spiro[fluorene-9,9′- xanthene] hybrid as efficient host for green thermally activated delayed fluorescence devices

Dyes and Pigments 166 (2019) 168–173

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Solution-processible 1,3,4-oxadiazole/spiro[fluorene-9,9′- xanthene] hybrid as efficient host for green thermally activated delayed fluorescence devices

T

Xiang-Hua Zhaoa,∗, Jing-Yuan Wanga, Jia-Xing Wua, Yue Lia, Guo-Dong Zoua, Guohua Xieb,∗∗, Zong-Qiong Linc, Ling-Hai Xied, Xin-Wen Zhangd, Jian-Feng Zhaoc,∗∗∗ a

College of Chemistry and Chemical Engineering, Xinyang Normal University, Xinyang, 464000, Henan, China Hubei Collaborative Innovation Center for Advanced Organic Chemical Materials Hubei Key Lab on Organic and Polymeric Optoelectronic Materials Department of Chemistry Wuhan University Wuhan, 430072, China c Key Laboratory of Flexible Electronics (KLOFE) & Institute of Advanced Materials (IAM), Jiangsu National Synergetic Innovation Center for Advanced Materials (SICAM), Nanjing Tech University, Nanjing, 211816, China d Key Laboratory for Organic Electronics and Information Displays & Institute of Advanced Materials (IAM), Jiangsu National Synergetic Innovation Center for Advanced Materials (SICAM), Nanjing University of Posts & Telecommunications, 9 Wenyuan Road, Nanjing, 210023, China b

ARTICLE INFO

ABSTRACT

Keywords: Spiro[fluorene-9,9′-xanthene 1,3,4-Oxadiazole Steric hindrance Thermally activated delayed fluorescence

A novel bulky 1,3,4-oxadiazole-based spirocyclic compound 2,5-bis(2-spiro[fluorene-9,9′-xanthene]-7-yl) phenyl)-1,3,4-oxadiazole (D- [2-SFXP]-o-OXD) has been synthesized successfully by incorporating spiro [fluorene-9,9′-xanthene] (SFX) into 2,5-biphenyl-1,3,4-oxadiazole reaction via Suzuki cross-coupling with Pd (PPh3)4 as catalyst. The electrochemical properties of the compound were researched via cyclic voltammetry (CV) and obtained the HOMO, LUMO, and energy gap (Eg, 3.57 eV). The green TADF OLEDs hosted D[2-SFXP]-oOXD and D[2-SFXP]-o-OXD:mCP by solution-processing techniques with maximum current efficiency (CE) and EQE (external quantum efficiency) of 15.2 cd A−1/4.8% and 25.9 cd A−1/8.5% as the doping concentration increasing from 5% to 10%, respectively. The electroluminescent (EL) spectra are independent of the concentration changes, which exhibits the three dimensional bulky D[2-SFXP]-o-OXD with large steric hindrance could successfully suppress the concentration quenching and excimer emission.

1. Introduction Organic light-emitting diodes (OLEDs) have been developed for several decades since the pioneering work of Tang and Vanslyke in 1987 [1], due to their enormous potential applications in full-color displays and white solid-state lighting [2–4]. In order to obtain high performance devices, the general method is to employ high internal quantum efficiency (IQE) emitters for many scientific workers to break through the theoretical 25% limit of conventional fluorescent emitters [5–8]. Thanks to the continuous devotions of numerous investigators, the significant advancement of phoshpor emitters with 100% IQE was first reported by Thompson and Forrest [9], which could harvest both signet and triplet excitons due to spin-orbit coupling caused by heavy metals. After delicately dispersed in suitable host materials carefully, the phosphor emitters based OLEDs with high performance have been extensive reported by many researchers [10]. Yet, the phosphorescent

precious-metal complexes often suffer from instability in full-color display and white solid-state lighting [11]. Moreover, due to the rather expensive and limited global resources of noble metals, purely organic thermally activated delayed fluorescence (TADF) metal-free emitters have increasingly attracted attention due to its high efficiency and low cost in the past few years [12,13]. For TADF emitters, they could not only harvest both triplet and singlet excitons through upconversion caused by thermally activated delayed fluorescence giving theoretical maximum IQE up to100% but also are inexpensive without using noblemetal complexes and comparable stability in practical application compared to that of phosphorescent OLEDs [6–13]. In order to achieve high efficiency, TADF emitters also should be dispersed into a suitable host to suppress concentration quenching through triplet-triplet annihilation and triplet-polaron quenching in solid state [14,15]. Therefore, an appropriate host is very important to realize high performance TADF OLEDs. 3D bulky host materials are

