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.

Time-resolved photoluminescence study of low-energy emission mechanisms in oligofluorene and polyfluorene films

“…In this regard, our result is consistent with the previous work by Lim et al, who found that the green emission was suppressed by reducing the close packing of molecular segments in polyfluorene copolymers [24].…”

A 10 Mb/s visible light communication system using a low bandwidth polymer light-emitting diode

“…This features two peaks at ~ 480 nm and 624 nm, significantly red-shifted with respect to the emission of all three components. While the emergence of "green emission bands" is well documented in the literature and attributed to a combination of ketone-defects and inter-chain effects [10], we consider that the relatively large red-shift of the second peak, that is observed in these spectra is likely to be related to formation of exciplex species between some of the polyfuorenes. The frontier energy levels of these materials are so close that it is hard to identify which specific polyfluorene is involved in the formation of this excited state and as such, further investigations are in progress.…”

Oligomers and Polymers Based on Bridged Phenylenes as Electronic Materials

“…[146,147] Introduction of 30-40 mol.-% of such units into fluorene copolymers 68a-d has been shown to suppress the long wavelength emission seen for the fluorene homopolymers. [148] The best EL efficiency (0.24 cd Á A À1 ) was seen for 68 which is, however, significantly below that of 4b (0.51 cd Á A À1 ). These effects are attributed to disruption of the chain packing, which not only hinders exciton migration to emissive defects, but also charge transport.…”