High and Balanced Bipolar‐Transporting Deep‐Blue HLCT Material for Efficient Monochrome and White OLEDs based on a Simple Phenanthroimidazole‐Dibenzothiophene Derivative

Abstract

The irreconcilable contradiction between wide bandgap and charge carriers injection/transport in deep‐blue organic electroluminescence materials is one of the largest bottlenecks restricting the development of organic light‐emitting diodes (OLEDs). Here, a multifunctional deep‐blue hybridized local and charge‐transfer material containing a phenanthroimidazole‐dibenzothiophene derivative that can act as an emitter as well as a host for phosphorescent OLEDs is reported. Owing to its high and balanced bipolar‐transporting ability, remarkable fluorescence efficiency, and suitable triplet energy level, the deep‐blue device with it as a neat emitter provides an external quantum efficiency (EQE) of 6.68%, while yellow and red phosphorescent OLEDs offer the EQEs of 25.35% and 22.19%, respectively. All of them exhibit low‐efficiency roll‐off characteristics. What's more amazing is that the fluorescent/phosphorescent hybrid white OLEDs achieve high power efficiencies and EQEs over 95.9 lm W−1 and 26.8%, and the values are still higher than 77.7 lm W−1 and 25.6% at the high luminance of 1000 cd m−2.

Bipolar Deep-Blue Host for High-Performance Multicolor OLEDs by Peripheral Auxiliary Group Strategy

“…Dibenzothiophene (DBT) as a rigid heteroaromatic group containing sulfur atoms (heavy-atom effect) displays good thermal stability, high triplet energy of 3.04 eV, and weaker electron-donating ability than carbazole, making its derivatives be used as luminescent materials and hosts for phosphorescent and TADF dopants. ,, The phenanthroimidazole (PI) group composed of phenanthrene and imidazole units has a rigid conjugated planar structure, bestowing its derivatives with a high luminous efficiency, high thermal stability, and bipolar-transporting characteristics. − ,, In the previous work,, we have realized a deep-blue nondoped device with the CIE coordinates of (0.152, 0.067) based on DBT-PI derivatives by regulating the electron-withdrawing ability of PI unit . Different from this, here we designed and synthesized two deep-blue DBT-PI fluorophores (PPISCF and PPISPhCz) with asymmetric molecular structures, where the peripheral auxiliary groups [electron-withdrawing group phenyfluoroform (PF) and electron-donating group phenylcarbazole (PCz)] were introduced at the C8-position of DBT unit, and the PI unit was incorporated at the C2-position.…”

“…The absorption and photoluminescence (PL) spectra of PPISCF and PPISPhCz were measured in dilute tetrahydrofuran (THF) solution (10 –5 M) for investigating their photophysical properties. It can be seen in Figure a that two similar absorption peaks at 261 nm for PISCF and PPISPhCz can be mainly ascribed to the π–π* transition of the phenyl ring, ,, and the long-wavelength absorption peaks at ∼336 and 363 nm should belong to the π–π* transition from the substituent anchored on the imidazole position to PI and the π–π* transition of the PI unit . The convex absorption band from 300 to 330 nm for PPISPhCz can originate from the π–π* transition of PCz moiety .…”

“…Both PPISCF and PPISPhCz show the moderately twisted molecular geometries with the torsional angles (36.7–46.5°) between DBT and adjacent phenyl groups (Figures and S2 of the Supporting Information), which is helpful to avoid red shifts of emission caused by molecular aggregation. Compared with the 2-(4-(dibenzo­[ b , d ]­thiophen-2-yl)­phenyl)-1-phenyl-1 H -phenanthro­[9,10- d ]­imidazole (PPIS), the peripheral linkage of PF has a small impact on the configuration in PPISCF molecules, while the dihedral angle between the benzene ring and DBT has a slight increase (∼10°) in PPISPhCz molecules. The dihedral angle between DBT and the peripheral group in PPISPhCz (44.8°) is higher than that in PPISCF (37.6°), resulting from the steric hindrance.…”

“…As shown in Scheme 1, the intermediates PPIB and DBTBr were obtained according to the previous reports, 33,40,42,43 while the compounds PPISCF and PPISPhCz were synthesized via two-step Suzuki−Miyaura cross-coupling reactions with the tetrakis(triphenylphosphine)palladium(0) as the catalyst.…”

“…To further understand the design strategy, molecular spatial configurations, frontier molecular orbitals (FMOs), and HOMO/LUMO energy levels of PPISCF and PPISPhCz 40 the peripheral linkage of PF has a small impact on the configuration in PPISCF molecules, while the dihedral angle between the benzene ring and DBT has a slight increase (∼10°) in PPISPhCz molecules. The dihedral angle between DBT and the peripheral group in PPISPhCz (44.8°) is higher than that in PPISCF (37.6°), resulting from the steric hindrance.…”