Incorporation of a photochromic moiety in a mixed-valent complex: Switching "On" and "Off' an intervalence electron transfer

A dinuclear ruthenium photochromic complex, based on the dithienylperfluorocyclopentene unit, has been prepared following a building block approach, by palladium-catalyzed Sonogashira-Hagihara cross-couplings. At 254 nm, a photostationary state occurs, characterized by 75 % of ring closure. Upon oxidation to mixed-valence species, the open isomer displays no intervalence (IV) band, while for the closed one, an IV band corresponding to a 0.025 eV electronic coupling parameter is observed, as well as a thermal reopening of the cyclohexadiene moiety. Extended Huckel calculations have been performed on model complexes in order to rationalize this efficient switching effect, and also the unexpected unstability of the oxidized closed isomer.