Corresponding author. Corresponding author. ∗∗∗ Corresponding author. E-mail addresses: [email protected] (X.-H. Zhao), [email protected] (G. Xie), [email protected] (J.-F. Zhao). ∗

∗∗

https://doi.org/10.1016/j.dyepig.2019.03.015 Received 15 January 2019; Received in revised form 6 March 2019; Accepted 6 March 2019 Available online 12 March 2019 0143-7208/ © 2019 Elsevier Ltd. All rights reserved.

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often designed and synthesized to resolve these problems for high-efficient TADF OLEDs. For example, Han and Duan used 3,6-di-tert-butylcarbazole/diphenylphosphine oxide hybrides as hosts with bulky steric hindrance for deep blue TADF OLEDs with maximum EQEs of 15.2% [16]. Duan and his co-workers untilized bulky indeno[2,1-b] carbazole/1,3,5-triazin hybrid as universal host to suppress the exciton concentration for obtaining high-efficient devices with maximum EQE of 23.2, 21.0, and 19.2% for orange, green, and sky-blue TADF OLEDs, respectively, even at 2000 cd/m2 [17]. Adachi's group employed bulky bis(2-(di(phenyl)phosphino)-phenyl)ether oxide (DPEPO) as host using a series of carbazoles/benzonitrile hybrides as emitters for deep-blue TADF OLEDs with maximum EQE of 10.3% [18]. For an excellent host material, there are some essential requirements of host materials for TADF emitters, such as higher triplet energy (T1) than that of guest, large spectral overlap between host emission and guest absorption, high thermal stability and stable morphology, good carrier transport properties, suitable highest occupied molecular orbital (HOMO) and lowest unoccupied molecular orbital (LUMO) levels alinged with those of neighboring layers [19]. Up to present, many scientific workers employed solution processing method for realizing high performance TADF OLEDs to reduce cost. For example, Su and his group employed benzene-1,3,5-triyltris(phenylmethanone)/9,9-dimethyl-9,10-dihydroacrid-ine as emitter to achieve green TADF OLEDs with EQE of ≈26% and ≈22% by vacuum-evaporating and solution-processing methods, respectively [20]. Kaji and his co-workers developed a series of adamantyl substituted 2-phenyl-1,3,5-triazine/acridine as emitters to obtain deep-blue TADF OLEDs with maximum EQEs of 22.1% by solution-processing [21]. Spirocyclic hybrids own many fascinating properties such as, high T1, two interrupted π-conjugated fragments and bulky 3D spiro-annulation structure due to their different ring sizes and orthogonal framework. The bulky 9,9′-spirobifluorene compounds are extensively used as host materials to suppress triplet-triplet annihilation and triplet-polaron quenching for high efficient OLEDs. For example, Xu and his group constructed a highly twisted and asymmetrical diphenylphosphine oxide/spiro[fluorene-9,9′-xanthene] (SFX) hybrid as universal host realizing TADF OLEDs with maximum EQE of 17.9%, 19.7%, 19.6%, 22.5%, 13.9%, and 19.0% and ηint of ≈100% for blue, green, yellowish green, yellow, orange, and complementary nearly white devices, respectively [22]. Whereafter, the group demonstrated triazine/ spiro[fluorene-9,12′- indeno[1,2-c]carbazole] as both emitter and host in a simple donor–acceptor systems for pure fluorescence white device with maximum EQE of 22.9% [23]. Herein, electron-deficient 1,3,4oxadiazole (OXD) unit was incorporated into electron-rich SFX core via Suzuki cross-coupling reaction, which would possess stable morphology and well balanced charge carriers. To the best of our knowledge, although there are many excellent 1,3,4-oxadiazole-based molecules have been reported in OLEDs [24–28], almost no 1,3,4-oxadiazole-based spiro-annulation compounds as hosts for TADF OLEDs have been explored in previous work. Thus, the search of 1,3,4-oxadiazole-based spiro-annulation compounds as host materials for OLEDs is very important to construct state-of-the-art models to uncover the relationship between molecular structure and device performance for cumulative enhancement of device efficiency and expediting the commercialization of OLEDs. So, a bulky 2,5-bis(2-spiro[fluorene-9,9′-xanthene]-7-yl) phenyl)-1,3,4-oxadiazole (D- [2-SFXP]-o-OXD) was designed and synthesized mixing mCP as co-host with 2,4,5,6-tetrakis(carbazol-9-yl)-1,3dicyano-benzene (4CzIPN) as the emitter for green TADF OLEDs [29a], obtaining the state-of-the-art EL performance with maximum current efficiency (CE) and EQE of 25.9 cd A−1 and 8.5%, respectively.

