Triads carotenoid-porphyrin-quinone

Numerous systems of this type, such as the carotenoid-porphyrin-quinone triad 94 [8.62a] (for PeT in a pentad see [8.62b]), have been extensively studied in many laboratories from the photochemical point of view and as models of natural photosynthetic centres [8.62-8.69, A. 10, A.20], especially in order to achieve very fast charge separation [8.69] and slow recombination, for instance in multiporphyrin... 

C-P-Q Triad Molecules. As discussed above, the natural reaction center has solved the problem of energy loss due to rapid charge recombination by employing a multistep electron transfer strategy. The same strategy may be applied to the porphyrin-quinone type systems. As we pointed out in 1982 [28], this requires the addition of a secondary electron donor or acceptor moiety. This strategy came to fruition in 1983 when we reported the synthesis of carotenoid-porphyrin-quinone (C-P-Q) triad 2 [29, 30]. This molecule features porphyrin and quinone moieties similar to those found in 1, but a... 

The first biomimetic triads were the P-Q-Q compounds of Nishitani et al. (1983). In the carotenoid-porphyrin-quinone (C-P-Q) triad of Moore ef o/. (1984), shown on the next page,... 

Many other supermolecules containing carotenoids have been synthesized and their photochemistry characterized (Osuka et al., 1990 Moore et al 1994 Cardoso et al, 1996), but not yet investigated with EMR. One of these, a carotenoid- porphyrin-quinone triad has been used to demonstrate light-induced vectorial electron transfer across an artificial photosynthetic hposome membrane with subsequent production of ATP catalyzed by FoF -ATPase (Steinberg-Yfrachetal., 1998). 

Many compounds containing a donor and an acceptor joined by a flexible linker have been synthesized (see Ref. 185 for an extensive list of references). The difficulty in using such compounds to study the distance dependence of ET is that the flexibility of the tinker precludes knowing the donor-acceptor separation precisely. Distances have been estimated from fluorescence quenching volumes (61) and NMR conformational studies (60) with such compounds. NMR studies indicated that the methylene tinker between the porphyrin and the quinone in carotenoid-porphyrin-quinone triads is in an extended (all-anti) conformation (60). The calculated value of p, based on this conformation for the series of methylene linkers, was... 

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]. 

Recently, a new type of C-P-Q triad was reported in which the carotenoid and quinone moieties were linked to the tetraarylporphyrin via basket handle linkages between opposite aryl groups [65]. This linkage positioned the carotenoid and quinone species above and below the plane of the porphyrin, rather than to the side, as confirmed by H-NMR studies. Evidence for photoinitiated electron transfer in this molecule was provided by incorporating it into a phospholipid bilayer and detecting a light-induced photocurrent. Similar experiments had previously been reported with the other C-P-Q triads discussed above [55],... 

Several triads (as well as more complex systems, tetrads, pentads, etc.) have been successfully developed making use of organic molecular components [211-213], some of which (porphyrins, quinones, carotenoids) are reminiscent of those found in the natural systems (for detailed accounts, see Volume III, Part 2, Chapters 1 and 2). Remarkable efficiencies and lifetimes of charge separation have been reached with such systems. Their potential has been impressively demonstrated by incorporation into liposomal membranes performing the photo-driven synthesis of ATP [214]. 

Triads 4-6 are similar to 2, with the exception that the methylene chain joining the porphyrin and quinone moieties has been increased to two, three, and four CHj groups, respectively. Molecules 9-11 are the corresponding P-Q systems which lack the carotenoid. Triad 7 features a porphyrin-quinone linkage as in 2, but a methylene group in the carotenoid-porphyrin linkage. 

Transfer of calcium cations (Ca2 + ) across membranes and against a thermodynamic gradient is important to biological processes, such as muscle contraction, release of neurotransmitters or biological signal transduction and immune response. The active transport can be artificially driven (switched) by photoinduced electron transfer processes (Section 6.4.4) between a photoactivatable molecule and a hydroquinone Ca2 + chelator (405) (Scheme 6.194).1210 In this example, oxidation of hydroquinone generates a quinone to release Ca2+ to the aqueous phase inside the bilayer of a liposome, followed by reduction of the quinone back to hydroquinone to complete the redox loop, which results in cyclic transport of Ca2 +. The electron donor/acceptor moiety is a carotenoid porphyrin naphthoquinone molecular triad (see Special Topic 6.26). 

Finally, couphng this porphyrin quinone with the carotenoid acid chloride 16 yielded the target triad 1 in 53% yield. 

Another interesting molecule is the triad of carotenoid, porphyrin, and quinone all linked together by covalent bonds (Figure 7.13). Excitation of the porphyrin transfers an electron to the quinone, making the latter negatively charged. The electron deficiency on the porphyrin is then satisfied by... 

Recently it has been reported (3 ) that in a triad molecule where a porphyrin is juxtaposed between a carotenoid and a quinone, a charge transfer donor-acceptor pair with a lifetime similar to that found experimentally in biological systems was produced on light irradiation. It was suggested that an electrical potential similar to the type developed in this donor-acceptor pair may be important in driving the chemical reactions in natural photosynthesis. 

As mentioned above, the natural photosynthetic reaction center uses chlorophyll derivatives rather than porphyrins in the initial electron transfer events. Synthetic triads have also been prepared from chlorophylls [62]. For example, triad 11 features both a naphthoquinone-type acceptor and a carotenoid donor linked to a pyropheophorbide (Phe) which was prepared from chlorophyll-a. The fluorescence of the pyropheophorbide moiety was strongly quenched in dichloromethane, and this suggested rapid electron transfer to the attached quinone to yield C-Phe+-Q r. Transient absorption studies at 207 K detected the carotenoid radical cation (kmax = 990 nm) and thus confirmed formation of a C+-Phe-QT charge separated state analogous to those formed in the porphyrin-based triads. This state had a lifetime of 120 ns, and was formed with a quantum yield of about 0.04. The lifetime was 50 ns at ambient temperatures, and this precluded accurate determination of the quantum yield at this temperature with the apparatus employed. 

Since the initial reports of the C-P-Q triads, a number of other molecules of the D-D -A or D -D-A types have been described. Triad 12, prepared by Wasielewski and coworkers, is a relative of the C-P-Q series in which the secondary donor is an aniline derivative (D), rather than a carotenoid [63]. The bicyclic bridges were introduced in order to add rigidity to the system. The fluorescence lifetime of the porphyrin moiety of 12 was found to be <30ps. This result is consistent with rapid electron transfer to the quinone to yield D-P+-QT. This result was confirmed by transient absorption measurements. The absorption results also revealed that this intermediate charge separated state decays with a rate constant of 1.4 x 1010 s-1 to a final charge separated state D+-P-Qr. Thus, the decay pathways are similar to those shown in Fig. 3 for the C-P-Q triads. This final state has a lifetime of 2.45 ps in butyronitrile (which is similar to that found for 4 in acetonitrile) [44], and is formed with a quantum yield of about 0.71. Thus, the efficiency of the transfer analogous to step 4 in Fig. 3 for this molecule is also about 0.71. 

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