Synthesis and characterization of a photochromic magnesium(II) coordination polymer based on a naphthalene diimide ligand.

Naphthalene diimides, which are planar, chemically robust and redox-active, are an attractive class of electron-deficient dyes, which can undergo a single reversible one-electron reduction to form stable radical anions in the presence of electron donors upon irradiation. This makes them excellent candidates for organic linkers in the construction of photochromic coordination polymers. Such a photochromic one-dimensional linear coordination polymer has been prepared using N,N'-bis(3-carboxyphenyl)naphthalene-1,8:4,5-tetracarboximide (H2BBNDI). Crystallization of H2BBNDI with magnesium nitrate in an N,N'-dimethylformamide (DMF)/ethanol/H2O mixed-solvent system under solvothermal conditions afforded the one-dimensional coordination polymer catena-poly[[bis(dimethylformamide-κO)magnesium(II)]-bis[µ-N-(3-carboxylatophenyl)-N'-(3-carboxylphenyl)naphthalene-1,8:4,5-tetracarboximide-κ2O:O']], [Mg(C28H13N2O8)2(C3H7NO)2]n. The asymmetric unit contains half of a magnesium cation, one HBBNDI- ligand and one DMF molecule. Two partially deprotonated HBBNDI- ligands bridge two magnesium cations to form a one-dimensional chain. Strong inter-chain π-π interactions between the naphthalene rings of the HBBNDI- ligand and the imide rings of adjacent chains provide a two-dimensional structure. The supramolecular three-dimensional framework is stabilized by π-π interactions between naphthalene rings of neighbouring two-dimensional supramolecular networks. The complex exhibits a reversible photochromic behaviour, which may originate from the photoinduced electron-transfer generation of radicals in the HBBNDI- ligand.