N-Benzoimidazole/Oxadiazole Hybrid Universal Electron Acceptors for Highly Efficient Exciplex-Type Thermally Activated Delayed Fluorescence OLEDs

Recently, donor/acceptor type exciplex have attracted considerable interests due to the low driving voltages and small singlet-triplet bandgaps for efficient reverse intersystem crossing to achieve 100% excitons for high efficiency thermally activated delayed fluorescence (TADF) OLEDs. Herein, two N-linked benzoimidazole/oxadiazole hybrid electron acceptors were designed and synthesized through simple catalyst-free C-N coupling reaction. 24iPBIOXD and iTPBIOXD exhibited deep-blue emission with peak at 421 and 459 nm in solution, 397 and 419 nm at film state, respectively. The HOMO/LUMO energy levels were −6.14/−2.80 for 24iPBIOXD and −6.17/−2.95 eV for iTPBIOXD. Both compounds could form exciplex with conventional electron donors such as TAPC, TCTA, and mCP. It is found that the electroluminescent performance for exciplex-type OLEDs as well as the delayed lifetime was dependent with the driving force of both HOMO and LUMO energy offsets on exciplex formation. The delayed lifetime from 579 to 2,045 ns was achieved at driving forces close to or larger than 1 eV. Two TAPC based devices possessing large HOMO/LUMO offsets of 1.09–1.34 eV exhibited the best EL performance, with maximum external quantum efficiency (EQE) of 9.3% for 24iPBIOXD and 7.0% for iTPBIOXD acceptor. The TCTA containing exciplex demonstrated moderate energy offsets (0.88–1.03 eV) and EL efficiency (~4%), while mCP systems showed the poorest EL performance (EQE <1%) and shortest delayed lifetime of <100 ns due to inadequate driving force of 0.47–0.75 eV for efficient exciplex formation.


INTRODUCTION
Organic light-emitting diodes (OLEDs) have been developed rapidly in recent years since the pioneer work on low-voltage fluorescence electroluminescence by Tang in 1987 (Tang andVanSlyke, 1987;Ma et al., 1998;Gong et al., 2010;Park et al., 2013;Zhang et al., 2016). According to spin statistics, the ratio for singlet and triplet excitons recombined from electrogenerated holes and electrons is 1:3 (Baldo et al., 1999;Segal et al., 2003). Thus, the first generation of traditional fluorescent OLEDs which solely harvest singlet excitons only shows 25% of maximum internal quantum efficiency (IQE) (Wen et al., 2005). On the other hand, the second generation of phosphorescent OLEDs (PHOLEDs) based on heavy metal complexes and third generation of thermally activated delayed fluorescence (TADF) OLEDs could both reach 100% IQE in theory by utilizing all singlet and triplet excitons through intersystem crossing (ISC) and reverse inter-system crossing (RISC), respectively (Baldo et al., 1998;Adachi et al., 2001;Su et al., 2008;Lo et al., 2009;Uoyama et al., 2012;Zhang and Forrest, 2012;Cao et al., 2017;Huang et al., 2018;Wu Q. et al., 2018). However, to avoid consuming noble metals and achieving reliable true-blue light, TADF OLEDs based on low-cost pure organic emitters have attracted increasing interests as an alternative mechanism to PHOLEDs. TADF emission is realized by an upconversion process from lower energy triplet states to slightly higher energy singlet states by endothermic reverse inter-system crossing process Cao et al., 2017;Huang et al., 2018;Wu Q. et al., 2018). Therefore, a small singlet-triplet energy bandgap ( E ST ) is required for TADF emitters.
It is reported that the small E ST could be attained in (i) intramolecular charge transfer featured single molecule with twisted donor-acceptor structured for effective spatial isolation between the highest occupied molecular orbital (HOMO) and lowest unoccupied molecular orbital (LUMO) on the relevant hole and electron transporting moieties, and (ii) bimolecularexciplex which contains an electron-donor material mixed with an electron-acceptor material through intermolecular charge transfer characteristics (Cai and Su, 2018;Liu et al., 2018;Sarma and Wong, 2018). High external quantum efficiency (EQE) of 20% for red, 29% for orange, 38% for green, and 37% for lightblue TADF OLEDs have been achieved in single-molecule TADF emitters Chen et al., 2018;Wu T.-L. et al., 2018;Zeng et al., 2018). However, the development of bimolecular TADF lags far behind. Most exciplex-type TADF OLEDs showed maximum EQE close to 10% (Jankus et al., 2014;Liu et al., 2015a,b;Oh et al., 2015;Hung et al., 2016Hung et al., , 2017Jeon et al., 2016), with only one example approaching to 18% .

