Difluoroboron-Enabled Thermally Activated Delayed Fluorescence

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Difluoroboron-Enabled Thermally Activated Delayed Fluorescence Guijie Li, Weiwei Lou, Dan Wang, Chao Deng, and Qisheng Zhang ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.9b08107 • Publication Date (Web): 07 Aug 2019 Downloaded from pubs.acs.org on August 7, 2019

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ACS Applied Materials & Interfaces

Difluoroboron-Enabled Thermally Activated Delayed Fluorescence Guijie Li,1,* Weiwei Lou,1 Dan Wang,2 Chao Deng,2* Qisheng Zhang2 1State

Key Laboratory Breeding Base of Green Chemistry-Synthesis Technology, College of Chemical

Engineering, Zhejiang University of Technology, Hangzhou, 310014, P. R. China 2MOE

Key Laboratory of Macromolecular Synthesis and Functionalization, Department of Polymer

Science and Engineering, Zhejiang University, Hangzhou 310027, P. R. China E-mail: [email protected]; [email protected].

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ABSTRACT: A new series of tetracoordinated boron-enabled thermally activated delayed fluorescence (TADF) materials with a donor-acceptor-BF2 type framework was designed and conveniently synthesized. Difluoroboron plays a critical role and acts as a key to coordinate with the latent acceptor of the 2-(4-phenylpyridin-2-yl)phenol (PPyPOH) moiety to realize TADF. TADF materials are air-stable and have a high photoluminescence quantum yield of up to 99%. NOBF2-Cz- and NOBF2-DPCz-doped blue OLEDs demonstrated EQEs of 11.0% with CIE coordinates of (0.14, 0.16) and 15.8% with (0.14, 0.28), respectively, and high brightness of 6761 and 19383 cd/m2 could be achieved. Moreover, the blue OLED doped with NOBF2-DPCz and the green OLED doped with NOBF2-DMAC achieved operational lifetimes at 50% of initial luminance (L0 = 500 cd/m2), LT50, of 54 and 920 h, respectively. This work indicates that these tetracoordinated difluoroboron molecules can act as efficient and stable TADF materials for OLED applications. KEYWORDS: tetracoordinated difluoroboron compound, thermally activated delayed fluorescence, organic light-emitting diode, blue OLED, dihedral angle

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■ INTRODUCTION Organic light-emitting diodes (OLEDs)1–6 play an important role in the development of next generation display and lighting technologies because of their many unique and outstanding merits, such as low-cost fabrication, high color quality, and fast response.7 According to spin statistics, the ratio of the electrically generated singlet and triplet excitons is 1:3.8 Thus, 25% of internal quantum efficiency (IQE) is obtained for OLEDs that use conventional fluorescent materials, due to non-radiative decay from most triplet excitons to produce undesired heat. The IQE can be increased through upconversion by triplet-triplet annihilation (TTA) process to realize a theoretical maximum of 62.5%.9 By contrast, OLEDs that employ phosphorescent materials, such as iridium(III)10–12 and platinum(II)13–20 complexes, can potentially achieve 100% IQE because of the strong spin-orbit coupling (SOC) of the heavy metal complexes to enable efficient intersystem crossing (ISC) and harvest all electrogenerated excitons.3 Recently, another promising approach was presented using all excitons known as thermally activated delayed fluorescence (TADF),5,21–23 in which a small triplet-to-singlet energy gap (ΔEST) is required to facilitate a fast reverse intersystem crossing (RISC) from the lowest excited triplet state (T1) to singlet state (S1), subsequently resulting in radiative decay from S1 to ground state (S0). Thus, the TADF materials have the potential to harvest both singlet and triplet excitons, and realize a theoretical IQE of unity in electroluminescent (EL) devices without the assistance of previous heavy metals. Conventionally, donor-acceptor (D-A) molecular systems are most widely utilized for the design of TADF materials. These synstems enable the spatial separation of the highest occupied molecular orbital (HOMO) and lowest unoccupied molecular orbital (LUMO), resulting in a small ΔEST and a short TADF lifetime. In the last few years, several types of acceptors have been successfully developed24–37 and great progress has been made for the TADF-based OLEDs. However, a new design for efficient TADF materials with long operational lifetime and high brightness in the device setting is still challenging and highly desired.38–41 Organoboron compounds have been intensively studied and extensively applied as optoelectronic materials given their diverse optical and electronic properties.42–46 As a result of the vacant pz orbital, the central boron atoms in the tricoordinated organoboron segments, such as triarylboron,47 3 / 21



