Spiro-type host materials with rigidified skeletons for RGB phosphorescent OLEDs

Article abstract of DOI:10.1039/d0tc01389g

A universal host material for red, green and blue phosphors raises challenges for the fabrication of efficient phosphorescent organic light-emitting diodes (PHOLEDs). Herein, two spiro host materials, namely 3-(4,6-diphenyl-1,3,5-triazin-2-yl)-9′,9′-dimethyl-9′H-spiro[fluorene-9,5′-quinolino[3,2,1-de]acridine] (QAF-TRZ) and 3′-(4,6-diphenyl-1,3,5-triazin-2-yl)-10-phenyl-10H-spiro[acridine-9,9′-fluorene] (STF-TRZ), are designed and synthesized. The designed strategy is more appropriate and rarely found among the reported spiro-based skeletons for organic light-emitting diodes (OLEDs). Both materials exhibited excellent electroluminescence performance. In particular, the RGB three-color PHOLEDs based onQAF-TRZandSTF-TRZas hosts and FIrpic, Ir(ppy)₂acac, and Ir(MDQ)₂acac as emitters are fabricated. The blue PHOLEDs based onQAF-TRZrealize a maximum external quantum efficiency (EQE) of 19.4%, and remain 17.2% at 1000 cd m^?2. On the other hand, green PHOLEDs show maximum EQEs of 21% and 19% forQAF-TRZandSTF-TRZ, respectively, and both devices exhibit low efficiency roll-off. The red devices obtain EQEs of 22.6% and 19.4% based onQAF-TRZandSTF-TRZ, respectively, in which the former has among the most efficient value. Consequently, these results confirm that host materials incorporating the spiro-motif with sufficiently high triplet energy are promising for obtaining high performance RGB-based PHOLEDs. The better device performance based onQAF-TRZstems from its better hole transport capability, resulting in a superior charge recombination efficiency.

Full text of DOI:10.1039/d0tc01389g

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Page 1 of 22
Journal of Materials Chemistry C
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DOI: 10.1039/D0TC01389G
Spiro-Type Host Materials with Rigidified Skeleton for RGB Phosphorescent
OLEDs
Aziz Khan, ‡ Xing Chen, ‡ Sarvendra Kumar, Sheng-Yi Yang, You-Jun Yu, Wei Luo, Zuo-Quan
Jiang,* Man-Keung Fung,* and Liang-Sheng Liao
A. Khan, X. Chen, S. Kumar, S. Y. Yang, Y. J. You, W. Luo, Prof. M. K. Fung, Prof. Z. Q. Jiang,
Prof. L. S. Liao
Institute of Functional Nano & Soft Materials (FUNSOM), Jiangsu Key Laboratory for Carbon-
Based Functional Materials & Devices, Soochow University, 199 Ren'ai Road, Suzhou, 215123,
Jiangsu, PR China
E-mail: zqjiang@suda.edu.cn; mkfung@suda.edu.cn
[‡] These authors contributed equally to this work.
Keywords: phosphorescent organic light-emitting diodes; RGB colors; high efficiency, spiro-type
host; bipolar.
Abstract
The universal host material for red, green and blue is still a big challenge for fabricating
efficient phosphorescent organic light-emitting diodes (PHOLEDs). Herein, two spiro host
materials namely 3-(4,6-diphenyl-1,3,5-triazin-2-yl)-9',9'-dimethyl-9'H-spiro[fluorene-9,5'-
quinolino[3,2,1-de]acridine] (QAF-TRZ) and 3'-(4,6-diphenyl-1,3,5-triazin-2-yl)-10-phenyl-
10H-spiro[acridine-9,9'-fluorene] (STF-TRZ) are designed and synthesized. The designed
strategy is more appropriate and rarely found among the reported spiro-based skeleton for organic
light-emitting diodes (OLEDs). Both materials exhibited excellent electroluminescence
performance. In particular, the RGB three-color PHOLEDs based on QAF-TRZ, STF-TRZ as
hosts and FIrpic, Ir(ppy)2acac, Ir(MDQ)2acac as emitters are fabricated. The blue PHOLEDs

