Photoactive dyads

2 Photoactive Dyads. - Many new molecular dyads, comprising donor-spacer-acceptor systems, have been described in the recent literature. These systems are intended to reproduce the essential electron-transfer steps occurring in natural photosynthetic organisms by eliminating as many components as possible. The main events in the natural apparatus involve light-induced electron 

Hybrid systems have been constructed in which a metal complex is covalently linked to an organic species so as to produce a donor-acceptor dyad, with either subunit functioning as the chromophore. Thus, ruthenium(II) tris(2,2 -bipyridyl) complexes have been synthesized bearing appended anthraquinone or tyrosine functions. Both systems enter into intramolecular electron-transfer reactions. With an appended anthraquinone moiety, direct electron transfer occurs from the triplet excited state of the metal complex to the quinoid acceptor. This is not the case with tyrosine, which is an electron donor, but the metal complex can be photooxidized by illumination in the presence of an added acceptor. The bound tyrosine residue reduces the resultant ruthenium(III) tris(2,2 -bipyridyl) complex 

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Figure 6. (a) Schematic representation of the four-module format of an ionically switchable photoactive dyad. This requires a shield module owing to the exceptional sensitivity of its emissive state towards collisional quenching. However, this emission can be put to good use, as discussed in the text. In many respects however, the situation in Figure 1 is applicable here. Note that the relative position of the dyad with respect to the shield can depend on whether the receptor is ion-bound or not. (continued)... 

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-... 

Harriman et al. [187] subsequently described a series of photoactive dyads bearing pyrene and metal (M = Ru or Os ) tris(2,2 -bipyridine) terminals bridged by an ethynylene group (Fig. 2.24). For the Os -based dyads, the triplet state re-... 

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