Realization of Highly Efficient Red Phosphorescence from Bis-Tridentate Iridium(III) Phosphors

Article abstract of DOI:10.1021/acs.inorgchem.9b01383

Bis-tridentate Ir(III) metal complexes bring forth interesting photophysical properties, among which the orthogonal arranged, planar tridentate chelates could increase the emission efficiency due to the greater rigidity and, in the meantime, allow strong interligand stacking that could deteriorate the emission efficiency. We bypassed this hurdle by design of five bis-tridentate Ir(III) complexes (1-5), to which both of their monoanionic ancillary and dianionic chromophoric chelate were functionalized derivative of 2-pyrazolyl-6-phenylpyridine, i.e. pzpyphH₂ parent chelate. Hence, addition of phenyl substituent to the pyrazolyl fragment of pzpyphH₂ gave rise to the precursors of monoanionic chelate (A1H-A3H), on which the additional tert-butyl and/or methoxy groups were introduced at the selected positions for tuning their steric and electronic properties, while precursors of dianionic chelates was judiciously prepared with an isoquniolinyl central unit on pziqphH₂ in giving the red-shifted emission (cf. L1H₂ and L2H₂). Factors affected their photophysical properties were discussed by theoretical methods based on DFT and TD-DFT calculation, confirming that the T₁ excited state of all investigated Ir(III) complexes shows a mixed metal-to-ligand charge transfer (MLCT), intraligand charge transfer (ILCT), ligand-to-ligand charge transfer (LLCT), and ligand-centered (LC) transition character. In contrast, the poor quantum yield of 3 is due to the facilitation of the nonradiative decay in comparison to the radiative process. As for potential OLED applications, Ir(III) complex 2 gives superior performance with max. efficiencies of 28.17%, 41.25 cd·A^-1 and 37.03 lm·W^-1, CIE_x,y = 0.63, 0.37 at 50 mA cm^-2, and small efficiency roll-off.

Full text of DOI:10.1021/acs.inorgchem.9b01383

Article
pubs.acs.org/IC
Cite This: Inorg. Chem. XXXX, XXX, XXX−XXX
Realization of Highly Efficient Red Phosphorescence from Bis-
Tridentate Iridium(III) Phosphors
Premkumar Gnanasekaran,†,⊥ Yi Yuan,‡,§,⊥ Chun-Sing Lee,‡,*,§ Xiuwen Zhou,*,∥ Alex K.-Y. Jen,*,‡
and Yun Chi*,†,‡
†Department of Chemistry and Frontier Research Center on Fundamental and Applied Sciences of Matters, National Tsing Hua
University, Hsinchu 30013, Taiwan
‡Department of Materials Science and Engineering and Department of Chemistry, City University of Hong Kong, Kowloon, Hong
Kong SAR
§Center of Super-Diamond and Advanced Films (COSDAF), City University of Hong Kong, Kowloon, Hong Kong SAR
∥School of Mathematics and Physics, The University of Queensland, Brisbane, Queensland 4072, Australia
S
* Supporting Information
ABSTRACT: Bis-tridentate Ir(III) metal complexes bring
forth interesting photophysical properties, among which the
orthogonal arranged, planar tridentate chelates could increase
the emission efficiency due to the greater rigidity and, in the
meantime, allow strong interligand stacking that could
deteriorate the emission efficiency. We bypassed this hurdle
by design of five bis-tridentate Ir(III) complexes (1−5), to
which both of their monoanionic ancillary and dianionic
chromophoric chelate were functionalized derivative of 2-
pyrazolyl-6-phenylpyridine, i.e. pzpyphH2 parent chelate.
Hence, addition of phenyl substituent to the pyrazolyl
fragment of pzpyphH2 gave rise to the precursors of
monoanionic chelate (A1H−A3H), on which the additional
tert-butyl and/or methoxy groups were introduced at the selected positions for tuning their steric and electronic properties,
while precursors of dianionic chelates was judiciously prepared with an isoquniolinyl central unit on pziqphH2 in giving the red-
shifted emission (cf. L1H2 and L2H2). Factors affected their photophysical properties were discussed by theoretical methods
based on DFT and TD-DFT calculation, confirming that the T1 excited state of all investigated Ir(III) complexes shows a mixed
metal-to-ligand charge transfer (MLCT), intraligand charge transfer (ILCT), ligand-to-ligand charge transfer (LLCT), and
ligand-centered (LC) transition character. In contrast, the poor quantum yield of 3 is due to the facilitation of the nonradiative
decay in comparison to the radiative process. As for potential OLED applications, Ir(III) complex 2 gives superior performance
with max. efficiencies of 28.17%, 41.25 cd·A−1 and 37.03 lm·W−1, CIEx,y = 0.63, 0.37 at 50 mA cm−2, and small efficiency roll-
off.
green and red phosphors have all been used for commercial
OLED devices, while corresponding TADF emitters are
considered as the future replacements.
1. INTRODUCTION
Organic light emitting diodes (OLEDs) have been considered
as the emerging technology for flat panel displays and solid-
state luminaries of the 21st century. Since the first report on
electroluminescence,1 the OLED emitters have evolved from
fluorescent2 and phosphorescent3−8 ultimately to thermally
activated delayed fluorescent (TADF).9−13 Among these three
generations of designs, the fluorescent materials utilize only the
singlet excitons, which set the highest possible internal
quantum efficiency to only 25%. In contrast, both the
phosphorescent and TADF emitters take advantage of the
fast (both forward and reversed) intersystem crossing induced
by the inherent heavy atom effect as well as the small gap
between the singlet and triplet excited states, which, in theory,
give a unitary internal quantum efficiency for the fabricated
OLED devices. Nowadays, the blue fluorescent emitter and
Moreover, all these OLED emitters required good stability
to survive the harsh condition applied during electrical
excitation, which remained to be the central focus in this
arena. In this regard, for the phosphorescent emitters, changing
the traditional design, cf. Ir(III) complexes with three
bidentate chelates (tris-bidentate), to those with only two
tridentate chelates (bis-tridentate) would offer the respective
phosphors with improved stability due to the multidentate
coordination nature of chelates.14 To date, the tris-bidentate
Ir(III) phosphors are common; for example, homoleptic
Received: May 12, 2019
© XXXX American Chemical Society
A
DOI: 10.1021/acs.inorgchem.9b01383
Inorg. Chem. XXXX, XXX, XXX−XXX

Inorganic Chemistry
Article
[Ir(ppy)3] and heteroleptic [Ir(ppy)2(acac)] have been
considered as the excellent dopants for various monochromic
and white-emitting OLED applications (cf. Scheme 1).15,16 On
Scheme 2. Structural Drawings of Dianionic Chelates,
pzpyph2− and pzpypz2−, and Monoanionic Chelates, dpyx−,
phpyim−, and impyim−, Respectively
Scheme 1. Drawings of Ir(III) Complexes [Ir(ppy)3],
[Ir(ppy)2(acac)], and [Ir(dpyx)(dppy)]
the other hand, the best known example of bis-tridentate
Ir(III) complex is [Ir(dpyx)(dppy)], bearing chelates derived
from 1,3-di(2-pyridyl)-4,6-dimethylbenzene (dpyx)H and 2,6-
diphenylpyridine (dppy)H2, despite poor luminescent effi-
ciency.17,18 In fact, the design challenge in bis-tridentate
complexes lies on how to control the energy gap and inherent
properties of luminescent metal complexes, which are deemed
necessary for device fabrication using vacuum deposition. The
next endeavor is finding a high yield route to incorporate these
tailor-made tridentate chelates (both monoanionic and
dianionic) around the Ir(III) metal center in a systematic
fashion.19,20
Recently, we have described utilization of the precursors of
dianionic tridentate chelates with one or two pyrazolyl units
(cf. pzpyphH2 and pzpypzH2) in synthesizing the bis-tridentate
Ru(II), Os(II), and Ir(III) complexes.20,21 Our examinations
hinted that, independent to the central metal atoms, it is
essential to first incorporate a neutral or monoanionic chelate
to the employed metal reagents, followed by addition of
dianionic chelate in affording the bis-tridentate arrangement.
This strategic sequence is important as the first added chelate
with less negative charge will coordinate to the metal reagents
in giving intermediate of high synthetic yield. For instance,
2,2′:6,2″-terpyridine (tpy) and functional chelates reacted with
MCl3, M = Ru and Os, in giving [(tpy)MCl3], which can
convert to the bis-tridentate complexes upon addition of
functional derivatives of pzpyphH2 and pzpypzH2, affording
excellent sensitizers suited for making dye-sensitized solar cells
(DSSC).22−25 Moreover, the bis-tridentate Ir(III) complexes
[Ir(dpyx)(dppy)] and [Ir(fpbpy)(dppy)] were known to be
produced by first treatment of IrCl3·3H2O with (dpyx)H or 6-
(5-trifluoromethylpyrazol-3-yl)-2,2′-bipyridine (fpbpy)H in
giving isolable dimer complex [Ir(dpyx)(μ-Cl)Cl]2 or hypo-
thetical [Ir(fpbpy)(μ-Cl)Cl]2, followed by conversion to the
bis-tridentate complexes [Ir(dpyx)(dppy)] and [Ir(fpbpy)-
(dppy)] upon addition of (dppy)H2.17,26 In fact, this
sequential transformation remained useful for other mono-
anionic chelates with imidazolium carbene unit, i.e., phpyim−
and impyim− (Scheme 2), which are the key ancillaries in
giving efficient blue-emitting bis-tridentate Ir(III) phos-
phors.27−29
monoanionic due to removal of the pyrazolic NH fragment.
