Heavy Atom Effect of Bromine Significantly Enhances Exciton

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Organic Electronic Devices

Heavy Atom Effect of Bromine Significantly Enhances Exciton Utilization of Delayed Fluorescence Luminogens Shifeng Gan, Shimin Hu, Xiang-Long Li, Jiajie Zeng, Dongdong Zhang, Tianyu Huang, Wenwen Luo, Zujin Zhao, Lian Duan, Shi-Jian Su, and Ben Zhong Tang ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.8b05389 • Publication Date (Web): 03 May 2018 Downloaded from http://pubs.acs.org on May 4, 2018

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ACS Applied Materials & Interfaces

Heavy Atom Effect of Bromine Significantly Enhances Exciton Utilization of Delayed Fluorescence Luminogens Shifeng Gan,†,# Shimin Hu,†,# Xiang-Long Li,† Jiajie Zeng,† Dongdong Zhang,‡ Tianyu Huang,‡ Wenwen Luo,† Zujin Zhao,*,† Lian Duan,*,‡ Shi-Jian Su,*,† and Ben Zhong Tang*,†,§ †

State Key Laboratory of Luminescent Materials and Devices, South China University of Technology, Guangzhou 510640, China. ‡

Key Lab of Organic Optoelectronics and Molecular, Engineering of Ministry of Education, Department of Chemistry, Tsinghua University, Beijing 100084, China. §

Department of Chemistry, Hong Kong Branch of Chinese National Engineering Research Center for Tissue Restoration and Reconstruction, The Hong Kong University of Science and Technology, Clear Water Bay, Kowloon, Hong Kong, China.

KEYWORDS: thermally activated delayed fluorescence, exciton utilization, spin-orbit coupling, heavy atom effect, aggregation-induced emission, organic light-emitting diodes

ABSTRACT: Raising triplet exciton utilization of pure organic luminescent materials is of significant importance for efficiency advance of organic light-emitting diodes (OLEDs). Herein, by introducing bromine atom(s) onto a typical molecule (bis(carbazol-9-yl)-4,5-dicyanobenzene) with thermally activated delayed fluorescence, we demonstrate that the heavy atom effect of bromine can increase spin-orbit coupling and promote the reverse intersystem crossing, which endow the molecules with more distinct delayed fluorescence. In consequence, the triplet exciton utilization is improved greatly as the increase of bromine atoms, affording apparently advanced external quantum efficiencies of OLEDs. Utilizing enhancement effect of bromine atoms on delayed fluorescence should be a simple and promising design concept for efficient organic luminogens with high exciton utilization.

INTRODUCTION In organic light-emitting diodes (OLEDs), electroluminescence (EL) originates from the radiative decay of excitons formed in organic emitters upon recombination of electrons and holes injected from electrodes. The electron−hole recombination can generate singlet and triplet excitons in a ratio of 1 : 3, supposing that the recombination is equally efficient and the spin-statistics is unaffected by other processes.1,2 Since the conversion of triplet exciton energy into photons is forbidden, common fluorescent materials can only harvest singlet excitons for light emission, resulting in a theoretical limit of 25% for internal quantum efficiency (IQE) of fluorescent OLEDs. In order to get access to a nearly 100% IQE, many approaches have been exploited to utilize triplet excitons. Phosphorescent OLEDs based on noble-metal complexes can reach three times higher IQE than fluorescent devices, owing to the mixing of singlet and triplet states via efficient spin-orbit coupling (SOC) of heavy metals.3,4 However, the use of noble metals such as iridium and platinum naturally increases the cost of device fabrication.5 Besides, blue or deep-blue phosphorescent materials are far from satisfactory yet, leading to short operating lifetimes and inferior device performances.6,7

By adopting the processes that affect spin-statistics, researchers have proposed some strategies to utilize triplet excitons of metal-free fluorescent emitters, such as triplet-triplet annihilation (TTA),8 hybridized local and charge transfer (HLCT),9,10 exciton−polaron interaction (EPI)11 and thermally activated delayed fluorescence (TADF).12−17 However, TTA emitters can merely afford a maximum IQE of 62.5% in principle;18 the design of HLCT molecules remains challenging yet;15 device fabrication with EPI materials encounters lots of difficulties because of their bimolecular processes.19–21 In recent times, TADF emitters attract considerable attentions as they can efficiently harvest triplet excitons for light emission by triplet-to-singlet up-conversion, which occurs via reverse intersystem crossing (RISC).22−25 In order to realize RISC process, the highest occupied molecular orbital (HOMO) and lowest unoccupied molecular orbital (LUMO) of TADF emitters have to be dissociated, which brings about small energy splitting (∆EST) between lowest triplet excited state (T1) and lowest singlet excited state (S1).19−25 Designing an electron donor−acceptor (D−A) system is a widely accepted principle to realize effective HOMO−LUMO separation, and thus small ∆EST. But the molecular conjugation and oscillator strength will be

