Excimer emission induced by metal ion coordination in 1,8-naphthalimide-tethered iminopyridine ligands

Ligands 2-5, containing the light-emitting subunit 1,8-naphthalimide, have been prepared and their photophysical properties studied by absorption and emission spectroscopy. Ligand 2 interacts in solution with several cations according to a 1 : 2 (metal : ligand) stoichiometry, the 1 : 3 species being not favoured probably because of steric hindrance, while ligands 3-5 form 1 : 3 adducts with all the investigated metal ions. The interaction of ligands 2-5 with metal ions induces considerable variations on the photophysical properties of the light-emitting subunit. The coordination of genuine transition metal ions (FeII, CoII, NiII, CuII) causes the emission intensity to decrease in all the investigated systems, while ZnII or CdII induce a fluorescence enhancement (system 2) or the formation of a new band in the emission spectra (systems 3-5) which can be ascribed to an intramolecular excimeric species. Excimeric emission is not observed in the complexes of 2, possibly because the ethylenic chain bridging the naphthalimide and the iminopyridine units is too short to allow the intramolecular interaction. The excimeric species disappears on increasing the metal ion (ZnII or CdII) concentration, as a result of the "disassembling" of the 1 : 3 complexes and the consecutive formation of 1 : 2 and 1 : 1 species, in which the intramolecular interaction is less probable or no longer possible. The appearance and disappearance of an excimer band in the emission spectrum can be described as a convenient way to monitor a metal-driven assembling/disassembling process.