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10H-phenoxaborin,48 9,10-dihydro-9,10-diboraanthracene49 and their derivatives, exhibit strong π-electron-accepting properties via p-π conjugations to facilitate intramolecular charge transfer (ICT). Hence, tricoordinated boron acts as an ideal acceptor for TADF materials (Figure 1A).47–62 Generally, at least one bulky aryl group is needed to prevent the boron center attacking by nucleophilic reagents and avoid decomposition in air for these molecules.48,62 Consequently, tetracoordinated boron with octet stability was developed and demonstrated as an ideal hub for a spiro linker without affecting the electronic properties of the donor and acceptor in TADF materials (Figure 1B).63–65 The tetracoordinated boron ordinarily has no contribution to the frontier orbitals for its electron positive and lower-lying electronic transitions. In this work, we report a new molecular design of boron-enabled TADF materials with a D-A-BF2 framework type. This design employs the boron as a key to coordinate with the latent acceptor in the precursor, which enables spatial separation of the HOMO and LUMO, and enhances the ICT character to attain TADF (Figure 1C). Tetracoordinated difluoroboron materials are air-stable, and the boron can significantly adjust the electronic property of the latent acceptor, and enhance the molecular rigidity to achieve high photoluminescence quantum yield (PLQY).

Figure 1. Molecular designs for boron-based TADF materials. (A) TADF materials using tricoordinated boron as an acceptor. (B) TADF materials using tetracoordinated boron as a linker. (C) Boron-enabled TADF materials using boron as a key to coordinate with the latent acceptor in the precursor and attain TADF.

■ RESULTS AND DISCUSSION Synthesis and Characterization. The synthetic routes and molecular structures of the D-A-BF2 type TADF materials are shown in Figures 2a and 2b. Initially, Buchwald−Hartwig amination of the aryl 4 / 21


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chloride ClPPyPOCH3 with amine-type donors of carbazole (Cz), 3,6-di-tert-butylcarbazole (DTCz), 3,6-diphenylcarbazole (DPCz), or dimethylacridine (DMAC), provided the DPPyPOCH3 compounds. The products were demethylated using boron tribromide (BBr3) to allow the precursors of DPPyPOH containing 2-(4-phenylpyridin-2-yl)phenol (PPyPOH) moiety to function as the latent acceptor. Finally, boronation of the DPPyPOH with cheap boron trifluoride diethyl etherate (Et2O:BF3) in the presence of triethylamine or diisopropylethylamine provided the desired tetracoordinated difluoroboron compounds NOBF2-Cz, NOBF2-DTCz, NOBF2-DPCz, and NOBF2-DMAC in excellent yields (93%–99%). These processes are concise and robust without using inaccessible diaryl or triaryl organoboron reagents66–68, thereby enabling us to prepare the materials in gram-scales. All the difluoroboron compounds are air-stable, and they can be purified through column chromatography on silica gel. Difluoroboron compounds were characterized by 1H,

13C, 19F, 11B

NMR, and high-resolution mass spectroscopy

(HRMS). Unfortunately, from 1H NMR spectra (see Supporting Information), a little impurity was observed in the difluoroboron compounds, which could not removed by column chromatographic separation and recrystallization. Theoretical Calculations. Density functional theory (DFT) and time-dependent DFT (TD-DFT) calculations were carried out to investigate molecular geometries and the HOMO and LUMO distributions of the newly-developed difluoroboron compounds. The fully optimized structures were calculated via B3LYP functional at the 6–311G** level in the gas phase. As shown in Figure 2b, the HOMOs are primarily localized on the electron-donating groups Cz, DTCz, DPCz and DMAC. Also, a small portion of HOMOs distribute on the phenylene groups for all the compounds except NOBF2-DMAC. The HOMO level of NOBF2-DMAC (-5.33 eV) is much higher than these of NOBF2-Cz (-5.70 eV), NOBF2-DTCz (-5.59 eV) and NOBF2-DPCz (-5.60 eV), because the aromaticity of the Cz, DTCz and DPCz units make them rather weak donors in comparison to the DMAC unit. The LUMOs are entirely localized on the electron-accepting PPyPO moieties. Notably, no LUMO distributions were observed on the boron atoms. All difluoroboron compounds exhibit similar dihedral angles between pyridyl and phenylene rings (α = 35.8–38.0°) in their optimized S0 structures. However, the donors significantly affect the dihedral angles (β) between the phenylene ring and the 5 / 21



