Short-Axis Substitution Approach Selectively Optimizes Electrical Properties of Dibenzothiophene-Based Phosphine Oxide Hosts

Abstract

Two dibenzothiophene (DBT)-based phosphine oxide hosts, named 4-diphenylphosphoryl dibenzothiophene (DBTSPO) and 4,6-bis(diphenylphosphoryl) dibenzothiophene (DBTDPO), were prepared by short-axis substitution with the aim to selectively adjust electrical properties. The combined effects of short-axis substitution and the involvement of electron-donating S atom in conjugation effectively suppress the influence of electron-withdrawing diphenylphosphine oxide (DPPO) moieties on the frontier molecular orbitals and the optical properties. Therefore, DBTSPO and DBTDPO have the nearly same hole injection ability and the excited energy levels, while more electron-transporting DPPOs and the symmetrical configuration endow DBTDPO with enhanced electron-injecting/transporting ability. As the result, on the basis of this short-axis substitution effect, the selective adjustment of electrical properties was successfully realized. With the high first triplet energy level (T(1)) of 2.90 eV, the suitable energy levels of the highest occupied molecular orbital and the lowest unoccupied molecular orbital of -6.05 and -2.50 eV and the improved carrier-transporting ability, DBTDPO supported its blue- and white-emitting phosphorescent organic light-emitting diodes as the best low-voltage-driving devices reported so far with the lowest driving voltages of 2.4 V for onset and <3.2 V at 1000 cd m(-2) (for indoor lighting) accompanied with the high efficiencies of >30 lm W(-1) and excellent efficiency stability.

The First Tandem, All-exciplex-based WOLED

“…More recently, we have demonstrated a high efficiency yellow exciplex with TCTA as the HT layer and a triazine-based molecule (3P-T2T)12 as the ET layer. To further reduce the LUMO energy and retain the high triplet energy, we herein introduced diphenylphosphine oxide as the electron-withdrawing group as well as the conjugation-blocking group202122 to make a new ET material PO-T2T (Fig. 1) for this work (see Supplementary online information for synthesis/characterization).…”

High efficiency and low driving voltage blue/white electrophosphorescence enabled by the synergistic combination of singlet and triplet energy of bicarbazole derivatives

“…In general, the bipolar molecules containing both p-type and n-type moieties can facilely achieve high performance even in the most challengable blue phosphorescent OLEDs (PhOLEDs). By choosing reasonable linkage, the resulted materials with both electron donating/withdrawing groups can minimize intramolecular charge transfer (ICT) and thus gain high triplet energy (E T ) level as well as matched HOMO/LUMO levels for carrier injections [13][14][15][16][17][18]. For example, Lee's group reported a new bipolar host molecule CbBPCb (E T = 2.77 eV), which can achieve very high EQE of 30.1% in sky-blue PhOLEDs [19].…”

Dimeric SFX host materials for red, green and blue phosphorescent organic light-emitting devices

“…To overcome the poor charge transporting properties, it is very important to choose donor or acceptor active groups for the design of high-efficiency host materials. Up to date, various holetransporting groups such as carbazole [11][12][13][14], triphenylamine [15][16][17] and electron-transporting units such as phosphine oxide [7,[18][19][20][21], oxadiazol [22,23] and triazine [24] have been applied into host materials.…”

“…In order to keep the triplet energy level of host materials, various backbones of hosts was used, such as benzene [25,26], fluorenes [21,27], diarylfluorene [28,29], tetraarylsilane [9,10], spiro-systems [19,[30][31][32][33][34], and other heteroatomic compounds such as dibenzofuran and dibenzothiophene [18,[35][36][37][38][39]. Among them, diarylfluorene and spiro-systems have obvious advantage of high thermal and morphological stability for host materials.…”

“…These results indicated that SFXs have excellent transport properties that can effectively balance hole and electron density in the emitting layer. However, the effect of conformation on device performance was also interesting and was specially investigated rarely [8,18,19].…”