Electron transfer theory design

In summary, at nanostructured tin-oxide semiconductor-aqueous solution interfaces, back ET to molecular dyes is well described by conventional Marcus-type electron-transfer theory. The mechanistic details of the reaction, however, are remarkably sensitive to the nature of the semiconductor-dye binding interactions. The mechanistic differences point, potentially, to differing design strategies for kinetic optimization of the corresponding liquid-junction solar cells. 

This review highlights recent studies of synthetic, covalently linked multicomponent molecular devices which mimic aspects of photosynthetic electron transfer. After an introduction to the topic, some of the salient features of natural bacterial photosynthetic reaction centers are described. Elementary electron transfer theory is briefly discussed in order to provide a framework for the discussion which follows. Early work with covalently linked photosynthetic models is then mentioned, with references to recent reviews. The bulk of the discussion concerns current progress with various triad (three-part) molecules. Finally, some even more complex multicomponent molecules are examined. The discussion will endeavor to point out aspects of photoinitiated electron transfer which are unique to the multicomponent species, and some of the considerations important to the design, synthesis and photochemical study of such molecules. 

The basic challenge of artificial photosynthesis is to design and prepare synthetic systems which mimic this natural process. We will consider the progress which has been made to date, but first it is desirable to review a little basic electron transfer theory. 

The concept of redox, either in its elementary form (that is, as the loss or gain of oxygen) or as electron transfer, is a constant theme throughout the syllabus and the associated practical work. The list suggests plenty of varied examples, designated by R, to reinforce the theory. 

Historically, the development of ET theory has been based on inorganic systems, in which the (metal-ion) redox centers are surrounded by coordinated ligands [52]. In those cases in which no new metal-ligand bonds are formed or bond breakage is observed, the interaction between the redox centers is weak (usually Hda < 200 cm-1), and such reactions are conventionally designated as outer-sphere (OS) electron transfer [52, 53]. 

An appreciation of the basic parameters of electron tunneling theory and a survey of the values of these parameters in natural systems allows us to grasp the natural engineering of electron transfer proteins, what elements of their design are important for function and which are not, and how they fail under the influence of disease and mutation. Furthermore, this understanding also provides us with blueprint for the design of novel electron transfer proteins to exploit natural redox chemistry in desirable, simplified de novo synthetic proteins (Robertson et al., 1994). 

CiolowskiJ (1990) Scaling Properties of Topological Invariants. 153 85-100 Clark T (1996) Ab Initio Calculations on Electron-Transfer Catalysis by Metal Ions. 177 1 - 24 Cohen MH (1996) Strenghtening the Foundations of Chemical Reactivity Theory. 183 143-173 Collet A, Dutasta J-P, Lozach B, Canceill J (1993) Cydotriveratrylenes and Cryptophanes Their Synthesis and Applications to Host-Guest Chemistry and to the Design of New Materials. 165 103-129... 

If we return to ligand field theory, recall that the d electrons for an octahedral complex lie in the 2g and eg levels, also designated as tt and a respectively to equate with the character of bonding in which they participate in a complex exhibiting both donor/acceptor bonding character. For electron transfer, a metal d electron needs to move from a location in a -it or a orbital on one metal ion to a it or a orbital on the other metal ion. Generally, it will be more favourable for an electron to move between orbitals of the same symmetry (or from like to like orbitals) that is tt->tt or ct ct transitions are energetically favoured. Further, the character of tt and a orbitals differ, and so the electron transfer process will be affected by the nature of the donor and acceptor orbitals. Models predict that the d-ir orbitals are more exposed than da orbitals, thus more able to interact with orbitals on a different metal complex, and as a result it is anticipated that tt tt electron transfer should occur faster than a a electron transfer. Let s examine this for a Ru(III)/Ru(II) and a Co(III)/Co(II) couple. 

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