Homogeneous electron/energy transfer

ZnO (suspension) sensitizes the photoreduction of Ag" by xanthene dyes such as uranin and rhodamine B. In this reaction, ZnO plays the role of a medium to facilitate the efficient electron transfer from excited dye molecules to Ag" adsortei on the surface. The electron is transferred into the conduction band of ZnO and from there it reacts with Ag. In homogeneous solution, the transfer of an electron from the excited dye has little driving force as the potential of the Ag /Ag system is —1.8 V (Sect. 2.3). It seems that sufficient binding energy of the silver atom formed is available in the reduction of adsorbed Ag" ions, i.e. the redox potential of the silver couple is more positive under these circumstances. 

Similar to homogeneous electron-transfer processes, one can consider the observed electrochemical rate constant, k, , to be related to the electrochemical free energy of reorganization for the elementary electron-transfer step, AG, by... 

Figure 4.5, Potential energy diagrams for the homogeneous electron transfer reaction between an aromatic radical-anion and a second aromatic with a frangible R-X bond, (a) The situation where back electron transfer and bond cleavage have similar free energy of activation, (b) The situation where the RX radical-anicm has high energy and the R-X bond has low dissociation ertergy.

The photo-oxidation of PE sensitized by DC A in homogeneous solution followed by reduction of the reaction mixture with sodium sulfite solution gave the ene product pinocarveol 14 and the non-ene products myrtenal 15, epoxide 16 and aldehyde 17, as shown in Fig. 21. The ene product and the non-ene products have been proposed to be derived from the energy-transfer and electron-transfer pathways, respectively [177-181], The product distributions in acetonitrile is given in Fig. 21. 

Interestingly, in a homogeneous series of dyads where triplet energy transfer (Eq. 3.12), electron transfer (Eq. 3.13), and hole transfer (Eq. 3.14) processes... 

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