Effects of Different Eelectron-withdrawing Moieties on the General Photoelectric Properties of Fluorene-based Dimers

Here, 9,9,9'-tris(4-butoxyphenyl)-9H-9'H-2,2'-difluorene(DF) was synthesized as a reference for compa-rison, and diphenyl sulfone and triphenylphosphine oxygen were introduced to the 9-position of fluorene through a C-H coupling reaction to produce two fluorene-based oligomers 9',9'-[sulfonylbis(4,1-phenylene)]bis[9,9,9'-tris-(4-butoxyphenyl)-9H,9'H-2,2'-bifluorene](DF)₂SO₂ and phenylbis(4-{9,9',9'-tris(4-butoxyphenyl)-9H,9'H-[2,2'-bif-luorene]-9-yl}phenyl)phosphine oxide[(DF)₂PO]. Solid powders of all the three compounds exhibit excellent thermal stability with thermal temperature at 5% mass loss of 375, 429 and 383℃ for (DF)₂SO₂, (DF)₂PO and DF. In addition, owing to the distorted molecular structure and weak electron-absorbing ability of the acceptor, (DF)₂SO₂ and (DF)₂PO do not have obvious intramolecular charge transfer characteristics, and exhibit stable localized 394 nm/375 nm fluorescence emission peaks in different polar solvents. The absolute luminescence quantum efficiencies of (DF)₂SO₂, (DF)₂PO and DF solid powders are 20.83%, 10.03% and 59.46%. Compound DF has the highest quantum yield as an electron donor. The chromaticity coordinates of the blue OLED devices based on DF and DF₂SO₂ fabricated by solution spin coating were (0.19, 0.10) and (0.19, 0.11), which were closest to the deep blue region, and the corresponding maximum external quantum efficiencies are 1.45% and 0.87%, respectively, which are higher than that of (DF)₂PO(0.25%) and consistent with the difference in the solid-state quantum efficiency between them.