Metal-Induced Assembling/Disassembling of Fluorescent Naphthalenediimide Dericatives Signalled by Excimer Emission

The new quadridentate bis-chelating ligands 2 and 3 display in solution the typical absorption and emission properties expected for naphthalenediimide derivatives. Spectrophotometric studies show that systems 2 and 3 interact with ZnII, CdII and CuI in CHCl3 or MeCN solution according to an apparent 1:1 stoichiometry. Molecular modelling, ESI-MS and 1H-NMR experiments point out that the complex species formed in the presence of stoichiometric amounts of metal ion are non-helical [2+2] adducts. The metal induced self-assembling process is signalled by an intense excimer-type emission due to the intramolecular interaction of two naphthalenediimide subunits which face each other in the [2+2] adduct, as shown by molecular modelling studies. In the presence of excess metal ion, a disassembling process takes place, leading to a dinuclear complex with a 2:1 metal:ligand stoichiometry, in which the intramolecular excimer is no longer allowed to form, since the interaction between the naphthalenediimide subunits has been lost. Thus, the overall metal-induced assembling/disassembling process is signalled by the appearance and disappearance of the excimer band in the emission spectrum.