Insights into Photoinduced Electron Transfer Between [Ru(mptpy)2]4+ (mptpy = 4'-(4-methylpyridinio)-2,2':6',2"-terpyridine) and [S2O8]2– : Computational and Experimental Studies

The mechanism and electron transfer dynamics of the reaction [RuII(mptpy)2]4+ + hν + [S2O8]2− → [RuIII(mptpy)2]5+ + SO42− + SO4−• were studied using various computational (density functional and exciton interaction theories) and experimental (transient absorption, static and time-resolved fluorescence spectroscopy, and other) techniques. The results were compared with those recently reported for [Ru(bpy)3]2+ dye [ref 18]. It was found that the excitation energy of [Ru(mptpy)2]4+ is about 0.4−0.5 eV smaller than that of [Ru(bpy)3]2+, which is consistent with the measured absorption maxima of 445 and 507 nm, for [Ru(bpy)3]2+ and [Ru(mptpy)2]4+, respectively. The smaller excitation energy in [Ru(mptpy)2]4+ correlates with much slower electron transfer rates to persulfate compared to [Ru(bpy)3]2+. The quenching of the photoexcited [Ru(mptpy)2]4+ by [S2O8]2− occurs via a unimolecular mechanism with formation of a weak ion-pair complex {[Ru(mptpy)2]4+···([S2O8]2−)n}, where n = 1 and 2. The initial photon is absorbed by the [Ru(mptpy)2]4+ fragment forming an MLCT state, e.g., the bright singlet state S1. This S1 state undergoes a fast spin−orbit coupling induced intersystem crossing to a lower-lying triplet and rapid subsequent relaxation down to the lowest triplet T1 via internal conversion and collisions with solvent molecules. At this stage, the electron transfer from [Ru(mptpy)2]4+ to a loosely attached [S2O8]2− occurs in a dark reaction via elongation of the O−O peroxo bond of the oxidant [S2O8]2−. The electron transfer lifetimes in water are calculated to be 1/κ1 = 199.4 ns and 1/κ2 = 108.4 ns, for the 1:1 and 1:2 complexes, respectively. The computed electron transfer lifetimes (1/κ1) are in reasonable agreement with their experimental values of 298 and 149 ns for the 1:1 and 1:2 complexes, respectively. The effect of solvent polarity on electron transfer rates is found to be significant: the less polar acetonitrile slows the rate by an order of magnitude compared to water.