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

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 (T1)
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.