Polymer Light Technology Corp. 2-Bromo-9-fluorenone, phenol, methanesulfonic acid, n-butyllinthium tetrakistriphenyl-phosphine palladium(0), and 2-isopropoxy-4,4,5,5-tetramethyl-1,3,2-dioxaborolane were used without further purification. 2,5-Bis(2-bromophenyl)-1,3,4oxadiazole, 2-bromo-spiro[fluorene-9,9′-xanthene] and 2-(spiro [fluorene- 9,9′-xanthene]-2-yl)-1,3,2-dioxaborinane were synthesized according to the literature reports [24,29b,30]. All reactions involving organometallic reagents were carried out under nitrogen except special illustration. 1H (400 MHz) and 13C NMR (100 MHz) spectra measurements were recorded on a Bruker 400 MHz spectrometer in CDCl3. MALDI-TOF MS spectrum was carried out by reflective mode. The Ultraviolet–visible (UV–vis) absorption and photoluminescence (PL) spectra were recorded on a Shimadzu UV-3600 and Shimadzu RF5301(PC)S luminescence spectrometer at 25 °C under atmosphere, respectively. Cyclic voltammetric measurements (CVs) were researched on a CHI660C Electrochemical Workstation in acetonitrile solution with 0.1 mol L−1 of Bu4NPF6 at room temperature, using a platinum sheet working electrode, a reference electrode (Ag/Ag+, referenced against ferrocene/ferrocenium (FOC)), and a platinum wire counter electrode. The HOMO, LUMO, and HOMO-LUMO energy gap (Eg) of the compound were calculated by the equations HOMO = −[Eox−E(Fc/ Fc+) + 4.8] eV, LUMO = −[Ered−E(Fc/Fc+) + 4.8] eV, and E(Fc/ Fc+) is about 0.0118 V. The geometry of the ground state structure of the compound was optimized with the assistance of density functional theory (DFT) calculations, obtaining the HOMO and LUMO spatial distributions of the compound. 2.2. Synthesis of 2,5-bis(2-spiro[fluorene-9,9′-xanthene]-7-yl)phenyl)1,3,4-Oxadiazole (D[2-SFXP]-o-OXD) A mixture of 2-(spiro[fluorene-9,9′-xanthene]-2-yl)-1,3,2-dio- xaborinane (2.7480 g, 6.0 mmol), 2,5-bis(4-bromophenyl)-1,3,4- oxadiazole (1.134 g, 3.0 mmol), Pd(PPh3)4 (0.0347 g, 0.03 mmol), degassed toluene (120 mL) and K2CO3 (2 M, 15.0 mL) were added into a three-neck flask under N2 atmosphere. Then, the mixture was refluxed and avoided light exposure for 48 h. Afterwards, the reaction was cooled to room temperature, and then cold water was added and extracted with dichloromethane for several times. The organic extracts were collected and over anhydrous Na2SO4. The organic phase was condensed by rotary evaporation under vacuum and purified through column chromatography on silica gel with the mixture of ethyl acetate and petroleum ether to afford white solid product; yield: 58%. MALDITOF (EI) m/z 882.491 [M+]; 1H NMR (400 MHz, CDCl3, ppm): δ 7.65 (dd, J = 9.20 Hz, 4H), 7.43 (m, 4H), 7.32 (dd, J = 4.80 Hz, 5H), 7.22 (d, J = 23.20 Hz, 1H), 7.14 (m, 7H), 7.07 (m, 7H), 6.99 (s, 2H), 6.68 (t, J = 7.60 Hz, 4H), 6.40 (d, J = 9.20 Hz, 4H). 13C NMR (100 MHz, CDCl3): δ (ppm) 164.18, 155.58, 154.15, 151.38, 142.07, 140.71, 139.84, 138.40, 130.90, 129.78, 128.72, 128.39, 128.01, 127.85, 127.88, 126.33, 125.42, 124.88, 123.43, 122.66, 120.04, 119.79, 116.71. Anal. Calcd for C64H38N2O3: C, 87.05; H, 4.34; N, 3.17; found: C, 86.66; H, 3.99; N, 3.47%. 3. Results and discussions 3.1. Synthesis and thermal stability The 1,3,4-oxadiazole-based spirocyclic compound has been synthesized by incorporating SFX into 2,5-biphenyl-1,3,4-oxadiazole according to our previously work via Suzuki cross-coupling with Pd (PPh3)4 as catalyst (Scheme 1) [3]. The molecular structure is characterized by LC-MS, 1H NMR and 13C NMR, which has good solubility in common solvents in dichloromethane, tetrahydrofuran and chloroform. The combination of electron-deficient OXD and electron-rich SFX is anticipated to balance electron and hole injection/transportation, and obtain good thermal and morphological stability. In order to research the thermal and morphological properties, the compound was