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plane of the corresponding donor, where NOBF2-DMAC has a much larger torsion angle of 90.0° compared with the other three compounds that have only 50.5–53.2° because of the big steric hindrance of the DMAC substituent. This condition results in a small overlap between HOMO and LUMO of NOBF2-DMAC, leading to a much smaller ΔEST value of 0.01 eV as compared with NOBF2-Cz, NOBF2-DTCz, and NOBF2-DPCz with 0.24, 0.28 and 0.18 eV, respectively. The orthogonal D-A conformation of NOBF2-DMAC also weakens the p-π conjugation between DMAC and phenylene ring, thereby leading to a small oscillator strength (f = 0.0001).

Figure 2. (a) The synthetic route of the difluoroboron compounds through C-N cross-coupling, demethylation and 6 / 21


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difluoroboronation reactions. (b) Molecular structures, theoretical HOMO/LUMO distributions and HOMO/LUMO levels of the four difluoroboron compounds. The HOMO/LUMO levels are obtained from their redox potentials.

To investigate the effect of difluoroboron (BF2) units, DFT calculations were also performed for the DPPyPOH precursors, the results and comparisons with the difluoroboron compounds are shown in Figure S1. All the DPPyPOH precursors exhibit similar molecular geometries, α and β dihedral angles, and HOMO distributions compared with the difluoroboron compounds because of the existence of intramolecular hydrogen bond (N…H–O). However, the LUMOs of the precursors are mainly localized on 2-phenylpyridine rings, only a little on phenyloxy moieties. By contrast, the LUMOs are well-distributed on the PPyPO moieties for the difluoroboron compounds, because of the strong electron-withdrawing ability of the BF2 unit, which results in the LUMO level of the difluoroboron compound is much deeper than that of the corresponding precursor. This study indicates that the BF2 units significantly affect the LUMOs of the difluoroboron compounds. Photophysical Properties. Figure 3a shows the UV/Vis absorption and PL spectra of the newly developed difluoroboron compounds in toluene at room temperature; and their photophysical data are given in Table 1. The prominent absorption bands at 283–295 nm are assigned to π-π* transitions, and the broad shoulders at 358–375 nm are identified as the ICT transitions from the amine-type donors to the PPyPO moieties. The PL spectra exhibit bathochromic shift as increasing the electron-donating ability of the donor group and display relatively narrow full width at half maximum (FWHM) of 68–72 nm for the blue emitters. The compounds exhibit blue and green emission in toluene at room temperature (Figure S2). The ICT characteristics of the compounds are supported by the PL spectra in various solvents. The absorption spectra are almost independent of solvent polarity (Figure S3), whereas the PL spectra exhibit significant redshifts in higher-polarity solvents (Figure S4). We also measured the fluorescence spectra and phosphorescence spectra at 77 K in toluene (Figure S5), and the energy level of singlet (ES) and triplet (ET) were calculated from the onset of the fluorescence and phosphorescence spectra, respectively 27 (Table 1). The ΔEST values were calculated as 0.20, 0.20, 0.22, and 0.08 eV for NOBF2-Cz, NOBF2-DTCz, NOBF2-DPCz, and NOBF2-DMAC, respectively, which is consistent with the trend of the TD-DFT calculations discussed above. The ΔEST values are relatively smaller than those reported in the literatures,37 which indicates that the difluoroboron compounds might be TADF 7 / 21



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materials.