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based on QAF-TRZ realize a maximum external quantum efficiency (EQE) of 19.4%, and remain
17.2% at 1000 cd/m2. On the other hand, green PHOLEDs shows maximum EQEs of 21% and
19% for QAF-TRZ and STF-TRZ, respectively, and both devices exhibit low efficiency roll-off.
Red devices obtain EQEs of 22.6 % and 19.4 % based on QAF-TRZ and STF-TRZ, respectively,
in which the former has among the most efficient value. Consequently, these results confirm host
materials incorporating spiro-motif having sufficiently high triplet energy are promising to obtain
high-performances of RGB based PHOLEDs. The better device performance based on QAF-TRZ
stems from its better hole transport capability, hence a superior charge recombination efficiency.
Introduction
Organic light-emitting diodes (OLEDs) have attracted widespread interest in both
academia and industry, because of the fascinating applications in flat panel display and solid-state
lighting.1-5 Generally, in fluorescent OLEDs the non-radiative process limits the internal quantum
efficiency (IQE) to 25% while the external quantum efficiency (EQE) hardly exceeds 5%.6 To
resolve this issue, Forrest et al. represented phosphorescent organic light-emitting diodes
(PHOLEDs) containing platinum (II) or iridium (III) complexes, which can successfully obtain
100% IQE via spin-orbit coupling, utilizing both singlet and triplet excitons.7-11 Since then, many
efforts have been made to improve device performance and stability. On the other hand,
phosphorescent OLEDs doped with host material is also highly demanded to mitigate triplet-triplet
annihilation and concentration quenching phenomena.12, 13 Host materials usually have two design
strategies. Firstly, to prevent the reverse energy transfer from guest to host, the host materials must
have a higher triplet energy level (ET) than the guest. Secondly, to reduce the turn-on voltage and
facilitate effective injection of hole and electron, suitable highest occupied molecular orbitals

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(HOMOs) and the lowest unoccupied molecular orbitals (LUMOs) of the host to adjacent material
layers are highly required.14-19
Compared with unipolar host,20, 21 bipolar host materials attracted more attention because
of both the hole and electron transport elements can be found in a single molecule.22-27 In this
regard, many groups strived to design bipolar host materials with good efficiency7, 22, 23, 28. Among
them, spiro-type29-36 bipolar hosts are of particular interest due to the following features: i) the 3-
D configuration can enhance the thermal and morphological stabilities, ii) the functionalization of
spiro-hosts is flexible, i.e. chemist has the room to modify not only the acceptor unit but also donor
unit of the hemisphere, iii) this modification can balance hole/electron transport. In designing
bipolar compounds in the most classic 9,9’-spirobifluorene, many electron-rich spiro backbones
have been developed with appended electron acceptor (Scheme 1a).
Herein, two novel spiro-type bipolar host materials namely 3-(4,6-diphenyl-1,3,5-triazin-
2-yl)-9',9'-dimethyl-9'H-spiro[fluorene-9,5'-quinolino[3,2,1-de]acridine] (QAF-TRZ) and 3'-
(4,6-diphenyl-1,3,5-triazin-2-yl)-10-phenyl-10H-spiro[acridine-9,9'-fluorene
(STF-TRZ)
containing 9,9-dimethyl-10-phenyl-9,10-dihydroacridine and triphenylamine donor linking with
strong triazine acceptor through spiro bridge are prepared, respectively. In comparison, QAF-TRZ
with new rigidified donor moiety has a prominent effect on photophysical, thermal properties and
electroluminescence (EL) device performance. We systematically investigated both materials as
universal hosts in blue/green/red PHOLEDs. As a result, the devices based on QAF-TRZ show
superior performances and lower efficiency roll-off than STF-TRZ, indicating the potential of this
new spiro-structure for PHOLED host.

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Experimental Section
Compound 1 and 2' were purchased while compound 2 was synthesized according to proposed
literature.37, 38
3-bromo-9',9'-dimethyl-9'H-spiro[fluorene-9,5'-quinolino[3,2,1-de]acridine] (3)
The compound 2, (10-(2-bromophenyl)-9,9-dimethyl-9,10-dihydroacridine) (2.0 g, 5.6 mmol) was
dissolved in THF (40 mL) under inert environment. The n-butyl lithium (1.6 M solution in hexane
2.2 mL, 5.5 mmol) was added slowly at -78 oC, and the reaction mixture was stirred for 1 hour at
the same condition. Then THF solution (20 mL) of 3-bromo-9H-fluoren-9-one (1.1 g, 4.6 mmol)
was added and further stirred for 2 hours at -78oC. It is then stirred overnight at room temperature.
After completion, the reaction mixture was quenched with water (5 mL). The THF was evaporated
and the obtained viscous solid was extracted with dichloromethane (DCM) and washed with water
(30 mL  3). The organic portion was collected and purified through column about 1.8 g (72%)
until white solid was obtained. which was used in the next reaction after purification. The
intermediate
3-bromo-9-(2-(9,9-dimethylacridin-10(9H)-yl)phenyl)-9H-fluoren-9-ol
was
dissolved in acetic acid (30 mL) refluxed at 80 oC for 20 minutes. Then hydrochloric acid (8 mL)
was added slowly to the reaction mixture. After 4 hours, white solid was appeared. It was filtered
off and washed first with water and then petroleum ether (PE). Column chromatography was used
1
for further purification: PE/ DCM (7/3, v/v) (1.6 g, 97%). H NMR (400 MHz, CDCl3) δ (ppm)
8.05 (d, J = 1.8 Hz, 1H), 7.88 (d, J = 7.6 Hz, 1H), 7.83 (d, J = 1.7 Hz, 1H), 7.72 (dd, J = 12.3, 4.3
Hz, 1H), 7.67 (d, J = 7.9 Hz, 1H), 7.60 (ddd, J = 8.1, 6.1, 1.5 Hz, 2H), 7.54 – 7.46 (m, 1H), 7.39
– 7.30 (m, 2H), 7.27 – 7.18 (m, 2H), 7.13 (d, J = 7.7 Hz, 1H), 7.07 (dd, J = 8.2, 1.8 Hz, 1H), 7.02
– 6.94 (m, 1H), 6.85 (td, J = 7.6, 3.0 Hz, 2H), 6.65 (ddd, J = 9.6, 7.9, 1.4 Hz, 1H), 6.45 (ddd, J =
13
9.0, 7.8, 1.2 Hz, 1H), 1.99 (s, 3H), 1.39 (s, 3H). C NMR (100 MHz, CDCl3) δ (ppm) 154.60,