Hence, a total of three functional designs (cf. A1H−A3H)
were introduced, with an aim to probe both the steric and
electronic effects imposed by the introduced substituents.
Furthermore, the second tridentate chelate precursors are also
prepared using a central isoquinolinyl entity for reducing the
ligand-centered ππ* (or intraligand charge transfer, ILCT)
energy gap and giving efficient red emission, i.e., L1H2 and
L2H2. Preparation of the bis-tridentate Ir(III) emitters was
next conducted using the literature strategy. From the
viewpoint of molecular design, this work represents an
interesting example on the bis-tridentate Ir(III) phosphors
with chelate deriving from essentially identical origin. Hence,
these Ir(III)-based materials could enrich the prospective of
OLED emitters with high efficiency.
2. RESULTS AND DISCUSSION
2.1. Synthesis and Characterization. All needed
tridentate chelates for this investigation are summarized in
Scheme 3. They consist of both the monoanionic ancillaries
An− (n = 1−3) and dianionic chelates Ln2− (n = 1 and 2); the
latter possesses a relatively smaller ligand-centered ππ* or
ILCT energy gap in comparison to that of An− ancillaries,
attributed to the replacement of central pyridyl unit in An−
with the isoquinolinyl fragment in Ln2− with greater π-
conjugation. It is interesting to note that both the tridentate
An− and Ln2− chelates are derived from the original design of
pzpyphH2.
The precursor of N-phenyl-substituted ancillary chelate,
pzPhpyphH (A1H), was synthesized using a multistep protocol
depicted in Scheme 4. First, 2,6-dibromopyridine was treated
with n-BuLi, followed by addition of N,N-dimethyl acetamide
at −78 °C in affording 2-acetyl-6-bromopyridine, which was
next coupled with phenylboronic acid to yield 2-acetyl-6-
phenylpyridine. Subsequent Claisen condensation of 2-acetyl-
6-phenylpyridine with ethyl trifluoroacetate produced a
diketone intermediate that, in turn, underwent reaction with
phenyl hydrazine and, next, with acid-catalyzed hydrazone
cyclization to afford the desired precursor A1H. Analogously,
two more precursors of functional ancillaries, namely
pzPhpyBphBH (A2H) and pzPhpyphO2H (A3H), one with a
With this motivation, we proceed to investigate the bis-
tridentate Ir(III) complexes bearing monoanionic chelate such
as pzPhpyph− and functional analogues. We anticipated that
addition of a phenyl substituent to the peripheral pyrazolate of
pzpyphH2 in giving pzPhpyphH as shown in Scheme 3 should
convert the corresponding tridentate chelate from dianionic to
B
DOI: 10.1021/acs.inorgchem.9b01383
Inorg. Chem. XXXX, XXX, XXX−XXX

Inorganic Chemistry
Article
Scheme 3. Structural Drawings of the Studied Precursors of Chelates, pzpyphH (AnH), n = 1−3, and pziqphH2 (LnH2), n = 1
and 2
Scheme 4. Synthetic Protocols to pzPhpyphH (A1H)
Chelate
central isoquinolinyl unit. Hence, Sonogashira cross-coupling
a
was first employed to link 2-bromobenzaldehyde and phenyl-
acetylene in forming 2-(phenylethynyl)benzaldehyde (I-1),30
which was treated with t-butylamine in affording N-tert-butyl-
1-(2-(phenylethynyl)phenyl)methanimine (I-2).31 Cyclization
of I-2 in the presence of CuI afforded the 3-phenylisoquinoline
(I-3), to which the catalytic acylation with paraldehyde
afforded 1-(3-phenylisoquinolin-1-yl)ethan-1-one (I-4). Sub-
sequently, this acetyl isoquinoline derivative undergoes Claisen
condensation with ethyl trifluoroacetate in giving the diketone
intermediate, to which the hydrazine cyclization afforded the 3-
phenyl-1-(3-(trifluoromethyl)-1H-pyrazol-5-yl)isoquinoline
(L1H2). The 3,5-dimethoxy substituted pziqphO2H2 (L2H2)
was obtained in a similar fashion, which is expected to have an
even more reduced ππ* energy gap compared to that of the
parent precursor L1H2.
a
Experimental conditions: (i) n-BuLi, dimethylacetamide, diethyl
ether; (ii) phenyl boronic acid, Pd(dppf)Cl2, K2CO3, toluene:water:-
ethanol (7:1.5:1.5), reflux; (iii) NaOEt, ethyl trifluoroacetate, THF,
reflux; (iv) phenyl hydrazine, p-TsOH·H2O, ethanol, reflux; (v)
concentrated HCl, ethanol, reflux.
With both chelates in hands, the demanded bis-tridentate
Ir(III) complexes 1−5 were obtained by first treatment of one
equiv of AnH (i.e., A1H−A3H) with IrCl3·3H2O in refluxing
ethanol for 12 h. After that, the ethanol was removed under
vacuum to afford a residue. To this uncharacterized material
were added an equal molar ratio of the second chelating
precursors LnH2 (i.e., L1H2 and L2H2), an excess of sodium
acetate as catalyst, and decalin as solvent. The resulting
mixture was heated to reflux for 48 h, to which the stepwise
transformations are depicted below:
tert-butyl group located at both the central pyridyl and
peripheral phenyl units and the second with the 3,5-dimethoxy
substituted peripheral phenyl group for adjusting electronic
properties, were prepared using relevant starting materials,
following the analogous procedures depicted in Scheme 4.
Next, synthesis of 3-phenyl-1-(3-(trifluoromethyl)-1H-pyr-
azol-5-yl) isoquinoline, pziqphH2 (L1H2) is depicted in
Scheme 5, to which the procedures are slightly more
complicated than those described for the precursors of
monoanionic An− ancillaries, as needed for construction of
a
Scheme 5. Synthetic Protocols to pziqphH2 (L1H2) Chelate
a
Experimental conditions: (i) phenyl acetylene, Pd(PPh3)2Cl2, CuI, dry Et3N, 50 °C; (ii) tert-butylamine, room temperature; (iii) CuI, DMF, 100
°C; (iv) FeSO4·7H2O, paraldehyde, CF3CO2H, t-BuO2H, CH3CN, reflux; (v) NaOEt, ethyl trifluoroacetate, THF, reflux; (vi) hydrazine hydrate, p-
TsOH·H2O, ethanol, reflux.
C
DOI: 10.1021/acs.inorgchem.9b01383
Inorg. Chem. XXXX, XXX, XXX−XXX

Inorganic Chemistry
Article
Scheme 6. Structural Drawings of the Studied Bis-Tridentate Ir(III) Metal Complexes 1−5
IrCl3·3H2O + AnH → “[Ir(An)Cl2]” + HCl + 3H2O
(1)
“[Ir(An)Cl2]“ + LnH2 + NaOAc
→ [Ir(An)(Ln)] + 2NaCl + HOAc
(2)
Upon chromatographic separation and recrystallization, the
expected charge-neutral bis-tridentate Ir(III) complexes with
general formula [Ir(An)(Ln)] were isolated in moderate to
high yields. Scheme 6 presents the molecular drawings of these
Ir(III) complexes. It is also notable that two Ir(III) complexes
2 and 5, both with two tert-butyl substituted ancillary A2−,
tend to afford the best product yields of >80%, in comparison
to other ancillary (i.e., A1− and A3−) without the tert-butyl
substituent (39 ∼ 45%). It is expected that the tert-butyl
substituents of A2H can block the unwanted side reactions,
particularly on the central pyridinyl unit, and giving better
yields in comparison to those using precursors A1H and A3H.
1
After then, these Ir(III) complexes were characterized by H
and 19F NMR spectroscopic methods, mass spectrometry, and
elemental analyses.
Figure 1. Structural drawing of 1 with thermal ellipsoids shown at the
30% probability level. Selected bond distances: Ir−C(1) = 2.005(2),
Ir−C(22) = 2.020(2), Ir−N(1) = 2.000(2), Ir−N(2) = 2.154(2), Ir−
N(4) = 2.001(2), and Ir−N(5) = 2.092(2) Å. Bond angles: C(1)−Ir−
N(2) = 157.31(8), C(22)−Ir−N(5) = 159.26(8), and N(1)−Ir−
N(4) = 176.51(8)°.
Single crystal X-ray analyses of Ir(III) complex 1, i.e.
[Ir(A1)(L1)], were also conducted to reveal their true identity.
