Suppression of Green Emission in a New Class of Blue-Emitting Polyfluorene Copolymers with Twisted Biphenyl Moieties

Authors


  • We thank the Engineering and Physical Sciences Research Council (EPSRC (UK)) and the European Commission for financial support. The work in Mons is supported by the Belgian Federal Government “InterUniversity Attraction Pole in Supramolecular Chemistry and Catalysis (PAI 5/3)” and the Belgian National Fund for Scientific Research (FNRS-FRFC). E. H. and D. B. are FNRS Research Fellow and Senior Research Associate, respectively. S. F. was funded by the Institute of Materials Research & Engineering (IMRE), Singapore. F. C. was a Royal Society University Research Fellow (URF) at the time of this work.

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.

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