A porphyrin-doped polymer catalyzes selective, light-assisted water oxidation in seawater



Publication Details

Chen, J., Wagner, P. W., Tong, L., Wallace, G. G., Officer, D. L. & Swiegers, G. F. (2012). A porphyrin-doped polymer catalyzes selective, light-assisted water oxidation in seawater. Angewandte Chemie - International Edition, 51 (8), 1907-1910.


In a classic experiment, Naruta and co-workers demonstrated in 1994 that the dimanganese complexes 1 (Scheme 1) facilitate water oxidation catalysis yielding dioxygen (O2) at potentials above 1.2 V vs. Ag/AgCl.[1] The corresponding, unconnected Mn-porphyrin monomers were, however, catalytically inactive.[1] Subsequent work suggested that OO bond formation leading to O2 generation by 1 involved a concerted interaction between two short-lived, high-valent MnV=O intermediates at each of the porphyrins, presumably during conformational flexing of the dimer.[2] The transience and brief lifetime of these intermediates likely rendered the free monomers inactive. Catalytic actions like those of 1, which encompass a synchronized, cooperative interplay between two or more catalytic groups, are of significant fundamental and practical interest.[3] For example, enzymes are believed to employ synchronous protein motions to cooperatively harness reactive intermediates that are often too short-lived to be utilized in other classes of catalyst.[3–5] This may explain how they can catalyze some reactions that cannot be catalyzed outside of biology. The question that arises is: how can one design simple, practical abiological molecular catalysts to synchronously harness very short-lived reactive intermediates? In a previous report[6] we described an approach to this problem that involved drastically concentrating the corresponding monomeric catalytic groups within a limited volume. This may conceivably cause some small but statistically significant proportion of the monomers to be adventitiously ideally placed to facilitate cooperative catalysis. If the monomer-bound reactive intermediates are too short-lived to be sequestered and exploited in any other way, then only the product deriving from cooperative catalysis should be obtained.[6] Here we report the application of this “statistical proximity” approach[6] to water oxidation catalyzed by Mn-porphyrins. We show that concentration of the sulfonated, monomeric Mn-porphyrin 2 (Scheme 1), which is normally catalytically inactive,[7] within a thin layer of poly(terthiophene) (PTTh) yields a remarkable light-assisted catalyst with a low overpotential for water oxidation at pH 7. The catalyst selectively oxidizes water before chloride in seawater. Mn-porphyrin monomer 2 was uniformly incorporated as an anionic counter-ion into a thin PTTh film during the electrochemical polymerization of TTh monomer in ethanol/ dichloromethane (1:1 by volume) containing 2 (see Supporting Information). PTTh-2 was deposited as a composite film onto indium tin oxide (ITO) glass or flexible ITO-coated poly(ethylene terephthalate) (PET) sheet. Figure S1 (Supporting Information) shows the flexible electrode obtained when PTTh-2 was coated on ITO-PET. UV/vis measurements confirmed the incorporation of 2 in the coating. Energydispersive X-ray mapping indicated that 2 was uniformly dispersed in the coating (Figure S3). Elemental analysis indicated a high density of 2 within the PTTh-2, with the mole ratio of 2 (identified by Mn+S):terthiophene (identified by S) being ca. 1:3. The PTTh-2 films were then studied as putative working electrodes in photocatalytic oxygen generation from water. Cyclic voltammograms (CVs) of the PTTh-2/ITO glass electrode were taken with and without illumination using SoLux daylight MR16 halogen light bulbs (12 V, 50W, 248) in an aqueous 0.1m Na2SO4 electrolyte. Figure 1 depicts the data that was obtained. As can be seen, the CVs with and without light are significantly different. Substantially larger currents were observed positive of 0.68 V with illumination than without illumination. This is a region in which we have previously observed water oxidation in comparable systems.[8] Moreover, the reduction peakAin Figure 1 was also observed only under illumination. In our experience, peaks of this type are often characteristic of adsorbed dioxygen.[6] To study the peak at A, we conditioned the PTTh-2 coating in aqueous 0.1m Na2SO4, by maintaining it at 0.8 V (I), 0.9 V (II), or 1.0 V (III) for 1 h, and then immediately thereafter, performing a linear sweep voltammogram (LSV). The resulting LSV data are shown as the inset in Figure 1. As can be seen, under these conditions the broad peak at A resolves into two separate peaks: a large peak A’ and a small Scheme 1. Diporphyrin 1 and monomer porphyrin 2. Ar=4-tBuC6H4, 2,4,6-Me3C6H2, or C6F5.

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