Covalent functionalisation with alkyl tails is a common method for supporting molecular catalysts and photosensitisers onto lipid bilayers, but the influence of the alkyl chain length on the... Show moreCovalent functionalisation with alkyl tails is a common method for supporting molecular catalysts and photosensitisers onto lipid bilayers, but the influence of the alkyl chain length on the photocatalytic performances of the resulting liposomes is not well understood. In this work, we first prepared a series of rhenium-based CO2-reduction catalysts [Re(4,4'-(CnH2n+1)(2)-bpy)(CO)(3)Cl] (ReCn; 4,4'-(CnH2n+1)(2)-bpy=4,4'-dialkyl-2,2'-bipyridine) and ruthenium-based photosensitisers [Ru(bpy)(2)(4,4'-(CnH2n+1)(2)-bpy)](PF6)(2) (RuCn) with different alkyl chain lengths (n=0, 9, 12, 15, 17, and 19). We then prepared a series of PEGylated DPPC liposomes containing RuCn and ReCn, hereafter noted C-n, to perform photocatalytic CO2 reduction in the presence of sodium ascorbate. The photocatalytic performance of the C-n liposomes was found to depend on the alkyl tail length, as the turnover number for CO (TON) was inversely correlated to the alkyl chain length, with a more than fivefold higher CO production (TON=14.5) for the C-9 liposomes, compared to C-19 (TON=2.8). Based on immobilisation efficiency quantification, diffusion kinetics, and time-resolved spectroscopy, we identified the main reason for this trend: two types of membrane-bound RuCn species can be found in the membrane, either deeply buried in the bilayer and diffusing slowly, or less buried with much faster diffusion kinetics. Our data suggest that the higher photocatalytic performance of the C-9 system is due to the higher fraction of the more mobile and less buried molecular species, which leads to enhanced electron transfer kinetics between RuC9 and ReC9. Show less
Artificial photosynthesis has experienced rapid developments aimed at producing photocatalytic systems for the synthesis of chemical energy carriers. Conceptual advances of solar fuel systems have... Show moreArtificial photosynthesis has experienced rapid developments aimed at producing photocatalytic systems for the synthesis of chemical energy carriers. Conceptual advances of solar fuel systems have been inspired by improved understanding of natural photosynthesis and its key operational principles: (a) light harvesting, (b) charge separation, (c) directional proton and electron transport between reaction centres and across membranes, (d) water oxidation and (e) proton or CO2 reduction catalysis. Recently, there has been a surge of bio-inspired photosynthetic assemblies that use liposomes as nanocompartments to confine reaction spaces and enable vectorial charge transport across membranes. This approach, already investigated in the 1980s, offers in principle a promising platform for solar fuel synthesis. However, the fundamental principles governing the supramolecular assemblies of lipids and photoactive surfactant-like molecules in membranes, are intricate, and mastering membrane-supported photochemistry requires thorough understanding of the science behind liposomes. In this review, we provide an overview of approaches and considerations to construct a (semi)artificial liposome for solar fuel production. Key features to consider for the use of liposomes in solar fuel synthesis are highlighted, including the understanding of the orientation and binding of different components along the membrane, the controlled electron transport between the reaction centres, and the generation of proton gradients as driving force. Together with a list of experimental techniques for the characterisation of photoactive liposomes, this article provides the reader with a roadmap towards photocatalytic fuel production at the interface of lipid membranes and aqueous media. Show less
