Circularly Polarized Electroluminescence of Thermally Activated

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Circularly Polarized Electroluminescence of Thermally Activated Delayed Fluorescence-Active Chiral Binaphthyl-Based Luminogens Yuxiang Wang,† Yu Zhang,‡ Wenrui Hu,† Yiwu Quan,*,‡ Yunzhi Li,*,§ and Yixiang Cheng*,† †

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Key Lab of Mesoscopic Chemistry of MOE and Jiangsu Key Laboratory of Advanced Organic Materials, School of Chemistry and Chemical Engineering, and ‡Key Laboratory of High Performance Polymer Material and Technology of Ministry of Education, Department of Polymer Science and Engineering, School of Chemistry and Chemical Engineering, Nanjing University, Nanjing 210023, China § School of Chemistry and Chemical Engineering, Linyi University, Linyi 276000, China S Supporting Information *

ABSTRACT: Two pairs of thermally activated delayed fluorescence (TADF)active chiral luminogens (R/S-1 and R/S-2) can be achieved by introducing D− A-type groups to chiral BINOL skeletons. The resulting chiral luminogens can exhibit aggregation-induced emission properties in THF−water mixtures and TADF emission in a doped-film state. The absolute photoluminescence quantum yield (ΦPL) and delayed fluorescence lifetimes (τdelayed) were measured to be 18.5% and 1.03 μs for R-1 and 15.7% and 0.97 μs for R-2. However, only R/S-1 with the fixed conjugation structure can emit circularly polarized luminescence signals, and glum can reach 1.6 × 10−3 in toluene solution and 9.2 × 10−4 in the neat film. Most importantly, R/S-1 was chosen as the emitting layers for orangered circularly polarized organic light-emitting diodes, which can display low turnon voltage (Von) of 3.4 V, high maximum brightness (Lmax) up to 40 470 cd m−2, moderate external quantum efficiency of 4.1%, as well as circularly polarized electroluminescence signal with gEL = −0.9 × 10−3/+1.0 × 10−3. KEYWORDS: circularly polarized luminescence, thermally activated delayed fluorescence, donor−accepter structures, BINOL, circularly polarized organic light-emitting diodes

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INTRODUCTION

It is well acknowledged that pure organic dyes with a thermally activated delayed fluorescence (TADF) property have received more and more attention in a new generation of optoelectronic materials since Adachi et al. first found their application in high-performance OLED devices in 2012.24−28 TADF-based OLED emitters can take advantage of both the triplet (T1) and singlet (S1) excitons for luminescence through reverse intersystem crossing (RISC) because of the small S1 to T1 energy gap (ΔEST),29−33 which means theoretically 100% internal quantum efficiency. So far, reports about CP-OLEDs based on TADF-active chiral emitters are very few.34−38 Recently, Chen et al. reported a pair of aromatic-imide-based TADF enantiomers containing chiral 1,2-diaminocyclohexane. The CP-OLEDs constructed by doped films of these molecules exhibited high external quantum efficiencies (EQEs) up to 19.8% and CP electroluminescence (CP-EL) signals with gEL = 2.3 × 10−3.36 Meanwhile, Tang’s group synthesized a series of aggregation-induced emission enhancement-active chiral binaphthalene-based TADF emitters and CP-OLEDs were fabricated by using doped and nondoped films of these emitters. The doped CP-OLED devices could exhibit higher

Recently, circularly polarized luminescence (CPL) has raised increasing interest owing to its potential application in chiral recognition sensors,1−3 optical data storage,4 optical spintronic communication,5,6 chiral light-emitting transistor,7 and especially for 3D display8 in the future. Normally, circularly polarized (CP) light is achieved by light-emitting devices with a polarizer and a quarterwave plate, which gives rise to energy loss and complex device structures.9,10 With the development of organic light-emitting diodes (OLEDs) technology, more and more works have been devoted to CP organic lightemitting diodes (CP-OLEDs) from chiral CPL emitters, which can provide a more simple and efficient way to generate CPL. In the past few years, there have been only some precursory works on CP-OLEDs by using chiral fluorescence polymers11−14 and chiral lanthanide complexes as well as chiral Ir(III)/Pt(II) phosphorescent complexes as OLED emitters.8,15−18 However, these chiral polymers and metal complexes could not achieve large electroluminescence (EL) dissymmetry factor (gEL) and high device efficiency at the same time. Recently, much attention has been paid to various chiral small organic molecules as CPL emitters because of the easy organic synthesis, smart structure modification at a welldefined molecule level, and tunable emission wavelength.19−23 © XXXX American Chemical Society

