Photosensitized electron transfer reactions

Cobalt(III) cage complexes can also perform as electron transfer agents in the photoreduction of water.180181 Because of the kinetic inertness of the encapsulated cobalt(II) ion, the cobalt(II)/co-balt(III) redox couple can be repeatedly cycled without decomposition. Thus these complexes are potentially, useful electron transfer agents, e.g, in the photochemical reduction of water, in energy transfer and as relays in photosensitized electron transfer reactions.180,181 The problem of the short excited-state lifetimes of these complexes can be circumvented by the formation of Co(sep)3+ ion pairs, so that the complexes can be used as photosensitizers for cyclic redox processes.182 183... 

Photosensitized Electron-Transfer Reactions in Organized Systems... 

Charged colloids and water-in-oil microemulsions provide organized environments that control photosensitized electron transfer reactions. Effective charge separation of the primary encounter cage complex, and subsequent stabilization of the photoproducts against back electron transfer reactions is achieved by means of electrostatic and hydrophobic interactions of the photoproducts and the organized media. 

Figure 1. Conversion of light energy to chemical potential by means of photosensitized electron transfer reactions.

A recently reported regioselective photoreduction of benzoates by photosensitized electron transfer reaction was applied to nucleosides [85]. In presence of jV-methyl-carbazole as the electron donor sensitizer and in an isopropanol, water solution, w-trifluoromethylbenzoates of adenosine 83 or benzoates of uridine 84 give deoxygenated products in good yields (73 %). 

Photosensitized electron transfer reactions conducted in the presence of molecular oxygen occasionally yield oxygenated products. The mechanism proposed to account for many of these reactions [145-147] is initiated by electron transfer to the photo-excited acceptor. Subsequently, a secondary electron transfer from the acceptor anion to oxygen forms a superoxide anion, which couples with the donor radical cation. The key step, Eq. (18), is supported by spectroscopic evidence. The absorption [148] and ESR spectra [146] of trans-stilbene radical cation and 9-cyanophenanthrene radical anion have been observed upon optical irradiation and the anion spectrum was found to decay rapidly in the presence of oxygen. 

The contents of this chapter have been organized into two main sections. Section 2, describes the photosensitized electron transfer reaction where either acceptor or donor molecule is regenerated after the initial PET processes. This includes reaction originating both from radical cations and radical anions. Section 3, deals with the intermolecular addition reaction between donor-acceptor pairs. 

Therefore, the main aim in all the photosensitized electron-transfer reactions, including oxygenations, is to prolong the lifetime of the intermediate radical ions maximizing their cage escape efficiency, such that the dark chemistry of the oxidized and/or reduced species can be more easily controlled. 

Magnetic Field Effects on Photosensitized Electron Transfer Reactions in the Presence of TiCk and CdS Loaded Particles. No effect of magnetic field is seen up to 4000 G. 502... 

The separation of photoproducts formed in photosensitized electron transfer reactions is essential for efficient energy conversion and storage. The organization of the components involved in the photo-induced process in interfacial systems leads to efficient compartmentalization of the products. Several Interfaclal systems, e.g., lipid bllayer membranes (vesicles), water-in-oil mlcroemulslons and a solid SIO2 colloidal Interface, have been designed to accomplish this goal. 

In the water-ln-oll mlcroemulslon the separation of photoproducts is achieved by means of the hydrophilic-hydrophobic nature of the products. A two compartment model system to accomplish the photo-decomposition of water is described. Photosensitized electron transfer reactions analogous to those occurring in the two half-cells are presented. In these systems the phase transfer of one of the photoproducts into the contlnuotis oil phase is essential to stabilize the photoproducts. 

In this article we will discuss several approaches to the design of organized and controlled photosensitized electron transfer reactions. Special emphasis will be given to the utilization of the stored energy In the photodecoDq>osltlon of water (eq.2). 

Since these interfaces are usually constructed of charged detergents a diffuse electrical double layer is produced and the interfacial boundary can be characterized by a surface potential. Consequently, electrostatic as well as hydrophilic and hydrophobic interactions of the interfacial system can be designed. In this report we will review our achievements in organizing photosensitized electron transfer reactions in different microenvironments such as bilayer membranes and water-in-oil microemulsions.In addition, a novel solid-liquid interface, provided by colloidal Si02 particles in an aqueous medium will be discussed as a means of controlling photosensitized electron transfer reactions. 

The photosensitized electron transfer reaction forms the reduced lipophilic electron acceptor BNA which is ejected into the continuous organic phase and thus separated from the oxidized product. In order to monitor the entire phase transfer of the reduced acceptor, BNA, a secondary electron acceptor, p-dlmethyl-amlnoazobenzene (dye),was solubilized in the continuous oil phase. The photochemically induced electron transfer reaction in this system results in the reduction of the dye (0 = 1.3 x 10 3). Exclusion of the sensitizer or EDTA or the primary electron acceptor, BNA, from the system resulted in no detectable reaction. Substitution of the primary acceptor with a water soluble derivative, N-propylsulfonate nicotinamide, similarly results in no reduction of the dye. These results indicate that to accomplish the cycle formulated in Figure 6A the amphiphilic nature of the primary electron acceptor and its phase transfer ability in the reduced form are necessary requirements. 

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