Figure 3. (a) UV–vis and PL spectra of the difluoroboron compounds in toluene solution at room temperature. (b) PL spectra of the difluoroboron compounds in 10 wt% emitter:DPEPO doped film measured at room temperature under N2. (c) Transient PL decay curves of the difluoroboron compounds in 10 wt% emitter:DPEPO doped film measured at room temperature under N2. (d) Cyclic voltammograms of the difluoroboron compounds measured in anhydrous N,N-dimethylformamide (DMF) in nitrogen atmosphere; Silver wire, platinum wire and glassy carbon were used as pseudoreference electrode, counter electrode and working electrode respectively; scan rate is 300 mV/s.

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Table 1. Photophysical and Electrochemical Properties of the Difluoroboron Compounds λabs [nm]

λPL [nm]

λFWHM [nm]

ΦPLc [%]

sola

sola/filmb

sola/filmb

filmb

NOBF2-Cz

283,366

449 / 475

68 / 74

99

NOBF2-DTCz

295,375

473 / 488

71 / 77

NOBF2-DPCz

288,374

471 / 496

72 / 81

NOBF2-DMAC

283,358

540 / 542

92 / 104

Compound

τpd [ns]/

HOMO/

ES/ETf

ΔESTg

kRISCh

LUMOe [eV]

[eV]

[eV]

[103 s-1]

6.7 / 132

-5.70 / -2.99

3.08 / 2.88

0.20

8.4

74

6.3 / 126

-5.59 / -3.00

3.01 / 2.81

0.20

4.2

70

6.3 / 110

-5.60 / -3.00

3.04 / 2.82

0.22

5.2

65

6.2 / 57

-5.33 / -3.01

2.85 / 2.77

0.08

d [µs]

τd

10.7

Measured in toluene solution at room temperature. 10 wt%-doped thin film in DPEPO at room temperature. Absolute PLQY measured using an integrating sphere in nitrogen atmosphere on PTI QM-40 system with excitation at 360 nm. dPL lifetimes of prompt (τ ) and delayed (τ ) decay portions for the 10 wt%-doped film measured at room temperature p d using semiconductor laser (371 nm) on Horiba deltaflex01 system via TCSPC technique and using nitrogen laser (337 nm) on PTI QM-40 spectrofluorometer via strobe technique, respectively. eHOMO and LUMO levels were calculated by utilizing Cp2Fe+/Cp2Fe values of 4.8 eV below the vacuum level. fSinglet (ES) and triplet (ET) energies estimated from onset wavelengths of the fluorescent and phosphorescent spectra at 77 K in toluene solution, respectively. gΔEST = ES – ET. hSee Table S1. a

b

The

PL

spectra

of

the

c

difluoroboron

compounds

doped

in

host

bis[2-(diphenylphosphino)phenyl]ether oxide (DPEPO, ET = 3.0 eV) were also measured at room temperature, as illustrated in Figure 3b. The PL spectra become slightly broader and show a smaller redshift compared with those in toluene because of the higher polarity of the host DPEPO.69 Photoluminescence quantum yields (ΦPL) of NOBF2-Cz, NOBF2-DTCz, NOBF2-DPCz and NOBF2-DMACC in DPEPO are 99%, 74%, 70% and 65%, respectively, at room temperature in nitrogen atmosphere (Table 1); the ΦPL of prompt and delay portion of the four compounds were also measured in toluene, which were 81%/17%, 54%/13%, 56%/14% and 48%/17% respectively (Table S1). The relatively low ΦPL of NOBF2-DMAC can be attributed to its small f. The ΦPL of all the DPPyPOH precursors in DPEPO films were also measured, however, nearly no luminescence could be detected (ΦPL < 1%), which demonstrates that the presence of the BF2 unit plays a critical role in the realization of highly efficient TADF. Then the transient PL decay curves of the difluoroboron compounds in doped DPEPO films were measured at room temperature. As shown in Figures 3c, S6 and S7, and Table 1, all four curves contain prompt components with lifetime (τp) of 6.2–6.7 ns, corresponding to the fluorescence generation from S1 to S0, and also a long-delayed components with τd of 57–132 µs, which are the emission from S1 to T1 transition, then upconversion back to S1 and radiative decay to S0. These results demonstrate that the delayed fluorescence occurred via the RISC process. The shorter τd of 9 / 21



