Photodriven water oxidation initiated by a surface bound chromophore-donor-catalyst assembly

Abstract

In photosynthesis, solar energy is used to produce solar fuels in the form of new chemical bonds. A critical step to mimic photosystem II (PS II), a key protein in nature's photosynthesis, for artificial photosynthesis is designing devices for efficient light-driven water oxidation. Here, we describe a single molecular assembly electrode that duplicates the key components of PSII. It consists of a polypyridyl light absorber, chemically linked to an intermediate electron donor, with a molecular-based water oxidation catalyst on a SnO₂/TiO₂ core/shell electrode. The synthetic device mimics PSII in achieving sustained, light-driven water oxidation catalysis. It highlights the value of the tyrosine-histidine pair in PSII in achieving efficient water oxidation catalysis in artificial photosynthetic devices.

Scheme 1. (Top) Structure and direction of electron flow in the dye sensitized photoelectrosynthesis cell (DSPEC), FTO|SnO2/TiO2|–RuIIP(TPA)(Cat)2+. The structure omits the external, stabilizing polymer, 4,5-difluoro-2,2-bis(trifluoromethyl)1,3-dioxole (AF). In the final assembly, Cat is a derivative of a Ru(ii)-2,2′-bipyridine-6,6-dicarboxylate based catalyst for water oxidation and TPA is a derivative of tri-phenyl amine. (Bottom) Illustrating the related components in PSII with arrows indicating the direction of light-driven electron transfer following excitation of the external chromophore.

Fig. 1. Absorption spectra for FTO|TiO2|–RuIIP(Cat)2+, before (black), and after addition of TPA to give FTO|TiO2|–RuIIP(TPA)(Cat)2+. Spectra of the related electrodes, FTO|TiO2 (gray) and FTO|TiO2|-TPA, after TPA deposition (blue) are also shown. The spectra were obtained at room temperature in air.

Fig. 2. (A) Variations in surface coverage with time for the electrodes, FTO|TiO2|–RuIIP2+|AF and FTO|TiO2|–RuIIP(TPA)2+|AF with 455 nm, 475 mW cm⁻² photolysis over 16 h periods in aqueous, pH 7, 0.1 M phosphonate buffers in 0.4 M NaClO₄. Loss from the surface was monitored by following absorbance changes at 450 nm for the chromophore with corrections for light scattering by TiO₂. (B) Current density–time (j–t) traces over 150 s dark–light cycles for water oxidation by FTO|SnO2/TiO2|–RuIIP(Cat)2+|AF (black) and FTO|SnO2/TiO2|–RuIIP(TPA)(Cat)2+|AF (red) at an applied bias of 0.6 V vs. NHE in 0.1 M phosphonate buffers in 0.4 M NaClO₄ at pH 7.0. (C) A 3 h photoelectrochemical water oxidation cycle for FTO|SnO2/TiO2|–RuIIP(TPA)(Cat)2+|AF illuminated under the same conditions as in (b) compared to an electrode without the TPA donor. (D) IPCE (Incident Photon-to-Electron Conversion Efficiency) results for FTO|SnO2/TiO2|–RuIIP(TPA)(Cat)2+|AF, at an applied bias of 0.6 V at pH 7.0 in a 0.1 M phosphate buffer. A 400 nm cut-off filter was used to mimic the conditions used in the current–time experiments.

Fig. 3. Transient absorption spectra for the assemblies, (A) FTO|SnO2/TiO2|–RuIIP2+|(AF), (B) FTO|SnO2/TiO2|–RuIIP(TPA)2+|(AF), (C) FTO|SnO2/TiO2|–RuIIP(Cat)2+|(AF), and (D) FTO|SnO2/TiO2|–RuIIP(TPA)(Cat)2+|(AF). Kinetics were evaluated at 470 nm at the ground state bleaches, note (E), and at 680 nm, in (F), for absorption by the TPA radical. The samples were excited at 400 nm with pulse energies of 100–300 μJ cm⁻² in air.

Scheme 2. Redox potential diagram based on kinetic studies of the component assemblies and of the final assembly, FTO|SnO2/TiO2|–RuIIP(TPA)(Cat)2+|(AF)vs. NHE, for the first step in the water oxidation cycle. The range of potentials for the three electrons transfer activation of the catalyst is also shown.