As shown in Figure 1, both the tridentate A1− and L12−
chelates are coordinated to the central Ir(III) atom in a
mutually orthogonal fashion, to which the overall molecular
arrangement resemble that of the homoleptic bis-tridentate
Ir(III) complexes reported earlier.32 The dianioinc chelate L1
has C(22)−Ir−N(5) bite angle of ∼159° and isoquinolinyl Ir−
N(4) distance of 2.001(2) Å, the latter is much shorter
compared to the adjacent pyrazolyl Ir−N(5) distance of
2.092(2) Å and Ir−C(22) distance of 2.020(2) Å. Similarly, an
C(1)−Ir−N(2) bite angle of ∼157° was also observed for
monoanionic A2− ancillary, except the existence of a
lengthened pyrazolyl Ir−N(2) distance of 2.154(2) Å due to
the attachment of N-phenyl substituent. Finally, this phenyl
appendage from A1− chelate is perfectly residing over the
isoquinolinyl ring of L12− chelate, showing notable through-
space π−π interaction. The calculated center-to-center
distance between the phenyl and isoquinolinyl group is
approximately 3.648 Å, coinciding to those of π−π stacking
distances reported in relevant supramolecularly caged Ir(III)
complexes.33−38
Figure 2. Absorption and emission spectra of Ir(III) complexes 1−5
recorded in CH2Cl2 at room temperature.
2.2. Photophysical Properties. The UV−visible absorp-
tion and emission spectra of the bis-tridentate Ir(III)
complexes 1−5 in CH2Cl2 solution were shown in Figure 2,
and photophysical data were depicted in Table 1. The intense
absorptions from 340 to 450 nm with relatively higher
extinction coefficient are contributed mainly by ππ* transitions
within 2-(1-pheny-pyrazolyl)-6-phenyl pyridine (An−) ancillary
and 1-pyrazolyl-3-phenyl isoquinoline chromophoric chelate
(Ln2−). The broadened absorption centered at around 475 nm
was attributed to the spin-allowed metal-to-ligand charge
D
DOI: 10.1021/acs.inorgchem.9b01383
Inorg. Chem. XXXX, XXX, XXX−XXX

Inorganic Chemistry
Article
Table 1. Absorption and Emission Properties of Ir(III) Complexes 1−5 Recorded in CH2Cl2 at Room Temperature
a
b
bc
,
abs λ [nm] (ε × 10−4 M−1cm−1
)
em λmax [nm]
Φ [%]
τobs [μs]
kr [×105 s−1
]
knr [×105 s−1
]
1
2
3
4
5
254 (4.68), 301 (3.7), 345 (1.7), 452 (0.31)
253 (4.55), 307 (3.48), 346 (1.76), 454 (0.308)
308 (3.81), 373 (1.38), 470 (0.25)
290 (3.72), 375 (1.36), 470 (0.26)
290 (5.09), 358 (2.09), 476 (0.33)
593, 645, 704
595, 649, 707
592, 643, 705
612, 661, 735
617, 666, 735
98
100
15
90
100
7.42
6.19
0.69
4.46
1.31
1.61
2.2
2.01
2.7
0.026
−
12
0.22
−
3.74
a
b
All absorption spectra were measured using a 10−5 M of CH2Cl2 solution at room temperature; Emission spectra were measured in degassed
CH2Cl2 solution at room temperature. 4-Dicyanomethylene-2-methyl-6-(4-dimethylaminostyryl)-4H-pyran in DMSO (Q.Y. = 80% and λmax = 637
c
nm), which was employed as a standard.
Table 2. Electrochemical and Energy Gap Data of Studied Ir(III) Complexes 1−5
e
Measurement
DFT
a
1/2
a
b
c
d
Eox
[V]
Ered
[V]
1/2
HOMO [eV]
optical gap [eV]
LUMO [eV]
HOMO [eV]
LUMO [eV]
H−L gap [eV]
1
2
3
4
5
0.53 [0.10]
0.46 [0.13]
0.30 [0.07]
0.27 [0.14]
0.25 [0.21]
−2.30 [0.10]
−2.38 [0.12]
−2.32 [0.09]
−2.37 [0.11]
−2.42 [0.12]
−5.33
−5.26
−5.10
−5.07
−5.05
2.17
2.17
2.21
2.14
2.13
−3.16
−3.09
−2.89
−2.93
−2.92
−5.47
−
−5.20
−5.18
−
−1.84
−
−1.76
−1.82
−
3.63
−
3.44
3.36
−
a
E
1/2 refers to (Epa + Epc)/2, where Epa and Epc are the anodic and cathodic potentials, respectively, referenced to the redox couple (Fc/Fc+ = −4.8
eV), while the corresponding values of ΔEp = Epa − Epc are given in square brackets. The oxidation wave and reduction wave measured in CH2Cl2
and THF solution, respectively. HOMO = −(Eox + 4.8) eV. Optical gap (eV) = 1240/phosphorescent onset (nm). LUMO = HOMO + optical
b
c
d
e
gap. PBE1PBE/6-31G**(LANL2DZ with ECP for Ir) level with PCM for modeling the CH2Cl2 solvent.
Table 3. Calculated Excitation Energy (λ), Oscillator Strength (f), Main Orbital Contribution and Charge Characters of the
Lowest Singlet and Triplet Excited States for the Optimized Ground-State Geometries of Ir(III) Complexes 1, 3, and 4, at
a
PBE1PBE/6-31G**(LANL2DZ with ECP for Ir) Level with PCM for Modeling the CH2Cl2 Solvent
state
S1
λ (nm)
f
orbital contribution (>20%)
assignment
1
3
4
450
0.0056
HOMO → LUMO+1 (70%)
HOMO → LUMO (23%)
HOMO → LUMO (52%)
HOMO−1 → LUMO (30%)
HOMO → LUMO+1 (68%)
HOMO → LUMO (28%)
HOMO → LUMO+1 (43%)
HOMO → LUMO (21%)
HOMO → LUMO (91%)
HOMO → LUMO (45%)
HOMO−1 → LUMO (23%)
MLCT (24%), LLCT, ILCT, LC
T1
S1
584
490
584
0
MLCT (16%), LLCT, ILCT, LC
MLCT (21%), LLCT, ILCT, LC
MLCT (16%), LLCT, ILCT, LC
0.0091
0
T1
S1
T1
488
592
0.0220
0
MLCT (21%), LLCT, ILCT, LC
MLCT (13%), LLCT, ILCT, LC
a
The percentage of MLCT character of each MO transition was calculated from the decrease in the electron distribution on Ir atom. Complete
orbital contribution and their assignments were collated in Table S4.
transfer (1MLCT) transition, while the shoulder that extended
into the longer wavelength region was due to the spin-
forbidden MLCT absorption. The metal-induced spin−orbit
coupling effect allows these spin-forbidden states mixed with
spin-allowed singlet states. Therefore, the excitations from
these states to the ground state (S0) are weakly allowed with
low oscillator strengths.
Upon excitation, the first three bis-tridentate Ir(III)
complexes 1−3, i.e., all those with L12− chelate, showed that
the structured emission profile with first peak maximum
occurred in a narrow regime between 592 and 598 nm,
together with two more lower-energy shoulders, one at ∼645
nm, while the second appeared at ∼705 nm. Both the
structured profile and the lowered radiative rate constant (kr)
of 1.31−2.2 × 105 s−1 suggest the dominance of the intraligand
charge transfer (ILCT) and ligand-to-ligand charge transfer
(LLCT) transition character, together with MLCT transition
contribution at the excited state manifolds. Similarly,
complexes 4 and 5 display an analogous structured pattern,
but with a more red-shifted 0−0 emission peak wavelength (cf.
at 612 and 617 nm) and vibronic sidebands, revealing the
strong influence of dimethoxyphenyl peripheral group at this
3,5-dimethoxyphenyl substituted L22− chelate.
3
2.3. Electrochemical Properties. The redox properties of
complexes 1−5 were studied using cyclic voltammetry, to
which their CV traces and numeric data are depicted in Figure
S2 and Table 2. As can be seen, all oxidation and reduction
peaks are reversible, among which the Ir(III) complex 1 shows
the most positive oxidation potential at 0.53 V. Moreover,
complex 2 shows a further cathodic shift to 0.46 V, due to the
addition of two t-butyl substituent at A2− ancillary.
Subsequently, Ir(III) complexes 3, 4, and 5 show less positive
oxidation potential at 0.30, 0.27, and 0.25 V respectively, due
to the introduction of 3,5-dimethoxyphenyl substituent
irrespective to their location, and this confirmed the metal-
centered oxidation process.
E
DOI: 10.1021/acs.inorgchem.9b01383
Inorg. Chem. XXXX, XXX, XXX−XXX

Inorganic Chemistry
Article
Figure 3. Key molecular orbitals calculated based on the optimized ground-state geometry (energy level and contribution from Ir atom for electron
distribution are given in the parentheses), at PBE1PBE/6-31G**(LANL2DZ with ECP for Ir) level with PCM for modeling the CH2Cl2 solvent
(for more orbitals see Figure S3 in the Supporting Information).
Alternatively, the reduction potentials also followed a similar
trend as revealed in their oxidation potentials, among which
the A2− ancillary of 2 and 5 induce more negative potentials in
comparison to the A1 ancillary of 1 and 4, whereas the L22−
chelate of 4 and 5 with 3,5-dimethoxy substituents is also
capable to give more negative potentials with respect to both 1
and 2 with L12− chelate. All these observations are consistent
with the higher electron donating capability exerted by the tert-
butyl and methoxy groups. After then, HOMO energy level of
all Ir(III) complexes was calculated using the equation HOMO
= −(Eox + 4.8) eV, while LUMO was calculated using the
HOMO data and optical gap obtained from the onset of
emission. All these data were listed in Table 2 for scrutiny.
