Synthesis and characterization of kinked and hyperbranched carbazole/fluorene-based copolymers

Abstract

To decrease the aggregation phenomena and improve the luminescent properties of polyfluorenes (PFs), a series of novel kinked and hyperbranched carbazole units were copolymerized by the Suzuki coupling polycondensation reaction to introduce disorder packing into the copolymer backbones. The thermal, photophysical, electrochemical, and electroluminescent properties of the copolymers were investigated. All of these polymers possess excellent thermal stability with onset decomposition temperatures at 407-466 degrees C and glass transition temperatures at 109-140 degrees C. Photoluminescent (PL) studies showed that these polymers were promising blue light-emitting materials, which exhibited high quantum efficiencies in solution and solid states. Long wavelength emissions at 500-600 nm, which were typical for PFs due to their self-aggregation in the solid state, were suppressed in these polymers. Annealing studies under air showed both improved thermal and photoluminescence stability of the polymers. It was proven that HOMO energy levels of the copolymers can be enlarged by increasing the carbazole content in the electrochemical measurements; hence, the hole injection was greatly enhanced. Pure-blue electroluminescence (EL) spectra with narrow fwhm (full width at the half-maximum) values (39-42 nm) and negligible low-energy excimer emission bands were successfully achieved, indicating that these copolymers could be good candidates for blue light-emitting materials.