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NOBF2-DMAC than that of the other compounds can be explained by the considerably smaller ΔEST value (0.08 eV), resulting in a larger RISC rate. Electrochemical Properties. The electrochemical properties of the difluoroboron compounds were investigated using cyclic voltammetry (CV) and differential pulsed voltammetry (DPV) using ferrocenium/ferrocene (Cp2Fe+/Cp2Fe) as internal reference in anhydrous N,N-dimethylformamide (DMF) solutions under nitrogen atmosphere (Figure 3d, Table S2). All compounds show well-defined reversible reduction processes with nearly the same reduction potentials, which is consistent with their LUMO distributions on the same PPyPO acceptors with levels of about −3.00 eV. These findings demonstrate that the PPyPO moieties can act as stable boron-enabled acceptors. By contrast, their oxidation processes are strongly affected by the structures of the compounds. NOBF2-Cz and NOBF2-DTCz have irreversible reduction processes; however, NOBF2-DPCz and NOBF2-DMAC exhibit reversible reduction processes, thereby facilitating enhanced stability in device settings. The oxidation potentials gradually decrease with the increase in the electron-donating ability of the donors (Table S1), which indicates that the oxidation processes mainly occur on the amine-type donor portions, consistent with the results of the DFT calculations. The HOMO levels of NOBF2-Cz, NOBF2-DTCz, NOBF2-DPCz, and NOBF2-DMAC are −5.70, −5.59, −5.60, and −5.33 eV, respectively, according to oxidation potentials. EL Properties. The EL properties of difluoroboron compounds in device settings were investigated. The device structures and energy level diagrams are shown in Figures 4a and 4b, the EL properties of the OLEDs are shown in Figures 4c–4f, the molecular structures of the materials utilized in the devices are displayed in Figure S8, and the device performances are summarized in Table 2. Blue OLEDs were fabricated with the structure of ITO/HATCN (10 nm)/NPB (30 nm)/TCTA (10 nm)/mCBP (10 nm)/mCBP:dopant (10%−30%, 30 nm)/PPT (5 nm)/Li2CO3:Bepp2 (5%, 35 nm)/Li2CO3 (1 nm)/Al (100 nm) using NOBF2-Cz, NOBF2-DTCz, and NOBF2-DPCz as dopants, respectively. Here, ITO and Al were utilized as the anode and cathode respectively. HATCN and Li2CO3 were used as the hole and electron injection layers, whereas NPB, TCTA, and Li2CO3:Bepp2 were used as the hole- and electron-transporting layers. mCBP was employed as the electron-blocking layer, whereas PPT was 10 / 21


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acted as the hole-blocking layer. The electrogenerated triplet excitons can be confined inside of the emitting layer effectively because of the higher LUMO level of the TCTA (–2.3 eV) and lower HOMO level of the PPT (–6.6 eV) than the host mCBP (–2.7 and –6.2 eV for LUMO and HOMO, respectively), and the higher T1 energies of PPT (3.07 eV) and mCBP (2.90 eV) compared with the dopants (2.81–2.88 eV),. The green OLED was fabricated with the following structure: ITO /HATCN (10 nm)/TAPC (65 nm)/ CBP:NOBF2-DMAC (10%–20%, 20 nm)/Bepp2 (10 nm)/Li2CO3:Bepp2 (5%, 30 nm)/Li2CO3(1 nm)/Al(100 nm), wherein the TAPC, CBP and Bepp2 were employed as hole transporting, host and hole-blocking materials, respectively.

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Figure 4. Performance of difluoroboron-enabled TADF OLEDs doped with 10% emitters. (a) Device structures and energy level diagrams of blue OLEDs. (b) Device structure and energy level diagram of green OLED. (c) Electroluminescent spectra. (d) External quantum efficiency versus luminance. (e) Current density and luminance versus voltage characteristics. (f) Electroluminescent color coordinates on the CIE 1931 chromaticity diagram. HTL, hole-transporting layer. EML, emissive layer. ETL, electron-transporting layer. 12 / 21