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152.92, 150.51, 149.26, 144.90, 141.43, 139.18, 138.61, 136.98, 136.35, 135.57, 134.29, 131.71,
131.01, 129.11, 128.68, 128.42, 127.40, 126.79, 126.59, 125.41, 124.47, 124.03, 123.71, 123.61,
123.04, 122.51, 121.40, 120.37, 118.50, 116.91, 57.36, 36.93, 31.81, 23.02. MS (EI) m/z: 525.08
[M+].
3'-bromo-10-phenyl-10H-spiro[acridine-9,9'-fluorene] (3')
The intermediate 3' was synthesized according to the similar procedure as explained above for
compound 3 except changing the corresponding reagents. Which was further purified from column
1
chromatography with eluents: PE/ DCM (8/2, v/v) (1.3 g, 90%). H NMR (400 MHz, CDCl3 δ
(ppm) 7.97 (d, J = 1.7 Hz, 1H), 7.83 – 7.71 (m, 3H), 7.62 (t, J = 7.5 Hz, 1H), 7.53 (d, J = 8.4 Hz,
2H), 7.50 – 7.39 (m, 3H), 7.32 (dd, J = 18.3, 9.5 Hz, 2H), 7.01 – 6.92 (m, 2H), 6.62 (t, J = 7.5 Hz,
2H), 6.42 (dd, J = 16.2, 8.5 Hz, 4H). 13C NMR (100 MHz, CDCl3) δ (ppm) 156.81, 155.39, 141.34,
140.96, 137.93, 131.16, 129.17, 128.56, 127.74, 127.38, 125.97, 124.01, 123.17, 121.69, 120.65,
120.17, 114.79, 56.66. MS (EI) m/z: 485.24 [M+].
9',9'-dimethyl-3-(4,4,5,5-tetramethyl-1,3,2-dioxaborolan-2-yl)-9'H-spiro[fluorene-9,5'-
quinolino[3,2,1-de]acridine] (4)
A mixture of 3-bromo-9',9'-dimethyl-9'H-spiro[fluorene-9,5'-quinolino[3,2,1-de]acridine] (1.3 g,
2.4 mmol), 4,4,40,40,5,5,50,50-octamethyl-2,20-bi(1,3,2- dioxaborolane) (1.2 g, 4.9 mmol), 1,1'-
bis(diphenylphosphino)ferrocene-palladium(II) (Pd(dppf)2Cl2) 87.0 mg, 0.1 mmol), potassium
acetate (1.1 g, 12.0 mmol), and anhydrous 1,4-dioxane (50 mL) was degassed and then refluxed
under an inert atmosphere for 24 hours at 80 oC. The reaction mixture was washed three times with
water (30 mL  3) after dilution with DCM. In order to remove water completely from the organic
part, anhydrous MgSO4 was added. The solvent of crude product was dried through rotary
evaporator, which was purified using column chromatography to obtain a white powder with the