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weakened, which undermines radiative decay and emission efficiency in turn. Concerning the important role of SOC in organometallic phosphors and even pure organic phosphors,26-28 we consider that we can harness SOC to benefit to delayed fluorescence, which has been ignored in design of pure organic emitters. According to equation (1): λ ∝ Hso / ∆EST (1), the first-order mixing coefficient (λ) between singlet and triplet states is proportional to spin-orbit interaction (Hso) and inversely proportional to ∆EST. Hence, the enhancement of SOC can facilitate not only intersystem crossing (ISC) but also RISC.12,29−31 Given the dominative 75% triplet excitons direct generated via electron−hole recombination, more triplet excitons can be converted to singlet excitons by the promoted RISC than singlet excitons that are turned back to triplet ones by ISC. Eventually, the radiative singlet excitons are increased in device, namely the exciton utilization is enhanced. Since Hso is proportional to Z4 (Z is the atomic number, the presence of heavy halogen atoms (e.g. bromine (Br), iodine (I)), can increase SOC and thus favor RISC, which means the requirement on ∆EST can be lowered in molecular design. As a proof of concept, herein, we propose a new strategy to increase triplet exciton utilization based on heavy atom effect of halogens. Since I atom has weaker bond strength than Br atom, we introduce Br atom(s) to a typical TADF molecule, bis(carbazol-9-yl)-4,5dicyanobenzene (2CzPN, Figure 1), reported by Adachi and coworkers,12 and investigate the photoluminescence (PL) and EL properties of the resulting luminogens to validate our hypothesis. The findings demonstrate that the presence of Br can apparently promote SOC and RISC. Eventually, the triplet excitons can be harvested more efficiently indeed, which greatly improves the device efficiencies.

Figure 1. Molecular structures of the luminogens and crystal structures of 2BrCzPN and 2CzPN.

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RESULTS AND DISCUSSION The molecular structures of 2CzPN derivatives bearing one and two Br atoms (BrCzCzPN and 2BrCzPN) are shown in Figure 1. Details of synthetic routes (Scheme S1), procedures and characterization data are given in Supporting Information. 2BrCzPN and BrCzCzPN have high thermal decomposition temperatures at 358 oC and 318 oC (Figure S1), respectively, which are comparable to that of 2CzPN (336 oC), suggesting the presence of Br(S) causes no obvious damage to their thermal stability. Single crystals of 2BrCzPN and 2CzPN are grown from dichloromethane/methanol mixtures and analyzed by X-ray single-crystal diffraction. The torsion angles between dicyanobenzene (PN) and carbazole (Cz) or 3-Br-carbazole (BrCz) moieties are relatively large (52.6−67.0o) (Figure 1), which can lower the overlap of HOMOs and LUMOs, and facilitate the occurrence of delayed fluorescence.13,32−35 No obvious π−π stacking is found in crystals of 2BrCzPN and 2CzPN, due to their highly twisted molecular conformations, while multiple C−Br…π interactions exist in 2BrCzPN (Figure S2), which may lead to red-shifted emission. 2BrCzPN and BrCzCzPN show similar absorption spectra to 2CzPN (Figure S3). The absorption bands ranging from 300 to 350 nm belong to π−π* transition of carbazole-centered units,36,37 while the absorption bands at ~366 nm result from the twisted intramolecular charge transfer (TICT) from Cz to PN.6,7,36,37 In dilute THF solutions, 2BrCzPN, BrCzCzPN and 2CzPN show PL peaks at

Figure 2. (A) PL spectra of 2BrCzPN in THF-water mixtures with different water fractions (fw). Inset: photos of 2BrCzPN in THFwater mixtures (fw = 0 and 90 %), taken under the illumination of a UV lamp (365 nm). (B) Plots of (I/Io – 1) versus water fractions in THF-water mixtures. I0 is the PL intensity in pure THF. (C) PL and (D) EL spectra of 2BrCzPN, BrCzCzPN and 2CzPN doped in CBP host (6 wt%).