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All the devices showed low turn-on voltage of 2.5–3.3 V, exhibiting blue and green EL with no emission from the host and other layers, thereby indicating that all excitons were successfully confined within the emitting layers. The EL spectra of the OLEDs doped with 10% emitters (devices 1–3 and 6) shifted to the short wavelength region compared with their corresponding PL spectra in doped DPEPO films because of the lower polarity of the host materials of the devices. Blue OLED doped with 10% NOBF2-Cz (device 1) exhibited a maximal external quantum efficiency (EQE) of 11.0% and a relatively narrow FWHM of 68 nm with CIE 1931 coordinates of (0.14, 0.16), and a maximal luminescence (Lmax) of 6761 cd/m2 could be achieved. The peak EQE and Lmax could be further enhanced by 10% NOBF2-DTCz and NOBF2-DPCz-doped blue OLEDs (devices 2 and 3), whose peak EQE is 12.7% and 15.8% and Lmax is 8208 and 19383 cd/m2, respectively. These device efficiencies exceed the theoretical limit of fluorescent emitter-based OLEDs (5%–7.5%),27 which suggest the utilization of the triplet excitons through an efficient RISC process from T1→S1 (TADF) in these devices. The Lmax of the NOBF2-DPCz-doped blue OLED is remarkably higher than those of the reported boron-based blue TADF OLEDs, most of which were less than 10000 cd/m2,48,52,53–57,59 except the recently reported blue OLED doped with 20% oxygen-bridged boron compound TDBA-DI to show Lmax of 47680 cd/m2.70 Devices 1-3 shown heavy efficiency roll-off (Figure 4d), owing to the relatively large ΔEST (0.20–0.22 eV) and long excited-state lifetimes (110–132 μs) of the NOBF2-Cz, NOBF2-DTCz, and NOBF2-DPCz, which result in serious TTA and singlet-triplet annihilation (STA) processes. Next, the dopant concentration effect on the device performance was investigated. As expected, elevated concentration resulted in redshift of the EL spectra and decrease of the EQE because of concentration quenching (devices 3–5). However, FWHM remained unchanged and the Lmax doubled for device 5 compared with device 3. In addition, 10% NOBF2-DMAC-doped green OLED (device 6) demonstrated a peak EQE of 13.2% and a Lmax of 35350 cd/m2 with CIE 1931 coordinates of (0.29, 0.60). As a result of the small ΔEST (0.08 eV) and short τd (57 μs) of NOBF2-DMAC, the TTA and STA processes could be significantly suppressed; thus, device 6 exhibited much lower efficiency roll-off (Figure 4d). These results demonstrate that the difluoroboron compounds can act as efficient TADF materials in OLED applications for display and lighting. 13 / 21



Table 2. Summary of OLED Characteristics Using Difluoroboron Compounds as Dopants Device 1 2 3 4 5 6 7

Dopant 10% NOBF2-Cz 10% NOBF2-DTCz 10% NOBF2-DPCz 20% NOBF2-DPCz 30% NOBF2-DPCz 10% NOBF2-DMAC 20% NOBF2-DMAC

Von [V]

λEL [nm]

FWHM [nm]

Lmax [cd/m2]

ηEQE [%]

ηc [cd/A]

ηp [lm/W]

CIE (x, y)

3.0 2.5 3.3 2.8 2.7 2.6 2.4

467 471 483 491 495 523 530

68 64 65 65 66 75 89

6761 8208 19383 28256 38305 35350 45925

11.0 12.7 15.8 13.4 10.2 13.2 10.3

12.6 40.6 25.3 27.2 20.6 40.8 33.4

12.5 44.0 22.7 29.4 24.0 44.2 38.9

(0.14, 0.16) (0.14, 0.21) (0.14, 0.28) (0.16, 0.40) (0.18, 0.45) (0.29, 0.60) (0.34, 0.58)

Von: turn-on voltage at 1 cd/m2. λEL: EL emission maximum. FWHM: full width at half-maximum of the EL spectrum. Lmax: maximum luminance. ηEQE: maximum external EL quantum efficiency. ηc: maximum current efficiency. ηp: maximum power efficiency. CIE: Commission Internationale de l’Éclairage color coordinates measured at 20 mA/cm2. 100 20% NOBF2-DPCz (device 4)

90 L/L 0 (%)

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

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L0=500 cd/m2 LT50=54 h

80

20% NOBF2-DMAC (device 7) L0=500 cd/m2 LT50=920 h

70 60 50

0

200

400 600 Time (h)

800

1000

Figure 5. Relative luminance vs operational time of NOBF2-DPCz-based blue OLED and NOBF2-DMAC-based green OLED.

