Photophysical Characterization of a Chromophore/Water Oxidation Catalyst Containing a Layer-by-Layer Assembly on Nanocrystalline TiO₂ Using Ultrafast Spectroscopy

Abstract

Femtosecond transient absorption spectroscopy is used to characterize the first photoactivation step in a chromophore/water oxidation catalyst assembly formed through a "layer-by-layer" approach. Assemblies incorporating both chromophores and catalysts are central to the function of dye-sensitized photoelectrosynthesis cells (DSPECs) for generating solar fuels. The chromophore, [Rua(II)](2+) = [Ru(pbpy)2(bpy)](2+), and water oxidation catalyst, [Rub(II)-OH2](2+) = [Ru(4,4'-(CH2PO3H2)2bpy)(Mebimpy)(H2O)](2+), where bpy = 2,2'-bipyridine, pbpy = 4,4'-(PO3H2)2bpy, and Mebimpy = 2,6-bis(1-methylbenzimidazol-2-yl)pyridine), are arranged on nanocrystalline TiO2 via phosphonate-Zr(IV) coordination linkages. Analysis of the transient spectra of the assembly (denoted TiO2-[Rua(II)-Zr-Rub(II)-OH2](4+)) reveal that photoexcitation initiates electron injection, which is then followed by the transfer of the oxidative equivalent from the chromophore to the catalyst with a rate of kET = 5.9 × 10(9) s(-1) (τ = 170 ps). While the assembly, TiO2-[Rua(II)-Zr-Rub(II)-OH2](4+), has a near-unit efficiency for transfer of the oxidative equivalent to the catalyst, the overall efficiency of the system is only 43% due to nonproductive photoexcitation of the catalyst and nonunit efficiency for electron injection. The modular nature of the layer-by-layer system allows for variation of the light-harvesting chromophore and water oxidation catalyst for future studies to increase the overall efficiency.

Mechanistic Insights into Light‐Activated Catalysis for Water Oxidation

“…Quenching of the excited state of the PS is observed in chromophore‐catalyst dyads obtained by combination of a Ru(bpy) 3 2+ ‐type PS and both Ru(tpy)(bpy)(H 2 O) (where tpy = 2,2′:6′,2′′‐terpyridine) and Ru(Mebimpy)(bpy)(H 2 O) (where Mebimpy = 2,6‐bis(1‐methylbenzimidazol‐2‐yl)pyridine) WOCs, either covalently linked or supramolecularly assembled , . In these systems, quenching of the Ru(II) PS excited state takes place through electronic energy transfer to the Ru‐WOC (Equation ) and competes with the expected photoinduced electron transfer to the acceptor , . *PS quenching was also observed within a covalent dyad based on a Ru(bpy) 3 2+ ‐type PS and a Co‐salophen WOC …”

Artificial Molecular Photosynthetic Systems: Towards Efficient Photoelectrochemical Water Oxidation

“…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.…”

“…Molecular WOCs have moderate absorptioni nt he visible region,a nd therefore, electron injection can be hinderedb ye xcitation of the WOC, leadingt of ast thermal deactivation withoutp hotocurrentg eneration. [77,83,87] The ultrafasti nterfacial electron injection dynamics of functionalized metal oxide nanoparticles has been studied using time-resolved terahertz probe spectroscopy. [72] Recombination to sensitizers Although the efficiency of electron injectioni su nity,t ypical DSPEC devices suffer from low efficiency (< 1%).…”

“…Various methods have been reported for the surface functionalization of nanocrystalline wide‐bandgap inorganic semiconductors: co‐adsorption of sensitizers and catalysts, adsorption of covalently tethered sensitizer–catalyst assemblies, electropolymerization, Nafion‐mediated deposition, and stepwise deposition methods involving Zr IV ions . For example, in layer‐by‐layer deposition by ligation of Zr IV ions, the time‐resolved spectroscopy of zirconium‐mediated molecular assembled films showed that the back‐ET from TiO 2 (e − ) to the WOC or the oxidized photosensitizer on the surface is an order of magnitude slower if assembly takes place in the following order: TiO 2 , sensitizer, Zr IV ions, then WOC .…”

“…Moreover, the contribution of ET from the chromophores to the WOCs in the quenching of the excited state should also be considered. Molecular WOCs have moderate absorption in the visible region, and therefore, electron injection can be hindered by excitation of the WOC, leading to fast thermal deactivation without photocurrent generation . The ultrafast interfacial electron injection dynamics of functionalized metal oxide nanoparticles has been studied using time‐resolved terahertz probe spectroscopy …”