Received: April 22, 2019 Accepted: June 26, 2019 Published: June 26, 2019 A

DOI: 10.1021/acsami.9b07005 ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX

Research Article

ACS Applied Materials & Interfaces

9.0 Hz, 2H), 7.10 (dd, J = 8.4, 1.8 Hz, 2H), 6.72 (d, J = 1.6 Hz, 2H), 3.39 (s, 6H). 13C NMR (100 MHz, CDCl3): δ 165.93, 156.73, 150.99, 134.19, 132.80, 130.73, 130.24, 128.12, 127.28, 126.76, 125.94, 125.89, 125.20, 122.27, 121.69, 120.98, 119.01, 51.88. MS (ESI, m/ z): calcd for C36H24Br2O6Na+ [M + Na]+, 734.98; found, 734.85. Synthesis of R/S-4. R/S-3 (3.00 g, 4.21 mmol) was dissolved in 40 mL of 1,4-dioxane. KOH (2.36 g, 42.11 mmol) aqueous solution (20 mL) was added to the reaction. After stirring at room temperature overnight, hydrochloric acid (2.0 mol L−1) was used to neutralize the solution. The acquired solid was filtered and washed with water and diethyl ether. R/S-4 (2.46 g, 85%) can be collected as a white solid after vacuum-drying. 1H NMR (400 MHz, DMSO-d6): δ 8.06 (d, J = 9.0 Hz, 2H), 8.00 (d, J = 8.1 Hz, 2H), 7.64 (d, J = 8.4 Hz, 2H), 7.48− 7.44 (m, 2H), 7.37−7.30 (m, 4H), 7.18 (d, J = 9.0 Hz, 2H), 7.12 (d, J = 8.5 Hz, 2H), 6.73 (d, J = 1.8 Hz, 2H). 13C NMR (100 MHz, DMSO-d6): δ 166.08, 156.22, 152.33, 133.94, 133.27, 130.86, 130.50, 128.69, 127.44, 126.99, 126.11, 125.75, 125.44, 124.09, 123.26, 120.94, 118.76. MS (ESI, m/z): calcd for C36H19Br2O6− [M − H]−, 682.95; found, 682.90. Synthesis of R/S-5. R/S-4 (2.00 g, 2.92 mmol) was dissolved in 20 mL of trifluoroacetic acid. Trifluoroacetic anhydride (3.68 g, 17.54 mmol) was added dropwise to the reaction at 0 °C. After stirring at room temperature overnight, the mixture was added slowly to 200 mL of saturated NaHCO3 aqueous solution. After extraction with DCM and purification by column chromatography (petroleum ether/DCM = 1:1), R/S-5 (1.74 g, 92%) can be afforded as a light-yellow solid. 1H NMR (400 MHz, CDCl3): δ 9.21 (s, 2H), 8.25 (dd, J = 13.7, 8.4 Hz, 4H), 7.57 (ddd, J = 8.1, 6.8, 0.9 Hz, 4H), 7.46−7.41 (m, 4H), 7.24 (d, J = 8.0 Hz, 2H), 7.12 (d, J = 1.7 Hz, 2H). 13C NMR (100 MHz, CDCl3): δ 177.45, 156.60, 149.95, 136.32, 130.50, 129.99, 129.87, 129.79, 129.61, 128.21, 127.37, 126.05, 125.25, 121.18, 119.92, 118.36. MS (ESI, m/z): calcd for C36H17Br2O4+ [M + H]+, 648.95; found, 648.90. Synthesis of R/S-1. R/S-5 (400 mg, 0.62 mmol), phenoxazine (452 mg, 2.47 mmol), t-BuONa (356 mg, 3.70 mmol), Pd(dba)2 (35 mg, 0.06 mmol), and (t-Bu)3P (0.29 mL, 10 wt % in toluene, 0.12 mmol) were dissolved in 20 mL of anhydrous toluene, and the reaction was stirred at 110 °C under Ar atmosphere for 16 h. The solvents were removed by rotary evaporation, which was followed by further purification through column chromatography (petroleum ether/DCM/ethyl acetate = 40:10:1) to collect R/S-1 (426 mg, 81%) as a red solid. 1H NMR (400 MHz, CDCl3): δ 9.23 (s, 2H), 8.58 (d, J = 8.4 Hz, 2H), 8.25 (d, J = 8.3 Hz, 2H), 7.58−7.54 (m, 2H), 7.46− 7.42 (m, 2H), 7.30−7.25 (m, 4H), 6.99 (d, J = 1.8 Hz, 2H), 6.66− 6.60 (m, 8H), 6.49 (ddd, J = 8.1, 6.9, 2.1 Hz, 4H), 5.89 (dd, J = 8.0, 1.0 Hz, 4H). 13C NMR (100 MHz, CDCl3): δ 177.30, 158.38, 150.12, 145.67, 143.99, 136.39, 133.16, 130.56, 129.93, 129.89, 129.69, 126.02, 125.92, 125.26, 123.16, 122.10, 121.31, 120.83, 120.00, 118.37, 115.81, 113.48. HRMS (ESI, m/z): calcd for C58H33N2O6+ [M + H]+, 853.2333; found, 853.2339. Elementary analysis: Calcd for C58H32N2O6: C, 81.68; H, 3.78; N, 3.28. Found: C, 81.58; H 3.86; N, 3.26%. α25 D = −234/204 for R/S-1. Synthesis of R/S-6. (R/S)-2,2′-bis(methoxymethoxy)-1,1′-binaphthalene (3.00 g, 8.01 mmol) was dissolved in 80 mL of anhydrous diethyl ether. n-BuLi (12.82 mL, 2.5 mol L−1 in hexane, 32.05 mmol) was added dropwise to the above solution at 0 °C under Ar atmosphere. After stirring at room temperature for 2 h, 4-fluoroN,N-dimethylbenzamide (5.36 g, 32.05 mmol) was added at −40 °C under Ar atmosphere. Then, the reaction was stirred at room temperature overnight; 60 mL saturated NH4Cl aqueous solution was added. Then, ethyl acetate was used to extract the aqueous phase. The organic phase was concentrated by rotary evaporation followed with column chromatography (petroleum ether/ethyl acetate = 10:1) purification to afford R/S-6 (2.13 g, 43%) as a white solid. 1H NMR (400 MHz, CDCl3): δ 8.08 (s, 2H), 8.01−7.97 (m, 4H), 7.95 (d, J = 8.2 Hz, 2H), 7.50 (ddd, J = 8.1, 6.9, 1.1 Hz, 2H), 7.40 (ddd, J = 8.2, 6.9, 1.2 Hz, 2H), 7.30 (d, J = 8.4 Hz, 2H), 7.14−7.09 (m, 4H), 4.58 (s, 4H), 2.64 (s, 6H). 13C NMR (100 MHz, CDCl3): δ 194.10, 167.17, 164.63, 150.71, 134.88, 133.83, 133.82, 133.62, 133.01, 132.91, 130.74, 130.05, 128.86, 128.21, 126.11, 126.02, 115.69,

