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Coumarin-based Thermally Activated Delayed Fluorescence Emitters with High External Quantum Efficiency and Low Efficiency Roll-off in the Devices Jia-Xiong Chen, Wei Liu, Cai-Jun Zheng, Kai Wang, Ke Liang, Yizhong Shi, Xue-Mei Ou, and Xiao-Hong Zhang ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.6b15816 • Publication Date (Web): 20 Feb 2017 Downloaded from http://pubs.acs.org on February 22, 2017

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ACS Applied Materials & Interfaces

Coumarin-based Thermally Activated Delayed Fluorescence Emitters with High External Quantum Efficiency and Low Efficiency Roll-off in the Devices Jia-Xiong Chen,† Wei Liu,‡,§ Cai-Jun Zheng,*,ǁ,‡ Kai Wang,†,‡ Ke Liang,† Yi-Zhong Shi,† Xue-Mei Ou,*,† and Xiao-Hong Zhang,*,†

†

Institute of Functional Nano & Soft Materials (FUNSOM), Jiangsu Key Laboratory

for Carbon-Based Functional Materials & Devices, Soochow University, Suzhou, Jiangsu, 215123, P.R. China ǁSchool of Optoelectronic Information, University of Electronic Science and Technology of China (UESTC), Chengdu, 610054, P.R. China ‡

Nano-organic Photoelectronic Laboratory and Key Laboratory of Photochemical

Conversion and Optoelectronic Materials, Technical Institute of Physics and Chemistry, Chinese Academy of Sciences, Beijing, 100190, P.R. China §

College of Materials Science and Engineering, Beijing Institute of Technology,

Beijing, 100081, P.R. China

KEYWORDS: thermally activated delayed fluorescence, coumarin derivatives, organic light emitting diodes, high external quantum efficiency, low efficiency roll-off. 1

ACS Paragon Plus Environment


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ABSTRACT: Thermally activated delayed fluorescence (TADF) emitters have attracted much interest for their great applications in organic light emitting diodes (OLEDs), but the TADF OLEDs are limited by large efficiency roll-offs. In this study, we

report

two

coumarin-based

3-methyl-6-(10H-phenoxazin-10-yl)-1H-isochromen-1-one

TADF (PHzMCO)

emitters, and

9-(10H-phenoxazin-10-yl)-6H-benzo[c]chromen-6-one (PHzBCO), with relatively high photoluminescence quantum yields (PLQYs) and extremely small singlet-triplet splittings. OLEDs using these two TADF compounds as the emitters respectively demonstrate high external quantum efficiencies of 17.8% for PHzMCO and 19.6% for PHzBCO, which are the highest among the reported coumarin derivatives-based OLEDs. More importantly, these devices based on PHzMCO and PHzBCO remained 10.3% and 12.9% at 10000 cd m-2, respectively, showing relatively low efficiency roll-offs at high brightness. These results reveal that the TADF emitters with high PLQYs can effectively reduce the efficiency roll-off in the devices.

1. INTRODUCTION Organic light emitting diodes (OLEDs) are considered as one of the most promising candidates for display and light applications, due to their high efficiency, flexibility, high brightness and full-color emission.1-3 In the devices, electrogenerated excitons are generated as singlet and triplet excitons with a ratio of 1:3.4-5 Traditional fluorescent OLEDs can only harvest singlet excitons and result in limitations of 25% 2