2. Experimental section 2.1. Materials and methods All the solvents and chemical materials were purchased from Xi'an 169

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Scheme 1. Synthetic route to D[2-SFXP]-o-OXD.

Fig. 1. TGA (a) and DSC (b) curves of D[2-SFXP]-o-OXD.

measured by thermogravimetric analysis (TGA) and differential scanning calorimeter (DSC) under nitrogen atmosphere at a heating rate of 10 °C/min (Fig. 1). The TGA and DSC curves revealed good decomposition temperature (Td) at 198 °C and stable morphology for no crystallization and melting phenomena ranging from 40 to 150 °C, implying potential application in stable OLEDs. Although the molecular structure of D[2-SFXP]-o-OXD is similar to SBF-o-OXD reported by Yang and Ma, the Td of the former (198 °C) is lower than that of the latter (401 °C) [24], which is probably resulted from twisted structure of SFX lowering the thermal stability of D[2-SFXP]-o-OXD. However, the morphology of D[2-SFXP]-o-OXD is higher than that of carbazole/oxadiazole (97 °C) and triphenylamine/oxadiazole hybrids (84 °C) [28,31], which is ascribed to the 3D bulky spiro-annulation strucutre of D[2-SFXP]-o-OXD.

around 229, 272 and 316 nm in the four solvents, corresponding to the π→π* transitions of xanthene, mixed n→π* transitions of xanthene and π→π* transitions of fluorene, and n→π* transitions from xanthene to fluorene, respectively [22,32]. Both of the weaker absorption peaks around 272 and 316 nm for D[2-SFXP]-o-OXD remarkably shift bathochromicly by 10 nm compared to those of SFX (corresponding to 262 nm and 306 nm) [32], which is ascribed to the enlarged π-conjugation between fluorene and 2,5-biphenyl-1,3,4-oxadiazole moities. It is notable that π→π* transition absorption from electron donor fluorene to electron acceptor oxadiazole unit is almost disappeared, which implies both of the linkage mode of the two moieties and distorted orthogonal framework of SFX breaks effectively the π -conjugation between the two parts [24,27,28]. Furthermore, the PL spectra exhibit nearly no dependence on the solvent polarity, with less than 5 nm shifts from cyclohexane to acetonitrile upon increasing the solvent polarity (Fig. 2), which implies the intramolecular charge transfer is weak [24]. In addition, the T1 of D[2-SFXP]-o-OXD is 2.48 eV calculated from its phosphorescent spectrum in DCM frozen glass at 77 K (Fig. S1), while the S1 estimated from the optical absorption threshold is about 3.60 eV. The energy gap (ΔEST) estimated by the energy level between T1 and S1 is about 1.12 eV.