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4

10

A

4

4

10

B

2BrCzPN

BrCzCzPN

10

2

10

1

10

0

THF

0

5

10

15

10

3

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2

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THF

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Time (µs)

400

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Time (ns)

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PL intensity (au)

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Doped in CBP 2BrCzPN BrCzCzPN 2CzPN

D 10

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10 5

Time (µs) 10

THF

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20

fw (90%)

3

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PL intensity (au)

3

PL intensity (au)

PL intensity (au)

10

2CzPN

C

fw (90%)

fw (90%)

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200

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800

1000

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2BrCzPN in CBP

E

300 K 270 K 250 K 220 K 200 K 100 K

PL intensity (au)

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PL intensity (au)

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BrCzCzPN in CBP

F

300 K 270 K 250 K 220 K 200 K 100 K

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0

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Time (µs)

Time (µs)

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Time (µs)

Figure 3. PL decay spectra of (A) 2BrCzPN, (B) BrCzCzPN and (C) 2CzPN in THF-water mixtures (fw = 0 and 90 vol%). (D) PL decay spectra of 2BrCzPN, BrCzCzPN and 2CzPN doped in CBP host (6 wt%). Temperature-dependent PL decay spectra of (E) 2BrCzPN and (F) BrCzCzPN doped in CBP host (6 wt%).

495, 502 and 501 nm, with low fluorescence quantum yields (ФF) of are 5.7, 6.1 and 12.5%, respectively. The ФF values in nonpolar toluene are 5.9%, 10.2% and 19.4% for 2BrCzPN, BrCzCzPN and 2CzPN, respectively, which are slightly higher than those in THF, owing to the weakened TICT effect. With the addition of water to THF solutions, the emissions are red-shifted and decreased, owing to enhanced TICT effect along with increased solvent polarity.38,39 However, when the water fraction (fw) exceeds 60%, the emissions are increased and blue-shifted (Figure 2A, 2B and S4). 2BrCzPN, BrCzCzPN and 2CzPN exhibit strong PL peaks at 494, 500 and 499 nm with enhanced ФFs of 9.6%, 14.2% and 17.8%, respectively, at a fw of 90%. Since these luminogens are insoluble in water, they must have aggregated at a high fw (≥ 70 %). The intramolecular motion is restricted, which blocks nonradiative decay channel and increases the emission.40,41 In order to confirm their TICT property, PL spectra in different solvents with varied polarity are measured. The PL peaks are redshifted progressively along with the increase of polarity (Figure S5), which can verify their TICT property.38,39 These results indicate that 2CzPN and its derivatives are of TICT and AIE characteristics. Figure 3A displays the fluorescence decay curves of 2BrCzPN and apparent delayed fluorescence is observed in THF and aggregated state. For 2BrCzPN, the delayed lifetime (τd) in aggregate (7.8 µs) is longer than that in THF (6.8 µs), and the ratio of delayed component (Rd) in aggregate is also higher than that in THF. The delayed fluorescence of BrCzCzPN in different states holds the same trend (Figure 3B). All of both cases

suggest that the aggregate formation can enhance delayed fluorescence.32−35 Unlike 2BrCzPN and BrCzCzPN, 2CzPN shows no obvious delayed fluorescence in THF and aggregate (Figure 3C), indicating the RISC is relatively more difficult in 2CzPN than in 2BrCzPN and BrCzCzPN. In neat films, 2BrCzPN, BrCzCzPN and 2CzPN exhibit PL peaks at 508, 509 and 503 nm (Figure S6), with ФFs of 26.6, 40.9 and 55.8%, respectively. The doped films of 2BrCzPN, BrCzCzPN and 2CzPN in 4,4’-di(9H-carbazol-9yl)-1,1’-biphenyl (CBP) (6 wt%) host show blue-shifted emissions at 490, 481 and 474 nm, with higher ФFs of 54.3, 55.5 and 60.0%, respectively. In doped films, the TICT effect of the luminogenic molecules is weakened because of the less polar host matrix (CBP), while in neat films, the TICT effect becomes stronger as the neat films are formed by the high polar D-A molecules themselves. So, the blue-shifted emissions and increased ФFs in doped films are mainly caused by the reduced TICT effect. 2BrCzPN and BrCzCzPN in doped films exhibit prominent delayed fluorescence with greatly elongated lifetimes of 84.5 and 78.4 µs, respectively (Figure 3D and Table 1), which are much longer than that of 2CzPN in doped film (25.9 µs). The Rd of 2BrCzPN (91.5%) is much higher than that of 2CzPN (61.2%). And the rate of RISC (kRISC) of 2BrCzPN (1.34 × 105 s−1) is larger than that of 2CzPN (0.79 × 105 s−1). In doped film, the intramolecular motion of the luminogens is restricted by spatial constraint and thus the nonradiative decay channel is blocked. Therefore, S1 has long enough lifetime to undergo ISC and then RISC processes, furnishing apparently delayed fluorescence. This is

