Solution-Processed Highly Efficient Bluish-Green Thermally Activated

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Solution-Processed Highly Efficient Bluish-Green Thermally Activated Delayed Fluorescence Emitter Bearing Asymmetric Oxadiazole-Difluoroboron Double Acceptor Di Zhou, Denghui Liu, Xu Gong, Huili Ma, Gaowei Qian, Shaolong Gong, Guohua Xie, Weiguo Zhu, and Yafei Wang ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.9b07511 • Publication Date (Web): 12 Jun 2019 Downloaded from http://pubs.acs.org on June 12, 2019

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Solution-Processed

ACS Applied Materials & Interfaces

Highly

Efficient

Bluish-Green

Thermally Activated Delayed Fluorescence Emitter Bearing Asymmetric Oxadiazole-Difluoroboron Double Acceptor Di Zhou†‡, Denghui Liu†, Xu Gong‖, Huili Maξ, Gaowei Qian†, Shaolong Gong‖, Guohua Xie‖*, Weiguo Zhu†*, Yafei Wang†Δ* †National

Experimental Demonstration Center for Materials Science and Engineering, Jiangsu Key Laboratory of Environmentally Friendly Polymeric Materials, Jiangsu Collaborative Innovation Center of Photovoltaic Science and Engineering, Jiangsu Engineering Laboratory of Light-Electricity-Heat Energy-Converting Materials and Applications, School of Materials Science & Engineering, Changzhou University, Changzhou 213164, China. E-mail: [email protected]; [email protected] ‖Hubei Key Lab on Organic and Polymeric Optoelectronic Materials, Department of Chemistry, Wuhan University, Wuhan 430072, China. E-mail: [email protected] ‡College of Chemistry, Xiangtan University, Xiangtan 411105, China ΔKey Laboratory of Advanced Display and System Applications of Ministry of Education, Shanghai University, 149 Yanchang Rd., Shanghai 200072, China ξKey Laboratory of Flexible Electronics (KLOFE) & Institute of Advanced Materials (IAM) Nanjing Tech University (NanjingTech), 30 South Puzhu Road, Nanjing 211816, China. KEYWORDS. Thermally activated delayed fluorescence; Difluoroboron complex; Oxadiazole unit; Acridine unit; Organic light-emitting diodes ABSTRACT: Difluoroboron (BF2)-containing dyes have attracted great interests owing to their exceptionally high luminescence efficiency and good electron-withdrawing properties. However, only few reports on difluoroboron-based thermally activated delayed fluorescence (TADF) have been addressed. In this contribution, a novel BF2-containing TADF molecule of BFOXD, which contains two acceptor fragments of oxadiazole (OXD) and BF2 and one donor unit of 9,9-dimethylacridine, was synthesized and characterized. For comparison, the precursor of OHOXD bearing one acceptor unit was also investigated. Both molecules clearly show TADF characteristics with sky-blue emission in solution and film state. Additionally, OHOXD undergoes excited-state intramolecular proton transfer (ESIPT) coupled intramolecular charge transfer (ICT) processes. Using 9-(4-tert-butylphenyl)-3,6-bis(triphenylsilyl)-9Hcarbazole (CzSi) as the host, the organic light-emitting diodes (OLEDs) fabricating via solution process show maximum external quantum efficiency (EQE) of 2.98 and 13.8% for OHOXD- and BFOXD-based devices, respectively. While the bipolar TADF host of 10-(4-((4-(9H-carbazol-9-yl)phenyl)sulfonyl)phenyl)-9,9-dimethyl-9,10-dihydroacridine (CzAcSF) is utilized instead of CzSi, the OHOXD- and BFOXD-based devices exhibit better performances with the maximum EQE of 12.1% and 20.1%, respectively, which renders the most efficient and the bluest emission ever reported for the BF2-based TADF molecules. This research demonstrates that introduction of one more acceptor unit into TADF molecule could play a positive effect on the emission efficiency, which opens a new way for design high efficiency TADF molecules.