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internal quantum efficiency (IQE). Thus, in the past decade, phosphorescent OLEDs are always in the spotlight for their full exciton utilization. However, the noble metal ions used in phosphorescent emitters significantly increase their costs and further hindered their practical application.6-8 In 2012, Adachi et al. reported a new pathway of thermally activated delayed fluorescence (TADF) to achieve full exciton utilization without the assistance of noble metal ions.9 With extremely small singlet-triplet splittings (∆ESTs), the non-radiative triplet excitons can be converted to radiative singlet excitons via efficient reverse intersystem crossing (RISC) process on TADF emitters. Thus a 100% IQE can be archived in the devices.10 Till now, it is common to construct TADF emitters using the D-A (electron-donor and electron-acceptor) structure molecules, as they can minimize the overlap of the highest occupied molecular orbital (HOMO) and the lowest unoccupied molecular orbital (LUMO) in the molecules.1, 11-13 Based on this strategy, plenty of highly efficient TADF emitters have been reported. And the devices based on these TADF emitters successfully realized extremely high maximum external quantum efficiencies (EQEs) over 20%.14-17 However, most of these devices suffer serious efficiency roll-offs, exhibiting quite low EQEs at a practical brightness of 1,000 cd m-2.17-18 This is the major challenge for TADF emitters. It was considered that such high efficiency roll-offs of these devices should be mainly ascribed to the low photoluminescence quantum yields (PLQYs) caused by the isolated structure of the D-A structure molecules.19-20 Thus, constructing the TADF emitters with high PLQYs and extremely small ∆ESTs simultaneously is clearly one of the possible 3



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approaches to address this issue. As the naturally occurring and synthetic compounds, coumarin derivatives are widely reported for different applications due to their intrinsic high PLQYs.21-24 Especially in OLEDs, they have even been used as the initial electroluminescent (EL) materials.25-26

In

1989,

Tang

et al.

reported

3-(2-benzothiazolyl)-7-diethylaminocoumarin

a coumarin

(coumarin

540)

as

derivative an

of

efficient

fluorescent emitter in the device.27 Since then, the coumarin derivatives are extensively investigated as the emitters in OLEDs. In 2003, Chen et al. reported three coumarin derivatives with relatively high PLQYs of 0.48, 0.77 and 0.86 as the fluorescent emitters in the devices.28 In 2013, our group developed a coumarin derivative with a high PLQY of 0.79 as the blue emitter for hybrid white OLEDs.29 In 2016, Patil et al. synthesized 7-(9H-carbazol-9-yl)-4-methylcoumarin with a high PLQY of 0.70 in a doped matrix, and used it for deep-blue OLED.30 These results further proved the naturally high PLQYs of the coumarin derivatives, and indicate they should be the excellent candidates to build the TADF emitters with high PLQYs. In

this

work,

we

report

two

novel

3-methyl-6-(10H-phenoxazin-10-yl)-1H-isochromen-1-one

coumarin

derivatives,

(PHzMCO)

and

9-(10H-phenoxazin-10-yl)-6H-benzo[c]chromen-6-one (PHzBCO), as the TADF emitters. These two coumarin derivatives are composed of D-A structure with phenoxazine as the electron-donor and benzopyrone as the electron-acceptor. In consequence of the similar molecular structures, these two coumarin derivatives exhibit similar photophysical properties, HOMO, LUMO distributions and close 4


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energy levels. Both materials show high PLQYs of 0.47 and 0.52 and extremely small ∆ESTs of 0.018 and 0.006 eV for PHzMCO and PHzBCO respectively. As expected, these two coumarin derivatives TADF emitters show high maximum EQEs of 17.8% for PHzMCO and 19.6% for PHzBCO in the devices. Moreover, both two devices exhibit relatively low efficiency roll-off at high brightness (remaining 10.3% and 12.9% at 10000 cd m-2, respectively). These results not only reveal the feasibility of the coumarin derivatives as the TADF emitters, but also provide an approach to avoid the high efficiency roll-off in TADF OLEDs.

2. RESULTS AND DISCUSSION 2.1 Synthesis. The synthesis routes of two compounds are shown in Scheme 1. For PHzMCO, the acidamide unit was first synthesized by 4-bromo-2-iodobenzoic acid (M1) and aniline.

