Carbazole is a heterocyclic tricyclic aromatic organic compound consisting of two six-membered benzene rings fused on either side of a five-membered nitrogen-containing ring. A large number of carbazole derivatives have been designed and synthesized and organic electronic devices based on these derivatives such as organic light emitting diodes (OLEDs), have been investigated. The optical and electrical properties of carbazoles are affected by substitution on the 2-, 3-, 6-, 7- and 9H-positions. Many carbazole derivatives have sufficiently high triplet energy to make them an efficient host where they can serve as red, green, or blue triplet emitters. Highly fluorescent and stable carbazole-based compounds were synthesized and characterized. Substitution of carbazoles at 3- and 6- position by tert-butyl group enhanced the solubility. Suitably susbtituted carbazoles form highly stable fluorescent organic nanoparticles. The emission of these nanoparticles was reversibly switched on/off in the blue-green and orange-red regions from a change in the ratio of the tetrahydrofuran/water system used in their preparation.