INTRODUCTION

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ACS Applied Materials & Interfaces 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

Since Adachi and co-workers harvested both singlet and triplet excitons in organic light-emitting diodes (OLEDs) with purely organic thermally activated delayed fluorescence (TADF) materials in 2012,1 TADF has become a very popular participant as the organic electroluminescent materials for harnessing triplet excitons.2-5 Intrinsic TADF involves the reverse intersystem crossing (rISC) of a triplet (T1) exciton to the singlet state (S1), followed by emission from the singlet state.6-9 Compared with traditional phosphorescent materials bearing heavy metal atoms such as platinum and iridium, TADF materials can theoretically achieve 100% internal quantum efficiency without a metal, which improves its costeffectiveness and environmental friendliness. Over the past several years, substantial progresses have been made in shifting the emission of TADF materials from blue to red.10-16 Usually, a very small energy gap (ΔEST) between S1 and T1 can facilitate the rISC and may result in TADF.17-20 With this in mind, research on TADF molecules has largely been focused on the twisted donor (D)–acceptor (A) skeletons, which can efficiently decrease the ΔEST through spatially separated highest occupied molecular orbital (HOMO) and lowest unoccupied molecular orbital (LUMO).21-23 Among these TADF molecules, phenylamine,24-26 carbazole,27-29 acridine30,31 and 32,33 phenoxazine are usually regarded as the donor moieties, whereas sulfonyldibenzene,34-36 benzophenone,37-39 1,3,5triazine,40,41 pyrazine-2,3-dicarbonitrile42,43 and cyano groups4446 are the major acceptor moieties. Generally, there is only one acceptor moiety in these reported TADF molecular framework, such as D-A, D-A-D or star-shaped of D3-A. How about several acceptor units in one TADF molecule? To this end, we would like to explore the effect of the number of acceptor unit on the photophysical property of the TADF emitter. Difluoroboron (BF2)-containing chromophores are of particular interest in organic semiconductors due to their exceptional electron-accepting abilities, high extinction coefficients and high-efficiency luminescence in both the solution and solid state.47,48 However, it is unclear why BF2containing dyes have not become popular in OLEDs, let alone in TADF-OLEDs. To the best of our knowledge, there are only a couple of examples of BF2-based TADF materials.49-54 Adachi and co-workers reported a D-A-D boron difluoride curcuminoid derivative that showed TADF with near-infrared emission (700800 nm) and a maximum external quantum efficiencies (EQE) of 10% in an OLED.55 Although some difluoroboron βdiketonate-based small molecules and polymers also showed TADF properties,56,57 their device performances have not been reported. Notably, these reported BF2-based TADF molecules are β-diketonate (OBO) complexes and their emissions were mainly in the range of long wavelength (> 500 nm). Therefore, we seek to apply difluoroboron (BF2)-based chromophores in TADF materials and devices, especially for the short wavelength emission (< 500 nm).

Chart 1. Molecular structure of OHOXD and BFOXD Based on the above considerations, in this context, we would like to introduce BF2 group into the D-A molecule. This molecule complexation with BF2 is expected to reduce the ΔEST and improve the emissive efficiency because of the increased

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electron-withdrawing ability and molecular rigid construction. With this in mind, one BF2-based TADF molecule bearing two acceptor moieties was proposed. Due to the planar structure and excellent electronic transport properties, 2,5-diphenyl-1,3,4oxadiazole (OXD) was chosen as the first acceptor moiety, although little attention has been paid to its application as a moiety for the TADF emitter.58-60 Then, BF2 unit was integrated as the second acceptor moiety to adjust the photophysical property. Therefore, the compound, named BFOXD (Chart 1), containing both acceptor fragments of β-ketoiminate (NBO) and OXD and one donor group of 9,9-dimethylacridine has been prepared. For comparison, the precursor OHOXD bearing one acceptor group was also investigated. The photophysical properties of both compounds were explored both by experimental and theoretical methods. Compounds OHOXD and BFOXD clearly showed TADF with sky-blue emission in solution and film state, evidenced by their steady-state fluorescent spectra. In addition, OHOXD displayed excitedstate intramolecular proton transfer (ESIPT) and intramolecular charge transfer (ICT) characteristics. Encouraged by the TADF sensitized fluorescence emission,61 OLEDs employing the TADF material CzAcSF as the host exhibited the maximum EQE values of 12.1% and 20.1% for the OHOXD-based and BFOXD-based devices, respectively. This research reveals the best result reported for the BF2-based TADF materials (Figure 1), and it paves the way for the design of novel high-efficiency TADF molecules.

