Light-emitting devices, using ruthenium(II)(4,7-diphenyl-1,10-phenanthroline)(3) ([Ru(dphphen)(3)](2+)) as dopant and emitting center have been investigated, in a structure of indium-tin-oxide glass (TTO)/poly(N-vinylcarbazole) (PVK):2-(4-biphenyl)-5-(4-terbutylphenyl)-1,3,4-oxadiazole (PBD):[Ru(dphphen)(3)](2+)/tris-(8-hydroxyquinoline) aluminum (Alq(3))/Al. Instead of the long response time observed at high doping concentration, instantaneous light emission from [Ru(dphphen)(3)](2+) is obtained at a doping concentration at which the electronic coupling between the [Ru(dphphen)(3)](2+) molecules is eliminated by the host matrix. This is attributed to the direct exciton formation at [Ru(dphphen)(3)](2+) molecule by charge trapping from PVK and PBD. The emission is isolated to [Ru(dphphen)(3)](2+) by inserting a layer of 2,9-dimethyl-4,7-diphenyl-1,10-phenanthroline (BCP) between PVK:PBD: [Ru(dphphen)3]2+ and Alq3 layers. (C) 2004 Elsevier B.V. All rights reserved.