EQE up to 9.3% and the nondoped CP-OLED devices could produce larger gEL up to +0.06/−0.06.37 Therefore, further investigation on the effective chiral induction and transfer mechanism between chiral moiety and TADF-active emitters is of great significance and a challenge for developing CP-OLEDs with excellent performance. Axially chiral 1,1′-binaphthol (BINOL) has been assigned to one of the most important chiral molecules because of its stable chiral configuration and high chiral induction. So far, our group discovered a series of well-defined binaphthyl fluorescence dyes for CPL materials by selectively functionalizing at the skeletal structure of the chiral BINOL moiety.39−43 Recently, we reported CP-OLEDs based on aggregationinduced emission (AIE)-active chiral binaphthyl-conjugated polymers with gEL as high as 0.024, but the device efficiency was unsatisfying.44 A combination of chiral binaphthyl and TADF may produce highly efficient CP-OLEDs, but the molecule design of chiral binaphthyl-based TADF molecules is homogeneous.35,37 In this paper, we further designed two pairs of chiral TADF-active enantiomers R/S-1 and R/S-2 by extending the conjugated skeletons of chiral BINOL. Both R/ S-1 and R/S-2 showed AIE properties in tetrahydrofuran (THF)−H2O mixtures and TADF emission in a doped-film state. However, only the rigid molecule R/S-1 can exhibit CPLactive emission, and the luminescent dissymmetry factors (glum) can be up to 1.6 × 10−3 in toluene solution and 9.2 × 10−4 in neat films. Most importantly, the CP-OLEDs incorporating R/S-1 chosen as chiral emitting layers (EMLs) can display CP-EL signals with gEL of −0.9 × 10−3/+1.0 × 10−3.

