Electron-hopping process

The occurrence of intramolecular electron-hopping processes will now be discussed in detail, considering on an empirical basis systematic variation of relevant structural factors in the substrates. First, however, a few theoretical and experimental aspects will be summarized. 

Thus, in the present approach, the major focus is on the question of how we can influence the external parameters like solvent and counterion and the intrinsic structural parameters within the systems A-l-A to force the electron-hopping process into the timescale of the experiment, or at least to establish clearly the borderline cases. That we are still looking at an electron-hopping process in the case of effective charge delocalization over the entire molecule and not at a pure resonance phenomenon may be reassured by VIS/NIR spectroscopy of the neutral and charged species the absorption of a single chromophore should be detected unless a very fast process > 1012 Hz is taking place. 

The second-order character of the electron hopping process is responsible for the nonlinear expression of the term that depicts the field effect. 

It is somewhat surprising that electrochromic devices have also been reported. TiC>2 is not electroactive at potentials where the molecular components are. However, electron transport to the underlying ITO surface through an electron-hopping process drives the electrochemical process. A very fast electrochromic device has been developed, further suggesting considerable potential of these assemblies for commercial applications. 

Modified TiC>2 surfaces have also found application in the design of fast elec-trochromic devices. The influence of the substrate on the behavior of interfacial assemblies is well illustrated in this book. However, it is important to realize that the electrochromic behavior observed for modified TiC>2 surfaces was not expected. The oxidation and reduction of attached electrochromic dyes are not mediated by the semiconductor itself but by an electron-hopping process, not unlike that observed for redox polymers, where the electrochemical reaction is controlled by the underlying indium-tin oxide (ITO) contact. These developments show that devices based on interfacial assemblies are a realistic target and that further work in this area is worthwhile. 

They also used a random-walk treatment to describe the electron-hopping process coupled to physical diffusion [vii]. 

This last equation is valid as long as the diffusion front of the diffusing species in solution phase remains within the electrode coating, a condition that applies for times shorter than 10-20 msec (Miller and Majda, 1986,1988). Dynamics of electron hopping processes have been recently modeled by Denny and Sangaranarayan (1998) using kinetic Ising model formalism. 

It will be an interesting problem to analyze the intersoliton hopping mechanism assisted by a motion of the dopant molecule. We try to formulate here such an electron hopping process between two (CH), chains (Yamabe et al., 1984b), as illustrated in Fig. 14. 

Application of an external magnetic field of 12 kG gives better resolution of the two patterns at room temperature and establishes that the tetrahedral A site spins align antiparallel and the B site spins parallel to the applied field [28], The lines from the B site cations are broader than those from the A site because of the electron-hopping process. Study of the relative line broadening led to a value for the relaxation time of the hopping at room temperature of t = M ns. 

Also, the available data show that the conductivity of the obtained films decreases at low potential values ( " < — 1 V/SCE) as the film thickness increases . In contrast, the polymer conductivity seems to be unchanged at positive potential values. This evokes a voltage-dependent conductivity similar to that described for simple polypyrrole films. Anyhow, the incorporation of the complexes into the polymer films imparts a typical redox conductivity to these materials that allows their use over a large potential range. One can even assert that at the potentials where many of the effects are expected, the electron hopping process between macrocyclic complex sites dominates the global charge transport mechanism. 

In all cases, the films were obtained by oxidative electropolymerization of the cited substituted complexes from organic or aqueous solutions. The mechanism of metalloporphyrin Him formation was suggested to be a radical-cation induced polymerization of the substituents on the periphery of the macrocycle. As it was reported for the case of polypyrrole-based materials ", cyclic voltammetry and UV-visible spectroscopy with optically transparent electrodes were extensively used to provide information on the polymeric films (electroactivity, photometric properties, chemical stability, conductivity, etc.). Based on the available data, it appears that the electrochemical polymerization of the substituted complexes leads to well-structured multilayer films. It also appears that the low conductivity of the formed films, combined with the cross-linking effects due to the steric hindrance induced by the macrocyclic Ugand, confers to these materials a certain number of limitations such as the limited continuous growth of the polymers due to the absence of electronic conductivity of the films. Indeed, the charge transport in many of these films acts only by electron-hopping process between porphyrin sites. 

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