Porphyrin-quinone systems

M. Antolovich, P. J. Keyte, A M. Oliver, M. N. Paddon-Row, J. Kroon, J. W. Verhoeven, S. A Jonker, J. M. War-man, Modelling Long-Range Photosynthetic Electron Transfer in Rigidly Bridged Porphyrin-Quinone Systems ,/. Phys. Chem. 1991, 95,1933-1941. 

Investigation of photoinduced intramolecular ET processes in such systems has in general shown that, compared to analogous porphyrin/quinone systems, i. charge separation occurs with higher efficiency and ii. charge-separated states have longer lifetimes (Scheme 9.4). Such a behavior is the result of the combination of two... 

Modelling of the intermediate step of the charge separation process during photosynthesis with porphyrin-quinone systems... 

A number of covalently bound porphyrin-quinone systems have recently been synthesized as models for carefully spaced donor-acceptor systems. These are designed to test the possibility of controlling electron transfer rates by spatial separation of donor-acceptor pairs. Among these exceedingly clever studies (30) are molecules designed to separate the donor and acceptor by rigid insulating molecules, for example, 2 (31),... 

Modelling of the Intermediate Step of the Charge Separation Process During Photosynthesis with Porphyrin-Quinone Systems... 

In these devices, the acceptor is joined to two donors which can in principle transfer electrons to it with comparable efficiencies. From the point of view of long-lived charge separation, molecules of this type offer no apparent advantages over simple D-A systems. However, as mentioned above, they may be useful in some other situations, especially as components of larger devices when A is a 2-electron acceptor. There are a few examples of porphyrin-quinone systems with this structure in the literature. 

The basic photophysical behavior of porphyrin-quinone systems is illustrated by dyad 8, in which a naphthoquinone derivative is linked to a porphyrin via an amide bond [61], The photophysics of this molecule has been investigated using techniques... 

The ionic separation may not be the only factor giving rise to the behavior of 19, as the photoinduced electron transfer rate constant does not change appreciably, even when the driving force is increased by 0.4 eV through introduction of zinc into the porphyrin macrocycle. In this connection, very rapid photoinduced electron transfer has been observed in other zinc-containing porphyrin-quinone systems [80, 86], and photoinduced electron transfer in zinc-containing porphyrin dyads has... 

We reported the first example of such a compound, porphyrin-fullerene dyad 23, in 1994 [134, 135]. Since that paper, a number of examples of dyads consisting of fullerenes linked to porphyrins or phthalocyanines have been prepared [136, 137-159]. Some of these, such as 24, use amide linkages related to those employed in some porphyrin-quinone systems [137, 149, 152]. In dyad 25, the moieties are linked by an extended, rigid bridge, of the type that has been shown to facilitate rapid long-range electron transfer in other systems [146, 147]. A number of dyads feature pyrrolidine-functionalized fullerenes, as in 26 and 27 [90, 139, 145, 148]. 

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). 

It is surprising that the rate of photodriven electron transfer in 17 is as great as it is. It was noted above that simple electron transfer theories predict an exponential dependence of electron transfer rates on donor-acceptor separation. Calculations based on an estimate of the donor-acceptor distance in 17 and the quantitative dependence of electron transfer on distance found for other porphyrin-quinone systems [27, 62-64] suggest that the quantum yield of formation of C-P -QA(OMe)2-Qr should be near zero. It seems likely, then, that the dimethoxynaphthalene 7t-electron system and perhaps the bicyclic bridge are playing some role in the electron transfer process. 

In 1993, Kuroda and coworkers reported a unique porphyrin-quinone system using cyclodextrin-sandwiched porphyrin 138 as a photodonor.In this model. CD is fixed above the porphyrin plane, and the electron acceptor receives an electron from the photoexcited poiphyrin in... 

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