Photoswitching of the magnetic interaction between a copper(II) ion and a nitroxide radical by using a photochromic spin coupler

Photoswitching of an intramolecular spin exchange interaction between a copper(II) ion and a nitroxyl radical by using a metal complex of diarylethene has been studied by means of ESR spectroscopy. As a coordination ligand, a diarylethene with a 1,10-phenanthroline ring and nitronyl nitroxide radical was synthesized. Mixing the diarylethene ligand with [Cu(hfac)(2)] (hfac = hexafluoroacetylacetone) in toluene led to a hypsochromic shift of the absorption maxima of the closed-ring isomer due to complexation. ESR measurement in toluene at room temperature of the open-ring isomer of the Cu-II complex gave a spectrum that is a superposition of the spectra from the nitroxide radical and Cu-II. When the sample was irradiated with 366 nm light, a new peak due to large exchange interaction appeared between those of the nitroxyl radical and Cu-II. This ESR measurement revealed that the magnitude of the spin exchange interaction was changed by more than 160-fold by photoirradiation. This is the largest magnetic photoswitching phenomenon recorded in diarylethene systems.