title: Modulating the optoelectronic properties of large, conjugated, high-energy gap, quaternary phosphine oxide hosts: impact of the triplet-excited-state location.
abstract: 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.
authors.affiliation: Key Laboratory of Functional Inorganic Material Chemistry, Ministry of Education, School of Chemistry and Materials, Heilongjiang University, 74 Xuefu Road, Harbin 150080, P.R. China.
authors.forename: Zhen
authors.initials: Z
authors.lastname: Zhang
authors.forename: Zhensong
authors.initials: Z
authors.lastname: Zhang
authors.forename: Runfeng
authors.initials: R
authors.lastname: Chen
authors.forename: Jilin
authors.initials: J
authors.lastname: Jia
authors.forename: Chunmiao
authors.initials: C
authors.lastname: Han
authors.forename: Chao
authors.initials: C
authors.lastname: Zheng
authors.forename: Hui
authors.initials: H
authors.lastname: Xu
authors.forename: Donghui
authors.initials: D
authors.lastname: Yu
authors.forename: Yi
authors.initials: Y
authors.lastname: Zhao
authors.forename: Pengfei
authors.initials: P
authors.lastname: Yan
authors.forename: Shiyong
authors.initials: S
authors.lastname: Liu
authors.forename: Wei
authors.initials: W
authors.lastname: Huang
keywords: electrophosphorescence; optoelectronic properties; phosphine oxides; quaternary systems; triplet states