Molecular dyads, examples

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]. 

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... 

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... 

However, the complexity of the natural systems containing multiple chromophores and the difficulty of the measurements on such a short time scale make the pathway assignments uncertain. Compounds 1 and 4 are examples of simple, well defined molecular dyads which were designed to help in the understanding of the carotenoid to chlorophyll singlet-singlet energy transfer process in natural systems. 

Figure 2.S. Examples of some of the early molecular dyads used to examine light-induced electron transfer.

Figure 2.8. Examples of molecular dyads used to study intramolecular triplet energy transfer.

Figure 2.18. Examples of molecular dyads bearing central diethynylated aromatic groups as the bridge.

Fullerenes have shown particular promise as acceptors in molecular electronics, and numerous interesting TTF/Cgo ensembles have been reported.42 For example, Orduna and co-workers75,76 prepared the TTF/C60 dyad 13 and observed photoinduced electron-transfer from the TTF to the fullerene. Martin et al 1 observed two separate one-electron transfer events in their conjugated dyads 14 (where n = 2). The TTF-porphyrin-fullerene triad 15, prepared by Carbonera et al.7 showed long-lived photoinduced charge separation. 

Axes of rotation are among the most common of molecular symmetry operations. A onefold axis is a rotation by a full turn, equivalent to the identity. A twofold rotation axis, as in the example of the water molecule, is sometimes called a dyad. Cyclopropane has a threefold axis perpendicular to the plane containing the carbon atoms it also has three twofold axes. Can you visualize them ... 

The conjugation in the molecular wire may be disrupted or modulated to create systems with different properties. For example, a porphyrin Ceo donor-acceptor system linked with a conjugated binaphthyl unit, has a preference for the atropi-somer where the fullerene unit is closer to the porphyrin system, thus increasing the through space interactions [82]. The charge transfer process on a dyad containing a crown ether in the linker structure can be modulated by complexation/ decomplexation of sodium cations [83] but even more interesting is the construction of supramolecular systems where the donor and acceptor moieties are... 

Some molecular components can adopt several different states as a function of several inputs [28], These compounds are of particular interest for their potential as processing devices. Flavylium salts, for instance, interconvert between a bewildering array of isomers as a result of pH, photonic and thermal inputs [29]. A more elegant example is the interconversion of 5, 6, 7-tri-tert-butyl-1, 4-anthraquinone (11) between itself and three isomeric states (12, 13, 14) by photochemical and electrochemical means (Fig. 6) [30]. This system could be considered a dyad of an electrochemically active quinone and a photochemically active tri-ferf-butyl-substituted polyacene, but, as the conjugation of the entire system is important for its properties, it is treated as a single unit. Each state has a different absorbance spectrum, and the spectral changes for each transformation were measured (Fig. 6). Unfortunately, the conditions required for the electrochemical and photochemical transformations differ, so a single sample cannot be readily interconverted between all four states. 

Some examples of donor-acceptor dyads for studies of photoinduced electron transfer across molecular bridges are shown in Fig. These systems are usually... 

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