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

The tetra-azamacrocyclic nickel(II) complex [NiII(4)]2+, has been synthesized, structurally characterized and investigated as a fluorescence switch. [NiII(4)]2+, which in solution displays an emission spectrum typical of naphthalene derivatives, undergoes a deprotonation at moderately basic pH values (pKa=8.8 in 4:1 MeCN–water solution) involving the dienic moiety of the macrocyclic system. As the deprotonated species, [NiII(8)]+, is distinctly less emissive than its dienic analogue, the fluorescence of [NiII(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.