ABSTRACT
Coupling the notoriously non-emissive complex [Ru(tpy)(bpy)Cl]Cl (tpy = 2,2':6',2''-terpyridine, bpy = 2,2'-bipyridine) to a C12 alkyl chain via an amide linker on the 4' position of the terpyridine yielded a new amphiphilic ruthenium complex showing red emission and chloride-dependent aggregation properties. This emissive complex is highly cytotoxic in A549 non-small lung cancer cells where it can be followed by confocal microscopy. Uptake occurs within minutes, first by insertion into the cellular membrane, and then by migration to the peri-nuclear region.
ABSTRACT
Recently, the addition of negatively charged liposomes was shown to increase the quantum yield of the photocatalytic reduction of 5,5'-dithio(2-nitrobenzoic acid) (H2DTNB) to 2-nitro-5-thiobenzoic acid (H2NTB) by triethanolamine using meso-tetra(4-(N-methylpyridinium)porphyrinato zinc chloride as photosensitizer. In this work, we investigate in detail the kinetics of this photocatalytic reaction both in homogeneous solution and at the surface of negatively charged liposomes, to unravel the effects of liposomes on the mechanism of the photoreaction. In homogeneous solution, the reaction is initiated by oxidative quenching. Both static (singlet) and dynamic (triplet) quenching of the photosensitizer contribute to the formation of the photoproduct. In these conditions, the reaction is limited by the low efficiency of reductive regeneration of the photosensitizer, compared to charge recombination. Upon adsorption of the positively charged photosensitizer to the negative surface of the liposomes, however, both static and dynamic oxidative quenching become ineffective due to electrostatic repulsion of the dianionic DTNB2- from the negatively charged membrane. In such conditions, photoreduction occurs via reductive quenching, showing that the addition of liposomes can truly modify the mechanism of photocatalyzed redox reactions.
ABSTRACT
Liposomes are interesting scaffolds for photocatalysis. In particular, charged liposomes were shown to increase the quantum efficiency of photocatalytic reactions involving charged porphyrin photosensitizers and charged electron acceptors. In this work, the effects of adding positively charged liposomes (DMPC/eDMPCCl 1:1) on the mechanism of the photocatalytic reduction of methyl viologen (MV(2+)) by cysteine in the presence of sodium meso-tetra-(4-sulfonato)porphyrinatozinc (Na41) were probed by modeling UV-vis spectroscopy data using a steady-state approximation. By varying the concentration of methyl viologen, we found that the liposomes not only prevent the formation of a 1:1 complex between ground-state photosensitizer 1(4-) and MV(2+) but also that they increase the cage-escape yield in the excited state. Furthermore, the electrostatic repulsion between the liposomes and MV(2+) diminishes by 1 order of magnitude the rate of oxidative quenching of the photosensitizer triplet excited state ((T)1(4-)) by MV(2+). By varying the amount of sacrificial electron donor (cysteine), the effect of liposome addition on the charge recombination reactions could also be studied. Because of the positive charge borne by the photoproduct MV(â¢+), it was also repelled from the membrane, which significantly slows charge recombination at the surface of the liposome. Overall, compared to a liposome-free solution, the rates of most elementary steps of the photocatalytic reduction of MV(2+) by cysteine are strongly modified when the negative photosensitizer is adsorbed on a positively charged liposome surface. These results not only explain the much higher efficiency of the liposome-containing system but also illustrate the power of supramolecular chemistry for the tuning of photocatalysis.
ABSTRACT
Unidirectional photocatalytic electron transfer from a hydrophilic electron donor encapsulated in the interior of a liposome, to a hydrophilic electron acceptor on the other side of the membrane, has been achieved using the simple membrane-soluble electron relay 1-methoxy-N-methylphenazinium (MMP(+)). The total amount of photoproduct (>140 nmol) exceeds the number of moles of MMP(+) present (125 nmol), thus showing that the transport of electrons is catalytic.