Molecular Water Oxidation Catalysis Characterization of Subnanosecond Processes and Ruthenium "Green Dimer" Formation

Dye-sensitized photoelectrochemical cells were prepared with popular ruthenium sensitizer (RuP) and ruthenium catalyst (RuCAT) coadsorbed on mesoporous titania. The cells were studied in 0.1 M Na₂SO_4(aq) by spectroscopic methods, including femtosecond transient absorption spectroscopy. The formation of RuCAT dimer can be observed by the naked eye due to the change of color from dark-red to green. The dimer displays a characteristic absorption feature with λmax ≈ 670–680 nm and its formation was found to be accelerated upon irradiation. Electron injection from RuP into titania occurs partially from the excited singlet state decaying on the ultrafast time scale (<0.2 ps) and partially from the triplet state with a time constant of several tens of ps. The decay of the excited RuCAT dimer takes place with a main component of about 1 ps. The quenching of the oxidized RuP by electron transfer from RuCAT is observed with a time constant 150–200 ps and is independent of the excitation fluence. This fast first step of catalyst oxidation further explains the chronoamperometry data recorded for photoanodes made of coadsorbed RuP and RuCAT. Finally, RuCAT in solution shows a remarkable short lifetime of the excited state, with a longest component of about 20 ps.