The mechanism and electron transfer dynamics of the reaction [Ru
II(mptpy)
2]
4+ +
hν + [S
2O
8]
2− → [Ru
III(mptpy)
2]
5+ + SO
42− + SO
4−• 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 [S
2O
8]
2− occurs via a unimolecular mechanism with formation of a weak ion-pair complex {[Ru(mptpy)
2]
4+···([S
2O
8]
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-l
ying 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 [S
2O
8]
2− occurs in a dark reaction via elongation of the O−O peroxo bond of the oxidant [S
2O
8]
2−. The electron transfer lifetimes in water are calculated to be 1/κ
1 = 199.4 ns and 1/κ
2 = 108.4 ns, for the 1:1 and 1:2 complexes, respectively. The computed electron transfer lifetimes (1/κ
1) 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.