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Table 1. Photophysical properties of 2BrCzPN, BrCzCzPN and 2CzPN. b

a

compound

c

d

λem (nm)/ФF (%)/τd (µs)/Rd (%)

λabs

e

f

(nm)

THF

fw (90%)

2BrCzPN

366

495/5.7/6.8/7.6

494/9.6/7.8/12.1

BrCzCzPN

366

502/6.1/3.4/4.3

500/14.2/6.3/10.4

481/55.5/78.2/87.7

2CzPN

368

501/12.5/−/−

499/17.8/−/−

474/60.0/25.9/61.2

a

−5

doped film 490/54.3/84.5/91.5

b

∆ET (eV)

kISC

g 7

kRISC 1

g 5

SOC 1

1

(×10 s− )

(× 10 s− )

9.95

1.34

0.62

0.30

9.52

0.90

0.49

0.35

3.01

0.79

0.33

0.24

h

(cm− )

c

In THF solution (10 M). Fluorescence quantum yield determined by a calibrated integrating sphere. Delayed decay components for the compounds measured under nitrogen. dThe ratio of delayed components. eIn THF-water mixture with a water fraction of 90%. f Estimated from the fluorescence and phosphorescence spectra at 77 K of the doped film. gkISC = intersystem crossing decay rate from S1 to T1, kRISC = reverse intersystem crossing decay rate from T1 to S1. hCalculated by TD-DFT at B3LYP/TD-FC at gas phase.

the reason for the finding that the delayed fluorescence becomes more pronounced in doped films (or aggregates) than in solutions.42 It is noteworthy that the τd and Rd are increased apparently from 2CzPN to BrCzCzPN and to 2BrCzPN, no matter in solutions or doped films. This interesting phenomenon manifests that the introduction of Br atoms has unquestionably expedited the RISC process, leading to more prominent delayed fluorescence. On the other side, the Rds of 2BrCzPN and BrCzCzPN are increased from 100 to 300 K (Figure 3E and 3F), which is attributed to the fact that high temperature can accelerate RISC process.20,43,44 The results confirm that the presence of Br atom(s) indeed has positive effect on the delayed fluorescence of these luminogens. To have a deep understanding of their photophysical property, density functional theory (DFT) calculation is conducted on these luminogens. Their HOMOs and LUMOs are clearly separated because of the large dihedral angles between donor and acceptor groups.13 The LUMOs are mainly located on PN units, while the HOMOs are delocalized on Cz or BrCz moieties (Figure S6). The calculated ΔESTs of these luminogens in gas phase are decreased slightly from 0.34 to 0.32 eV as the addition of Br atom (Figure 4). The experimental ΔESTs of these luminogens dispersed in CBP films are estimated from the fluorescence spectra and the phosphorescence spectra at 77 K

(Figure S7). The ΔESTs of 2BrCzPN and BrCzCzPN are measured to be 0.24 and 0.30 eV, respectively, being smaller than that of 2CzPN (0.35 eV), which is consistent with the calculated results. This indicates that 2BrCzPN and BrCzCzPN can undergo RISC process more easily than 2CzPN,45,46 although the ΔESTs are actually still larger than many TADF emitters.47,48 In order to deepen insight into their obvious difference in kRISC values, time-dependent DFT (TD-DFT) at BDF program package is carried out to calculate the SOC values of these luminogens.49 From Figure 4, the S1 has a predominate charge transfer (CT) nature, while the T1 is a mixed locally-excited (LE) and CT state. This difference in the nature of the excited states results in significant SOC matrix element values. The SOC value of 2BrCzPN (0.62 cm‒1) is about twofold larger than that of 2CzPN (0.33 cm‒ 1 ), which leads to a larger kRISC of the former. Similarly, BrCzCzPN also shows a larger SOC (0.49 cm‒1) than 2CzPN, which is consistent with its larger kRISC. Our experimental results suggest that the higher SOC can lead to a faster RISC process and thus more distinct delayed fluorescence, which is in excellent agreement with computational results from Kim and co-workers.19−21 These findings provide direct evidences to prove the important enhancement effect of Br atoms on the delayed fluorescence.

