Microwave-assisted synthesis of 3-hydroxy-4-pyridinone/naphthalene conjugates. Structural characterization and selection of a fluorescent ion sensor


Two novel 3-hydroxy-4-pyridinone/naphthalene conjugates (L1 and L2) with different distances between the chelating and the fluorescent moieties were synthesized using conventional heating and microwave irradiation achieving a shorter reaction time. The structure of both compounds was confirmed by X-ray crystallography, revealing that these compounds were isolated as hydrochloride salts in dihydroxypyridinium forms. In solution and in the presence of a base, the tautomeric keto forms may be obtained as it was elucidated by NMR analysis. The dihydroxypyridinium form of L1 exhibits fluorescence at 450 nm, both in ACN and DMSO, whereas the corresponding keto form exhibits fluorescence at 365 nm. In contrast, the dihydroxypyridinium form of L2 only fluoresces in DMSO, exhibiting a band at 340 nm, while the keto form is non-fluorescent. These distinct fluorescent behaviors reveal that the tautomeric form in which the ligands are isolated and the distance between the chelating and fluorescent functions strongly influences their fluorescence properties. Ligand L1 exhibits better fluorescence properties and its fluorescence intensity is quenched in the presence of variable concentration of Cu(2+), Zn(2+), and Fe(3+), thus making it suitable to be used as ion sensor. (C) 2010 Elsevier Ltd. All rights reserved.



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Silva, AMG; Leite, A; Andrade, M; Gameiro, P; Brandao, P; Felix, V; de Castro, B; Rangel, M

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Financial support from FCT through project PTDC/QUI/67915/2006 is gratefully acknowledged. Andreia Leite (SFRH/BD/30083/2006) thanks FCT for Ph.D. grant. We also thank Dr. Andrea Carneiro from CENTI, V.N. Famalicao, for making available to us a CEM Discover microwave reactor. The Bruker Avance II 400 spectrometer is part of the National NMR network and was purchased under the framework of the National Programme for Scientific Re-equipment, contracREDE/1517/RMN/2005, with funds from POCI 2010 (FEDER) and (FCT).

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