Dendrimeric synthetic antenna complexes have been constructed using transition metal complexes of Ru or Os, bridged by oligopyridine‐type ligands [37]. The choice of connecting ligand is important because it does not merely bring together mononuclear complexes into a closely packed ensemble, but it determines the overall nanoscale architecture of the dendrimeric structure and controls both the local electronic properties of the metal‐based units and the electronic coupling between them [34]. Earth‐abundant first‐row transition metal complexes feature much less prominently in the field of light harvesting compared to 4d and 5d elements, but research efforts are currently directed toward changing this paradigm [38]. Multiporphyrin arrays, particularly utilizing zinc porphyrins, represent another common pattern in the context of bioinspired light‐harvesting complexes [33, 34]. On the other hand, light‐harvesting dendrimers constructed using solely organic subunits have also been explored [42]. Figure 3.5 depicts two examples representative of ideas mentioned above. Another molecular approach worth mentioning is based on host–guest constructions, where molecules such as organic dyes or transition metal complexes are accommodated within internal cavities of dendrimers [45, 46]. Research and development of artificial light‐harvesting systems is an active field with immense potential and diversity that goes far beyond what can be covered in the present chapter, so the interested reader is referred to the primary literature for more details.
Figure 3.5 Examples of synthetic approaches to multi‐chromophore arrays: (a) a nine‐porphyrin array unit comprising a central free‐base porphyrin core that acts as final acceptor and is surrounded by eight energy‐donating zinc porphyrins [43].
Source: Choi et al. [41]. (b) Dendrimer consisting of a terrylenediimide (TDI) core with four attached perylenemonoimides (PMI) and eight peripheral naphthalenemonoimides (NMI) [44].
Source: Balzani et al. [34].
3.4 Charge Separation and Electron Transfer
Referring back to Figure 3.2 that depicts the main components involved in oxygenic photosynthesis, it is useful to translate that scheme into a corresponding energy/electron flow diagram, the so‐called Z‐scheme of photosynthesis shown in Figure 3.6. The essential features are the utilization of two charge separation events (at P680 of PS‐II and P700 of PS‐I) with distinct potentials and the closely spaced electron transfer cascades that contribute to stabilization of charge separation and unidirectionality of electron transfer. It is noted that purple bacteria utilize only a type II reaction center that lacks the water oxidizing ability of PS‐II, whereas green sulfur bacteria utilize only type I centers, related to PS‐I. A one‐step process is in principle harder to apply to water splitting because of constraints placed on the reduction potential of the excited reaction center chromophore: it should be more positive than the water oxidation potential yet more negative than the hydrogen evolution potential. This creates limitations regarding the minimum excitation energy required to drive a single‐step water splitting process. By contrast, the two‐step process embodied in the Z‐scheme of oxygenic photosynthesis relaxes these constraints by utilizing two photons per electron transferred from water to the final electron acceptor and hence being able to use sunlight of lower energy that what would have been necessary otherwise. In the context of artificial photosynthesis, an implementation of the Z‐scheme (for instance, in multi‐junction photovoltaic devices) would similarly offer higher flexibility in the choice of materials and redox linkers/mediators, requiring only that the excited‐state potential of the reaction center at the oxidative side be lower than that of the reaction center at the reducing side.
Figure 3.6 Simplified Z‐scheme of natural oxygenic photosynthesis, showing how two photons are used per electron flowing from the terminal donor (H2O) to the terminal acceptor (NADP+) of the light‐dependent reactions.
Both photosystems have homodimeric structures and exhibit high similarity in the proteins and cofactors comprising their core regions, suggestive of their common evolutionary origin. In the following we will focus on the enzyme responsible for water oxidation, PS‐II (see Figure 3.7). Crystallographic structures of PS‐II are mostly available from thermophilic cyanobacteria such as Thermosynechococcus elongatus and Thermosynechococcus vulcanus. Conventional X‐ray diffraction (XRD) studies, which first yielded a PS‐II crystallographic model in 2001 [47] and make use of synchrotron X‐ray radiation, have more recently been supplanted by approaches that utilize X‐ray free‐electron laser (XFEL) femtosecond pulses [48, 49]. Through a long series of XRD studies [50–56], the highest‐resolution cyanobacterial PS‐II crystallographic models currently stand at 1.9 Å [55] and 1.87/1.85 Å [56]. Presently available XFEL models have still not achieved comparable resolution, but they have opened the way for probing intermediate states of the water oxidation cycle [57-63]. Higher‐plant PS‐II structures that resolve internal cofactors have so far been reported from cryo‐electron microscopy at comparatively lower resolution [64, 65].
Figure 3.7 Photosystem II from T. vulcanus (PDB ID: 3WU2, a) and major redox‐active components within a PS‐II monomer involved in the main‐pathway electron transfer indicated with red arrows (b).
Each monomer of cyanobacterial PS‐II consists of 20 protein chains, a large number of organic cofactors such as chlorophylls, pheophytins, carotenoids, lipids, and quinones, and a number of inorganic cofactors that include hemes, a nonheme iron, calcium and chloride ions, and the tetramanganese–calcium cofactor (Mn4CaOx) of the OEC. Four transmembrane proteins comprise the core of the enzyme and host all major nonprotein cofactors; these are denoted D1, D2, CP43, and CP47. The PS‐II core proteins are surrounded by additional proteins that may be either permanently or transiently attached [64, 66–70]. In contrast to the highly conserved core proteins, extrinsic or auxiliary proteins have larger variation between different species. The redox‐active cofactors that participate in electron transfer from water to plastoquinone QB are arranged in quasi‐symmetric branches (Figure 3.7). Importantly, only one branch is considered