Suppression of green emission in a new class of blue-emitting polyfluorene copolymers with twisted biphenyl moieties

Abstract

The color purity of polyfluorene-based blue-emitting polymers is often compromized by "long-wavelength" green emission bands, attributed to polymer interchain species first and more recently to formation of emissive fluorenone defects. Here, we study the nature and the suppression of such bands via characterization of a new class of polyfluorene derivatives modified by insertion of functional groups at the bridging points (position C9), so as to increase inter-ring torsion angles. We find that the solid-state photoluminescence spectra of random copolymers of the modified polyfluorenes and the homopolymer display a progressive decrease of the long-wavelength emission. Electroluminescence spectra also show efficient suppression of such bands in the copolymers with a concentration of 'twisted' comonomer units of 40% or greater. Quantum-chemical calculations on model oligomers address the influence of the bridging unit on the torsion angles, and the resulting excited-state properties; the impact on molecular packing is also explored with force-field calculations. We conclude that increase of intra-biphenyl torsion angles is a viable strategy for suppression of long-wavelength emission bands in polyfluorenes. © 2005 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.