Poly(9,9-dioctylfluorene-2,7-diyl-co-2,8-dihexyldibenzothiophene-S,S-dio xide-3,7-diyl) copolymers (pF-S-6 8-10) of varying composition have been synthesized by Suzuki-Miyaura polymerization. The hexyl substituents on the dibenzothiophene-S,S-dioxide (S) units improve the solubility or the copolymers and increase the dihedral angles in the backbone; this shifts the emission deep into the blue (lambda(max) 420 nm for films of 10) and increases the photoluminescence quantum yield compared with previous pF-S copolymers containing non-alkylated S units. The backbone twist restricts formation of the intramolecular charge transfer (ICT) state for low incorporation ratios of S-6 units. The triplet energy of these new copolymers increases as the percentage of the S-6 unit increases (i.e., 15, 30, 50%: 8 -> 9 -> 10). The alternating copolymer 10 has a sufficiently high triplet energy (E-T 2.46 eV for onset of phosphorescence) to host a green phosphorescent iridium guest emitter, as demonstrated in electroluminescence studies which showed emission exclusively from the guest complex.