Inversion of Macromolecular Helicity

Another interesting and unique feature of dynamic helical polymers is the reversible helix inversion (helix-helix (PM) transition) between right- and left-handed helical conformations regulated by external stimuli, such as a change in temperature, solvent or by irradiation with light. Because the extremely high sensitivity of dynamic helices to a chiral environment arising from a high cooperativity, the formation of an excess of the preferred helical sense can be altered, resulting in the inversion of helicity. Biological polymers such as DNA [111] and polypeptides [112] with specific sequences are known to undergo inversion of the helicity driven by a change in the salt concentration and temperature, respectively. Some static helical polymers (poly-5) [21] and chloral oligomers (11, 12) [22, 23] also exhibit a transition in their helicities, but their processes are not reversible, and racemization. Several synthetic, dynamic helical polymers exhibit a reversible PM transition by changing the external conditions, such as temperature (31, 70-75), solvent (29, 30, 33a,b, 34b, 76) or by light irradiation (77).

Zentel and Mager found that the helix-sense of an optically active polyisocya-nate copolymer containing a photosensitive azobenzene side group (77) can be switched upon the photochemical trans-cis isomerization [113]. The CD spectral pattern of the copolymer was completely inverted upon photoirradiation.

Scheme 11.6

Fig. 11.17 (A) An illustration of the PM transition of a rigid polysilylene with P- and M-73 helical motif. (B) CD and UV absorption spectra of 78 at —40 °C (solid line) and —5 °C (dotted line) in isooctane. The right-and left-handed helices of 78 are not enantiomers, but diastereomers because of the presence of enantiopure pendants. Therefore, their CD spectra differ from one another after the helicity inversion. (Reproduced with permission from Ref. 114. Copyright 2000 American Chemical Society.)

Fig. 11.17 (A) An illustration of the PM transition of a rigid polysilylene with P- and M-73 helical motif. (B) CD and UV absorption spectra of 78 at —40 °C (solid line) and —5 °C (dotted line) in isooctane. The right-and left-handed helices of 78 are not enantiomers, but diastereomers because of the presence of enantiopure pendants. Therefore, their CD spectra differ from one another after the helicity inversion. (Reproduced with permission from Ref. 114. Copyright 2000 American Chemical Society.)

Fujiki and coworkers synthesized a series of homopolymers and copolymers of optically active helical polysilanes to develop chiral switchable materials based on the inversion of helicity. They found that poly((S)-3,7-dimethyloctyl-3-methylbutylsilane) (78), a family of rod-like helical polysilanes, undergoes a thermo-driven PM transition through a transition temperature (Tc) at —20 °C in isooctane; above and below the Tc, the polymer showed opposite Cotton effect signs to each other (Fig. 11.17B) [47, 114]. The inversion of helicity is sensitive to the structure of pendants, and an analogous polysilane (79) bearing a slightly different b-branched achiral side chain showed no inversion of the CD from —90 to +80 °C. Although the origin of the PM transition remains obscure, Fujiki et al. reported a double-well ("W") shape potential energy curve for 78, which may be responsible for the thermo-driven PM transition. In contrast, 79 showing no PM transition exhibits an unclear double well potential curve. Fujiki et al. further demonstrated that it is possible to control the PM transition temperature by co-polymerization with appropriate achiral monomers (70, 71) or by changing the molecular shape of the hydrocarbon solvents [47].

Green et al. also reported designer polyisocyanates (72, 73) showing an inversion of the helicity with a desired Tc in dilute solution by the copolymerization of paired structurally different enantiomers, which are in competition with each other in helical sense preferences [115]. They further applied this concept to the lyotropic LC state formed by poly(n-hexyl isocyanate) (19) using the thermo-driven switchable polyisocyanates as chiral dopants. The addition of 73 showing a Tc near 30 °C to a nematic solution of 19 gave rise to a typical finger texture above and below the Tc due to a cholesteric LC phase. Changing the temperature

Fig. 11.18 Schematic illustration of the macromolecular helicity inversion in dilute solution and 2-D crystal state. (A) CD and absorption spectra of d-30 measured in THF, chloroform, and benzene. The helix-sense of d-30 in benzene inverts to the opposite helix-sense in THF and chloroform. (B) Rodlike helical 30 self-assembles into 2-D helix bundles with the controlled helicity upon exposure of each organic solvent vapor. The one-handed 2-D helix bundles of d-30 further invert to the opposite handedness by exposure to benzene vapor on the substrates. AFM images of 2-D self-assembled d-30 on HOPG and helical structures of d-30 proposed by AFM and X-ray analyses are shown. (Reproduced with permission from Ref. 117. Copyright 2006 American Chemical Society.)

