Shorter Alkyl Chains Enhance Molecular Diffusion and Electron Transfer Kinetics between Photosensitisers and Catalysts in CO 2 -Reducing Photocatalytic Liposomes.

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 photocatalytic performances of the resulting liposomes is not well understood. In this work, we first prepared a series of rhenium-based CO ₂ -reduction catalysts [Re(4,4'-(C _n H _2n+1 ) ₂ -bpy)(CO) ₃ Cl] (ReC _n ; 4,4'-(C _n H _2n+1 ) ₂ -bpy=4,4'-dialkyl-2,2'-bipyridine) and ruthenium-based photosensitisers [Ru(bpy) ₂ (4,4'-(C _n H _2n+1 ) ₂ -bpy)](PF ₆ ) ₂ (RuC _n ) with different alkyl chain lengths (n=0, 9, 12, 15, 17, and 19). We then prepared a series of PEGylated DPPC liposomes containing RuC _n and ReC _n , hereafter noted C _n , to perform photocatalytic CO ₂ 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 RuC _n 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 RuC ₉ and ReC ₉ .
(© 2021 The Authors. Chemistry - A European Journal published by Wiley-VCH GmbH.)