diyl-co-2,8-di(O-methylenecyclohexyl)dibenzothiophene-S,S-dioxide-3,7-diyl) p(F m-S OCy) in three different co-monomer ratios, P1-3, give deep blue electroluminescence peaking at 415 nm, with the ratio of 70 : 30 p(F m : S OCy) producing a maximum external quantum efficiency (EQE) h ext, max 2.7%, whilst the ratio of 85 : 15 gave the highest maximum brightness L max of 81 cd m À2 , with CIE coordinates (0.17, 0.12) The analogous para series poly(9,9-dihexylfluorene-2,7-diyl-co-2,8-di(O-methylenecyclohexyl)dibenzothiophene-S,S-dioxide-3,7-diyl) p(F p-S OCy) and poly(9,9-dihexylfluorene-2,7-diylco-2,8-dihexyloxydibenzothiophene-S,S-dioxide-3,7-diyl) p(F p-S O6) in two different ratios, P4-7, produced blue emission peaking at ca. 450 nm. The ratio of 70 : 30 F : S units consistently gave better devices than the corresponding 50 : 50 co-polymers. It was also observed that co-polymers incorporating the bulkier S OCy derivatives gave more efficient and brighter devices, with polymer P5 attaining a remarkable h ext, max 3.2%, 4.4 cd A À1 , 3.4 lm W À1 and maximum brightness 2500 cd m À2 with CIE (0.16, 0.18).