Then

the

resulting

acidamide

(M2)

was

converted

to

6-bromo-3-methyl-1H-isochromen-1-one (M3) via a cyclization process by adding pentane-2,4-dione. While for PHzBCO, the benzocoumarin unit was ﬁrst synthesized as benzene ring substituted the iodine atom on M1 via Suzuki reaction. Then the resulting 5-bromo-[1,1'-biphenyl]-2-carboxylic acid (M4) was self-cyclized to form 9-bromo-6H-benzo[c]chromen-6-one (M5). Finally, both target TADF materials were synthesized via Buchwald-Hartwig cross-coupling reaction between M3 (or M5) and the electron donor unit phenoxazine. The structures of PHzMCO and PHzBCO are further confirmed by mass spectroscopy (MS) and nuclear magnetic resonance (NMR) 5



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spectroscopy.

2.2 Theoretical calculations and electrochemical properties. To estimate the frontier molecular orbitals and the ∆EST values of the designed molecules, density functional theory (DFT) was performed at the B3LYP/6-31G (d) level for two compounds. As shown in Figure 1, the HOMOs of PHzMCO and PHzBCO are mainly localized on the electron-donating phenoxazine groups, and LUMOs are distributed over the electron-deficient benzopyrone units, showing separation clearly. The theoretical ∆EST values are calculated to be 0.03 and 0.01 eV for PHzMCO and PHzBCO, respectively, suggesting the efficient RISC can be achieved in both emitters. The electrochemical properties of PHzMCO and PHzBCO were then investigated by cyclic voltammetry (shown in Figure S1 and Table 1). The HOMO and LUMO energy levels are obtained from the onset of oxidation and reduction of two compounds with respect to that of ferrocene.31 The HOMO levels are calculated to be ˗5.28 and ˗5.38 eV for PHzMCO and PHzBCO, respectively; while LUMO levels are calculated to be -2.82 and -2.97 eV, respectively. The lower HOMO and LUMO levels of PHzBCO are attributed to the conjugation enhancement of the additional phenyl group.

2.3 Photophysical Properties. The room-temperature UV-Vis absorption and emission spectra of PHzMCO and 6


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PHzBCO in toluene are shown in Figure 2 and listed in Table 1. Both two materials exhibit broad absorptions from 366 to 450 nm and a locally excited transition absorption from 290 to 360 nm. The broad absorption is attributed to the intramolecular charge transfer transition from phenoxazine group to coumarin group. The fluorescence and phosphorescence spectra of 8 wt% PHzMCO and PHzBCO doped in m-bis(N-carbazolyl)benzene (mCP) film at 77 K are shown in Figure S2. Accordingly, the ∆ESTs of two compounds are estimated to be 0.018 eV for PHzMCO and 0.006 eV for PHzBCO, similar with that obtained from the DFT calculations. The extremely small ∆EST will benefit the up-conversion from non-radiative triplet excitons to radiative singlet excitons and indicate that two molecules may possess highly efficient TADF properties. To further confirm the TADF behaviors of two compounds, the transient PL decays spectra were measured on 8 wt% PHzMCO or PHzBCO doped mCP film. As shown in Figure 3, a prompt decay with a lifetime of 2.8 ns and a delayed decay with a lifetime of 17.9 µs are obtained for PHzMCO at 300 K, while the PHzBCO-doped film exhibits a prompt lifetime of 3.3 ns and a delayed lifetime of 9.3 µs at the same condition. In addition, both the delayed lifetime of PHzMCO and PHzBCO film gradually decrease with the increasing temperature from 200 K to 300 K. The enhanced delayed decays reveal that the up-conversion of excitons from the lowest triplet excited state (T1) to the lowest singlet excited states (S1) increases with increasing temperature. The observed results further confirm that PHzMCO and PHzBCO are TADF materials. 7



