Mixed-valence devices

Figure 11.7 Potential energy diagrams for a two-component mixed valence device with (a) negligible, (b) weak and (c) strong electronic coupling. The dashed curves represent unperturbed zero-order states. The horizontal axis represents generalised nuclear coordinates, including contributions from both ligand atom positions and solvent sphere.

The potential development of optoelectronic devices based on nonlinear polarization of the MMCT transitions of mixed-valence complexes has been the subject of an investigation 100). The hyperpolarizabilities measured for the complexes [(CN)5Ru(jU.-CN)Ru(NH3)5] and [(T75-C5H5)Ru(PPh3)2( U-CN)Ru(NH3)5]3+ are among the largest known for a solution species. The tunability of MMCT energy is an added advantage for the construction of nonlinear optical devices. 

Selective perturbation of metal-metal coupling by either inner-sphere or outer-sphere manipulations will continue. Aside from increasing our understanding of metal-metal superexchange coupling, these studies will probe the nature of solvent-solute interactions and perhaps be applicable to the design of molecular devices. The synthesis of novel nonlinear optical materials that incorporate mixed-valence metal systems will be the subject of much interest 100). 

The experimental evidence for the existence of bistable mixed-valence isomers 12A and 12B and their switching times of about one trillion per second as inferred from temperature-dependent coalescence of carbonyl stretching bands may be eventually employed to provide a solid basis for the fabrication of molecular electronic devices. 

Suitable candidates for these investigations have mobile charge compensating counterions and contain mixed-valence redox centers. Beyond its theoretical importance, solid networks are important for chemical sensing [128], as well as for the development of charge storage, electrochromic, and molecular electronics devices [129]. Faulkner and Kulesza have successfully applied microelectrodes to study solid-state redox transitions in... 

Preliminary results of a light-variable transmission electrochromic device in acid aqueous solution which uses Prussian blue as the electrochromic material to operate in a mode complementary to pANI have been reported by Duek et al [32]. Prussian blue exhibits interesting bicolour electroch-romism as shown below (Fe"Fe" is the mixed valence Prussian blue (PB) compound, its reduced form Fe Fe is Everitt salt (ES) and its oxidized form Fe Fe is Berlin green (BG)) ... 

Electron-spin-based phenomena are attracting attention for the possibility of logic devices based on spin rather than on the flow of charge for possible quantum information applications. Retention of a spin lifetime in the material is important for these considerations. The conduction-band states at the zone center are approximately spin eigenstates, and therefore, they have relatively long spin lifetimes because they are made up of s-type orbitals. This is in contrast to valence-band states that are made up of p-type orbitals for which the spin-orbit interaction mixes the orbital part of the p state with the spin part of the electron wave... 

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