2.4. Theoretical Investigation. The electronic structures
of Ir(III) complexes 1, 3, and 4 were investigated in order to
understand the nature of the absorption and emission bands
and the effect of the ligand modification on the optical
properties. Details of computational methods are described in
the Experimental Section.
The calculated energy levels of HOMO and LUMO and
their energy gap for complexes 1, 3, and 4 were tabulated in
Table 2. The calculated HOMO energies for all three
complexes showed the same trend as the observed values,
that is, the HOMO energies of 3 and 4 (−5.20 eV and −5.18
eV) are higher than that of 1 (−5.47 eV). This is due to the
addition of electron-donating methoxy groups compared to 1.
Furthermore, LUMO energies and the gap calculated by DFT
are less physically meaningful than HOMO and the calculated
gap should not be expected to be an approximation to the
excitation energy. Instead, the excitation energy should be
more properly described by, e.g., TD-DFT.
The calculated excitation energies in terms of wavelength
and dominant orbital transitions for the lowest singlet- and
triplet- excited states by TD-DFT are collated in Table 3, and
the important frontier molecular orbitals are shown in Figure
3. The calculated excitation energies to the lowest singlet
excited state (S1) for complexes 1, 3, and 4 are 450, 490, and
488 nm, respectively, and were closely related to the observed
values of the experimental absorption peaks shown in Figure 2
and Table 1 (452, 470, and 470 nm for 1, 3, and 4
respectively). This indicated the lowest Franck−Condon
transition calculated by TD-DFT predicted a right trend for
the lowest absorption peak of the investigated Ir(III)
complexes. Moreover, the calculated energy of the lowest
triplet excited state (T1) on the Franck−Condon state (S0
equilibrium geometry) for complexes 1, 3, and 4 are 584, 584,
and 592 nm, respectively, which are well related to the
experimental phosphorescence peaks (593 nm, 592 nm, 612
nm for 1, 3, and 4, respectively).
However, the calculated energies of the T1 excited state for
1, 3, and 4 based on the optimized triplet geometry are 793,
717, and 764 nm (see Table S3 in the Supporting
Information), respectively, showing a different trend from the
experimental phosphorescence peaks. This indicates the real
structure of the emitting species is an intermediate between the
calculated S0 and T1 equilibrium geometries for the
investigated Ir(III) complexes. This finding has been reported
extensively for other Ir(III) complexes and the main cause was
reported as the lowest triplet state existed in a shallow
anharmonic potential energy well.39,40
LUMO of all three complexes is mainly localized at the Ln2−
chelate, while HOMO of all three complexes is more
delocalized at the metal atom and both An− and Ln2− chelates.
F
DOI: 10.1021/acs.inorgchem.9b01383
Inorg. Chem. XXXX, XXX, XXX−XXX

Inorganic Chemistry
Article
Figure 4. (a) Schematic device configuration, energy level diagram, and chemical structures of the employed materials for the OLEDs based on
complexes 2 and 5 as emitters. (b) Current density−voltage−luminance (J−V−L) characteristics, (c) current efficiency and power efficiency versus
luminance characteristics, and (d) external quantum efficiency versus luminance correlations of the devices (insert: EL spectra at 50 mA cm−2).
The T1 → S0 electronic excitation of 1 and 4 is mainly
contributed by both HOMO−1 and HOMO to LUMO orbital
transitions, whereas, that of 3 is mainly contributed by HOMO
to both LUMO and LUMO+1 transition. The T1 excited states
of all three complexes show a mixed MLCT, ILCT, LLCT, and
LC transition characters.
phenyl)-[1,1′:3′,1″-terphenyl]-3,3″-diyl)dipyridine (TmPyPB,
60 nm)/LiF (1 nm)/Al (100 nm), in which the CBP host
codeposited with 5 wt % of Ir(III) complex was constituted the
emitting layer (EML). TAPC, TmPyPB and TCTA were
employed as the hole-transporting layer (HTL), electron-
transporting layer (ETL) and exciton-blocking layer, respec-
tively. The schematic device structure, the energy diagram
along with the corresponding molecular structures of these
functional materials used in the device are depicted in Figure
4a.
The OLEDs using these Ir(III) complexes as dopants emit
bright red emission with Commission Internationale de
L’Eclairage (CIE) coordinates (0.63, 0.37) and (0.66, 0.34)
for 2 and 5, respectively. Besides, the EL spectra of all studied
devices (Figure 4d) exhibit similar profile to their correspond-
ing PL spectra, without any emission derived from the CBP
host or any adjacent layer, indicating efficient energy transfer
from host to the Ir(III) dopant and well confinement of the
electron-generated excitons within the EML. Current density−
voltage−luminance (J−V−L) characteristics of these devices
are shown in Figure 4b. Complex 2-based device shows better
J−V characteristics with turn-on voltages (Von, defined as the
voltage at a brightness of 1 cd m−2) below 3.5 V and low
operation voltage (at a brightness of 100 cd m−2) around 3.9
V. In contrast, the device of 5 exhibits a lowered current
density at the same driving voltage, which can be explained by
the carrier trapping effect caused by the much shallower
HOMO levels relative to that of the CBP host. The efficiency-
luminance curves of the investigated devices are depicted in
parts c and d of Figure 4, and corresponding numeric data are
summarized in Table 4. In general, the device based on 2
Moreover, the theoretical and experimental results show that
adding electron donating 3,5-dimethoxphenyl fragment on the
L12− ligand of 1 (forming L22− of 4) red-shifted the emission
band, while adding 3,5-dimethoxphenyl fragment at the A1−
ligand of 1 (forming A3− of 3) affected negligibly on the
emission wavelength, i.e., failed to show the expected redshift.
Note that 4 has both HOMO and LUMO localized on 3,5-
dimethoxphenyl thus this group at L22− ligand decreases
notably the ππ* transition energy, while 3 has no LUMO
localization on 3,5-dimethoxphenyl of A3−, resulting a
negligibly change in the emission wavelength. Moreover,
emission quantum yield is also reduced to only 15% in 3 from
nearly unitary in complexes 1 and 2. From Table 1, it is clearly
that this drop of emission quantum yield in complex 3 is
associated with the increase of knr, which were reported for the
vibronic coupling process or the thermal population to
nonradiative metal-centered states of Ir(III) complexes.41−43
2.5. Electroluminescent Properties. The high PLQYs of
Ir(III) complexes 2 and 5 inspired us to further investigate
their EL properties. Thus, the red-emitting OLEDs with the
following structure were fabricated: indium tin oxide (ITO)/
4,4′-(cyclohexane-1,1-diyl)bis(N,N-di-p-tolylaniline) (TAPC,
40 nm)/tris(4-(9H-carbazol-9-yl)phenyl)amine (TCTA, 10
nm)/5 wt % of 2 or 5 doped in 4,4′-di(9H-carbazol-9-yl)-
1,1′-biphenyl (CBP, 20 nm)/3,3′-(5′-(3-(pyridin-3-yl)-
G
DOI: 10.1021/acs.inorgchem.9b01383
Inorg. Chem. XXXX, XXX, XXX−XXX

Inorganic Chemistry
Article
Table 4. EL Performance of the OLEDs Based on Complexes 2 and 5
a
b
b
b
c
c
dopant
Von [V]
EQE [%]
C
[cd A−1
]
PE [lm W−1
]
λmax [nm]
CIE(x,y)
2
5
<3.5
<4.0
28.17/21.16
24.29/18.88
41.25/32.14
23.7/19.51
37.03/22.07
16.04/9.32
598
613
0.63, 0.37
0.66, 0.34
a
b
c
Turn on voltages at 1 cd m−2; Order of measured efficiency values: maximum, then values at 1000 cd m−2; Data recorded at 50 mA cm−2.
(A3H) were prepared using the method established for A1H, while 3-
(3,5-dimethoxyphenyl)-1-(3-(trifluoromethyl)-1H-pyrazol-5-yl)-
isoquinoline (L2H2) was prepared using the method established for
L1H2.54 1H and 19F NMR spectra were measured with a Varian
Mercury-400 instrument. UV−vis absorption and emission spectra
were recorded using the Hitachi (U-4100) spectrophotometer and the
Edinburgh (FS920) fluorometer, respectively. The nanosecond time-
resolved studies were performed using an Edinburgh FL 900 time-
correlated single photon-counting (TCSPC) system. Cyclic voltam-
metry was measured using a Model 600E Electrochemical Analyzer,
CH Instruments. The elemental analysis was carried out on a Heraeus
CHN-O Rapid Elementary Analyzer. Mass spectra were recorded on a
JMS-T200GC AccuTOF GCx instrument operating in low resolution
electron impact (LREI) or fast atom bombardment (FAB) mode.
