Andrea Pannwitz

ABSTRAKT: Natürliche Photosynthese wird durch Sonnenlicht angetrieben, wobei ein großer Teil des sichtbaren Spektrums durch die Lichtsammelkomplexe (LHCs) absorbiert wird, welche die Energie anschließend an das Reaktionszentrum weiterleiten. Inspiriert von der Natur haben wir eine Lichtsammel‐ und Energieübertragungskaskade in biomimetischen Lipid‐Doppelschichten von Liposomen aus DPPC (1,2‐Dipalmitoyl‐sn‐glycero‐3‐phosphocholin) aufgebaut, wobei wir Fluorescein, 2‐(3,6‐Dihydroxy‐9H‐xanthen‐9‐yl)‐5‐dodecanamidobenzoesäure (FlC ₁₂ ) als primären Absorber und Eosin Y, Hexadecyl‐2‐(2,4,5,7‐tetrabrom‐3,6‐dihydroxy‐9H‐xanthen‐9‐yl)benzoat (EYC ₁₆ ) als Energieakzeptor in der Membran verankert haben, um Sauerstoff zu sensibilisieren und so die reaktive Sauerstoffspezies ¹ O ₂ zu erzeugen. Diese wird als Oxidationsmittel genutzt, um das Modellsubstrat Nicotinamidadenindinukleotid (NADH) innerhalb der kompartimentierenden Liposomen‐Nanoreaktoren zu oxidieren. Es wurde beobachtet, dass unser metallfreies LHC‐System nur einen geringen Einfluss auf die Photooxidationsgeschwindigkeit von NADH hat, wenn die Nanoreaktormembran symmetrisch funktionalisiert ist. Im Gegensatz dazu führt eine asymmetrische Funktionalisierung der Membranen der Liposomen‐Nanoreaktoren zu einer Beschleunigung um 16 bis 27 %, wenn mehrfarbige Leuchtdioden (LED) bzw. simuliertes Sonnenlicht verwendet werden.

In nature, photosynthesis is driven by solar light and a large proportion of the visible spectrum is absorbed by the light harvesting complexes (LHCs), which then transfer the energy to the reaction center. Inspired by nature, we implemented a light harvesting energy transfer cascade within biomimetic lipid bilayers of liposomes built with DPPC (1,2‐dipalmitoyl‐sn‐glycero‐3‐phosphocholine), using membrane‐anchored fluorescein, 2‐(3,6‐dihydroxy‐9H‐xanthen‐9‐yl)‐5‐dodecanamidobenzoic acid (FlC ₁₂ ) as primary absorber and membrane anchored eosin Y, hexadecyl 2‐(2,4,5,7‐tetrabromo‐3,6‐dihydroxy‐9H‐xanthen‐9‐yl)benzoate (EYC ₁₆ ), as energy acceptor to sensitize oxygen and generate the reactive oxygen species ¹ O ₂ . Finally, the model substrate nicotinamide adenine dinucleotide (NADH) is oxidized by ¹ O ₂ within the compartmentalizing liposome nanoreactors. It was observed that our metal‐free LHC system has only a minor effect on the photooxidation rate of NADH when the nanoreactor membrane is functionalized symmetrically. By contrast, asymmetric membrane functionalization of the liposome nanoreactor membranes leads to acceleration by 16% to 27% when using multi‐colored light emitting diodes (LED) or simulated solar light, respectively.

Molecule-in-matrix systems such as Langmuir films with photosensitizers and hydrogen evolution reaction (HER) catalysts may exhibit heterogeneity in photocatalytic activity across large-area films. Here, we introduce a confined illumination approach and simultaneous localized detection of light-driven hydrogen (H₂) evolution to directly correlate information on structural organization with catalytic performance of Langmuir–Blodgett and Langmuir–Schaefer films with incorporated [Mo₃S₁₃]^2– catalyst and a Ru-based photosensitizer. Derived from a coating of optical fibers used in scanning near-field optical microscopy (SNOM), the outside of the glass sheath of the microelectrode was modified with a silver coating. Coupling light into the coated glass sheath, local illumination with suppressed light loss, and visible light excitation directly beneath the microsensor apex could be achieved, resulting in a 3.4-fold higher photon flux density compared to the uncovered microelectrode. Using platinum-black-modified microelectrodes as sensitive H₂ microsensors, a bifunctional probe serving simultaneously as a local light waveguide and an H₂ microsensor, enables quantitative detection of photogenerated H₂ with spatial resolution. This is demonstrated here for films comprising a phospholipid matrix or a π-conjugated rigid molecular scaffold coembedding Ru-based photosensitizers and [Mo₃S₁₃]^2– catalysts. This enables not only the local quantification of H₂, but also the determination of the apparent quantum efficiencies (AQEs) of the different film architectures.

We report the governing principles that regulate the activity of light-driven CO₂ reduction by a molecular photosensitizer bis(2,2′-bipyridine)-(4,4′-dinonyl-2,2′-bipyridine)-ruthenium(II) (RuC₉) and a molecular catalyst (5,10,15,20-tetra(4-methylphenyl)porphinato)cobalt(II) (CoTTP) in supramolecular assembly within the lipid bilayers of liposomes suspended in water. We tested six different lipids with membranes in either the gel phase, fluid phase, or at the transition between both states, as well as zwitterionic or negatively charged headgroups. The correlation of the membrane rigidity with light-driven catalysis performance is not conclusive for the investigated set of lipid membranes, but molecular dynamics simulations elucidate how catalyst efficiency increases with the distance from the membrane center as well as their calculated vertical reduction energies. Luminescence quenching studies revealed that mainly dynamic quenching was observed with the highest quenching efficiency found with 1,2-dimyristoyl-sn-glycero-3-phosphocholine (DMPC) and 1,2-dipalmitoyl-sn-glycero-3-phospho-(1′-rac-glycerol)(sodium salt) (DPPG)-based liposomes, in agreement with the results of the best performance in photocatalysis and the computational insights. A variation of cations did not show any significant influence on the performance, as opposed to electrochemical studies. The overall mechanistic findings of this study provide design principles for light-driven CO₂ reduction by molecular components in liposomes.

