[Fe-S] clusters, nature's modular electron transfer units, are often arranged in chains that support long-range electron transfer. Despite considerable interest, the design of biomimetic artificial systems emulating multicluster-binding proteins, with the final goal of integrating them in man-made oxidoreductases, remains elusive. Here, we report a novel bis-[4Fe-4S] cluster binding protein, DSD-Fdm, in which the two clusters are positioned within a distance of 12 Å, compatible with the electronic coupling necessary for efficient electron transfer. The design exploits the structural repeat of coiled coils as well as the symmetry of the starting scaffold, a homodimeric helical protein (DSD). In total, eight hydrophobic residues in the core of DSD were replaced by eight cysteine residues that serve as ligands to the [4Fe-4S] clusters. Incorporation of two [4Fe-4S] clusters proceeds with high yield. The two [4Fe-4S] clusters are located in the hydrophobic core of the helical bundle as characterized by various biophysical techniques. The secondary structure of the apo and holo proteins is conserved; further, the incorporation of clusters results in stabilization of the protein with respect to chemical denaturation. Most importantly, this de novo designed protein can mimic the function of natural ferredoxins: we show here that reduced DSD-Fdm transfers electrons to cytochrome c, thus generating the reduced cyt c stoichiometrically.

A semiconducting porphyrin polymer that is solution processable and soluble in organic solvents has been synthesized, and its spectroscopic and electrochemical properties have been investigated. The polymer consists of diarylporphyrin units that are linked at meso-positions by aminophenyl groups, thus making the porphyrin rings an integral part of the polymer backbone. Hexyl chains on two of the aryl groups impart solubility. The porphyrin units interact only weakly in the ground electronic state. Excitation produces a local excited state that rapidly evolves into a state with charge-transfer character (CT) involving the amino nitrogen and the porphyrin macrocycle. Singlet excitation energy is transferred between porphyrin units in the chain with a time constant of ca. 210 ps. The final CT state has a lifetime of several nanoseconds, and the first oxidation of the polymer occurs at ca. 0.58 V vs. SCE. These properties make the polymer a suitable potential excited state electron donor to a variety of fullerenes or other acceptor species, suggesting that the polymer may find use in organic photovoltaics, sensors, and similar applications.

A zinc and a free base phthalocyanine-fulleropyrrolidine dyad in which the chromophores are linked by a phenylethynyl group have been prepared using a new synthetic route, and their photoelectrochemical properties have been investigated. The zinc dyad is readily soluble in a variety of solvents, and its spectroscopic properties have been determined in toluene and benzonitrile. In toluene, excitation of the zinc phthalocyanine is followed by rapid establishment of an equilibrium between the phthalocyanine and fullerene excited states. These excited states decay mainly to the ground state and the respective triplet states. The fullerene triplet then transfers its energy to form the phthalocyanine triplet. About 20% of the phthalocyanine excited states lead to formation of a charge-separated state. In benzonitrile, the same decay pathways are observed, but photoinduced electron transfer is much faster, and generates the charge separated state with a quantum yield of ≥ 85%. The charge separated state has a lifetime of 2.8 ns in toluene and 94 ps in benzonitrile.

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