Ni(II)porphyrins as pH dependent light-driven coordination-induced spin-state switches (LD-CISSS) in aqueous solution

Abstract

A water-soluble Ni(II)-porphyrin substituted with a covalently attached azopyridine ligand was synthesized. Upon irradiation with violet and green light, the azo unit performs a reversible cis–trans isomerization. This geometry change triggers a coordination/de-coordination of the pyridine nitrogen at the central nickel(II) ion. The concomitant change in coordination number at the Ni ion in turn switches the spin state between high and low spin, a process we coined a “light-driven coordination-induced spin state switch (LD-CISSS). To increase the coordination power of the pyridine, particularly in aqueous environments, we introduced an electron donating OH group in 4-position. With increasing pH, the hydroxyl group is deprotonated, further enforcing coordination. We report on the properties of this pH-dependent spin switch, particularly the magnetic properties.