Rational molecular design of deep blue thermally activated delayed fluorescent emitters for high efficiency fluorescent and hyperfluorescent devices

Journal Pre-proof Rational molecular design of deep blue thermally activated delayed fluorescent emitters for high efficiency fluorescent and hyperfluorescent devices

Yu Jin Kang, Ju Hui Yun, Jun Yeob Lee PII:

S1566-1199(19)30631-7

DOI:

https://doi.org/10.1016/j.orgel.2019.105604

Reference:

ORGELE 105604

To appear in:

Organic Electronics

Received Date:

27 October 2019

Accepted Date:

21 December 2019

Please cite this article as: Yu Jin Kang, Ju Hui Yun, Jun Yeob Lee, Rational molecular design of deep blue thermally activated delayed fluorescent emitters for high efficiency fluorescent and hyperfluorescent devices, Organic Electronics (2019), https://doi.org/10.1016/j.orgel.2019.105604

This is a PDF file of an article that has undergone enhancements after acceptance, such as the addition of a cover page and metadata, and formatting for readability, but it is not yet the definitive version of record. This version will undergo additional copyediting, typesetting and review before it is published in its final form, but we are providing this version to give early visibility of the article. Please note that, during the production process, errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain. © 2019 Published by Elsevier.

Journal Pre-proof

Journal Pre-proof

Rational molecular design of deep blue thermally activated delayed fluorescent emitters for high efficiency fluorescent and hyperfluorescent devices Yu Jin Kang+, Ju Hui Yun+, Jun Yeob Lee* School of Chemical Engineering, Sungkyunkwan University 2066, Seobu-ro, Jangan-gu, Suwon, Gyeonggi, 440-746, Korea E-mail: [email protected] Y. J. Kang and J. H. Yun contributed equally. Keywords: t-butyldelayed fluorescence fluorescent device energy transferhyperfluorescence Abstract Two deep blue thermally activated delayed fluorescent (TADF) emitters with peripheral alkyl branches surrounding the donor and acceptor moieties were synthesized to explore ideal design of deep blue TADF emitters for fluorescence and hyperfluorescence in terms of quantum efficiency. The TADF emitters having the multiple alkyl branches protecting the donor and acceptor moieties performed better than the emitter without the alkyl branch or with the alkyl branch only in the donor or acceptor unit. The TADF emitters with the peripheral alkyl branches both in the donor and acceptor units showed a high quantum efficiency above 22.0%, a deep blue color coordinate of (0.15, 0.20), and doping concentration independent emission color as TADF emitters, and 13.7% quantum efficiency in the TADF sensitized hyperfluorescent device. Therefore, the multiple alkyl modification method of both donor and acceptor units of TADF emitters can resolve the issues of TADF emitters for ideal device performances.

1

Journal Pre-proof

Introduction Organic light-emitting diodes (OLEDs) are in the main stream of display technology because of unique device characteristics such as high contrast ratio, short response time, and design freedom. However, OLEDs suffer from poor device performances like low efficiency and short lifetime in blue devices. Therefore, it is urgently needed to upgrade the efficiency and lifetime of blue OLEDs simultaneously[1-4]. There are several solutions to overcome the efficiency and lifetime issues of blue OLEDs, which are phosphorescent OLEDs (PhOLEDs)[5-10], thermally activated delayed fluorescent (TADF) OLEDs[11-15], and TADF sensitized hyperfluorescent OLEDs[16-18]. These three technologies have a potential as the high efficiency blue OLED system although each technology has merits and demerits. Currently, PhOLEDs are ahead of other blue OLED technologies in terms of efficiency and lifetime[19-23]. However, the device characteristics of the blue TADF OLEDs and hyperfluorescence OLEDs have been rapidly advanced for the last couple of years although the lifetime of the TADF devices is still very short[24-28]. Therefore, more materials which can upgrade the device performances of TADF OLEDs and hyperfluorescence OLEDs are strongly demanded. In particular, an organic compound which can afford high efficiency and lifetime in the TADF OLEDs and hyperfluorescence OLEDs would be ideal in the search of high performance blue OLEDs. In this work, a molecular design protecting the donor and acceptor moieties of TADF emitters using peripheral alkyl branches were proposed as a strategy to resolve the low external quantum efficiency (EQE), poor color purity and doping concentration dependent color shift issues of TADF and hyperfluorescence OLEDs. Two TADF emitters with multiple alkyl branches surrounding the donor-acceptor core structure, 9,9'-(5-(4,6-bis(4-(tert-butyl)phenyl)1,3,5-triazin-2-yl)-1,3-phenylene)bis(3,6-di-tert-butyl-9H-carbazole) (tDCztTrz) and 9,9'-(5(4,6-bis(3,5-di-tert-butylphenyl)-1,3,5-triazin-2-yl)-1,3-phenylene)bis(3,6-di-tert-butyl-9H-

