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 [Ru-II(mptpy)(2)](4+) + hv + [S2O8](2) -> [Ru-III(mptpy)(2)](5+) + SO42 + SO4-center dot 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+)center dot center dot center dot([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 static, 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 he 1/kappa(1) = 199.4 ns and 1/kappa(2) = 108.4 us, for the 1:1 and 1:2 complexes, respectively. The computed electron transfer lifetimes (1/kappa(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 he significant: the less polar acetonitrile slows the rate by an order of magnitude compared to water.