Synthesis and Characterization
Scheme 1 shows the synthetic routes and molecular structures of 24iPBIOXD and iTPBIOXD. The two compounds could be facilely synthesized by a simple one-step catalyst free C-N coupling reaction. This nucleophilic substitution reaction was carried out in DMSO solvent with K 2 CO 3 base at high yields over 80% by using di/tri-fluorine substituted oxadiazole derivatives as electrophiles and 2-phenyl-1H-benzo[d]imidazole as nucleophiles. The considerably high yields and environmentally eco-friendly conditions demonstrated the superiority than common metal-catalyzed Ullman reactions (Son et al., 2008;Liu et al., 2011;Volz et al., 2013). In addition, the directly connection of the isomeric N-linked benzoimidazole to the central phenyl ring avoided the complicated multistep ring-closing synthetic process for the normal C-linked benzoimidazole in traditional electron transport material of 2,2,2-(1,3,5phenylene)-tris(1-phenyl-1H-benzoimidazole) (TPBI) or its derivatives. The chemical structures of the new compounds were fully characterized by 1 H NMR, 13 C NMR, mass spectrometry (MALDI-TOF) and element analysis ( Figure S1). The good thermal stability of the two compounds was confirmed by thermogravimetric analysis (TGA) and differential scanning calorimetry (DSC) (Figure 1). The decomposition temperatures (T d , corresponding to a 5% weight loss) from TGA curves for 24iPBIOXD and iTPBIOXD were determined at 443 and 461 • C, respectively. Additionally, the melting point (T m ) of iTPBIOXD was observed at 327 • C, which was much higher than 286 • C of 24iPBIOXD. The glass transition temperature (T g ) of both materials can be detected from the second heating cycles from DSC, with values of 126 • C for 24iPBIOXD and 165 • C for iTPBIOXD, indicating their reasonable thermal stability.

Photophysical Properties
The room temperature UV-Vis absorption and photoluminescence (PL) spectra of 24iPBIOXD and iTPBIOXD SCHEME 1 | Synthesis of compounds 24iTPBIOXD and iTPBIOXD. in CH 2 Cl 2 solution are shown in Figure 2A. Both compounds exhibited an intense absorption with peaks at 289 and 283 nm in solution, 297 and 288 nm in film, respectively, which can be ascribed to the π-π * transition of molecules. The optical bandgap (E g ) was calculated to be 3.34 eV for 24iPBIOXD and 3.22 eV for iTPBIOXD, according to the film-state absorption edge. On the other hand, 24iPBIOXD and iTPBIOXD showed unimodal photoluminescence peaking at 421 and 459 nm in solution, whereas significantly blue-shift to 397 and 419 nm in film state (Table 1). By analyzing the highest-energy vibronic sub-band of low-temperature fluorescence and phosphorescence spectrum ( Figure 2B), the singlet (E S ) and triplet (E T ) energy levels could be determined to be 3.31/2.55 and 3.18/2.53 eV for 24iPBIOXD and iTPBIOXD, respectively. In addition, the E S /E T energy levels of three hole-transport electron donor materials were also calculated to be 3.54/2.95 eV for mCP, 3.79/2.82 eV for TAPC, and 3.66/2.84 eV for TCTA ( Figure 2C). The PL spectra for the neat film of electron donors such as mCP, TAPC, and TCTA, the two new electron-acceptors of 24iPBIOXD and iTPBIOXD as well as their corresponding mixtures in a 1:1 weight ratio were investigated. As shown in Figure 3 and Figure S2, all blended films showed bathochromic shifted PL spectra compared with the emission of neat 24iPBIOXD/iTPBIOXD and the corresponding donor-material, indicating the successful formation of exciplex (Zhang T. et al., 2015). In addition, it is found that exciplex based on 24iPBIOXD acceptors all exhibited about 20-30 nm blue-shifted emission than iTPBIOXD based exciplex systems. The exciplex emission color could be tuned from deep-blue of mCP:24iPBIOXD with peak at 419 nm to light-blue of TCTA:24iPBIOXD (501 nm) and further to green of TAPC:24iPBIOXD (518 nm). Besides, transient photoluminescence (PL) measurements were carried out for all six exciplexes (Figure 4). The exciplexes comprising TAPC or TCTA donor all possessed significantly longer delayed decay lifetime, with values of 579 ns for TCTA:24iPBIOXD, 1,907 ns for TCTA:iTPBIOXD, 1,520 and 2,045 ns for TAPC:24iPBIOXD, TAPC:iTPBIOXD exciplex, respectively. However, the mCP:24iPBIOXD and mCP:iTPBIOXD exciplex systems displayed greatly shorter delayed decay lifetime of only 42 and 72 ns (Table 1). Besides, the temperature dependent PL transients for the representative TAPC:24iPBIOXD and TCTA:iTPBIOXD exciplexes (Figure S3) both demonstrated a more significant decay from 100 to 300 K at the longer lifetime range, suggesting the potential existence of endothermic reverse inter-system crossing. It is expected the obvious variations on delayed decay time for different exciplexes may demonstrate some relationships with the device efficiency in exciplex-TADF OLEDs.