2

Journal Pre-proof carbazole) (tDCz2tTrz), were developed for universal use in deep blue TADF OLEDs and hyperfluorescence OLEDs. The tDCztTrz and tDCz2tTrz emitters showed a high EQE over 22%, a deep blue emission color of (0.15,0.20), and doping concentration independent emission color as TADF emitters, and provided high EQE over 13% in the hyperfluorescence OLEDs.

Results and discussion Among the many blue TADF emitters reported until now, triazine type blue TADF emitters are differentiated from other emitters in that they have a potential as both high efficiency and long lifetime blue emitters. However, the triazine type emitters have a drawback of strong intermolecular interaction with other molecules or by themselves, which shifts the emission color to long wavelength at high doping concentration or by doping in a highly polar host. Therefore, the color purity of the blue devices with the triazine type emitters is degraded in the TADF OLEDs. Moreover, it works poorly in the hyperfluorescence OLEDs due to significant Dexter energy transfer from the TADF emitter to a fluorescent emitter caused by close contact between planar triazine unit and fluorescent emitter. As the Dexter energy transfer degrades the EQE of the hyperfluorescence OLEDs due to triplet exciton quenching of the TADF sensitizer, the challenging issue of the Dexter energy transfer should be handled for high EQE in the hyperfluorescence OLEDs. Our idea of avoiding the intermolecular interaction and orbital overlap is to introduce bulky alkyl branches in the periphery of the donor-acceptor core structure of the TADF emitters to separate two molecules. The alkyl branch was a t-butyl unit, and the donor-acceptor core structure was 9,9'-(5-(4,6-diphenyl-1,3,5-triazin-2-yl)-1,3-phenylene)bis(9H-carbazole) (DCzTrz)[26]. Two DCzTrz derivatives, tDCztTrz and tDCz2tTrz, with multiple t-butyl units around the DCzTrz core were designed and synthesized. The tDCz2tTrz had an additional tbutyl unit in the phenyl unit of diphenyltriazine compared with the tDCztTrz. The multiple t3

Journal Pre-proof butyl units around the core may separate the TADF molecules by large size of the substituent, eventually weakening intermolecular interaction with other molecules or by themselves. 9,9'(5-(4,6-bis(4-(tert-butyl)phenyl)-1,3,5-triazin-2-yl)-1,3-phenylene)bis(9H-carbazole) (CztTrz) possessing the t-butyl substituents only in the acceptor moiety[29] and 9,9'-(5-(4,6-diphenyl1,3,5-triazin-2-yl)-1,3-phenylene)bis(3,6-di-tert-butyl-9H-carbazole) (tCzTrz) having the tbutyl substituents only in the donor moiety were used as reference materials to study the effect of peripheral t-butyl branches on the device performances and to establish a design rule of TADF emitters for high efficiency and long lifetime. The synthesis of the two emitters was conducted by a multi-step synthetic process including Grignard reaction of the t-butyl substituted phenyl unit and Suzuki coupling reaction of the tbutyl substituted carbazole as described in Scheme 1. Two emitters were prepared in good yields over 60% and general purification processes of column chromatography, recrystallization, and temperature gradient sublimation afforded highly pure TADF emitters. Purity level of the compounds identified by high performance column chromatography was over 99%. Quantum chemical calculation of the TADF emitters was carried out using Gaussian molecular simulation software equipped with B3LYP 6-31G basis set. Molecular calculation results of tDCztTrz and tDCz2tTrz are presented in Figure 1. Overall distribution of the highest occupied molecular orbital (HOMO) and the lowest unoccupied molecular orbital (LUMO) was not affected by the t-butyl branches as projected from the HOMO localization on the donor parts and LUMO localization on the acceptor parts. The HOMO and LUMO were effectively overlapped at the central phenyl unit of the two emitters. The HOMO and LUMO projected from the quantum chemical calculation results were determined using experimental measurement by cyclic voltammetry (CV). The HOMO/LUMOs of tDCztTrz, and tDCz2tTrz were -6.19/-3.48 eV and -6.17/-3.40 eV, respectively, from the oxidation and reduction data of the emitters in Figure 2. Similar 4

