Electron Transfer in the Solid State

Mikami et al. investigated the photochemical Diels-Alder reaction of anthracenes with in the solid state. Irradiation of a mixture of and 9-methylanthracene led to the formation of mono- and bisadducts. No formation of 9-methylanthracene dimers as in solution was observed. The reaction did not work with less bulky anthracenes. The reaction rate depends on the ionization potential of the anthracenes. With decreasing ionization potential, the Diels-Alder reaction of proceeds much more easily. Therefore, the reaction may proceed via photoinduced electron transfer from anthracenes to the triplet excited state of Q . The energetics for the photoinduced electron transfer in the solid state are significantly different from those in solution where solvation plays an important role. Such a difference leads to the different reactivity of the anthracene derivatives in the solid state as compared to that in solution. 

The radical anions of dialkyl sulfoxides (or sulfones) may be obtained by direct capture of electron during y-irradiation. It was shown that electron capture by several electron acceptors in the solid state gave anion adducts 27. It was concluded276 that these species are not properly described as radical anions but are genuine radicals which, formed in a solid state cavity, are unable to leave the site of the anions and exhibit a weak charge-transfer interaction which does not modify their conformation or reactivity appreciably, but only their ESR spectra. For hexadeuteriodimethyl sulfoxide in the solid state, electron capture gave this kind of adduct 278,28 (2H isotopic coupling 2.97 G is less than 3.58 G normally found for -CD3). 

Electron transfer, a fundamental chemical process underlying all redox reactions, has been under experimental and theoretical study for many years [1-6]. Theoretical studies of such processes seek to understand the ways in which their rate depends on donor and acceptor properties, on the solvent, and on the electronic coupling between the states involved. The different roles played by these factors and the way they affect qualitative and quantitative aspects of the electron transfer process have been thoroughly discussed in the past half-century. This kind of processes, which dominate electron transitions in molecular systems, is to be contrasted with electron transport in the solid state, that is, in metals and semiconductors. Electrochemical reactions that involve both molecular and solid-state donor/acceptor systems, bridge the gap between these phenomena [6]. Here, electron transfer takes place between quasi-free electronic states on one side, and bound molecular electronic states on the other. 

In order to observe the important structure-dependent anisotropic spin-spin interactions, such as the dipolar interaction, D, within radical pairs and to prevent spin lattice relaxation from destroying the spin polarization, it is necessary to examine the radical pairs in the solid state at low temperatures. Photosynthetic model systems based on chlorophyll or porphyrin electron donors have the interesting, but unfortunate property that the efficiency of their light-initiated, singlet state electron transfer reactions is negligibly low whenever they are dissolved in solid solutions. Stated more precisely, the decay rates of chlorophyll and porphyrin excited singlet states are much faster than the rates of electron transfer from these donors to most electron acceptors in the solid state. 

R. Lazzaroni, M. Logdiund, S. Stafstrom, W. R. Salaneck, and J. L. Bredas. The poly-3-hexylthio-phene/NOPF6 system a photoelectron spectroscopy study of electronic structural changes induced by the charge transfer in the solid state, J. Chem. Pins. 93 4433 (1990). 

Fig. 10-1. Electron transfer mechanism for a chain process of iodo-de-diazoniation in the solid state (after Gougoutas, 1979).

State decarbonylation reaction in total synthesis was reported recently in the case of natnral prodnct (+)-herbetenolide, which farther illustrates the exquisite control that the solid state may exert on the chemical behavior of the otherwise highly promiscuous reactive intermediates. As word or caution, it should be mentioned that intramolecular quenching effects known to act in solution can also affect that reaction in the solid state. Recently reported examples include the well-known intramolecular P-phenyl and electron transfer quenching. ... 

The theory on the level of the electrode and on the electrochemical cell is sufficiently advanced [4-7]. In this connection, it is necessary to mention the works of J.Newman and R.White s group [8-12], In the majority of publications, the macroscopical approach is used. The authors take into account the transport process and material balance within the system in a proper way. The analysis of the flows in the porous matrix or in the cell takes generally into consideration the diffusion, migration and convection processes. While computing transport processes in the concentrated electrolytes the Stefan-Maxwell equations are used. To calculate electron transfer in a solid phase the Ohm s law in its differential form is used. The electrochemical transformations within the electrodes are described by the Batler-Volmer equation. The internal surface of the electrode, where electrochemical process runs, is frequently presented as a certain function of the porosity or as a certain state of the reagents transformation. To describe this function, various modeling or empirical equations are offered, and they... 

If, on the other hand, the electron transfer in solution is determined by some rearrangement within the ion-pair structure, it is crucial to investigate the feasibility of electron transfer for an immobilized ion-pair structure in the solid state. 

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