Molecular systems intramolecular electron transfer

Wigner rotation/adiabatic-to-diabatic transformation matrices, 92 Electronic structure theory, electron nuclear dynamics (END) structure and properties, 326-327 theoretical background, 324-325 time-dependent variational principle (TDVP), general nuclear dynamics, 334-337 Electronic wave function, permutational symmetry, 680-682 Electron nuclear dynamics (END) degenerate states chemistry, xii-xiii direct molecular dynamics, structure and properties, 327 molecular systems, 337-351 final-state analysis, 342-349 intramolecular electron transfer,... 

Intraanchor reactions, conical intersection, two-state systems, 437-438 Intramolecular electron transfer, electron nuclear dynamics (END), 349-351 Intrinsic reaction coordinate (IRC), direct molecular dynamics, theoretical background, 358-361... 

Therefore, the role that the ferrocenium group plays in the in vitro cytotoxicity appears to be that of an intramolecular electron acceptor. The inertness of the non-phenolic compound 7 to pyridine in this model system shows that a phenolic group is necessary for the reaction to take place. Likewise, for the unconjugated 20a-c, chemical reduction of the Fe(III) atoms was not observed suggesting that the electron transfer process occurs through a coupling in the molecular Ti-system. Thus, as soon as an adequate base is available, a substantially fast intramolecular electron transfer may occur, thereby leading to the oxidation of the phenolic moiety made easier because of its displacement by the reaction of the phenoxy cation with the pyridine base [153-155]. 

Finally, it is important to note (Section 5.3.6) that electrochemistry and UV-Vis absorption spectra of molecular dyads or triads based on metal polypyridines show that electronic interactions between the components of the systems discussed above are too small to influence ground-state behavior. Nevertheless, they are sufficient to allow for very fast intramolecular electron transfer when electronically excited. In fact electronic coupling of 0.002-0.005 eV would be quite enough, but hardly detectable electrochemically. Detailed studies of electrochemistry and spectroscopy of these supramolecular systems and their components are, nevertheless, essential for the understanding of the energetics of photoinduced intramolecular electron and energy transfer reactions. 

As has been discussed above, molecular clusters produced in a supersonic expansion are preferred model systems to study solvation-mediated photoreactions from a molecular point of view. Under such conditions, intramolecular electron transfer reactions in D-A molecules, traditionally observed in solutions, are amenable to a detailed spectroscopic study. One should note, however, the difference between the possible energy dissipation processes in jet-cooled clusters and in solution. Since molecular clusters are produced in the gas phase under collision-free conditions, they are free of perturbations from many-body interactions or macro-molecular structures inherent for molecules in the condensed phase. In addition, they are frozen out in their minimum energy conformations which may differ from those relevant at room temperature. Another important aspect of the condensed phase is its role as a heat bath. Thus, excess energy in a molecule may be dissipated to the bulk on a picosecond time-scale. On the other hand, in a cluster excess energy may only be dissipated to a restricted number of oscillators and the cluster may fragment by losing solvent molecules. 

Related molecular dyads have been constructed in which a metal complex, often ruthenium(II) tris(2,2 -bipyridine) or similar, functions as chromophore and an appended organic moiety acts as redox partner. Other systems " have been built from two separate metal complexes. Each of these systems shows selective intramolecular electron transfer under illumination. Rates of charge separation and recombination have been measured in each case and, on the basis of transient spectroscopic studies, the reaction mechanism has been elucidated. The results are of extreme importance for furthering our understanding of electron-transfer reactions and for developing effective molecular-scale electronic devices. The field is open and still highly active. 

Hendrickson and co-workers have continued to probe the dynamics of electron transfer in molecular systems in the solid state. Mossbauer and specific-heat data on biferrocenium [(C5H5)Fe(C5H4 C5H4)Fe(C5H5)] salts indicate that intramolecular electron transfer is controlled by lattice dynamics. The tri-iodide salts show valence localization up to 350 K by Mbssbauer data. The room-temperature crystal structure is centrosymmetric and evidently disordered. 

The uv/vis/nir spectra of mixed-valence species exhibit unique low-energy intervalence charge transfer (IVCT) transitions which are absent in the fully reduced and oxidised complexes. Qass (valence localised) mixed-valence systems have broad, weak, structureless IVCT bands which are sensitive to molecular environment (eg solvent ). Detailed study of these bands in trapped valence species allows os to calculate intramolecular electron transfer rates. Class III (valence delocalised) systems have more intense low-energy transitions which are often structured and are largely insensitive to solvent. We will consider the degree of metal-metal interactions within class III species by analysis of low-energy absorption transitions. 

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