Journal Pre-proof HOMO levels were observed in the tDCztTrz and tDCz2tTrz emitters because the same donors were included in the molecular structure. However, the LUMO became shallow in the tDCz2tTrz by electron donating effect of two t-butyl units in the phenyl unit of diphenyltriazine. The measurement data are summarized in Table 1. Photophysical characterization of the two emitters was performed to study the TADF related properties. Firstly, photoluminescence (PL) analysis to determine the singlet energy (ES) and triplet energy (ET) was performed (Figure 3(a) and (b)). Room temperature PL measurement of fluorescence and low temperature PL measurement of phosphorescence provided the ES and ET of the TADF emitters. Solution samples with the TADF emitters dissolved in toluene were used to characterize the fluorescence and phosphorescence. The ES/ET values of tDCztTrz and tDCz2tTrz were 3.03/2.99 and 3.04/3.02 eV, respectively. There was little difference of the ES and ET values in the two emitters. A very small singlet-triplet energy gap (EST) of 0.04 and 0.02 eV was observed in the tDCztTrz and tDCz2tTrz emitters. Strong donor character of the t-butyl modified carbazole strengthening charge transfer (CT) property reduced the EST. The strong CT character of the tDCztTrz and tDCz2tTrz emitters was reflected in the ultraviolet-visible (UV-vis) absorption data exhibiting weak CT absorption from 350 nm to 420 nm. PL emission behavior of the emitters was further examined by characterizing transient PL decay from excited state. Transient PL characterization data of the tDCztTrz and tDCz2tTrz emitters are shown in Figure 4. Both tDCztTrz and tDCz2tTrz emitters can be categorized into typical TADF emitters considering the fast prompt fluorescence and slow delayed fluorescence in the transient PL decay. Excited state lifetimes for delayed fluorescence were 55.7 and 118.7 s in the tDCztTrz and tDCz2tTrz emitters, respectively. PL quantum yield (PLQY) of the two emitters was characterized by absolute PLQY measurement using an integrating sphere. The PLQY of tDCztTrz and tDCz2tTrz was 0.10 under oxygen, while it was increased to 0.54 and 0.59 under nitrogen, respectively. The dramatic increase of the 5

Journal Pre-proof PLQY under nitrogen reflects triplet exciton contribution to singlet emission, supporting the TADF character of the two emitters. Material characterization data of the TADF emitters are summed up in Table 1. The photophysical properties of the emitters characterized by PL analysis suggested that the tDCztTrz and tDCz2tTrz emitters are potentially good TADF emitters, which allowed the fabrication of TADF devices using the two emitters. The emitters were doped in the same bis[2-(diphenylphosphino)phenyl] ether oxide (DPEPO) host for full harvesting of triplet excitons by up-conversion process. Doping concentration of the emitters was optimized from 10% to 30%. Basic electrical and photometric data represented by current density and luminance are presented in Figure 5. As is clearly documented in several publications[30-33], gradual increase of current density and luminance by increased doping concentration was detected in the two devices by facilitated carrier hopping between TADF emitters at high doping concentration. EQE of the TADF devices is presented in Figure 6. In addition to the EQE of the tDCztTrz and tDCz2tTrz devices, the EQE data of tCzTrz, and CztTrz devices were also added for comparison in Table 2. Maximum EQE of the tDCztTrz and tDCz2tTrz devices was 22.6%, which was much higher than those of tCzTrz (17.4%) and CztTrz (14.5%) devices. This indicates that the t-butyl units only around the donor moiety or the acceptor moiety are not useful. Simultaneous protection of the donor and acceptor moieties using alkyl branches is critical to the EQE of the TADF devices. The rather high EQE of the TADF devices might be due to horizontal dipole orientation of the emitters. Doping concentration dependent electroluminescence (EL) spectra of the tDCztTrz and tDCz2tTrz devices are presented in Figure 7(a) and (b). Although the doping concentration was changed from 10% to 30%, there was little change of the EL spectra. This behavior is quite different from the device data of common TADF emitters showing doping concentration dependent red-shift of the EL spectrum[26,34,35]. As described in the material design concept, 6

