Isomeric N‐Linked Benzoimidazole Containing New Electron Acceptors for Exciplex Forming Hosts in Highly Efficient Blue Phosphorescent OLEDs

Abstract

Since the pioneering work of Tang et al., organic light-emitting diodes (OLEDs) unveil a bright future for the applications in new-generation solid-state lightings, full-color flat-panel displays, and flexi ble displays due to their low power consumption, brilliant colors, fast response time, and self-emission. [1,2] According to the emission mechanism, the phosphorescent OLEDs (PHOLEDs) or thermally activated delayed fluorescence (TADF) OLEDs, which can simultaneously harvest both singlet and triplet excitons through intersystem crossing (ISC) or reverse intersystem crossing (RISC), to reach maximum internal quantum efficiency (IQE) of 100%, are expected to achieve higher efficiencies than the traditional fluorescent devices with theoretical IQE of only 25%. [2] However, in order to suppress the concentration quenching and triplettriplet annihilation in phosphorescent or TADF OLEDs, the phosphorescent or pure organic TADF emitter has to be dispersed into a suitable organic host matrix to achieve high device efficiency. [3] Thus, the exploration of host materials is essential to acquire ideal phosphorescent/TADF OLEDs. [3,4] In comparison to the traditional single-carrier-transport host materials, bipolar hosts comprising of both hole-and electron-transport moieties have been widely reported to broaden exciton recombination zones, extend the lifetime of excitons, improve charge carrier injection and transport balance, and thus to increase the device performance and reduce efficiency roll-off. [3][4][5] It is well proved that the strategy of using bipolar hosts with extremely low singlet and triplet bandgap (ΔE ST ) is an effective way to lower driving voltage and drastically increase the power efficiency of the devices owing to better matched frontier energy levels of host materials with those of the adjacent hole transporting materials and electron transporting materials. [6] This is due to the fact that the high first triplet energy level (T 1 ) can significantly prevent reverse energy transfer from the phosphorescent dopant to host and the relatively low first singlet energy level (S 1 ) is able to remarkably improve and balance carrier injection and transport in the emissive layers, 1,3,5-tris(2-(pyridin-2-yl)-1H-benzo[d]imidazol-1-yl)benzene (i TPyBIB), 1,1′,1′′-(pyridine-2,4,6-triyl)tris(2-phenyl-1H-benzo[d]imidazole) (i TPBIPy), and 1,1′,1′′-(pyridine-2,4,6-triyl)tris(2-(pyridin-2-yl)-1H-benzo[d]imidazole) (i TPyBIPy) are designed and synthesized. Compared to the commercial electron-transport 2,2,2-(1,3,5-phenylene)-tris(1-phenyl-1H-benzimidazole) (TPBI) with the C-atom in benzoimidazole linked to the central phenyl ring, the introduction of isomeric N-linkage to the central phenyl or pyridine ring in three compounds remarkably simplifies the synthesis to a one-step CN coupling reaction. iTPyBIB, iTPBIPy, and iTPyBIPy exhibit comparable highest occupied molecular orbital (≈6.0 eV) and lowest unoccupied molecular orbital (≈2.5 eV) energy levels, similar triplet energy levels of ≈2.65 eV with TPBI, whe...

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

“…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 24 i PBIOXD and i TPBIOXD, 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 2,5-diphenyl-1,3,4-oxadiazole, along with mildly distribution over the penta-heterocyclic imidazoles, suggesting the weak electron-withdrawing property to gently participate electron-transport for the imidazoles.…”

“…The electron-withdrawing oxadiazole (OXD) unit has been extensively applied in donor-acceptor type bipolar transport host materials, single molecule intramolecular charge transfer type TADF emitters as well as electron transport materials (Tao et al, 2011; Mondal et al, 2013; Olivier et al, 2017; Cooper et al, 2018; Yao et al, 2018; Zhang et al, 2018). By combining OXD building block with our previously reported isomeric N -linkaged benzoimidazole (Hu et al, 2017), both 24 i PBIOXD, and i TPBIOXD exhibited deep HOMO level of ~-6.15 eV, facilitating the exciplex formation with general electron donor materials of TAPC, TCTA, and mCP due to the compatible HOMO and LUMO energy levels between donor and acceptor materials. The gradient energy offsets ranging from 0.47 to 1.34 eV correlated well with the delayed lifetime and EL efficiencies in exciplex type TADF OLEDs.…”

Exciplex hosts for blue phosphorescent organic light-emitting diodes

“…The exciplex host of mCP:1,1 ,1 -(pyridine-2,4,6triyl)tris(2-phenyl-1H-benzo[d]imidazole (iTPBIPy) can also be classified as a weak p-type and strong n-type hosts combined exciplex [49]. 2,2,2-(1,3,5-phenylene)tris(1-phenyl-1H-benzimidazole (TPBI) is well known as a weak n-type host, and it was modified with a pyridine unit to strengthen the electron transport character of TPBI.…”

Highly efficient exciplex-based OLEDs incorporating a novel electron donor

“…However, it is relatively rare to select an electron donor for fabricating an exciplex system in order to achieve high performance OLEDs, because most researchers pay more attention to designing electron acceptor materials, e.g., star-shaped 1,3,5-triazine/cyano hybrid molecule CN-T2T, 18 heptazine derivative HAP-3MF, 13 benzimidazole-triazine-based electron acceptor PIM-TRZ, 19 and isomeric N-linked benzoimidazole materials (iTPyBIB, iTPBIPy, and iTPyBIPy). 20 These electron acceptor materials have shown significant roles in highly efficient exciplex systems. In contrast, the common electron donors widely utilized in exciplex systems are mainly restricted to commercial materials, such as 1,3-di-9-carbazolylbenzene (mCP), 15 1,1-bis[4-[N 0 ,N 0 -di(p-tolyl)amino]-phenyl]cyclohexane (TAPC), 17 4,4 0 ,4 00 -tris[3-methylphenyl(phenyl)amino] triphenylamine (m-MTDATA), 11 and N,N,N-tris(4-(9-carbazolyl)phenyl)amine (TCTA).…”