Lipid bilayer membranes are ubiquitous in natural chemical conversions. They enable self-assembly and compartmentalization of reaction partners and it becomes increasingly evident that a thorough fundamental understanding of these concepts is highly desirable for chemical reactions and solar energy conversion with artificial systems. This minireview focusses on selected case studies from recent years, most of which were inspired by either membrane-facilitated light harvesting or respective charge transfer. The main focus is on highly biomimetic liposomes with artificial chromophores, and some cases for polymer-membranes will be made. Furthermore, we categorized these studies into energy transfer and electron transfer, with phospholipid vesicles, and polymer membranes for light-driven reactions.
Multichromophoric systems based on a Ru II polypyridine moiety containing an additional organic chromophore are of increasing interest with respect to different lightdriven applications. Here, we present the synthesis and detailed characterization of a novel Ru II photosensitizer, namely [(tbbpy) 2 Ru((2-(perylen-3-yl)-1H-imidazo[4,5-f][1,10]phenanthrolline))](PF 6 ) 2 RuipPer, that includes a merged perylene dye in the back of the ip ligand. This complex features two emissive excited states as well as a long-lived (8 μs) dark state in acetonitrile solution. Compared to prototype [(bpy) 3 Ru] 2 + -like complexes, a strongly altered absorption (ɛ = 50.3 × 10 3 M À 1 cm À 1 at 467 nm) and emission behavior caused by the introduction of the perylene unit is found. A combination of spectro-electrochemistry and timeresolved spectroscopy was used to elucidate the nature of the excited states. Finally, this photosensitizer was successfully used for the efficient formation of reactive singlet oxygen.
Confinement of reaction spaces was achieved in a biomimetic manner by using liposome vesicles that are based on phospholipid bilayer membranes, similar to cellular compartments. Encapsulation of photosensitizer (PS) and substrate within the inner aqueous compartment of liposomes accelerated the photosensitized model reaction of nicotinamide adenine dinucleotide (NADH) conversion to its oxidized form (NAD + ) by one order of magnitude compared to classical homogeneous reaction conditions. Furthermore, it was found that the reaction proceeds around 40 % faster when the photosensitizer is dissolved in the inner aqueous compartment instead of being embedded within the phospholipid bilayer which is attributed to the diffusion behavior of singlet oxygen which acts as oxidant in this reaction. These experimental findings will allow for reaction and molecular systems design for photochemical and catalytic conversions and will be relevant in the context of creating artificial cellular compartments such as fully artificial chloroplasts.
Two are better than one: In a joint study, groups in two laboratories at TU Braunschweig and Ulm University present a novel bichromophoric RuII complex. This unique photosensitizer was obtained by fusing a ruthenium polypyridine chromophore with an additional perylene dye. This photosensitizer exhibits a strong absorptivity in the visible region and an enhanced excited state lifetime of 8 μs. In addition, this complex enables the efficient production of singlet oxygen. More information can be found in the Research Article by S. Rau, S. Tschierlei, et al. (DOI: 10.1002/chem.202103609).
Calculations of Förster
Resonance Energy Transfer (FRET)
often neglect the influence of different chromophore orientations
or changes in the spectral overlap. In this work, we present two computational
approaches to estimate the energy transfer rate between chromophores
embedded in lipid bilayer membranes. In the first approach, we assess
the transition dipole moments and the spectral overlap by means of
quantum chemical calculations in implicit solvation, and we investigate
the alignment and distance between the chromophores in classical molecular
dynamics simulations. In the second, all properties are evaluated
integrally with hybrid quantum mechanical/molecular mechanics (QM/MM)
calculations. Both approaches come with advantages and drawbacks,
and despite the fact that they do not agree quantitatively, they provide
complementary insights on the different factors that influence the
FRET rate. We hope that these models can be used as a basis to optimize
energy transfers in nonisotropic media.
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