Journal Pre-proof the peripheral t-butyl units would hinder intermolecular donor-acceptor interaction, prohibiting the EL spectrum change according to doping concentration. A blue color coordinate of (0.15, 0.20) was maintained at all doping concentrations. Hyperfluorescent OLEDs were also developed using the tDCztTrz and tDCz2tTrz emitters in comparison with tCzTrz and CztTrz emitters. The EQE of the hyperfluorescent OLEDs of the four emitters is plotted in Figure 8. Fluorescent blue emitter of the hyperfluorescent OLEDs was 2,5,8,11-tetra-tert-butylperylene (TBPe). The tDCztTrz and tDCz2tTrz based hyperfluorescent OLEDs showed high EQE of 13.7%, whereas the tCzTrz and CztTrz emitters exhibited EQE of 10.4% and 5.9%, respectively. The EQE was comparatively high in the tDCztTrz and tDCz2tTrz devices, suggesting the superiority of the alkyl branch approach for high EQE in both TADF and hyperfluorescent OLEDs. The EQE improvement in the hyperfluorescent OLEDs by the alkyl modifying design method was analyzed from the viewpoint of Dexter energy transfer from the TADF emitter to TBPe because main role of the alkyl branch would be to separate TADF emitters from TBPe. The Dexter energy transfer was indirectly investigated by monitoring delayed fluorescence of the TADF emitters. A well-known compound, 4,4′-bis(N-carbazolyl)-1,1′-biphenyl (CBP), with a singlet energy of 3.32 eV and a triplet energy of 2.49 eV[36] was doped in the TADF emitters to study the Dexter energy transfer. As the singlet energy transfer from the TADF emitters to CBP is difficult by high singlet energy of CBP, the change of the delayed fluorescence component can be correlated with triplet energy transfer. Transient PL decay data in Figure 9 apparently indicate that the delayed fluorescent component of CztTrz is largely suppressed by CBP doping, but that of tDCz2tTrz is weakly affected even after CBP doping. This result suggests that the Dexter energy transfer is effectively suppressed by peripheral t-butyl units surrounding the donor-acceptor core structure[37,38].

Conclusions 7

Journal Pre-proof In conclusion, two deep blue TADF emitters with peripheral alkyl branches surrounding the donor and acceptor moieties were synthesized to explore ideal design of deep blue TADF emitters for fluorescence and hyperfluorescence in terms of EQE. The TADF emitters with the peripheral alkyl branches both in the donor and acceptor units showed a high quantum efficiency above 22.0%, a deep blue color coordinate of (0.15, 0.20), and doping concentration independent emission color as TADF emitters, and 13.7% quantum efficiency in the TADF sensitized hyperfluorescent device. Therefore, the multiple alkyl modification method of both donor and acceptor units of TADF emitters can resolve the issues of TADF emitters for ideal device performances.

Experimental Synthesis 2,4-bis(4-(tert-butyl)phenyl)-6-chloro-1,3,5-triazine and 2-chloro-4,6-bis(3,5-di-tertbutylphenyl)-1,3,5-triazine were synthesized according to our previous work[29].

