Molecular dyads

Recent literature contains many examples of the construction of cascades [56], Usually they are made by the covalent linking of monomer dyes, which allows strict control of their stoichiometry. The pyrene-Bodipy molecular dyads and triads are examples [57]. Efficient energy flow was reported in a purpose-built cascade molecule bearing three distinct chromophores attached to the terminal acceptor [58]. A combinatorial approach with the selection of the best hits can be applied using the assembly of fluorescent oligonucleotide analogs [59]. 

Ziessel R, Goze C, Ulrich G, Cesario M, Retailleau P, Harriman A, Rostron JP (2005) Intramolecular energy transfer in pyrene-bodipy molecular dyads and triads. Chemistry 11 7366-78... 

Here, rDA is the donor-acceptor distance, H% 0) is the interaction at constant distance r0, and [1 is the so-called attenuation factor. In vacuum, values of [1 are relatively large in the range of 2-5 A-1 [21]. Consequently, at donor-acceptor distances commonly found in molecular dyads, the through space couplings will be negligible. 

Because of the presence of a well-defined energy gap between the conduction and the valence band, semiconductors are ideally suited for investigation of the interfacial interactions between immobilized molecular components and solid substrates. In this chapter, interfacial assemblies based on nanocrystalline TiOz modified with metal polypyridyl complexes will be specifically considered. It will be shown that efficient interaction can be obtained between a molecular component and the semiconductor substrate by a matching of their electronic and electrochemical properties. The nature of the interfacial interaction between the two components will be discussed in detail. The application of such assemblies as solar cells will also be considered. The photophysical processes observed for interfacial triads, consisting of nanocrystalline TiO 2 surfaces modified with molecular dyads, will be discussed. Of particular interest in this discussion is how the interaction between the semiconductor surface and the immobilized molecular components modifies the photophysical pathways normally observed for these compounds in solution. 

In the design of such supramolecular dyads, a number of prerequisites should to be considered. In order to obtain a true supramolecular assembly there needs to be substantial interaction between the different components of the assembly. There should, however, not be any substantial changes in the physical properties of these components, but their combination should lead to some new and novel characteristics. The combination of components should have properties over and above those of the separate components, without destroying their individual characters. Molecular dyads may, for example, contain a photosensitizer and an electron or energy donor or acceptor. An example of such a combination of a sensitizer and an electron donor is the Ru-PTZ dyad [14] shown in Figure 6.21. In this assembly, the ruthenium center is the sensitizer, S, and the phenothiazine... 

A variation of the surface structure will also be of great interest in the study of the electron transfer processes of surface-bound molecular dyads. As outlined above,... 

Photovoltaic Devices with OPV4—Ceo- The increased lifetime of the charge-separated state, which extends into the millisecond time domain, opens the possibility of using the OPVrt-Coo dyads as the active material in a photovoltaic device. As an important difference with previous bulk heterojunction cells, the covalent linkage between donor and acceptor in these molecular dyads restricts the dimensions of the phase separation between the oligomer and the fullerene that could freely occur in blends of the individual components. This can be considered as a primitive attempt to obtain more ordered and better-defined phase-separated D-A networks. 

As illustrated by nature and shown schematically in Fig. 4, an artificial photosynthetic system will require several molecular components that are spatially juxtaposed to favor the various tasks at hand, i.e., light harvesting, charge separation, catalysis for HER and OER. This next section will cover some of the molecular dyads, triads and larger assemblies prepared to address some of these challenges. 

Multicomponent assemblies in which a photoredox-active metal-polypyridine unit is combined with electron acceptors and/or donors show very rich photo-induced electron transfer reactivity [38]. Such species are often called molecular dyads (triads, tetrads. ..). Electron transfer usually occurs from an excited metal-polypyridine unit M to an attached acceptor ... 

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. 

Polynuclear complexes, molecular dyads, triads, and other supermolecules composed of redox- and photo-active metal polypyridine units have a great promise as components of future molecular electronic or photonic devices as optical switches, relays, memories, etc. [38, 46],... 

Argazzi et al. followed that strategy to elaborate a nanocrystalline solar cell which incorporates a molecular dyad (HI) based on ruthenium bipyridine as a sensitizer and phenothiazine as a donor (Figure 19) [109]. 

Figure 19. Molecular dyad based on ruthenium bipyridine and phenothiazine [109],...

Many bridging ligands have been constructed that contain porphyrin subunits. Two examples are given to show the versatility of this structural motif. Porphyrins have been used for many years as models for the reactive site in the photosynthetic reaction center because of the resemblance to the natural components and the ability to vary the spectroscopic and redox properties of these chromophores by the use of bulky substituents and by their coordination to different transition metals.192 Molecular dyads of Znn/Irin and Auni/Irm and triads of Znn/Irni/Auin have been synthesized using the porphyrin bridging ligand (66).193-195... 

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. 

Photoinduced Electron Transfer in Molecular Dyads and Higher-order Analogues - Light-induced electron transfer from a donor to an acceptor, followed by successive transfer of the redox equivalents across the membrane, has... 

Photoactive Dyads. - Many new molecular dyads, comprising donor-spacer-acceptor systems, have been described in the recent literature. These systems are intended to reproduce the essential electron-transfer steps occurring in natural photosynthetic organisms by eliminating as many components as possible. The main events in the natural apparatus involve light-induced electron... 

Durrant J. R., Haque S. A. and Palomares E. (2006), Photochemical energy conversion from molecular dyads to solar cells , Chem. Comm., 3279-3289. 

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