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EXPERIMENTAL SECTION

Measurements and Materials. Nuclear magnetic resonance (NMR) spectra were taken on a 400 MHz Bruker spectrometer with tetramethylsilane as the internal standard. Ultraviolet−visible (UV− vis) spectra were obtained by using a Shimadzu UV-3600 spectrophotometer. Fluorescence spectra were tested from a HORIBA Scientific FluoroMax-4 Spectrofluorometer. The transient photoluminescence (PL) decay spectra and absolute PL quantum yield were measured by a HORIBA Fluorolog-3 3D-Spectrofluorometer. Low-temperature fluorescence and phosphorescence spectra were measured by a HITACHI F-4600 fluorescence spectrophotometer with a Dewar. Circular dichroism (CD) spectra were measured from a JASCO J-810 spectropolarimeter. CPL/CP-EL spectra were acquired from a JASCO CPL-300 spectrofluoropolarimeter. Thermogravimetric analysis (TGA) and differential scanning calorimetry (DSC) were carried out by using a PerkinElmer Pyris-1 instrument under N2 atmosphere. Cyclic voltammetry (CV) was measured on a CHI660D (Shanghai CH Instrument Company, China) in CH2Cl2 solution with a supporting electrolyte (0.1 M TBABF4) at room temperature. The solvents and reagents used in this paper were purchased from common reagent companies with A.R. grade. Synthesis of R/S-3. (R/S)-1,1′-binaphthyl-2,2′-diol (2.00 g, 6.99 mmol) and K2CO3 (2.41 g, 17.46 mmol) were dissolved in 10 mL of anhydrous dimethylformamide (DMF) and heated to 100 °C. Anhydrous DMF solution (10 mL) of methyl 4-bromo-2fluorobenzoate (4.07 g, 17.46 mmol) was added to the reaction. After stirring at 110 °C for 12 h, the mixture was added into 40 mL of saturated NH4Cl aqueous solution. Then, ethyl acetate was used to extract the aqueous phase. The organic phase was dried by rotary evaporation followed with column chromatography (petroleum ether/ ethyl acetate = 10:1) purification to afford R/S-3 (3.88 g, 78%) as a white solid. 1H NMR (400 MHz, CDCl3): δ 7.95 (d, J = 9.0 Hz, 2H), 7.91 (d, J = 8.1 Hz, 2H), 7.57 (d, J = 8.4 Hz, 2H), 7.44 (ddd, J = 8.1, 6.5, 1.5 Hz, 2H), 7.33 (ddd, J = 14.0, 10.7, 4.8 Hz, 4H), 7.23 (d, J = B

DOI: 10.1021/acsami.9b07005 ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX

Research Article

ACS Applied Materials & Interfaces Scheme 1. Chemical Structures and Synthesis Procedures of R/S-1 and R/S-2

Figure 1. (a) Illustration of chemical structures of R/S-1 and R/S-2. (b) Calculated HOMO and LUMO spatial distributions of S-1 and S-2. 115.47, 99.84, 56.60. MS (ESI, m/z): calcd for C38H28F2O6Na+ [M + Na]+, 641.18; found, 641.10. Synthesis of R/S-2. Phenoxazine (195 mg, 1.07 mmol) and tBuOK (125 mg, 1.12 mmol) were dissolved in 5 mL of anhydrous DMF and then heated to 100 °C under Ar atmosphere. R/S-6 (300 mg, 0.48 mmol) in 5 mL of anhydrous DMF was added to the above solution. After the mixture was stirred at 100 °C for 12 h, 40 mL of saturated NH4Cl aqueous solution was added. Then, ethyl acetate was used to extract the aqueous phase. The organic phase was dried by rotary evaporation followed with column chromatography (petroleum

ether/DCM/ethyl acetate = 40:10:4) purification to collect R/S-2 (389 mg, 85%) as an orange solid. 1H NMR (400 MHz, CDCl3): δ 8.19 (t, J = 4.1 Hz, 6H), 8.00 (d, J = 8.1 Hz, 2H), 7.54−7.50 (m, 2H), 7.43 (dd, J = 10.4, 4.8 Hz, 6H), 7.33 (d, J = 8.5 Hz, 2H), 6.70−6.53 (m, 12H), 5.91 (d, J = 7.4 Hz, 4H), 4.64 (s, 4H), 2.68 (s, 6H). 13C NMR (100 MHz, CDCl3): δ 194.52, 150.74, 144.04, 137.05, 135.04, 133.39, 133.04, 131.11, 130.87, 130.09, 128.95, 128.39, 126.12, 123.13, 121.90, 115.73, 113.17, 99.96, 56.65. HRMS (ESI, m/z): calcd for C62H44N2O8Na+ [M + Na]+, 967.2990; found, 967.2983. C

