Tetraphenylsilane derivatives spiro-annulated by triphenylamine/carbazole with enhanced HOMO energy levels and glass transition temperatures without lowering triplet energy: Host materials for efficient blue phosphorescent OLEDs

Two new host materials, SiBSTPA and SiBSCz, were designed and synthesized based on 4,4′-bis(triphenylsilyl)-biphenyl (BSB). Their thermal, electrochemical, electronic absorption and photoluminescent properties were fully investigated. The introduction of spiro-annulated triphenylamine/carbazole moieties on 4,4′-bis(triphenylsilyl)-biphenyl (BSB) increases the HOMO energy levels from -6.49 eV (BSB) to -5.30 eV for SiBSTPA and -5.56 eV for SiBSCz, and accordingly facilitates hole injection from the nearby hole-transporting layer. Compared to 4,4′-bis(triphenylsilyl)-biphenyl (BSB), higher glass transition temperatures (T_g) were observed at 133 °C for SiBSTPA and 129 °C for SiBSCz, owing to the rigid spiro-annulated structures. Meanwhile, the perpendicular conformation between the triphenylamine or carbazole plane and the biphenyl plane effectively prevents the extension of the π-conjugation and consequently causes no depreciation of their triplet energies (ca. 2.75 eV). Phosphorescent organic light-emitting devices (PhOLEDs) with the following configuration: ITO/NPB/TCTA/EML/TAZ/LiF/Al were fabricated by using the two host materials and the blue emitter bis[2-(4′,6′-difluorophenyl)pyridinato-N,C ²′]iridium(iii) picolate (FIrpic) as the guest. These devices exhibited good performance with the maximum current efficiency of 21.4 cd A ^-1 and the maximum power efficiency of 15.6 lm W^-1.