Chromo and Fluorogenic Properties of Some Azo-Phenol Derivatives and Recognition of Hg2+ Ion in Aqueous Medium by Enhanced Fluorescence

Azo-phenol based receptors, having an azo core incorporated with naphthol and substituted (-H, -NO2, -CH3, and -OCH3) benzothiazole units have been synthesized and their detailed optical properties have been investigated in different media. The nitro substituted azo derivative, 2, has shown, categorically, promising optical behavior in the absence as well as presence of different transition metal ions in aqueous-acetonitrile solution and has illustrated greater sensitivity, using the naked-eye for the detection of Hg2+ ion by enhanced fluorescence. The mechanism of metal-ion interaction has been established by absorption, emission, FT-IR, and (HNMR)-H-1 spectroscopic experiments that indicated favorable coordination of Hg2+ metal ion by the phenolic oxygen atom, the azo nitrogen atom adjacent to the naphthol ring, and the thiazole nitrogen of benzothiazole ring, obviously in a terdentate manner leading to the formation of a five-membered chelate ring. The 2:1 stoichiometry and association constants, K-assoc(abs.) = 1.04 +/- 0.3 x 10(5) M-2 and K-assoc(em.) = 1.51 x 10(5) M-2 have been estimated with the help of Job's plot and the Benesi-Hildebrand method. The cis-trans isomerism and azo-hydrazone type of tautomerism by the photoinduced proton transfer reaction and also metal induced (Hg2+ ion) proton transfer reaction from the hydrazone form in the present organic molecular systems have been confirmed by photoirradiation and fluorescence excitation experiments and by H-1 and C-13 NMR spectral analysis.