Photocatalytic Hydrogen Generation by Vesicle-Embedded [FeFe]Hydrogenase Mimics: A Mechanistic Study

Copper-Catalyzed Sulfimidation in Aqueous Media: a Fast, Chemoselective and Biomolecule-Compatible Reaction

Performing transition metal-catalyzed reactions in cells and living systems has equipped scientists with a toolbox to study biological processes and release drugs on demand. Thus far, an impressive scope of reactions has been performed in these settings, but many are yet to be introduced. Nitrene transfer presents a rather unexplored new-to-nature reaction. The reaction products are frequently encountered motifs in pharmaceuticals, presenting opportunities for the controlled, intracellular synthesis of drugs. Hence, we explored the transition metal-catalyzed sulfimidation reaction in water for future in vivo application. Two Cu(I) complexes containing trispyrazolylborate ligands (Tpx ) were selected, and the catalytic system was evaluated with the aid of three fitness factors. The excellent nitrene transfer reactivity and high chemoselectivity of the catalysts, coupled with good biomolecule compatibility, successfully enabled the sulfimidation of thioethers in aqueous media. We envision that this copper-catalyzed sulfimidation reaction could be an interesting starting point to unlock the potential of nitrene transfer catalysis in vivo.

Initial Quenching Efficiency Determines Light-Driven H2 Evolution of [Mo3 S13 ]2- in Lipid Bilayers

Nature uses reactive components embedded in biological membranes to perform light-driven photosynthesis. Here, a model artificial photosynthetic system for light-driven hydrogen (H2 ) evolution is reported. The system is based on liposomes where amphiphilic ruthenium trisbipyridine based photosensitizer (RuC9 ) and the H2 evolution reaction (HER) catalyst [Mo3 S13 ]2- are embedded in biomimetic phospholipid membranes. When DMPC was used as the main lipid of these light-active liposomes, increased catalytic activity (TONCAT ~200) was observed compared to purely aqueous conditions. Although all tested lipid matrixes, including DMPC, DOPG, DPPC and DOPG liposomes provided similar liposomal structures according to TEM analysis, only DMPC yielded high H2 amounts. In situ scanning electrochemical microscopy (SECM) measurements using Pd microsensors revealed an induction period of around 26 minutes prior to H2 evolution, indicating an activation mechanism which might be induced by the fluid-gel phase transition of DMPC at room temperature. Stern-Volmer-type quenching studies revealed that electron transfer dynamics from the excited state photosensitizer are most efficient in the DMPC lipid environment giving insight for design of artificial photosynthetic systems using lipid bilayer membranes.

Vesicles as a Multifunctional Microenvironment for Electrochemiluminescence Signal Amplification

Vesicles as a typical interface-rich microenvironment can promote the reaction rate and the intermediate stability, which are promising for introduction in electrochemiluminescence (ECL) signal amplification. In this work, a kind of multilamellar vesicle obtained from sodium bis(2-ethylhexyl) sulfosuccinate (AOT) was used to modify the electrode surface. The AOT vesicle-modified microenvironment could significantly enhance the ECL performances for the luminol/O2 system in a neutral medium. The mechanism study demonstrated that the nanoscale multilamellar vesicles could maintain the vesicle structure on the electrode surface, which substantially improved the electron transfer and reaction rate, luminescence efficiency of the excited-state 3-aminophthalate anion, and stability of the superoxide anion radical. Alternatively, such a multifunctional microenvironment was also able to enhance the ECL signals from the tris(2,2'-bipyridine)ruthenium(II) (Ru(bpy)32+)/tripropylamine (TPrA) system. Moreover, another dodecyl dimethyl(3-sulfopropyl) ammonium hydroxide inner salt (DSB)-based vesicle was constructed to further verify the versatility of the vesicle-modified microenvironment for ECL signal amplification. Our work not only provides a versatile microenvironment for improving the efficiency of various ECL systems but also offers new insights for the microenvironment construction using the ordered assemblies in ECL fields.

