Fluorene-based copolymers for Solid-State Lighting: correlation between molecular structure and photophysical properties

Efficient white electroluminescence can be obtained from a fully miscible blend of a blue polyfluorene derivative (PF) and a new red- and green-emitting polymers, hereafter core-copolymers, bearing a 4,7-bis(thiophen-2-yl)benzothiadiazole and a benzothiadiazole respectively as bridging core between two identical polymeric arms synthesized by Suzuki coupling reaction of the dibromine derivative of such chromophores and essentially borolane-ended alternated copolymers of triphenylammine disubstituted fluorene with dialkylsubstituted fluorene.1 The chemical composition of the materials is a key factor to tune the emission colour, particularly the white shade which dramatically depends on the percentage of the three copolymers constituting the blend. A deep chemical characterization of three compounds has been carried out combining different techniques: in particular, the MALDI-TOF MS analysis of the neat blue copolymer and core-copolymers and of the their narrow polydisperse fractions obtained by SEC fractionation. By applying the off-line SEC/MALDI-TOF MS coupling method to the crude synthesized materials, reliable information on both their composition and on the side reactions occurring during the synthesis of the PF pre-polymers are obtained.2 We were able to discriminate among the different situations: core-type and end-type copolymers for high and intermediate molecular weight range respectively, and blue copolymer for the lower molecular weight fractions. Photophysical characterization (absorption and photoluminescence) of diluted solution of the different fractions have been carried out to investigate intramolecular energy transfer mechanism as a function of both the molecular weight and the acceptor dye position within the polymer chain (core or end-type copolymers). These results totally agree with the different compositions determined by their MALDI mass spectra