Bis-tridentate IrIII Phosphors Bearing Two Fused Five-Six-Membered Metallacycles: A Strategy to Improved Photostability of Blue Emitters

Article abstract of DOI:10.1002/chem.201903707

Iridium complexes bearing chelating cyclometalates are popular choices as dopant emitters in the fabrication of organic light-emitting diodes (OLEDs). In this contribution, we report a series of blue-emitting, bis-tridentate Ir^III complexes bea

Full text of DOI:10.1002/chem.201903707

A Journal of
Accepted Article
Title: Bis-tridentate Ir(III) Phosphors Bearing Two Fused Five-Six
Membered Metallacycles; Strategy to Improved Photostability of
Blue Emitters
Authors: Yun Chi, Ling-Yang Hsu, Qiumin Liang, Zhiheng Wang, Hsin-
Hung Kuo, Wun-Shan Tai, Shi-Jian Su, Xiuwen Zhou, and Yi
Yuan
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10.1002/chem.201903707
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Bis‐tridentate Ir(III) Phosphors Bearing Two Fused Five‐Six Membered Metallacycles; Strategy
to Improved Photostability of Blue Emitters
Ling‐Yang Hsu,a Qiumin Liang,b Zhiheng Wang,b Hsin‐Hung Kuo,a Wun‐Shan Tai,a Shi‐Jian Su,b,*
Xiuwen Zhou,c,* Yi Yuan,d Yun Chi,a,d,*
a Department of Chemistry and Frontier Research Center on Fundamental and Applied Sciences
of Matters, National Tsing Hua University, Hsinchu 30013, Taiwan
b
State Key Laboratory of Luminescent Materials and Devices and Institute of Polymer
Optoelectronic Materials and Devices, South China University of Technology, Guangzhou 510640,
China, E‐mail: mssjsu@scut.edu.cn
c School of Mathematics and Physics, The University of Queensland, Brisbane, Queensland 4072,
Australia, E‐mail: uqxzho13@uq.edu.au
d Department of Materials Science and Engineering and Department of Chemistry, City University
of Hong Kong, Hong Kong SAR, E‐mail: yunchi@cityu.edu.hk
Abstract
Iridium metal complexes with chelating cyclometalates are popular choices as dopant
emitters in the fabrication of Organic Light‐emitting Diodes (OLEDs). In this contribution, we
reported a series of blue‐emitting, bis‐tridentate Ir(III) complexes bearing chelates with two fused
five‐six membered metallacycles, which are in sharp contrast to the traditional designs of
tridentate chelates bearing the alternative, fused five‐five metallacycles. Five Ir(III) metal
complexes Px‐21 − 23, and Cz‐4 and Cz‐5 were synthesized, in which the coordinated dicarbene
pincer chelate contains a methylene spacer and the dianionic chromophoric chelate is installed
with either a phenoxy or carbazolyl appendage for tuning coordination arrangement. All these
tridentate chelates afforded peripheral ligand‐metal‐ligand bite angles of 166 − 170, which are
− 1 −
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larger than the typical bite angle of 153 – 155 observed for the five‐five coordinated tridentate
counterparts, confirming a reduced geometrical distortion in octahedral frameworks.
Photophysical measurements and TD‐DFT studies verified the inherent transition characteristics
that gave high emission efficiency, while photodegradation experiments confirmed the improved
stability in comparison to the benchmark fac‐[Ir(ppy)3] in degassed toluene at RT. Phosphorescent
OLED devices were also fabricated, among which the carbazolyl functionalized emitter Cz‐5
exhibited the best performance among all studied bis‐tridentate phosphors, showing max.
external quantum efficiency (EQEmax) of 18.7% and CIEx,y coordinates of (0.145, 0.218), and a
slightly reduced EQE of 13.7% at 100 cd·m‐2 due to the efficiency roll‐off.
− 2 −
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Organic light‐emitting diodes (OLEDs) using either the transition metal based phosphors[1]
or thermally activated delayed fluorescent (TADF) emitters[2] are expected to exhibit 100%
internal quantum efficiency which stands for the highest limits that could be achieved. Despite of
the successful development of green and red‐emitting materials, the performances of blue
and/or true‐blue emitters is still lagging behind, and remains to be an active area of current
research endeavors.[3] However, given that the bis‐tridentate Ir(III) emitters possess the relatively
more robust metal‐chelate bonding interaction,[4] these phosphors may tolerate the excessive
injected electronic energy during device operation and provide a possible solution to the urgently
needed, stable blue phosphors.[5] On the other hand, N‐heterocyclic carbene (NHC) chelates have
been actively studied in both chemistry and materials science,[6] and their strong σ‐donating
character and π‐acceptor ability could simultaneously raise the lowest unoccupied molecular
orbital (LUMO) and reduce the highest occupied molecular orbital (HOMO).[7] Among all studied
examples, the multi‐dentate carbene chelates,[8] including the di‐carbene pincer,[4] are of
uttermost important due to their capability in forming the multiple coordination interaction to
the central transition metal atom. Hence, a wide range of Pt(II) and Ir(III) metal based carbene
complexes were actively pursued.[4, 9] In addition, the stronger metal‐carbene interactions are
known to destabilize the metal‐centered (MC) dd excited states, making these complexes a
promising choice in the development of metal based OLED phosphors, particularly the efficient
blue and true‐blue emitters for possible OLED applications.[10]
However, from the viewpoint of structural design, the carbene pincer chelates can be
classified to three distinctive forms, namely: classes A, B and C as depicted in Scheme 1. The class
A composes of two carbene appendages such as imidazolylidene directly linked to the central
phenyl (or pyridinyl) fragment with the pre‐organized meta‐connections.[11] Subsequent metal
complexation could produce a planar metal‐chelate bonding motif that possesses two fused five‐
membered metallacycles. Alternatively, both peripheral carbene fragments in B are separated
from the central phenyl (or pyridinyl) group with a methylene (‐CH2‐) linker and, hence, this class
of chelates forms two six‐membered metallacycles upon coordination to metal atom.[12] With
− 3 −
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possession of the sp3 hybridized methylene linkers, a distorted (non‐planar) backbone is
subsequently expected. Moreover, these methylene linkers might offer an enlarged CCC bite
angle for the pincer chelate, such that its angular strain can be somewhat relaxed to afford a
reduced coordination distortion,[13] which could be important to the successful design of blue and
true‐blue OLED emitters with higher emission efficiency. To our dissatisfaction, the intended Ir(III)
complexes bearing the class B di‐carbene pincer chelates cannot be realized in all attempts. Hence,
we diverted to a third design C with an anticipation that the fused five‐six membered
metallacycles could provide a fine balance between synthetic feasibility and reduced structural
distortion. Moreover, its structural design is conceptual akin to the cyclometalated aryl carbene
bearing an additional methylene‐linked phosphine, that allowed an easy access to multi‐nuclear
coordinated heterometallic complexes.[14]
Scheme 1. Di‐carbene pincer chelates with coordination geometries A, B and C.