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1
eluents: PE/ DCM (4/6, v/v) (1.1 g, 78%). H NMR (400 MHz, CDCl3) δ (ppm) 8.37 (s, 1H), 8.17
(s, 1H), 7.96 (dd, J = 14.1, 7.5 Hz, 1H), 7.80 – 7.69 (m, 1H), 7.68 – 7.54 (m, 2H), 7.53 – 7.42 (m,
2H), 7.38 – 7.29 (m, 2H), 7.18 (ddd, J = 23.6, 14.6, 8.3 Hz, 3H), 6.94 (t, J = 7.5 Hz, 1H), 6.82 (dd,
J = 9.2, 4.6 Hz, 2H), 6.67 – 6.60 (m, 1H), 6.44 (t, J = 7.2 Hz, 1H), 1.98 (s, 3H), 1.47 (s, 6H), 1.39
(s, 3H), 1.32 (s, 6H).13C NMR (100 MHz, CDCl3) δ (ppm) 153.73, 149.96, 139.28, 138.62, 136.99,
136.41, 135.24, 134.68, 134.19, 134.14, 129.23, 128.24, 128.06, 127.42, 127.13, 127.06, 126.90,
126.67, 126.46, 126.37, 124.39, 123.98, 123.85, 123.27, 123.02, 122.86, 122.11, 120.38, 120.34,
118.42, 118.38, 116.98, 116.92, 84.05, 83.72, 57.68, 36.92, 31.77, 31.66, 25.00, 24.83, 24.79,
23.02. MS (EI) m/z: 573.29 [M+].
10-phenyl-3'-(4,4,5,5-tetramethyl-1,3,2-dioxaborolan-2-yl)-10H-spiro[acridine-9,9'
fluorene] (4')
The compound 4' was synthesized according to the similar procedure mention above for the
compound 4 only by changing the corresponding reagents. In order to obtain pure product, it was
1
passed through column chromatography with the eluents: PE/ DCM (4/6, v/v) (0.8 g, 66%). H
NMR (400 MHz, CDCl3) δ (ppm) 8.30 (s, 1H), 7.88 (d, J = 7.5 Hz, 1H), 7.81 – 7.69 (m, 3H), 7.60
(t, J = 8.0 Hz, 1H), 7.53 (d, J = 8.1 Hz, 2H), 7.46 (dd, J = 14.3, 7.5 Hz, 2H), 7.39 (t, J = 8.0 Hz,
1H), 7.25 (d, J = 7.4 Hz, 1H), 6.97 – 6.89 (m, 2H), 6.57 (t, J = 7.5 Hz, 2H), 6.39 (dd, J = 13.7, 7.3
13
Hz, 4H), 1.42 (s, 12H). C NMR (100 MHz, CDCl3) δ (ppm) 159.53, 156.68, 141.17, 138.95,
135.10, 131.15, 128.40, 127.87, 127.58, 127.21, 126.33, 125.66, 125.35, 124.54, 120.54, 120.18,
114.63, 83.90, 56.97, 24.94. MS (EI) m/z: 533.35 [M+].

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3-(4,6-diphenyl-1,3,5-triazin-2-yl)-9',9'-dimethyl-9'H-spiro[fluorene-9,5'-quinolino[3,2,1-
de]acridine] (QAF-TRZ)
The
compounds
9',9'-dimethyl-3-(4,4,5,5-tetramethyl-1,3,2-dioxaborolan-2-yl)-9'H-
spiro[fluorene-9,5'quinolino [3,2,1-de]acridine] (0.8 g,1.5 mmol), 2-chloro-4,6-diphenyl-1,3,5
triazine (0.5 g, 1.8 mmol), tetrakis(triphenylphosphine)palladium(0) (50.0 mg, 0.05 mmol) and
potassium carbonate (0.5 g, 3.7 mmol) were dissolved in 1,4-dioxane/ H2O (40: 4), and refluxed
for 24 hours at 85 oC. The mixture was diluted with DCM and washed thrice with distilled water
(100 mL). The organic layer was dried and purified through column chromatography using
1
PE/DCM (7/3, v/v) to obtain a pale white powder (0.8 g, 80%). H NMR (600 MHz, CDCl3) δ
(ppm) 9.25 (s, 1H), 9.02 (s, 1H), 8.87 (dd, J = 14.1, 7.3 Hz, 2H), 8.73 (d, J = 7.1 Hz, 2H), 8.36 (d,
J = 8.1 Hz, 1H), 8.14 (d, J = 7.6 Hz, 1H), 7.93 (d, J = 7.5 Hz, 1H), 7.79 (d, J = 8.0 Hz, 1H), 7.73
(d, J = 8.0 Hz, 1H), 7.70 – 7.48 (m, 8H), 7.36 (s, 1H), 7.31 (t, J = 7.2 Hz, 1H), 7.20 (t, J = 8.9 Hz,
1H), 7.15 (d, J = 7.6 Hz, 1H), 6.98 (t, J = 7.5 Hz, 1H), 6.83 (dd, J = 13.1, 7.2 Hz, 2H), 6.69 (dd, J
13
= 14.3, 7.9 Hz, 1H), 6.49 (dd, J = 18.7, 7.8 Hz, 1H), 1.97 (s, 3H), 1.39 (s, 3H). C NMR (150
MHz, CDCl3) δ (ppm) 171.73, 158.31, 155.01, 154.51, 150.38, 143.51, 142.24, 139.26, 138.69,
137.70, 137.03, 136.67, 136.16, 135.75, 134.32, 132.58, 131.96, 129.64, 129.42, 127.81, 127.04,
126.63, 124.51, 124.31, 123.39, 123.08, 122.39, 120.95, 118.56, 117.02, 57.79, 36.98, 31.84,
23.08. MALDI-TOF (m/z) calculated for C49H34N4 [M+]: 678.27, found: 678.22. Anal. Cacd for
C49H34N4 C, 86.70; H, 5.05: N, 8.25 found C, 85.92; H, 4.97: N, 8.07.
3'-(4,6-diphenyl-1,3,5-triazin-2-yl)-10-phenyl-10H-spiro[acridine-9,9'-fluorene] (STF-TRZ).
A mixture of 10-phenyl-3'-(4,4,5,5-tetramethyl-1,3,2-dioxaborolan-2-yl)-10H-spiro[acridine-9,9'-
fluorene] (0.7 g, 1.3 mmol), 2-chloro-4,6-diphenyl-1,3,5 triazine (0.4 g, 1.6 mmol), tetrakis
(triphenylphosphine) palladium (0) (50.0 mg, 0.04 mmol) and potassium carbonate (0.4 g, 3.2