Theoretical Calculations and Electrochemical Properties
In order to gain insights into the frontier molecular orbital and excited states level distribution of 24iPBIOXD and iTPBIOXD, density functional theory (DFT) calculation was conducted at the B3LYP level (Francl et al., 1982;Becke, 1988;Lee et al., 1988). From the optimized geometry shown in Figure 5, the dihedral angles between the central phenyl and oxadiazole ring were 22.0 and 50.3 • for 24iPBIOXD and iTPBIOXD, respectively, the values between the benzoimidazoles and the central phenyl rings ranged from 50.4 to 77.4 • , indicating a twisted structure for both compounds. Furthermore, in the ground state, the highest occupied molecular orbital (HOMO) were almost completely located on one of the ortho-positioned phenylbenzoimidazole units, indicating the electron-donating characteristics of N-linked phenylbenzoimidazole, which was quite different from the C-isomerized phenylbenzoimidazole containing TPBI (Hu et al., 2017). And the lowest unoccupied molecular orbital (LUMO) were mainly localized on 2,5diphenyl-1,3,4-oxadiazole, along with mildly distribution over  the penta-heterocyclic imidazoles, suggesting the weak electronwithdrawing property to gently participate electron-transport for the imidazoles. Similar distribution can be observed in the highest occupied natural transition orbital (HONTO) and the lowest unoccupied natural transition orbital (LUNTO) at singlet excited state. It should be noted that the HONTO distribution at triplet excited state was completely different from S 0 and S 1 for both compounds, which was mainly delocalized through the 2,5-diphenyl-1,3,4-oxadiazole skeleton, similar with the LUNTO distribution. The electrochemical features were measured by cyclic voltammetry (CV) (Figure 6). Both compounds exhibited reversible reduction whereas undetectable oxidation behavior. The LUMO energy levels calculated from the onset of reduction curves for 24iPBIOXD and iTPBIOXD were measured to be −2.80 and −2.95 eV, while the HOMO energy levels calculated from the different between the LUMO and optical bandgaps (E g ) were evaluated to be −6.14 and −6.17 eV, respectively. The values were in good agreement with the theoretical calculation. Besides, the energy levels for electron donor materials were also measured, with HOMO estimated from the onset of electrooxidation curves and LUMO calculated from HOMO and optical bandgaps. The HOMO/LUMO energy level values for mCP, TAPC, and TCTA were −5.67/−2.20, −5.05/−1.61, and −5.19/−1.92 eV, respectively. The deep HOMO and LUMO for the two new electron acceptors of 24iPBIOXD and iTPBIOXD, provided sufficient driving forces on HOMO/LUMO energy offsets for the exciplex formation ( Figure 6A). As shown in Figure 7A, the HOMO energy level offsets between the electron donor of TAPC, TCTA, or mCP and the electron acceptors of 24iPBIOXD/iTPBIOXD were calculated to be 1.09/1.12, 0.95/0.98, or 0.47/0.5 eV, and the corresponding LUMO offsets were 1.19/1.34, 0.88/1.03, or 0.6/0.75 eV, respectively. It is noted in both acceptor systems, the TAPC donor based exciplex presented the highest driving force, followed by TCTA, while the mCP donor demonstrated the lowest HOMO/LUMO offsets.