Hence, we report preparation of carbene pincer chelates [(bmipmi)H3·(PF6)2] and
[(fmipmi)H3·(PF6)2], together with two distinctive class of chromophoric chelates, namely:
functional 6‐pyrazolyl‐2‐phenoxy pyridine [(pzyPx)H2] and 9‐(6‐(3‐(trifluoromethyl)‐1H‐pyrazol‐5‐
yl)pyridin‐2‐yl)‐9H‐carbazole [(pzyCz)H2], all are capable to form the another fused, five‐six
membered metallacycles. The structural drawings of these chelates are depicted in Scheme 2.
After then, we utilized these chelates in assembly of blue‐emitting Ir(III) phosphors with
minimized angular strain energy. The corresponding design is expected to offer a relatively higher
ligand field strength and emission efficiency in comparison to the tridentate chelates of class A
− 4 −
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with fused five‐five coordination architecture, the latter is expected to possess a smaller
peripheral CCC bite angle.[15] We anticipated that this geometry with two fused, five‐six
membered metallacyclic chelates may lead to efficient metal phosphors for successful fabrication
of OLED devices.
Scheme 2. Structural drawings of tridentate chelates capable to form fused five‐six membered
metallacycles.
For a direct comparison, it has been reported that the bis‐tridentate [Ru(tpy)2]2+ (tpy =
2,2’:6’,2”‐terpyridine) is virtually non‐emissive in solution at RT, whereas the tris‐bidentate
counterpart [Ru(bpy)3]2+ possesses a decent orange emission under identical condition.[16] This
change of emission quantum yield has been attributed to the small N‐Ru‐N bite angle of 158
incurred for the tridentate tpy chelate, to which the larger distortion away from that of ideal
octahedral arrangement (180) has caused a weakened ligand field and stabilized MC dd excited
state in [Ru(tpy)2]2+. Hence, new tridentate chelates has been designed to enlarge the
coordination angles as close as possible to 180 in the first‐row metal complexes for increasing
emission efficiency.[17] In theory, this manipulation may also offer a solution in strengthening the
crystal field strength of the third‐row metal elements, which is pivotal to the high performances
of blue‐emitting Ir(III) phosphors.
Results and Discussion
− 5 −
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Preparation and characterization. The asymmetrical carbene pincer derivatives, i.e. di‐
hexafluorophosphate salts of 1‐(3‐(trifluoromethyl)‐5‐((3‐methyl‐1H‐imidazol‐3‐ium‐1‐
yl)methyl)phenyl)‐3‐methyl‐1H‐imidazol‐3‐ium, [(fmipmi)H3·(PF6)2], and 1‐(3‐(t‐butyl)‐5‐((3‐
methyl‐1H‐imidazol‐3‐ium‐1‐yl)methyl)phenyl)‐3‐methyl‐1H‐imidazol‐3‐ium, [(bmipmi)H3·(PF6)2],
were synthesized from commercially available starting materials (3‐methyl‐5‐
(trifluoromethyl)phenyl)methanol and (3‐methyl‐5‐(t‐butyl)phenyl)methanol, respectively. The
experimental procedures were depicted in Scheme 3. It involved initial bromination of alcohol
with N‐bromosuccinimide in presence of triphenylphosphine to afford corresponding bromo
derivatives.[18] Two imidazoles were next added to the 1‐bromo‐3‐(bromomethyl)benzene
skeleton using literature approaches.[19] Finally, addition of methyl iodide and metathesis with
KPF6 afforded the pro‐chelates (fmipmi)H3·(PF6)2 and [(bmipmi)H3·(PF6)2]. Moreover, a total of
four dianionic chromophoric chelates, namely: functional 6‐pyrazolyl‐2‐phenoxy pyridine
[(pzyPx)H2] and 9‐(6‐(3‐(trifluoromethyl)‐1H‐pyrazol‐5‐yl)pyridin‐2‐yl)‐9H‐carbazole [(pzyCz)H2],
were also prepared, which are known in giving bis‐tridentate Ir(III) phosphors with true‐blue to
sky‐blue emission according to precedent in literature.[20]
Scheme 3. Synthetic route to pincer chelate (fmibmi)H3·(PF6)2: (i) NBS, PPh3, THF, RT; (ii) imidazole,
K2CO3, acetonitrile, reflux; (iii) imidazole, K2CO3, CuO, DMSO, 150 C; (iv) MeI, acetonitrile, 110 C;
(v) saturated KPF6(aq)
.
− 6 −
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The targeted bis‐tridentate Ir(III) complexes Px‐21 − 23 and Cz‐4 and Cz‐5 with dual fused
five‐six metallacycles were next synthesized using the one‐pot synthetic procedures. Accordingly,
heating of equal amount of carbene pincer and dianionic pro‐chelates and IrCl3·3H2O in refluxing
propionic acid with K2CO3 promoter afforded the aforementioned Ir(III) metal complexes in yields
comparable to those reported for their pristine Ir(III) counterparts Px‐14 and Cz‐2, i.e. those with
the alternative, fused five‐five membered metallacyclic carbene pincer chelate.[21] Structural
drawings of all these bis‐tridentate Ir(III) complexes are depicted in Scheme 4.
Scheme 4. Schematic structural drawings of the studied bis‐tridentate Ir(III) complexes.
Single crystal X‐ray analyses on Ir(III) complexes Px‐22 and Cz‐4 were next examined. Their
structural drawings are depicted in Figures 1 and 2, together with selected bond distances and
− 7 −
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angles. As can be seen, the sp3‐hybridized methylene fragment of the carbene pincer in Px‐22
and Cz‐4 has induced a slight rotation of adjacent carbene, giving a dihedral angle of (37.08 and
24.32) vs. the central phenyl fragment. Concurrently, C‐Ir‐C bite angle between both peripheral
carbene units are measured to be 165.68 and 168.44 in Px‐22 and Cz‐4, which are notably larger
than that observed in the traditional carbene pincer chelates (153 ‐ 155) without the methylene
bridge,[22] showing a reduced geometrical distortion for this class C chelates in comparison to that
of class A. Apparently, this increase of C‐Ir‐C bite angle of carbene pincer chelate is caused by the
larger bite angle of the six membered metallacycle (89.11(12) ‐ 89.26(19)) in comparison to that
of five membered metallacycle (78.66(12) ‐ 79.34(19)), the latter is analogous to that of
bidentate phenyl‐carbene cyclometalate, i.e. 78, observed in the alternative tris‐bidentate Ir(III)
complexes.[23] Finally, the carbene adjacent to the methylene linker exhibits an elongated Ir‐C
distance, i.e. Ir‐C(17) = 2.075(3) Å in Px‐22 and Ir‐C(11) = 2.068(5) Å in Cz‐4, in comparison to the
second carbene unit within the same molecule, c.f. Ir‐C(27) = 2.050(3) Å and Ir‐C(1) = 2.041(5) Å,
as well as Ir‐C distances observed in typical fused five‐five carbene pincer chelate in relevant Ir(III)
complexes (2.04 Å ‐ 2.05 Å),[22b] while the central phenyl group (2.015(4) Å and 2.020(5) Å)
remained to exhibit the shortest Ir‐C distance among all Ir‐C bonds detected in both Ir(III)
complexes Px‐22 and Cz‐4.
