Multi-step electron-transfer process

In Fig. 13.13, the CR rates in the Marcus inverted region are much slower than the CS rates from both the singlet and triplet excited states in the Marcus normal region. This allows a subsequent electron transfer from an additional electron donor such as ferrocene (Fc) to ZnP+ in the triad molecule (Fc-ZnP+ -C60 ) to produce the final CS state, Fc+-ZnP-C60 , in competition with the back electron transfer in the initial CS states [41]. Such multi-step electron-transfer processes are expanded to the tetrad molecule (Fc-ZnP-H2P-C60) as shown in Fig. 13.16a [50], In the final CS state, Fc+-ZnP-H2P-C60 , charges are separated at... 

The best molecule mimicking multi-step electron-transfer processes in the photo synthetic reaction center so far reported is a ferrocene-meso, meso-linked porphyrin trimer-fullerene pentad [Fc-(ZnP)3-C60] in Fig. 13.16b, where the C60 and the ferrocene (Fc) are tethered at both the ends of (ZnP)3 (R = 46.9 A)... 

The multi-step electron transfer process in namral photosynthesis has been utilized in the construction of various triads using porphyrin, metalloporphyrin, fullerene, and imide as basic components for harvesting solar energy as electrical energy and for photoreduction of water to get clean fuel hydrogen [6, 7]. Recently, tetrads, pentads and hexads have been constructed using porphyrin, fullerene, and a chromophoric unit as basic components for fast energy transfer process. 

Indirect electrochemical reactions usually involve a multi-electron-transfer system that consists of a set of electron transmission units (Fig. 4). Although the overall feature of an electron-transfer process in indirect electrosynthetic reactions is understandable, each step of the electron transmission has not yet been elucidated [10]. 

The theoretical understanding of electron-transfer processes in molecular systems is still lagging behind experiment. This is understandable in view of the intrinsic complexity of these systems. It appears that in order to design an efficient light-driven electron-transfer system, the complete assembly has to be considered. Electron transfer can be very rapid, but so is recombination. For the systems considered in this chapter to be useful in practical applications, one needs to consider the fate of both the electron and hole formed in the initial step. The multi-bridged systems that have recently been constructed may be the first step in that direction. 

The kinetics of formation of nitroprusside from [Fe VCN)5(H20)] indicate a mechanism of complex formation in which outer-sphere reduction to [Fe (CN)5(Fl20)] precedes substitution."" Reduction of the dimeric pentacyanoferrate(III) anion [Fe2(CI io]" by thiourea is a multi-stage process the first step is one-electron transfer to give [Fe2(CN)io], which dissociates to give [Fe(CN)5(tu)]2- and [Fe(CN)5(H20)] -.""... 

Many redox reactions at electrodes involve transfer of more than one electron. It is agreed that such processes usually involve several consecutive one-electron steps rather than a simultaneous multi-electron transfer. The kinetics of the overall reaction (and hence the current flowing) are complicated by such factors as the lifetimes of the transient intermediate species. 

Here, the longer arrow indicates the direction of the preferred electron transfer from the metal to the substrate (S), and the shorter arrow indicates the direction of the reverse transfer. It is obvious that four protons accompanied by the water molecule rearrangement cannot be transferred in one synchronous step. Owing to the high degree of electron delocalization in the polynuclear metal complexes, these complexes are more suitable for multi-electron processes. 

We shall not discuss here the mechanism of either of these reactions they were mentioned only to show the complexity of typical electrode reactions. It is evident then that the equations of electrode kinetics derived in Chapter E must be generalized to describe multi step electrode processes, which involve the transfer of several electrons. 

Many of the models for electron-transfer (ET) reactions discussed in this work assume the following 1) just one electron is transferred, 2) the transfer occurs from donor to acceptor in a single step, and 3) the bridge is rigid during the process. Recent experimental and theoretical advances indicate that these assumptions are insufficient in many circumstances. Indeed, multi-electron, multistate, and dynamic bridge effects enrich the subject substantially. In this chapter we shall examine the influence of these effects on chemical and biological ET reactions. 

It should be realized that a photochemically induced reaction may have a multi-step mechanism in which, perhaps, only one step may involve light absorption. For example, an excited molecule may transfer an electron to some acceptor molecule in its ground state to produce two odd-electron species. Both these free radicals may then take part in subsequent dark reactions. Although it is often stated that photochemistry is relatively insensitive to temperature, this is strictly only correct for the initial, light absorbing step and the rapid internal rearrangements of the excited state. Subsequent processes may be very susceptible to temperature effects. 

The photocatalytic properties and electron/photon-induced processes related to natural systems treated in Chapters 13 and 14 have been researched in depth. Different single fundamental multi-electron catalytic processes and photoexcited state electron-transfer reactions, both in polymer matrixes, are described in relation to photosynthesis (Section 13.2). It is now necessary to combine these reactions step by step to produce artificial photosynthetic systems. Some photoinduced energy-transfer processes (photooxidations) have now reached the level of practical application for wastewater cleaning (Section 13.4) and should be extended to other reactions induced by irradiation with visible light. 

In the previous chapter we have introduced the case of multiple-electron transfers (multi-E mechanisms). As discussed then, depending on the formal potentials of the different electrochemical steps comproportiona-tion/disproportionation reactions may be thermodynamically favourable and may affect the voltammetry if the electron transfers are not reversible, the diffusion coefficients of the species are different, there is mass transport by migration or other chemical reactions take place. For example, let us consider the case of two consecutive reduction processes (the EE mechanism) where the formal potential of the second step is much more negative than the first one ... 

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