Tuning charge recombination rate constants through inner-sphere coordination in a copper(I) donor-acceptor compound

The coordination compounds [Cu(bpy-MV2+)(PPh3)(2)](PF6)(3), where bpy-MV2is the 1-(4-(4'-methyl-2,2'-bipyridin-4-yl)butyl)-1'-methyl-4,4'-bipyridine diium(2+) cation, and [Cu(dmb)(PPh3)(2)](PF6), where dmb is 4,4'-dimethyl-2 ,2'-bipyridine, have been prepared and characterized. Visible light (417 nm ) excitation of [Cu(bpy-MV2+)(PPh3)(2)](3+) at room temperature leads to ra pid intramolecular electron transfer, k(cs) > 1 x 10(8) s(-1), to form a ch arge-separated state with an electron localized on the pendant viologen gro up and a copper(II) metal center, abbreviated [Cu-II-MV.+]. This state reco mbines to ground-state products with first-order rate constants that can be tuned with solvent over a similar to 10(7)-10(5) s(-1) range. The activati on parameters were determined from temperature-dependent electron-transfer data with Arrhenius analysis. A model is proposed wherein a solvent molecul e is coordinated to Cu(II) in the charge-separated state, [(S)Cu-II-bpy-MV. +]. Visible light excitation of [Cu(dmb)(PPh3)(2)](PF6) in argon-saturated dichloromethane produces long-lived photoluminescent excited states, tau = 80 ns, that are dynamically quenched by the addition of Lewis basic solvent s. The measured quenching constants each correlate well with the lifetime o f the charge-separated state measured after excitation of [Cu(bpy-MV2+)(PPh 3)(2)](3+) in the corresponding solvent.