On the other hand, the dianionic chelates pzyPx and pzyCz also exhibited enlarged C‐Ir‐N
bite angles of 170.00(11) and 169.89(17) in Px‐22 and Cz‐4, respectively. Although these
structural data are similar to those revealed in the analogous Ir(III) complexes [Ir(mbib)(pzpyOphF)]
and Cz‐0,[20, 21b] their peripheral bite angles are notably wider than the basic 2‐pyrazolyl‐6‐
phenylpyridine class of tridentate chelates which are known to form the alternative, fused five‐
five metallacycles.[24] Hence, the current design with both chelates bearing fused five‐six
membered metallacycles afforded not only the blue‐shifted emission, but also a ligand
arrangement capable to exhibit strengthened crystal field, as well as higher emission efficiency
and increased durability.
− 8 −
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Photophysical Data. UV‐Vis absorption and emission spectra of all studied Ir(III) complexes
were measured in CH2Cl2 at RT, for which the spectra and numeric data are depicted in Figure 3
and Table 1, respectively. In general, they exhibited absorption pattern where all peaks of  350
nm with ε  104 M‐1cm‐1 were assigned to the ligand centered (ππ*) transitions, while the weaker
absorption band at the region > 350 nm and the onset extending into 430 nm were ascribed to
the mixed, singlet and triplet metal‐to‐ligand charge transfer (MLCT) and ligand centered ππ*
transitions. Moreover, carbazolyl complexes can be easily distinguished from phenoxy
counterparts by the appearance of an intense absorption at 366 nm, which is attributed to the
carbazolyl fragment. Its higher π‐conjugation seems to affect the lower energy MLCT band, to
which a relatively higher extinction coefficient was observed for complexes Cz‐4 and Cz‐5 in
comparison to the phenoxy derivatives Px‐21 to Px‐23.
Remarkably, all studied Ir(III) metal complexes exhibited photoluminescence quantum yields
(PLQY) greater than 87 % in degassed CH2Cl2 solution at RT and with shorter radiative lifetime (τrad
< 9.32 μs) that consistent of efficient phosphorescence. Firstly, the Px series of Ir(III) metal
complexes showed a relatively much blue shifted emission compared to that of the Cz series,
which reflected the intrinsic energy gap and degree of π‐conjugation of pzyPx and pzyCz chelates.
Moreover, there is also a gradual blue shift in emission, coupled with a decrease in PLQY and
increase in radiative lifetime in these phosphors. As expected, their photophysical properties
agreed well with their structural modifications, i.e. the replacement of electron rich t‐butyl with
electron deficient CF3 substituent at the central phenyl fragment of pincer carbene chelate (c.f.
Px‐21 → Px‐22) would reduce the electron density at the Ir(III) metal center, which resulted a
reduction in MLCT contribution at the excited states, giving both the blue‐shifted emission and
relatively lengthened radiative lifetime. Alternatively, introduction of t‐butyl substituent at the
pyridyl site of dianionic chromophoric chelate (c.f. Px‐22 → Px‐23; Cz‐4 → Cz‐5) destabilized the
empty π‐orbital of this tridentate chelate, giving blue‐shifted emission and lengthened radiative
lifetime. Finally, comparison of the photophysical data between the pair of samples Px‐14 / Px‐
21 and Cz‐2 / Cz‐5 suggested that the fused five‐six membered carbene would cause a notable
− 9 −
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red‐shifting in emission, together with improved quantum yield and shortened radiative lifetime
in comparison to those with the fused five‐five membered carbene pincer chelates;[21] all of these
photophysical characteristics will be further elaborated and discussed in the section of theoretical
calculation.
Electrochemistry. The redox properties were studied using cyclic voltammetry, to which
their CV traces and numeric data are depicted in Figure S1 and Table S1 of supporting information.
As can be seen, all oxidation peaks of Px derivatives are irreversible, while that of Cz series of
derivative are quasi‐reversible, among which Cz‐4 showed the least positive oxidation peak
potential at 0.61 V and were coincided with the longest emission wavelength detected among all
the studied samples. This result is attributed to the better electron donation property of
carbazolyl appendage of Cz series of complexes in reference to the Px series of complexes.
Moreover, variation of oxidation peak of Px‐21 and Px‐22 is also in good agreement with their
structural character, to which the electron withdrawing CF3 substituent on the carbene pincer of
Px‐22 lowered the electron density at Ir(III) metal center, giving more positively shifted peak
potential. Alternatively, despite of having identical carbene pincer chelates, the carbazolyl
substituted Cz‐4 and Cz‐5 showed a less positive oxidation potential in comparison that that of
phenoxy derivatives Px‐22 and Px‐23, confirming the strong influence of dianionic chelate to the
virtually metal‐centered oxidation process. As for the electrochemical reductions, all Ir(III)
complexes except for Cz‐4 showed reduced intensity and very similar peak potentials, implicating
ligand centered reduction processes. This seemingly abnormal behavior of Cz‐4 agrees with the
greatest red shifting in emission among all studied Ir(III) complexes, and implicated the greatest
stabilization of the LUMO due to the lack of electron donating t‐butyl substituent on the adjacent
pyridyl unit of Cz‐5.
Photostability. As indicated in Table 1, all studied Ir(III) complexes showed sufficiently high
Td as indicated by thermal gravimetric analysis (TGA) data, confirming their basic thermal
− 10 −
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robustness. Next, photolysis in degassed toluene was executed using xenon lamp irradiation at
620 W·m−2 and 35 °C. As depicted in Figure 4, only Px‐21 exhibited a slightly inferior stability in
comparison to the green‐emitting benchmark fac‐[Ir(ppy)3], while all other complexes have
displayed relatively better stability. It is notable that the poor stability of Px‐21 may be originated
from the t‐butyl substituent on the carbene pincer chelate, as the analogous Px‐14 showed an
even worse stability than Px‐21 under the condition examined. Table S2 presents the
decomposition rate constants calculated based on a first order uni‐molecular process. As can be
seen, Px‐22, Px‐23, Cz‐4 and Cz‐5 were calculated to be approx. 1.24, 1.3, 2.17 and 2.36 times
more stable than that of fac‐[Ir(ppy)3]; implicating possession of better photostability against the
green‐emitting fac‐[Ir(ppy)3], which should behave as the most photostable Ir(III) metal complex
judged according to its coordination design. Moreover, both Px‐21 and Cz‐5 were 1.2 and 2.5
times more stable than their counterparts Px‐14 and Cz‐2 bearing fused five‐five carbene
ancillaries,[21] confirming the superiority in photostability of the currently employed carbene
pincer ancillaries. Finally, the Cz series of phosphors were also confirmed to be more stable and
robust than that of Px series of derivatives, giving new perspectives for designing molecules.
