Molecular Topology Tuning of Bipolar Host Materials Composed of Fluorene‐Bridged Benzimidazole and Carbazole for Highly Efficient Electrophosphorescence

Abstract

Two new molecules, CzFCBI and CzFNBI, have been tailor-made to serve as bipolar host materials to realize high-efficiency electrophosphorescent devices. The molecular design is configured with carbazole as the hole-transporting block and N-phenylbenzimidazole as the electron-transporting block hybridized through the saturated bridge center (C9) and meta-conjugation site (C3) of fluorene, respectively. With structural topology tuning of the connecting manner between N-phenylbenzimidazole and the fluorene core, the resulting physical properties can be subtly modulated. Bipolar host CzFCBI with a C connectivity between phenylbenzimidazole and the fluorene bridge exhibited extended π conjugation; therefore, a low triplet energy of 2.52 eV was observed, which is insufficient to confine blue phosphorescence. However, the monochromatic devices indicate that the matched energy-level alignment allows CzFCBI to outperform its N-connected counterpart CzFNBI while employing other long-wavelength-emitting phosphorescent guests. In contrast, the high triplet energy (2.72 eV) of CzFNBI imparted by the N connectivity ensures its utilization as a universal bipolar host for blue-to-red phosphors. With a common device configuration, CzFNBI has been utilized to achieve highly efficient and low-roll-off devices with external quantum efficiency as high as 14 % blue, 17.8 % green, 16.6 % yellowish-green, 19.5 % yellow, and 18.6 % red. In addition, by combining yellowish-green with a sky-blue emitter and a red emitter, a CzFNBI-hosted single-emitting-layer all-phosphor three-color-based white electrophosphorescent device was successfully achieved with high efficiencies (18.4 %, 36.3 cd A(-1) , 28.3 lm W(-1) ) and highly stable chromaticity (CIE x=0.43-0.46 and CIE y=0.43) at an applied voltage of 8 to 12 V, and a high color-rendering index of 91.6.

Development of Materials for Blue Organic Light Emitting Devices

“…Furthermore, by utilizing CzFNBI as host, white PhOLEDs with uniformly separated RGB peaks were also realized. Thus, high color stability all‐phosphor three‐color WOLEDs could be successfully achieved using this bipolar host and was one of the best results obtained at that time …”

“…Furthermore, two bipolar host materials, CzFCBI and CzFNBI (Scheme ) by employing fluorene as a bridge between the carbazole and N‐phenylbenzimidazole moieties were developed . Thus, the electronic coupling between the hole‐transporting carbazole and electron‐transporting phenylbenzimidazole moieties could be effectively blocked, thereby limiting the extension of molecular conjugation, and consequently a high triplet energy (2.72 eV) was obtained.…”

Hosts for High‐Performance Phosphorescent Organic Light‐Emitting Diodes Based on Carbazole Derivatives

“…On the other hand, the carbazole segment is attached to other segments with different electron affinities to adjust the E T value and charge-transporting prop-A C H T U N G T R E N N U N G erties, and to produce ambipolar hosts. [13] Common electron-deficient segments include 1,3,5-triazine, [14] 1,3,4-oxadiazole, [15,16] phosphine oxide, phosphine sulfide, [17,18] benzimidazole, [19] and others. An ambipolar host can not only broaden the exciton-formation zone, consequently reducing the efficiency roll-off, but also simplify the device structure by using a single emitting layer comprised of an ambipolar host and two or more phosphors.…”

High thermal-stability benzocarbazole derivatives as bipolar host materials for phosphorescent organic light-emitting diodes

“…Subsequently, the desired products (DYNS and DYSN) were obtained by the palladium catalyzed BuchwaldeHartwig cross coupling reaction of benzocarbazole species (1e3 and 2e4) and bromide intermediate 2-(4-bromophenyl)-1-phenyl-1H-benzo[d]imidazole in 60% yield. The two target compounds were identified and characterized by 1 H and 13 C NMR as well as the high-resolution mass spectrometry (HRMS).…”