Synthesis and characterization of thermally stable blue light-emitting polyfluorenes containing siloxane bridges

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 % siloxane-bridged fluorene unit) and PSiloBg3 (containing 3 mol % siloxane-bridged fluorene unit). PSiloBg1 and PSiloBg3 exhibited good solubility in common organic solvents, thermal stability up to 420 degreesC, and facile film formation. The glass transition temperatures of the bridged polymers (106 and 110 degreesC, respectively) were higher than that of the homo poly(dihexylfluorene) (PDHF). In particular, PSiloBg3 exhibited a polymerization yield (96%) and a molecular weight (MW = 185 000) higher than those of PDHF (polymerization yield = 62%, MW = 82 000). Interestingly, after the bridged polymers had been annealed at 150 degreesC for 4 h in air, their PL spectra showed no significant increase in vibronic structures at 450 and 475 nm and no evidence of aggregation formation and excimers at wavelengths of >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.