A series of four new statistical copolymers of 9,9-dihexylfluorene and 9-fluorenone with well-defined structures and a new fluorene-fluorenone-fluorene trimer model compound were synthesized and used to investigate the photophysics, origin of the green emission, and electroluminescence of this class of light-emitting materials. We show that the new oligofluorene trimer with a central fluorenone moiety is an excellent model of the green-emitting chromophore in polyfluorenes. From systematic studies of the steady-state photoluminescence (PL) and PL decay dynamics of solutions of the fluorenone-containing copolymers and oligomer and thin films of the copolymers, we show that the controversial 535-nm green emission band originates from the fluorenone defects in single-chain polyfluorenes and not from intermolecular aggregates or excimers. The green emission, centered at 535 nm, was observed in dilute toluene solutions of all fluorenone-containing copolymers and oligomer; it was long-lived with a single-exponential PL lifetime of ∼5 ns, compared to a lifetime of 240-400 ps for the blue emission. The PL decay dynamics of the 535-nm emission from thin films of all copolymers was also well-described by a single-exponential lifetime of 5-6 ns. The observed increased intensity of the green emission with increased intermolecular interactions in solution or solid state can be explained by the increased excitation energy transfer from fluorene segments to the fluorenone moieties. Bright green electroluminescence (EL) centered at 535 nm was achieved from single-layer copolymer lightemitting diodes (LEDs), ITO/PEDOT/copolymer/Al, with luminances of 1600-3340 cd/m 2 that varied with fluorenone content. The EL data suggest that the fluorene-fluorenone copolymers are very promising materials for green LEDs.