No solid-state fluorescence is observed for 9-(diethylamino) benzo [a] phenoxazin-5-one (Nile Red). However, 9-dibutylamino-6-{perfluoro [4-methyl-3-(1-methylethyl)-2-penten]-2- oxy} benzo [a] phenoxazin-5-one showed fluorescence maximum at 717nm in solid state with fluorescence quantum yield 0.024. X-ray crystallographic analysis suggests that prevention of network π− π interactions by the bulky fluorine-containing and dibutylamino ...