Aside from quantum efficiency, operational lifetime is also a critical metric of the device performance. Several boron-based TADF materials have been developed; however, the stabilities of the boron-based TADF OLEDs are still unclear. Thus, operational lifetime measurements were also carefully performed and the time-relative luminance curves are shown in Figure 5. The blue OLED doped with 20% NOBF2-DPCz (device 4) exhibited an operational lifetime of 54 h at 50% of an initial luminance (L0 = 500 cd/m2), LT50. The operational lifetime can be further improved by using a stable hole-blocking material to replace the PPT. The 20% NOBF2-DMAC-doped green OLED (device 7) achieved an operational lifetime LT50 of 920 h at L0 of 500 cd/m2 (Figure 5). This operational lifetime 14 / 21


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represented an approximately threefold improvement compared with the reported green OLED doped with a tricoordinated boron-based TADF material CzDBA.74 This study indicates that the difluoroboron compounds can be stable emitters for OLED applications. ■ CONCLUSION In summary, we designed and developed a new series of tetracoordinated boron-enabled TADF materials with a D-A-BF2-type framework, the necessity of the BF2 unit were supported by the DFT calculations and the photophysical measurements. The materials were prepared conveniently and they were air-stable with high PLQYs in solution. EQEs of 11.0% with CIE coordinates of (0.14, 0.16) and 15.8% with (0.14, 0.28) could be achieved for the NOBF2-Cz and NOBF2-DPCz-doped blue OLEDs, and also exhibited high brightness of 6761 and 19383 cd/m2, respectively. High brightness of 45925 cd/m2 could be achieved for device 7. NOBF2-DPCz-doped blue OLED and the NOBF2-DMAC-doped green OLED also achieved operational lifetimes, LT50, of 54 and 920 h with L0 of 500 cd/m2, respectively. This study provides a new route for the development of boron-based TADF materials and demonstrates that the tetracoordinated difluoroboron molecules can act as efficient and stable TADF materials. ■ EXPERIMENTAL SECTION Synthesis. Detailed synthesis and analysis of the difluoroboron compounds is described in the Supporting Information. Theoretical Calculation. Gaussian 09 program package was used for all quantum chemical calculations. Molecular geometries were optimized with the DFT method. B3LYP functional with the 6-311G** basis set was used to describe all atoms. Based on the optimized geometric structures, the dihedral angles of the molecules were obtained. TD-DFT was then employed to obtain the vertical transitions of the S1 and T1 states based on the corresponding S0 geometries at same theoretical level.62 Photophysical Measurements. The absorption spectra were measured on an Agilent 8453 UV−VS Spectrometer. The excitation wavelength used to measure the emission spectra and quantum yields is 360 nm on PTI QM-40 system. Semiconductor laser (371 nm) was used for the fluorescent lifetime measurements on Horiba deltaflex01 system via TCSPC technique. Nitrogen laser (337 nm) was 15 / 21



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used for the TADF lifetime measurements on PTI QM-40 system via strobe technique. The DPEPO films were prepared through spin coating. The samples measured at room temperature in nitrogen atmosphere. Low temperature (77 K) emission spectra were measured in toluene or in DPEPO film cooled with liquid nitrogen. Electrochemistry. Electrochemical measurements (cyclic voltammetry and differential pulsed voltammetry) were performed using a CH1760E electrochemical analyzer according to our previous report.19 Device Fabrication and Characterization. All devices were fabricated by vacuum thermal evaporation and tested outside glove box after encapsulation. Please see Suooirting Information for details. ■ ASSOCIATED CONTENT Supporting Information The Supporting Information is available free of charge on the ACS Publications website at DOI: xxx. Detailed synthetic procedures, 1H,

13C, 19F, 11B

NMR and HRMS data for the newly-developed

difluoroboron compounds; photo showing PL of the difluoroboron compounds in toluene solution under irradiation by a UV lamp (Figure S1); absorption spectra of the difluoroboron compounds measured at room temperature in various solvents (Figure S2); fluorescence spectra of the difluoroboron compounds measured at room temperature in various solvents (Figure S3); fluorescence and phosphorescence spectra of the difluoroboron compounds measured at 77 K in toluene solution (Figure S4); electrochemical properties of the difluoroboron compounds (Table S1); and operational lifetime of the NOBF2-DPCz-based blue OLED and the NOBF2-DMAC-based green OLED (Figure S5). ■ AUTHOR INFORMATION Corresponding Author *E-mail: [email protected]. *E-mail: [email protected]. ORCID Guijie Li: 0000-0002-0740-2235 16 / 21


