Tuning the optoelectronic properties of polyfluorenes by copolymerisation with thiophene moieties

A series of alternating copolymers of the type A-alt-B, where A is 9,9-bis(2′-ethylhexyl)fluorene and B is a thiophene-based moiety, was synthesised via the palladium-catalysed Suzuki coupling. These copolymers were characterised by optical and electrochemical methods and used in the fabrication of light-emitting diodes. We find that upon increasing the conjugation length of the B moiety, from a thiophene unit to 2,5-bis(2′-vinyl thienyl)thiophene, the fluorescence quantum yields are reduced both in solution (from 51 to 11%) and in solid state. Both absorption and emission are red-shifted, with the solid state emission maximum increasing from 476 nm (green) to 600 nm (orange). When the B unit is changed from thiophene to thiophene-S,S-dioxide, there is a significant increase in both the ionisation potential and electron affinity, and a concurrent reduction of the solution fluorescence efficiency (down to 16%). These polymers appear promising materials for optoelectronic applications.