Binuclear coordination complex of open merocyanine form of photochromic spiropyran with MnII(hfac)2 having high spin (S = 5) ground state

Abstract

Photochromic molecules can transform from colorless closed to colored open forms by coordination with metal ions. Interaction of photochromic 1,3,3-trimethylindolino-β-naphthopyrylospiran (TMI-NPS) with manganese(II) hexafluoroacetylacetonate in o-dichlorobenzene results in the formation of deep red-violet solution indicating a coordination-induced transition from closed to colored open form. Precipitation of crystals yields {Mn^II(hfac)₂·(μ₂-TMI-NPS)}₂ dimers (1) in which two Mn(II) atoms of hexafluoroacetylacetonate are bridged by oxygen atoms of two TMI-NPS ligands in the open form. Magnetic studies show a weak ferromagnetic exchange between Mn^II spins through oxygen atoms of TMI-NPS with estimated exchange interaction (J) of +1.07 cm⁻¹. A positive Weiss temperature of +8 K also supports ferromagnetic coupling. As a result, Mn^II spins arrange parallel to each other at 2 K, and correspondingly, these dimers transfer to a high spin (S = 5) state. DFT calculations support noticeable mixing of the metal and ligand orbitals and essential positive charge on the TMI-NPS ligands, which can provide ferromagnetic exchange between Mn^II spins.