Porphyrin light-induced charge separation

Porphyrins are ideal for investigations of light-induced charge separation and oxygen transport in vesicular solutions. Their intense Soret band at 400 nm allows measurement at concentrations down to 10 M their fluorescence spectra being detectable even at 10 M. Photo- and redox chemistries are extremely variable depending on the central metal ion and substituents. ... 

Light-induced charge separation was treated in the porphyrin section 4.4, activities of entrapped dyes in section 4.5. In the following, we discuss the effect of vesicle viscosity on photoreactions and photochemical labelling. 

SDS micelles have also been used as a basis for light-induced charge separation processes. A hydrophobic, photooxidizable dye (e.g., a zinc porphyrinate) was, for example, dissolved in an anionic SDS micelle with copper(II) counterions. Upon excitation with visible light an electron was transferred first and very fast to the copper(II) coating, which then was reoxidized by anionic ferricyanide in the bulk water phase. The reduced ferrocyanide ion formed did not react with the oxidized porphyrin, because the anionic micelles and reductant repelled each other and the ferrocyanide was highly diluted by ferricyanide (Fig. 2.5.5). The energy of sunlight has thus initiated a simple vectorial reaction in a primitive membranous system. 

Only the lateral assembly produces an 850 nm signal upon flashing, indicating formation of a porphyrin anion radical presumably resulting from light-induced charge separation. 

More complex triad porphyrin compounds containing a carotenoid or chlorophyll-like substituent in addition to the quinone have also been synthesized. One example is given in Scheme 7.2.9. Light-induced charge separations are fast in such molecules (10 °-10" s ), but the recombination rates are almost equally fast (10 -10 ° s ). Neither carotenes nor chlorophylls help in the stabilization of charge-separated states. Laser flash photolysis of chloro-... 

The efficiency of light-induced charge separation and electron-transport through the bilayer lipid walls have been examined recent-ly3.3 3.39 for a variety of redox systems in lipid vesicles. Of particular interest has been the redox reactions sensitized by porphyrins such as chlorophyll-a and derivatives of RuCbpy) ". Gratzel, Fendler and others have investigated recently electron-transfer and... 

The porphyrin pairs can then be used for light-induced charge separation, the aqueous volume in between can be filled up with phenol relay systems. Such systems will be especially useful if they are transferred from electrodes to colloidal particles, such as gold or silicate (Figure 4). 

Figure 7.2.5 Light-induced electron transfers in amide- and ester-linked porphyrin-quinone pairs. The resulting long-lived ESR spectrum shows porphyrin as well as semi-quinone anion radical signals, (a) n = 2, x = VH (h) n = 3, x = NH observed signals for intramolecular charge transfer (c, d) calculated spectra for the sum of porphyrin and quinone radicals (e) asymmetrical signal for intramolecular charge separation. (From McIntosh et al., 1983.)...

Diatninoporphyrin was reported to form face-to-face aggregate tubules with an outer diameter around 30 nm and an average length of 800 nm in aqueous solution at pH S [716], Light irradiation to the tubules was observed to induce a formation of porphyrin cation and anion radicals by charge separation (51). 

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