Modulating the optoelectronic properties of large, conjugated, high-energy gap, quaternary phosphine oxide hosts: impact of the triplet-excited-state location

The purposeful modulation of the optoelectronic properties was realised on the basis of a series of the large, conjugated, phosphine oxide hosts 9,9-bis-{4'-[2-(diphenylphosphinoyl)phenoxy]biphenyl-4-yl}-9H-fluorene (DDPESPOF), 9,9-bis-{3'-(diphenylphosphinoyl)-4'-[2-(diphenylphosphinoyl)phenoxy]biphenyl-4-yl}-9H-fluorene (DDPEPOF), 9-[4'-(9-{4'-[2-(diphenylphosphoryl)phenoxy]biphenyl-4-yl}-9H-fluoren-9-yl)biphenyl-4-yl]-9H-carbazole (DPESPOFPhCz) and 9-[4'-(9-{3'-(diphenylphosphoryl)-4'-[2-(diphenylphosphoryl)phenoxy]biphenyl-4-yl}-9H-fluoren-9-yl)biphenyl-4-yl]-9H-carbazole (DPEPOFPhCz). The last two are quaternary with fluorenyls as linking bridges, diphenylphosphine oxide (DPPO) moieties as electron acceptors and diphenylethers and carbazolyls as two different kinds of electron donors. Owing to the fine-organised molecular structures and the mixed indirect and multi-insulating linkages, all of these hosts achieve the same first triplet energy levels (T1) of 2.86 eV for exothermic energy transfer to phosphorescent dopants. The first singlet energy levels (S1) and the carrier injection/transportation ability of the hosts were accurately modulated, so that DPESPOFPhCz and DPEPOFPhCz revealed extremely similar optoelectronic properties. However, the T1 state of the former is localised on fluorenyl, whereas the carbazolyl mainly contributes to the T1 state of the latter. A lower driving voltages and much higher efficiencies of the devices based on DPESPOFPhCz indicated that the chromophore-localised T1 state can suppress the quenching effects through realising independent contributions from the different functional groups to the optoelectronic properties and the embedding and protecting effect on the T1 states by peripheral carrier transporting groups.