9,9'-(5-bromo-1,3-phenylene)bis(3,6-di-tert-butyl-9H-carbazole) Sodium hydride (6.22 g, 259 mmol) washed with n-hexane was added to dimethylformamide (DMF, 20 ml). 3,6-Di-tert-butyl-9H-carbazole (31.9 g, 114 mmol) dissolved in DMF (60 ml) was added to the solution dropwisely. After 30 min, 1-bromo-3,5-difluorobenzene (10.0 g, 51.8 mmol) was added and the mixture was refluxed overnight. The reaction was quenched with distilled water and the mixture was extracted with methylene chloride (MC). The crude product was purified by column chromatography and a white powder was obtained as a final product. Yield : 29.2 g, 79.1%, MS(APCI) 712.70 m/z . 1H NMR (500 MHz, CDCl3): δ 8.133 (s, 4H), 7.797 (s, 2H), 7.768 (s, 1H), 7.508-7.460 (m, 8H), 1.463 (s, 36H). LC/MS (m/z): found, 712.70 ([M + H]+); Calcd. for C46H51BrN2, 711.81. 8

Journal Pre-proof

9,9'-(5-(4,4,5,5-tetramethyl-1,3,2-dioxaborolan-2-yl)-1,3-phenylene)bis(3,6-di-tert-butyl9H-carbazole) 9,9'-(5-Bromo-1,3-phenylene)bis(3,6-di-tert-butyl-9H-carbazole) (5.0 g, 7.02 mmol), bis(pinacolato) diboron (2.14 g, 8.43 mmol), potassium acetate (2.76 g, 28.1 mmol) and [1,1′bis(diphenylphosphino)ferrocene]dichloropalladium(II) (0.15 g, 0.21 mmol) were dissolved in 1,4-dioxane (60 ml) and refluxed for 5 hours. The reaction mixture was extracted with MC and purified by column chromatography. A white powder was obtained as a product. Yield : 3.1 g, 58.2%, MS(APCI) 759.35 m/z . 1H NMR (500 MHz, CDCl3): δ 8.072 (s, 4H), 7.793 (s, 2H), 7.695 (s, 1H), 7.457 (d, 4H, J=8.5 Hz), 7.304 (d, 4H, J=8.0 Hz), 1.463 (s, 36H), 1.356 (s, 12H). LC/MS (m/z): found, 759.35 ([M + H]+); Calcd. for C52H63BN2O2, 758.88.

9,9'-(5-(4,6-bis(4-(tert-butyl)phenyl)-1,3,5-triazin-2-yl)-1,3-phenylene)bis(3,6-di-tert-butyl9H-carbazole) (tDCztTrz) 2,4-Bis(4-(tert-butyl)phenyl)-6-chloro-1,3,5-triazine (0.5 g, 1.32 mmol) and 9,9'-(5-(4,4,5,5tetramethyl-1,3,2-dioxaborolan-2-yl)-1,3-phenylene)bis(3,6-di-tert-butyl-9H-carbazole) (1.20 g, 1.58 mmol) were dried in vacuum and dissolved in THF (20 ml). Tetrakis(triphenylphosphine) palladium(0) (0.076 g, 0.066 mmol) and 2M potassium carbonate aqueous solution (10 ml) were added to the solution. The mixture was refluxed overnight and extracted with MC. The crude product was purified by column chromatography using an eluent of n-hexane/MC. After vacuum train sublimation, the product was obtained as a white powder. Yield : 0.94 g, 73.4%, MS(APCI) 977.58 m/z . 1H NMR (500 MHz, CDCl3): δ 9.033 (s, 2H), 8.650 (d, 4H, J=8.5 Hz), 8.188 (s, 4H), 7.998 (s, 1H), 7.559-7.499 (m, 12H), 1.486 (s, 36H), 1.373 (s, 18H). 13C NMR (125 MHz, CDCl3): δ 172.09, 170.60, 156.58, 143.55, 140.46, 9

Journal Pre-proof 140.35, 139.25, 133.43, 129.16, 128.19, 125.89, 125.60, 124.18, 123.88, 116.63, 109.45, 35.31, 35.00, 32.23, 31.39, 29.92. MS (m/z): found, 976.5765 ([FAB]+); Calcd. for C69H77N5, 976.3832.