4.1.1. Synthesis of Complex 1. A mixture of IrCl3·3H2O (100 mg,
0.28 mmol) and pzPhpyphH (A1H, 102 mg, 0.28 mmol) in 25 mL of
ethanol was heated at 110 °C for 12 h. After removal of ethanol, the
pziqphH2 (L1H2, 96 mg, 0.28 mmol) and NaOAc (465 mg, 5.67
mmol) were dissolved in 20 mL of decalin and refluxed for 48 h. Next,
the solvent was removed under vacuum and residue was dissolved in
CH2Cl2. The solution was washed with deionized water, dried over
anhydrous MgSO4, and stripped to dryness. Further purification was
conducted by silica gel column chromatography eluting with a 1:2
mixture of ethyl acetate and hexane, affording a orange red
[Ir(A1)(L1)] (93 mg, 0.11 mmol); yield: 39%. Relevant bis-tridentate
Ir(III) complexes 2−5 were synthesized using similar experimental
procedures.
achieved excellent performance with a maximum external
quantum efficiency (EQEmax) of 28.17%, a maximum current
efficiency (CEmax) of 41.25 cd A−1, and a maximum power
efficiency (PEmax) of 37.03 lm W−1, respectively. These
efficiency are comparable to the state-of-the art values for
the Ir(III) complex-based red PhOLEDs.44−53 By contrast, the
device based on 5 exhibits slightly lowered peak efficiency with
an EQEmax of 24.29%, a CEmax of 23.7 cd A−1 and a PEmax of
16.04 lm W−1. However, it is worth noting that the efficiency
roll-off of this device is much lower than that of 2 based device
at high brightness, especially above 3000 cd m−2, presumably
due to the relatively higher radiative transition rate constant of
5.
3. CONCLUSION
In summary, a series of red-emitting bis-tridentate Ir(III)
phosphors were synthesized and characterized by spectro-
scopic, structural and electrochemical methods, and by DFT
and TD-DFT calculations. The employed structural mod-
ification involves addition of bulky substituents (e.g., a phenyl
group at chelate A1−, a pair of tert-butyl substituents at the
chelate A2− with an intention to reduce intermolecular
stacking interactions, and even addition of two electron
donating methoxy groups in either A3− or L22− for possible
red-shifting of emission wavelength. Photophysical and DFT
and TD-DFT analyses demonstrate that the T1 excited state of
all investigated Ir(III) complexes show a mixed MLCT, ILCT,
LLCT and LC transition character, as confirmed by the nearly
unitary quantum yields (except 3), structured emission pattern,
and relatively high radiative rate (kr) of (1.31−2.7) × 105 s−1.
The poor quantum yield of 3 is due to the facilitation of the
nonradiative decay in comparison to the radiative process.
Red-emitting OLED devices were fabricated using representa-
tive Ir(III) complexes 2 and 5 as the dopant emitters. Both of
them showed very high efficiency and significantly reduced
roll-off at high luminance, among which complex 2 gives the
best performances with maximum efficiencies of 28.17%, 41.25
cd·A−1, and 37.03 lm·W−1, and CIEx,y = 0.63, 0.37 at 50 mA
cm−2. These observations confirmed a notion that both the
bulky tert-butyl and perpendicular arranged phenyl groups
have effectively boosted the performances of OLED devices.
Moreover, even in absence of strong field carbene based
chelating unit, which is the common building block of blue-
emitting Ir(III) phosphors, the bis-tridentate Ir(III) architec-
tures composed of only pyridine- and pyrazolate-based donors
can still behave as efficient design of the red-emitting OLED
phosphors.
1
4.1.2. Selected Spectroscopic Data of 1. H NMR (400 MHz,
acetone-d6): δ 8.67 (d, J = 9.3 Hz, 1H), 8.31 (d, J = 7.8 Hz, 1H),
8.16−8.09 (m, 3H), 8.04 (t, J = 8.0 Hz, 1H), 7.98 (s, 1H), 7.84−7.74
(m, 2H), 7.70 (d, J = 7.7 Hz, 1H), 7.67 (d, J = 7.8 Hz, 1H), 7.43 (s,
1H), 6.87−6.76 (m, 3H), 6.72 (t, J = 7.5 Hz, 1H), 6.66 (t, J = 7.3 Hz,
1H), 6.52 (t, J = 7.5 Hz, 2H), 6.45 (d, J = 6.7 Hz, 2H), 6.01 (d, J = 7.5
Hz, 1H), 5.80 (d, J = 7.7 Hz, 1H). 19F NMR (376 MHz, acetone): δ
−59.85 (s, 3F), −60.28 (s, 3F). MS [FAB]: m/z 894.2, M+. Anal.
Calcd for C40H23F6IrN6: C, 53.75; H, 2.59; N, 9.40. Found: C, 53.93;
H, 2.57; N, 9.54.
4.1.3. Selected Crystal Data of 1. C40H23F6IrN6; M = 894.84;
monoclinic; space group = P21/n; a = 13.5264(3), b = 17.4386(4),
and c = 14.0191(3) Å; β = 90.0487(6)°; V = 3306.84(13) Å3; Z = 4;
ρcalcd = 1.844 Mg·m−3; F(000) = 1744; crystal size = 0.320 × 0.134 ×
0.108 mm3; λ(Mo Kα) = 0.71073 Å; T = 150(2) K; μ = 4.113 mm−1;
27203 reflections collected, 9617 independent reflections (Rint
=
0.0334), restrain/parameter = 0/478, GOF = 1.034, final R1[I >
2σ(l)] = 0.0224, and wR2 (all data) = 0.044; Largest difference peak
and hole = 0.712 and −0.593 e·Å−3.
Complex 2 was isolated as orange red [Ir(A2)(L1)], yield: 83%.
Selected spectroscopic data follow. 1H NMR (400 MHz, acetone-d6):
δ 8.70−8.65 (m, 1H), 8.47 (d, J = 1.7 Hz, 1H), 8.25 (d, J = 1.7 Hz,
1H), 8.14 (s, 1H), 8.13−8.10 (m, 1H), 8.08 (s, 1H), 7.83−7.76 (m,
3H), 7.72 (dd, J = 7.8, 1.2 Hz, 1H), 7.44 (s, 1H), 6.85 (td, J = 7.5, 1.2
Hz, 1H), 6.80 (m, 2H), 6.67 (td, J = 7.3, 1.3 Hz, 1H), 6.62 (dd, J =
8.1, 2.1 Hz, 1H), 6.53 (tt, J = 7.6, 1.1 Hz, 1H), 6.44 (d, J = 7.2 Hz,
2H), 6.06 (dd, J = 7.4, 1.2 Hz, 1H), 5.73 (d, J = 8.1 Hz, 1H). 1.65 (s,
9H), 1.15 (s, 9H). 19F NMR (376 MHz, acetone): δ −59.86 (s, 3F),
−60.20 (s, 3F). MS [FAB]: m/z 1006.3, M+. Anal. Calcd for
C48H39F6IrN6: C, 57.30; H, 3.91; N, 8.35. Found: C, 57.67; H, 3.84;
N, 8.63.
4. EXPERIMENTAL SECTION
4.1. General Information and Materials. All reactions were
performed under a nitrogen atmosphere and solvents were distilled
from appropriate drying agents prior to use. Commercially available
reagents were used without further purification unless otherwise
stated. Both 4-(tert-butyl)-2-(3-(tert-butyl)phenyl)-6-(1-phenyl-5-(tri-
fluoromethyl)-1H-pyrazol-3-yl)pyridine (A2H) and 2-(3,5-dimethox-
yphenyl)-6-(1-phenyl-5-(trifluoromethyl)-1H-pyrazol-3-yl)pyridine
Complex 3 was isolated as orange red [Ir(A3)(L1)], yield: 43%.
Selected spectroscopic data follow. 1H NMR (400 MHz, acetone-d6):
δ 8.62 (d, J = 7.7 Hz, 1H), 8.22 (d, J = 7.6 Hz, 1H), 8.15 (d, J = 8.1
Hz, 1H), 8.07 (d, J = 7.4 Hz, 1H), 8.02 (s, 1H), 7.96 (s, 2H), 7.75 (m,
H
DOI: 10.1021/acs.inorgchem.9b01383
Inorg. Chem. XXXX, XXX, XXX−XXX

Inorganic Chemistry
Article
2H), 7.67 (d, J = 7.6 Hz, 1H), 7.41 (s, 1H), 6.99 (s, 1H), 6.81 (d, J =
7.1 Hz, 3H), 6.66 (t, J = 7.2 Hz, 1H), 6.52 (t, J = 7.4 Hz, 2H), 6.43 (s,
1H), 5.93 (d, J = 7.3 Hz, 1H), 5.72 (s, 1H), 3.65 (s, 3H), 2.55 (s,
3H). 19F NMR (376 MHz, acetone): δ −59.87 (s, 3F), −60.09 (s,
3F). MS [FAB]: m/z 954.2, M+. Anal. Calcd for C42H27F6IrN6O2: C,
52.88; H, 2.85; N, 8.81. Found: C, 53.11; H, 2.69; N, 8.87.
results of (TD-)DFT calculations including the
Cartesian coordinates of the optimized ground-state
geometries of the three studied Ir(III) metal complexes
(PDF)
Accession Codes
Complex 4 was isolated as red [Ir(A1)(L2)], yield: 45%.
CCDC 1908679 contains the supplementary crystallographic
data for this paper. These data can be obtained free of charge
via www.ccdc.cam.ac.uk/data_request/cif, or by emailing
data_request@ccdc.cam.ac.uk, or by contacting The Cam-
bridge Crystallographic Data Centre, 12 Union Road,
Cambridge CB2 1EZ, UK; fax: +44 1223 336033.
