Porphyrins Covalently Linked to Quinones

Nevertheless, the inverse region is observed in the particular case where the electron donor and the electron acceptor are held apart by a bridge (e.g. porphyrins covalently linked to quinones). 

The vast majority of the dyad models for photosynthetic electron transfer have consisted of synthetic porphyrins covalently linked to quinones. The first such models were reported in the late 1970 s. Kong and Loach prepared the ester-linked dyad 2 in 1978 [38], and the amide 3 was reported by Tabushi and coworkers in 1979 [39]. A large number of these P-Q systems have now appeared in the literature. The reader is referred to several reviews [13, 34, 40], including the recent review by Connolly and Bolton [41] for a complete compilation of these results. 

Photoinduced intramolecular electron tunneling was observed also in some other porphyrin containing bridge molecules, such as porphyrin covalently linked to phenolphthalein [308], dimethylaniline — mesoporphyrin II — quinone triad [309], Zn porphyrin-viologen-quinone triad [310], carotenoid — porphyrin -diquinone tetrad [311]. The influence of conformational state of porphyrin-viologen bridge molecules on the rate of PET reactions was studied in Ref. [312]. 

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) ... 

The simplest covalently linked systems consist of porphyrin linked to electron acceptor or donor moiety with appropriate redox properties as outlined in Figure 1. Most of these studies have employed free base, zinc and magnesium tetrapyrroles because the first excited singlet state is relatively long-lived (typically 1-10 ns), so that electron transfer can compete with other decay pathways. Additionally, these pigments have relatively high fluorescence quantum yields. These tetrapyrroles are typically linked to electron acceptors such as quinones, perylenes , fullerenes , acetylenic fragments (14, 15) and aromatic spacers and other tetrapyrroles (e.g. boxes and arrays). 

In the natural photosynthetic reaction center, ubiquinones (QA and QB), which are organized in the protein matrix, are used as electron acceptors. Thus, covalently and non-covalently linked porphyrin-quinone dyads constitute one of the most extensively investigated photosynthetic models, in which the fast photoinduced electron transfer from the porphyrin singlet excited state to the quinone occurs to produce the CS state, mimicking well the photo synthetic electron transfer [45-47]. However, the CR rates of the CS state of porphyrin-quinone dyads are also fast and the CS lifetimes are mostly of the order of picoseconds or subnanoseconds in solution [45-47]. A three-dimensional it-compound, C60, is super-... 

This model was used successfully to explain the different solvent dependencies of the rates of photoinduced charge for the dyad 23(8) and the naphthalene analogue (i.e., the methoxy groups are replaced by hydrogens) [67], Equation 27 has recently been used to explain the interesting contrasting photoinduced electron transfer properties of the two closely related covalently linked porphyrin-naphthoquinone dyads 80 and 81 [178]. It was found that dyad 80, whose quinone carbonyl groups are 6.7 A from the center of the porphyrin donor, exhibits photoinduced ET whose... 

The first covalently linked porphyrin-based systems for mimicry of photosynthetic electron transfer were reported in the late 1970s. These were porphyrin-quinone dyad molecules 6 and 7 [45-47]. Subsequent to these reports, a very large number of porphyrin-quinone dyads have been studied, and still work in this field continues. Connolly and Bolton reviewed the work in the 1970s and 1980s [48]. Since that... 

Recently a number of covalently linked porphyrin-quinone systems such as IS (Malaga et al., 1984) or 16 (Joran et al., 1984) have been synthesized in order to investigate the dependence of electron-transfer reactions on the separation and mutual orientation of donor and acceptor. These systems are also models of the electron transfer between chlorophyll a and a quinone molecule, which is the essential charge separation step in photosynthesis in green plants. (Cf. Section 7.6.1.) Photoinduced electron transfer in supra-molecular systems for artificial photosynthesis has recently been summarized (Wasielewski, 1992). 

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