Covalently linked donor-acceptor system

The half-deprotected product was obtained in 65% yield. Of course, such a yield is insufficient from a synthetic point of view, the photovariant of the redox reaction is not simple instrumentally, and the duration of the reaction (5 h) is too long. Nevertheless, this approach is promising, and deserves attention and development. Thus, the photochemical method proved to be successful in the removal of protecting groups based on covalently linked donor-acceptor systems (Lee and Falvey 2000). 

Although covalently linked donor-acceptor systems of small organic chromo-phores have been studied for some time in order to uncover the basic principles of electron and energy transfer, the first covalently linked cyclic tetrapyrroles were reported by Gouterman, Dolphin and coworkers in 1972 [27]. In 1976 the first dimeric chlorophyll-based models were reported. Structure la, based upon pyropheophorbide-a, was prepared by Boxer and Closs [28], whereas the pheo-phorbide-a derivative lb was reported by Wasielewski, Studier and Katz [29]. 

Fig. 15. Example of a covalently linked donor-acceptor system featuring a photochromic control moiety 11) for the downregulation of photoinduced electron transfer under intense light conditions. Adapted from Ref. (133).

Some references of reviews besides the ones already cited are given [1,3, 5-9, 19, 23-25, 28, 31, 33]. Organometallic photochemistry [36] was excellently treated in [37] and may be compared with inorganic photochemistry to gain further inspiration [38-40]. A recent multiauthored book strongly overlaps with the subject matter of the present section, and should certainly be consulted [41]. Electron transfer reactions play a determinant role in many photocatalytic processes several recent reviews and books may be cited on this topic [42-44]. The photochemistry of the M-CO bond [45] and the theme of photocatalysis by transition metal complexes [46] have recently been reviewed. Covalently linked donor-acceptor systems for mimicry of photosynthetic energy transfer have been discussed in [47]. Several special issues of Coordination Chemistry Reviews have been devoted to the photochemistry and photophysics of coordination compounds [48-50], and a special issue to photochemistry [51]. Further developments in photochemistry were the subject of a special issue of Chemical Reviews [52]. Practical considerations useful for designing photochemical experiments may be found in [53]. 

Fig. 2 Examples of covalently linked donor-acceptor systems (dyads) used for the study of photoinduced electron transfer across organic spacers. Boxes are drawn to identify the photoexcitable chromophore (left), the bridge (center), and the acceptor unit (right).

Flamigni, L. and M.R. Johnston (2001). Photoinduced electron transfer in a non-covalently linked donor-acceptor system A bis-porph5rinic host and a naphthalene diimide guest. Afew. J. Chem. 25(11), 1368-1370. 

Figure 15.19 Molecular structures of some covalently linked donor-acceptor systems.

The study of photoinduced ET in covalently linked donor-acceptor assemblies began with comparatively simple dyad systems which contain a transition metal center covalently linked to a single electron donor or acceptor unit [26]. However, work in this area has naturally progressed and in recent years complex supramolecular assemblies comprised of one or more metal complexes that are covalently linked to one or more organic electron donors or acceptors have been synthesized and studied [27-36]. Furthermore, several groups have utilized the useful photoredox properties of transition metal complexes to probe electron and energy transfer across spacers comprised of biological macromolecules such as peptides [37,38], proteins [39,40], and polynucleic acids [41]. 

In many cases it will be seen, that the simple five-step scheme given above is not sufficient to describe more complex systems, i.e., donor-acceptor couples in solution or covalently linked donor acceptor couples. More sophisti-... 

Gust, D, and Moor, T.A. (eds) (1989) Covalently linked donor-acceptor photosynthetic model systems. Tetraedron 45 (special issue)... 

One of the most versatile electron-accepting molecules is the quinonoid compound, and the redox reaction of the quinone-hydroquinone couple is one of the most thoroughly studied proton-coupled electron transfer systems of organic molecules. Quinones show the reversible two-step le reduction in aprotic organic solvents (Fig. 2). One-electron addition to quinone forms the semiquinone radical with five n electrons. The stability of the semiquinone form is affected by the existence of a minute amount of proton, which appears as the large shift of the reduction potentials in the positive direction. This implies that quinonoid compoimds are representative acceptor molecules of which redox properties are influenced by external perturbation, such as protonation and solvation (Fig. 2). They are employed in covalently and noncovalently linked donor-acceptor systems of particular interest in the study of proton-coupled electron transfer and photoin-duced electron transfer. ... 

Covalently linked donor-acceptor molecules. One can gain greater control of the kinetics of electron transfer reactions in these systems by fixing the distance between donor and acceptor, and by introducing a third redox molecule, which serves as either a secondary donor or accrator, into the chain. For example, we have shown that a Ru(bpy)32+-diquat + donor-... 

Paddon-Row MN (1994) Investigating long-range electron-transfer processes with rigid, covalently linked donor-(norbomylogous bridge)-acceptor systems. Acc Chem Res 27 18-25... 

Paddon-Row MN (2003) Superexchange-mediated charge separation and charge recombination in covalently linked donor-bridge-acceptor systems. Aust J Chem 56 729-748... 

M. N. Paddon-Row, Investigating Long-Range Electron-Transfer Processes With Rigid, Covalently Linked Donor-(Norbornylogous Bridgel-Acceptor Systems , Acc. Chem. Res. 1994, 21,18-25. 

A large number of covalently linked systems are currently being synthesized and investigated, differing in the nature of A, B, and L, as well as in the number of functional units in the supramolecular system (nuclearity). It is common to call simple two-component donor-acceptor systems such as that of Eq. 2 dyads , and progressively more complex systems triads , tetrads , pentads , etc.. Systems where all the A and B units are organic molecules are dealt with in Chapter 1 of this section. The present chapter deals with systems where at least one of the A/B functional units is a transition metal coordination compound. From this definition, however, are excluded (a) systems where A and/or B are porphyrins or related species (dealt with in Chapter 2) and (b) systems of high nuclearity with dendritic structures (dealt with in Chapter 9). 

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). 

Films prepared by co-deposition of a polycyclic aromatic hydrocarbon (PAH) and an electron-poor PAH (EPPAH) (Fig. 27) exhibit an oblique arrangement reflecting the formation of epitaxial composite layers of the electron donors and acceptors [81]. Interestingly, on top of a layer of donor molecules, within the same layer between two physisorbed donor molecules, acceptor molecules are co-adsorbed in a well-defined arrangement. When the acceptor co-crystallizes, the second PAH layer is stabilized (Fig. 27). These studies were extended to covalently linked electron donor-acceptor systems [82,83]. 

Covalently Linked, Sigma Donor-Acceptor Systems... 

---