Spiro-type TADF emitters based on acridine donors and anthracenone acceptor

Abstract

Two rigid spiro-type TADF emitters (CBZANQ, PXZANQ) were designed and synthesized via a two-step route. CBZANQ and PXZANQ both contain an anthracenone acceptor along with a carbazole-acridine-fused donor and a phenoxazine-acridine-fused donor, respectively, where each donor and acceptor are linked by a spiro bridge to form D-sigma-A structures with an orthogonal arrangement. Both CBZANQ and PXZANQ have the difference between the singlet and triplet energy values (Delta E-ST) of 0.05 eV and 0.03 eV, respectively, which are sufficiently small for efficient triplet harvesting. CBZANQ with a weak electron donor (carbazole) exhibited a weak intramolecular charge transfer (ICT) character, resulting in a low photoluminescence quantum yield (PLQY) of 18%. On the other hand, when combined with a strong electron donor (phenoxazine), PXZANQ exhibited a strong ICT character, resulting in a high PLQY of 71%. Organic light-emitting diode (OLED) devices based on CBZANQ emitter and bis[2-(diphenylphosphino)phenyl] ether oxide (DPEPO) host exhibited sky-blue electroluminescence (EL) at 492 nm with a low external quantum efficiency (EQE) of 6.7%. Conversely, the OLED devices fabricated using PXZANQ emitter and DPEPO host exhibited green emission at 528 nm, along with a high EQE of 22.1%. Additionally, the efficiency of PXZANQ-based devices remained high at high luminance, indicating relatively small efficiency roll-off (13.9% EQE under 500 cd/m(2)).