Side chain tuning

The optical properties can be tuned by variations of the chromophores (e.g. type of side-chains or length of chromophorc). The alkyl- and alkoxy-substituted polymers emit in the bluc-gnecn range of the visible spectrum with high photolu-inincsccncc quantum yields (0.4-0.8 in solution), while yellow or red emission is obtained by a further modification of the chemical structure of the chromophores. For example, cyano substitution on the vinylene moiety yields an orange emitter. 

A second method of activating the acid for esterification (see Section 7.6) is as the mixed anhydride. The mixed-anhydride reaction had been employed decades ago for preparing activated esters. However, it was never adopted because of its unreliability and the modest yields obtained. The method was fine-tuned (Figure 7.12), after reliable information on the properties of mixed anhydrides was acquired (see Section 2.8). Tertiary amine is required for esterification of the mixed anhydride to occur. The method is generally applicable, except for derivatives of asparagine, glutamine, and serine with unprotected side chains. The base also prevents decomposition that occurs when the activated derivative is a Boc-amino acid (see... 

In the other subdivision, water activation occurs in the first step of the enzymatic cycle. This activation is achieved by a carboxylate group in aspartic hydrolases (Fig. 3.10), Zn2+ and a carboxy group in metallopep-tidases (Fig. 3.12 ), a histidine side chain in calcium-dependent hydrolases (Fig. 3.14), or a Zn2+ in carbonic anhydrase (Fig. 3.15). The substrate, on the other hand, is polarized (activated) by a carboxy group in aspartic hydrolases or by a cation in metallopeptidases and calcium-dependent hydrolases. In this manner, the reactivity of both the water molecule and the substrate is enhanced and fine-tuned to drive formation of a tetrahedral intermediate that will break down to form the hydrolysis products. 

In summary, the configuration of the desired product is controlled by the planar-chiral imine and ketimine ligand backbone. The selectivity of the reaction depends on both the chiral center and the communication of the side-chain with the ligand backbone. We tuned the side-chain to increase the enantioselectivity up to 90% ee. In the case of the amino alcohol ligands, chiral cooperativity is also observed. However, the influence of the planar chirality is much lower, whereas central chirality is dominant in this instance. In most cases the enantioselectivity is lower than for the ketimines. 

It is interesting to note that the CD spectrum of 11 can be tuned by changing the oxidation states of the TTF units in the side-chains of 11. Polymer 11 shows reversible interconversion between three univalent and two very broad mixed-valence redox states that have different chiroptical properties. 

Recent studies have validated the idea that the superhelix model is responsible for chiroptical switching of polysilane aggregates in a series of diarylpolysilanes, 28-30, in which only the length of the para-substituent in the aryl side chain varies, decreasing monotonically in the order of 28 (ethyl), 29(rc-propyl), 39 (n-butyl) with the intention to effect a chirality switch by reduction of d alone [82]. The proper structural tuning results in a p/d ratio,... 

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