3.2. Optical properties The ultraviolet–visible (UV–vis) absorption and photoluminescence (PL) spectra are tested in diluted dichloromethane (DCM), ethyl acetate, cyclohexane and acetonitrile (Fig. 2). As can be seen, the absorption spectra profiles are almost identical and independent of the solvent polarity, which indicates the Franck–Condon excited state of D [2-SFXP]-o-OXD suffered from a rather small dipolar change to the ground state [27]. There are three absorption peaks of the compound

3.3. The frontier orbital properties of the compound To obtain further insight into the relationships between molecular structure and properties of this compound, the compound is calculated by DFT calculations at molecular level and the results are listed in Table 1. The optimized structure shows that the highest occupied molecular orbital (HOMO) is mainly delocalized over SFX, one of the phenyl fragments in 2,5-biphenyl-1,3,4-oxadiazole unit and slightly extended to the 1,3,4-oxadiazole ring, while the lowest unoccupied molecular orbital (LUMO) is basically distributed on 2,5-biphenyl1,3,4-oxadiazole and slimly broadened to one of the fluorene group. The small degree of spatial overlap between HOMO and LUMO in this molecule is vital for high luminescence efficiency [33]. Cyclic voltammetry measurements were employed to estimate the value of the HOMO and LUMO energy levels (Table 2). The HOMO and LUMO levels of D[2-SFXP]-o-OXD were evaluated to be −6.02 and −2.45 eV, respectively. The energy gap (Eg, 3.57 eV) was obtained from the difference of HOMO and LUMO.

Fig. 2. The UV–vis and PL spectra of D[2-SFXP]-o-OXD in different solvents. 170

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Table 1 HOMO and LUMO surfaces from DFT calculations.

Table 2 Electrochemical properties and triplet energy of D[2-SFXP]-o-OXD.

Table 3 Device performance of the green TADF OLEDs.

Compound

HOMO (eV)

LUMO (eV)

Eg (eV)

T1 (eV)

D[2-SFXP]-o-OXD

−6.02

−2.45

3.57

2.48

4. Electroluminescent properties

Device

Host

La (cd m−2)

CEa(cd A−1)

PEa (lm W−1)

EQEa (%)

A B

D[2-SFXP]-o-OXD mCP: D[2-SFXP]o-OXD

11286 16468

15.2 25.9

4.3 8.5

4.8 8.5

a The efficiencies of L, CE, PE and EQE of prototype devices at maximum values.

To evaluate the novel 1,3,4-oxadiazole/spiro[fluorene-9,9′-xanthene] hybrid D[2-SFXP]-o-OXD as host for green TADF OLEDs, devices A and B with architectures of ITO/PEDOT:PSS (60 nm)/D[2-SFXP]- oOXD:4CzIPN (95:5, 50 nm)/TmPyPB (65 nm)/Liq (1 nm)/Al (100 nm) and ITO/PEDOT:PSS (60 nm)/mCP: D[2-SFXP]-o-OXD:4CzIPN (45:45:10, 50 nm)/TmPyPB (65 nm)/Liq (1 nm)/Al (100 nm) were initially fabricated, respectively (shown in Fig. 3). The devices data were listed in Table 3. In the device, the commercially available PEDOT:PSS is composed of a conductive polymer poly(3,4-ethylenedioxy-thiophene) doped with poly(styrene sulfonic acid) that is used as the holeinjection layer (HIL). Liq (8-Hydroxyquinolinolato-lithium) serves as the electron-injection layer (EIL), and 1,3,5-tri[(3-pyridyl)-phen-3-yl]benzene (TmPyPB) as the electron-transporting layers (ETL). The easily

available 2,4,5,6-tetra(9H-carbazol-9-yl)isophthalonitrile is used as green TADF guest. The EL spectra of devices A and B are shown in Fig. 4, which own nearly the same profiles with almost no shift and excimer emission as the host changes from D[2-SFXP]-o-OXD to co-host mCP: D[2-SFXP]-o-OXD and doping concentration increases from 5% to 10%. This indicates the distorted linkage between SFX and 2,5-biphenyl-1,3,4-oxadiazole with 3D bulky steric hindrance could suppress the concentration quenching and excimer emission successfully. The luminance–voltage–current density curves (L–V–J) characteristics, and power efficiency or EQE-current density of devices A and B are shown

Fig. 3. (a) Schematic configuration of the OLEDs,(b) Energy-band diagrams of the OLEDs, (c) Molecular structures of the main organic materials. 171

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Fig. 4. The normalized EL spectra of the devices for green TADF OLEDs. Device A: solid square black line; device B: solid circle dot red line. (For interpretation of the references to color in this figure legend, the reader is referred to the Web version of this article.)

