Mechanistic Insights into the Charge Transfer Dynamics of Photocatalytic Water Oxidation at the Lipid Bilayer–Water Interface

Abstract

Photosystem II, the natural water-oxidizing system, is a large protein complex embedded in a phospholipid membrane. A much simpler system for photocatalytic water oxidation consists of liposomes functionalized with amphiphilic ruthenium(II)-tris-bipyridine photosensitizer (PS) and 6,6′-dicarboxylato-2,2′-bipyridine-ruthenium(II) catalysts (Cat) with a water-soluble sacrificial electron acceptor (Na2S2O8). However, the effect of embedding this photocatalytic system in liposome membranes on the mechanism of photocatalytic water oxidation was not well understood. Here, several phenomena have been identified by spectroscopic tools, which explain the drastically different kinetics of water photo-oxidizing liposomes, compared with analogous homogeneous systems. First, the oxidative quenching of photoexcited PS* by S2O8 2– at the liposome surface occurs solely via static quenching, while dynamic quenching is observed for the homogeneous system. Moreover, the charge separation efficiency after the quenching reaction is much smaller than unity, in contrast to the quantitative generation of PS+ in homogeneous solution. In parallel, the high local concentration of the membrane-bound PS induces self-quenching at 10:1–40:1 molar lipid–PS ratios. Finally, while the hole transfer from PS+ to catalyst is rather fast in homogeneous solution (k obs > 1 × 104 s–1 at [catalyst] > 50 μM), in liposomes at pH = 4, the reaction is rather slow (k obs ≈ 17 s–1 for 5 μM catalyst in 100 μM DMPC lipid). Overall, the better understanding of these productive and unproductive pathways explains what limits the rate of photocatalytic water oxidation in liposomal vs homogeneous systems, which is required for future optimization of light-driven catalysis within self-assembled lipid interfaces.

Alignment and photooxidation dynamics of a perylene diimide chromophore in lipid bilayers

“…Quenching of homogeneously dissolved [ 1 ] 2+ in 1 : 1 (V : V) acetonitrile–water mixture occurred with a Stern–Volmer constant with different values for the quenching of the emission spectrum's intensity and lifetime with K SV ( I 0 / I ) = 17.0 ± 2.9 and K SV ( τ 0 / τ ) = 0.04 ± 0.02, respectively, which indicates a static quenching process as the intensity-based Stern–Volmer constant is positive and the lifetime-based Stern–Volmer constant is close to zero. 7,31,32 Upon incorporation of [ 1 ] 2+ into liposomes the intensity and lifetime-based Stern-Volmer constants differ from each other with K SV ( I 0 / I ) = 20.9 ± 1.9 and K SV ( τ 0 / τ ) = −0.17 ± 0.02, respectively. The lifetime-based Stern–Volmer constant appears to be slightly negative, however, when comparing the actual lifetimes, no significant effect is observed, as the lifetimes stay constant within the experimental error of the lifetime measurements.…”

“…7). 7,31,32 Such static quenching processes are typical for singlet emitters with short excited state lifetimes where a pre-associated chromophore–quencher adduct needs to be formed for the quenching process to take place (Table 3). 31…”

“…Based on the extinction coefficient in acetonitrile/water and neglecting the scattering effect we estimate that the overall concentration of indicates a static quenching process (Figure 7). 7,31,32 Such static quenching processes are typical for singlet emitters with short excited state lifetimes where a pre-associated chromophore-quencher adduct needs to be formed for the quenching process to take place (Table 3). From the Stern Volmer constants and the lifetime of the chromophore in absence of quencher, the quenching constant k q can be calculated as follows: 31,33 k q = K SV •τ 0 .…”

“…7). 7,31,32 Such static quenching processes are typical for singlet emitters with short excited state lifetimes where a pre-associated chromophore-quencher adduct needs to be formed for the quenching process to take place (Table 3). 31 From the Stern-Volmer constants and the lifetime of the chromophore in the absence of a quencher, the quenching constant k q can be calculated as follows: 31,33…”

AIE Polymer Micelle/Vesicle Photocatalysts Combined with Native Enzymes for Aerobic Photobiocatalysis

“…Polymer micelles and vesicles are also core–shell structures with a hydrophilic polymer shell and a hydrophobic polymer core formed by simple self-assembly of amphiphilic block copolymers (in vesicles, the core and the shell correspond, respectively, to the hydrophobic inner part and the hydrophilic outer layers of the membrane). It is worth mentioning that recently, photocatalysis on the lipid membranes of liposomes has been reported to mimic and understand natural photosynthesis and the role of the membrane in it. − Here, in polymer micelles and vesicles, the hydrophobic block can be designed as a photocatalytic polymer, and the hydrophobic cores constitute then nanophotoreactors. The mass transportation of hydrophilic NADH/NAD + within the hydrophilic polymer layer could be free.…”