Donor-acceptor linked assemblies

Photoinduced processes of the donor-acceptor linked molecules in condensed condition are important, since these photoactive molecules are expected to be used in a condensed form such as fine particles and assembled membranes. Thus, the investigations on the fine particles of the donor-acceptor dyad molecules give fruitful discussion on the light-ener conversion systems. 

In the case of oligothiophene-fullerene dyads, oligothio-phene and fullerene moieties seem to be located at the closest position since both oligothiophene and fullerene moieties are hydrophobic. On the other hand, aniline-fullerene dyad is considered to form a spontaneous self-assembly in such a way that the hydrophobic fullerene moieties come together, leaving the polar aniline moiety outside, in which the distance between aniline in a dyad and Qg in another dyad will be similar to that of the dyad molecule in solution. These findings indicate that the self-assembly of donor-acceptor linked molecules in the fine particles is an important factor governing the rates of the CS and CR processes. 

The feasibility of intramolecular electron- and energy-transfer depends on distance and is usually studied in covalently linked systems. However, donor-acceptor dyads can be also arranged by self-assembly what resembles the situation of electron transfer in biological systems. Artificial dyads tethered by a small number of hydrogen bonds immediately dissociate in methanol or water. To improve the binding while keeping the reversibility, a photoinducible electron donor-acceptor dyad linked by a kinetically labile bond was designed. [19]... 

The study of photoinduced ET in covalently linked donor-acceptor assemblies began with comparatively simple dyad systems which contain a transition metal center covalently linked to a single electron donor or acceptor unit [26]. However, work in this area has naturally progressed and in recent years complex supramolecular assemblies comprised of one or more metal complexes that are covalently linked to one or more organic electron donors or acceptors have been synthesized and studied [27-36]. Furthermore, several groups have utilized the useful photoredox properties of transition metal complexes to probe electron and energy transfer across spacers comprised of biological macromolecules such as peptides [37,38], proteins [39,40], and polynucleic acids [41]. 

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. 

The -electron-deficient pyridinium ring has played an important role in template-directed synthesis in which the interaction of a -electron acceptor and a -electron donor facilitate self-assembly. This has led to the construction of a variety of supramolecular assemblies including catenanes (linked rings) and rotaxanes (wheel and axle) . 

Supramolecular chemistry is concerned with assemblies of molecules held together by non-covalent forces, such as hydrogen bonds, donor-acceptor interactions between aromatic stacks, ion-ion, ion-dipole, dipole-dipole and van der Waals attractions [1, 2]. Molecules can also be gathered around a metal this is the realm of coordination chemistry. Last, molecules can be linked together without the need for any chemical bond the so-called physical or mechanical bond is found in catenanes (species formed of interlocked rings) and rotaxanes (Figure 1). 

Increasingly systems are investigated in which the photocatalyst, the electron acceptor, and/or the electron donor are linked through molecular bridges. In case of molecular systems, such an appropriate assembly of suitable molecular components capable of performing light-induced functions can be called a photochemical molecular device (Balzani et al., 1987). Such photochemical... 

Two general approaches of modeling photosynthesis can be considered, one involving mimicking the functions of the photosynthetic reaction center by means of synthetic analogs [25-27]. To this extent the synthesis of linked multicomponent donor-acceptor assemblies could lead to charge separation by means of sequential ET processes as outlined in Eqs. (1) to (4), where S is the light-active component and A and D represent electron acceptor and donor units, respectively. 

More and more attentions have been paid to H-bonding-based electron transfer in recent years. These systems have been extended to the donor-acceptor assemblies linked by two-point H-bonds, triple H-bonds, or multiple H-bonds. In this section, we will first present the important mechanistic progress in the last two decades and then discuss recent representative examples based on H-bonding. 

The cage structure 6 is composed of two face-to-face porphyrins linked at both sides via six hydrogen bonds to two triaminopyridine entities. The distance between the cofacial porphyrins is estimated to be 10 A. The initial nieso 5,15-diuracil-substituted porphyrin has two rotameric forms, a syn and an anti, due to the relative orientations of the two uracil groups with respect to the porphyrin plane. In the self-assembling process, the syn rotamer yielded the major component, the cage structure, whereas the anti conformer yielded a zig-zag strand structure. As for the properties expected by such systems, metalation of one of the porphyrins allows for the creation of a self-assembled donor-acceptor system while complexation of the porphyrins with zinc(Il) allows for the cage cavity to be used as a... 

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