Ester dyads

As shown below, there are essentially four types of symmetrical, ester triad mesogenic structures, and three types of ester dyad mesogenic structures ... 

The vast majority of the dyad models for photosynthetic electron transfer have consisted of synthetic porphyrins covalently linked to quinones. The first such models were reported in the late 1970 s. Kong and Loach prepared the ester-linked dyad 2 in 1978 [38], and the amide 3 was reported by Tabushi and coworkers in 1979 [39]. A large number of these P-Q systems have now appeared in the literature. The reader is referred to several reviews [13, 34, 40], including the recent review by Connolly and Bolton [41] for a complete compilation of these results. 

This method has been applied to the rigid 3-bond dyad, DNM[3]M, (Fig. 37), in which the maleate ester group (M) serves as the acceptor (the DCV acceptor group cannot be used because the energies of the singlet and triplet +DMN[3]DCV CS states all lie above the 3[ DMN[3]DCV] locally excited triplet state, see Fig. 29). Benzophenone (BP) was used as the sensitiser, and a highly polar solvent... 

Condensing racemic l,2-methano[70]fullerene-71-carboxylic acid obtained by hydrolysis of its tert-butyl ester (( )-260, Scheme 1.25),243 with an amino derivative of zinc tetraphenylporphyrin, Imahori, Yamazaki, Sakata, and coworkers synthesized a C7o-containing dyad.355,446 Comparison with the C60-analogue showed that photoinduced electron transfer from the singlet excited Zn-porphyrin to the fullerene is faster in the C70 derivative. 

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. 

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). 

Mechanistic and structural investigations have revealed that while some esterase antibodies promote direct attack by hydroxide on the ester function [ 15], others unexpectedly may also use a covalent mechanism involving a serine-histidine dyad strongly reminiscent of the catalytic triad of serine proteases [16]. 

C.Ober, J.I. Jin, and R.W.Lenz, Thermotropic polyesters with either dyad or triad aromatic ester mesogenic units and flexible polymethylene spacers in the main chain, Pol3mier J. 14 9 (1982). 

Analysis of the NMR spectrum of a polymer made under these conditions can give information about the stereochemistry of two or three of the dyads at the end of the chain. When methyl iodide terminates the anionic polymerization of MMA, the resulting terminal unit is -CH2 CMe2 COOMe which resembles that produced when a particular azo ester is used as a radical initiator for the monomer (see section 3.2.2). 

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