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The PL spectra of PHzMCO or PHzBCO doped mCP films were further measured with doping ratios from 4% to 16% and their corresponding PLQYs were measured by using integrating sphere as well. As listed on Table S1 and shown in Figure S3, with the increasing doping concentrations, the peaks of PL spectra are slightly red shift from 490.8 to 496.8 nm for PHzMCO and from 495.8 to 504.0 nm for PHzBCO, which should be ascribed to the increased intermolecular interactions between the dopants. The PLQYs of the films first raise then fall with the increasing doping concentrations for both the emitters. Both PHzMCO and PHzBCO doped mCP films show highest PLQYs of 0.47 and 0.52 at 8 wt% doping ratios. The trend can be attributed to the interaction between the inadequate utilization of excitons under low doping concentration and evident exciton quenching under high doping concentration. These ideal PLQY values are comparable and even higher than some other coumarin derivatives such as coumarin 6 (PLQY=0.31 measured under same condition),23 indicating the high PLQY characteristics of TADF coumarin derivatives are well maintained.

2.4 Electroluminescence Properties. To investigate the EL properties of two compounds, multilayer devices were fabricated with the optimized structures of ITO/MoO3 (1 nm)/TAPC (35 nm)/TCTA (10 nm)/mCP:x wt% PHzMCO or PHzBCO (20 nm)/TmPyPb (45 nm)/ LiF (1 nm)/Al. The doping ratios are further optimized from 4 to 16 wt%. In both devices, ITO (indium tin oxide) and LiF/Al were used as the anode and the cathode; MoO3 was the 8


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hole-injection layer; TAPC (1,1-Bis[4-[N,N-di(p-tolyl)amino]phenyl]-cyclohexane) and TmPyPb (1,3,5-tri[(32pyridyl)phen-3-yl]benzene) were the hole-transporting layer

and

the

electron-transporting

layer,

respectively;

TCTA

(4,4’,4’’-tris(carbazol-9-yi)triphenylamine) was the exciton-blocking layer. As shown in Figure S4, with the doping ratio increasing from 4 to 16 wt%, both the emitters exhibit slightly red-shift EL spectra, similar with their PL spectra. The EQEs of all devices are summarized in Table S1. Both the emitters show the highest EQE with a doping ratio of 8 wt%, which is consistent with the PLQY results. With a low doping ratio of 4 wt%, relatively low EQEs of 12.1% for PHzMCO and 16.5% for PHzBCO are obtained, which should be ascribed to the inadequate exciton utilization under low doping concentration. Whereas with high doping ratios of 12 and 16 wt%, the efficiency declines should be caused by the interaction and aggregation between PHzMCO and PHzBCO molecules in the EML.32 The EL characteristics of two 8 wt%-doping OLEDs are presented in Figure 4 and summarized in Table S2. Two coumarin-based devices display green emissions with the peaks at 508 and 520 nm and the CIE coordinates of (0.26, 0.50) and (0.32, 0.50) for PHzMCO and PHzBCO, respectively. And the EL spectra of the two devices show little change with the increased luminance, revealing the good stability of the emitters. Based on the same device structure, the device based on PHzMCO obtains a maximum luminance of 22000 cd m-2 at 8.3 V, a maximum current efficiency (CE) of 52.07 cd A-1, a maximum power efficiency (PE) of 48.01 lm W-1 and a maximum EQE of 17.8%. While, the PHzBCO-based device exhibits even better performance 9



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with a maximum luminance of 35560 cd m-2 at 8.2 V and the maximum CE, PE and EQE of 61.23 cd A-1, 60.09 lm W-1 and 19.6%, respectively. These high efficiencies confirm the excellent TADF characteristics of the two emitters. And the improved properties of the PHzBCO-based device may be attributed to the higher PLQY of PHzBCO than that of PHzMCO. Owning to the contribution of the high PLQYs as well as the small ∆ESTs, a high ratio of original singlet excitons will radiatively decay and avoid intersystem crossing (ISC) to triplet excitons; and original triplet excitons will up-convert to singlet excitons via effective RISC process, as well as the ISC triplet excitons, decreasing the aggregation quenching and triplet-triplet annihilation at high exciton concentration.32 Thus, these two devices exhibit low EQE roll-off at high brightness. The EQEs of the PHzMCO and PHzBCO-based devices slightly decreased to 15.3% and 17% at the luminance of 1000 cd m-2. And even at the luminance of 10000 cd m-2, the EQEs remain as high as 10.3% and 12.9% for PHzMCO and PHzBCO, respectively. As listed in Table 2, such low efficiency roll-offs are quite remarkable for TADF OLEDs. And this work also proved the TADF emitters with high PLQYs and extremely small ∆ESTs can effectively avoid the high efficiency roll-off in TADF OLEDs. Further investigations were also carried out to clarify the origins of the low efficiency roll-offs for our devices. The transient lifetimes of triplet excited states were measured for two emitters at 100 K. The low temperature can suppress the RISC process from triplet to singlet states. As shown in Figure S5, T1 lifetimes of 384 µs and 474 µs are obtained at 100 K for PHzMCO and PHzBCO, respectively. These 10