Molecule-in-matrix systems such as Langmuir films with photosensitizer and hydrogen evolution reaction (HER) catalyst may exhibit heterogeneities in photocatalytic activity across large-area films. Here, we introduce a confined illumination approach and simultaneous localized detection of light-driven hydrogen (H2) evolution to directly correlate information on structural organization with catalytic performance at Langmuir-Blodgett and Langmuir-Schäfer films with incorporated [Mo₃S₁₃]²⁻ catalyst and Ru-based photosensitizer. Derived from coating of optical fibers used in scanning near-field optical microscopy (SNOM), the outside of the glass sheath of the microelectrode was modified with a silver coating. Coupling light into the coated glass sheath, local illumination with suppressed light loss and visible light excitation directly beneath the microsensor apex, resulting in a 3.4-fold higher photon flux density compared to the uncovered microelectrode. Using platinum-black modified microelectrodes as sensitive H2 microsensors, a bifunctional probe serving simultaneously as local light waveguide and H2 microsensor, enables quantitative detection of photogenerated H2 with high spatial resolution. This is demonstrated here for films comprising a phospholipid matrix or a π-conjugated rigid molecular scaffold co-embedding Ru-based photosensitizers and [Mo₃S₁₃]²⁻ catalysts. This enables not only the local quantification of H2, but also to determine the apparent quantum efficiencies (AQEs) of the different film architectures.

A UiO-67 MOF functionalized with [Ru(bpy) ₃ ] ²⁺ and Mn ²⁺ ions is active in light-driven amine oxidation. The oxidation via photosensitized singlet oxygen and electron transfer to manganese was evaluated.

We report a biomimetic system for light‐driven CO₂ conversion in lipid bilayers using a [FeFe]‐hydrogenase‐inspired molecular dyad (PS‐CAT) acting simultaneously as photosensitizer and catalyst. The molecular design of PS‐CAT consists of both light‐harvesting and catalytically active moieties in a single molecule. This structure allows for efficient charge transfer between these two moieties. The PS‐CAT consists of only an organic chromophore and an iron complex. Photocatalytic reduction of CO₂ to CO (CO₂RR) as well as H₂ production (HER) was traced over 56 h to determine CO₂RR/HER selectivity. The presence of water in the lipid bilayer system allows for CO₂RR selectivity >99 % over the competitive HER, outperforming previous molecular, and liposome‐based systems for light‐driven CO₂ to CO conversion. The product selectivity in CO₂RR (e.g., CO, HCOO⁻, and CH₄) was determined via gas chromatography and NMR spectroscopy with ¹³C‐labeled carbon sources. The Stern–Volmer quenching studies on the initial light‐driven electron‐transfer revealed static quenching and indicated a preassembly of the PS‐CAT with the electron donor at the membrane‐water interface.

The four Zr₈O₆‐based metal‐organic frameworks (MOFs), porous coordination network (PCN) 221 M, comprising earth‐abundant metalloporphyrin tetracarboxylate (M‐TCPP, M: 2 H, Zn, Ni, and a mixture of 1:1 Zn and Ni), are investigated for light‐driven H₂ evolution reaction (HER) in water. Under the irradiation of a 405 nm light emitting diode source and in the presence of triethanolamine (TEOA) as a sacrificial electron donor, the photocatalytic HER activity of PCN 221 varies with the metal center in the porphyrin linker. Among the tested MOFs, the Zn‐porphyrin derivative (PCN 221 Zn) produces H₂ at a TON = 4, which is about seven times greater than that of homogeneous Zn‐TCPP (0.6) and superior to its 2 H (2.7), Ni (0.40), and ZnNi (1.6) analogs. Detailed photochemical studies via time‐resolved and steady‐state spectroscopy reveal two distinct charge transfer pathways: Direct H₂ evolution from Zn‐TCPP itself, and electron transfer from the Zn‐TCPP photosensitizer to the Zr₈O₆ SBU catalytic sites. The improved HER performance of PCN 221 Zn is attributed to its favorable features, such as optical absorption, excited‐state properties, and charge separation dynamics, as well as the coordination of TEOA. This study provides fundamental insights into the design of MOF‐based heterogeneous photocatalysts exploiting earth‐abundant metal‐based porphyrin for solar fuel generation.

In natural photosynthesis, light‐driven electron transfer across the thylakoid membrane enables efficient charge separation and the confinement of reaction spaces for generating NADPH and CO ₂ and oxidation of water. These reactions are complementary redox reactions and require different reaction conditions for optimal performance. However, current artificial photosynthesis studies only take place in the bulk and are sensitive toward oxygen and air, which limits their applicability under aerated and water‐splitting conditions. Herein, we report light‐driven electron transfer across a lipid bilayer membrane of liposome vesicles via a rigid oligoaromatic molecular wire that allows to electronically connect an oxidation and reduction reaction which are spatially separated by the membrane. The molecular wire has a simple, symmetric, easy‐to‐synthesize design based on benzothiadiazole and fluorene units and absorbs in the visible spectrum which makes it suitable for solar energy conversion. The model reactions in this study are light‐driven NADH oxidation on one side of the membrane and light‐driven reduction of an organic water‐soluble dye in the bulk phase of liposomes. Additionally, the system is active in both aerobic and anaerobic atmospheres, rendering it ideal for aerobic conditions or reactions that produce oxygen such as solar‐driven water splitting and artificial photosynthesis applications.