Transfer processes intramolecular, reversible

The reversible first-order reaction (1.47) can be converted into an irreversible A X process by scavenging X rapidly and preventing its return to A. Thus the intramolecular reversible electron transfer in modified myoglobin (Sec. 5.9)... 

In order to perform controlled and reversible movements and to behave as a machine, the envisaged molecular system should have a mobile and a fixed component one of the components should be redox active and the oxidized and reduced states should have almost comparable stability and should be connected by a reversible, and possibly fast, electron transfer process. The two oxidation states should display a different topological affinity with respect to the other component, so that a redox change can induce a modification of the topology of the whole molecular system, generating an intramolecular motion. The occurrence of fast and reversible movements also requires that the interaction between the mobile and the fixed part is based on... 

Such a sacrificial mechanism, although fully successful, is less appealing than the intramolecular one because it leads to the formation of waste products. However, instead of using a sacrificial reductant, that is, an electron donor molecule that undergoes a fast decomposition reaction after electron transfer has taken place, a reversible reductant, giving rise to a stable oxidized form, may be successfully employed, provided that the back electron transfer process can be slowed down by a wise choice of the partners. 

Substitution reactions of cis-Ru(bipy)2X2 (X = F, Cl, or Br) with methyl-substituted pyridines have been studied in various non-aqueous solvents and the results indicate a dissociative mechanism via [Ru(bipy)2X] as the intermediate. The outer-sphere electron transfer between [Ru(NH3)g] and [(NH3)5Co02Co(NH3)s] has been monitored. Flash photolysis at the intense metal-to-ligand CT band (508 nm) of the pyrazine complex [(NH3)5Ru"(pyr)Cu"] causes an intramolecular electron transfer to [(NH3)5Ru (pyr)Cu ]. This allowed the unimolecular reverse electron-transfer process to be monitored. 

The lack of a counterion implies intramolecular charge compensation and therefore mixed-valence character for [(Fe )2Fe OL3 ] 16a. This was unambiguously confirmed by a Mossbauer spectrum which exhibits two quadruple doublets with a peak area ratio of 1 2. Cyclic voltammetric investigation of the redox-active iron centers of neutral 16a shows a reversible three-potential one-electron transfer process. The half-wave potentials of -635 and -1230 mV correspond to the redox processes [(Fe 02Fe OL3 ] [Fe (Fe )20L3 ] [(Fe lsOL ] , whereas the oxidation of... 

An interesting property of SALAI and its substituted derivatives is their ability to colour markedly under UV irradiation. This photochromism has been attributed to the possible formation of a geometric isomer of the quinoid tautomer. On the other hand, these compounds also exhibit thermochromiSm, which consists of a thermally induced change in colour which increases with an increase of the temperature. This phenomenon has been attributed to intramolecular hydrogen transfer, such as that depicted in Scheme 1. Both processes are reversible and mutually exclusive for the same compound in a given crystalline form. However, since the same anil may exhibit polymorphism, it may be thermochromic in one crystalline form and photochromic in the other. Therefore, these processes should be correlated with differences in the crystal structure rather than with inherent properties of the molecule. All the crystallographic results, which will be reported in Section 20.1.2.3, confirm the earlier hypothesis that molecules which exhibit thermo-chromism are planar, while the others are non-planar, as shown in Figure 1. 

Datta A, Mandal D, Pal SK, Bhattacharyya K. Intramolecular charge transfer processes in confined systems. Nile Red in reverse tnicelles.J Phys Chem B. 1997 101(49) 10221 — 10225. http //dx.doi.org/10.1021/jp971576m. 

Hazra, R. and Sarkar, N. 2001. Intramolecular charge transfer processes and solvation dynamics of Coumarin 490 in reverse micelles. Chem. Phys. Lett. 342, 303-311. 

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