Theoretical investigation. The electronic properties of five Ir(III) complexes (Px‐14, Px‐21 −
23, and Cz‐4) were investigated in order to understand the nature of the absorption and emission
bands and effect of the ligand modification on the photophysical properties. Details of
computational methods are described in the experimental section.
The TD‐DFT calculation results for the lowest singlet‐ and triplet‐ excited states are illustrated
in Table 2, and selected frontier molecular orbitals are shown in Figure S4. The calculated energy
to the lowest singlet excited state (S1) for complexes Px‐14, 21‐23, and Cz‐4 are 356 nm, 365 nm,
355 nm, 352 nm, and 371 nm, respectively, showing the same trend as the experimental lowest‐
energy absorption peaks (392 nm, 401 nm, 392 nm, 381 nm, and 414 nm for Px‐14, 21 − 23, and
Cz‐4 respectively, c.f. Figure 3 and Table 1), and with the deviation as approx. 0.3 eV. Hence, these
lowest Frank‐Condon transitions as calculated by TD‐DFT predicted a right trend for the lowest
− 11 −
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absorption peak of the investigated Ir(III) complexes. What is more, the lowest triplet excited state
(T1) for complexes Px‐14, 21 − 23, and Cz‐4 are calculated to be 417 nm, 418.3 nm, 417.9 nm,
416.8 nm, and 424 nm, respectively, which shows the same trend as that of the experimental
phosphorescence peaks (476 nm, 484 nm, 473 nm, 470 nm, and 494 nm for Px‐14, 21 − 23, and
Cz‐4 respectively, c.f. Figure 3 and Table 1), with the deviation as about 0.4 eV.
To characterize the S1 and T1 excited states, we listed their main contributing orbitals in Table
2, while their detailed orbital illustrations are illustrated in Figure S4. The S1 state is predominantly
contributed by HOMO to LUMO transition (≥ 94% for all complexes). The HOMO of all investigated
complexes is delocalized at the metal atom and both ligands, whereas the LUMO is mainly
localized at the dianionic tridentate chelate bearing either phenoxy or carbazolyl fragment.
Therefore, the S1 excited state of all complexes is a mixture of ligand‐centered ππ*, MLCT, ligand‐
to‐ligand charge transfer (LLCT), and intra‐ligand charge transfer (ILCT) characters. However, the
T1 state is contributed by a list of orbital transitions such as HOMO‐3 to LUMO, HOMO‐2 to LUMO,
and HOMO‐2 to LUMO+1, without a single dominant component. To obtain a much clear and
compact orbital representation for the T1 excited state, we applied natural transition orbitals
(NTOs) analysis[25] to express the T1 state as a single pair of orbitals that are shown in Figure 5.
The NTOs of the T1 excited state is localized only at the metal atom and dianionic chelate, which
shows this state is predominant by ligand‐centered ππ* character mixed with small components
of MLCT and ILCT. Hence, this NTO result unambiguously confirmed the dominance of dianionic
chelate in controlling of the emission energy gap; in contrast, the carbene pincer is acting only as
the spectator group and is less influential to the detected emission wavelength.
To understand the effect of ligand modifications on the emission energy, we analyzed how
ligand modifications affect the energy level of the main molecular orbitals involved in the T1 state.
As shown in Figure S4, the introduction of methylene spacer in carbene pincer chelate (c.f. Px‐14
→ Px‐21) destabilize both the HOMO‐2 and LUMO but more on the former thus leading to a red‐
shifted emission (note HOMO‐2 to LUMO transition has the greatest contribution to the T1 excited
state of Px‐21, Px‐22 and Px‐23). The use of electron‐deficient CF3 substituent replacing the
− 12 −
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electron‐rich t‐butyl group at the central phenyl fragment of carbene pincer chelate (c.f. Px‐21 →
Px‐22) stabilizes the HOMO‐2 notably by 0.1 ‐ 0.3 eV but affects little on the LUMO and, thus,
resulting a blue‐shifted emission. Moreover, the introduction of electron‐rich t‐butyl at the
pyridyl site of dianionic chromophoric chelate (c.f. Px‐22 → Px‐23) results in a destabilized LUMO
(by 0.05 eV) but negligible change on HOMO‐2 and thus also a blue‐shifted emission. The change
from phenoxy to carbazolyl substituent on the dianionic chelate, where LUMO is mainly localized
(c.f. Px‐23 → Cz‐4), leads to a significantly stabilized LUMO (by 0.2 eV) but little change on HOMO
‐2 and HOMO‐3, resulting a significant red shift in emission.
In addition, the ligand modification of Px complexes (c.f. Px‐21 → Px‐23) results in a
decreased contribution of Ir atom on the electron distribution of HOMO‐2 orbital (shown in Figure
S4), and thus a decreased MLCT character of the T1 state as it is mainly contributed by HOMO‐2
to LUMO transition, further leading to a lengthened radiative lifetime.
Electroluminescence. To investigate their electroluminescent (EL) features, Px‐21 − 23, Cz‐4
and Cz‐5 were used as dopant to fabricate phosphorescent OLED devices with a configuration of
ITO (95 nm)/TAPC (30 nm)/mCP (20 nm)/10 wt% dopant:DPEPO (25 nm)/DPEPO (10 nm)/TmPyPB
(40 nm)/LiF (1 nm)/Al (120 nm), as shown in Figure 6. 4,4'‐(Cyclohexane‐1,1‐diyl)bis(N,N‐di‐p‐
tolylaniline) (TAPC) and 3,3'‐(5'‐(3‐(pyridin‐3‐yl)phenyl)‐[1,1':3',1''‐terphenyl]‐3,3''‐diyl)dipyridine
(TmPyPB) are served as hole and electron transport layers, respectively. 1,3‐Di(9H‐carbazol‐9‐
yl)benzene (mCP) and bis[2‐(diphenylphosphino)phenyl]ether oxide (DPEPO) make contributions
to restrict excitons and carriers and thus improve the device efficiency. To ensure complete energy
transfer from the host to the guest via Förster and Dexter energy transfer pathways, a doping
concentration of 10 wt% was adopted. The device characteristics are given in Table 3 and Figure
7, respectively.
