-dimethylpyridin-4-amine.
(Diphenylamino)phenyl)boronic acid and
(4-
(4-(bis(4-
fluorophenyl)amino)phenyl)boronic acid are commercially available.
Particle size distribution was measured using nanoparticle tracking
analysis (NTA) instrument PMX 120 ZetaView. The SEM images were
taken on scanning electron microscope LEO 1530 FEG operating at 5.00
kV.
X-ray crystallography
Figure 4. a) Normalized EL spectra and b) EQE-luminance characteristics of
solution-processed OLEDs based on 1, 2 and 3 at their optimized doping
concentration. c) Normalized EL spectra and d) EQE-luminance characteristics
of solution-processed energy down-conversion fluorescent OLEDs in which 1
and rubrene were both doped in the EML and used as the sensitizer and emitter,
respectively.
A yellow block-shaped crystal of 3 with dimension of 0.39×0.21×0.08 mm3
was mounted on a MiTeGen MicroMount (100-micron aperture) with Dow
Corning HVG. The X-ray diffraction data of this crystal were collected on a
Bruker D8 VENTURE Fixed-Chi X-ray diffractometer equipped with a Mo
Ka (0.71073 Å ) radiation, a PHOTON 100 CMOS detector, and an Oxford
Cryostream 700 low-temperature device, operating at T = 200(2) K,
controlled by using APEX3 software suite v2015.5-2 (Bruker-AXS, 2015).
The unit cell was initially determined by the fast Fourier transform method.
The frames were integrated with the Bruker SAINT v8.34A (Bruker-AXS,
2013) software package using a wide-frame algorithm. Data were
corrected for absorption effects using the Multi-Scan method (SADABS).
The ratio of minimum to maximum apparent transmission was 0.403. The
structures were solved by using SHELXT 2014/5 software (Sheldrick,
2015) and refined by using SHELXT 2014/7 software.[14] CCDC 2022575
(for 3) contains the supplementary crystallographic data for this paper.
These data are provided free of charge by The Cambridge
Crystallographic Data Centre at https://www.ccdc.cam.ac.uk.
Conclusion
Sky-blue luminescent arylgold(III)-TADF complexes (1 and 2) with
emission peak maxima at 470484 nm in thin films were
successfully developed through a strategic consideration of
substituent effects on the pincer C^N^C ligand scaffold. These
arylgold(III)-TADF complexes show high emission quantum yields
of up to 0.82 and lifetimes less than 1 s. Sky-blue gold(III) TADF
OLEDs have been achieved with high EQEs of up to 15.25%,
narrow-band emission with FWHM of 6467 nm (28252953 cm-
1), and good color purity with CIE coordinates of (0.16, 0.23). A
white OLED with CIE coordinates of (0.32, 0.36), CRI of 69.44
and maximum EQE of 13.35% was also successfully fabricated
using the down-conversion technique with the combination of the
emission from 1 and rubrene. Although the device performance
shown by these blue emissive gold(III) complexes still has not met
the standard for commercial applications, their high efficiency and
the true blue emission color purity altogether indicate the
appealing potential of gold(III)-TADF emitters in the future
Preparation and characterization of gold(III) complexes 1–3
The synthetic protocol of 13 is the same as that in the previous studies[2,15]
with the use of cyclometallated gold(III) hydroxides and amino-containing
phenylboronic acids. The precursor cyclometallated gold(III) hydroxides
[Au(C^N^C)(OH)][15] were synthesized using the corresponding
cyclometallated gold(III) chlorides[16] as precursor which were prepared
through transmetallation from organomercury chlorides according to the
literature. The detailed synthetic procedures and NMR spectra were given
in the Supporting Information.
6
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FULL PAPER
2,6-Bis(2,4-difluorophenyl)-N,N-dimethylpyridin-4-amine: 1H NMR
(500 MHz, CD2Cl2): δ 8.07-8.03 (m, 2H), 7.03-6.99 (m, 2H), 6.97 (s, 2H),
6.95-6.91 (m, 2H), 3.08 (s, 6H); 19F NMR (376 MHz, CDCl3): δ 110.4,
112.1; 13C NMR (125 MHz, CDCl3): δ 163.04 (dd, J = 11.3, 247.5 Hz),
160.7 (dd, J = 12.5, 251.3 Hz), 155.37, 152.59, 132.60, 125.01 (d, J = 15.0
Hz), 111.73 (d, J = 11.3 Hz), 106.20 (d, J = 8.8 Hz), 104.33 (t, J = 25.0 Hz),
39.59; HRMS (EI): m/z Calcd. for C19H14F4N2 (M+): 346.1093, found:
346.1058.
