Porphyrin donors

Helms A, Heiler D, McLendon G (1992) Electron transfer in bis-porphyrin donor-acceptor compounds with polyphenylene spacers shows a weak distance dependence. J Am Chem Soc 114 6227-6238... 

Figure 17. Three dyads possessing Zn(ll) porphyrin donor and Fe(lll) porphyrin acceptor linked by (from top to bottom) H-bonded bridge, a partially unsaturated bridge and a saturated bridge.1341...

The bis-dienes 13-15 form ladder-type bis-fullerene adducts. Cycloadditions proceed in good yields also with electron-deficient dienes such as 21-23. Porphyrin donor systems such as 25 or 30 or quinoidal acceptor systems such as 19 and 20 were introduced to study the properties of the charge transfer in Cjq donor-acceptor systems. 

Figure 4.12 Examples of bichromophoric molecules used for the study of intramolecular electron transfer, (a) Dimethoxynaphthalene electron donors dicyanoethylene electron acceptor with rigid spacers (b) porphyrin donor and quinone acceptors separated by flexible spacers...

Sanders and coworkers have prepared a cyclic molecule featuring two zinc porphyrin donors linked to two pyromellitimide acceptors as a precursor to 32 [99, 100]. The photochemical and photophysical properties were not discussed. 

Figure 45. A clip-shaped dyad 55 for investigating superexchange through aromatic systems. The resorcinol 56 becomes complexed to 55 by H-bonding interactions, shown schematically by 57. Fluorescence quantum yields (CCI4) reveal that the porphyrin donor fluorescence is strongly quenched in 57, indicating that ET from the porphyrin to the quinone is occurring through the complexed resorcinol by a superexchange mechanism [134],...

This model was used successfully to explain the different solvent dependencies of the rates of photoinduced charge for the dyad 23(8) and the naphthalene analogue (i.e., the methoxy groups are replaced by hydrogens) [67], Equation 27 has recently been used to explain the interesting contrasting photoinduced electron transfer properties of the two closely related covalently linked porphyrin-naphthoquinone dyads 80 and 81 [178]. It was found that dyad 80, whose quinone carbonyl groups are 6.7 A from the center of the porphyrin donor, exhibits photoinduced ET whose... 

An interesting and much more complex system is 50, which consists of a rigid array containing a porphyrin donor and two electron acceptors a quinone and a methyl... 

It is noteworthy that the rate of reduction of the Zn porphyrin j-radical cation by the copper(I) complex is much faster than direct charge recombination between the porphyrin radicals, although the reaction exoergonicity for Eq. 19 is very much less than that for Eq. 18. The faster rate for Eq. 19 may arise because of the closer proximity of the reactants and/or because electron transfer occurs via a different route. In addition, /ci9 shows some dependence on the porphyrin donor. This effect could be due to a different spin multiplicity of the precursor excited state. 

The studies described in this section were started shortly after the X-ray crystal structure of the RC of Rh. viridis was disclosed [73]. During these years, the role of the so-called accessory bacteriochlorophyll BCh was under debate [79]. In particular, the possibility was considered that it could play the role of a superexchange relay between SP and BPh (see Figure 22). In this respect, the copper(I)-complexed [2]rotaxane Cu.20+ represented a functional artificial model of the SP/BPh/BCh triad, the central Cu complex fragment between the Zn porphyrin donor and the Au porphyrin acceptor mimicking the function of BCh between SP and BCh. However, the kinetic scheme shown in Figure 22a has been revised, being now quite firmly established that (at least at room temperature) BCh is directly involved in the electron transfer reaction the transfer from the electronically excited special pair SP to BCh takes about 3 ps, and the next transfer step to the BPh, 0.65 ps [80]. In the earlier experiments, detection of the intermediate state SP+BCh was prevented by its relatively slow population and fast decay. 

The self-assembled diad Zn P-PH2P consisting of a zinc porphyrin donor and a free base porphyrin acceptor (Scheme 7.4) was studied by time-resolved fluorescence [21]. The driving force of the assembly is the site selective binding of an imidazole connected to a free base porphyrin. Evidence for Forster back transfer was obtained from the analysis of the fluorescence decay (Fig. 7.8) and the relevant rate was quantitatively evaluated for the first time. The transfer efficiency [13] is 0.98, and the rate constants for direct and back transfer were found to be 24.4 x 10 s and 0.6 X 10 s respectively. These values are consistent with the Forster energy transfer mechanism. 

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