Searching for new fluorescence switches: naphthalene-containing metal complexes whose emission can be controlled by pH variations

The tetra-azamacrocyclic nickel(II) complex [Ni-II(4)](2+), has been synthesized, structurally characterized and investigated as a fluorescence switch. [Ni-II(4)](2+), which in solution displays an emission spectrum typical of naphthalene derivatives, undergoes a deprotonation at moderately basic pH values (pK(a) = 8.8 in 4:1 MeCN-water solution) involving the dienic moiety of the macrocyclic system. As the deprotonated species, [Ni-II(8)](+), is distinctly less emissive than its dienic analogue, the fluorescence of [Ni-II(4)](2+) in a 4:1 MeCN-water solution is quenched by addition of a base and revived by addition of an acid. So, a reversible variation of the emission intensity can be obtained by simply varying the pH of the solution. (C) 2000 Elsevier Science S.A. All rights reserved.