Abstract
The water-soluble tin(IV) porphyrin dichlorido-5,10,15,20-tetrakis(p-carboxyphenyl)-porphyrinato-tin(IV) (SnTPPC, 1) was synthesised as a mimic of biological chlorophyll photosensitisers. In natural photosynthesis, chlorophyll pigments start the multi-electron transfer processes resulting in water-oxidation and NADP(+)-reduction. The photochemical properties of compound 1 were characterised by measuring absorption and fluorescence spectra. Electrochemical measurements in water revealed well-suited redox potentials of 1 for both proton-reduction to H2 as well as water-oxidation to O2. The tin(IV) porphyrin was then used as a photosensitiser in model systems for light-induced proton-reduction in aqueous solution, where an optimization of the experimental conditions was carried out to achieve reaction rates comparable to those found for [Ru(bipy)3](2+), a standard dye in artificial photosynthesis. By employing UV/Vis-spectroelectrochemistry, we found that the porphyrin ligand of 1 is redox non-innocent in water. A complex set of reduction reactions of the porphyrin macrocycle occurs during photocatalytic experiments involving the ligand's chlorin form as a key intermediate. On the basis of these results, a potential reaction sequence for light-driven H2-formation is formulated, where the reductive quenching of 1 forms the initial reaction step and reduced forms of 1 serve as hydride transfer agents to the H2 evolution catalyst. The spectroscopic, electrochemical and catalytic properties of SnTPPC make this compound class an attractive, affordable and easily accessible choice for photosensitisers in artificial photosynthetic systems. Finally, the detected complicated redox reactions of the porphyrin ring in water offer a possible explanation of why the chlorophylls of P680 or P700 are carefully wrapped in a water-free part of the PSII and PSI proteins.
Highlights
Many designs for solar energy conversion are based on lightdriven water-splitting.[1,2] The first step of any such system is light absorption followed by charge separation
We found that the synthesis and purification of the carboxyphenyl-porphyrin complex of tin(IV) were far easier and yielded much better results
Aside from impurities caused by inorganic salts like carbonates and sulphates, an additional reason for the difficult purification of the sulphonato derivative appeared to be that the reported preparation methods for the tetrakis( p-sulfophenyl)porphyrin (TPPS) ligand yield mono, di, and trisulfonated porphyrins,[17] which are very hard to separate from the desired ligand bearing four –SO3À-groups
Summary
Many designs for solar energy conversion are based on lightdriven water-splitting.[1,2] The first step of any such system is light absorption followed by charge separation. Chlorophyll molecules are key players during this process, as they act both as light absorbing molecules and electron transfer agents In this way, the biological system generates the strongly oxidising potentials needed for water-oxidation in Photosystem II (PSII) and the strong reducing agent for NADP+-reduction in PSI via light absorption by chlorophyll dyes.[3,4]. The biological system generates the strongly oxidising potentials needed for water-oxidation in Photosystem II (PSII) and the strong reducing agent for NADP+-reduction in PSI via light absorption by chlorophyll dyes.[3,4] In their electronic ground states, both P680 and P700 are poor electron donors, but in their excited states electron transfer from the chlorophylls to the primary acceptors within PSI and PSII takes place with a quantum efficiency of nearly 100%. After light-induced oxidation (oxidative quenching), the oxidised forms of the pigments, P680+ and P700+, are available as ground state oxidation agents within the respective electron transfer chains to drive the oxidation of water (in PSII) or plastocyanin (in PSI), respectively
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