Photophysical Characterization of a Helical Peptide Chromophore-Water Oxidation Catalyst Assembly on a Semiconductor Surface Using Ultrafast Spectroscopy

We report a detailed kinetic analysis of ultrafast interfacial and intra-assembly electron transfer following excitation of an oligoproline scaffold functionalized by chemically linked light-harvesting chromophore [Ru(pbpy)(2)(bpy)](2+) (pbpy = 4,4'-(PO3H2)(2)-2,2'-bipyridine, bpy = 2,2'-bipyridine) and water oxidation catalyst [Ru(Mebimpy)(bpy)OH2](2+) (Mebimpy = 2,6-bis(1-methylbenzimidazol-2-yl)pyridine). The oligoproline scaffold approach is appealing due to its modular nature and helical tertiary structure. They allow for the control. of electron transfer distances in chromophore-catalyst assemblies for applications in dye-sensitized photoelectrosynthesis cells (DSPECs). The proline chromophore-catalyst assembly was loaded onto nanocrystalline TiO2 with the helical structure of the oligoproline scaffold maintaining the controlled relative positions of the chromophore and catalyst. Ultrafast transient absorption spectroscopy was used to analyze the kinetics of the first photoactivation step for oxidation of water in the assembly. A global kinetic analysis of the transient absorption spectra reveals that photoinduced electron injection occurs in 18 ps and is followed by intra-assembly oxidative activation of the water oxidation catalyst on the hundreds of picoseconds time scale (k(ET) = 2.6 X 10(9) s(-1); tau = 380 ps). The first photoactivation step in the water oxidation cycle of the chromophore-catalyst assembly anchored to TiO2 is complete within 380 ps.