History electron transfer processes

The sensitization of electrodes to visible light by dye molecules is an old area of science with a rich history [2]. A dye-sensitized photoefifect was measured at a semiconductor surface as early as 1887 in Vienna [3]. The accepted mechanisms for the dye sensitization of electrodes emerged from photoelectrochemical studies in the 1960s and 1970s [4-6]. These studies were motivated by a desire to quantify interfacial electron transfer processes and develop cells useful for solar energy conversion. The two most common approaches are shown schematically in Figure 1. 

An outer-sphere electron-transfer process should be independent of the nature of the electrode and of the electrolyte medium. Nevertheless, experimental electron-transfer rates are strongly dependent on the electrodes and of their surface history. 

The ability of a nltro group in the substrate to bring about electron-transfer free radical chain nucleophilic subsdnidon fSpj li at a saniratedcarbon atom is well documented. Such electron transfer reacdons are one of the characterisdc feanires of nltro compounds. Komblum and Russell have established ihe Spj l reaction independently the details of the early history have been well reviewed by them. The reacdon of -nitrobenzyl chloride v/ith a salt of nitro ilkane is in sharp contrast to the general behavior of the ilkyladon of the carbanions derived from nitro ilkanes here, carbon ilkyladon is predominant. The carbon ilkyladon process proceeds via a chain reacdon involving anion radicals and free radicals, as shovmin Eq. 5.24 and Scheme 5.4 fSpj l reacdoni. 

The early history of redox initiation has been described by Bacon.23 The subject has also been reviewed by Misra and Bajpai,207 Bamford298 and Sarac.2,0 The mechanism of redox initiation is usually bimolecular and involves a single electron transfer as the essential feature of the mechanism that distinguishes it from other initiation processes. Redox initiation systems are in common use when initiation is required at or below ambient temperature and drey are frequently used for initiation of emulsion polymerization. 

Since electroanalytical response ultimately depends on electron transfer between an electrode material and a solution species, any intervening layers or films are of obvious consequence. As noted earlier, carbon materials are prone to adsorption, and it is not trivial to prepare carbon surfaces that are not contaminated with adsorbed layers, usually of adventitious organic impurities. In order to avoid poor or irreproducible responses from carbon electrodes, the user must either find conditions where surface impurities have negligible or reproducible effects on the process of interest, or prepare the carbon surface in a way that avoids surface films. As is evident from the history of electroanalytical chemistry since the DME, surface cleanliness is a major issue with solid electrodes, no less so for carbon. 

Electrode Pretreatment. There is ample evidence that the rate of electron transfer at a solid electrode is sensitive to the surface state and previous history of the electrode. An electrode surface that is not clean usually will manifest itself in a voltage-sweep experiment to give a decrease in the peak current and a shift in the peak potential. Various pretreatment methods have been employed to clean or activate the surface of electrodes the process is intended to produce an enhancement of the reversibility of the reaction (i.e., produce a greater rate of electron transfer).97 This activation or cleaning process may function in two ways by removing adsorbed materials that inhibit electron transfer and by altering the microstructure of the electrode surface. 

Koko Maeda, contributed a chapter Hexaarylbiimidazolyl The Choice Initiator for Photopolymers, History of Synthesis and Investigations of Photochemical Properties , and Yi Lin, Andong Liu, Alexander D. Trifunac and Vadim Krongauz c chapter Investigation of Electron Transfer Between Hexaarylbiimidazoles and a Visible Sensitizer in Krongauz and Trifunac s Processes in Photo-Reactive Polymers, Chapman Hall (1995). 

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