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mmol) was dissolved in 1,4-dioxane/ H2O (40: 4) refluxed over night at 85 oC. The crude organic
layer was subjected to column chromatography for further purifications using PE/ DCM (7/3, v/v)
(0.68 g, 81%). 1H NMR (600 MHz, CDCl3) δ (ppm) 9.15 (s, 1H), 8.81 (d, J = 6.9 Hz, 4H), 8.69
(d, J = 8.0 Hz, 1H), 8.05 (d, J = 7.5 Hz, 1H), 7.73 (t, J = 7.6 Hz, 2H), 7.68 – 7.56 (m, 8H), 7.54
(d, J = 7.8 Hz, 2H), 7.47 (dd, J = 17.7, 10.3 Hz, 2H), 7.33 (t, J = 7.4 Hz, 1H), 6.94 (t, J = 7.7 Hz,
2H), 6.58 (t, J = 7.4 Hz, 2H), 6.47 (d, J = 6.9 Hz, 2H), 6.39 (d, J = 8.4 Hz, 2H). 13C NMR (150
MHz, CDCl3) δ (ppm) 171.83, 171.64, 160.63, 156.52, 141.28, 140.99, 139.87, 138.70, 136.25,
135.95, 132.51, 131.17, 131.08, 129.54, 128.97, 128.84, 128.65, 128.49, 127.77, 127.74, 127.39,
125.96, 125.87, 124.25, 120.63, 120.53, 120.40, 114.73, 57.03. MALDI-TOF (m/z) calculated for
C46H30N4 [M+]: 638.24, found: 638.18. Anal. Cacd for C46H30N4 C, 86.49; H, 4.73: N, 8.77 found
C, 85.58; H, 4.69: N, 8.39.
Results and Discussion
Synthesis and Characterization.
The synthetic routes for two compounds are depicted in Scheme 1b. The intermediate 3 and 3′
were synthesized through a three-step consecutive reaction under the same condition. Similarly,
the intermediate 4 and 4′ were synthesized through the Miyaura borylation reaction.39 Finally, the
QAF-TRZ and STF-TRZ were successfully obtained in good yields through the Suzuki-Miyaura
coupling reaction with 2-chloro-4,6-diphenyl-1,3,5 triazine and corresponding reagents 4 and 4′.
To make it highly pure, the products were further sublimated under vacuum after column
chromatography before used in OLED devices. The chemical structures of QAF-TRZ and STF-
1
13
TRZ were fully characterized through HNMR, CNMR spectroscopy, mass spectrometry, and
elemental analysis.

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Scheme 1 (a) The derivatization way of donor/acceptor host materials in the spiro-skeleton. (b)
The synthetic routes for QAF-TRZ and STF-TRZ.
Thermal properties
The thermogravimetric analysis (TGA) and differential scanning calorimetry (DSC) under inert
atmosphere were demonstrated in Fig. S1a and Fig. S1b for both compounds. The measured
thermal properties and values are depicted in Table 1. The spiro annulated structure reveals high
decomposition temperatures (Td) with 5% weight loss observed at 430 and 424 oC, while the glass
o
transition temperature (Tg) are 195 and 158 C for QAF-TRZ and STF-TRZ, respectively.

