Chiroptical sensing

Figure 9.8) [70]. This work is part of a broader activity in the field of binaphthyl-based chiral macrocycles for chiroptical sensing and chiral nanostructuring [71-87]. The macrocycle 23 incorporates a Binol unit. In order to counterbalance the inherent distortion brought about by the binaphthyl units into the macro-cyclic framework, sp carbon atoms, imparting a higher conformational freedom with respect to sp- or sp -hybridized carbon atoms, have been introduced. 

In this section, we comprehensively focused on the controlled synthesis, chiroptical characterization, and manipulation of optically active poly(dialkyl-silane)s. Although many artificial polymers adopting preferential screw sense... 

The reported determinations of nonequivalence sense for configurationally known compounds are presented or referenced in this section. The NMR method of absolute configuration determination, when it works, is often more reliable and positive than chiroptic... 

Moreover, introduction of additional silane with chiral/achiral side groups to the 16-based copolymers permits fine control of the transition temperature (Tc) between -64 and +79°C. These results may open new methodologies in designing PM-transition polymers that may be promising in diverse screw-sense related applications, such as thermo-driven chiroptical switch and memory, a thermo-driven switching chiral separation column, and molecular recognition ability. 

A knowledge and understanding on the nature of the helix switching may be helpful in designing PM-transition capability and characteristics in diverse screw-sense inversion related applications, such as chiroptical switches and memory, switchable chiral separation columns, molecular recognition and molecular motor applications. 

Fujiki, M. 2000. Hehx magic Thermo-driven chiroptical switching and screw-sense inversion of flexible rod helical polysilylenes. / Am Chem Soc 122 3336. 

Poly (PDBF) s synthesized by helix-sense-selective polymerization using FlLi-Sp and FlLi-DDB complexes did not show detectable CD or optical activity in solution. However, a thin film made by drop-casting the solution onto a quartz plate indicated intense CD absorptions. The spectra of the polymers synthesized with (+)- and (-)-DDB were almost mirror images. The CD spectra are not due to film anisotropy but to conformational chirality of polymer chains. Because the polymers showing clear CD bands in film did not show chiroptical properties in solution, molecular aggregation may amplify and stabilize the single-handed helical conformation induced... 

Helix-sense-selective polymerization of achiral carbodii-mides 159 and 160 was performed using optically active titanium(IV) complex (161). " ° The anthracene-containing poly-159 slowly racemized in toluene at 80 °C. Below the racemization temperature, poly-159 exhibited reversible switching of chiroptical properties driven by temperature and solvent. ° The switching is due to a change in the orientation of pendent anthracene moieties, and the kinetically controlled helicity during the polymerization seems to remain unchanged. 

Helical Poly(phenylacetylenes). The most widely studied helical-substituted polyacetylenes are based on poly(phenylacetylene) with chiral ring substituents. Pol5unerization of chiral phenylacetylenes was first reported in 1995 (72). 4-(-)-Menthoxycarbonylphenylacetylene (41) was subjected to the polymerization with several transition metal catalysts such as [(nbd)RhCl]2 and WCle. The resultant Rh-based polymer shows a large optical rotation and intense CD effects in the electric absorption region of the main chain. The polymer, thus, exists in a helical conformation with an excess of one-handed screw-sense. Poly(phenylacetylene) with small chiral pendants, poly(42), in contrast, displays poorer chiroptical properties. Interestingly, an increase in temperature steeply increases the optical rotation of poly(41) if the polymer is produced with a W catalyst. Such a drastic enhancement of chiroptical properties is not observed in the case of Rh-based poly(41). 

FIGURE 14.4 An equilibrium between diastereomers with (M)- or (P)-helicity, and a preferred sense of dynamic helicity due to the intramolecular transmission of chirality in propellershaped tertiary derivative (R,R)-syn-7 leading to a chiroptical readout. 

The chiral groups substituted p-conjugated polyacelylenes form helical structure with a predominantly one-handed screw sense exhibits simultaneous changes in both optical and chiroptical properties [125, 126]. For example, the pendant chiral oxazoline moiety in poly(phenylacetylene) [127] induces an excess one handedness helical conformation in the main chain of the polymer. 

Chiral phosphorus-containing dendrimers which have ferrocene molecules at the periphery have been the subject of work by Majoral and co-workers. These materials will have obvious applications in catalysis. Layer-by-layer dendrimers, which are built up containing chiral ferrocenylphosphine molecules, have also been prepared, and their chiroptical and electrochemical properties have been investigated. A recent article covers the basic chemistry of ferrocene-containing liquid-crystalline polymers which stems from the extensive work of Deschenaux, who essentially pioneered the area. A ferrocene end-capped dendrimer has also been used in carbon monoxide sensing. The actual dendrimer, shown as 19, has 48 ferrocene molecules on the periphery. 

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