1
Spectroscopic data follow. H NMR (400 MHz, acetone-d6): δ 8.66
(d, J = 8.4 Hz, 1H), 8.23 (d, J = 7.4 Hz, 1H), 8.11 (m, 3H), 8.05 (s,
1H), 8.01 (d, J = 7.8 Hz, 1H), 7.95 (s, 1H), 7.79−7.76 (m, 2H), 7.62
(d, J = 7.6 Hz, 1H), 7.43 (s, 1H), 7.04 (s, 1H), 6.79 (s, 2H), 6.70 (t, J
= 7.3 Hz, 1H), 6.56 (t, J = 7.5 Hz, 1H), 6.54−6.49 (m, 1H), 6.46 (d, J
= 6.8 Hz, 1H), 5.90 (s, 1H), 5.74 (d, J = 7.5 Hz, 1H), 3.77 (s, 3H),
3.04 (d, J = 8.5 Hz, 3H). 19F NMR (376 MHz, acetone): δ −59.70 (s,
3F), −60.22 (s, 3F). MS [FAB]: m/z 954.2, M+. Anal. Calcd for
C42H27F6IrN6O2: C, 52.88; H, 2.85; N, 8.81. Found: C, 52.77; H,
2.84; N, 8.81.
AUTHOR INFORMATION
Corresponding Authors
■
*(Y.C.) E-mail: yunchi@cityu.edu.hk.
*(C.-S.L.) E-mail: apcslee@cityu.edu.hk.
*(X.Z.) E-mail: x.zhou6@uq.edu.au.
*(A.K.-Y.J.) E-mail: alexjen@cityu.edu.hk.
Complex 5 was isolated as red [Ir(A2)(L2)], yield: 80%. Selected
spectroscopic data follow. 1H NMR (400 MHz, acetone-d6): δ 8.68−
8.63 (m, 1H), 8.38 (s, 1H), 8.17−8.05 (m, 3H), 8.02 (s, 1H), 7.82−
7.70 (m, 3H), 7.42 (s, 1H), 7.05 (s, 1H), 6.78 (t, J = 7.3 Hz, 2H),
6.61 (dd, J = 8.0, 1.6 Hz, 1H), 6.55 (t, J = 7.5 Hz, 1H), 6.43 (d, J =
7.1 Hz, 2H), 5.90 (s, 1H), 5.65 (d, J = 8.0 Hz, 1H), 3.77 (s, 3H), 3.04
(s, 3H), 1.65 (s, 9H), 1.15 (s, 9H). 19F NMR (376 MHz, acetone): δ
−59.71 (s, 3F), −60.11 (s, 3F). MS [FAB]: m/z 1066.3, M+. Anal.
Calcd for C50H43F6IrN6O2: C, 56.33; H, 4.07; N, 7.88. Found: C,
56.49; H, 3.82; N, 8.05.
ORCID
Chun-Sing Lee: 0000-0001-6557-453X
Xiuwen Zhou: 0000-0002-7449-9380
Alex K.-Y. Jen: 0000-0002-9219-7749
Yun Chi: 0000-0002-8441-3974
4.2. Procedures for Theoretical Calculation. The electronic
structure and electronic excitations of Ir(III) complexes 1, 3, and 4
were investigated by methods based on DFT55,56 and TD-DFT.57−59
The equilibrium S0 geometries of complexes 1, 3, and 4 were
optimized based on the X-ray structure of 1 (cf. Table S1 of
Supporting Information). Both the geometry optimization and TD-
DFT calculations were performed with the PBE1PBE60,61 functional
combined with the polarizable continuum model (PCM)62−65 to
include solvent effects of CH2Cl2, using Gaussian 09 codes.66 The 6-
31G(d,p)67 basis set was used for light elements such as hydrogen,
carbon, nitrogen, oxygen, and fluorine, while the LANL2DZ68 basis
set with the Los Alamos National Laboratory (LANL)69 effective core
potentials (ECPs) was used for iridium. Mulliken population analysis
was used to calculate the contribution from Ir atom for the electron
distribution of each molecular orbital, the decrease of which in a
molecular orbital transition was used to estimate the percentage of
MLCT character. The T1 geometry was optimized by unrestricted
DFT based on equilibrium S0 geometries. The results of
benchmarking calculations using different functionals for geometry
optimization and electronic excitations were listed in Tables S1 and
S2, respectively.
Author Contributions
⊥These authors had equal contributions.
Notes
The authors declare no competing financial interest.
ACKNOWLEDGMENTS
■
This work was supported by the funding from Ministry of
Science and Technology (MOST), featured areas research
program within the framework of the Higher Education Sprout
Project administrated by Ministry of Education (MOE) of
Taiwan, and City University of Hong Kong, Hong Kong SAR.
Single crystal X-ray structural analysis was conducted by Dr.
G.-H. Lee of Instrumentation Center, National Taiwan
University. X.Z. is supported by a University of Queensland
Development Research Fellowship and a computation grant
(rw25) from the National Computational Infrastructure of
Australia.
4.3. OLED Device Fabrication and Measurement. Devices
were fabricated on ITO-coated glass substrates with a sheet resistance
of 15 Ω sq−1. Before device fabrication, ITO substrates were swabbed
with Decon-90 solution and cleaned with deionized water, dried in an
oven at 120 °C, and treated with UV-ozone for 20 min before
transferring into deposition chamber. All the organic materials, Liq,
and Al were deposited at the rate of 1−2, 0.1, and 5 Å·s−1,
respectively, with a base pressure of 10−6 Torr. The current−voltage
characteristics were measured using a Keithley 2400 source meter.
Electroluminescence spectra were collected using a photonic
multichannel analyzer PMA-12 (Hamamatsu C10027−01), which
can detect in the spectral region 200−950 nm. All the measurements
were carried out under ambient atmosphere at room temperature.
REFERENCES
■
(1) Tang, C. W.; VanSlyke, S. A. Organic electroluminescent diodes.
Appl. Phys. Lett. 1987, 51, 913−915.
(2) Tagare, J.; Vaidyanathan, S. Recent development of phenan-
throimidazole-based fluorophores for blue organic light-emitting
diodes (OLEDs): an overview. J. Mater. Chem. C 2018, 6, 10138−
10173.
(3) Chou, P.-T.; Chi, Y. Phosphorescent dyes for organic light-
emitting diodes. Chem. - Eur. J. 2007, 13, 380−395.
(4) Xiang, H.; Cheng, J.; Ma, X.; Zhou, X.; Chruma, J. J. Near-
infrared phosphorescence: materials and applications. Chem. Soc. Rev.
2013, 42, 6128−6185.
(5) Sasabe, H.; Kido, J. Development of high performance OLEDs
for general lighting. J. Mater. Chem. C 2013, 1, 1699−1707.
(6) Chi, Y.; Tong, B.; Chou, P.-T. Metal Complexes with Pyridyl
Azolates: Design, Preparation and Applications. Coord. Chem. Rev.
2014, 281, 1−25.
(7) Yang, X.; Zhou, G.; Wong, W.-Y. Functionalization of
phosphorescent emitters and their host materials by main-group
elements for phosphorescent organic light-emitting devices. Chem.
Soc. Rev. 2015, 44, 8484−8575.
ASSOCIATED CONTENT
* Supporting Information
The Supporting Information is available free of charge on the
ACS Publications website at DOI: 10.1021/acs.inorg-
chem.9b01383.
■
S
Detailed synthetic procedures of chelates, original
electrochemical data, and benchmarking and detailed
I
DOI: 10.1021/acs.inorgchem.9b01383
Inorg. Chem. XXXX, XXX, XXX−XXX

Inorganic Chemistry
Article
(8) Li, T.-Y.; Wu, J.; Wu, Z.-G.; Zheng, Y.-X.; Zuo, J.-L.; Pan, Y.
Rational design of phosphorescent iridium(III) complexes for
emission color tunability and their applications in OLEDs. Coord.
Chem. Rev. 2018, 374, 55−92.
Phosphors with Bis-Tridentate Chelating Architecture for High
Efficiency OLEDs. J. Mater. Chem. C 2015, 3, 3460−3471.
(28) Kuei, C.-Y.; Liu, S.-H.; Chou, P.-T.; Lee, G.-H.; Chi, Y. Room
Temperature Blue Phosphorescence; A Combined Experimental and
Theoretical Study on the Bis-tridentate Ir(III) Metal Complexes.
Dalton Trans. 2016, 45, 15364−15373.
(29) Lin, J.; Chau, N.-Y.; Liao, J.-L.; Wong, W.-Y.; Lu, C.-Y.; Sie, Z.-
T.; Chang, C.-H.; Fox, M. A.; Low, P. J.; Lee, G.-H.; Chi, Y. Bis-
Tridentate Iridium(III) Phosphors Bearing Functional 2-Phenyl-6-
(imidazol-2-ylidene)pyridine and 2-(Pyrazol-3-yl)-6-phenylpyridine
Chelates for Efficient OLEDs. Organometallics 2016, 35, 1813−1824.
(30) Roesch, K. R.; Larock, R. C. Synthesis of Isoquinolines and
Pyridines by the Palladium/Copper-Catalyzed Coupling and Cycliza-
tion of Terminal Acetylenes and Unsaturated Imines: The Total
Synthesis of Decumbenine B. J. Org. Chem. 2002, 67, 86−94.