Blue EL emissions with peak max. ranging from 472 nm to 482 nm were achieved with CIE
coordinates of around (0.15, 0.23). The device based on Px‐21 exhibits the best performance with
max. EQE of 18.0%, followed by the devices based on Px‐22 and Px‐23 with max. EQE of 15.0%
− 13 −
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and 11.4%, respectively, to which their EL profiles are consistent with the PL spectra of the
respective Px series of phosphors. The device efficiency increases as the radiative lifetime of
exciton decreases. The device efficiency based on Px‐21 − 23 remain 11.3%, 9.4% and 6.0% at 100
cd·m‐2, respectively. On the other hand, the device based on Cz‐5 exhibits much superior
performance among all the devices with max. EQE of 18.7% and CIE coordinates of (0.145, 0.218).
The EQEs of the devices based on Cz‐4 and Cz‐5 remain 12.9% and 13.7% at 100 cd·m‐2. Severe
efficiency roll‐off at high current density may be attributed to triplet‐triplet annihilation (TTA)
which is commonly seen in phosphorescent devices.[26] The longer lifetime of excitons may lead
to an increased TTA, resulting in lower efficiency and unsatisfactory roll‐off.[27]
Introduction of t‐butyl substituent at the pyridyl site of dianionic chromophoric chelate (c.f.
Px‐22 → Px‐23; Cz‐4 → Cz‐5) exerts greater influence on Px series of phosphors due to the more
flexible phenoxy for the reason that the exciton lifetime increases from 5.66 to 8.45 μs in doped
DPEPO, while there is little difference in exciton lifetime of Cz‐4 and Cz‐5 (3.11 to 3.19 μs).
Compared with the device based on Px‐23, the device based on Cz‐5 exhibits relatively higher
efficiency, which could be ascribed to the shorter radiative lifetime and more rigid and planar
coordinative structure for carbazolyl fragments. Furthermore, they also contributed to the
enhanced electron density at the iridium metal center which may facilitate the metal to ligand
electron transfer in the excited state. The more rigid and planar ligand may also result the
relatively increased metal‐chelate bond strength, giving a second reason for the improved device
performance and suppressed efficiency roll‐off in Cz‐5‐based OLED devices. Considering the
effective exciton restriction but the low charge mobility of the high triplet‐energy electron‐
transporting‐dominant host DPEPO, the unbalanced carrier may lead to a narrow exciton
recombination zone close to the interface between mCP and DPEPO, which may result in severe
efficiency roll‐off when increasing the current density.[28] One strategy in alleviating this problem
is the employment of gradient dopant architecture, which has been proofed useful in the
29]
fabrication of many true‐blue emitting OLEDs,[6b,
phosphors.[21b]
even for the relevant bis‐tridentate
− 14 −
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Conclusion
In conclusion, Ir(III) metal complexes with both tridentate chelates bearing the fused five‐six
membered metallacycle were synthesized and employed for fabrication of blue‐emitting
phosphorescent OLEDs. The corresponding tridentate chelates offer an enlarged peripheral
ligand‐metal‐ligand bite angle (c.f. 166 − 170) in comparison to the tridentate chelates with the
alternative five‐five coordination geometry (c.f. 153 – 155). The expended bite angles are
expected to afford a near ideal octahedral geometry and, hence, giving improved ligand field
strength. The associated higher stability is also confirmed by the independent photolysis
experiments, to which the initial evaluation confirmed the better stability in reference to the
benchmark fac‐[Ir(ppy)3] and those with fused five‐five carbene pincer chelate. Also, application
of corresponding Px and Cz series of emitters as dopants, the max. EQE of the fabricated OLED is
qualitatively related to the radiative lifetime recorded either in solution or doped DPEPO, to which
the best set of data was obtained for Cz‐5, showing max. EQE of 18.7% and with CIEx,y of (0.145,
0.218). Hence, the carbazolyl fragment of Cz‐based Ir(III) complexes is capable to afford a reduced
exciton lifetime in comparison to that of Px series of emitters with alternative cyclometalating
phenoxy unit. Finally, although the obtained performances are still lagging behind the commercial
standards, these Ir(III) phosphors bearing relevant bis‐tridentate architecture, particular to those
with double fused five‐six membered metallacycles, are still capable to serve as the promising
design for achieving better efficiency and durability at the same time.
Experimental section
General Information and Materials. All reactions were carried out under a nitrogen
atmosphere and the solvents were dried prior to use. Commercially available chemicals were
used without purification unless otherwise stated. 1H and 19F NMR spectra were measured on a
Varian Mercury‐400 instrument. Elemental analysis was carried out on a Heraeus CHN‐O Rapid
Elementary Analyzer. Field desorption (FD) mass spectra were recorded on a JEOL JMS‐T200GC
− 15 −
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AccuTOF GCx instrument. Cyclic voltammetry was conducted on a CHI621A Electrochemical
Analyzer. Ag/Ag+ (0.01 M AgNO3) electrode was employed as reference electrode. The oxidation
and reduction potentials were measured using a platinum wire as working electrode with 0.1 M
of NBu4PF6 in CH2Cl2 and a gold wire with 0.1 M of NBu4PF6 in THF, respectively. The potentials
were referenced externally to the ferrocenium/ferrocene (Fc+/Fc) couple. The photolysis
experiments were performed using Atlas Suntest CPS+ Xenon Test instrument.
Preparation of bis‐tridentate Ir(III) complexes. A mixture of [(bmipmi)H3·(PF6)2] (221 mg,
0.37 mmol), [(pzyPxpF3)H2] (138 mg, 0.37 mmol), IrCl3·3H2O (130 mg, 0.37 mmol) and K2CO3 (1.02
g, 7.38 mmol) in propionic acid (30 mL) was refluxed under nitrogen for 12 hours. After removal
of solvent in vacuo, the residue was extracted into ethyl acetate (100 mL), and the organic layer
was washed with water (20 mL x 3), dried over anhydrous Na2SO4 and evaporated to dryness.
Afterwards, the obtained solid was purified by column chromatography eluting with a blend of
hexane: ethyl acetate (3:1 to 2:1) to give a yellow powder of Px‐21 (89.5 mg, 28%). Other Ir(III)
metal complexes Px‐22 and Px‐23 and Cz‐4 and Cz‐5 were obtained in 25  33% of yields using
similar procedures.
