• donor–acceptor systems;
  • electrophosphorescence;
  • heterocycles;
  • phosphorous;
  • triplet energy levels


A series of donor (D)–π–acceptor (A)-type phosphine-oxide hosts (DBFxPOPhCzn), which were composed of phenylcarbazole, dibenzofuran (DBF), and diphenylphosphine-oxide (DPPO) moieties, were designed and synthesized. Phenyl π-spacer groups were inserted between the carbazolyl and DBF groups, which effectively weakened the charge transfer and triplet-excited-state extension. As the result, the first triplet energy levels (T1) of DBFxPOPhCzn are elevated to about 3.0 eV, 0.1 eV higher than their D[BOND]A-type analogues. Nevertheless, the electrochemical analysis and DFT calculations demonstrated the ambipolar characteristics of DBFxPOPhCzn. The phenyl π spacers hardly influenced the frontier molecular orbital (FMO) energy levels and the carrier-transporting ability of the materials. Therefore, these D[BOND]π[BOND]A systems are endowed with higher T1 states, as well as comparable electrical properties to D[BOND]A systems. Phosphorescent blue-light-emitting diodes (PHOLEDs) that were based on DBFxPOPhCzn not only inherited the ultralow driving voltages (2.4 V for onset, about 2.8 V at 200 cd m−2, and <3.4 V at 1000 cd m−2) but also had much-improved efficiencies, including about 26 cd A−1 for current efficiency, 30 Lm W−1 for power efficiency, and 13 % for external quantum efficiency, which were more than twice the values of devices that are based on conventional unipolar host materials. This performance makes DBFDPOPhCzn among the best hosts for ultralow-voltage-driven blue PHOLEDs reported so far.