OLED fabrication
Materials: PEDOT:PSS [poly(3,4-ethylenedioxythiophene):poly(styrene
sulfonic acid)] (Clevios P AI 4083) was purchased from Heraeus; OTPD,
PVK, OXD-7, PYD2, DPEPO, TPBi, and LiF from Luminescence
Technology Corp; Aluminum pellets from Kurt J Lesker. All these materials
were used as received and their structural drawings are given in the
Supporting Information (Figure S17).
1: Yield: 57 mg (48%). 1H NMR (500 MHz, CD2Cl2): δ 7.45 (d, J = 8.5 Hz,
2H, Hd), 7.27 (t, J = 8.5 Hz, 4H, Hg), 7.14 (d, J = 8.0 Hz, 4H, Hf), 7.07-7.04
(m, 4H, Ha, e), 7.00 (t, J = 7.5 Hz, 2H, Hh), 6.92 (dd, J = 2.5, 7.5 Hz, 2H,
Hc), 6.66-6.61 (m, 2H, Hb), 3.16 (s, 6H, NMe2); 19F NMR (470 MHz,
CD2Cl2): δ –109.4, –111.6; 13C NMR (150 MHz, CD2Cl2): δ 170.45, 164.03
(dd, J = 10.5, 256.9 Hz), 161.70 (dd, J = 10.5, 259.5 Hz), 158.72 (d, J =
6.2 Hz), 158.04, 148.59, 145.04, 143.00, 134.62, 133.77, 129.51, 126.07,
124.03, 122.53, 117.81 (d, J = 20.0 Hz), 102.81 (d, J = 19.4 Hz), 102.50 (t,
J = 26.5 Hz), 40.10; MS (FAB): m/z 785.3 (M+); Anal. Calcd. (%) for
C37H26AuF4N3: C 56.57, H 3.34, N 5.35; found: C 57.04, H 3.28, N 5.29.
Substrate cleaning: Glass slides with pre-patterned ITO electrodes used
as substrates of OLEDs were cleaned in an ultrasonic bath of Decon 90
detergent and deionized water and rinsed with deionized water. Then they
were cleaned in sequential ultrasonic baths of deionized water, acetone,
and isopropanol, and subsequently dried in an oven for 1 h.
Fabrication and characterization of OLEDs: Aqueous solutions of
PEDOT:PSS were spin-coated onto the cleaned ITO-coated glass
substrates and baked at 120 °C for 20 min to remove the residual water
solvent in a clean room. For blue OLEDs and down-conversion devices,
the crosslinkable OTPD was spin-coated on top of the PEDOT:PSS layer
and heated at 200 °C for 30 min to carry out crosslinking inside a N2-filled
glove box. The crosslinked OTPD was then subjected to spin
chlorobenzene solvent for three times to remove the unreacted moieties.
Afterwards, blends of PYD2: 1 or 2 (for blue OLEDs) or PYD2: 1: rubrene
(for down-conversion devices) were spin-coated from chlorobenzene atop
the OTPD layer inside the same N2-filled glove box. The thickness for all
EMLs was approximately 60 nm. After annealed at 70 °C for 30 min, all
devices were subsequently transferred into a Kurt J. Lesker SPECTROS
vacuum deposition system without exposing to air. In the vacuum chamber,
organic materials of DPEPO and TPBi were thermally deposited in
sequence at a rate of 0.1 nm s-1. Finally, LiF and Al were thermally
deposited at rates of 0.03 and 0.2 nm s-1, respectively. For the 3-based
OLEDs, the blend of PVK:OXD-7:3 was directly spin-coated from
chlorobenzene atop the PEDOT:PSS layer inside the N2-filled glove box.
After annealed at 110 °C for 60 min, all devices were subsequently
transferred into a Kurt J. Lesker SPECTROS vacuum deposition system
without exposing to air. In the vacuum chamber, TPBi was thermally
deposited at a rate of 0.1 nm s-1. Finally, LiF and Al were thermally
deposited at rates of 0.03 and 0.2 nm s-1, respectively.