Spectral data of Px‐21: 1H NMR (400 MHz, acetone‐d6, 298K): δ 8.17 (t, J = 7.7 Hz, 1 H),
7.82 (d, J = 7.7 Hz, 1 H), 7.75 (m, 1 H), 7.41 (d, J = 1.7 Hz, 1 H), 7.26 (dd, J = 8.2 ,1.1 Hz, 1 H), 7.19
(d, J = 1.9 Hz, 1 H), 7.11 (d, J = 1.7 Hz, 1 H), 7.01 (s, 1 H), 6.96 (d, J = 8.2 Hz, 1 H), 6.93  6.89 (m, 1
H), 6.87 (d, J = 2.2 Hz, 1 H), 6.69 (d, J = 1.9 Hz, 1 H), 6.36 (d, J = 2.2 Hz, 1 H), 6.08 (d, J = 14.0 Hz, 1
H, methylene), 5.23 (d, J = 14.0 Hz, 1 H, methylene), 2.97 (s, 3 H), 2.75 (s, 3 H), 1.40 (s, 9 H). 19F
NMR (376 MHz, acetone‐d6, 298K): δ ‒60.19 (s, 3 F), ‒62.10 (s, 3 F). FD MS: m/z 871.2 (M+). Anal.
calcd. for C35H30F6IrN7O: C, 48.27, H, 3.47, N, 11.26. Found: C, 48.56, H, 3.33, N, 11.29.
Spectral data of Px‐22: 1H NMR (400 MHz, acetone‐d6, 298K): δ 8.25 (t, J = 8.0 Hz, 1 H), 7.91
(d, J = 2.3 Hz, 1 H), 7.87 (d, J = 8.0 Hz, 1 H), 7.66 (s, 1 H), 7.37 (s, 1 H), 7.32 (d, J = 8.0 Hz, 1 H), 7.26
(d, J = 1.8 Hz, 1 H), 7.03 (d, J = 7.2 Hz, 2 H), 7.00  6.90 (m, 2 H), 6.75 (d, J = 1.8 Hz, 1 H), 6.29 (d, J
− 16 −
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= 2.3 Hz, 1 H), 6.14 (d, J = 14.4 Hz, 1 H, methylene), 5.41 (d, J = 14.4 Hz, 1 H, methylene), 3.03 (s,
3 H), 2.77 (s, 3 H). 19F NMR (376 MHz, acetone‐d6, 298K): δ ‒60.36 (s, 3 F), ‒61.50 (s, 3 F), ‒62.21
(s, 3 F). FD MS: m/z 883.0 (M+). Anal. calcd. for C32H21F9IrN7O: C, 43.54, H, 2.40, N, 11.11. Found:
C, 43.93, H, 2.37, N, 11.15.
Selected crystal data of Px‐22: C40H37F9IrN7O5; M = 1058.96; T = 150(2) K; (Mo‐K) = 0.71073
Å; monoclinic; space group = P21/n; a = 15.4885(8) Å, b = 15.0157(7) Å, c = 17.7743(10) Å; β =
97.0621(17)°; V = 4102.4(4) Å3; Z = 4; Calcd. = 1.715 Mg·m3; F(000) = 2096; crystal size =0.122 x
0.118 x 0.112 mm3; µ = 3.349 mm1; 35531 reflections collected, 9400 independent reflections
(Rint = 0.0320), data / restraints / parameters = 9400 / 0 / 565, GOF = 1.079, final R1[I > 2(I)] =
0.0254 and wR2(all data) = 0.0631.
Spectral data of Px‐23: 1H NMR (400 MHz, acetone‐d6, 298K): δ 8.00 (d, J = 1.7 Hz, 1 H), 7.90
(d, J = 2.0 Hz, 1 H), 7.66 (s, 1 H), 7.37 (s, 1 H), 7.34 (d, J = 1.7 Hz, 1 H), 7.26 (d, J = 1.9 Hz, 1 H), 7.14
(s, 1 H), 7.03 (d, J = 8.4 Hz, 1 H), 6.98  6.94 (m, 2 H), 6.75 (d, J = 1.9 Hz, 1 H), 6.31 (d, J = 2.0 Hz, 1
H), 6.15 (d, J = 14.4 Hz, 1 H, methylene), 5.41 (d, J = 14.4 Hz, 1 H, methylene), 3.04 (s, 3 H), 2.77
(s, 3 H), 1.52 (s, 9 H). 19F NMR (376 MHz, acetone‐d6, 298K): δ ‒60.31 (s, 3 F), ‒61.47 (s, 3 F), ‒
62.17 (s, 3 F). FD MS: m/z 939.3 (M+). Anal. calcd. for C36H29F9IrN7O: C, 46.05, H, 3.11, N, 10.44.
Found: C, 45.82, H, 2.93, N, 10.34.
Spectral data of Cz‐4: 1H NMR (400 MHz, acetone‐d6, 298K): δ 8.65 (s, 1 H), 8.56 (d, J = 8.8
Hz, 1 H), 8.36 (t, J = 8.0 Hz, 1 H), 8.29 (dd, J = 8.0, 1.0 Hz, 1 H), 8.03 (dd, J = 8.0, 1.0 Hz, 1 H), 7.93
 7.90 (m, 1 H), 7.88 (dd, J = 8.8, 1.6 Hz, 1 H), 7.82 (d, J = 2.1 Hz, 1 H), 7.64 (s, 1 H), 7.40 (s, 1 H),
7.28 (d, J = 1.8 Hz, 1 H), 7.15 (s, 1 H), 6.81 (d, J = 2.1 Hz, 1 H), 6.74 (d, J = 1.8 Hz, 1 H), 6.36 (d, J =
1.8 Hz, 1 H), 6.06 (d, J = 14.4 Hz, 1 H, methylene), 5.43 (d, J = 14.4 Hz, 1 H, methylene), 3.14 (s, 3
H), 2.52 (s, 3 H). 19F NMR (376 MHz, acetone‐d6, 298K): δ ‒60.34 (s, 3 F), ‒61.42 (s, 3 F), ‒61.50
(s, 3 F), ‒61.56 (s, 3 F). FD MS: m/z 1024.3 (M+). Anal. calcd. for C39H23F12IrN8: C, 45.75, H, 2.26,
N, 10.94. Found: C, 45.63, H, 2.33, N, 10.89.
− 17 −
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Selected crystal data of Cz‐4: C40H25Cl2F12IrN8; M = 1108.78; T = 150(2) K; (Mo‐K) = 0.71073
Å; monoclinic; space group = C2/c; a = 24.9130(8) Å, b = 17.2160(5) Å, c = 20.7020(7) Å; β =
117.0189(9)°; V = 7910.0(4) Å3; Z = 8; Calcd. = 1.862 Mg·m3; F(000) = 4320; crystal size =0.217 x
0.202 x 0.131 mm3; µ = 3.611 mm1; 25605 reflections collected, 9083 independent reflections
(Rint = 0.0328), data / restraints / parameters = 9083 / 46 / 586, GOF = 1.064, final R1[I > 2(I)] =
0.0369 and wR2(all data) = 0.0963.