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values are much shorter than the common T1 lifetimes of triplet–triplet annihilation (TTA)-induced delayed fluorescence (in the microsecond range),33 revealing that TTA process can be effectively suppressed in the PHzMCO and PHzBCO-based devices and efficiency roll-offs will be decreased. Moreover, hole only devices (HODs) and electron only devices (EODs) were subsequently fabricated to study the exciton transport and formation in the EML with the structure of ITO/MoO3 (12 nm)/ mCP:8 wt% PHzMCO or PHzBCO (50 nm)/MoO3 (12 nm)/Al and ITO/Al (40 nm)/LiF (1 nm)/ mCP:8 wt% PHzMCO or PHzBCO (50 nm)/LiF (1 nm)/Al, respectively. As shown in Figure S6, although the current densities of HODs are still slightly higher than that of EODs for both EMLs due to the intrinsic hole transporting properties of mCP host,32,34 the electron transporting properties of the EMLs have been remarkably improved via doping either PHzMCO or PHzBCO. These better charge balances can effectively extend the exciton recombination area and decrease the excitons concentrations in both EMLs, resulting in a decline of exciton-exciton and exciton-polaron annihilations33 and finally realizing extremely small efficiency roll-offs in both devices.

3. CONCLUSION In conclusion, two TADF emitters (PHzMCO and PHzBCO) are developed based on coumarin derivatives with high PLQYs of 0.47 and 0.52 and small ∆ESTs of 0.018 and 0.006 eV, respectively. The OLEDs using PHzMCO and PHzBCO as the emitters exhibit maximum EQE/PE/CE of 17.8%/52.07 cd A-1/48.01 lm W-1 and 11



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19.6%/61.23 cd A-1/60.09 lm W-1 with the CIE coordinates of (0.26, 0.50) and (0.32, 0.50), respectively. Moreover, relatively low EQE efficiency roll-offs are realized in the devices, remaining EQEs of 10.3% and 12.9% at 10000 cd m-2 for PHzMCO and PHzBCO, respectively. The results prove the bright prospects of coumarin derivatives on TADF emitters, and reveal that emitters with high PLQY as well as extremely small ∆EST can remarkably decrease the efficiency roll-offs in TADF OLEDs.

4. EXPERIMENTAL SECTION 4-bromo-2-iodo-N-phenylbenzamide (M2). A solution of 4-bromo-2- iodobenzoic acid (1.63 g, 5 mmol) in anhydrous dichloromethane (50 mL) was slowly added sulfuryl dichloride (1.18 g, 10 mmol) under argon. Subsequently, anhydrous DMF (0.6 mL) was added to the solution and stirring at room temperature for 3h. Then the solvent was removed in vacuo, and the residue was dissolved in anhydrous dichloromethane (60 mL). Next, aniline (0.7 g, 7.5 mmol) and NEt3 (1 g, 10 mmol) were added, and the reaction mixture was continued stirring at room temperature for 1h. After completion of the reaction, the solution was washed with saturated aqueous NaHCO3. The organic layer was dried over Na2SO4, evaporated, and purified by column chromatography to give a solid product (1.62 g, 81%): 1H NMR (400 MHz, DMSO) δ 10.45 (s, 1H), 8.16 (s, 1H), 7.76 - 7.67 (m, 3H), 7.44 (d, J = 8.1 Hz, 1H), 7.36 (t, J = 7.7 Hz, 2H), 7.12 (t, J = 7.4 Hz, 1H); MS (EI) m/z: [M]+ calced for C13H9BrINO 400.89, found 400.98.

