Redox-switchable direction of photoinduced electron transfer in an Ru(bpy)₃²⁺ - Viologen dyad

Quenching of the ³MLCT excited state of [Ru(bpy)₃]²⁺ (bpy = bipyridine) by the reduction products (MV⁺ and MV⁰) of methyl viologen (MV²⁺) was studied by a combination of electrochemistry with laser flash photolysis or femtosecond pump-probe spectroscopy. Both for the bimolecular reactions and for the reactions in an Ru(bpy)₃²⁺-MVⁿ⁺ dyad, quenching by MV⁺ and MV⁰ is reductive and gives the reduced ruthenium complex [Ru(bpy)₃]⁺, in contrast to the oxidative quenching by MV²⁺. Rate constants of quenching (k_q), and thermal charge recombination (k_rec) and cage escape yields (φ_ce) were determined for the bimolecular reactions, and rates of forward (k_f) and backward (k_b) electron transfer in the dyad were measured for quenching by MV²⁺, MV⁺, and MV⁰. The reactions in the dyad are very rapid, with values up to k_f= 1.3 × 10¹² s^-1 for * Ru(bpy)₃²⁺-MV⁺. In addition, a long-lived (τ=15ps) vibrationally excited state of MV⁺ with a characteristically structured absorption spectrum was detected; this was generated by direct excitation of the MV⁺ moiety both at 460 and 600 nm. The results show that the direction of photoinduced electron transfer in a Ru(bpy)₃-MV molecule can be switched by an externally applied bias.