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The reduction of CO2 to renewable fuels must be coupled to a sustainable oxidation process to devise a viable solar fuel-producing device. In photoelectrochemical cells, water oxidation to O2 is the predominant oxidation reaction and typically requires a pair of light absorbers or an applied bias voltage when coupled to CO2 reduction. Here, we report a bias-free photoelectrochemical device for simultaneous CO2 reduction to formate and alcohol oxidation to aldehyde in aqueous conditions. The photoanode is constructed by co-immobilisation of a diketopyrrolopyrrole-based chromophore and a nitroxyl-based alcohol oxidation catalyst on a mesoporous TiO2 scaffold, providing a precious metal-free dye-sensitised photoanode. The photoanode was wired to a biohybrid cathode consisting of the CO2 reduction enzyme formate dehydrogenase integrated into a mesoporous indium tin oxide electrode. The bias-free cell delivers sustained photocurrents of up to 30 µA cm−2 under visible-light irradiation, resulting in simultaneous aldehyde and formate production. Our results show that single light absorber photoelectrochemical cells can be used for parallel fuel production and chemical synthesis from CO2 and waste streams in the absence of an external bias.

Multi-redox catalysis requires the accumulation of more than one charge carrier and is crucial for solar energy conversion into fuels and valuable chemicals. In photo(electro)chemical systems, however, the necessary accumulation of multiple, long-lived charges is challenged by recombination with their counterparts. Herein, we investigate charge accumulation in two model multi-redox molecular catalysts for proton and CO2 reduction attached onto mesoporous TiO2 electrodes. Transient absorption spectroscopy and spectroelectrochemical techniques have been employed to study the kinetics of photoinduced electron transfer from the TiO2 to the molecular catalysts in acetonitrile, with triethanolamine as the hole scavenger. At high light intensities, we detect charge accumulation in the millisecond timescale in the form of multi-reduced species. The redox potentials of the catalysts and the capacity of TiO2 to accumulate electrons play an essential role in the charge accumulation process at the molecular catalyst. Recombination of reduced species with valence band holes in TiO2 is observed to be faster than microseconds, while electron transfer from multi-reduced species to the conduction band or the electrolyte occurs in the millisecond timescale. Finally, under light irradiation, we show how charge accumulation on the catalyst is regulated as a function of the applied bias and the excitation light intensity., Using transient spectroelectrochemical techniques, we investigate multiply reduced states of molecular catalysts on titania photoelectrodes as a function of the applied bias and the light intensity.

Multi-redox catalysis requires the accumulation of more than one charge carrier and is crucial for solar energy conversion into fuels and valuable chemicals. In photo(electro)chemical systems, however, the necessary accumulation of multiple, long-lived charges is challenged by recombination with their counterparts. Herein, we investigate charge accumulation in two model multi-redox molecular catalysts for proton and CO2 reduction attached onto mesoporous TiO2 electrodes. Transient absorption spectroscopy and spectroelectrochemical techniques have been employed to study the kinetics of photoinduced electron transfer from the TiO2 to the molecular catalysts in acetonitrile, with triethanolamine as the hole scavenger. At high light intensities, we detect charge accumulation in the millisecond timescale in the form of multi-reduced species. The redox potentials of the catalysts and the capacity of TiO2 to accumulate electrons play an essential role in the charge accumulation process at the molecular catalyst. Recombination of reduced species with valence band holes in TiO2 is observed to be faster than microseconds, while electron transfer from multi-reduced species to the conduction band or the electrolyte occurs in the millisecond timescale. Finally, under light irradiation, we show how charge accumulation on the catalyst is regulated as a function of the applied bias and the excitation light intensity.

Multi-redox catalysis requires the transfer of more than one charge carrier and is crucial for solar energy conversion into fuels and valuable chemicals. In photo(electro)chemical systems, however, the necessary accumulation of multiple, long-lived charges is challenged by recombination with their counterparts. Herein, we investigate charge accumulation in two model multi-redox molecular catalysts for proton and CO2 reduction attached onto mesoporous TiO2 electrodes. Transient absorption spectroscopy and spectroelectrochemical techniques have been employed to study the kinetics of photoinduced electron transfer from the TiO2 to the molecular catalysts in acetonitrile, with triethanolamine as the hole scavenger. At high light intensities, we detect charge accumulation in the millisecond timescale in the form of multi-reduced species. The redox potentials of the catalysts and the capacity of TiO2 to accumulate electrons play an essential role in the charge accumulation process at the molecular catalyst. Recombination of reduced species with valence band holes in TiO2 is observed to be faster than microseconds, while electron transfer from multi-reduced species to the conduction band or the electrolyte occurs in the millisecond timescale. Finally, under light irradiation, we show how charge accumulation on the catalyst is regulated as a function of the applied bias and the excitation light intensity.