Figure 1. EQE-wavelength plots of reported BF2-based TADF emitters. RESULTS AND DISCUSSION Synthesis and characterization As depicted in Scheme 1, using triethylamine as the base, 2hydroxybenzohydrazide was reacted with 4-bromobenzoyl chloride to yield an intermediate of N'-(4-bromobenzoyl)-2hydroxybenzohydrazide, followed by an intramolecular cyclization in the presence of SOCl2 to afford the key precursor 2-(5-(4-bromophenyl)-1,3,4-oxadiazol-2-yl)phenol (1). The Buchwald–Hartwig coupling between compound 1 and 9,9dimethyl-9,10-dihydroacridine provided OHOXD in 46% yield. Then, boron coordination reaction was carried out in anhydrous dichloromethane in the presence of boron trifluoride diethyl etherate to obtain BFOXD in 84% yield. The structures of OHOXD and BFOXD were confirmed by 1H NMR, 13C NMR, 19F NMR, HRMS or X-ray single crystal analyses


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(Figure S9-S17). In order to further confirm the structure of BFOXD, absolute energy value was evaluated by theoretical calculation which shows that BFOXD has more stable molecular structure. Both compounds have satisfied solubility in common organic solvents, such as, dichloromethane, O N H OH

Br

O NH2

Cl

 Br

Br O OH

chloroform and THF. Their thermogravimetric curves imply good thermal stability with the decomposition temperatures (5% weight loss) of 385 oC and 382 oC for OHOXD and BFOXD, respectively (Figure S1).

H N



N N

i) Et3N, CH2Cl2

O

ii) SOCl2

OH

N N

1

(t-Bu)3P, Pd2dba3, t-BuONa

O OH N N

N

OH-OXD

O OH N N

N

O

Et3N, CH2Cl2 BF3•Et2O

O F

B

N N F BF-OXD

N

Scheme 1. Synthetic route for compounds OHOXD and BFOXD.

Figure 2. Single crystal structure of OHOXD: a) from an axel perspective; b) side view; c) molecular stacking.



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Figure 3. Molecular structure, HOMO and LUMO distributions based on ground state (S0) at the B3LYP/6-31G(d) level for OHOXD and BFOXD, the excitation energy of singlet (S1) and triplet (T1) were evaluated by using TD-DFT/BMK/6-31G(d) based on optimized S1-geometry. Table 1. Photophysical parameters for OHOXD and BFOXD aλ aλ bE cE dΔE eτ (ns)/ fФ gHOMO gLUMO abs em S1 T1 ST p τd (μs) (%) (eV) (eV) (nm) (nm) (eV) (eV) (eV) OHOXD 317, 368 473 3.2 3.04 0.16 19/1.9 0.30 -5.29 -2.35 BFOXD 318, 360, 386 473 3.12 3.03 0.09 22/4.3 0.66 -5.31 -2.42 a: In toluene at room temperature; b: the onset of the fluorescence spectra in toluene at room temperature; c: the onset of the phosphorescence spectra in toluene at 77 K; d: calculated by fluorescent and phosphorescent spectra; e: in dichloromethane at room temperature; f: in doped film for the host and guest; g: in dichloromethane solution, EHOMO = -(Eox + 4.8) eV, ELUMO = -(Eoptg-EHOMO) eV.

