Twisted conformations in complexes of N-(3-imidazol-1-yl-propyl)-1,8-naphthalimide and fluorescence properties

Abstract

A series of complexes of N-(3-imidazol-1-yl-propyl)-1,8-naphthalimide (L) with divalent ions of manganese, cobalt, zinc, cadmium and mercury are structurally characterized. The metal complexes [ML₂Cl₂] {M = Zn (1), Cd (2), Hg (3)} are isomorphous and have a distorted tetrahedral geometry with a bend conformation of L. The thiocyanate complex [ZnL₂(SCN)₂] has a distorted tetrahedral geometry with L in bent conformation but with a different geometry from that in the structure of complex 1. The manganese and cobalt thiocyanate complexes [ML₄(SCN)₂]·2CH₃CN (M = Mn, Co) are isomorphous and have a distorted octahedral geometry with the thiocyanate ligands occupying the axial positions. The cadmium complex [CdL₃(SCN)₂DMF]·DMF has a distorted octahedral geometry with thiocyanate ligands in the axial positions. The tetrahedral complexes [ML₂Cl₂] {M = Zn (1), Cd (2)} in the solid state show emission at shorter wavelengths than the single emission peak observed from the ligand, whereas the fluorescence emission of [ML₂(SCN)₂] {M = Zn (1), Cd (2)} occurred at longer wavelengths than L. On the other hand, [HgL₂Cl₂] (3) showed a single emission peak with higher intensity but at 31 nm shorter wavelength than the emission peak of the parent ligand. Two types of bend orientations of the ligand L, namely parallel arrangement of imidazole with the 1,8-naphthalimide ring and non-parallel arrangement in the tetrahedral complexes are observed. The former case favors intra-molecular charge transfer to show shorter wavelength emission, whereas the non-parallel arrangement facilitates exciplex leading to emission at longer wavelengths.