Dinuclear Cu-II-patellamide complexes (patellamides are naturally occurring cyclic pseudo-octapeptides) are known to be efficient catalysts for hydrolysis reactions of biological importance, for example, those of phosphatase, carbonic anhydrase, and glycosidase. However, the biological role of patellamides is still unknown. Patellamides were originally extracted from the sea squirt Lissoclinum patella, but are now known to be ribosomally expressed by the blue-green algae Prochloron that live in symbiosis with L. patella. In a further step to unravel the metabolic significance of the patellamide complexes, the question as to whether these are also formed inside Prochloron cells is addressed. In this study, a biocompatible patellamide-fluorescent dye conjugate has been introduced into living Prochloron cells and, by means of flow cytometry and confocal microscopy, it is shown that Cu-II ions are coordinated to patellamides in vivo.

The patellamides (cyclic pseudo-octapeptides) are produced by Prochloron, a symbiont of the ascidians, marine invertebrate filter feeders. These pseudo-octapeptides are present in the cytoplasm and a possible natural function of putative metal complexes of these compounds is hydrolase activity, however the true biological role is still unknown. The dinuclear CuII complexes of synthetic patellamide derivatives have been shown in in vitro experiments to be efficient hydrolase model catalysts. Many hydrolase enzymes, specifically phosphatases and carboanhydrases, are ZnII-based enzymes and therefore, we have studied the ZnII and mixed ZnII/CuII solution chemistry of a series of synthetic patellamide derivatives, including solution structural and computational work, with the special focus on model phosphatase chemistry with bis-(2,4-dinitrophenyl)phosphate (BDNPP) as the substrate. The ZnII complexes of a series of ligands are shown to form complexes of similar structure and stability compared to the well-studied CuII analogues and the phosphatase reactivities are also similar. Since the complex stabilities and phosphatase activities are generally a little lower compared to those of CuII and since the concentration of ZnII in Prochloron cells is slightly smaller, we conclude that the CuII complexes of the patellamides are more likely to be of biological importance.

Dinuclear Cu

Cyclic pseudo-peptides, derived from marine metabolites found in the genus Lissoclinum bistratum and Lissoclinum patella and produced by their symbiont Prochloron have attracted scientific interest in the last two decades. Their structural properties and solution dynamics were analyzed in detail, elaborate synthetic procedures for the natural products and synthetic derivatives were developed, the biosynthetic pathways were studied and it now is possible to produce them biosynthetically. A major focus in the last decade was on their CuII – more recently also on the ZnII – coordination chemistry, as a number of studies have indicated that dinuclear CuII and ZnII complexes of cyclic peptides may be involved in the ascidians’ metabolism. Solution equilibria of various derivatives of the synthetic ligands in presence of CuII were studied thoroughly and the solution structures were determined by EPR spectroscopy and spectra simulation in combination with molecular mechanics and DFT calculations. Recent in vitro studies indicate that the dicopper(II) complexes are phosphatase, glycosidase, lactamase and very efficient carbonic anhydrase model systems. First in vivo studies with a patellamide derivative containing an appended fluorescence tag suggest that CuII is coordinated to the patellamides in the Prochloron cells.

The patellamides (cyclic pseudo-octapeptides) are produced by Prochloron, a symbiont of the ascidians, marine invertebrate filter feeders. These pseudo-octapeptides are present in the cytoplasm and a possible natural function of putative metal complexes of these compounds is hydrolase activity, however the true biological role is still unknown. The dinuclear Cu

Prochloron, a blue-green algae belonging to ancient prokaryotes, produces, like other cyanobacteria, cyclic pseudo-peptides, which are also found in its obligate symbiont ascidiae (Lissoclinum patellum). Although research has focused for some time on the putative metabolic function of these cyclic peptides, to date it is still not understood. Their role might be connected to the increased concentrations of divalent metal ions, especially Cu(II), found in ascidiae. Dinuclear copper(II) complexes of cyclic pseudo-peptides revealed a broad hydrolytic capacity, including carboanhydrase and phosphatase activity. This study reports their β-lactamase as well as α- and β-glycosidase activity with kcat=(11.34±0.91)ˑ10(-4)s(-1) for β-lactamase, kcat=(1.55±0.13)ˑ10(-4)s(-1) for α-glycosidase and kcat=(1.22±0.09)ˑ10(-4)s(-1) for β-glycosidase activity.

Substituted benzyloxycarbonyloxyl radicals were generated by sensitised photolyses of benzyl oxime carbonates. EPR spectroscopy showed they ring closed exclusively by spiro-cyclisation onto the ipso-C-atoms of the aromatic rings. β-Scission of the alkoxycarbonyloxyls to CO2 and benzyloxyl radicals increasingly competed and became dominant above 270 K. The first rate parameters for spiro-cyclisations of O-centred radicals onto aromatics were obtained by the steady-state kinetic EPR method. Pentafluoro-substitution of the ring substantially reduced the spiro-cyclisation rate. Activation barriers of the spiro-cyclisations were computed by DFT to be about half those of the alternative ortho-cyclisations. Consideration of the TS structures suggested the preference for spiro- over ortho-cyclisation resulted from better overlap of the oxyl SOMO with the aromatic π-system during spiro closure.