DOI: 10.1021/acsami.9b07005 ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX

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ACS Applied Materials & Interfaces

Table 1. Photophysical Properties of R-1 and R-2

Elementary analysis: Calcd for C62H44N2O8: C, 78.80; H, 4.69;, N, 2.96. Found: C, 78.70; H, 4.87; N, 2.85%. α25 D = −88/70 for R/S-2. Device Fabrication and Characterization. Indium tin oxide (ITO)-coated glass substrates were washed with deionized water, acetone, and ethanol under ultrasonication. After 20 min of UV− ozone treatment, the substrates were deposited by spin-coating of PEDOT/PSS and the emitting orange layer (dissolved in chlorobenzene) in sequence. The electron-transporting layer (TPBI), electron injection layer (Ca), and cathode (Ag) were highvacuum (1 × 10−5 and 5 × 10−4 Pa for organic and metal materials) thermal-evaporated onto the devices with the deposition rate of 0.5, 0.5 and 1 Hz s−1. A Keithley 2636A system SourceMeter was used to test the current−voltage characteristics and brightness of the devices under a nitrogen atmosphere. Photo Research PR-655 SpectraScan was used to record the EL spectra at ambient conditions.

compound R-1

R-2

λabsa (nm) 450, 405, 322, 264 402, 323, 293

ΦPL (%)

τpromptd (ns)

τdelayedd (μs)

ΔESTe (eV)

587b,568c

2.3b, 18.5c

29.1

1.03

0.059

547b, 530c

5.9b, 15.7c

18.5

0.97

0.076

λPL (nm)

In THF solutions. bIn neat films. cIn doped films (15 wt % in TCTA). dLifetimes measured in doped films. eObtained from the fluorescence and phosphorescence spectra onsets in toluene solutions at 77 K.

a

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322, and 264 nm, and R-2 has three bands at 402, 323, and 293 nm. The absorption bands below 300 nm can be ascribed to the absorption of BINOL and PXZ units. The absorption peaks of R-1 and R-2 at 450 and 402 nm in the longwavelength region can be regarded as the intramolecular charge-transfer (ICT) absorption bands between PXZ and chiral binaphthyl acceptors. As shown in Figures S5 and S6, R1 and R-2 showed almost no emission in the THF solution. As poor solvent H2O was gradually added, obvious fluorescence emission enhancement can be observed at 590 nm for R-1 and 542 nm for R-2, and the PL intensity can be enhanced by 59and 40-fold, whereas the water fraction (f w) was increased to 99% (Figures S5 and S6), which demonstrated characteristic AIE properties for both R-1 and R-2. Compared with R-2, the emission wavelength of R-1 appears to have a 38 nm red shift, which can be due to a more rigid conjugation skeleton in R-1. Herein, we also measured the PL spectra of R-1 and R-2 in the spin-coated films, orange and yellow emission can be observed at 587 nm for R-1 and 547 nm for R-2 in neat films with the absolute PL quantum yield (ΦPL) of 2.3 and 5.9%, respectively (Figure 2a). Meanwhile, the emissions of R-1 and R-2 were blue-shifted to 568 and 530 nm in the co-doped films (15 wt % in 4,4′,4″-tri-9-carbazolyltriphenylamine, TCTA), and the ΦPL were greatly enhanced to 18.5 and 15.7%, which can be attributed to the reduced ICT effect and dipole−dipole interactions caused by a nonpolar matrix environment.45,46 In order to well understand the photophysical process, lowtemperature PL properties of R-1 and R-2 were investigated (Figure S8). The experimental ΔEST values of R-1 and R-2 were obtained from the fluorescence and phosphorescence spectra onsets in toluene solutions at 77 K. As depicted in Figure S8, the ΔEST values are 0.059 and 0.076 eV for R-1 and R-2, which are small enough for effective thermally promoted RISC from T1 to S1 and provide possibility for TADF emission. We measured the transient PL spectra of R-1 and R-2 in neat and doped films at room temperature as shown in Figures 2b and S7. The transient decay curves of the R-1/R-2 in doped films can be fitted by double-exponential functions with prompt fluorescence lifetimes (τprompt) of 29.1/18.5 ns and delayed fluorescence lifetimes (τdelayed) of 1.03/0.97 μs. The temperature-dependent transient PL spectra of R-1 and R2 in doped films (Figure S9) revealed that the components of the delayed fluorescence for both of the luminogens were enhanced with the increasing of temperature, which directly supported the TADF nature. On the basis of the reported methods,24,47,48 various rate constants and efficiencies of different photophysical processes can be calculated by the data of ΦPL and lifetimes. As listed in Table S3, R-1 and R-2 show quantum yields of delayed fluorescence components