in Fig. 5. For green TADF OLEDs, device A shows a maximum current efficiency (CE), power efficiency (PE) and EQEs of 15.2 cd A−1, 4.3 lm W−1 and 4.8%, respectively. Device B employed the mixture of mCP: D [2-SFXP]-o-OXD:4CzIPN as co-host with 10% weight of 4CzIPN as guest, exhibiting a maximum CE, power efficiency (PE) and EQE of 25.9 cd A−1, 8.5 lm W−1 and 8.5%, respectively. This implies that the poorer performance of device A might probably be due to the electron injection/transportation ability of D[2-SFXP]-o-OXD higher than that of hole injection/transportation ability caused by the strong electron-deficient ability of 1,3,4-oxadiazole. Moreover, the increasing concentration of the guest properly could also enhance the performance of device without changing the EL spectrum. The high performance of SFX/OXD hybrid based TADF-OLEDs is on-going by improving the hole injection/transportation ability of the compound via modifying molecular structure and optimizing the device configuration. 5. Conclusions In this work, a 1,3,4-oxadiazole/spiro[fluorene-9,9′-xanthene] hybrid was constructed successfully by introducing spiro[fluorene-9,9′xanthene] (SFX) into 2,5-biphenyl-1,3,4-oxadiazole via Suzuki crosscoupling reaction. The DSC curves demonstrate the 3D bulky spiroannulation structure of D[2-SFXP]-o-OXD is helpful to improve morphological stability. The green TADF OLEDs based on D[2-SFXP]-o-OXD and D[2-SFXP]-o-OXD:mCP were processed with maximum EQEs of 4.8% and 8.5% along with the doping concentration increasing from 5% to 10% without any changes of the EL spectra, which indicates the 3D bulky spiro-annulation structure of D[2-SFXP]-o-OXD could restrict the concentration quenching and excimer emission successfully. It is expected that 3D bulky spirocyclic SFX/OXD hybrids based-TADF OLEDs with high performance could be achieved by optimizing the molecular structure and device construction according to the aforementioned guidelines in this work.

Fig. 5. (a) Luminance–voltage–current density, (b) power efficiency-current density, (c) EQE-current density of devices A and B.

Appendix A. Supplementary data Supplementary data to this article can be found online at https:// doi.org/10.1016/j.dyepig.2019.03.015 References

6. Conflicts of interest

[1] Tang CW, Vanslyke SA. Organic electroluminescent diodes. Appl Phys Lett 1987;51:913–5. [2] Zhao XH, Zhang ZS, Qian Y, Yi MD, Xie LH, Hu CP, et al. A bulky pyridinylfluorenefuctionalizing approach to synthesize diarylfluorene-based bipolar host materials for efficient red, green, blue and white electrophosphorescent devices. J Mater Chem C 2013;1:3482–90. [3] Zhao XH, Xie GH, Liu ZD, Li WJ, Yi MD, Xie LH, et al. A 3-dimensional spirofunctionalized platinum(ii) complex to suppress intermolecular pi-pi and pt center dot center dot center dot pt supramolecular interactions for a high-performance electrophosphorescent device. Chem. Commun. 2012;48:3854–6. [4] (a) Tao Y, Yuan K, Chen T, Xu P, Li HH, Chen RF, et al. Thermally activated delayed fluorescence materials towards the breakthrough of organoelectronics. Adv Mater 2014;26:7931–58; (b) Wang Z, Wang Z, Zhou Y, Gu P, Liu G, Zhao K, et al. Structure engineering:

The authors declare that they have no conflicts of interest. Acknowledgements For the financial support, we thank the National Natural Science Foundation of China (No. 61405170, 21502091, 21502092), Natural Science Foundation of Jiangsu Province (14KJB430017), the Key Laboratory of Functional Inorganic Material Chemistry (Heilongjiang University), Ministry of Education, P. R. China and the Nanhu Scholars Program for Young Scholars of XYNU for financial support. 172

Dyes and Pigments 166 (2019) 168–173

X.-H. Zhao, et al.

