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Achieving Efficient Blue Delayed Electrofluorescence by Shielding Acceptor with Carbazole Units Zong Cheng, Zhiqiang Li, Yincai Xu, Jixiong Liang, Chunhui Lin, Jinbei Wei, and Yue Wang ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.9b07820 • Publication Date (Web): 10 Jul 2019 Downloaded from pubs.acs.org on July 17, 2019

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ACS Applied Materials & Interfaces

Achieving Efficient Blue Delayed Electrofluorescence by Shielding Acceptor with Carbazole Units Zong Cheng, Zhiqiang Li, Yincai Xu, Jixiong Liang, Chunhui Lin, Jinbei Wei* and Yue Wang* State Key Laboratory of Supramolecular Structure and Materials, College of Chemistry, Jilin University, Changchun 130012, P. R. China.

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ABSTRACT: The design and synthesis of blue thermally activated delayed fluorescence (TADF) emitters that have high electroluminescence (EL) efficiency and low efficiency roll-off feature remains a great challenge. Herein, we developed a facile and efficient strategy by shielding acceptors with carbazole units for constructing high performance blue TADF emitters. Benzonitrile (BN), 9, 9-diphenylacridan (DPAc), carbazole (Cz) were adopted as acceptor, donor and protector, respectively, to build two TADF emitters named DPAc-DCzBN and DPAcDtCzBN. The nondoped organic light-emitting diodes (OLEDs) of DPAc-DCzBN as emitter exhibited a standard sky-blue emission with CIE (Commission Internationale de L’Eclairage) coordinates of (0.16,0.26), high external quantum efficiency (EQE) of 20.0% and low efficiency roll-off (EQEs of 19.5%, 16.1% and 12.6% at 100, 500 and 1000 cd m‒2, respectively) which is an outstanding nondoped blue TADF OLEDs. The doped device of DPAc-DtCzBN displayed a pure blue emission and corresponding CIE coordinates is (0.16, 0.15). Meanwhile, it also demonstrated high and stabilized EQE value of 23.1%, 18.3% and 11.5% at maxima, 100 and 500 cd m‒2, respectively, which is a quite high level among the pure-blue TADF OLEDs. This study testifies the feasibility of our strategy in constructing high performance TADF electroluminescent materials.

Keywords: TADF, OLEDs, D-A emission molecule, blue electroluminescence, low efficiency roll-off

1. INTRODUCTION Recently, Adachi and coworkers proposed and employed thermally activated delayed fluorescence (TADF) molecules as emitters to fabricate high performance organic light-emitting


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diodes (OLEDs) owing to the capacity of utilizing 100% electron injection induced excitons through reverse intersystem-crossing (RISC) process.1-6 Highly efficient TADF emitters with green,7-8 yellow,9-11 and red12-15 colors have been successfully constructed and applied in fluorescent OLEDs (FOLEDs), but easily-synthesized and superior sky-blue (CIEy≤0.3, Commission Internationale de l’Eclairge) and pure blue (CIEy≤0.15) emitters are still rare.16-20 Between the lowest singlet excited state (S1) and triplet excited state (T1), a sufficiently small energy gap (△EST) is critical to achieve the RISC process and obtain effective TADF characteristics. It was demonstrated that minimization of △EST has been achieved by separating the highest occupied molecular orbitals (HOMOs) and the lowest unoccupied molecular orbitals (LUMOs). The rational design of electron donor-acceptor (D-A) structure can endow the molecules with TADF feature based on intramolecular charge transfer (ICT) state. It is well known that ICT states often generate emission at longer wavelength region, which is the intrinsic issue for developing blue TADF emitters.21-23 Most TADF OLEDs suffer from the efficiency roll-off since long lifetimes of delayed excited states. Especially for blue TADF OLEDs (CIEy≤0.3), the delicate balance between color purity and TADF characteristics always causes unsatisfactory performance. Previous studies demonstrated that the triplet-triplet annihilation (TTA) and triplet-polaron annihilation (TPA) is mainly responsible for the efficiency roll-off in OLEDs upon exciton density increasing.24-27 For TADF emitters, the accomplishment of TTA and TPA processes are based on intermolecular short-range electron-exchange.28 Therefore, inhibiting intermolecular electron-exchange should be an efficient strategy to suppress efficiency roll-off. In principle, shortening the lifetimes of T1 excitons can reduce the opportunities of TTA and TPA.29-31 For instance, Adachi and co-workers have developed two highly efficient blue TADF emitters named 3Cz2DPhCzBN and