Mechanistic Insights into the Charge Transfer Dynamics of Photocatalytic Water Oxidation at the Lipid Bilayer-Water Interface

Photosystem II, the natural water-oxidizing system, is a large protein complex embedded in a phospholipid membrane. A much simpler system for photocatalytic water oxidation consists of liposomes functionalized with amphiphilic ruthenium(II)-tris-bipyridine photosensitizer (PS) and 6,6'-dicarboxylato-2,2'-bipyridine-ruthenium(II) catalysts (Cat) with a water-soluble sacrificial electron acceptor (Na₂S₂O₈). However, the effect of embedding this photocatalytic system in liposome membranes on the mechanism of photocatalytic water oxidation was not well understood. Here, several phenomena have been identified by spectroscopic tools, which explain the drastically different kinetics of water photo-oxidizing liposomes, compared with analogous homogeneous systems. First, the oxidative quenching of photoexcited PS* by S₂O₈^2- at the liposome surface occurs solely via static quenching, while dynamic quenching is observed for the homogeneous system. Moreover, the charge separation efficiency after the quenching reaction is much smaller than unity, in contrast to the quantitative generation of PS⁺ in homogeneous solution. In parallel, the high local concentration of the membrane-bound PS induces self-quenching at 10:1-40:1 molar lipid-PS ratios. Finally, while the hole transfer from PS⁺ to catalyst is rather fast in homogeneous solution (k_obs > 1 × 10⁴ s^-1 at [catalyst] > 50 μM), in liposomes at pH = 4, the reaction is rather slow (k_obs ≈ 17 s^-1 for 5 μM catalyst in 100 μM DMPC lipid). Overall, the better understanding of these productive and unproductive pathways explains what limits the rate of photocatalytic water oxidation in liposomal vs homogeneous systems, which is required for future optimization of light-driven catalysis within self-assembled lipid interfaces.

Interfacial Characterization of Ruthenium-Based Amphiphilic Photosensitizers

Nonreactive surfactant molecules have long been used and characterized for a wide range of applications in industries, life science, and everyday life. Recently, new types of functional amphiphilic molecules have emerged that bear another function, for example, a light-absorbing action, or catalytic properties. However, the surfactant properties of these molecules remain to date essentially unknown. In this context, we investigated here the interfacial activity of photocatalytic surfactants based on a ruthenium(II) tris-bipyridine core, functionalized with two alkyl tails. We realized a systematic characterization of the surfactant properties of these molecules at a water-air interface and studied the effect of the alkyl chain length and of the counterions (hexafluorophosphate or chloride) on these properties. Our data demonstrate that ruthenium surfactants with chloride counteranions form a denser layer at the interface, but their surfactant properties can dramatically deteriorate when the chain length of the alkyl tail increases, leading to simple hydrophobic molecules with poor surfactant properties for the longest chains (C17). These findings pave the way for a better use and understanding of photocatalytic soft interfaces.

Supramolecular strategies in artificial photosynthesis

Artificial photosynthesis is a major scientific endeavor aimed at converting solar power into a chemical fuel as a viable approach to sustainable energy production and storage. Photosynthesis requires three fundamental actions performed in order; light harvesting, charge-separation and redox catalysis. These actions span different timescales and require the integration of functional architectures developed in different fields of study. The development of artificial photosynthetic devices is therefore inherently complex and requires an interdisciplinary approach. Supramolecular chemistry has evolved to a mature scientific field in which programmed molecular components form larger functional structures by self-assembly processes. Supramolecular chemistry could provide important tools in preparing, integrating and optimizing artificial photosynthetic devices as it allows precise control over molecular components within such a device. This is illustrated in this perspective by discussing state-of-the-art devices and the current limiting factors - such as recombination and low stability of reactive intermediates - and providing exemplary supramolecular approaches to alleviate some of those problems. Inspiring supramolecular solutions such as those discussed herein will incite expansion of the supramolecular toolbox, which eventually may be needed for the development of applied artificial photosynthesis.

Photocatalytic Hydrogen Generation by Vesicle-Embedded [FeFe]Hydrogenase Mimics: A Mechanistic Study

Artificial photosynthesis—the direct photochemical generation of hydrogen from water—is a promising but scientifically challenging future technology. Because nature employs membranes for photodriven reactions, the aim of this work is to elucidate the effect of membranes on artificial photocatalysis. To do so, a combination of electrochemistry, photocatalysis, and time‐resolved spectroscopy on vesicle‐embedded [FeFe]hydrogenase mimics, driven by a ruthenium tris‐2,2′‐bipyridine photosensitizer, is reported. The membrane effects encountered can be summarized as follows: the presence of vesicles steers the reactivity of the [FeFe]‐benzodithiolate catalyst towards disproportionation, instead of protonation, due to membrane characteristics, such as providing a constant local effective pH, and concentrating and organizing species inside the membrane. The maximum turnover number is limited by photodegradation of the resting state in the catalytic cycle. Understanding these fundamental productive and destructive pathways in complex photochemical systems allows progress towards the development of efficient artificial leaves.