These results indicate that they are potentially applicable in OLEDs via either solution-process or thermal vacuum deposition. The single crystal of OHOXD was obtained by slow evaporation from a mixture of dichloromethane and methanol (Figure 2). As shown in Figure 2a, the distance between the hydroxyl proton and the oxadiazol nitrogen atom is 1.930 Å, clearly implying an intramolecular H-bond. Similar to a previous report,62,63 the 2-(1,3,4-oxadiazol-2-yl)phenol moiety is almost coplanar due to hindered rotation from the fixed Hbond. The 1H NMR data further confirmed the intramolecular H-bond, which shows a typical phenyl hydroxyl proton resonance signal of δ 10.20 ppm (Figure S11).62,63 The phenyl and acridine units are almost perpendicular to each other (θ1 = 81.45o), while the dihedral angle between the phenyl and oxadiazole units is 17.44o (θ2). Notably, the intramolecular Hbond plays a key role in the molecular geometry. Theoretical calculation It is well known that ΔEST is proportional to the overlap between the HOMO and LUMO levels. To evaluate the frontier molecular orbitals and energy levels, the molecular geometries of OHOXD and BFOXD in the ground state and the lowest excited singlet state were optimized using density functional theory (DFT) and time-dependent DFT (TDDFT) at the B3LYP/6-31G(d) level; while the TDDFT/BMK/6-31G(d) method was employed to deal with the ΔEST, which can provide

a good description to the triplet states of BF2 derivatives.64 As depicted in Figure 3, the acridine and diazole moieties in the optimized structures of all compounds are nearly orthogonal, which is essential for effectively separating the HOMO and LUMO distributions. Notably, the HOMOs of the both compounds are localized on the acridine moiety (donor), whereas the LUMOs of both OHOXD and BFOXD are distributed mainly on the 2,5-diphenyl-1,3,4-oxadiazole moiety (acceptor). The calculated ΔEST values for OHOXD and BFOXD are evaluated to be 0.19 and 0.008 eV, respectively. At the same time, the HOMO and LUMO distributions of both compounds have clear overlap on the phenyl ring between donor and acceptor, which can lead to efficient light emission due to the increased oscillator strength. Photophysical properties The steady-state absorption and photoluminescence (PL) spectra of OHOXD and BFOXD in dilute toluene (10-5 M) are shown in Figure 4, and their photophysical parameters are summarized in Table 1. Both compounds possess an intense absorption band at approximately 317 nm, which is assigned to π-π* transitions in the OXD moiety.59,62 In addition, the broad and weak absorption bands in the range of 350-450 nm are attributed to the ICT transition between the donor (acridine) and the acceptor (diphenyloxadiazole) unit. Compared to the OHOXD, compound BFOXD exhibits clearly stronger and redshifted absorption spectrum in the long wavelength, which could be ascribed to the increased acceptor ability and


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conjugated degree after introduction of BF2 moiety. At an excitation

Figure 4. Steady-state absorption and fluorescence spectra of OHOXD and BFOXD in toluene and in CzAcSF (10%) at room temperature. wavelength of 360 nm, the steady-state photoluminescence spectra of OHOXD and BFOXD are almost identical, and both show the maximum emission peaks at 472 nm at room temperature. To further understand the fluorescence of OHOXD, the solvent polarity-dependent absorption and

emission behaviors were evaluated at room temperature (Figures S2-S3 and Table S1). It is noted that the absorption spectra of OHOXD change slightly with the increasing solvent polarity, implying this parameter has a negligible effect at the ground state. Conversely, a strong solvatochromic effect was observed based on the CT character of the excited state caused by the donor–acceptor structure of the acridine and OXD moieties. According to previous reports, this phenomenon is attributed to the ESIPT coupled with ICT processes.62,65 Similar to most TADF molecules, BFOXD also displays a clear solvatochromic effect due to strong ICT effect (Figures S4-S5 and Table S2). The PL spectra of OHOXD and BFOXD doped into CzAcSF (10%) host present slight red shift with the maximum emission peak at 479 and 488 nm, respectively. The PL quantum yields (Ф) of OHOXD and BFOXD were 16 and 21% in air, respectively. At 77 K, the phosphorescent emission spectra exhibit similar shapes to those of the PL profiles at room temperature (Figure S6). Based on the onsets of the fluorescence (room temperature) and low-temperature phosphorescence spectra (77 K) in toluene, the ΔEST values of OHOXD and BFOXD are estimated to be 0.16 and 0.09 eV. It is noted that introduction of one more acceptor group into TADF molecule can effectively decrease the ΔEST. To further explore the photophysical and TADF properties, the transient PL spectra of both compounds in toluene under

