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

Abstract

We report a detailed kinetic analysis of ultrafast interfacial and intraassembly electron transfer following excitation of an oligoproline scaffold functionalized by chemically linked light-harvesting chromophore [Ru(pbpy) 2 (bpy)] 2+ (pbpy = 4,4′-(PO 3 H 2 ) 2 -2,2′-bipyridine, bpy = 2,2′-bipyridine) and water oxidation catalyst [Ru-(Mebimpy)(bpy)OH 2 ] 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 TiO 2 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 × 10 9 s −1 ; τ = 380 ps). The first photoactivation step in the water oxidation cycle of the chromophore−catalyst assembly anchored to TiO 2 is complete within 380 ps.

Artificial Molecular Photosynthetic Systems: Towards Efficient Photoelectrochemical Water Oxidation

“…Although the quenching of the excited state of the photosensitizer unit with the adjacent catalyst was not discussed, the extremely short linkage (methylene spacer) in the single molecular system might be suitable for efficient ET coupled with PT following a fast injection, as the slow accumulation of oxidative equivalents on a metal center of a WOC can potentially be the bottleneck in DSPEC devices. An oligoproline functionalized with a RuP 2+ chromophore derivative and a Ru(bpy)(Mebimpy)(OH 2 ) 2+ WOC was loaded onto TiO 2 , and the interfacial and facial ET dynamics were analyzed by transient femtosecond absorption . In the assembly system, electron injection from the excited state of the chromophore unit to TiO 2 is followed by the intra‐assembly ET from the WOC moiety to photogenerated Ru III species.…”

“…Moreover,t he interfacial and surface ET/PT kinetics of each step should be optimized for high efficiency.Inf act, the quantum yield of water splitting in DSPEC devices is low because the interfacial charger ecombination reactioni sm uch faster than the forwardE T, including the catalytic four-electron oxidation of water to dioxygen. [71] Various methods have been reported for the surfacef unctionalization of nanocrystalline wide-bandgapi norganic semiconductors:c o-adsorption of sensitizers and catalysts, [71][72][73][74][75] adsorptiono fc ovalently tethered sensitizer-catalyst assemblies, [76,77] electropolymerization, [78,79] Nafion-mediated deposition, [80,81] and stepwise deposition methods involving Zr IV ions. [82][83][84] For example, in layer-by-layerd eposition by ligation of Zr IV ions, the time-resolved spectroscopy of zirconium-mediated molecular assembled films showedt hat the back-ET from TiO 2 (e À )tothe WOC or the oxidized photosensitizer on the surface is an order of magnitude slower if assembly takes place in the followingo rder:T iO 2 ,s ensitizer,Z r IV ions, then WOC.…”

“…by transientf emtoseconda bsorption. [76] In the assembly system,e lectron injection from the excited state of the chromophore unit to TiO 2 is followed by the intra-assembly ET from the WOC moiety to photogenerated Ru III species. Using ag lobal analysis, the intra-assembly ET was found to occur with ar ate constant of 2.6 10 9 s À1 (t = 380 ps), giving an overall efficiency of 49 %f or the initial photoexcitation process.…”

Recent advances in ruthenium complex-based light-driven water oxidation catalysts

“…Structure-performance relationship exploration that is complicated by variable experimental conditions, have received less attention, and should be heavily pursued by some advanced spectroscopy techniques [60][61][62]. The more stable grafting techniques of WOCs such as elelctropolymerization and solvent casting [63] rather than phosphate linking are in great demand.…”

Extending Long‐lived Charge Separation Between Donor and Acceptor Blocks in Novel Copolymer Architectures Featuring a Sensitizer Core

“…Alternatively, polymer chemistry offers an elegant route to tailored photo‐ and electrochemically active macromolecules, which a priori provide charge percolation pathways, can perform charge accumulation as well as a phase segregation of the donor/acceptor domains—in close resemblance of modern organic photovoltaics . The versatility to design and construct macromolecular photosystems is evident from the reports on Ru‐decorated oligopeptides, poly(styrene)s, poly(acrylates)s, poly(thiophene)s, poly(fluorene) or poly(fluorene‐ co ‐thiophene) . Notably, most of the early examples comprised statistical assemblies and/or grafted macromolecules, mainly due to the available polymerization techniques (and their limitations) at that time.…”