9,9'-(5-(4,6-bis(3,5-di-tert-butylphenyl)-1,3,5-triazin-2-yl)-1,3-phenylene)bis(3,6-di-tertbutyl-9H-carbazole) (tDCz2tTrz) The synthetic route of tDCz2tTrz was the same as that of tDCztTrz. 2-Chloro-4,6-bis(3,5-ditert-butylphenyl)-1,3,5-triazine (0.6 g, 1.22 mmol) was used instead of 2,4-bis(4-(tertbutyl)phenyl)-6-chloro-1,3,5-triazine. The product was obtained as a yellowish white powder. Yield : 0.86 g, 64.7%, MS(APCI) 1089.40 m/z . 1H NMR (500 MHz, DMSO): δ 9.022 (s, 2H), 8.660 (s, 4H), 8.181 (s, 4H), 8.124 (s, 1H), 7.713-7.696 (m, 6H), 7.527 (d, 4H, J=9.0 Hz), 1.489 (s, 36H), 1.418 (s, 36H). 13C NMR (125 MHz, CDCl3): δ 172.64, 171.01, 151.48, 143.70, 140.36, 140.33, 138.81, 135.43, 127.32, 126.26, 124.30, 124.18, 124.02, 123.44, 116.60, 109.54, 35.26, 35.00, 32.23, 31.66. MS (m/z): found, 1088.6636 ([FAB]+); Calcd. for C77H93N5, 1088.5958.

Device fabrication and measurement Device fabrication and measurement were described in our previous work[29]. The optimized device structure was ITO/PEDOT:PSS (60 nm)/TAPC (20 nm)/mCP (10 nm)/DPEPO:tDCztTrz or tDCz2tTrz (25 nm)/TSPO1 (5 nm)/TPBi (40 nm)/LiF (1.5 nm)/Al (200nm). PEDOT:PSS is poly(3,4-ethylenedioxythiophene):poly(styrenesulfonate), TAPC is 4,4′-cyclohexylidenebis[N,N -bis(4-methylphenyl) aniline], mCP is 1,3-bis(Ncarbazolyl)benzene, TSPO1 is diphenylphosphine oxide-4-(triphenylsilyl)phenyl and TPBi is 1,3,5-tris(N-phenylbenzimidazole-2-yl)benzene. tDCztTrz and tDCz2tTrz were doped in DPEPO host as doping concentration of 10% to 30%. All layers was thermally evaporated under vacuum pressure of 1.0 × 10−6 torr. 10

Journal Pre-proof

References [1]. Y. Wang, Y. Hua, X. Wu, L. Zhang, Q. Hou, F. Guan, N. Zhang, S. Yin, X. Cheng, Org. Electron. 2008, 9, 692. [2]. J. Shi, C. W. Tang, Appl. Phys. Lett. 2002, 80, 3201. [3]. J. Y. Hu, Y. J. Pu, F. Satoh, S. Kawata, H. Katagiri, H. Sasabe, J. Kido, Adv. Funct. Mater. 2014, 24, 2064. [4]. M. Zhu, C. Yang, Chem. Soc. Rev. 2013, 42, 4963. [5]. J. Lee, H. F. Chen, T. Batagoda, C. Coburn, P. I. Djurovich, M. E. Thompson, S. R. Forrest, Nat. Mater. 2016, 15, 92. [6]. H. Sasabe, J. Takamatsu, T. Motoyama, S. Watanabe, G. Wagenblast, N. Langer, O. Molt, E. Fuchs, C. Lennartz, J. Kido, Adv. Mater. 2010, 22, 5003. [7]. J. Zhuang, W. Li, W. Su, Y. Liu, Q. Shen, L. Liao, M. Zhou, Org. Electron. 2013, 14, 2596. [8]. X. C. Hang, T. Fleetham, E. Turner, J. Brooks, J. Li, Angew. Chem. Int. Ed. 2013, 52, 6753. [9]. Y. Im, S. Y. Byun, J. H. Kim, D. R. Lee, C. S. Oh, K. S. Yook, J. Y. Lee, Adv. Funct. Mater. 2017, 27, 1603007. [10]. K. S. Yook, J. Y. Lee, Adv. Mater. 2012, 24, 3169. [11]. Q. Zhang, B. Li, S. Huang, H. Nomura, H. Tanaka, C. Adachi, Nat. Photon. 2014, 8, 326. [12]. S. Hirata, Y. Sakai, K. Masui, H. Tanaka, S. Y. Lee, H. Nomura, N. Nakamura, M. Yasumatsu, H. Nakanotani, Q. Zhang, K. Shizu, H. Miyazaki, C. Adachi, Nat. Mater. 2015, 14, 330. [13]. W. Song, T. Kim, J. Y. Lee, Y. Lee, H. Jeong, J. Ind. Eng. Chem. 2018, 68, 350. 11