Figure 4. Natural Transition Orbital (NTO) pairs for the representative excited states of the luminogens with energy levels and energy splitting (∆EST) and spin orbit coupling (SOC) matrix elements for the pair of relevant states, calculated by TD-DFT at B3LYP/6-31g(d,p) at gas phase. Hole and particle wave functions with the largest weight (ʋ) are placed below and above the arrows, respectively.

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The electrochemical behaviors of these luminogens are investigated cyclic voltammetry (CV) in dichloromethane with 0.1 M tetrabutylammonium hexafluorophosphate as the supporting electrolyte at a scan rate of 50 mV s−1. 2BrCzPN and BrCzCzPN show almost the same electrochemical behaviors as 2CzPN (Figure 5). They experience reversible oxidation processes with peak potentials at

HOMO/LUMO 2BrCzPN −5.40/−2.39 eV BrCzCzPN −5.41/−2.41 eV 2CzPN −5.54/ −2.57 eV

Current

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0.0

0.5

1.0

1.5

Potential (V)

Figure 5. Cyclic voltammograms of 2BrCzPN, BrCzCzPN and 2CzPN, measured in dichloromethane with 0.1 M tetrabutylammonium hexafluorophosphate as the supporting electrolyte at a scan rate of 50 mV s−1.

about ~1.3 V, while reduction processes are hardly observed. From oxidation onset potentials and optical bandgaps, the HOMO energy levels are calculated to be ‒ 5.40, ‒5.41 and ‒5.54 eV, and LUMO energy levels are ‒ 2.39, ‒2.41 and ‒2.57 eV for 2BrCzPN, BrCzCzPN and 2CzPN, respectively. These results disclose that the introduction of Br atom(s) has slightly raised the energy levels of these luminogens but exerts no apparent impact on their electrochemical stability. Since 2BrCzPN and BrCzCzPN have larger SOC and faster kRISC than 2CzPN, it is envisioned that they are able to harvest more triplet excitons in OLEDs, and thus give rise to higher EL efficiencies. Therefore, by vacuum deposition, we fabricate devices with a three-layer configura-

tion: ITO/TAPC (25 nm)/emitter (35 nm)/TmPyPB (55 nm)/LiF (1 nm)/Al (device I), where the doped films of 2BrCzPN, BrCzCzPN or 2CzPN in CBP (6 wt%) function as emitters, and TAPC and TmPyPB serve as hole- and electron-transporting layers, respectively. 2BrCzPN and BrCzCzPN show EL emissions at 499 nm (CIEx,y = 0.22, 0.46) and 492 nm (CIEx,y = 0.19, 0.38), respectively, close to their PL emissions in doped films. 2CzPN shows EL peak at 487 nm (CIEx,y = 0.18, 0.33). These results indicate Br atom can cause slight red-shifts in EL spectra (Figure 2B). The device of 2BrCzPN shows the best maxima current (ηC,max), power (ηP,max) and external quantum (ηext,max) efficiencies of 31.5 cd A−1, 30.3 lm W−1 and 12.3%, respectively. The device of BrCzCzPN also exhibits good ηC, ηP and ηext of 27.1 cd A−1, 25.1 lm W−1 and 10.8%, respectively (Figure 6). However, the EL efficiencies of the device of 2CzPN are relatively inferior (20.0 cd A−1, 19.6 lm W−1 and 8.9%). These results elucidate that the device performance is improved under the identical device configuration with the increase of Br atoms in the molecules (Table 2), which is consistent with recent findings from Yang et al.50 In comparison with 2CzPN, the advanced EL performances of 2BrCzPN and BrCzCzPN are mainly attributed to their larger SOC and kRISC. The relatively lower ФF but better ηext,max of 2BrCzPN are sound evidences for the enhanced utilization of excitons generated under electrical excitation. In general, the maximum fraction of excitons involving radiative decay (ηST) in OLEDs can be estimated according to equation (2):

