Magnetic and Photochromic Properties of a Manganese(II) Metal-Zwitterionic Coordination Polymer

The solvothermal reaction of Mn(ClO₄)₂, NaN₃, and a rigid viologen-tethered tetracarboxylic acid (1,1′-bis(3,5-dicarboxyphenyl)-4,4′-bipyridinium chloride, [H₄L]Cl₂) led to a coordination polymer of formula [Mn₄(L)(N₃)₆(H₂O)₂]_n. X-ray analysis revealed a 3D coordination structure. The Mn(II) ions are connected by mixed azide and carboxylate bridges to give 2D layers, which are pillared by the viologen tether of the zwitterionic ligand. Magnetic analyses suggested that the compound features antiferromagnetism and field-induced metamagnetism. The compound also shows photochromic and photomagnetic properties. The long-range magnetic ordering is owed to the spin-canting structure of the Mn(II)-azide-carboxylate layer; the photochromism involves the formation of viologen radicals via photoinduced electron transfer, and the photomagnetism is related to the interactions between the metal ion and the photogenerated radicals. The study demonstrates a strategy for the design of new multifunctional materials with photoresponsive properties.