Journal Pre-proof [14]. S. K. Jeon, H. L. Lee, K. S. Yook, J. Y. Lee, Adv. Mater. 2019, 31, 1803524. [15]. T. Hatakeyama, K. Shiren, K. Nakajima, S. Nomura, S. Nakatsuka, K. Konoshita, J. Ni, Y. Ono, T. Ikuta, Adv. Mater. 2016, 28, 2777. [16]. H. Nakanotani, T. Higuchi, T. Furukawa, K. Masui, K. Morimoto, M. Numata, H. Tanaka, Y. Sagara, T. Yasuda, C. Adachi, Nat. Commun. 2014, 5, 4016. [17]. S. Y. Byeon, D. R. Lee, K. S. Yook, J. Y. Lee, Adv. Mater. 2019, 31, 1803714. [18]. J. S. Jang, S. H. Han, H. W. Choi, K. S. Yook, J. Y. Lee, Org. Electron. 2018, 59, 236. [19]. Y. Zhang, J. Lee, S. R. Forrest, Nat. Commun. 2014, 5, 5008. [20]. J. Lee, C. Jeong, T. Batagoda, C. Coburn, M. E. Thompson, S. R. Forrest, Nat. Commun. 2017, 8, 15566. [21]. M. Kim, J. Y. Lee, Adv. Funct. Mater. 2014, 24, 4164. [22]. H. Cho, J. Lee, J. I. Lee, N. S. Cho, J. H. Park, J. Y. Lee, Y. Kang, Org. Electron. 2016, 34, 91. [23]. H. Shin, J. H. Lee, C. K. Moon, J. S. Huh, B. Sim, J. J. Kim, Adv. Mater. 2016, 28, 4920. [24]. D. Zhang, M. Cai, Y. Zhang, D. Zhang, L. Duan, Mater. Horiz. 2016, 3, 145. [25]. L. S. Cui, Y. L. Deng, D. P. K. Tsang, Z. Q. Jiang, Q. Zhang, L. S. Liao, C. Adachi, Adv. Mater. 2016, 28, 7620. [26]. M. Kim, S. K. Jeon, S. H. Hwang, J. Y. Lee, Adv. Mater. 2015, 27, 2515. [27]. D. H. Ahn, S. W. Kim, H. Lee, I. J. Ko, D. Karthik, J. Y. Lee, J. H. Kwon, Nat. Photon. 2019. [28]. H. Noda, H. Nakanotani, C. Adachi, Sci. Adv. 2018, 4, eaao6910. [29]. Y. J. Kang, J. Y. Lee, Dyes Pigm. 2017, 138, 176. [30]. R. J. Holmes, B. W. D'Andrade, S. R. Forrest, X. Ren, J. Li, M. E. Thompson, Appl. Phys. Lett. 2003, 83, 3818. [31]. Y. J. Cho, K. S. Yook, J. Y. Lee, Adv. Mater. 2014, 26, 6642. [32]. C. W. Seo, J. H. Yoon, J. Y. Lee, Org. Electron. 2012, 13, 341. 12

Journal Pre-proof [33]. J. Yang, J. Shen, J. Appl. Phys. 1998, 84, 2105. [34]. Y. H. Park. Y. Kim, H. Sohn, K. An, J. Phys. Org. Chem. 2012, 25, 207. [35]. M. Kim, S. K. Jeon, S. H. Hwang, S. S. Lee, E. Yu, J. Y. Lee, Chem. Commun. 2016, 52, 339. [36]. V. Jankus, C. Winscom, A. P. Monkman, J. Chem. Phys. 2009, 130, 074501. [37]. J. H. Park, S. H. Back, H. M. Jeong, D. J. Ahn, Macromol. Res. 2018, 26, 1099. [38]. H. J. Jang, J. Y. Lee, J. Kwak, D. Lee, J.-H. Park, B. Lee, Y. Y. Noh, J. Inf. Disp. 2019, 20, 1.

