Naphthoquinone derivatives as receptors for the chromogenic sensing of metal cations and anions

The behaviour of the naphthoquinone derivatives 4-(4-dimethylamino-phenyl)-[1,2]naphthoquinone (L-1) and 4-[4-(1,4,7,10,13-pentaoxa-16-aza-cyclooctadec-16-il)-phenyl]-[1,2]naphthoquinone (L-2) as chromogenic chemosensors of metal cations and anions in acetonitrile has been studied. L-1 and L-2 are highly coloured receptors with a broad band centred at 534 (log epsilon=3.80) and 532 nm (log epsilon=3.86), respectively, and attributed to an intra-ligand charge transfer band involving the amine and the carbonyl groups. L-1 shows a hypsochromic shift of the absorption band in the visible zone in presence of Cu2+, Fe3+ and Hg2+. Additionally, L-2 suffers bleaching induced by the metal cations Pb2+ and Ba2+. In both cases, the data suggested metal cation coordination through the nitrogen atom of the aniline group. H-1 NNIR studies in the presence of certain metals are also in agreement with this interpretation. log K for the formation of the corresponding [M(L)](2+) complexes have been determined from nonlinear least-squares treatment of the titration profiles. Studies in the presence of anions were also carried out. In acetonitrile a chromogenic sensing of the anions HSO4- was observed using the metal complex Ba-L-2. (C) 2005 Elsevier Ltd. All rights reserved.