ηST = ηext / ηint = ηext / (ϒ × ηout × ФF)

(2)

where ϒ is the efficiency for electron−hole recombination (ideally 1.0), ηout is the light−outcoupling efficiency (typically 20‒30%) and ФF is the PL quantum yield of the emitting layer. According to the equation, the ηSTs of 2BrCzPN, BrCzCzPN and 2CzPN are calculated to be 90.6, 77.8 and 59.3%, respectively, given a mean ηout of 25%. By comparing these ηSTs, we can find that introducing Br atom(s) to 2CzPN have indeed greatly enhanced the utilization of excitons, and thus given rise to better device

Figure 6. (A) Luminance−voltage−current density and (B) current efficiency−luminance−power efficiency characteristics of the OLEDs based on the new luminogens. Device configuration: ITO/TAPC (25 nm)/emitter (35 nm)/TmPyPB (55 nm)/LiF (1 nm)/Al; emitter: doped films of 2BrCzPN, BrCzCzPN or 2CzPN in CBP host (6 wt%).

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Table 2. Key values of OLEDs based on 2BrCzPN, BrCzCzPN and 2CzPN.a

emitter

device I

device II device III

Von

λEL

ηc,max

ηp,max

(V)

(nm)

(cd A−1)

(lm W−1)

ηext, max

CIE (x,y)

(%)

6 wt% 2BrCzPN : CBP

3.3

499

31.5

30.3

12.3

(0.22, 0.46)

6 wt% BrCzCzPN : CBP

3.3

492

27.1

25.1

10.8

(0.19, 0.38)

6 wt% 2CzPN : CBP

3.3

487

20.0

19.6

8.9

(0.18, 0.33)

6 wt% 2BrCzPN : mCP

3.7

492

29.5

23.2

13.8

(0.19, 0.36)

6 wt% 2CzPN : mCP

3.7

488

25.1

19.7

10.8

(0.18, 0.35)

15 wt% 2BrCzPN : CzTRZ

2.9

500

40.4

50.0

16.2

(0.22, 0.42)

15 wt% 2CzPN: CzTRZ

3.5

500

28.2

23.9

11.3

(0.22, 0.44)

Abbreviations: Von = turn-on voltage at 1 cd m−2; ηc,max = maximum current efficiency; ηp,max = maximum power efficiency; ηext,max = maximum external quantum efficiency; CIE = Commission Internationale de I’Eclairage. Device configuration: ITO/TAPC (25 nm)/emitter (35 nm)/TmPyPB (55 nm)/LiF (1 nm)/Al; emitter: doped film of 2BrCzPN, BrCzCzPN or 2CzPN in CBP host (6 wt%) (device I); ITO/TAPC (25 nm)/emitter (35 nm)/TmPyPB (55 nm)/LiF (1 nm)/Al; emitter: doped film of 2BrCzPN or 2CzPN in mCP host (6 wt%) (device II); ITO/HATCN (5 nm)/NPB (30 nm)/mCBP (10 nm)/emitter (30 nm)/CzTRZ (5 nm)/BPBiPA (30 nm)/LiF (0.5 nm)/Al (150 nm) (emitter = doped film of 2BrCzPN or 2CzPN in CzTRZ host (15 wt%) (device III). ITO = indium tin oxide; TAPC = 1,1’-bis(di-4tolylaminophenyl) cyclohexane; TmPyPB = 1,3,5-tri(m-pyrid-3-yl-phenyl)benzene; mCP = 1,3-bis(carbazol-9-yl)benzene; HATCN = dipyrazino[2,3-f:2',3'-h]quinoxaline-2,3,6,7,10,11-hexacarbonitrile; NPB = N,N-bis(1-naphthalenyl)-N,N'-diphenyl-[1,1'-biphenyl]-4,4'diamine; mCBP = 3,3'-di(9H-carbazol-9-yl)-1,1'-biphenyl; CzTRZ = 9-(3'-(4,6-diphenyl-1,3,5-triazin-2-yl)-[1,1'-biphenyl]-3-yl)-9Hcarbazole; BPBiPA = 9,10-bis(4-(2-phenyl-1H-benzo[d]imidazol-1-yl)phenyl) anthracene. a