13

Journal Pre-proof List of tables Table 1. Electrochemical and photophysical properties of tDCztTrz and tDCz2tTrz. Table 2. Quantum efficiency, power efficiency and color coordinate of tDCztTrz, tDCz2tTrz, tCzTrz and CztTrz

14

Journal Pre-proof

Table 1. Electrochemical and photophysical properties of tDCztTrz and tDCz2tTrz Electrochemical properties

Photophysical properties

HOMO (eV)

LUMO (eV)

Band gap (eV)

ES (eV)[a]

ET (eV)[b]

ΔEST (eV)

PLQY (%)

Delayed fluorescence lifetime (μs)

tDCztTrz

-6.19

-3.48

2.71

3.03

2.99

0.04

54

55.7

tDCz2tTrz

-6.17

-3.40

2.77

3.04

3.02

0.02

59

118.7

[a] Singlet energies were calculated from edge of fluorescent spectra in 1 wt% doped polystyrene film. [b] Triplet energies were calculated from edge of phosphorescent spectra in tetrahydrofuran solution at 77K.

15

Journal Pre-proof Table 2. Quantum efficiency, power efficiency and color coordinate of tDCztTrz, tDCz2tTrz, tCzTrz and CztTrz Quantum efficiency (%)

Power efficiency (lm/W)

Maximum

100 cd/m2

Maximum

100 cd/m2

tDCztTrz

22.6

15.5

25.6

11.5

(0.15, 0.20)

tDCz2tTrz

22.6

17.6

27.3

15.2

(0.15, 0.20)

tCzTrz

17.4

14.5

27.6

16.2

(0.17, 0.28)

CztTrz

14.5

3.3

8.7

1.0

(0.15, 0.12)

16

Color coordinate

Journal Pre-proof List of figures Scheme 1. Synthetic procedure of tDCztTrz and tDCz2tTrz Figure 1. HOMO and LUMO distribution of tDCztTrz and tDCz2tTrz Figure 2. Cyclic voltammetry (CV) curves of tDCztTrz and tDCz2tTrz Figure 3. Photoluminescence (PL) spectra of (a) tDCztTrz and (b) tDCz2tTrz Figure 4. Transient PL curves of tDCztTrz and tDCz2tTrz Figure 5. Current density and luminance plots of (a) tDCztTrz and (b) tDCz2tTrz Figure 6. External quantum efficiency (EQE) plots of (a) tDCztTrz and (b) tDCz2tTrz Figure 7. Electroluminascence (EL) spectra of (a) tDCztTrz and (b) tDCztTrz Figure 8. EQE of the hyperfluorescent OLEDs of tDCztTrz, tDCz2tTrz, tCzTrz and CztTrz Figure 9. Transient PL curves of (a) CztTrz and (b) tDCz2tTrz doped with CBP 1wt%

17

Journal Pre-proof H N O

Br

Br

F

F

NaH, DMF

N

O

B

N

N

Br

N N

Cl

O

O

O

N

Cl

O

KOAc, PdCl2(dppf) 1,4-Dioxane

N

O

N

B B

N Cl

Mg, I2, THF

N N

N K2CO3, Pd(PPh3)4 THF/H2O

Cl

N N

N N

tDCztTrz

O

N

Cl N Cl

O

N

Br

N N

B

N Cl

Mg, I2, THF

N N

N K2CO3, Pd(PPh3)4 THF/H2O

Cl

N N

N N

tDCz2tTrz

Scheme 1

18

B

O

N

Journal Pre-proof

Figure 1

19

Journal Pre-proof

Figure 2

20

Journal Pre-proof (a)

(b)

Figure 3

21

Journal Pre-proof

Figure 4

22

Journal Pre-proof (a)

(b)

Figure 5

23

Journal Pre-proof (a)

(b)

Figure 6

24

Journal Pre-proof (a)

(b)

Figure 7

25

Journal Pre-proof

Figure 8

26

Journal Pre-proof (a)

(b)

Figure 9

27

Journal Pre-proof

Declaration of interests ☒ The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper. ☐The authors declare the following financial interests/personal relationships which may be considered as potential competing interests:

Journal Pre-proof

-

Deep blue TADF and TADF sensitized fluorescent organic light-emitting diodes

-

High efficiency in the blue organic light-emitting diodes by t-butyl modified emitter

-

Molecular design of the sensitizer with t-butyl blocking groups for high efficiency