(31) Jeganathan, M.; Pitchumani, K. Synthesis of substituted
isoquinolines via iminoalkyne cyclization using Ag(i) exchanged
K10-montmorillonite clay as a reusable catalyst. RSC Adv. 2014, 4,
38491−38497.
(32) Lin, J.; Wang, Y.; Gnanasekaran, P.; Chiang, Y.-C.; Yang, C.-C.;
Chang, C.-H.; Liu, S.-H.; Lee, G.-H.; Chou, P.-T.; Chi, Y.; Liu, S.-W.
Unprecedented Homoleptic Bis-Tridentate Iridium(III) Phosphors:
Facile, Scaled-Up Production, and Superior Chemical Stability. Adv.
Funct. Mater. 2017, 27, 1702856.
(33) Costa, R. D.; Orti, E.; Bolink, H. J.; Graber, S.; Housecroft, C.
E.; Constable, E. C. Light-emitting electrochemical cells based on a
supramolecularly-caged phenanthroline-based iridium complex. Chem.
Commun. 2011, 47, 3207−3209.
(34) Li, P.; Shan, G.-G.; Cao, H.-T.; Zhu, D.-X.; Su, Z.-M.; Jitchati,
R.; Bryce, M. R. Intramolecular π Stacking in Cationic Iridium(III)
Complexes with Phenyl-Functionalized Cyclometalated Ligands:
Synthesis, Structure, Photophysical Properties, and Theoretical
Studies. Eur. J. Inorg. Chem. 2014, 2014, 2376−2382.
(35) Qu, X.; Liu, Y.; Si, Y.; Wu, X.; Wu, Z. A theoretical study on
supramolecularly-caged positively charged iridium(iii) 2-pyridyl
azolate derivatives as blue emitters for light-emitting electrochemical
cells. Dalton Trans. 2014, 43, 1246−1260.
(36) Housecroft, C. E.; Constable, E. C. Over the LEC rainbow:
Colour and stability tuning of cyclometallated iridium(III) complexes
in light-emitting electrochemical cells. Coord. Chem. Rev. 2017, 350,
155−177.
(37) Namanga, J.; Gerlitzki, N.; Smetana, V.; Mudring, A.-V.
Supramolecularly-caged green emitting ionic Ir(III)-based complex
with fluorinated C^N ligands and its application in light emitting
electrochemical cells. ACS Appl. Mater. Interfaces 2018, 10, 11026−
11036.
(9) Uoyama, H.; Goushi, K.; Shizu, K.; Nomura, H.; Adachi, C.
Highly efficient organic light-emitting diodes from delayed
fluorescence. Nature 2012, 492, 234−238.
(10) Adachi, C. Third-generation organic electroluminescence
materials. Jpn. J. Appl. Phys. 2014, 53, No. 060101.
(11) Wong, M. Y.; Zysman-Colman, E. Purely Organic Thermally
Activated Delayed Fluorescence Materials for Organic Light-Emitting
Diodes. Adv. Mater. 2017, 29, 1605444.
(12) Kim, J. H.; Yun, J. H.; Lee, J. Y. Recent Progress of Highly
Efficient Red and Near-Infrared Thermally Activated Delayed
Fluorescent Emitters. Adv. Opt. Mater. 2018, 6, 1800255.
(13) Cai, X.; Su, S.-J. Marching Toward Highly Efficient, Pure-Blue,
and Stable Thermally Activated Delayed Fluorescent Organic Light-
Emitting Diodes. Adv. Funct. Mater. 2018, 28, 1802558.
(14) Chi, Y.; Chang, T.-K.; Ganesan, P.; Rajakannu, P. Emissive Bis-
Tridentate Ir(III) Metal Complexes: Tactics. Coord. Chem. Rev. 2017,
346, 91−100.
(15) Kim, K.-H.; Moon, C.-K.; Lee, J.-H.; Kim, S.-Y.; Kim, J.-J.
Highly Efficient Organic Light-Emitting Diodes with Phosphorescent
Emitters Having High Quantum Yield and Horizontal Orientation of
Transition Dipole Moments. Adv. Mater. 2014, 26, 3844−3847.
(16) Lee, T.; Caron, B.; Stroet, M.; Huang, D. M.; Burn, P. L.; Mark,
A. E. The molecular origin of anisotropic emission in an organic light-
emitting diode. Nano Lett. 2017, 17, 6464−6468.
(17) Wilkinson, A. J.; Puschmann, H.; Howard, J. A. K.; Foster, C.
E.; Williams, J. A. G. Luminescent Complexes of Iridium(III)
Containing N∧C∧N-Coordinating Terdentate Ligands. Inorg. Chem.
2006, 45, 8685−8699.
(18) Williams, J. A. G.; Wilkinson, A. J.; Whittle, V. L. Light-emitting
iridium complexes with tridentate ligands. Dalton Trans. 2008, 2081−
2099.
́
́
(19) Esteruelas, M. A.; Gomez-Bautista, D.; Lopez, A. M.; Onate, E.;
̃
Tsai, J.-Y.; Xia, C. η1-Arene Complexes as Intermediates in the
Preparation of Molecular Phosphorescent Iridium(III) Complexes.
Chem. - Eur. J. 2017, 23, 15729−15737.
(20) Kuo, H.-H.; Zhu, Z.-L.; Lee, C.-S.; Chen, Y.-K.; Liu, S.-H.;
Chou, P.-T.; Jen, A. K.-Y.; Chi, Y. Bis-tridentate Iridium(III)
Phosphors with Very High Photostability and Fabrication of Blue-
Emitting OLEDs. Adv. Sci. 2018, 5, 1800846.
(21) Chi, Y.; Wu, K.-L.; Wei, T.-C. Ruthenium and Osmium
Complexes That Bear Functional Azolate Chelates for Dye-Sensitized
Solar Cells. Chem. - Asian J. 2015, 10, 1098−1115.
(22) Chou, C.-C.; Wu, K.-L.; Chi, Y.; Hu, W.-P.; Yu, S. J.; Lee, G.-
H.; Lin, C.-L.; Chou, P.-T. Ru(II) Sensitizers with Heteroleptic
Tridentate Chelates for Dye-Sensitized Solar Cells. Angew. Chem., Int.
Ed. 2011, 50, 2054−2058.
(23) Hsu, C.-W.; Ho, S.-T.; Wu, K.-L.; Chi, Y.; Liu, S.-H.; Chou, P.-
T. Ru(ii) sensitizers with a tridentate heterocyclic cyclometalate for
dye-sensitized solar cells. Energy Environ. Sci. 2012, 5, 7549−7554.
(24) Wu, K.-L.; Ho, S.-T.; Chou, C.-C.; Chang, Y.-C.; Pan, H.-A.;
Chi, Y.; Chou, P.-T. Engineering of Osmium(II)-Based Light
Absorbers for Dye-Sensitized Solar Cells. Angew. Chem., Int. Ed.
2012, 51, 5642−5646.
(38) Qu, Z.-Z.; Gao, T.-B.; Wen, J.; Rui, K.; Ma, H.; Cao, D.-K.
Cyclometalated Ir(iii) complexes [Ir(tpy)(bbibH2)Cl][PF6] and
[Ir(tpy)(bmbib)Cl][PF6]: intramolecular π···π interactions leading
to facile synthesis and enhanced luminescence. Dalton Trans. 2018,
47, 9779−9786.
(39) Younker, J. M.; Dobbs, K. D. Correlating Experimental
Photophysical Properties of Iridium(III) Complexes to Spin−Orbit
Coupled TDDFT Predictions. J. Phys. Chem. C 2013, 117, 25714−
25723.
(40) Jansson, E.; Norman, P.; Minaev, B.; Ågren, H. Evaluation of
low-scaling methods for calculation of phosphorescence parameters. J.
Chem. Phys. 2006, 124, 114106.
(41) Yersin, H.; Rausch, A. F.; Czerwieniec, R.; Hofbeck, T.; Fischer,
T. The triplet state of organo-transition metal compounds. Triplet
harvesting and singlet harvesting for efficient OLEDs. Coord. Chem.
Rev. 2011, 255, 2622−2652.
(25) Chou, C.-C.; Hu, F.-C.; Yeh, H.-H.; Wu, H.-P.; Chi, Y.;
Clifford, J. N.; Palomares, E.; Liu, S.-H.; Chou, P.-T.; Lee, G.-H.
Highly Efficient Dye-Sensitized Solar Cells Based on Panchromatic
Ruthenium Sensitizers with Quinolinylbipyridine Anchors. Angew.
Chem., Int. Ed. 2014, 53, 178−183.
(26) Chen, J.-L.; Wu, Y.-H.; He, L.-H.; Wen, H.-R.; Liao, J.; Hong,
R. Iridium(III) Bis-tridentate Complexes with 6-(5-Trifluoromethyl-
pyrazol-3-yl)-2,2’-bipyridine Chelating Ligands: Synthesis, Character-
ization, and Photophysical Properties. Organometallics 2010, 29,
2882−2891.
(42) Zhou, X.; Burn, P. L.; Powell, B. J. Bond Fission and Non-
Radiative Decay in Iridium(III) Complexes. Inorg. Chem. 2016, 55,
5266−5273.
(43) Zhou, X.; Powell, B. J. Nonradiative Decay and Stability of N-
Heterocyclic Carbene Iridium(III) Complexes. Inorg. Chem. 2018, 57,
8881−8889.