[5] [6]

[7] [8]

[9] [10]

[11] [12]

[13] [14]

[15]

[16] [17]

extending the length of azaacene derivatives through quinone bridges. J Mater Chem C 2018;6:3628–33; (c) Yang B, Xiao J, Wong JI, Guo J, Wu Y, Ong L, et al. Shape-controlled micro/ nanostructures of 9,10-diphenylanthracene (dpa) and their application in lightemitting devices. J Phys Chem C 2011;115:7924–7; (d) Gu PY, Zhao YB, He JH, Zhang J, Wang CY, Xu QF, et al. Synthesis, physical properties, and light-emitting diode performance of phenazine-based derivatives with three, five, and nine fused six-membered rings. J Org Chem 2015;80:3030–5; (e) Li G, Zhao YB, Li JB, Cao J, Zhu J, Sun XW, et al. Synthesis, characterization, physical properties, and oled application of single BN-fused perylene diimide. J Org Chem 2015;80:196–203. Tagare J andVaidyanathan S. Recent development of phenanthroimidazole-based fluorophores for blue organic light-emitting diodes (oleds): an overview. J Mater Chem C 2018;6:10138–73. Li J, Zhang R, Wang ZQ, Zhao B, Xie JJ, Zhang F, et al. Zig-zag acridine/sulfone derivative with aggregation-induced emission and enhanced thermally activated delayed fluorescence in amorphous phase for highly efficient nondoped blue organic light-emitting diodes. Adv Opt Mater 2018;6:1701256. Niu YH, Liu MS, Ka JW, Bardeker J, Zin MT, Schofield R, et al. Crosslinkable holetransport layer on conducting polymer for high-efficiency white polymer lightemitting diodes. Adv Mater 2007;19:300–4. Maheshwaran A, Sree VG, Park HY, Kim H, Han SH, Lee JY, et al. High efficiency deep-blue phosphorescent organic light-emitting diodes with cie x, y (< = 0.15) and low efficiency roll-off by employing a high triplet energy bipolar host material. Adv Funct Mater 2018;28:1802945. Baldo MA, O'Brien DF, You Y, Shoustikov A, Sibley S, Thompson ME, et al. Highly efficient phosphorescent emission from organic electroluminescent devices. Nature 1998;395:151–4. (a) Sarada G, Cho W, Maheshwaran A, Sree VG, Park HY, Gal YS, et al. Deep-blue phosphorescent ir(iii) complexes with light-harvesting functional moieties for efficient blue and white pholeds in solution-process. Adv Funct Mater 2017;27:1701002; (b) Jia B, Lian H, Sun TJ, Guo HG, Cheng XZ, Wu J, et al. Efficient green phosphorescent organic light-emitting diodes enabled with new and thermally stable carbazole/pyridine derivatives as hosts. Dyes Pigments 2018;159:298–305. Lee SY, Yasuda T, Yang YS, Zhang QS, Adachi C. Luminous butterflies: efficient exciton harvesting by benzophenone derivatives for full-color delayed fluorescence OLEDs Angew. Chem Int Ed 2014;53:6402–6. Lin TA, Chatterjee T, Tsai WL, Lee WK, Wu MJ, Jiao M, et al. Sky-blue organic light emitting diode with 37% external quantum efficiency using thermally activated delayed fluorescence from spiroacridine-triazine hybrid. Adv Mater 2016;28:6976–83. Cui LS, Deng YL, Tsang DPK, Jiang ZQ, Zhang QS, Liao LS, et al. Controlling synergistic oxidation processes for efficient and stable blue thermally activated delayed fluorescence devices. Adv Mater 2016;28:7620–5. Ban XX, Jiang W, Sun KY, Lin BP, Sun YM. Self-host blue dendrimer comprised of thermally activated delayed fluorescence core and bipolar dendrons for efficient solution-processable nondoped electroluminescence. ACS Appl Mater Interfaces 2017;9:7339–46. Lin CC, Huang MJ, Chiu MJ, Huang MP, Chang CC, Liao CY, et al. Molecular design of highly efficient thermally activated delayed fluorescence hosts for blue phosphorescent and fluorescent organic light-emitting diodes. Chem Mater 2017;29:1527–37. Han CM, Duan CB, Yang WB, Xie MC, Xu H. Allochroic thermally activated delayed fluorescence diodes through field-induced solvatochromic effect. Sci Adv 2017;3. e1700904. Zhang DD, Zhao CG, Zhang YG, Song XZ, Wei PC, Cai MH, et al. Highly efficient full-color thermally activated delayed fluorescent organic light-emitting diodes:

[18] [19]

[20]

[21] [22] [23]

[24] [25] [26] [27] [28]

[29]

[30]

[31] [32]

[33]

