This paper describes a red dopant material, the cyanocoumarin derivative 9-cyano-10-(2-benzothiazolyl)-1,1,7,7-tetramethyl-2,3,6,7-tetrahydro-1H,5H,11H-benzo [l] pyrano[6,7,8-i j]quinolizin-11-one (RC545T), and its use in organic light-emitting diodes (OLEDs). Because RC545T exhibits (i) redshifted fluorescence (lambda(max)=607 nm) and (ii) a narrower full width at half-maximum (fwhm; 48 nm) relative to the corresponding signal for 4-(dicyanomethylene)-2-tert-butyl-6-(1,1,7,7-tetramethyljulolidyl-9-enyl)-4H-pyran (DCJTB; lambda(max)=605 nm; fwhm=73 nm), which is currently one of the most efficient red dopant materials, we could fabricate red OLEDs exhibiting improved color purity. A device incorporating RC545T as a dopant and rubrene as an assistant dopant in the configuration indium-tin oxide (ITO)/N,N'-bis(1-naphthyl)- N,N'-diphenyl-1,1'-biphenyl-4,4'-diamine (65 nm)/tris(8-hydroxyquinoline)aluminum (Alq(3)):15 wt % rubrene:2 wt % RC545T (30 nm)/Alq(3) (30 nm)/LiF (0.8 nm)/Al (250 nm) exhibited a maximum luminance of 8000 cd/m(2) at 12.5 V, with near-saturated CIE coordinates of (0.64, 0.35), significantly better red color purity than that of the device featuring DCJTB as the dopant material [CIE coordinates: (0.63, 0.37)], thereby rendering RC545T a good red-emitting dopant for OLED applications. (C) 2008 The Electrochemical Society. [DOI: 10.1149/1.2990718] All rights reserved.