Solvent-dependent intramolecular electron transfer in a peptide-linked [Ru(bpy)3]2+-C60 dyad

A novel peptide-linked [RU(bpy)(3)](2+)-C-60 dyad is shown to undergo an intramolecular photoinduced electron transfer in chlorinated hydrocarbons that causes quenching of the emission associated to the ruthenium metal-to-ligand charge-transfer excited state. Addition of a strong protic solvent, such as hexafluoro-2-propanol, leads to deactivation of the electron-transfer process with concomitant recovery of the emission to the extent recorded for a solution of a reference ruthenium complex lacking the fullerene moiety. This behavior is associated with a direct effect of the protic solvent on the secondary structure of the peptide spacer, whose preferred conformations in solution have been assessed by FT-IR and 2D NMR spectroscopy. Chlorinated hydrocarbons favor the peptide 3(10)-helical conformation, which provides efficient interactions between the ruthenium and C-60 chromophores, whereas the presence of a protic solvent produces helix unfolding, which hampers suitable spatial orientations of the chromophores for electron transfer. The reversibility of the phenomenon is also demonstrated and discussed.