Water oxidation in all oxygenic photosynthetic organisms is catalysed by the Mn₄CaO₄ cluster of Photosystem II. This cluster has inspired the development of synthetic manganese catalysts for solar energy production. A photoelectrochemical device, made by impregnating a synthetic tetranuclear-manganese cluster into a Nafion matrix, has been shown to achieve efficient water oxidation catalysis. We report here in situ X-ray absorption spectroscopy and transmission electron microscopy studies that demonstrate that this cluster dissociates into Mn(II) compounds in the Nafion, which are then reoxidized to form dispersed nanoparticles of a disordered Mn(III/IV)-oxide phase. Cycling between the photoreduced product and this mineral-like solid is responsible for the observed photochemical water-oxidation catalysis. The original manganese cluster serves only as a precursor to the catalytically active material. The behaviour of Mn in Nafion therefore parallels its broader biogeochemistry, which is also dominated by cycles of oxidation into solid Mn(III/IV) oxides followed by photoreduction to Mn²⁺.

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Nanoparticulate manganese oxides, formed in Nafion polymer from a series of molecular manganese complexes of varying nuclearity and metal oxidation state, are shown to effectively catalyze water oxidation under neutral pH conditions with the onset of electrocatalysis occurring at an overpotential of only 150 mV. Although XAS experiments indicate that each complex generates the same material in Nafion, the catalytic activity varied substantially with the manganese precursor and did not correlate with the amount of MnO x present in the films. The XAS and EPR studies indicated that the formation of the nanoparticulate oxide involves the dissociation of the complex into Mn(II) species followed by oxidation on application of an external bias. TEM studies of the most active films, derived from [Mn(Me3TACN)(OMe)3]+ and [(Me3TACN)2MnIII 2(μ-O)(μ-CH3COO)2]2+ (Me3TACN = N,N′,N″-trimethyl-1,4,7-triazacyclononane), revealed that highly dispersed MnO x nanoparticles (10–20 nm and 6–10 nm, respectively) were generated in the Nafion film. In contrast, the use of [Mn(OH2)6]2+ resulted in both a higher manganese oxide loading and aggregated nanoparticles with 30–100 nm approximate size, which were less effective water oxidation catalysts. Much higher turnover frequencies (TOFs) were observed for films derived from the two complexes, viz., ∼20 molecules of O2 per Mn per hour in dark and 40 molecules of O2 per Mn per hour under illumination at an overpotential of 350 mV, when compared with MnO x films made with [Mn(OH2)6]2+. This corresponds to a TOF > 100 molecules of O2 per Mn per second for a 10 nm MnO x nanoparticle. Thus, the catalytic activity is dependent on the ability to generate well-defined, dispersed nanoparticles. Electrochemical and spectroscopic methods have been used to follow the conversion of the molecular precursors into MnO x and to further evaluate the origin of differences in catalytic activity.

Soft X-ray absorption and resonant inelastic X-ray scattering at the Mn Ledge are established as tools for gaining electronic structural insights into water oxidation catalysis. The MnO x catalyst with the lowest d-d transitions, strongest charge transfer and a higher proportion of Mn 3+ over Mn 2+/4+ produces itinerant electrons that contribute to a higher catalytic activity.

Iron oxyhydroxide thin films electrochemically deposited from a non-aqueous medium using metal inorganic complexes as a metal ion precursor have been demonstrated as an efficient electrochemical water oxidation catalyst under near neutral as well as alkaline pH conditions.

Fungal endophytes are well-established sources of biologically active natural compounds with many producing pharmacologically valuable specific plant-derived products. This review details typical plant-derived medicinal compounds of several classes, including alkaloids, coumarins, flavonoids, glycosides, lignans, phenylpropanoids, quinones, saponins, terpenoids, and xanthones that are produced by endophytic fungi. This review covers the studies carried out since the first report of taxol biosynthesis by endophytic Taxomyces andreanae in 1993 up to mid-2020. The article also highlights the prospects of endophyte-dependent biosynthesis of such plant-derived pharmacologically active compounds and the bottlenecks in the commercialization of this novel approach in the area of drug discovery. After recent updates in the field of ‘omics’ and ‘one strain many compounds’ (OSMAC) approach, fungal endophytes have emerged as strong unconventional source of such prized products.

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