Covalently linked donor-acceptor molecules

Covalently linked donor-acceptor molecules. One can gain greater control of the kinetics of electron transfer reactions in these systems by fixing the distance between donor and acceptor, and by introducing a third redox molecule, which serves as either a secondary donor or accrator, into the chain. For example, we have shown that a Ru(bpy)32+-diquat + donor-... 

Molecular electron transfer is the basis for many important natural and commercial processes. During the past decade photochemists have relied upon supramolecular arrays of molecules to facilitate their understanding of the chemical and physical basis for this fundamentally important process. It therefore seems appropriate that several chapters in this volume examine thermally and photo-chemically induced electron transfer in supramolecular assemblies consisting of inorganic molecular building blocks such as covalently linked donor-acceptor dyads, transition metal clusters, and nanocrystalline semiconductor particles. 

Transmembrane charge separation by sequential electron transfer between adjacent redox centers is epitomized by studies utilizing covalently linked donor -sensitizer acceptor triad molecules that are vectorially organized across planar bilayer mem-... 

The principal methods for the immobilization of chemical receptors are (1) physical adsorption to a solid surface, (2) chemical adsorption (covalent attachment) to the surface, (3) affinity binding to physically or chemically boimd species, and (4) entrapment within a matrix. Since physical adsorption relies on relatively weak forces (van der Waals, ionic, solvation, donor/acceptor), molecules placed in this way may detach over time and/or exhibit nommiform biological activity becanse of a distribution of surface orientations/conformations. However, this method is clearly the simplest of the four and therefore often finds use. An example is the popular enzyme-linked immunosorbent assay (ELISA) used in medical diagnostics. 

One of the most versatile electron-accepting molecules is the quinonoid compound, and the redox reaction of the quinone-hydroquinone couple is one of the most thoroughly studied proton-coupled electron transfer systems of organic molecules. Quinones show the reversible two-step le reduction in aprotic organic solvents (Fig. 2). One-electron addition to quinone forms the semiquinone radical with five n electrons. The stability of the semiquinone form is affected by the existence of a minute amount of proton, which appears as the large shift of the reduction potentials in the positive direction. This implies that quinonoid compoimds are representative acceptor molecules of which redox properties are influenced by external perturbation, such as protonation and solvation (Fig. 2). They are employed in covalently and noncovalently linked donor-acceptor systems of particular interest in the study of proton-coupled electron transfer and photoin-duced electron transfer. ... 

In addition to the determination of distances at a supramolecular level, RET can be used to demonstrate the mutual approach of a donor and an acceptor at a supramolecular level as a result of aggregation, association, conformational changes, etc. The donor and acceptor molecules are generally covalently linked to molecular, macromolecular or supramolecular species that move toward each other or move away. From the variations in transfer efficiency, information on the spatial relation between donor and acceptors can thus be obtained. Because of its simplicity, the steady-state RET-based method has been used in many diverse situations as shown below5 . 

The donor and acceptor molecules can be covalently linked by a flexible or rigid spacer, conjugated to macromolecules, or simply mixed together. To illustrate some features of these approaches, it may be useful to display some simulated results. 

Another aspect of supramolecular photochemistry concerns the use of covalently linked bichromophoric and multichromophoric molecules. In the example of Figure 8.19 a two-step sequential, photoinduced electron transfer is observed in the trichromophoric species D-D-A. Local excitation of the cyanonaphthalene acceptor results in charge separation first to the amine and then to the methoxyaniline donor in times of a few tens of ns... 

The morphological problems associated with the BHJ solar cells, such as low concentration of percolating pathways which are needed in order to bring the separated charge carriers to their corresponding electrodes, have prompted the utilization of molecules in with the donor and the acceptor moieties were covalently linked. In this connection several examples of Pc-based polymers [161,162], Pc-C6o dyads [85,87,88] and triads [275] have been prepared and tested for photovoltaic applications, but the efficiencies of these systems have been proved to be still low. 

Figure 5. Long-range ET. Direct overlap between the donor and acceptor orbitals is negligible, and ET occurs by an indirect mechanism involving electron tunneling through the orbitals of the intervening medium, e.g., solvent molecules (upper) or a covalently linked saturated bridge (lower).

The simplest covalently linked system for studying photoinduced electron transfer consists of a porphyrin bonded to an electron acceptor or donor moiety with appropriate redox properties. Most of these studies have employed free-base, zinc, or magnesium porphyrins because the first excited singlet states of these molecules are relatively long-lived (typically 1-10 ns), so that electron transfer can compete with other decay pathways. In addition, they have relatively high fluorescence... 

A large number of covalently linked systems are currently being synthesized and investigated, differing in the nature of A, B, and L, as well as in the number of functional units in the supramolecular system (nuclearity). It is common to call simple two-component donor-acceptor systems such as that of Eq. 2 dyads , and progressively more complex systems triads , tetrads , pentads , etc.. Systems where all the A and B units are organic molecules are dealt with in Chapter 1 of this section. The present chapter deals with systems where at least one of the A/B functional units is a transition metal coordination compound. From this definition, however, are excluded (a) systems where A and/or B are porphyrins or related species (dealt with in Chapter 2) and (b) systems of high nuclearity with dendritic structures (dealt with in Chapter 9). 

Recently a number of covalently linked porphyrin-quinone systems such as IS (Malaga et al., 1984) or 16 (Joran et al., 1984) have been synthesized in order to investigate the dependence of electron-transfer reactions on the separation and mutual orientation of donor and acceptor. These systems are also models of the electron transfer between chlorophyll a and a quinone molecule, which is the essential charge separation step in photosynthesis in green plants. (Cf. Section 7.6.1.) Photoinduced electron transfer in supra-molecular systems for artificial photosynthesis has recently been summarized (Wasielewski, 1992). 

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