Synthesis of functionalised fluorescent dyes and their coupling to amines and amino acids

A series of novel functionalised benzophenoxazinones, analogues of Nile Red, is prepared and their fluorescence properties evaluated. The ring system is prepared by reaction of 5-diethylamino-2-nitrosophenol with 1,6-dihydroxynaphthalene followed by alkylation of the 2-hydroxy group with 6-bromohexanoic acid derivatives. Subsequent ester cleavage under a variety of conditions gives the acid 6. Replacement of the 9-diethylamino group with the N-ethyl-(3-sulfonylpropyl)amino group is carried out to increase water solubility and the resulting dye 14 has similar fluorescence properties. The acid 6 is coupled to a range of amino compounds.