Spectral data of Cz‐5: 1H NMR (400 MHz, acetone‐d6, 298K): δ 8.64 (s, 1 H), 8.53 (d, J = 8.8
Hz, 1 H), 8.24 (d, J = 1.6 Hz, 1 H), 8.11 (d, J = 1.6 Hz, 1 H), 7.91 (dd, J = 8.8, 1.8 Hz, 2 H), 7.80 (d, J
= 2.0 Hz, 1 H) 7.63 (s, 1 H), 7.37 (s, 1 H), 7.26 (d, J = 1.8 Hz, 1 H), 7.23 (s, 1 H), 6.79 (d, J = 2.0 Hz,
1 H), 6.71 (d, J = 1.8 Hz, 1 H), 6.36 (d, J = 1.8 Hz, 1 H), 6.04 (d, J = 14.4 Hz, 1 H, methylene), 5.41
19
(d, J = 14.4 Hz, 1 H, methylene), 3.11 (s, 3 H), 2.50 (s, 3 H), 1.57 (s, 9 H). F NMR (376 MHz,
acetone‐d6, 298K): δ ‒60.28 (s, 3 F), ‒61.36 (s, 3 F), ‒61.46 (s, 3 F), ‒61.49 (s, 3 F). FD MS: m/z
1080.1 (M+). Anal. calcd. for C43H31F12IrN8: C, 47.82, H, 2.89, N, 10.38. Found: C, 47.60, H, 2.82, N,
10.45.
Photophysical measurements. UV‐Vis spectra were recorded on a HITACHI U‐3900
spectrophotometer. The steady‐state emission spectra and lifetime studies were measured with
Edinburgh FL 900 photon‐counting system. Both wavelength‐dependent excitation and emission
responses of the fluorimeter were calibrated. To determine the photoluminescence quantum
yield in solution, samples were degassed using at least three freeze‐pump‐thaw cycles. All
photpphysical data were analyzed using a nonlinear least‐squares procedure in combination with
an iterative convolution method. The emission decays were fitted by the sum of exponential
functions with a temporal resolution of  300 ps after the deconvolution of instrument response
function.
X‐ray Structural Determination. Single crystal X‐ray diffraction data were recorded on a
Bruker D8 VENTURE diffractometer equipped with Oxford Cryostream 800+ controller. The data
− 18 −
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collection was executed using the SMART program. Refinement and data reduction were made
with the SAINT program. The structure was determined using the SHELXTL/PC program and
refined using full‐matrix least squares. Non‐hydrogen and hydrogen atoms were refined
anisotropically and with the riding mode with fixed parameters, respectively. The supplementary
crystallographic data of Px‐22 and Cz‐4, CCDC 1943429 and 1943265, can be obtained free of
charge
from
the
Cambridge
Crystallographic
Data
Center
via
www.ccdc.cam.ac.uk/data_request/cif.
Computational details of theoretical investigations: The geometry, electronic structure and
electronic excitations of complexes Px‐14, 21 ‐ 23, and Cz‐4 were investigated by methods based
on DFT[30] and TD‐DFT.[31] The corresponding ground state geometries were optimized based on
the X‐ray structure of Px‐14, Px‐22 and Cz‐4, respectively. Both the geometry optimization and
TD‐DFT calculations were performed with the PBE1PBE[32] functional combined with the
polarizable continuum model (PCM)[33] to include solvent effects of CH2Cl2, using Gaussian
codes.[34] The 6‐31G(d,p)[35] basis set was used for light elements such as hydrogen, carbon,
nitrogen, oxygen and fluorine, while LANL2DZ[36] basis set with the Los Alamos National
Laboratory (LANL)[37] effective core potentials (ECPs) was used for iridium. Mulliken population
analysis[38] was used to calculate the contribution from Ir atom for the electron distribution of
each molecular orbital using GaussSum software.[39] Natural transition orbitals (NTOs)[25] analysis
was adopted to obtain a clear and compact orbital representation for excited states described by
a list of orbital transitions without a single predominant one (e.g. T1 state of the complexes
investigated in this work). Avogadro software was used to visualize the orbitals present in this
work.[40]
Supporting Information
General experimental procedures of all measurements and calculations, synthetic
procedures of chelates, original electrochemical data and TD‐DFT results of the studied Ir(III)
metal complexes.
− 19 −
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Acknowledgements
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. XZ is supported by an Australian Research
Council Discovery Early Career Researcher Award (ARC DECRA DE190100144) and a computing
grant (rw25) from National Computational Infrastructure of Australia. XZ was also supported a
University of Queensland Development Research Fellowship.
− 20 −
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Figure 1. Structural drawing of Px‐22 with thermal ellipsoids shown at 30% probability level.
Selected bond length: Ir‐C(17) = 2.075(3), Ir‐C(21) = 2.015(3) , Ir‐C(27) = 2.050(3), Ir‐C(1) = 2.046(3),
Ir‐N(1) = 2.113(2) and Ir‐N(2) = 2.083(2) Å. Selected bond angle: C(17)‐Ir‐C(27) = 165.68(12), C(17)‐
Ir‐C(21) = 89.11(12), C(21)‐Ir‐C(27) = 78.66(12), C(1)‐Ir‐N(2) = 170.00(11), N(1)‐Ir‐N(2) = 78.08(10),
and C(21)‐Ir‐N(1) = 172.68(11).
Figure 2. Structural drawing of Cz‐4 with thermal ellipsoids shown at 30% probability level.
Selected bond length: Ir‐C(11) = 2.068(5), Ir‐C(4) = 2.020(5), Ir‐C(1) = 2.041(5), Ir‐C(17) = 2.039(5),
Ir‐N(6) = 2.142(4) and Ir‐N(7) = 2.088(4) Å. Selected bond angle: C(11)‐Ir‐C(1) = 168.44(18), C(11)‐
Ir‐C(4) = 89.26(19), C(4)‐Ir‐C(1) = 79.34(19), C(17)‐Ir‐N(7) = 169.89(17), N(6)‐Ir‐N(7) = 78.58(15)
and C(4)‐Ir‐N(6) = 174.45(17).
− 21 −
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1.0
0.8
0.6
0.4
0.2
0.0
Px-21
Px-22
Px-23
Cz-4
30
20
10
Cz-5
0
300
400
500
600
700
Wavelength (nm)
Figure 3. Absorption and emission spectra of the studied Ir(III) bis‐tridentate complexes, recorded
in CH2Cl2 at RT.
0.0
-0.1
Px-21
Px-22
-0.2
Px-14
Cz-2
fac-Ir(ppy)
Px-23
Cz-4
Cz-5
3
-0.3
0
10
20
30
40
50
60
Time (hrs)
Figure 4. The photo‐degradation response of the studied bis‐tridentate Ir(III) complexes.