2: Yield: 51 mg (42%). 1H NMR (500 MHz, CD2Cl2): δ 7.43 (d, J = 8.5 Hz,
2H, Hd), 7.12-7.09 (m, 6H, Ha, g), 7.01-6.97 (m, 6H, He, f), 6.92 (dd, J = 2.5,
7.0 Hz, 2H, Hc), 6.68-6.63 (m, 2H, Hb), 3.20 (s, 6H, NMe2); 19F NMR (470
MHz, CD2Cl2): δ –107.5, –109.9, –121.6; 19F NMR (470 MHz, CD2Cl2): δ –
107.5, –109.9, –121.6; 13C NMR (150 MHz, CD2Cl2): δ 170.00, 163.60 (dd,
J = 9.6, 256.1 Hz), 161.31 (dd, J = 10.9, 259.5 Hz), 158.48 (d, J = 239.6
Hz), 158.36, 158.33, 144.82, 144.42 (d, J = 2.5 Hz), 142.10, 134.21,
133.38, 125.37 (d, J = 7.9 Hz), 124.43, 117.39 (d, J = 19.1 Hz), 115.82 (d,
J = 22.4 Hz), 102.43 (d, J = 10.0 Hz), 102.12 (t, J = 26.6 Hz), 39.74; ESI-
MS: m/z 821.1 (M+); Anal. Calcd. (%) for C37H24AuF6N3: C 54.09, H 2.94,
N 5.11; found: C 54.21, H 2.92, N 5.02.
3: Yield: 59 mg (43%). 1H NMR (500 MHz, CD2Cl2): δ 7.98-7.92 (m, 3H,
Ha, b), 7.44 (d, J = 8.5 Hz, 2H, He), 7.28 (t, J = 7.5 Hz, 4H, Hh), 7.15 (d, J =
8.5 Hz, 4H, Hg), 7.07 (d, J = 8.5 Hz, 2H, Hf), 7.01 (t, J = 7.0 Hz, 2H, Hi),
6.94 (dd, J = 2.5, 7.0 Hz, 2H, Hd), 6.72-6.67 (m, 2H, Hc). 19F NMR (376
MHz, CDCl3): δ –105.3, –108.7; 13C NMR (150 MHz, CD2Cl2): δ 170.80,
164.91 (dd, J = 10.5, 258.0 Hz), 161.92 (dd, J = 10.5, 259.5 Hz), 160.00
(d, J = 6.0 Hz), 148.50, 145.49, 143.32, 141.52, 134.00, 133.21, 129.55,
125.99, 124.20, 122.70, 121.19 (d, J = 18.0 Hz), 118.36 (d, J = 18.0 Hz),
103.05 (t, J = 25.5 Hz); MS (FAB): m/z 742.1 (M+); Anal. Calcd. (%) for
C35H21AuF4N2: C 56.62, H 2.85, N 3.77; found: C 56.21, H 2.90, N 3.73.
Luminance-current-voltage characteristics, CIE coordinates, EL spectra,
current efficiency, power efficiency and EQE were measured using a
Keithley 2400 source-meter and an absolute external quantum efficiency
measurement system (C9920-12, Hamamatsu Photonics). All devices
were characterized at room temperature without encapsulation.
Photophysical measurements
All solvents for photophysical studies were of HPLC grade, except toluene
purified by distillation before photophysical use. Absorption spectra were
recorded on a Hewlett-Packard 8453 diode array spectrophotometer at
room temperature. Steady-state emission spectra were obtained on a
Horiba Fluorolog-3 spectrophotometer. Emission quantum yields () in
solution and thin films were measured with Hamamatsu C11347
Quantaurus-QY Absolute PL quantum yields measurement system. For
absorption and emission measurement, the concentration of the solution
is 210-5 mol dm-3. Solutions for photophysical studies were degassed by
using a high vacuum line in a two-compartment cell with five freeze-pump-
thaw cycles. The thin-film samples were prepared by drop-cast from a
chlorobenzene solution containing 4 wt% of AuIII complex with PMMA as
hosts. The solvent was evaporated at 80 °C and translucent films were
finally obtained. Low-temperature (77 K) emission spectra of complexes in
glassy and solid states were recorded in quartz tubes (4 mm internal
diameter) placed in a liquid nitrogen Dewar flask with quartz windows. The
glassy-state emission of the complex was measured in a mixed solvent
system of dichloromethane/methanol/ethanol (1:1:4, v/v/v). The emission
lifetime () measurements were performed on a Quanta Ray GCR 150-10
pulsed Nd:YAG laser system. Errors for  values (±1 nm),  (±10%), and
 (±10%) are estimated.