RESULTS AND DISCUSSION Synthesis and Thermal Properties. Scheme 1 presents the chemical structures and synthesis procedures of R/S-1 and R/S-2. R/S-3 was prepared by the C−O coupling reaction of the starting product R/S-BINOL with methyl 4-bromo-2fluorobenzoate. Then, R/S-5 was synthesized by esterolysis and acylation of R/S-3. R/S-6 could be obtained by the substitution reaction between 4-fluoro-N,N-dimethylbenzamide and R/S-BINOL-MOM. R/S-1 and R/S-2 could be achieved by the C−N coupling of R/S-5 and R/S-6 with phenoxazine in total yields of 49.4 and 36.6%. All the new compounds have been determined by NMR and mass spectra. As is evident from TGA and DSC curves in Figure S1, the 5% weight loss degradation temperatures (Td) of R-1 and R-2 are above 494 and 364 °C, and the glass-transition temperatures (Tg) are located at 212 and 217 °C, respectively. The high Td and Tg of the R/S-1 and R/S-2 revealed their good thermal and morphological stabilities, which are necessary for application in OLEDs. Besides, R/S-1 and R/S-2 showed good solubility in toluene, chlorobenzene, chloroform, and THF, which were suitable for solution-processed devices. Electrochemical Properties and Theoretical Calculations. The electrochemical behavior of R-1 and R-2 was measured by CV. As presented in Figure S2 and Table S1, the onset of oxidation potentials of R-1 and R-2 are 0.40 and 0.43 V, corresponding to HOMO energy levels (EHOMO) of −4.80 and −4.83 eV. The energy level gaps (Eg) of R-1 and R-2 are estimated to be 2.42 and 2.66 eV from the onset absorption wavelength. The LUMO energy levels (ELUMO) are −2.38 and −2.17 eV for R-1 and R-2 calculated from EHOMO and optical Eg. To gain full insight into the electronic structures of R/S-1 and R/S-2, density functional theory calculations were carried out with the Gaussian 09 package at the B3LYP/6-31+G** level. The optimized structures as well as the HOMO−LUMO spatial distributions of S-1 and S-2 are shown in Figure 1. For S-1 and S-2, the HOMO is occupying the phenoxazine (PXZ) moiety, whereas the LUMO is located on the chiral electronacceptor xanthenone or benzophenone moieties. Such clear separation of spatial electronic distribution in HOMO− LUMO is beneficial to the decrease of ΔEST, which is necessary for the efficient RISC process.27 In addition, the calculated HOMO/LUMO energy levels were −5.14/−2.74 and −5.10/−2.53 eV for S-1 and S-2, respectively. These results coincide well with the above CV experimental data. Photophysical Properties. The UV−vis absorption and PL emission spectra of R-1 and R-2 were measured in THF solution and spin-coating films (Table 1). As illustrated in Figure 2a, R-1 has four absorption bands situated at 450, 405, D

DOI: 10.1021/acsami.9b07005 ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX

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ACS Applied Materials & Interfaces

Figure 2. (a) Absorption (10−5 mol L−1 in THF) and PL (lines for neat films, lines, and symbols for 15 wt % films in TCTA) spectra of R-1 and R2. (b) Transient PL decay spectra of R-1 and R-2 in doped films at 300 K.

Figure 3. (a) CD (in neat films) and (b) CPL (in 10−4 mol L−1 toluene solutions, neat films, and doped films) spectra of R/S-1 and R/S-2.

Figure 4. (a) Energy level diagram and (b) molecular structures of the compounds used in the device.