6-bromo-3-methyl-1H-isochromen-1-one (M3)., M2 (1.3 g, 4 mmol), 1,3-diketone 12


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(0.5 mL, 4.8 mmol), copper(I) iodide (76 mg, 0.4 mmol), cesium carbonate (2.6 g, 4 mmol), and DMSO (20 mL) were added into a round-bottom flask (50 mL). The mixture was stirred at 100 °C for 30min under N2. Upon completion, water and dichloromethane were added to the flask. The organic layer were dried over Na2SO4, evaporated, and the crude product was purified by column chromatography on silica gel to give the desired product as a white solid (0.55 g, 58.33%): 1H NMR (600 MHz, CDCl3) δ 8.09 (d, J = 8.5 Hz, 1H), 7.55 (dd, J = 8.5, 1.8 Hz, 1H), 7.50 (d, J = 1.7 Hz, 1H), 6.18 (s, 1H), 2.29 (d, J = 0.4 Hz, 3H); MS (EI) m/z: [M]+ calced for C10H7BrO2 237.96, found 238.06.

3-methyl-6-(10H-phenoxazin-10-yl)-1H-isochromen-1-one (PHzMCO). M3 (0.48 g, 2 mmol), phenoxazine (0.44 g, 2.4 mmol), palladium (II) acetate (22.4 mg, 0.1 mmol) and triphenylphosphine (80 mg, 0.3 mmol) and cesium carbonate (1.32 g, 4 mmol) in 40 mL of toluene was added into a round-bottom flask (50 mL). Then, the mixture was stirred at 110 °C for 24 h. After completion of the reaction, water and dichloromethane were added to the cooled mixture. The organic layer was separated, and dried over Mg2SO4, and concentrated in vacuo. the residue solid was purified by column chromatography to give the product as green solid (0.48g, 70.3%): 1H NMR (400 MHz, CDCl3) δ 8.49 (d, J = 8.4 Hz, 1H), 7.45 (dd, J = 8.4, 1.9 Hz, 1H), 7.39 (d, J = 1.7 Hz, 1H), 6.79 - 6.69 (m, 4H), 6.65 (dd, J = 11.5, 5.2 Hz, 2H), 6.30 (s, 1H), 6.00 (d, J = 7.9 Hz, 2H), 2.34 (s, 3H); 13C NMR (151 MHz, CD2Cl2) δ 162.28, 156.18, 140.87, 132.99, 130.25, 123.73, 119.95, 113.99, 110.46, 103.41, 19.93. MS (EI) m/z: 13



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[M]+ calced for C22H15NO3 341.11, found 341.19.

5-bromo-[1,1'-biphenyl]-2-carboxylic acid (M4). 4-bromo-2-iodobenzoic acid (1.63 g, 5 mmol), phenylboronic acid (0.67 g, 5.5 mmol), and LiOH (0.27 g, 11 mmol) in degassed N-methyl-2-pyrrolidone/H2O (20 ml/20 ml) were stirred under nitrogen. Then Pd2(dba)3·CHCl3 (0.15 g, 0.15 mmol) was added, and the mixture was heated to 65 °C for 24 h. After cooling to room temperature, the reaction mixture was subjected to an acidic aqueous, followed by extraction with tert-butyl ether, and the organic layer was concentrated in vacuo to give an off-white residue which was purified by flash chromatography on silica gel using CH2Cl2 as eluent. 1H NMR (400 MHz, DMSO) δ 12.82 (s, 1H), 7.86 - 7.72 (m, 2H), 7.69 - 7.55 (m, 1H), 7.50 (dd, J = 10.2, 4.7 Hz, 1H), 7.45 - 7.33 (m, 4H). MS (EI) m/z: [M]+ calced for C13H9BrO2 275.98, found 276.06.