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3Ph2CzCzBN.32 By introducing a second type of donor (D2), the energy levels of locally excited triplet states (3LE) and CT triplet state (3CT) of TADF molecules can be properly tuned, thus facilitate the RISC progress. Extraordinarily high rate constants of RISC (kRISC) of 9.9 × 105 and 4.4 × 105 s-1 were achieved for 3Cz2DPhCzBN and 3Ph2CzCzBN, respectively, companying with extraordinarily low roll-off and high external quantum efficiency (EQE) of 20.9% and 17.9%. Obviously, blocking intermolecular electron exchange channels is another acceptable approach to obtain low efficiency roll-off. For T1 excitons, the two spin parallel electrons distribute on D and A moieties. Introducing rational blocking groups to separate D and A moieties may inhibit intermolecular electron exchange.33 Yasuda and co-workers demonstrate that there is no spin density distributions on the terminal substituents attached to sp3-hybridized carbon atom of acridan units in a D-A molecular.34 Therefore, high performance and remarkably low roll-off TADF OLEDs was attained with XAc-CM, MXAc-BF and MXAc-CM as emitters. However, the terminal substituents can only protect donors but acceptors are still exposed. Conventional bulky alky groups like tert-butyl groups are also introduced to emitters for suppressing intermolecular electron exchange but their electronic inertness causes poor carrier transporting ability which is disadvantageous to electroluminescence (EL).35-36 In this context, we proposed a facile strategy in order to construct blue emission D-A molecular mode (Figure 1), in which the D-A framework was composed of benzonitrile (BN) as acceptor and an aromatic amine derivative as donor. More importantly, on the terminals of D and A moieties conjugated protective groups (P1 and P2) are introduced. Nonplanar spiro-acridan based donor with two phenyls employed as block groups was adopted. Innovatively, carbazole (Cz) and tert-butyl carbazole (t-Cz) were selected as the protecting groups of acceptor. According to above idea we designed and synthesized two D-A molecules 2,6-di(carbazolyl)-4-


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(9,9-diphenylacridanyl)benzonitrile (DPAc-DCzBN) and 2,6-di(tert-butyl-carbazolyl)- 4- (9,9diphenylacridanyl)benzonitrile (DPAc-DtCzBN). 9,9-diphenylacridin (DPAc) has stronger electron donating property than Cz, therefore, in DPAc-DCzBN and DPAc-DtCzBN the intramolecular charge transfer process should be defined from DPAc to BN. Moreover, the mutually proper electron donating and withdrawing ability between DPAc and BN ensure that the ICT excited states of DPAc-DCzBN and DPAc-DtCzBN display blue emission. The conjugated Cz and phenyl groups with strong rigid feature are conducive to enhancements of emission efficiency and carrier mobility. In order to highlight the advantage of DPAc-DCzBN and

DPAc-DtCzBN,

we

synthesized

a

reference

compound

4-(9,9-

diphenylacridanyl)benzonitrile (p-ACN) according to the reported method (Scheme 1).37 Eventually, high performance sky-blue and pure blue TADF devices were fabricated with DPAcDCzBN and DPAc-DtCzBN as emitters. The nondoped sky-blue (0.16,0.26) device of DPAcDCzBN demonstrated maximum EQE value of 20.0% and remarkable efficiency stability: EQE values of 19.5%, 16.1% and 12.6% at 100, 500 and 1000 cd m‒2, respectively. The doped pure blue (0.16,0.15) device of DPAc-DtCzBN exhibited maximum EQE value of 23.1%, which remained 18.3% and 11.5% at 100 and 500 cd m‒2, respectively.

Figure 1. Proposed D-A molecular modes and molecular structures of DPAc-DCzBN and DPAc-DtCzBN.


