As a model reaction, we investigated the light-driven oxidation of NADH within the inner aqueous compartment of the liposome vesicles and the light-driven reduction of XTT (2,3-Bis(2-methoxy-4nitro-5-sulfophenyl)-2H-tetrazolium-5-carboxanilide sodium salt) in the outer aqueous bulk compartment. XTT is a well-known chemical reagent in biological assay to proof the presence of cellular reducing equivalents such as NADH. To enable transmembrane electron transfer across a lipid bilayer, we prepared DPPC liposomes that are impermeable to larger water-soluble molecules and integrated 1 mol% of the amphiphilic transmembrane chromophore [1] 2+ , as described above. The water-soluble electron donor NADH was added to the inner compartment of the liposome, while the water-soluble electron acceptor XTT was placed in the bulk or on the outer part of the liposome, forming so-called NADH/[1] 2+ /XTT liposomes (Figure ). Upon light irradiation, the absorption band at max = 470 nm gradually increased, corresponding to the electron uptake of XTT to generate its reduced form formazan (Fz), indicating that photoinduced electron transfer had occurred (Figure ). Simultaneously, the NADH absorption band at 340 nm is diminished, indicating oxidation of the electron donor. As there is significant spectral overlap with the absorption bands of [1] 2+ , the quantification of the transmembrane light-driven conversion was performed at the prominent Fz band at 470 nm.