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.　©　International　Union　of　Crystallography,　2017.