9-bromo-6H-benzo[c]chromen-6-one (M5). M4 (0.83 g, 3 mmol) and K2S2O8 (2.43 g, 9 mmol) were added into a 100 mL round-bottom flask in atmosphere, followed by addition of H2O/MeCN (30 mL, 1:1) mixture. Then the reaction mixture was heated to 50 °C for 18 h.

After reaction was complete, a saturated aqueous solution of

NaHCO3 was added to the reaction mixture, and extracted with ethyl acetate (3×30 mL). The combined organic solvent was concentrated in vacuo and purified by column chromatography to produce the desired product. 1H NMR (400 MHz, DMSO) δ 8.73 (d, J = 1.8 Hz, 1H), 8.46 (dd, J = 8.0, 1.4 Hz, 1H), 8.16 (d, J = 8.5 Hz, 1H), 14


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7.88 (dd, J = 8.5, 1.8 Hz, 1H), 7.61 (dd, J = 7.1, 1.3 Hz, 1H), 7.43 (ddd, J = 11.9, 6.7, 2.6 Hz, 2H). MS (EI) m/z: [M]+ calced for C13H7BrO2 273.96, found 274.09.

9-(10H-phenoxazin-10-yl)-6H-benzo[c]chromen-6-one (PHzBCO). Using a similar synthesis procedure for PHzMCO by a Buchwald-Hartwig cross coupling reaction, green solid (0.56g, 74.3%): 1H NMR (300 MHz, DMSO) δ 8.64 (s, 1H), 8.49 (dd, J = 15.6, 8.0 Hz, 2H), 7.73 (dd, J = 8.4, 1.4 Hz, 1H), 7.61 (t, J = 7.3 Hz, 1H), 7.48 (d, J = 7.9 Hz, 1H), 7.39 (t, J = 7.5 Hz, 1H), 6.84 - 6.65 (m, 6H), 6.01 (d, J = 7.6 Hz, 2H). 13

C NMR (75 MHz, DMSO) δ 160.22, 151.34, 145.41, 143.69, 138.14, 133.75,

131.74, 125.69, 125.31, 124.81, 124.29, 122.50, 121.07, 117.81, 116.00, 114.06. MS (EI) m/z: [M]+ calced for C25H15NO3 377.11, found 377.11.

AUTHOR INFORMATION Corresponding Author *E-mail: [email protected] *E-mail: [email protected] *E-mail: [email protected]

Supporting Information Descriptions of general methods for this work, cyclic voltammograms and fluorescence/phosphorescence spectra of PHzMCO and PHzBCO, PLQY/EQE and PL/EL spectra of PHzMCO and PHzBCO at different concentrations, transient PL 15



1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

decay curve of 8 wt% PHzMCO and PHzBCO doped mCP film at 100K, HODs and EODs for PHzMCO and PHzBCO, and the 1H NMR spectra of the intermediates and the emitters are supplied as Supporting Information. This material is available free of charge via the Internet at http://pubs.acs.org.

Notes The authors declare no competing financial interest.

Acknowledgements This work was supported by the National Key Research & Development Program (Grant No. 2016YFB0401002), the National Natural Science Foundation of China (Grant No. 51533005, 51373190), Jiangsu Provincial Natural Science Foundation (Grant No. BK20160308), Collaborative Innovation Center of Suzhou Nano Science & Technology, the Priority Academic Program Development of Jiangsu Higher Education Institutions (PAPD) and Qing Lan Project, P.R. China.

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Scheme 1. Synthetic routes and molecular structures of PHzMCO and PHzBCO.