(27) Tong, B.; Ku, H. Y.; Chen, I. J.; Chi, Y.; Kao, H.-C.; Yeh, C.-C.;
Chang, C.-H.; Liu, S.-H.; Lee, G.-H.; Chou, P.-T. Heteroleptic Ir(III)
J
DOI: 10.1021/acs.inorgchem.9b01383
Inorg. Chem. XXXX, XXX, XXX−XXX

Inorganic Chemistry
Article
(44) Feng, Y.; Li, P.; Zhuang, X.; Ye, K.; Peng, T.; Liu, Y.; Wang, Y.
A novel bipolar phosphorescent host for highly efficient deep-red
OLEDs at a wide luminance range of 1000−10 000 cd m−2. Chem.
Commun. 2015, 51, 12544−12547.
(45) Cui, L.-S.; Liu, Y.; Liu, X.-Y.; Jiang, Z.-Q.; Liao, L.-S. Design
and Synthesis of Pyrimidine-Based Iridium(III) Complexes with
Horizontal Orientation for Orange and White Phosphorescent
OLEDs. ACS Appl. Mater. Interfaces 2015, 7, 11007−11014.
(46) Li, G.; Feng, Y.; Peng, T.; Ye, K.; Liu, Y.; Wang, Y. Highly
efficient, little efficiency roll-off orange-red electrophosphorescent
devices based on a bipolar iridium complex. J. Mater. Chem. C 2015, 3,
1452−1456.
(47) Shih, C.-H.; Rajamalli, P.; Wu, C.-A.; Hsieh, W.-T.; Cheng, C.-
H. A Universal Electron-Transporting/Exciton-Blocking Material for
Blue, Green, and Red Phosphorescent Organic Light-Emitting Diodes
(OLEDs). ACS Appl. Mater. Interfaces 2015, 7, 10466−10474.
(48) Kim, K.-H.; Liao, J.-L.; Lee, S. W.; Sim, B.; Moon, C.-K.; Lee,
G.-H.; Kim, H. J.; Chi, Y.; Kim, J.-J. Crystal Organic Light-Emitting
Diodes with Perfectly Oriented Non-Doped Pt-Based Emitting Layer.
Adv. Mater. 2016, 28, 2526−2532.
(49) Lee, J.-H.; Shin, H.; Kim, J.-M.; Kim, K.-H.; Kim, J.-J. Exciplex-
Forming Co-Host-Based Red Phosphorescent Organic Light-Emitting
Diodes with Long Operational Stability and High Efficiency. ACS
Appl. Mater. Interfaces 2017, 9, 3277−3281.
(50) Jiang, B.; Zhao, C.; Ning, X.; Zhong, C.; Ma, D.; Yang, C. Using
Simple Fused-Ring Thieno[2,3-d]pyrimidine to Construct Orange/
Red Ir(III) Complexes: High-Performance Red Organic Light-
Emitting Diodes with EQEs up to Nearly 28%. Adv. Opt. Mater.
2018, 6, 1800108.
(51) Wang, Y.-K.; Li, S.-H.; Wu, S.-F.; Huang, C.-C.; Kumar, S.;
Jiang, Z.-Q.; Fung, M.-K.; Liao, L.-S. Tilted Spiro-Type Thermally
Activated Delayed Fluorescence Host for ≈100% Exciton Harvesting
in Red Phosphorescent Electronics with Ultralow Doping Ratio. Adv.
Funct. Mater. 2018, 28, 1706228.
(52) Yang, X.; Guo, H.; Liu, B.; Zhao, J.; Zhou, G.; Wu, Z.; Wong,
W.-Y. Diarylboron-Based Asymmetric Red-Emitting Ir(III) Complex
for Solution-Processed Phosphorescent Organic Light-Emitting Diode
with External Quantum Efficiency above 28%. Adv. Sci. 2018, 5,
1701067.
potentials for the prevision of solvent effects. Chem. Phys. 1981, 55,
117−129.
(63) Mennucci, B.; Tomasi, J. Continuum solvation models: A new
approach to the problem of solute’s charge distribution and cavity
boundaries. J. Chem. Phys. 1997, 106, 5151−5158.
(64) Barone, V.; Cossi, M. Quantum Calculation of Molecular
Energies and Energy Gradients in Solution by a Conductor Solvent
Model. J. Phys. Chem. A 1998, 102, 1995−2001.
(65) Tomasi, J.; Mennucci, B.; Cammi, R. Quantum Mechanical
Continuum Solvation Models. Chem. Rev. 2005, 105, 2999−3094.
(66) Frisch, M. J.; Trucks, G. W.; Schlegel, H. B.; Scuseria, G. E.;
Robb, M. A.; Cheeseman, J. R.; Scalmani, G.; Barone, V.; Mennucci,
B.; Petersson, G. A.; Nakatsuji, H.; Caricato, M.; Li, X.; Hratchian, H.
P.; Izmaylov, A. F.; Bloino, J.; Zheng, G.; Sonnenberg, J. L.; Hada, M.;
Ehara, M.; Toyota, K.; Fukuda, R.; Hasegawa, J.; Ishida, M.;
Nakajima, T.; Honda, Y.; Kitao, O.; Nakai, H.; Vreven, T.;
Montgomery, J. A.; Peralta, J. E.; Ogliaro, F.; Bearpark, M.; Heyd,
J. J.; Brothers, E.; Kudin, K. N.; Staroverov, V. N.; Kobayashi, R.;
Normand, J.; Raghavachari, K.; Rendell, A.; Burant, J. C.; Iyengar, S.
S.; Tomasi, J.; Cossi, M.; Rega, N.; Millam, J. M.; Klene, M.; Knox, J.
E.; Cross, J. B.; Bakken, V.; Adamo, C.; Jaramillo, J.; Gomperts, R.;
Stratmann, R. E.; Yazyev, O.; Austin, A. J.; Cammi, R.; Pomelli, C.;
Ochterski, J. W.; Martin, R. L.; Morokuma, K.; Zakrzewski, V. G.;
Voth, G. A.; Salvador, P.; Dannenberg, J. J.; Dapprich, S.; Daniels, A.
̈
D.; Farkas, O.; Foresman, J. B.; Ortiz, J. V.; Cioslowski, J.; Fox, D. J.
Gaussian 16, Revision A03; Gaussian Inc.: Wallingford, CT, 2016.
(67) Hariharan, P. C.; Pople, J. A. The influence of polarization
functions on molecular orbital hydrogenation energies. Theor. Chim.
Acta 1973, 28, 213−222.
(68) Weigend, F.; Ahlrichs, R. Balanced basis sets of split valence,
triple zeta valence and quadruple zeta valence quality for H to Rn:
Design and assessment of accuracy. Phys. Chem. Chem. Phys. 2005, 7,
3297−3305.
(69) Hay, P. J.; Wadt, W. R. Ab initio effective core potentials for
molecular calculations. Potentials for K to Au including the outermost
core orbitals. J. Chem. Phys. 1985, 82, 299−310.
(53) Lu, G.-Z.; Su, n.; Yang, H.; Zhu, Q.; Zhang, W.-W.; Zheng, Y.;
Zhou, L.; Zuo, J.-L.; Chen, Z.-X.; Zhang, H. Rapid room temperature
synthesis of red iridium(III) complexes containing four-membered Ir-
S-C-S chelating ring for highly efficient OLEDs with EQE over 30%.
Chem. Sci. 2019, 10, 3535−3542.
(54) Sloop, J. C.; Bumgardner, C. L.; Loehle, W. D. Synthesis of
fluorinated heterocycles. J. Fluorine Chem. 2002, 118, 135−147.
(55) Hohenberg, P.; Kohn, W. Inhomogeneous Electron Gas. Phys.
Rev. 1964, 136, B864−B871.
(56) Kohn, W.; Sham, L. J. Self-Consistent Equations Including
Exchange and Correlation Effects. Phys. Rev. 1965, 140, A1133−
A1138.
(57) Runge, E.; Gross, E. K. U. Density-Functional Theory for
Time-Dependent Systems. Phys. Rev. Lett. 1984, 52, 997−1000.
(58) Casida, M. E.; Jamorski, C.; Casida, K. C.; Salahub, D. R.
Molecular excitation energies to high-lying bound states from time-
dependent density-functional response theory: Characterization and
correction of the time-dependent local density approximation
ionization threshold. J. Chem. Phys. 1998, 108, 4439.
(59) Casida, M. E. Time-dependent density-functional theory for
molecules and molecular solids. J. Mol. Struct.: THEOCHEM 2009,
914, 3−18.
(60) Adamo, C.; Barone, V. Toward reliable density functional
methods without adjustable parameters: The PBE0 model. J. Chem.
Phys. 1999, 110, 6158−6170.
(61) Ernzerhof, M.; Scuseria, G. E. Assessment of the Perdew-Burke-
Ernzerhof exchange-correlation functional. J. Chem. Phys. 1999, 110,
5029−5036.
̌
(62) Miertus, S.; Scrocco, E.; Tomasi, J. Electrostatic interaction of a
solute with a continuum. A direct utilizaion of AB initio molecular
K
DOI: 10.1021/acs.inorgchem.9b01383
Inorg. Chem. XXXX, XXX, XXX−XXX