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Comparably, the QAF-TRZ has more favorable Td and Tg over the STF-TRZ for OLED
preparation in vacuum deposition due to the rigidification in spiro-skeleton.
Theoretical calculations
The theoretical calculations were performed to better understand the electronic properties of the
two compounds by using density functional theory (DFT) and time-dependent DFT (TD-DFT).
Ground-state geometries and excited state geometries were optimized at B3LYP/6-31G level.40
The highest occupied molecular orbital (HOMO) and the lowest unoccupied molecular orbital
(LUMO) of QAF-TRZ and STF-TRZ are mainly located at the donor and acceptor segment,
respectively (Fig. 1a). The energy level of the HOMO and LUMO of QAF-TRZ and STF-TRZ
are -5.04/-1.90 eV and -5.00/-1.74 eV, respectively (Fig. 1b); the fused strategy on donor leads to
deeper HOMO and higher singlet/triplet energies as unveiled from the photophysical
measurements (vide infra). Furthermore, the energy differences between S1 and T1 states for QAF-
TRZ and STF-TRZ are almost same. Based on the electron-hole distribution analysis,41 99.2%
and 99.3% of the HOMO-to-LUMO transition contributed to the S0 → S1 excitation for QAF-
TRZ and STF-TRZ, respectively, while that contributed to the S0 → T1 excitation was 98.0%
and 96.7%, respectively, for the QAF-TRZ and STF-TRZ.

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Fig. 1 (a) The distribution of HOMOs and LUMOs and the analysis for distribution of hole
(blue), electron (yellow) for S1 and T1 and (b) energy level diagram of HOMOs, LUMOs, S1 and
T1.
Photophysical properties.
The photophysical behavior of the QAF-TRZ and STF-TRZ, ultraviolet-visible (UV-vis) and
fluorescence spectra were recorded in toluene at room temperature. Phosphorescence spectra were
measured at frozen temperature (77 K) using the same solvent which are depicted in Fig. 2a and
Fig. 2b respectively. QAF-TRZ and STF-TRZ show very similar absorption peak at 305 nm
which is attributed to the - transitions. The HOMO-LUMO energy gaps were calculated from
the UV absorption onset, and both are 3.44 eV. The fluorescence peaks as observed in the
photoluminescence spectra (PL) for QAF-TRZ and STF-TRZ were 447 and 450 nm, respectively.
Apart from that, the photophysical properties of QAF-TRZ and STF-TRZ, were also measured
in neat film and their spectra are illustrated in Fig. S2. Notably, the absoption peak for QAF-TRZ
and STF-TRZ is blue-shifted as compared of the solution results as well as the triplet-energy
levels for QAF-TRZ and STF-TRZ are quite similar. Moreover, the triplet energies were
calculated from the vibronic band of the phosphorescence spectra at 77 K are 2.88 eV for QAF-

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DOI: 10.1039/D0TC01389G
TRZ, and 2.86 eV for STF-TRZ. Therefore, such triplet energies are high enough to prevent back
energy transfer from emitters.
Fig 2. (a) Normalized UV-vis absorption and fluorescence emission spectra in toluene at
room temperature. (b) Phosphorescence spectra in frozen toluene at 77 K for compounds
QAF-TRZ and STF-TRZ.
Table 1. Summary of the physical properties of QAF-TRZ and STF-TRZ
a
b
c
d
e
g
Compounds
Eg
ET
HOMO/LUMOf (eV) Td/Tg
Abs
Fl
Phos
(nm)
(nm)
(nm)
(eV)
(eV)
2.88
(oC)
QAF-TRZ
STF-TRZ
305
447
447
3.44
-5.67/-2.23
430/195
424/158
305
450
446
3.44
2.86
-5.36/-1.91
a
b
Calculated from the UV-vis absorption measured in toluene. Determined from the room
c
d
temperature fluorescence in toluene. Calculated from the 77 K phosphorescence in toluene.
e
Calculated from the UV-vis absorption in toluene. Calculated from the phosphorescence in
f
toluene. HOMO is calculated from the oxidation potential with respect to reference ferrocene;
LUMO is calculated from the energy gap plus HOMO. gTd: Decomposition temperature, Tg: glass
transition temperature.
Electrochemical Properties
To investigate the electrochemical behavior of the two host materials, QAF-TRZ and STF-TRZ,
cyclic voltammetry analysis was performed in anhydrous DCM using the ferrocene as the internal
standard and the parameters are summarized in Table 1. The oxidation potential values for the