173

extremely low efficiency roll-off utilizing a host with small singlet-triplet splitting. Acs Appl Mater Interfaces 2017;9:4769–77. Chan CY, Cui LS, Kim JU, Nakanotani H, Adachi C. Rational molecular design for deep-blue thermally activated delayed fluorescence emitters. Adv Funct Mater 2018;28:1706023. Li NQ, Fang YY, Li L, Zhao HR, Quan YW, Ye SH, et al. A universal solution-processable bipolar host based on triphenylamine and pyridine for efficient phosphorescent and thermally activated delayed fluorescence OLEDs. J Lumin 2018;199:465–74. Cai XY, Chen DJ, Gao K, Gan L, Yin QW, Qiao ZY, et al. Trade-off" hidden in condensed state solvation: multiradiative channels design for highly efficient solutionprocessed purely organic electroluminescence at high brightness. Adv Funct Mater 2018;28:1704927. Wada Y, Kubo S, Kaji H. Adamantyl substitution strategy for realizing solutionprocessable thermally stable deep-blue thermally activated delayed fluorescence materials. Adv Mater 2018;30:1705641. Li J, Ding DX, Tao YT, Wei Y, Chen RF, Xie LH, et al. A significantly twisted spirocyclic phosphine oxide as a universal host for high-efficiency full-color thermally activated delayed fluorescence diodes. Adv Mater 2016;28:3122–30. Lv XL, Zhang WZ, Ding DX, Han CM, Huang Z, Xiang SP, et al. Integrating the emitter and host characteristics of donor-acceptor systems through edge-spiro effect toward 100% exciton harvesting in blue and white fluorescence diodes. Adv Opt Mater 2018;6:1800165. Tao YT, Ao L, Wang Q, Zhong C, Yang CL, Qin JG, et al. Morphological stable spirobifluorene/oxadiazole hybrids as bipolar host materials for efficient green and red electrophosphorescence. Chem Asian J 2010;5:278–84. Wong KT, Ku SY, Cheng YM, Lin XY, Hung YY, Pu SC, et al. Synthesis, structures, and photoinduced electron transfer reaction in the 9,9 '-spirobifluorene-bridged bipolar systems. J Org Chem 2006;71:456–65. Mondal E, Hung WY, Dai HC, Wong KT. Fluorene-based asymmetric bipolar universal hosts for white organic light emitting devices. Adv Funct Mater 2013;23:3096–105. Chien CH, Kung LR, Wu CH, Shu CF, Chang SY, Chi Y. A solution-processable bipolar molecular glass as a host material for white electrophosphorescent devices. J Mater Chem 2008;18:3461–6. Tao YT, Wang QA, Yang CL, Zhong C, Qin JG, Ma DG. Multifunctional triphenylamine/oxadiazole hybrid as host and exciton-blocking material: high efficiency green phosphorescent oleds using easily available and common materials. Adv Funct Mater 2010;20:2923–9. a) Uoyama H, Goushi K, Shizu K, Nomura H, Adachi C. Highly efficient organic light-emitting diodes from delayed fluorescence. Nature 2012;492. 234-230; b) Xie LH, Liu F, Tang C, Hou XY, Hua YR, Fan QL, et al. Unexpected one-pot method to synthesize spiro fluorene-9,9'-xanthene building blocks for blue-lightemitting materials. Org Lett 2006;8:2787–90. Gu JF, Xie GH, Zhang L, Chen SF, Lin ZQ, Zhang ZS, et al. Dumbbell-shaped spirocyclic aromatic hydrocarbon to control intermolecular pi-pi stacking interaction for high-performance nondoped deep-blue organic light-emitting devices. J Phys Chem Lett 2010;1:2849–53. Tao YT, Wang Q, Yang CL, Wang Q, Zhang ZQ, Zou TT, et al. A simple carbazole/ oxadiazole hybrid molecule: an excellent bipolar host for green and red phosphorescent oleds. Angew Chem Int Ed 2008;47:8104–7. Zhao J, Xie GH, Yin CR, Xie LH, Han CM, Chen RF, et al. Harmonizing triplet level and ambipolar characteristics of wide-gap phosphine oxide hosts toward highly efficient and low driving voltage blue and green pholeds: an effective strategy based on spiro-systems. Chem Mater 2011;23:5331–9. Rajamalli P, Senthilkumar N, Gandeepan P, Huang PY, Huang MJ, Ren-Wu CZ, et al. A new molecular design based on thermally activated delayed fluorescence for highly efficient organic light emitting diodes. J Am Chem Soc 2016;138:628–34.