Electroluminescence Properties
To investigate the charge transport properties of the two new Nlinked isomeric benzoimidazole containing electron acceptors, single carrier electron-only device was prepared to find out the electron inject and transport properties of 24iPBIOXD and iTPBIOXD. The device structure was ITO/24iPBIOXD, iTPBIOXD, or TPBI (50 nm)/LiF (1 nm)/Al (150 nm), where the commercial electron transport material of 2,2,2-(1,3,5phenylene)-tris(1-phenyl-1H-benzoimidazole) (TPBI) with Clinkage in benzoimidazole was selected for comparison. As shown in Figure 8, at the same operating voltage, TPBI based device exhibited the highest current density among all the three devices. Since the LUMO energy of TPBI (2.7-2.9 eV)   (Bian et al., 2018;Jou et al., 2018) was almost the same as 24iPBIOXD and iTPBIOXD, which manifested their similar injection barrier for efficient electron injection. Therefore, the significantly higher current for TPBI indicated better electron transporting property than 24iPBIOXD and iTPBIOXD. On the other hand, the current density in iTPBIOXD device was slightly higher than 24iPBIOXD, as depicted in Figure 8, the LUMO level of iTPBIOXD was 0.15 eV lower than 24iPBIOXD, therefore a mildly efficient electron-injection could be attained in iTPBIOXD device due to its lower injection barriers. Thus, the electron-transport performance for both electron acceptors may be comparable.
The current density-voltage-luminance (J-V-L), electroluminescence (EL) spectra, together with the current and power efficiency, external quantum efficiency vs. luminance curves are shown in Figure 9. The device fabrication details are stated in Supporting Information. According to the key EL data listed in Table 2, the turn-on voltage for TAPC, TCTA, and mCP containing devices was gradiently increased from 2.8, 3.0   Figure 9B, devices A and B with TAPC donor depicted smooth exciplex-TADF emission, with EL peak at 519 and 556 nm, respectively, which is in agreement with the relevant PL spectra. Commission Internationale de L'Eclairage (CIE) values for device A and B was measured at (0.31, 0.55) and (0.43, 0.54), corresponding to green and yellow emission, respectively. The TCTA based device C and D both exhibited blueish-green emission, with a gentle shoulder peak at around 400 nm for 24iPBIOXD. The two mCP-based devices displayed blue emission with CIE x, y each at ∼0.20. However, the EL spectra of device E and F revealed bimodal emission showing comparable intensity for the two peaks. It is hypothesized that the inadequate HOMO and LUMO energy offsets (< ∼1 eV) for TCTA:24iPBIOXD, mCP:24iPBIOXD, and mCP:iTPBIOXD, resulted in the unexpected shorter wavelength EL emission peak, which was ascribed to pure mCP emission (Chiu and Lee, 2012;Shahalizad et al., 2017). In addition, since the E CT of TAPC and TCTA based exciplexes were lower than the triplet energy of both donor and acceptor materials, which was beneficial to restrict triplet excitons in the exciplex states for the efficient RISC. However, E CT of 2.8-2.96 eV ( Figure 7B) for mCP based exciplex was significantly higher than the triplet energy (∼2.55 eV) of the two electron acceptors, which provided a potential way for energy leakage from exciplex states to the T 1 excited state of 24iPBIOXD and iTPBIOXD. Thus, devices based on mCP donors demonstrated the lowest EL efficiency and inadequate TADF emission.

CONCLUSION
In summary, we have designed and synthesized two universal N-linked benzoimidazole/oxadiazole hybrid electron acceptors through a simple nucleophilic substitution reaction. Diverse deep-blue to yellow emissive exciplex could be formed between various conventional donor materials and the two acceptors due to their deep HOMO levels of ∼6.15 eV. The HOMO and LUMO energy level offsets which were also named as the driving forces for exciplex formation were gradiently increased from 0.47 to 1.12 and 0.6 to 1.34 eV in mCP, TCTA, to TAPC based exciplexes. We have found that both HOMO and LUMO offsets ≥1 eV was required to form efficient and stable intermolecular charge transfer exciplex. When the driving forces were as low as 0.47-0.75 eV, which is far <1 eV, the two mCP based exciplex demonstrated considerably short delayed component lifetime, with values of only 42 ad 72 ns for 24iPBIOXD and iTPBIOXD acceptors, respectively. Additionally, the exciplex-type device EQE was lower than 1%. When the driving forces were slightly lower or approaching 1 eV, the two TCTA exciplexes displayed moderate EL efficiency of about 4%. And the best EL performance was achieved in TAPC containing exciplex-type TADF OLEDs, with relatively low turn-on voltage of 2.8 V, maximum efficiency of 28.8 cd/A CE, 32.3 lm/W PE, and 9.3% for 24iPBIOXD acceptor and 22.1 cd/A CE, 23.4 lm/W and 7.0% for iTPBIOXD acceptor. Our results provide guidance on the exploration of efficient exciplex type TADF OLEDs.

AUTHOR CONTRIBUTIONS
WY, MZ, and DH designed and synthesized the materials. WY and MZ did most of the experimental work and data analyses. OLED device fabrication and electroluminescent performance studies were carried out by HY and NS. YT had the idea, led the project. WY and YT prepared the manuscript. All authors contributed to the manuscript preparation.