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DOI: 10.1039/D0TC01389G
compounds QAF-TRZ and STF-TRZ are +1.37 V and +1.05 V, respectively. The obtained results
indicate that QAF-TRZ is constructed with a rigid donor structure as compared to STF-TRZ. The
HOMO energy levels were estimated from the onset of the oxidation potential curves, to be -5.67/-
5.36 eV, respectively, while the LUMO energy levels were calculated from Eg (onset of UV-vis
spectra) and HOMO values, which were -2.23/-1.91 eV for QAF-TRZ and STF-TRZ,
respectively (Fig. S3).
Electroluminescence (EL) Properties.
To further evaluate the EL properties of these two host materials, firstly we fabricated blue
PHOLED devices based on FIrpic. As shown in Fig. 3a, the blue device structure is ITO/
1,4,5,8,9,11-hexaazatriphenylene-hexacarbonitrile (HAT-CN) (10 nm)/ 1,1-bis[4-[N, N-di(p-
tolyl)amino]phenyl]cyclohexane (TAPC) (40 nm)/ tris(4-(9H-carbazol-9-yl)phenyl)amine (TCTA)
(10 nm)/ QAF-TRZ or STF-TRZ: FIrpic 15 wt% (20 nm)/ 3,3'-(5'-(3-(pyridin-3-yl)phenyl)-
[1,1':3',1''-terphenyl]-3,3''-diyl)dipyridine (TmPyPB) (45 nm)/ (8-quinolinolato lithium) Liq (2
nm)/ Al (100 nm) (B1 for QAF-TRZ and B2 for STF-TRZ). The energy level diagrams of all the
blue, green and red devices fabricated in the present studies are displayed in Fig. 3b. ITO acts as
the anode while Al as the cathode. HAT-CN and Liq are the hole and electron-injection layers,
respectively. TAPC is employed as the hole transport layer because of its remarkable hole mobility
and TCTA plays the role as the electron blocking layer. Furthermore, TmPyPB is chosen as the
electron transport layer for better energy level alignment. The chemical structures of these
commercial materials are shown in Fig. S4. We all know that the ET of PHOLED host is higher
than that of the emitters for effective exciton confinement, better energy transfers from the host to
the guest and reducing the non-radiative transition. In this work, the ET of QAF-TRZ and STF-
TRZ are 2.88 and 2.86 eV, respectively, which tells that all are appropriate as the host for FIrpic

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DOI: 10.1039/D0TC01389G
(ET = 2.62 eV). We first fabricated the blue devices by varying the concentration of FIrpic from
10 to 20% (Table S1), and the optimized concentration was 15%. The current density-voltage-
luminance (J-V-L) characteristics for both B1 and B2 are shown in Fig. S5a and their device
performance is summarized in Table 2. Device B1 exhibits a maximum EQE, current efficiency
(CE), and power efficiency (PE) of 19.4%, 45 cd/A and 47.8 lm/W, respectively. The EQE can be
maintained as 17.2% at 1000 cd/m2. However, device B2 shows a maximum EQE, CE, and PE of
12.7%, 30.8 cd/A and 27.5 lm/W, which reveals an apparently lower device performance as
compared with B1 (Fig. 3c). Conventional hosts such as 1,3-Bis(N-carbazolyl)benzene (mCP)
and 2,6-bis(3-(carbazol-9-yl)phenyl)pyridine (26DCzppy) were also used for comparison. The
device results are summarized in Table S2. Among them, QAF-TRZ shows the best device
performance, making it a good candidate as a host material for PhOLED.
Fig. 3 (a) Device structure, (b) energy level diagrams of devices, (c) blue PHOLED performance

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DOI: 10.1039/D0TC01389G
and (d) hole-only and electron-only devices based on QAF-TRZ and STF-TRZ.
Table 2 Electroluminescence characteristics of the devices
Device
Va
(V)
CEb
(cd/A)
PEb
(lm/W)
EQEc
(%)
CIEd
(x,y)
B1
B2
G1
G2
R1
R2
3.4
3.5
2.8
2.9
3.8
3.8
45.0
30.8
80.0
72.0
26.5
29.0
47.8
27.5
91.0
76.0
24.1
25.3
19.4/18.3/17.2
12.7/12.3/11.3
21.0/20.4/20.0
19.0/18.9/18.8
22.6/21.8/17.1
19.4/19.0/15.5
(0.16,0.35)
(0.17,0.37)
(0.34,0.63)
(0.34,0.61)
(0.63,0.36)
(0.62,0.37)
b
aDriving voltage at 100 cd/m2; Maximum value of current efficiency and power efficiency;
d
cEQEs of maximum value, 100 cd/m2 and 1000 cd/m2; CIE (Commission International de
l'Eclairge); Measured at a driving current density at 5 mA/cm2
To further investigate the difference in the blue PHOLED performance under the same device
structure, we fabricated electron- and hole-only devices (EOD and HOD) to estimate their charge
mobilities. The device structures are as follows: ITO/ TmPyPB (20 nm)/ QAF-TRZ or STF-TRZ
(100 nm)/ TmPyPB (20 nm)/ Liq (2 nm)/ Al (100 nm) (as EOD) and ITO/ MoO3 (10 nm)/ QAF-
TRZ or STF-TRZ (100 nm)/ MoO3 (10 nm)/ Al (100 nm) (as HOD). As shown in Fig. 3d, the
J-V curves of QAF-TRZ and STF-TRZ almost overlap under the same driving voltages in the
EOD device, indicating that two host materials have the similar electron mobility. On the contrary,
QAF-TRZ shows a higher current density than STF-TRZ at a low driving voltage in the HOD.
Therefore, the better hole mobility of QAF-TRZ may more favor the hole-electron combination
for forming exciton in the device B1.