Computational details
The hybrid density functional, M06,[17] was employed for all calculations
using the program package G09.[18] The 6-31G* basis set[19] is used for all
atoms except Au, which is described by the Stuttgart relativistic
pseudopotential and its accompanying basis set (ECP60MWB).[20] Solvent
effect was also included by means of the polarizable continuum model
(PCM)[21] and default parameters are used for the solvent, toluene
(refractive index  = 1.4969). No symmetry constraints were applied in
geometry optimizations. For the singlet ground state (S0), the restricted
density functional theory (RDFT) formalism was employed. For the singlet
and triplet excited states, TDDFT were employed. Frequency calculations
were performed on the optimized structures to ensure that they are
minimum energy structures by the absence of imaginary frequency (i.e.
NImag = 0). Stability calculations were also performed for all the optimized
structures to ensure that all the wavefunctions obtained are stable.
The excited state energies at the optimized excited state geometries were
computed using a state-specific approach.[22] Relative excited state energy
gaps were computed using TDDFT within the Tamm-Dancoff
approximation (TDA)[23] to avoid the triplet instability problems.[24] The
radiative decay rate constants, kr, were computed at the optimized excited
7
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10.1002/chem.202003855
Chemistry - A European Journal
FULL PAPER
state geometries. Detailed procedures for radiative decay rate constant
calculations were reported in previous works.[25] SOC has been included
in the computations of triplet radiative decay rate constants.
F. Chotard, S. T. E. Jones, N. C. Greenham, M. Bochmann, D.
Credgington, A. S. Romanov, Nat. Commun. 2020, 11, 1-8.
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Adv. Funct. Mater. 2017, 27, 1603007.
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Acknowledgements
This work is supported by the Major Program of Guangdong Basic
and Applied Research (2019B030302009), the Basic Research
Program of Shenzhen (JCYJ20170818141858021 and
JCYJ20180508162429786), Innovation and Technology Fund
(PRP/071/19FX), Hong Kong Quantum AI Lab Limited, Hong
Kong Research Grants Council (HKU 17330416), and CAS-
Croucher Funding Scheme for Joint Laboratories. We thank The
University of Hong Kong's University Development Fund for
funding the Bruker D8 VENTURE Photon100 CMOS X-Ray
Diffractometer. We also thank Dr. K.-H. Low (The University of
Hong Kong) for assistance in solving the X-ray crystal structure of
complex 3.
[12] G. Cheng, F. Li, Y. Duan, J. Feng, S. Liu, S. Qiu, D. Lin, Y. Ma and S. T.
Lee, Appl. Phys. Lett. 2003, 82, 4224-4226.
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J. R. Cheeseman, G. Scalmani, V. Barone, B. Mennucci, G. A. Petersson,
H. Nakatsuji, M. Caricato, X. Li, H. P. Hratchian, A. F. Izmaylov, J. Bloino,
G. Zheng, J. L. Sonnenberg, M. Hada, M. Ehara, K. Toyota, R. Fukuda,
J. Hasegawa, M. Ishida, T. Nakajima, Y. Honda, O. Kitao, H. Nakai, T.
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Keywords: Gold • TADF • Sky-Blue OLEDs • WOLEDs • Pincer
ligands
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10.1002/chem.202003855
Chemistry - A European Journal
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Sky-blue emissive arylgold(III)-TADF complexes with emission maxima at 470484 nm,  of up to 82% and  < 1 s were successfully
developed through a strategic consideration of substituent effects on the pincer C^N^C ligands and triphenylamine donor. Sky-blue
emitting OLEDs have been achieved with these gold(III)-TADF emitters, showing high EQEs of up to 15.25%, narrow-band emission
and good color purity with CIE coordinates of (0.16, 0.23).
9
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