−1 and +3.5 × 10−4 for S-1 in neat films. Meanwhile, only R/S1 can emit mirror-imaged CPL signals in both solutions and films (Figure 3b). The CPL glum values are −1.2 × 10−3/+1.6 × 10−3 (581 nm), −7.1 × 10−4/+9.2 × 10−4 (587 nm) and −7.2 × 10−4/+8.2 × 10−4 (568 nm) in toluene solutions, neat films, and doped films, respectively (Figure S11). As for R/S-2, almost no CPL signal can be detected in either solutions or films (Figure S12). We propose that the fixed conjugation skeleton between chiral binaphthyl and the electron-acceptor xanthenone can promote the direct chirality transfer from chiral binaphthyl to TADF chromophore in R/S-1, which results in the CPL emission. On the contrary, the rotatable benzophenone structure in R/S-2 can limit the chirality transfer process, which leads to the silence in the CPL response.

(Φdelayed) of 5.3 and 11.8%. The efficiencies of intersystem crossing (ISC) (ΦISC) are 28.7 and 75.1% with the ISC rate (kISC) of 9.8 × 106 and 4.04 × 107 s−1, which promotes the ISC from S1 to T1. The efficiencies of RISC (ΦRISC) are 18.5 and 15.7% with the RISC rate (kRISC) of 1.4 × 106 and 4.2 × 106 s−1, which is of benefit to the efficient RISC process and delayed fluorescence emission. Chiroptical Properties. As is evident from Figure 3a, two pairs of chiral TADF emitters can exhibit good mirror-imaged CD signals in neat films. The Cotton effects in the shortwavelength region from 200 to 300 nm can be assigned to characteristic absorption of chiral binaphthyl. However, the cotton effects in the long-wavelength region can be observed only for R/S-1 at 450 nm (inset of Figure 3a), which can be regarded as the ICT absorption. The absorption dissymmetry factors (gabs) at 450 nm are calculated to be −5.8 × 10−4 for R E

DOI: 10.1021/acsami.9b07005 ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX

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ACS Applied Materials & Interfaces

Figure 5. (a) J−V−L, (c) CE−L, and (d) EQE−L characteristics of the CP-OLEDs based on R/S-1 in neat films and doped films (15 wt % in TCTA). (b) Normalized EL spectra of the CP-OLEDs at a voltage of 8 V.

Table 2. EL Performance of the CP-OLED Devicesa device

EML

Von [V]

Lmax [cd m−2]

CEmax [cd A−1]

EQEmax [%]

λEL [nm]

A B C D F

R-1 S-1 TCTA:15 wt % R-1 TCTA:15 wt % S-1 CBP:15 wt % R-1

3.1 3.0 2.7 2.7 3.4

2726 4212 9489 11 783 40 470

0.22 0.36 2.1 4.2 9.1

0.12 0.22 0.85 1.8 4.1

604 604 580 580 596

CIE [x, i] 0.57, 0.57, 0.49, 0.50, 0.56,

0.43 0.43 0.48 0.48 0.44

gEL [10−3] −0.42 0.58 −0.9 1.0

Abbreviations: EML = emitting layer, Von = turn-on voltage at 1 cd m−2, Lmax = maximum brightness, CEmax = maximum current efficiency, EQEmax = maximum EQE, λEL = maximum EL wavelength, CIE = Commission Internationale de L’Eclairage color coordinates measured at 8 V, gEL = EL dissymmetry factors at the maximum EL wavelength. a

EL Properties and CP-EL. Encouraged by the TADF and CPL properties of R/S-1, CP-OLEDs were fabricated to evaluate the EL performance of the enantiomers. The devices adopt the configuration (Figure 4): ITO/PEDOT:PSS (25 nm)/EML (35 nm)/TPBI (35 nm)/Ca (10 nm)/Ag (100 nm), where the EML refers to R-1, S-1, TCTA: 15 wt % R-1 and TCTA: 15 wt % S-1 for devices A, B, C, and D, respectively. Herein, TCTA was selected as the host material for the doped devices owing to its high triplet energy level, good hole transport ability, and excellent solution processability. The current density−voltage−luminance (J−V−L), current efficiency−luminance (CE−L), and external quantum efficiency−luminance (EQE−L) characteristics of the devices are shown in Figure 5, and the main EL parameters of the devices are outlined in Table 2. Device A/device B based on the neat film of R-1/S-1 exhibits the turn-on voltage (Von), maximum brightness (Lmax), maximum current efficiency (CEmax), and maximum external quantum efficiency (EQEmax) of 3.1/3.0 V, 2726/4212 cd m−2, 0.23/0.38 cd A−1, and 0.12%/ 0.22%, respectively. As for the doped devices, the Von, Lmax, CEmax, and EQEmax are 2.7 V, 9489 cd m−2, 2.1 cd A−1, and 0.85% for device C, 2.7 V, 11783 cd m−2, 4.2 cd A−1 and 1.8% for device D, respectively. Compared with the non-doped