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2. EXPERIMENTAL SECTION General Information. Bruker AVANCE III 500 MHz spectrometer was selected to measure the 1H NMR and 13C spectra with tetramethylsilane (TMS) utilized as internal standard. Thermo Fisher ITQ1100 GC/MS mass spectrometer was employed to measure the mass spectra. Flash EA 1112 spectrometer were used to perform the elemental analyses. Shimadzu RF-5301 PC spectrometer and Shimadzu UV-2550 spectrophotometer were adopted to record the PL emission spectra and UV-Vis absorption, respectively. The fluorescence and phosphorescent spectra taken at liquid nitrogen temperature (77 K) were recorded by Ocean Optics QE Pro with a 375 nm Ocean Optics LLS excitation source. Edinburgh FLS920 steady state fluorimeter equipping with an integrating sphere was employed to measure the absolute fluorescence quantum yields of both solutions and films. At a heating rate of 10 K min–1, NETZSCH DSC204 was utilized to measure the differential scanning calorimetric (DSC) of target molecules under nitrogen atmosphere. In the range of 25 to 800 C, TA Q500 thermogravimeter was selected to perform the thermogravimetric analyses (TGA) of target molecules under nitrogen atmosphere at a heating rate of 10 K min–1. BAS 100W Bioanalytical electrochemical work station was used to measure the electrochemical property with platinum disk as working electrode, platinum wire as auxiliary electrode, a porous glass wick Ag/Ag+ as pseudo reference electrode and ferrocene/ferrocenium as the internal standard. And 0.1 M solution of n-Bu4NPF6 in anhydrous CH2Cl2 and THF which is the supporting electrolyte was utilized to measure the oxidation and reduction potentials at a scan rate of 10 mV s-1. FLS980 fluorescence lifetime measurement system with 365 nm LED excitation source was selected to investigate the transient PL decay. Materials. All reagents were bought from Energy Chemical Co. and/or J&K scientific Ltd. Co. and immediately used without further purification. Schlenk technology was strictly performed


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under nitrogen conditions in all reactions. Syntheses process was showed below in detail. The final products were first purified by column chromatography, then temperature-gradient vacuum sublimation was utilized to further purify target molecules. Synthesis of 4-bromo-2,6-di(carbazol-9-yl)benzonitrile (compound 2a): 50 ml solution of anhydrous DMF (N,N- dimethylformamide) which contains 9H-carbazole (7.515 g, 45 mmol) was added dropwise into a mixture of t-BuOK (6.732g, 60 mmol) and 50 ml anhydrous DMF in a period of 15 min. After the system was stirring for 2h at ambient temperature, 20 ml anhydrous DMF solution containing 4-bromo-2,6-difluorobenzonitrile (4.36 g, 20 mmol) was injected dropwise into it within 15 min. The solution was stirred at 90°C for 12h and then cooled down. The resulted solution was poured into ice water (2000 g). The white powder solid was filtered out and dried in vacuum first, then it was further purified by column chromatography with using a mixture eluent of dichloromethane/petroleum ether (1:2), resulting in a white solid (7.48 g). Yield: 73%. ESI-MS (M): m/z: 511.22 [M]+ (calcd: 511.07). 1H NMR (500 MHz, Chloroform-d1) δ/ppm: 8.15 (d, J = 7.3 Hz, 4H), 7.93 (s, 2H), 7.54 – 7.50 (m, 4H), 7.41 – 7.36 (m, 8H).

13C

NMR (125 MHz, Chloroform-d) δ/ppm: 143.66, 140.37, 131.96, 129.12, 126.64, 124.35, 121.59, 120.88, 112.80, 109.72. Synthesis of 4-bromo-2,6-di(3,6-tert-butyl-carbazol-9-yl)benzonitrile (compound 2b): A similar procedure like synthesizing compound 2a was carried out except replacing carbazole with tert-butyl carbazole (12.573 g, 45 mmol), resulting in a white solid (13.42 g). Yield: 91%. ESIMS (M): m/z: 735.27 [M]+ (calcd: 735.32). 1H NMR (500 MHz, DMSO-d6) δ/ppm: 8.34 (s, 4H), 8.28 (s, 2H), 7.57 (d, J = 8.6 Hz, 4H), 7.41 (d, J = 8.7 Hz, 4H), 1.44 (s, 36H). 13C NMR (125 MHz, Chloroform-d) δ/ppm: 144.59, 144.23, 138.94, 131.15, 128.92, 124.54, 124.30, 116.96, 113.28, 111.22, 109.38, 34.96, 32.09.