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Chao Deng: 0000-0002-7063-1823 Qisheng Zhang: 0000-0002-0899-6856 Notes The authors declare no competing financial interest. ■ ACKNOWLEDGMENTS This work was supported by the National Natural Science Foundation of China (grant no. 21878276, 21602198, 51673164 and 51873183) and the Fundamental Research Funds for the Provincial Universities of Zhejiang (RF-A2019013). ■ REFERENCES (1) Tang, C. W.; VanSlyke, S. A. Organic Electroluminescent Diodes. Appl. Phys. Lett. 1987, 51, 913–915. (2) Burroughes, J. H.; Bradley, D. D. C.; Brown, A. R.; Marks, R. N.; Mackay, K.; Friend, R. H.; Burns, P. L.; Holmes, A. B. Light-Emitting Diodes Based on Conjugated Polymers. Nature 1990, 347, 539–541. (3) Baldo, M. A.; O’Brien, D. F.; You, Y.; Shoustikov, A.; Sibley, S.; Thompson, M. E.; Forrest, S. R. Highly Efficient Phosphorescent Emission from Organic Electroluminescent Devices. Nature 1998, 395, 151–154. (4) Ma, Y.; Zhang, H.; Shen, J.; Che, C. Electroluminescence from Triplet Metal-Ligand Charge-Transfer Excited State of Transition Metal Complexes. Synth. Met. 1998, 94, 245–248. (5) Uoyama, H.; Goushi, K.; Shizu, K.; Nomura, H.; Adachi, C. Highly Efficient Organic Light-Emitting Diodes from Delayed Fluorescence. Nature 2012, 492, 234–238. (6) Ai, X.; Evans, E. W.; Dong, S.; Gillett, A. J.; Guo, H.; Chen, Y.; Hele, T. J. H.; Friend, R. H.; Li, F. Efficient Radical-Based Light-Emitting Diodes with Doublet Emission. Nature 2018, 563, 536–540. (7) Forrest, S. R. The Path to Ubiquitous and Low-Cost Organic Electronic Appliances on Plastic. Nature 2004, 428, 911–918. (8) Rothberg, L. J.; Lovinger, A. J. Status of and Prospects for Organic Electroluminescence. J. Mater. Res. 1996, 11, 3174–3187. (9) Kondakov, D. Y.; Pawlik, T. D.; Hatwar, T. K.; Spindler, J. P. Triplet Annihilation Exceeding Spin Statistical Limit in Highly Efficient Fluorescent Organic Light-Emitting Diodes. J. Appl. Phys. 2009, 106, 124510 . (10) Baldo, M. A.; Lamansky, S.; Burrows, P. E.; Thompson, M. E.; Forrest, S. R. Very High-Efficiency Green Organic Light-Emitting Devices Based on Electrophosphorescence. Appl. Phys. Lett. 1999, 75, 4−6. (11) Adachi, C.; Baldo, M. A.; Thompson, M. E.; Forrest, S. R. Nearly 100% Internal Phosphorescence Efficiency in an Organic Light Emitting Device. J. Appl. Phys. 2001, 90, 5048–5051. (12) Lee, J.; Chen, H.-F.; Batagoda, T.; Coburn, C.; Djurovich, P. I.; Thompson, M. E.; Forrest,S. R. Deep Blue Phosphorescent Organic Light-Emitting Diodes with Very High Brightness and Effciency. Nature Mater. 2016, 15, 92–98. (13) Li, G.; She, Y. Tetradentate Cyclometalated Platinum Complexes for Efficient and Stable Organic Light-Emitting Diodes. Light Emitting Diode-An Outlook On the Empirical Features and Its Recent Technological Advancements. IntechOpen: London, 2018, 77–101. 17 / 21



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Difluoroboron-Enabled Thermally Activated Delayed Fluorescence

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