Fig. 11.18 Schematic illustration of the macromolecular helicity inversion in dilute solution and 2-D crystal state. (A) CD and absorption spectra of d-30 measured in THF, chloroform, and benzene. The helix-sense of d-30 in benzene inverts to the opposite helix-sense in THF and chloroform. (B) Rodlike helical 30 self-assembles into 2-D helix bundles with the controlled helicity upon exposure of each organic solvent vapor. The one-handed 2-D helix bundles of d-30 further invert to the opposite handedness by exposure to benzene vapor on the substrates. AFM images of 2-D self-assembled d-30 on HOPG and helical structures of d-30 proposed by AFM and X-ray analyses are shown. (Reproduced with permission from Ref. 117. Copyright 2006 American Chemical Society.)

thus allows one to control the mesoscopic cholesteric states of opposite twist sense.

Helical polyacetylenes bearing amino acids as the pendants also showed inversion of the helicity (29, 30, 34b) by changing the temperature or solvent, mainly resulting from the ''on and off'' fashion of the intramolecular hydrogen bonding between the pendant amide groups in nonpolar and polar solvents, respectively [49, 54, 63, 116]. The direct evidence for the macromolecular helicity inversion of a helical 30 in different solvents has been reported based on AFM observations of the diastereomeric helical structures (Fig. 11.18) induced in polar and nonpolar solvents, followed by deposition on graphite. The diastereomeric helical 30 further self-assembled into ordered, 2D helix-bundles with controlled molecular packing, helical pitch, and handedness on graphite upon exposure to each solvent. The macromolecular helicity of the helical macromolecules deposited on graphite from a polar solvent was further inverted into the opposite handedness by exposure to a specific nonpolar solvent, and these changes in the surface chir-ality based on the inversion of helicity could be visualized by AFM with molecular resolution [117], and the results were quantified by X-ray diffraction of the oriented liquid crystalline, diastereomeric helical polymer films.

Switching of the macromolecular helicity by chiral stimuli is a current challenging issue, but still remains rare, although such switching materials can be used for sensing the chirality of specific chiral guests. A poly(phenylacetylene) bearing an optically active (1R,2S)-norephedrine residue (80) was the first example of helix inversion induced by chiral stimuli [118]. The Cotton effect signs of 80 were inverted in the presence of excess (R)-mandelic acid ((R)-81), while the ICD of 80 hardly changed with an excess (S)-81, suggesting that 80 undergoes a transition from one helix to another in the presence of (R)-81.

Scheme 11.7

Introducing optically active cyclic host molecules, such as a-, b-, and g-cyclodextrin (CyD) residues to a dynamic helical polyacetylene backbone as the pendant groups (82) provides a conceptually new direct colorimetric detection-system for neutral chemical species including enantiomers as well as solvent and temperature based on the macromolecular helicity inversion (Fig. 11.19A). The helicity inversion was accompanied by a color change due to a change in the twist angle of the conjugated double bonds (tunable helical pitch) that was readily visible by the naked eye and could be quantified by absorption and CD spectroscopies. In particular, 82b bearing b-CyD residues is sensitive to achiral and chiral stimuli and exhibits an inversion of helicity induced by inclusion complexation with guest molecules into the chiral b-CyD cavity [119]. When 1-adamantanol (83) or (—)-borneol (84) was added to the 82b solution, the solution color immediately changed from yellow-orange to red accompanied by the inversion of the Cotton effect signs and a large red-shift of lmax (Fig. 11.19B), whereas, cyclooctanol (85) and cyclohexanol (86) neither produced such a dramatic color change in the solution nor the Cotton effect inversion. 82g also showed a similar CD inversion accompanied by a color change in response to the specific guest molecules capable of interacting with g-CyD. In addition, the racemic 45 and (R)-rich 45 of 50% ee could not induce a conformational change in 82b, resulting in almost no change in their absorption and CD spectra, while 82b is sensitive to the chirality of (S)-45, and (S)-rich 45 of 50% ee showed a sig-

Left-handed He fix Right-handed Helix

Fig. 11.19 (A) Schematic illustrations of a possible conformational change of 82b. (B) Visible color changes in 82b in DMSO-water (8/2, v/v) by the addition of 83-86. (C) CD and absorption spectral changes of 82b in alkaline water (pH 11.7)-DMSO (7/3, v/v) in the presence of 0-100% ee of 45 at 25 °C. (Reprinted with permission from Ref. 120. Copyright 2006 American Chemical Society.)

Wavelength (nm)

Fig. 11.19 (A) Schematic illustrations of a possible conformational change of 82b. (B) Visible color changes in 82b in DMSO-water (8/2, v/v) by the addition of 83-86. (C) CD and absorption spectral changes of 82b in alkaline water (pH 11.7)-DMSO (7/3, v/v) in the presence of 0-100% ee of 45 at 25 °C. (Reprinted with permission from Ref. 120. Copyright 2006 American Chemical Society.)

nificant change in the CD and absorption spectra as well as 100% ee of (S)-45 (Fig. 11.19C) [120]. The dynamic helical conformations of 82b showing opposite Cotton effect signs in DMSO and alkaline water could be further fixed by intramolecular crosslinking between the hydroxy groups of the neighboring b-CyD units in each solvent. The crosslink between the pendant CyD units suppressed the inversion of the helicity; therefore, the crosslinked 82bs showed no Cotton effect inversion [120].

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