Synthesis and characterization of fluorene-based blue light-emitting conjugated polymers

Abstract

The first novel polyfluorene copolymers containing siloxane bridges were synthesized by Ni(0)-mediated copolymerization between 2,7-dibromo-9,9’-dihexylfluorene and a bridged fluorene monomer containing siloxane linkages. Two such bridged copolymers were prepared: PSiloBg1 (containing 1 mol.% of siloxane-bridged fluorene unit) and PSiloBg_3 (containing 3 mol.% of siloxane-bridged fluorene unit). PSiloBg1 and PSiloBg3 showed good solubility in common organic solvents, thermal stability up to 420 ℃, and facile film formation. The glass transition temperatures of the bridged polymers (106 and 110℃ respectively) were higher than that of the homo poly(dihexylfluorene) (PDHF). In particular, PSiloBg3 exhibited a higher polymerization yield (96%) and molecular weight (M.W. = 185,000) compared to PDHF (polymerization yield = 62%, M.W. = 82,000). Interestingly, after the bridged polymers had been annealed at 150 ℃ for 4 h in air, their PL spectra showed no significant increase in vibronic structures at 450 nm and 475 nm and no evidence of aggregation formations and excimers at wavelengths over 500 nm. In addition, the full width at half maximum (fwhm) of the bridged polymers was very small (fwhm = 51-52 nm) compared to that of PDHF (fwhm = 85 nm). Collectively, the results show that the polyfluorenes with siloxane bridges exhibit thermally stable almost pure blue emission, making these polymers promising candidate materials for device applications. Novel blue polyfluorenes with polyhedral oligomeric silsesquioxanes(POSSs) were successfully synthesized by Ni(0)-mediated polymerization. The photoluminescence spectra of the copolyfluorenes containing POSSs at small contents exhibited no excimer and aggreation formations, and therefore resulted in thermally stable pure blue emission even after being annealed at 150℃ for 5h in air. Furthermore, PL quantum efficiencies of the polyfluorenes containing POSSs were highly enhanced compared to PDHF(polydihexylfluorene) in solid...