Structures and solid-state fluorescence of coordination compounds of metal halides and naphthalimide derivatives

Abstract

Naphthalimide derivatives are fluorophores that have been shown to have desired optical properties. When incorporated into a complex molecule, the naphthalimide derivative imparts fluorescence properties to the complex. This study focussed on the investigation of the structural characteristics and fluorescence properties of two families of coordination complexes comprised of metal halides and naphthalimide derivatives as fluorophores. The fluorophores N-(3-imidazol-1-yl-propyl)-1,8-naphthalimide (imnap) and N-(isonicotinylhydrazide)-1,8-naphthalimide (iz) were used. The crystal structure of imnap was determined, as well as the structures of ten new complexes formed from the combination of imnap and different metal halides from a solvent mixture containing dimethylformamide (DMF) with methanol or tetrahydrofuran. These complexes are formed from a combination of imnap and CoCl2, CoBr2, CuBr2, ZnBr2, ZnI2, CdBr2, CdI2, HgBr2 or HgI2 and they display a distorted tetrahedral geometry, with no solvent incorporated into the structures. The combination of imnap and CuCl2 results in a complex with a severely distorted tetrahedral geometry. When self-assembled from dimethyl sulfoxide (DMSO), ten new complexes are formed from the combinations of imnap and CoCl2, CuCl2, CuBr2, ZnCl2, ZnBr2, CdCl2, CdBr2, HgCl2, HgBr2 or HgI2, all exhibiting tetrahedral geometries, with DMSO molecules incorporated into the crystal structures. The complexes containing d10 metal ions showed fluorescence in the violet region of the spectrum, irrespective of the presence of solvent molecules in the structures, while imnap showed fluorescence in the blue region. The complexes containing paramagnetic metal ions showed fluorescence quenching, except for the CuBr2-containing complex. The crystal structures of the hydrated and anhydrous forms of iz, and the structures of twelve complexes comprised of iz and different metal halides were determined. All of these structures included either DMF or DMSO solvent molecules, depending on the solvent used for crystallisation. The combination of iz and ZnBr2 in DMF gave a distorted tetrahedral complex, while the combination of iz with HgBr2 or HgI2 resulted in distorted square planar complexes, with the structures incorporating DMF molecules. The combination of iz and CdBr2 in DMF resulted in a structure containing a halide-bridged polymer. When iz was combined with HgBr2 or HgI2 in DMSO, distorted square planar complexes formed, with both DMSO molecules incorporated into the structures. Lastly, the combination of iz with CoCl2, CoBr2, CuCl2, CuBr2, CdCl2 or CdBr2 in DMSO resulted in distorted octahedral complexes, with DMSO and water molecules incorporated into the structures. Due to the suspected loss of solvent molecules from the structures, pure bulk samples of the DMF-containing iz-complexes could not be obtained, and difficulty was also experienced in obtaining pure bulk samples of the DMSO-containing iz-complexes. Both the compound obtained from the combination of iz and CdCl2 in DMSO, and iz emitted in the blue region. The study showed that the structural characteristics and fluorescence properties of the complexes are affected by factors like the solvent of crystallisation, the metal ion, the organic ligand, as well as the halido ligand present.