devices, the doped devices achieve lower Von and much higher EQEmax, which can be attributed to more efficient charge carrier transport and alleviated exciton quenching effects.49,50 The EL spectra of the devices are also presented in Figure 5b. Orange-red EL centered at 604 and 580 nm can be detected for the nondoped devices and doped devices, respectively. Moreover, there has been almost no change for the EL spectra as the driving voltages ranged from 6 to 9 V (Figure S13), demonstrating that the CP-OLEDs are spectrally stable. In order to improve the efficiencies of the CP-OLEDs, we further chose 4,4′-dicarbazolyl-1,1′-biphenyl (CBP) as the host material. CBP is the widely used host material for green and red luminogens and has a high triplet energy level of 2.6 eV, which is sufficient to block the triplet excitons within the guest molecules.47 The devices have the structure of ITO/ PEDOT:PSS (25 nm)/CBP: x wt % R-1 (35 nm)/TPBI (35 nm)/Ca (10 nm)/Ag (100 nm), x = 10, 15, 20 and 25 for devices E, F, G, and H. The EL performances of devices E−H are shown in Figure 6. As can be seen from Figure 6a, devices E−H exhibit low Von of 3.3−4.0 V and high Lmax up to 40 470 cd m−2 (device F, CBP: 15 wt % R-1). Devices E−H emit orange-red EL with the peaks from 588 to 604 nm as the doping concentration increases. Figure 6c,d reveals that the F

DOI: 10.1021/acsami.9b07005 ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX

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Figure 6. (a) J−V−L, (c) CE−L, and (d) EQE−L characteristics of the CP-OLEDs based on R-1 doped in CBP. (b) Normalized EL spectra of the CP-OLEDs based on R-1 doped in CBP at a voltage of 8 V.

Figure 7. (a) CP-EL spectra and (b) curves of gEL values versus wavelength for CP-OLEDs based on R/S-1 neat films and doped films in TCTA.

optimized doping concentration of R-1 in CBP is 15 wt % (device F), which can result in CEmax of 9.1 cd A−1 and EQEmax of 4.1%. Furthermore, device F can preserve relative high CE and EQE above 8.5 cd A−1 and 3.8% at the luminance from 1000 to 5000 cd m−2. The efficiencies of device F are comparable to several reported TADF emitter-based orangered OLEDs.51−53 The CP-EL spectra of R/S-1 (devices A−D) in Figure 7a were recorded in a Jasco CPL-300 spectrometer. Both of the nondoped and doped devices can exhibit mirror-imaged CPEL signals among the region of the EL spectra. In order to measure the intensity of the CP-EL signals, the data of gEL were calculated and are presented in Figure 7b. The gEL values are −4.2 × 10−4/+5.8 × 10−4 at 604 nm for R/S-1 in nondoped devices and −0.9 × 10−3/+1.0 × 10−3 at 580 nm for R/S-1 in doped devices. These gEL values are analogous to the glum of R/ S-1 in films and the reported CP-EL of chiral TDAF molecules.36

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CONCLUSIONS

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ASSOCIATED CONTENT

In summary, two pairs of TADF-active enantiomers modified by extending the conjugated skeletons of chiral BINOL can exhibit typical AIE properties and long-lived delayed fluorescence emission in doped films. Only chiral binaphthyl enantiomers with the fixed conjugation structure can display CPL-active response in both toluene solution, and neat and doped films. The orange-red CP-OLEDs based on the doped films of R/S-1 can provide low Von of 3.4 V, high Lmax up to 40 470 cd m−2, moderate EQE of 4.1%, as well as CP-EL with gEL of −0.9 × 10−3/+1.0 × 10−3. This work supplies a new strategy on developing chiral TADF-based CPL materials for CPOLEDs.

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acsami.9b07005. G

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Thermal properties, electrochemical properties, photophysical properties and chiroptical properties of R/S-1 and R/S-2; NMR and HRMS Spectra of new compounds (PDF)

AUTHOR INFORMATION

Corresponding Authors

*E-mail: [email protected] (Y.Q.). *E-mail: [email protected] (Y.L.). *E-mail: [email protected] (Y.C.). ORCID

Yiwu Quan: 0000-0001-6017-1029 Yixiang Cheng: 0000-0001-6992-4437 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was supported by the National Natural Science Foundation of China (21674046 and 51673093). REFERENCES

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DOI: 10.1021/acsami.9b07005 ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX