Dyads and triads

Figure C 1.2.10. Representative examples of fullerene based donor-bridge-acceptor dyads and triads.

Use the dyad and triad fractions in Table 7.9 to calculate the average lengths of isotactic and syndiotactic sequences for the polymers of Fig. 7.10. Comment on the results. 

Since the total numbers of dyads and triads always occur as ratios in Eqs. (7.73) and (7.74), both the numerators and denominators of these ratios can be divided by the total number of dyads or triads to convert these total numbers into fractions That is, p-Jp = Pi/Ps- Thus the fractions in Table 7.9... 

This type of analysis adds nothing new to the picture already presented by the dyad and triad probabilities. It is somewhat easier to visualize an average sequence, however, although it must be remembered that the latter implies... 

Determination of Compositional Sequences. In the copolymerization of MMA and MAA or TBTM, compositional dyads and triads are generated. These sequences are determined by the relative concentration of monomers as well as by their relative reactivity. These compositional sequences characterize the material and allow predictions of activity based on structure by comparison with field tested polymers. 

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... 

The dyad and triad fractions each total unity by definition, that is... 

In general, conventional catalysts produce co-polymers with NB contents of less than 50 mol% with isolated or alternate NB units. Significantly, Shiono and co-workers demonstrated that Me2Si(Flu)( BuN)TiMe2 with dried MAO can form co-polymers with NB dyads and triads, and that the same complex in conjunction with Ph3CB(G6F5)4/ OctsAl provided a co-polymer with an NB content of 82mol% T = 237 °G) under polymerization at 80... 

Medium Effect on the Steric Dyad and Triad Composition of Polymers Produced by Initiation and Propagation at 250 and 225 K Using t-Butylmagnesium Compounds... 

As mentioned previously, photocycloaddition reactions are a useful method for obtaining fullerene derivatives fused directly to heterocycles. Such compounds have attracted much interest because they might have interesting properties, e.g., amino acid fullerene derivatives as biologically active compounds or pyrrolidino-fullerenes as key precursors for a great number of fullerenes donor-acceptor bridged dyads and triads. Fullerenes functionalized with silicon compounds are of great interest in materials science. The synthesis of these compounds will be described in this section. 

Also, Hossain and coworkers have reported photoinduced electron transfer in mixed metal Ru/Os dyads and triads having functionalized diimide ligands [46]. 

In contrast to the reference compounds 4a-c, the singlet excited state deactivates in the dyads and triads much faster than in the oFL references with rate constants typically on the order of 1012 s-1 (Fig. 8.13). These values match the quantitative quenching of the oFL fluorescence in 5 and 6. A comparison between the decay rates of the conjugates carrying one C60 or two C60 moieties, reveals a 2-fold acceleration of the o/igo(fluorene) deactivation in the latter. Conclusively, placing two Ceos instead of one onto the oFL backbone significantly accelerates the excited state decay. 

Sauvage J-P et al (1991) Photoinduced processes in dyads and triads containing a ruthe-nium(II) bis(terpyridine) photosensitizer covalently linked to electron-donor and acceptor groups. Inorg Chem 30 4230-4238... 

For further information on synthesis, properties, and applications of pyrro-lidinofullerenes, the chiral representatives of which were generally prepared and used as racemates, we refer to a recent review by Prato and Maggini.214 Fields in which many fullerene-fused pyrrolidines have been studied are biological and medicinal chemistry351 and, above all, advanced materials science.352,353 In the latter context, dyads and triads used for photoinduced charge separation between a fullerene acceptor moiety and electron donors like porphyrins, tetrathiafulvalenes, ferrocenes, or polyenes, are worth particular mentioning.354-356... 

Reductive and Oxidative Quenching Dyads and Triads with Donors and Acceptors... 

Some of the more donors that have incorporated into dyads and triads are ferrocene165, phenathiazine (PTZ),166 169 phenols170 and, more recently some tethered Mn... 

Table 2. Commonly used donor and acceptor moieties in dyads and triads.

Dyads and triads based on the photoactive, multibridging [Ru(bpz)3] (bpz = bipyrazine) complex directly bound to transition metal complexes were obtained by following the procedures previously reported for the generation of symmetric heptanuclear supermolecules (67-69). Such systems contain a tris(bpz)ruthenium (II) ion [RuJ attached to bis(bpy)chlororuthenium(II)/(III) [Rup], or penta-cyanoferrate(II)/(III) complexes via a bpz bridging ligand, as shown for the... 

Several different structural factors influence the properties of the mesophase in these polymers, including dipolar effects, the planarity and rigidity of the mesogenic unit, and its length-to-width ratio among others. These factors are difficult to quantify, either absolutely or relatively, but some idea of their influences can be obtained by comparing the properties of polymers with different mesogenic units when combined with the same flexible spacer. This comparison has already been made for the dyad and triad esters in Table 2, and in this section it will be extended to other types of liquid crystalline polymers which contain a common decamethylene spacer. 

With the conformational information in hand, the fluorescence lifetimes of the dyad and triad species were determined in dichloromethane solution in order to extract rate constants for photoinitiated electron transfer (e.g., step 2 in Figure 4) via Eq. (1). The results are tabulated in Table 1. Also included in the table are the distances from the center of the porphyrin macrocycle to the center of the quinone aryl ring and the distances from the edge of the porphyrin macrocycle (the nearest porphyrin meso carbon atom) to the edge of the quinone aryl ring (the carbon atom bearing the methylene group) (r e)-... 

There are not yet many reports describing the properties of sequentially ordered copolyesters. The reasons seem to be two-fold 1) Although relatively straightforward reactions are involved in each synthetic steps, preparation of the dyad and triad monomers is rather tedious and time-consuming and 2) as will be explained later, thermal stability of ester bonds formed is rather limited, which makes the study of the properties of the copolyesters difficult. The latter reason may cause significant degree of artefacts in the experimental data or observations. Therefore, comparison of the properties of sequentially ordered copolyesters with those of random counterparts can be made only based on limited experimental information gathered up to now by us(14-16) and others(17-20). 

The dyads and triads of ligand (66) were used to study charge-separated species as models for the photosynthetic reaction center. It was observed that charge separation depends on the thermodynamic parameters as well as the solvent system. The Zn/Ir/Au triad was shown to yield a fully charge separated species in toluene when excited in the visible region with a lifetime of 450 ns. 

Since a large number of n-add distributions are involved in the calculations, the program reports complete distributions only for dyads and triads. Collected distributions are reported for dyads, and tetrads. These quantities are of greater interest than the individual n-add distributions, since the properties of the central portions of unsymmetrical sequences are usually the same as those of the reversed sequences (e.g., the central units in AbScb and BCCBA sequences usually have identical chemical and physical properties.) Collected n-add distributions are thus calculated by adding appropriate n-add distributions. Examples of such calculations are shown below, where P1(n-add) defines a collected n-add distribution. 

Fullerenes, especially Qo, have been used as electron acceptors in covalently-linked donor-acceptor dyads and triads (see Part IV on Artificial Photosynthesis). A critical comparison has been made between the rates of... 

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