− 22 −
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Figure 5. Natural transition orbitals (NTOs) pairs expressing the T1 state of Ir(III) complexes Px‐14,
21 − 23 and Cz‐4, calculated at PBE1PBE/6‐31G** (LANL2DZ with ECP for Ir) level with PCM for
modeling the CH2Cl2 solvent.
− 23 −
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2.0
2.6
2.2
2.4
TAPC
mCP
2.7
TmPyPB
6.7
TAPC
LiF/Al
4.3
mCP
O P
P
O
O
ITO
4.8
DPEPO
5.5
DPEPO
5.6
5.9
6.1
TmPyPB
Figure 6. The energy level diagram of the phosphorescent OLEDs and the materials employed.
− 24 −
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12
1000
100
10
(a)
(b)
Px-21
Px-22
Px-23
10
10
8
6
4
2
0
1.0
0.8
0.6
0.4
0.2
0.0
1
1
400
500
600
700
800
Wavelength (nm)
0.1
0.1
1
10
100
1000
0
2
4
6
8
10
Voltage (V)
Luminance (cd m-2)
1000
(c)
(d)
12
10
8
Cz-4
Cz-5
10
100
10
1
1.0
0.8
0.6
0.4
0.2
0.0
6
1
4
2
400
500
600
700
800
Wavelength (nm)
0
0.1
0.1
1
10
100
1000
0
2
4
6
8
Luminance (cd m-2)
Voltage (V)
Figure 7. Current density–voltage–luminance (J–V–L) (a and c), EQE–luminance (b and d)
characteristics and EL spectra at 1 mA cm‐2 (inset) of the devices based on the developed
phosphors.
− 25 −
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Table 1. Essential photophysical data and thermal gravitational analysis (TGA) data of the studied bis‐tridentate Ir(III) complexes.
abs. λmax (nm) [ε  103][a]
em λmax (nm)[b]
484
 (%)[b, c]
100

obs (μs)[b]
rad (μs)[b]
2.70

obs (μs)[d] Td (°C)[e]
Px‐21
Px‐22
Px‐23
Px‐14
Cz‐4
271 (25.4), 307 (14.9), 342 (8.7), 401 (1.3)
271 (31.9), 312 (19.0), 340 (13.8), 392 (2.5)
271 (30.8), 309 (19.4), 341 (12.0), 381 (3.7)
272 (23.9), 316 (21.2), 327 (17.6), 392 (2.0)
285 (27.8), 368 (13.1), 414 (1.9)
2.70
5.72
8.13
3.4
3.74
5.66
8.45
‒
376
388
372
394
418
478
448
447 (sh), 473
446, 470
449 (sh), 476
494
100
5.72
87.2
100
9.32
3.4
100
2.08
2.44
3.20
2.08
3.11
3.19
‒
Cz‐5
286 (25.4), 365 (11.6), 405 (1.7)
484
96.3
83.5
2.53
Cz‐2
287 (30.6), 361 (15.2), 395 (2.4)
473
3.83
[a] UV‐Vis spectra was recorded in CH2Cl2 at a conc. of 10‐5 M at RT. [b] These data were recorded in degassed CH2Cl2 at 10‐5 M at RT. [c] Coumarin
e
102 (C102) in MeOH (Q.Y. = 87 % and λmax = 480 nm) were employed as standard. [d] Lifetime measured in doped DPEPO (10 wt%) at RT. [ ] Td
stands for the temperature with 5% loss of weight.
− 26 −
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Table 2. The calculated excitation energy (λ), oscillator strength (f), main orbital
contributions and charge characters of the lowest singlet and triplet excited state of
Ir(III) complexes Px‐14, 21 ‐ 23 and Cz‐4, at PBE1PBE/6‐31G** (LANL2DZ with ECP for
Ir) level with PCM for modeling the CH2Cl2 solvent.
State λ (nm)
f
Orbital contribution (>10%)
Assignment
S1
T1
S1
T1
356
0.01 HOMO → LUMO (94%)
HOMO‐3 → LUMO (56%)
LC, MLCT, LLCT, ILCT
417
0
HOMO‐2 → LUMO (15%)
HOMO‐2 → LUMO+1 (12%)
LC, MLCT, ILCT
365
0.04 HOMO → LUMO (96%)
HOMO‐2 → LUMO (35%)
LC, MLCT, LLCT, ILCT
LC, MLCT, ILCT
418.3
0
HOMO‐3 → LUMO (29%)
HOMO‐2 → LUMO+1 (14%)
S1
T1
S1
T1
S1
T1
355
417.9
352
0.05 HOMO → LUMO (97%)
LC, MLCT, LLCT, ILCT
LC, MLCT, ILCT
HOMO‐2 → LUMO (53%)
HOMO‐2 → LUMO+1 (14%)
0
0.06 HOMO → LUMO (97%)
LC, MLCT, LLCT, ILCT
LC, MLCT, ILCT
HOMO‐2 → LUMO (58%)
HOMO‐3 → LUMO (16%)
416.8
317
0
0.04 HOMO → LUMO (97%)
LC, MLCT, LLCT, ILCT
LC, MLCT, ILCT
HOMO‐2 → LUMO+1 (28%)
HOMO‐3 → LUMO (15%)
424
0
− 27 −
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Table 3. Summary of performance data for the OLED devices
voltage (V) / CE (lm·W‐1) / PE (cd·A‐1) / EQE (%)
dopant
Von[a] (V)
CIE(x, y)[b]
maximum
at 100 cd m‐2
at 500 cd m‐2
−
Px‐21
Px‐22
3.8
4.0
3.8 / 35.5 / 29.3 / 18.0
4.0 / 24.6 / 19.5 / 15.0
5.9 / 22.2 / 11.7 / 11.3
6.5 / 15.5 / 7.5 / 9.4
(0.162, 0.306)
(0.154, 0.240)
−
Px‐23
Cz‐4
4.1
3.6
3.7
4.1 / 17.7 / 13.6 / 11.4
4.0 / 29.2 / 23.0 / 14.9
3.7 / 30.1 / 25.9 / 18.7
7.5 / 9.3 / 3.9 / 6.0
5.2 / 25.3 / 15.3 / 12.9
5.1 / 21.8 / 13.5 / 13.7
−
(0.160, 0.226)
(0.154, 0.298)
(0.145, 0.218)
6.6 / 16.6 / 7.9 / 8.5
6.6 / 13.4 / 6.4 / 8.4
Cz‐5
[a]
V is obtained at 1 cd·m‐2; [b] CIE (x, y) is obtained at 1 mA·cm‐2.
on
− 28 −
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10.1002/chem.201903707
Chemistry - A European Journal
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