Switchable chirality

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. 

Figure 4.12 The structure of (a) [Pr4 (+)-(10) 9( X3-OH)](ClO4)2 and (b) [Pr3 (+)-(10) (1x3-0H)(H20)3] (C104)2 [18b]. (Reproduced with permission from O. Mamula, M. Lama, H.S. Evans and S. Shova, Switchable chiral architectures containing Prill ions an example of solvent-induced adaptive behavior, Angewandte Chemie International Edition, 2006, 45, 4940. Wiley-VCH Verlag GmbH Co. KgaA.)...

Hnang, Z. Lee, E. Kim, H. J. Lee, M. Aqneous nanofibers with switchable chirality formed of self-assembled dumbbell-shaped rod amphiphiles. Chem. Commun. 2009, 6819-6821. 

To conclude, the results obtained with helical polymer stmctures at surfaces pave the way toward new approaches not only for the construction of new chiral materials, such as chiral selectors and catalysts, but also for the rational design of novel switchable chiral surfaces based on inversion of helicity of macromolecules. 

Tunable and/or switchable chirality can be used for scientific and technical applications. The active parameter to switch chirality can be light, concentration, or temperature. Most commonly used is the photoisomerization of... 

Shinmyozu and co-workers prepared photo-switchable chiral receptor trans- Q9 by linking two chiral 2,2 -dihydroxy-l,l -binapthyl (BINOL) groups with a stiff-stilbene core [172]. This compound could be photochemically isomerized to cis-109. The tmns- Q9 binds fluoride or chloride ions to form 1 1 complexes with... 

In addition, the CD sign of chiral aggregates around the CD reversal point (THF/methanol 55%/45%) is easily switchable, depending on the solvent addition order either method I (methanol added into THF solution of the polysilane) or method II (THF solution of the polysilane added into methanol) (Figure 4.22). This CD switching may be related to the dependence of the... 

To understand how chirality is expressed, it is important to first describe the different thermotropic mesophase assemblies which can be formed by chiral discotics. Even though expression of chirality has been observed in thermotropic mesophases, the chiral expression occurs in a rather uncontrolled manner, and systems which are suitable for applications, for example, easily switchable columns/ferroelectric discotic liquid crystals, consequently have not yet been developed. Hence, the assembly of discotics in solution has received considerable attention. Supramolecular assemblies of discotic molecules in solution are still in their infancy and have not yet found commercial application, but they are of fundamental importance since they allow a detailed and focused investigation of the specific interactions that are required to express chirality at higher levels of organization. As such, the fundamental knowledge acquired from supramolecular assemblies in solution might formulate the design criteria for thermotropic chiral discotic mesophases and provide the necessary tools for the creation of functional systems. 

Irie et al. reported on the absolute reversible photocyclization reactions of achiral diarylethene 52 in chiral crystal leading to 53 in up to 99% ee. The asymmetric photochromic material has potential application in nonlinear optics, for example, in switchable second harmonic generation devices. [32]... 

Like in other chiroptical switches (Section 5.3.1), solvent polarity was found to play an important role. Diastereoselective cyclization was observed in THF and toluene, but not in nonpolar solvents such as n-hexane. Upon photoexcitation, diarylethenes 24 (Scheme 11) can adopt a planar and a twisted conformation, and photocyclization only proceeds through the planar conformation. In the case of chiral diarylethene 27a, there are two diastereomeric planar conformations leading to the diastereomers of the cyclic product 27b. The stereoselectivity in the photocyclization process is enhanced because of a decrease in the excited state energy of the unreactive twisted form, providing a relaxation pathway for the less favorable planar diastereoisomer in more polar solvents. Chiral photochromic diarylethenes are among the most prominent photoswitches known today, featuring nondestructive read-out, excellent reversibility, and the potential for construction of switchable molecular wires and modulation of liquid crystalline phases (see Section 5.5.3).[40,411... 

In these dynamic switchable systems, Green s majority rule is followed, with the group present at higher concentration or the isomer with the stronger chiral inductive effect controlling the helical conformation ofthe polymer.1791... 

The relationship between chirality and molecular-level motion is a complex one. Chirality is not an inherent requirement for generating directional motion and yet in some cases it is precisely what causes molecular level motion to proceed in one direction only. New generations of molecular machines will undoubtedly shed more light on this matter for instance in establishing the processes for which chirality is an absolute requirement in their design. Conversely, some molecular machines have the ability to dramatically influence the expression of chirality through controlled submolecular motion. This feature has potential application in data storage, displays and switchable catalysis. 

CuTPPS D-phenylalanine (removable) assembling pH 9.0 disassembling (chirality is permanent but pH switchable)... 

NQ )2—(Sc " (/ )-pybox)2] by 1 equiv ofDDQ results in reproducing NQ and Sc (7 )-pybox (110). Such formation and dispersion cycles of the chiral 7t-dimer complex in response to ET reduction and oxidation are highly reversible and can be repeated many times as shown in Fig. 40 (110). Thus, the effective redox control on building affinity of the chiral supramolecules affords reversible formation and dispersion of chiral assemblies in response to a simple external signal, for example, an electron, giving achiral-chiral switchability (111, 112). 

Figure 11.20 shows the dynamic behavior of the electrically switchable diffraction grating formed in the LC gel with 1 % gelator and 2% chiral dopant (period 40nm). The repeated switching of the first-order (+1) diffraction efficiency between 0 and 20 V of an applied square-wave electric field (10 s duration for field-off and field-on states) is relatively stable (Fig. 11.20a). Moreover, the change in diffraction efficiency in response to a pulse wave of...

M. Mathews, R.S. Zola, D.-K. Yang, Q. Li, Thermally, photochemically and electrically switchable reflection colors from self-organized chiral bent-cme liquid crystals. J. Mater. Chem. 21, 2098-2103 (2011)... 

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