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Fig. 4 (a) Green and (b) red PHOLEDs device performance for QAF-TRZ and STF-TRZ
Furthermore, green and red PHOLEDs were also fabricated with device structures of ITO/ HAT-
CN (10 nm)/ TAPC (40 nm)/ TCTA (10 nm)/ QAF-TRZ or STF-TRZ: Ir(ppy)2acac 10 wt% (20
nm)/ TmPyPB (45 nm)/ Liq (2 nm)/ Al (100 nm) and ITO/ HAT-CN (10 nm)/ TAPC (40 nm)/
TCTA (10 nm)/ QAF-TRZ or STF-TRZ: Ir(MDQ)2acac 6 wt% (20 nm)/ TmPyPB (50 nm)/ Liq
(2 nm)/ Al (100 nm) (G1, R1 for QAF-TRZ and G2, R2 for STF-TRZ). The ETs of green
(Ir(ppy)2acac) and red (Ir(MDQ)2acac) emitters are 2.4 eV and 2.0 eV, respectively, which are
much lower than those of the hosts, and therefore can realize effective energy transfer. The device
performance is shown in Fig. 4 and also summarized in Table 2. It can be seen that the maximum
EQE, CE, and PE of R1 and R2 are 22.6%, 26.5 cd/A, 24.1 lm/W and 19.4%, 29.0 cd/A, 25.3
lm/W, respectively. The EQE of R1 based on the QAF-TRZ is among the highest value in red
PHOLEDs. Also, the EQE can be kept as high as 17.1% at 1000 cd/m2. Moreover, the green
devices (G1 and G2) based on our two host materials exhibit maximum EQE, CE, PE, of 21.0%,
80 cd/A, 91 lm/W and 19%, 72 cd/A, 76 lm/W, respectively. Notably, G1 and G2 have quite
similar performance and low-efficiency roll-off. At a luminance of 1000 cd/m2, the EQE of devices

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G1 and G2 remain 20% and 18.8%, and merely decrease by 4% and 1% from their maximum
values, respectively. This low roll-off property is mainly attributed to the well-balanced charge
injection and better recombination arisen from the high ET of the hosts. The J-V-L characteristics
of green and red devices are exhibited in Fig. S5b and Fig. S5c, respectively. In addition, both
hosts almost lead to identical RGB EL emissions (Fig. S5d), i.e. 472 nm for B1 and B2, 524 nm
for G1 and G2, and 612 nm for R1 and R2. Thus, there is no any other emission peak in the EL
spectra, substantiating an efficient energy transfer from the hosts to the emitters.
Conclusion
We have designed and synthesized two host materials, QAF-TRZ and STF-TRZ, for red, green
and blue PhOLEDs. Both materials have an excellent thermal property with a high Td of over 420
oC. The EQE of the blue PHOLED reached nearly 20% when using QAF-TRZ as the host. The
maximum EQEs for the green PHOLEDs QAF-TRZ and STF-TRZ as the hosts were 21% and
19%, respectively. Both the green PhOLEDs exhibited a low-efficiency roll-off, in which the
EQEs merely decreased by 4% and 1%, respectively, from their maximum values when the devices
were driven to a luminance of 1000 cd/m2. Moreover, the red PHOLEDs exhibited maximum
EQEs of 22.6% and 19.4% with using QAF-TRZ and STF-TRZ as the hosts, respectively, in
which the former has the best EQE among all the red PhOLEDs. The superior device performance
is attributed to the new rigidified donor moiety with the spiro motif incorporating triazine (at para
position) type acceptors. Device based on QAF-TRZ even have higher efficiencies because it has
a higher hole mobility with better charge recombination. We believe our simple spiro-skeleton
strategy can facilitate the development of efficient and stable hosts for RGB based PHOLEDs.
Acknowledgments

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We acknowledge financial support from the National Key R&D Program of China (No.
2016YFB0400700), the National Natural Science Foundation of China (Nos. 51773141, 51873139
and 21572152) and the Natural Science Foundation of Jiangsu Province of China (BK20181442).
This project is also funded by Collaborative Innovation Center of Suzhou Nano Science &
Technology, the Priority Academic Program Development of Jiangsu Higher Education
Institutions (PAPD), the 111 Project of The State Administration of Foreign Experts Affairs of
China. We greatly thank Yang-Kun Qu for analyzing the molecular simulations.
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✔ High stability
✔ Low roll-off
✔ Efficient EQEs for RGB
STF-
TRZ
QAF-
TRZ