Diarylmethylene-bridged triphenylamine derivatives encapsulated with fluorene: Very high T_g host materials for efficient blue and green phosphorescent OLEDs

Two bridged triphenylamine/fluorene hybrids, BTPAF1 and BTPAF2, were designed and synthesized through Friedel-Crafts reaction. Their thermal, electrochemical, electronic absorption and photoluminescent properties were fully investigated. Very high glass transition temperatures (T_g) were observed at 204°C for BTPAF1 and 211°C for BTPAF2, owing to the introduction of rigid fluorene and bridged triphenylamine unit. The encapsulation of a fluorene unit at the para positions of bridged triphenylamine greatly enhances their electrochemical stability. The linkage by the quaternary carbon atom of the fluorene moiety (C-9) effectively prevents the extension of π-conjugation of the bridged triphenylamine core, and consequently means that the compounds have a high triplet energy of 2.86 eV. Phosphorescent organic light-emitting devices (PHOLEDs) fabricated by using the two compounds as the hosts and the blue emitter bis[2-(4′,6′-difluorophenyl)pyridinato-N, C^2′]iridium(iii) picolate (FIrpic) as the guest exhibit good EL performances with a maximum current efficiency of 20 cd A^-1, a maximum power efficiency of 14 lm W^-1, and a maximum external quantum efficiency of 9.4%. Green electrophosphorescent devices by using green-emitter iridium(iii) fac-tris(2-phenylpyridine) [Ir(ppy)₃] as guest and the two new compounds as the hosts display excellent EL performances with a maximum current efficiency of 75 cd A^-1, a maximum power efficiency of 60 lm W^-1, and a maximum external quantum efficiency of 19.5%. The device figures of merit, together with the excellent morphological and electrochemical stabilities, make the new compounds ideal host materials for PHOLEDs, especially for high-temperature applications of devices.