Charge separation, photoinduced processes

Figure 1. Energy diagram illustrating the possible pathways available in a photoinduced charge separation process between a donor (D) and acceptor (A).

Here Eq. 6 represents a photoinduced charge-separation process where the product state escapes the cage prior to (or in competition with) recombination. Equation 7 shows a schematic redox reaction involving a substrate S and one of the released components of the product state. This type of sequence uses photons to prepare a redox-active species that is not an excited state of the light-absorbing component. 

Photoinduced charge separation processes in the supramolecular triad systems D -A-A, D -A -A and D -A-A have been investigated using three potential energy surfaces and two reaction coordinates by the stochastic Liouville equation to describe their time evolution. A comparison has l n made between the predictions of this model and results involving charge separation obtained experimentally from bacterial photosynthetic reaction centres. Nitrite anion has been photoreduced to ammonia in aqueous media using [Ni(teta)] " and [Ru(bpy)3] adsorbed on a Nafion membrane. 

In order to continue the study of photoinduced charge separation processes in oligothiophene-based materials, Otsubo et al. recently synthesized two dual oligothiophene-fullerene [Ceol triads 2.132 and 2.133 (Chart 1.26), in which a quaterthiophene and an octithiophene unit were separated by a propyl chain... 

Similarly to other semiconducting materials, the photoluminescence of C-dots relies on radiative recombination of confined electrons and holes generated previously by efficient photoinduced charge separation processes. In addition, this emission is efficiently quenched by electron donor or acceptor molecules in solution, indicating that photo-excited carbon dots are both excellent electron donors and electron acceptors. ... 

From a consideration of thermodynamics, solar energy conversion by means of photoinduced charge separation followed by water splitting is a feasible process. 

Photoinduced electron transfer (PET) is often responsible for fluorescence quenching. This process is involved in many organic photochemical reactions. It plays a major role in photosynthesis and in artificial systems for the conversion of solar energy based on photoinduced charge separation. Fluorescence quenching experiments provide a useful insight into the electron transfer processes occurring in these systems. 

Photodiodes utilize principally the photophysical process of semiconductors. The most typical juctions to attain photoinduced charge separation are shown in Fig. 27 a c. If a photoexcited compound (P) is arranged with donor and/or acceptor on an electrode as shown in Fig. 25 (d), it must work as a kind of photodiode based on new principle of photochemical reaction. A polymer film must be most promising to construct such photoconversion element. 

The interface model predicts that Vcx in a dye cell will not be limited by because does not control the charge-separation process. Rather than having large potential gradients at equilibrium, as in conventional cells, the DSSC has only small and relatively insignificant values of and xbl. Illumination of a DSSC causes the potential gradients to increase, whereas in a conventional cell, they decrease upon illumination. Because the photoinduced increase in is practically eliminated by electrolyte ion redistribution, the photoinduced increase in p,neq can drive an efficient photoconversion process. 

Such a photoinduced charge separation can proceed effectively provided an electric field (potential gradient) has been established at the position where the primary photoexcitation takes place. In general, a potential gradient can be produced at the interface between two different substances (or different phases). For example, a very thin (ca. 50 A) lipid membrane separating two aqueous solutions inside the chloroplasts of green plants is believed to play the essential role in the process of photosynthesis, which is the cheapest and perhaps the most successful solar conversion system available. 

Thus, a number of processes may take place within supramolecular systems, modulated by the arrangement of the components excitation energy migration, photoinduced charge separation by electron or proton transfer, perturbation of optical transitions and polarizabilities, modification of redox potentials in ground or excited states, photoregulation of binding properties, selective photochemical reactions, etc. 

The D, A or PS units may be metal coordination centres. Metal to ligand charge transfer (MLCT) in metal complexes (such as Ru(n) or Re(l)-diimine centres) has been extensively used for generating PeT processes [8.64-8.68, A. 10, A.20]. Our own work has been concerned with photoinduced charge separation in macropoly-cyclic coreceptors containing both a photosensitive porphyrin group and binding sites for silver(i) ions as acceptor centres. Thus, complexation of silver ions by the... 

The Marcus Inverted Region (MIR) is that part of the function of rate constant versus free energy where a chemical reaction becomes slower as it becomes more exothermic. It has been observed in many thermal electron transfer processes such as neutralization of ion pairs, but not for photoinduced charge separation between neutral molecules. The reasons for this discrepancy have been the object of much controversy in recent years, and the present article gives a critical summary of the theoretical basis of the MIR as well as of the explanations proposed for its absence in photoinduced electron transfer. The role of the solvent receives special attention, notably in view of the possible effects of dielectric saturation in the field of ions. The relationship between the MIR and the theories of radiationless transitions is a topic of current development, although in the Marcus-Hush Model electron transfer is treated as a thermally activated process. 

It can be noticed that these processes are essentially charge recombinations, with a few examples of charge shifts [82], Photoinduced charge separations between neutral chromophores do not appear in this table, either because the range of free energies was not sufficient to reach the M.I.R., or because the M.I.R. was not observed this has carried on the trend already noted in the early 1980s, that photo-induced charge separations follow Rehm-Weller behaviour. 

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