Figure 5. Transient PL spectra of OHOXD (a) and BFOXD (b) in toluene at room temperature. Inset: Transient PL spectra of the compound. aerated and degassed conditions were investigated. As shown in Figure 5, the transient PL decay curves display obviously second-order exponential decays under degassed condition at room temperature. The decay curves contain the prompt components with lifetimes of 19 ns for OHOXD and 22 ns for BFOXD as well as the delayed components with lifetimes of 1.9 μs for OHOXD and 4.3 μs for BFOXD, respectively. Conversely, both compounds show single exponential decays with short lifetimes in the presence of oxygen (see the insets of Figures 5a and 5b). Moreover, the contributions of the delayed components of OHOXD and BFOXD were estimated to be 14% and 63%, respectively. To gain insight into the excited-state dynamics, femtosecond time-resolved transient absorption (TA) spectroscopy was performed for OHOXD and BFOXD in dichloromethane at room temperature. As shown in Figure 6, two main TA bands at about 504 (±4 nm) nm and 660 (±8 nm) nm were detected

forthe compound OHOXD in the range of 450-750 nm. With the increasing transient decay (starting from a transient decay of 1.07 ps), the short-wavelength band gradually declines and blue-shifts to 499 nm, whereas the long-wavelength band increases in intensity and red-shifts to 671 nm. At approximately 2.27 ps after excitation, an additional TA band was clearly observed at about 580 nm, suggesting a proton transfer process. Conversely, the TA spectra of BFOXD are different from those of OHOXD, in which the TA band at about 580 nm is absent due to the lack of a hydrogen proton. The absorption bands at approximately 500 nm and 670 nm were observed for BFOXD, and these bands can be attributed to intramolecular charge transfer.66 The TA dynamics of both compounds at short and long wavelengths show double exponential decays consisting of fast and slow absorption components (Figures 6e and 6f). The fast decay components of both compounds are assigned to the fast ISC process from S1 to



T1, while the slow decay components are attributed to the deactivation processes of their triplet states. Electrochemical Properties The HOMO and LUMO energy levels of OHOXD and BFOXD were measured by using cyclic voltammetry and optical bandgaps. As shown in Figure S7, both compounds only display the irreversible oxidation potential (Eox) in the range of 0⁓2.0 V. Owing to the same donor fragment, OHOXD and BFOXD exhibit almost identical Eox of 0.49 (vs. Fc/Fc+) and 0.51 V (vs. Fc/Fc+), respectively. According to the formula of EHOMO = -(Eox+4.8) eV, the HOMO energy levels were calculated to be 5.29 eV (OHOXD) and 5.31 eV (BFOXD). Based on the optical bandgap and HOMO energy levels, the LUMO energy levels are -2.35 eV and -2.42 eV for OHOXD and BFOXD, respectively. It is noted that BFOXD show a deeper LUMO energy level than that of OHOXD, implying that introduction of additional acceptor moiety into molecule have a clear effect on the LUMO energy level. Electroluminescent Properties To evaluate the electroluminescence (EL) performance of hydrogen- and boron difluoride-based TADF materials, OLEDs with the configuration of ITO/PEDOT:PSS (40 nm)/CzSi:dyes (OHOXD: Device I; BFOXD: Device II) (90:10, 50 nm)/DPEPO (10 nm)/TmPyPB (50 nm)/Liq (1 nm)/Al (100 nm)

