Charge-separation systems

This part of the photosynthetic chain can be mimicked by ruthenium-tyrosinate and manganese-tyrosinate complexes [80-82], In the excited state the ruthenium-polypyridine complex abstracts one electron from the tyrosinate moiety, yielding 

The other approach towards artificial photosynthesis does not model the natural photosystems directly, but instead links natural and synthetic components to achieve a system capable of photochemical water splitting [83]. 

Bellelli A, Branori M, Brzezinski P, Wilson MT. Photochemically induced electron transfer. Methods 2001 24 139-52. 

Millett F, Durham B. Design of photoactive ruthenium complexes to study interprotein electron transfer. Biochemistry 2002 41 11315-24. 

Pieroni O, Fissi A, Angehni N, Lenci F. Photoresponsive photopeptides. Acc Chem Res 2001 34 9-17. 

---

This review article attempts to summarize and discuss recent developments in the studies of photoinduced electron transfer in functionalized polyelectrolyte systems. The rates of photoinduced forward and thermal back electron transfers are dramatically changed when photoactive chromophores are incorporated into polyelectrolytes by covalent bonding. The origins of such changes are discussed in terms of the interfacial electrostatic potential on the molecular surface of the polyelectrolyte as well as the microphase structure formed by amphiphilic polyelectrolytes. The promise of tailored amphiphilic polyelectrolytes for designing efficient photoinduced charge separation systems is afso discussed. 

Microenvironmental effects such as electrostatic or hydrophobic ones provided by molecular assemblies or polymers on the photoinduced charge separation and electron relay are described in this chapter together with the solid phase and macroheterogene-ous photoinduced charge separation system constructed with polymer solids. 

On the basis of Hiickel s An +2) n- electron rule, all of these systems can be expected to be aromatic in nature. They do indeed exhibit varying degrees of aromatic stabilization depending on the nature and position of the heteroatom. They cannot all be represented by conventional classical structures. Structures (la)-(lc) can be represented by classical covalent bonded structures whereas those of the (Id) type form the nonclassical structures in the sense that they can be drawn only as charge-separated systems or biradicals in systems wherein X/Y are sulfur or selenium atoms, d- orbital participation in bonding is conceivable, leading to tetravalent sulfur or selenium. 

Cyclopropenethiones are readily available (Section II.F) and in many respects their chemistry is similar to that of the enones. Whilst X-ray results from diphenylcycloprop-enethione (362) imply little contribution of the dipolar form, MO calculations strongly support the aromatic charge-separated system with a n electron density on sulphur higher than on the oxygen in cyclopropenone. The infrared and Raman spectra of 362 and its perdeuterio derivative have been obtained and the bands assigned by normal coordinate analysis. When compared to the spectrum of diphenylcyclopropenone (14) shifts to 1786 and 1352cm are found °°. 

It may therefore be concluded that systems containing Zn porphyrins in solution with membrane-bound acceptors could be expected to be more practicable than other configurations in charge separation systems which rely upon generation of vesicle membrane separated redox products. 

DNA has also been applied for synkineses of molecular wires. One example applies aligned DNA fibers in which one acridine orange molecule intercalates per 10 base pairs. It shows photocurrents if a voltage is applied to the material placed between comb-form electrodes (Okahata et al., 1998). The dye-DNA complexes are probably not useful as parts of a charge separation system, but they clearly demonstrate extensive electronic communication between bound drug molecules—an influence that serves to raise the efficiency of transition dipole coupling at long distances. 

Nomoto, A., H. Mitsuoka, H. Oseki, and Y. Kobuke (2003). Porphyrin hetero-dimer as charge separating system for photocurrent generation. Chem. Commun. (9), 1074-1075. 

M. Narutaki, K. Takimiya, T. Otsubo, Y. Harima, H. Zhang, Y. Araki and O. Ito, Synthesis and photophysical properties of two dual oligothiophene-fullerene linkage molecules as photoinduced long-distance charge separation systems, J. Org. Chem., 71, 1761-1768 (2006). 

In the case of Au, nanoparticles are stable both under light irradiation and in the dark. Therefore, the charge separation system is applied to photovoltaic cells and photocatalysis. Photovoltaic cells may be a wet type or solid state. In the wet-type cell, a photoanode such as electrode/nSC/MNP (nSC = n-type semiconductor MNP = metal nanoparticle) [8] or a photocathode such as electrode/MNP/nSC [9] is used with an electrolyte containing a redox couple. Structures of solid-state cells are electrode/ nSC/MNP/HTM/electrode (HTM = hole transport material or p-type semiccmductor) [11] and electrode/nSC/MNP/electrode [12],... 

If one takes into account that in natural photosynthesis the lightharvesting systems and the charge-separating system pertain to different molecular entities, the combination of an artificial antenna system with an... 

It is hoped that the quasi-crystalline water framework between electron donors and electron acceptors will also fixate electron transfer agents, e.g. tyrosine or other phenol derivatives. Alignment may be possible. The distant pairs of electron donors and acceptors proposed in section 6 can thus be connected and hopefully produce useful, non-covalent charge separation systems. 

---