efficiency. In order to further justify the enhancement effect of Br atoms on device efficiency, more devices with different host materials and configurations are fabricated and investigated. Device II with a configuration of ITO/TAPC (25 nm)/emitter (35 nm)/TmPyPB (55 nm)/LiF (1 nm)/Al is fabricated, in which the emitter is composed of a doped film of 2BrCzPN or 2CzPN in a new host 1,3-bis(carbazol9-yl)benzene (mCP). The device II of 2BrCzPN shows ηC,max, power ηP,ma and ηext,max of 29.5 cd A−1, 23.2 lm W−1 and 13.8%, which are better than those obtained from the device of 2CzPN (25.1 cd A−1, 19.7 lm W−1 and 10.8%) (Figure S9). Moreover, an optimized device is fabricated as ITO/HATCN (5 nm)/NPB (30 nm)/mCBP (10 nm)/emitter (30 nm)/CzTRZ (5 nm)/BPBiPA (30 nm)/LiF (0.5 nm)/Al (150 nm) (device III), in which a bipolar host 9-(3'-(4,6diphenyl-1,3,5-triazin-2-yl)-[1,1'-biphenyl]-3-yl)-9Hcarbazole (CzTRZ) is used in order to improve device efficiency,51 because the TADF emitters doped in bipolar hosts can usually lead to higher EL efficiencies.52 As expected, the device III of 2BrCzPN furnishes improved ηC,max and ηext,max of 40.4 cd A−1 and 16.2%, respectively, which are higher than those of 2CzPN (28.2 cd A−1 and 11.3%). The better EL efficiencies of 2BrCzPN than 2CzPN in both devices II and III validate our hypothesis again. One thing should be mentioned is that device III of 2BrCzPN degrades much faster than device III of 2CzPN, measured at a luminance of 500 cd m−2 (Figure S10), probably due to the relatively weak Br‒C bond. So, a rational molecular design is required to increase molecular stability and avoid the crack of Br‒C bond for practical use.

CONCLUSION In conclusion, Br atoms are introduced into a typical TADF emitter 2CzPN in order to enlarge the SOC values

and thus the efficiency of delayed fluorescence. The generated new luminogens (2BrCzPN and BrCzCzPN) exhibit AIE, TICT and TADF characteristics. They have more obvious delayed fluorescence in aggregates than in solutions due to the restriction of intramolecular motion. As the increase of Br atoms, the SOC value becomes larger and RISC process is promoted, rendering the delayed fluorescence more distinct, which offers a direct evidence of the enhancement effect of SOC on delayed fluorescence. Consequently, the triplet exciton utilization is improved as the increase of Br atoms, affording greatly advanced EL efficiencies. Employing heavy atom effect of Br could be an effective approach to facilitate spin conversion and boost exciton utilization and device efficiency. However, the stability degradation of the devices based on Brsubstituted luminogens remains as a challenge. In our opinion, via rational molecular engineering and device optimization, the device stability could be improved. The useful information obtained in this work would help researchers to figure out a perfect strategy to increase device efficiency and stability at the same time when employing halogen-containing organic materials with delayed fluorescence or room temperature phosphorescence.

ASSOCIATED CONTENT Supporting Information. This Supporting Information is available free of charge via the Internet at http://pubs.acs.org. Experimental details, device fabrication, crystal data, TGA curves, single-crystal packing of 2BrCzPN, fluorescence and phosphorescence spectra, absorption spectra, PL decay spectra, HOMO and LUMO orbital distribution, device performance and degradation curves.

AUTHOR INFORMATION Corresponding Author

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*E-mail: [email protected] *E-mail: [email protected] *E-mail: [email protected] *E-mail: [email protected]

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Author Contributions #

S.G. and S.H. contributed equally to this work. (11)

Notes The authors declare no competing financial interest.

ACKNOWLEDGMENT This work was financially supported by the National Natural Science Foundation of China (21788102), the Nation Key Basic Research and Development Program of China (973 program, 2015CB655004) Founded by MOST, the Guangdong Natural Science Funds for Distinguished Young Scholar (2014A030306035), the Natural Science Foundation of Guangdong Province (2016A030312002), Science and Technology Project of Guangdong Province (2016B090907001), the Innovation and Technology Commission of Hong Kong (ITC-CNERC14SC01) and the Fundamental Research Funds for the Central Universities (2017ZD001).

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