Synthesis and Photophysical Characterization of Fluorescent Naphtho[2,3- d ]thiazole-4,9-Diones and Their Antimicrobial Activity against Staphylococcus Strains.

The chemical reaction of 2-(methylsulfinyl)naphtho[2,3- d ]thiazole-4,9-dione ( 3 ) using different amines, including benzylamine ( 4a ), morpholine ( 4b ), thiomorpholine ( 4c ), piperidine ( 4d ), and 4-methylpiperazine ( 4e ), produced corresponding new tricyclic naphtho[2,3- d ]thiazole-4,9-dione compounds ( 5a - e ) in moderate-to-good yields. The photophysical properties and antimicrobial activities of these compounds ( 5a - e ) were then characterized. Owing to the extended π-conjugated system of naphtho[2,3- d ]thiazole-4,9-dione skeleton and substituent effect, 5a - e showed fluorescence both in solution and in the solid state. The introduction of nitrogen-containing heterocycles at position 2 of the thiazole ring on naphtho[2,3- d ]thiazole-4,9-dione led to large bathochromic shifts in solution, and 5b - e exhibited orange-red fluorescence with emission maxima of over 600 nm in highly polar solvents. Staphylococcus aureus ( S. aureus ) is a highly pathogenic bacterium, and infection with its antimicrobial-resistant pathogen methicillin-resistant S. aureus (MRSA) results in serious clinical problems. In this study, we also investigated the antimicrobial activities of 5a - e against S. aureus , MRSA, and S. epidermidis . Compounds 5c with thiomorpholine group and 5e with 4-methylpiperazine group showed potent antimicrobial activity against these bacteria. These results will lead to the development of new fluorescent dyes with antimicrobial activity in the future.