Porphyrin-quinone assembly

Another example 145 of a porphyrin-quinone assembly via coordination was reported by Sakata and coworkers. The photodonor was two zinc porphyrins linked by a phenanthrene fragment and two pyridines bound to quinone ligated to the zinc porphyrin. A ditopic binding structure was determined by H NMR spectroscopy (AT , = 1.1 x 10 M in CH2CI2). According to a time-resolved fluorescence study, the ET rate constant was estimated to be 1.6 X 10 s within the conformationally restricted system. 

The lifetime of the final charge-separated ion pair, C -P-Q" , is about 300 ns in a dichloromethane solution (and about 2 jus in a butyro-nitrile solution). This lifetime is substantially longer when compared to porphyrin-quinone ion pairs, P -Q (ca. 100 to 200ps). Other D-S-A assemblies containing porphyrins as chromophores are exemplified in structures 3 and 4. In these triad assemblies an aniline residue provides the electron donor component [35], System 3, for example, exhibits structural rigidity, and thus defined spatial orientation between the donor and acceptor components is established. The sequence of ET reactions occurring upon excitation of the porphyrin is summarized by... 

The molecular assembly (7), consisting of a porphyrin chromophore linked to two quinones, which exhibit the proper ordering of redox potentials for stepwise ET, mimics the functionalities of the photosynthetic reaction center [38]. Transient absorption studies have revealed that upon excitation of the porphyrin, a charge-separatpd porphyrin-quinone ion pair with a lifetime of about 300 ps is formed. In a molecular assembly lacking the second quinone system, an ion pair with a lifetime of only 130ps is observed. The increase in the lifetime of the triad ion pair has been attributed to the multistep ET, leading to spatial separation of the photoproducts, which... 

The charge separation processes in photosynthesis involve photoinduced electron transfer from chlorophyll to quinone derivatives. A popular methodology of biomimetic chemistry for biological active sites is assembly of supposedly essential components via covalent linkage. TTius, a variety of covalently linked porphyrin-quinone derivatives such as 1 and 2 below have been prepared and photoinduced electron transfer therein investigated as models of photosynthetic electron transfer [12]. There are two major difficulties in the covalent approach to the preparation of this type of face-to-face porphyrin-quinone derivatives. The first is synthetic problems associated with the preparation of highly substituted quinone precursors and the porphyrin-quinone double coupling reactions. Second, there is little room for the systematic modification of the electtonic/steric structures of quinones. 

Electron transfer between porphyrins and quinones in non-bridge organized molecular assemblies... 

A chromophore such as the quinone, ruthenium complex, C(,o. or viologen is covalently introduced at the terminal of the heme-propionate side chain(s) (94-97). For example, Hamachi et al. (98) appended Ru2+(bpy)3 (bpy = 2,2 -bipyridine) at one of the terminals of the heme-propionate (Fig. 26) and monitored the photoinduced electron transfer from the photoexcited ruthenium complex to the heme-iron in the protein. The reduction of the heme-iron was monitored by the formation of oxyferrous species under aerobic conditions, while the Ru(III) complex was reductively quenched by EDTA as a sacrificial reagent. In addition, when [Co(NH3)5Cl]2+ was added to the system instead of EDTA, the photoexcited ruthenium complex was oxidatively quenched by the cobalt complex, and then one electron is abstracted from the heme-iron(III) to reduce the ruthenium complex (99). As a result, the oxoferryl species was detected due to the deprotonation of the hydroxyiron(III)-porphyrin cation radical species. An extension of this work was the assembly of the Ru2+(bpy)3 complex with a catenane moiety including the cyclic bis(viologen)(100). In the supramolecular system, vectorial electron transfer was achieved with a long-lived charge separation species (f > 2 ms). 

Molecular assembly 2, composed of a porphyrin chromophore covalently linked to a benzoquinone electron acceptor and a carotenoid polyene as electron donor provides an electron donor/photosensitizer/electron acceptor (D-S-A) configuration [34]. The sequential ET reactions occurring in the triad upon photoexcitation of the porphyrin center are outlined as follows (where C, P, and Q stand for the carotenoid polyene, porphyrin, and quinone components, respectively) ... 

Eugster, N., D.J. Fermin, and H.H. Girault (2003). Photoinduced electron transfer at liquid/liquid interfaces. D5mamics of the heterogeneous photoreduction of quinones by self-assembled porphyrin ion-pairs. J. Am. Chem. Soc. 125, 4862-4869. 

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