Porphyrin-quinone complexes

Tien and co-workers [100, 238] observed a photopotential and photocurrent arising in the planar BLM containing bridging molecules with a porphyrin bound covalently to a quinone (see System 44 of Table 1). The size of this porphyrin-quinone complex was not large enough to span across the whole of the membrane, therefore the mechanism of the arising photoeffect is most likely to be similar to those discussed in Sect. 2.4 for other BLMs doped with photosensitizers. 

As shown by Nango et al. [243] for the porphyrin-quinone complex as an example, even when an electron travels via intramolecular transfer only a part of its way across the membrane, the observed overall rate of transmembrane electron transport can be notably increased as compared to transport via diffusion only. 

Kong J. L. Y. and Loach P. A. (1980), Syntheses of covalently-linked porphyrin-quinone complexes , J. Heterocyclic Chem. 17, 737-744. 

The first example of a sideways-linked porphyrin-quinone complex was presented by Harriman, Sessler and coworker in 1992. Porphyrin 127 has a guanine subunit bound to one of four me.so positions, while p-benzoquinone as an electron acceptor is linked with a cytosine subunit to... 

Ibrm a Waison-Crick hydrogen-bonding inieraction. The association conslant derived from a H NMR study is 3.1 X lO M in CDiCli. Upon addition of die quinone. the fluorescence of the zinc porphyrin was quenched and its decay profile became biphasic with two exponential components of lifetimes, x, —1.5 ns and rs = 0.94ns, respectively. Both lifetimes did not depend on the concentration of the quinone, while the relative contribution of the shorter component incieased as the quinone concentration w as increa.sed.. suggesting a porphyrin-quinone complex formation. The rate constant of forward ET from photoexcited porphyrin to quinone was estimated to be 4.2 X lO s... 

Other intramolecular reactions showing an inverted region include those of several porphyrin-quinone complexes [21,f ] and of a ruthenated haem-protein [34,e]. These systems provide excellent data for the quantitative study of Marcus-type models cf. Sections 9.2.2.3 and 9.2.2.4 which follow. 

Kong, J., and Loach, P. A. (1978) Covalently-linked porphyrin quinone complexes as RC models, in P. C. Dutton, J. S. Leigh and H. Scarpa (eds.). Frontiers of Biological Energetics From Electrons to Tissues, Vol. 1, Academic, NY, pp. 73-82. 

Another example of compounds with the fixed mutual location of porphyrin and quinone are the porphyrin-quinone compounds with a rigid bridge. Charge photoseparation in P-L-Q molecules in which L is the trip-ticene bridge, P is tetraphenylporphin, TPP, or its zinc complex, and Q is benzoquinone, BQ, naphthoquinone, NQ, or anthraquinone, AQ, has been studied [55]. The distance between the centres of P and Q fragments in these... 

An entirely distinct series of model complexes has been carried out in order to show that metal porphyrins will actually bind to the type of substrate with which P-450 interacts. Hill, Macfarlane, Mann, and Williams (51) have studied molecular complex formation between such molecules as quinones and sterols and several metal porphyrins. The complexes between some of the porphyrins and sterols are remarkable strong. At the same time they have devised NMR methods for the elucidation of the structures of these complexes. 

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]. 

The distance dependencies of photoinduced electron transfer rates have been examined in anthracene-spacered porphyrin-quinone cyclophanes, and the same authors have also discussed the distance dependencies of photo-induced electron-transfer rates in benzene-, naphthalene-, and anthracene-spacered porphyrin-quinone cyclophanes and biphenylene-spacered porphyrin-quinone cyclophanes. Photoelectron transfer reactions of the porphyrin-quinone cyclophanes (3) and their zinc complexes have been examined, and in some cases at least interaction of the quinone carbonyl group with the zinc atom may be an alternative to through-space electron transfer. A study of intramolecular photoinduced electron transfer for the quinone-porphyrin cyclophane type (4) containing the especially strong acceptor 7,7,8,8-tetracyanoquinodimethane (TCNQ) has appeared." The distance dependence of the TCNQ and porphyrin is of particular interest, and to this end the corresponding 2,8-naphthalenediyl-TCNQ-porphyrin has been synthesised. 

Pulse radiolysis has been used to study elementary reactions of importance in photosynthesis. Early experiments provided rate constants for electron transfer reactions of carotenoid radical cations and radical anions with chlorophyll pigments.More recent experiments dealt with intramolecular electron transfer in covalently bound carotenoid-porphyrin and carotenoid-porphyrin-quinone compounds. Intramolecular electron transfer reactions within metalloproteins have been studied by various authors much of that work has been reviewed by Buxton, and more recent work has been published. Pulse radiolysis was also used to study charge migration in stacked porphyrins and phthalocyanines. Most of these studies were carried out by pulse radiolysis because this techruque allowed proper initiation of the desired processes and pemtitted determination of very high reaction rate constants. The distinct character of radiolysis to initiate reactions with the medium, in contrast with the case of photolysis, and the recent developments in pulse radiolysis techniques promise continued application of this technique for the study of porphyrins and of more complex chemical systems. 

More useful mechanistic information is obtained from intramolecular electron-transfer reactions if the kinetics for the electron-transfer step can be isolated from the effects of diffusion. The main stimulus for making such studies is the urge to design systems that mimic some of the essential features of the photosynthetic reaction centre complex and much attention has focussed on the study of porphyrin-based photoactive dyads. Thus, a series of N-alkylporphyrins linked to a quinolinium cation has been synthesized and found to display a rich variety of photoreactions. The singlet excited state of the quinolinium cation operates in both intramolecular energy- and electron-transfer reactions while the excited singlet state of the porphyrin transfers an electron to the appended quinolinium cation. Several new porphyrin-quinone dyads have been studied,including cyclophane-derived systems where the reactants are held in a face-to-face orienta-... 

A similar incremental effect of porphyrin-quinone separation was observed with the systems shown in Scheme 36 which were prepared by Wittig condensation of the meso-substituted porphyrin 116 (as the nickel complex) with the phosphorus ylide 117 Demethylation, reduction of the double bonds and then oxidation furnished the free base porphyrins 118 and 119a, b. The rate of photoinduced electron transfer in such systems showed an inverse exponential dependence on the length of the chain In order to demonstrate a multistep electron transfer the bis-quinone porphyrin 120 was prepared in which the pair of quinone rings provide a redox potential gradient and may thus stabilize charge separation. Comparison with the mono-quinone etioporphyrin 119a... 

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