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Synthesis of 2,6-di(carbazol-9-yl)-4-(9,9-diphenylacridin-10-yl)benzonitrile (DPAc-DCzBN): Compound 2a (0.512 g, 1 mmol), DPAc (0.50 g, 1.5 mmol), Cs2CO3 (1.30 g, 4 mmol) were added into anhydrous o-xylene (30 mL). After compound 2a and DPAc were fully dissolved, Pd2(dba)3 (0.92 g, 0.1 mmol) were added into the system. Thereafter, tri-tert-butylphosphine (TTBP) (0.20 g, 0.1 mmol) were injected into the system. The system was stirred and refluxed for 12 h and then cooled down to ambient temperature. The resulted solution was extracted with dichloromethane. The concentrated organic layer was evaporated and purified by column chromatography with a mixture eluent of dichloromethane/petroleum ether (1:1), resulting in a white powder (0.63 g) in 82% yield. ESI−MS m/z: 764.19 [M]+ (calcd: 764.29) Anal. Calcd for C56H36N4: C, 87.93; H, 4.74; N, 7.32. Found: C, 87.97; H, 4.72; N, 7.30. Td= 438 °C. 1H NMR (500 MHz, DMSO-d6) δ/ppm: 8.25 (dt, J = 7.8, 0.9 Hz, 4H), 7.68 (dd, J = 8.0, 1.2 Hz, 2H), 7.57 (ddd, J = 8.3, 7.2, 1.2 Hz, 4H), 7.37 – 7.28 (m, 12H), 7.22 – 7.17 (m, 8H), 6.88 (dd, J = 7.8, 1.4 Hz, 2H), 6.77 – 6.71 (m, 4H).

13C

NMR (125 MHz, Chloroform-d) δ/ppm: 149.73, 143.86,

143.82, 142.69, 140.52, 140.34, 130.42, 130.12, 128.10, 127.24, 127.11, 126.29, 125.67, 124.49, 123.85, 120.83, 120.67, 113.76, 109.82, 102.27, 58.85. Synthesis

of

2,6-di(3,6-tert-butyl-carbazol-9-yl)-4-(9,9-diphenylacridin-10-yl)benzonitrile

(DPAc-DtCzBN): A similar procedure like synthesizing DPAc-DCzBN was carried out except replacing compound 2a with compound 2b (0.737 g, 1 mmol), resulting in a white solid (0.841 g) in 85% yield. ESI−MS m/z: 988.48 [M]+ (calcd: 988.54). Anal. Calcd for C72H68N4: C, 87.41; H, 6.93; N, 5.66. Found: C, 87.35; H, 6.84; N, 5.81. Td= 445 °C. Melting point (m.p) =322 °C. 1H NMR (500 MHz, DMSO-d6) δ/ppm: 8.31 (d, J = 1.9 Hz, 4H), 7.62 – 7.56 (m, 6H), 7.32 (dt, J = 16.2, 7.4 Hz, 4H), 7.24 – 7.16 (m, 12H), 6.88 (d, J = 7.8 Hz, 2H), 6.76 (d, J = 7.8 Hz, 4H), 1.44 (s, 36H).

13C

NMR (125 MHz, Chloroform-d) δ/ppm: 149.29, 144.11, 143.67, 143.41, 142.25,


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140.56, 139.07, 130.41, 130.08, 128.05, 127.23, 127.03, 125.03, 123.90, 123.27, 116.71, 115.55, 113.92, 109.29, 103.35, 58.60, 34.90, 32.13. Synthesis of 4-(9,9-diphenylacridin-10(9H)-yl)benzonitrile (p-ACN): A similar procedure like synthesizing DPAc-DCzBN was carried out except replacing compound 2a with 4bromobenzonitrile (0.182 g, 1 mmol), resulting in a white solid (0.39 g) in 90% yield. ESI−MS m/z: 434.11 [M]+ (calcd: 434.18) Anal. Calcd for C56H36N4: C, 88.45; H, 5.10; N, 6.45. Found: C, 88.40; H, 5.03; N, 7.42. 1H NMR (500 MHz, DMSO-d6) δ/ppm: 8.08 – 8.02 (m, 2H), 7.34 – 7.23 (m, 8H), 7.13 (ddd, J = 8.4, 7.2, 1.6 Hz, 2H), 6.96 (td, J = 7.5, 1.2 Hz, 2H), 6.91 – 6.84 (m, 4H), 6.79 (dd, J = 7.8, 1.5 Hz, 2H), 6.41 (dd, J = 8.2, 1.2 Hz, 2H). 13C NMR (125 MHz, Chloroform-d) δ/ppm: 145.93, 145.65, 141.50, 134.40, 131.47, 131.00, 130.40, 130.29, 127.80, 127.07, 126.57, 121.33, 118.45, 114.95, 111.34, 56.99. Device Fabrication and Measurements. The indium tin oxide (ITO) glass substrates with a sheet resistance of 15 Ω per square were treated with plasma for 5 min, after cleaning with optical detergents, deionized water, acetone and isopropanol successively. Then they were transferred to a vacuum chamber in a nitrogen-filled glove box. Under high vacuum (

Achieving Efficient Blue Delayed ... - ACS Publications

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