Photochemically induced charge separation

FIGURE 13.3 Intersystem crossing induced by charge transfer (photochemically induced dynamic nuclear polarization [photo-CIDNP]). The arrows indicate the CIDNP pathway. The fnll-drawn line from the charge-separated state ends on the local triplet and the dashed line on the ground state. 

Photochemically induced charge separation from the CT complex by visible light irradiation. 

Since the reduction potential of MV2+/MV is low enough (—0.44 V at pH 7) to reduce protons, the presence of platinum as a catalyst in the solution containing MV 7 brings about hydrogen formation. Scheme 1 is a typical model of photo-induced charge separation and electron relay to yield H2. It also represents the half reaction cycles of the reduction site for the photochemical conversion shown in Fig. 3. 

Unfortunately, turnover control of PSII is more complicated than the above description would indicate. Because turnover of the S states is achieved via a photochemical reaction, the yield of the reaction depends on both the electron donors and the electron acceptors. The overall picture of electron transfer in PSII is shown in Figure 2 (II). Light induces a series of electron-transfer reactions that lead to the formation of progressively more stable charge-separated states. The dominant reaction under physiological conditions leads to a one-step advancement of the S state and reduction of the secondary quinone electron acceptor (Qb). In purified PSII preparations, however, the quinones are depleted and the QB site will mostly be unoccupied unless exogenous quinones are added. 

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. 

Fig. 5. Indirect redox titration of FeS-X (A) Light-induced EPR changes of P700 in TSF-I particles as a function of redox potential at pH 10 and at 15 K (B) Plot of the extent of dark decay of the EPR signals in (A) at pHs 8, 9 and 10 (C) Plot of the initial amplitude of the light-induced EPR changes in (A) at pHs 8, 9 and 10. Figure source Ke, Dolan, Sugahara, Hawkridge, Demeter and Shaw (1977) Electrochemical and kinetic evidence for a transient electron acceptor In the photochemical charge separation in photosystem I, in Photosynthetic Organelles (special issue of Plants Celt Physiol) pp. 195, 196.

The applicability of zeolite-based molecular multicomponent systems for light-induced charge separation or photochemical Hz evolution has been demonstrated impressively [97]. 

Photochemically Induced Charge Separation in Electrostatically Constructed Organic-Inorganic Multilayer Composites... 

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