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were fabricated via solution process. The energy-level diagram and the molecular structures of CzSi, DPEPO, TmPyPB and Liq are shown in Figure 7. Device I and device II show the EL spectra with a maximum peak at 468 nm and 496 nm, respectively. Compared to device I, device II exhibits a redshifted EL spectra due to the stronger intramolecular CT in BFOXD emitter. As expected, the EL spectra are similar with their correspondingly PL spectra, suggesting that the EL emission mainly generates from the emitter. The Commission Internationale de L'Eclairage (CIE) coordinates of the device I and device II are (0.17, 0.22) and (0.23, 0.43), respectively (Figure S8). As shown in Figure 8b, both devices exhibit relatively high driving voltages (recorded at a luminance of 1 cd cm-2). Device I presents a highest luminance of 218 cd m-2 and a maximum EQE of 2.98%. The device II exhibits a better performance with a highest luminance of 1647 cd m-2 and an EQE of 13.8%. Obviously, compound BFOXD has a much better device performance than that of OHOXD. This difference could be explained by the reasons: 1) introduction of BF2 moiety into compound OHOXD can effectively reduce the ΔEST; 2) the complexation with BF2 increases the molecular rigid, which plays a key role on suppressing the nonradiative transition. Obviously, the TADF molecule bearing two acceptor fragments show clearly higher efficiency.


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Figure 6. Time-resolved Transient absorption spectra of OHOXD and BFOXD in dichloromethane using 365 nm laser pulses at room temperature (a-d); Transient absorption dynamics for OHOXD at 506 nm and 668 nm (e) and for BFOXD at 483 nm and 671 nm (f). Inspired by the excellent device performance achieved by the TADF host,62,67 so we would like to fabricate OLEDs employing a TADF compound as the host. Considering the emission wavelength and triplet energy level of compounds OHOXD and BFOXD, the deep blue TADF emitter of CzAcSF (Figure 7) was selected as the host. To this end, the device with the structure of ITO/PEDOT:PSS (40 nm)/CzAcSF:dyes (OHOXD: device III; BFOXD: device IV) (90:10, 50 nm)/DPEPO (10 nm)/TmPyPB (50 nm)/Liq (1 nm)/Al (100 nm) were fabricated via solution process. Device III presents the EL spectra with an unstructured broad emission band centered at 480 nm (Inset of Figure 8). Compared with that of device III, the EL spectrum of device IV is red-shifted by 12 nm. The CIE coordinates of the OHOXDand BFOXD-based devices are (0.17, 0.28) and (0.21, 0.38), respectively (Figure S8), indicating sky-blue and bluish-green emission, respectively. Compared to the devices with CzSi host, device III and IV show a distinct reduced turn-on voltage (< 4

V), probably due to the bipolar property of TADF host. Correspondingly, the devices based on CzAcSF host also display better performances than that of the devices with CzSi host. Device III exhibits a current efficiency (CE) of 22.8 cd A-1, a power efficiency (PE) of 15.4 lm W-1 and an EQE of 12.1% (Figure 8 and Table 2). Encouragingly, the device IV presents better performances a CE of 46.9 cd A-1, a PE of 21.0 lm W-1 and an EQE of 20.1%. Even at a luminance of 1000 cd m-2, the EQE of the devices III and device IV are still 4.3% and 12.7%, respectively. According to the previous report, the TADF host CzAcSF can exhibit effectively up-conversion from triplet to singlet excitons via rISC process, followed by the Förster energy transfer from host to the dopant, which results in much better performance than the conventional host.[62,67] To the best of our knowledge, these results are among the best device performances reported for the hydroxy- and boron difluoride-based TADF materials.



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Figure 7. The molecular structures of CzSi, CzAcSF, DPEPO and TmPyPB and energy-level diagram (the red dotted line is CzSi and the blue dotted line is CzAcSF). Table 2. The performances of device III (OHOXD)and device IV (BFOXD).

Device

Vturn-on (V)

CIE (x,y)

EQE (%)

CE (cd A-1)

PE (lm W-1)

L (cd m-2)

Device III Device IV

Solution-Processed Highly Efficient Bluish-Green Thermally Activated

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