Table 1 Physical properties and calculated energy levels of the compounds. λabs

λem

∆EST

(nm)a

(nm)a

(eV)b

PHzMCO

320/395

510

0.018

PHzBCO

306/395

524

0.006

HOMO

LUMO

(eV) d

(eV)e

0.47

-5.28

0.52

-5.34

PLQYc

a

S1f

T1 g

-2.86

2.701

2.683

-2.97

2.786

2.780

Measured in toluene at room temperature. Measured as thin film doped in mCP at 77 K. c Measured as thin film doped in mCP. d Determined from the oxidation potential in 10-3 M DMF solution by cyclic voltammetry. e Determined from the reduction potential in 10-3 M DMF solution by cyclic voltammetry. f Estimated from the onset of fluorescence spectrum. g Estimated from the onset of phosphorescence spectrum b

23



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Page 24 of 27

Table 2. EL performance of representative green TADF OLEDs. EQEmax

EQE [%]

EQE [%]

[%]

at 1000 cd m-2

at 10000 cd m-2

PHzMCO

17.8

15.3

PHzBCO

19.6

Ac-VPN

Emitter

CIE

References

10.3

(0.26,0.50)

This work

17

12.9

(0.32,0.50)

This work

18.9

-

12.2

(0.23,0.50)

Ref. [4]

4CzCNPy

11.3

10.6

3.5a

(0.34,0.59)

Ref. [5]

PXZ-DPS

17.5

15.4

11a

-

Ref. [6]

DDCzIPN

18.9

15.6

＜1a

(0.22,0.46)

Ref. [10]

ACRDSO2

19.2

13

2a

(0.34,0.57)

Ref. [12]

DTCBPy

27.2

13.8

2.5a

(0.30,0.64)

Ref. [16]

TXO-PhCz

21.5

3.8

-

(0.31,0.56)

Ref. [17]

Px2BP

10.7

5

-

(0.37,0.58)

Ref. [18]

oPTC

19.9

11

4.7

(0.22,0.40)

Ref. [31]

a) Estimated from the graphs in the references.

O

a) O N O

PHzMCO O

b) O N O

PHzBCO

Figure 1. The structures and molecular orbitals of a) PHzMCO and b) PHzBCO.

24


PHzMCO PHzBCO

Normalized Absorbance (a.u.)

1.0

1.0

0.8

0.8

0.6

0.6

0.4

0.4

0.2

0.2

0.0

0.0 300

350

400

450

500

550

600

650

Wavelength (nm)

Figure 2. Normalized UV-vis absorption and fluorescence spectra for PHzMCO and PHzBCO in toluene at room temperature.

104

b) 200K 250K 300K

103

200K 250K 300K

104

PL Intensity (a.u.)

PL Intensity (a.u.)

a)

102

101

103

102

101

100

100 100

200

300

400

500

50

100

150

200

Time (µs)

Time (µs)

Figure 3. Transient PL decay curves for delayed emission of 8 wt% a) PHzMCO and b) PHzBCO doped mCP film at different temperatures.

b)

PHzMCO PHzBCO

100

102 50 101

Power Efficiency (%)

3

10

Current density (mA cm-2)

104

0

100 3

4

5

103

PHzMCO PHzBCO

150

6

7

8

102

101

100 100

9

101

101

102

103 -2

Voltage (V)

Luminance (cd m )

25


104

100

External Quantum Efficiency (%)

a)

Luminance (cd m-2)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


Normalized PL Intensity (a.u.)

Page 25 of 27


d)

c) 1.0

0.8

100 cd m-2 1000 cd m-2 10000 cd m-2

1.0

100 cd m-2 1000 cd m-2 10000 cd m-2

Normalized EL Intensity (a.u.)

Normalized EL Intensity (a.u.)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 26 of 27

0.6

0.4

0.2

0.0

0.8

0.6

0.4

0.2

0.0 400

500

600

700

400

500

Wavelength (nm)

Figure

4.

a)

600

700

Wavelength (nm)

Current

density-voltage-luminance

characteristics;

b)

The

EQE-luminance characteristics; and The EL spectra of the devices based on PHzMCO (c) and PHzBCO (d).

26


Page 27 of 27

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ToC figure

27


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