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Dynamic Helical Polymers Assisted by Noncovalent Bonding

Macromolecular helicity with an excess helical sense can also be induced in optically inactive, dynamically racemic helical polymers through specific noncovalent bonding interactions. This section mainly describes the helicity induction in poly(phenylacetylene)s through noncovalent chiral interactions.

Fig. 11.9 Schematic illustration of helicity induction in poly(phenyl-acetylene)s bearing various functional groups (36-41) upon complexation with chiral compounds.

11.3.2.1 Induced Helical Poly(phenylacetylene)s

A cis-transoidal, stereoregular poly((4-carboxyphenyl)acetylene) (36 in Fig. 11.9) was the first example of such a one-handed helical polymer induced by noncova-lent chiral acid-base interactions [66]. Upon complexation with chiral amines in DMSO, a dynamic one-handed helical conformation is immediately induced in the polymer, resulting in the appearance of a characteristic ICD in the long wavelength region of the polymer backbone (300-500 nm). The typical CD spectra of 36 in the presence of various optically active amines (42-46) in DMSO are shown in Fig. 11.10. The remarkable CD induction arises from a drastic change in the population of the interconvertible right- and left-handed helices of the polymer. Primary amines and amino alcohols of the same configuration give the same sign of the ICDs, reflecting the helix-sense of 36, and therefore, the Cotton effect sign of 36 can be used as a probe for sensing the chirality of various primary chiral amines. The magnitude of the ICD tends to increase with the increasing bulkiness of the amines [67]. In sharp contrast to the previously mentioned static and dynamic helical polymers assisted by covalent bonding, the helix-sense can be controlled by the chirality of small chiral molecules after polymerization.

Taking advantage of this helicity induction concept, a variety of chirality-responsive poly(phenylacetylene)s (37-41) has been synthesized by introducing a functional group as the pendant (Fig. 11.9) [68-72]. The sergeants and soldiers and majority rule effects are also observed for the noncovalent helicity induction in the poly(phenylacetylene)s [12, 67]. Among the poly(phenylacetylene)s prepared so far, a poly(phenylacetylene) (41) bearing the bulky aza-18-crown-6-ether, a typical host molecule in host-guest chemistry, as the functional pendant group was the most sensitive to the chirality of chiral molecules, such as amino acids,

Fig. 11.10 CD spectra of 36 upon complexation with chiral amines in DMSO. (Reproduced with permission from Ref. 67. Copyright 1997 American Chemical Society.)

and an almost one-handed helix was induced in 41 in the presence of 0.1 equiv. l-alanine (l-Ala) in acetonitrile (Fig. 11.11B) [72]. This extremely high sensitivity may be ascribed to the rigidity of the polymer backbone by the bulky pendant group, which may increase the helical segments separated by rarely occurring helix reversals. In addition, 41 showed an apparent ICD even with 0.01 equiv. of l-Ala, indicating a remarkable chiral amplification. Moreover, 41 showed the same Cotton effect signs upon complexation with all the common 19 l-amino acids, indicating that 41 is among the most sensitive and practically useful syn-

Fig. 11.11 (A) Schematic illustration of helicity induction on 41 with a small amount of l-AlaHClO4. (B) Titration curves of 41 with l-AlaHClO4 in acetonitrile at 25 °C and —10 °C. (C) Changes in ICD intensity (A£2nd) of 41 vs. the % ee of l-AlaHClO4 during the complexation with 41 in acetonitrile at 25 °C and —10 °C. (Reprinted with permission from Ref. 72. Copyright 2003 American Chemical Society.)

Fig. 11.11 (A) Schematic illustration of helicity induction on 41 with a small amount of l-AlaHClO4. (B) Titration curves of 41 with l-AlaHClO4 in acetonitrile at 25 °C and —10 °C. (C) Changes in ICD intensity (A£2nd) of 41 vs. the % ee of l-AlaHClO4 during the complexation with 41 in acetonitrile at 25 °C and —10 °C. (Reprinted with permission from Ref. 72. Copyright 2003 American Chemical Society.)

thetic receptors for detecting the amino acid chirality. More interestingly, even a 5% ee of Ala produced the full ICD in 41 as induced by the optically pure Ala (Fig. 11.11C). This majority rule effect of 41 enabled detecting an extremely small enantiomeric imbalance in the amino acids, for instance, Ala of less than 0.005% ee, showing an apparent ICD without derivatization.

The chiral recognition of charged biomolecules with synthetic receptor molecules in water through polar interactions is an attractive challenge, but still remains a very difficult problem, which prompted us to explore the one-handed helicity induction in chromophoric water-soluble 36-39 in water. These are poly-electrolytes and can interact with a variety of charged and noncharged biomole-cules involving amino acids, aminosugars, carbohydrates and peptides in water due to an ion condensation effect [73] and hydrophobic interactions, and the complexes exhibited characteristic ICDs in the UV-vis regions [74, 75]. Among the polyelectrolytes, 37b bearing the bulky phosphonate group as the pendant is the most sensitive induced helical polymer in water; the assay of 19 of the common free l-amino acids produced the ICDs of 37b with the same Cotton effect signs, which demonstrates that the polyelectrolyte is indeed the first powerful chirality-sensing probe in water.

H.3.2.2 Hierarchical Amplification of Helical-Sense Excess in Liquid Crystals

A positively charged polyelectrolyte, the hydrochloride of 39 (39-HCl) bearing an ammonium group, also formed an excess helical sense in the presence of various chiral acids such as 47 by a significant amplification of the chirality in water [76]. The polyelectrolyte function of the 39-HCl is crucial for such a high chiral amplification in water, because the neutral 39 is not sensitive to the chirality of chiral acids in organic media [70] and requires a large excess amount of chiral acids to exhibit the full ICD.

During the intensive exploration of the chirality amplification mechanism of 39-HCl, the polymer was found to form a lyotropic, nematic LC in concentrated water (>8 wt%) and the nematic LC phase converted to the cholesteric counterpart by doping with a tiny amount of optically active acids such as (S)-47 or 47 with a low ee [77]. This liquid crystallinity of 39-HCl is based on the main-chain stiffness in water as evidenced by its long persistence length (q) of 26.2 and 28 nm in the nematic and cholesteric LC states, respectively [78]. Interestingly, the helix-sense excess of 39-HCl induced by (S)-47 in dilute solution was further amplified in the LC state. The addition of increasing amounts of (S)-47 results in a tightening of the cholesteric helical pitch, that reached an almost constant value at 0.05 equiv. of (S)-47 in the LC state, whereas a larger amount of (S)-47 (ca 0.3 equiv.) was required in dilute solution for the full ICD. The 39-HCl exhibited a clear cholesteric LC phase showing well-defined fingerprint patterns even in the presence of 0.0005 equiv. of (S)-47 (Fig. 11.12B) and 0.1 equiv. of (S)-47 with 5% ee (Fig. 11.12C). In addition, 39-HCl exhibited a strong majority rule effect for the ee of 47 in the LC phase; the helical pitch decreased with the increasing ee and reached a constant value at about 10% ee. On the other hand, in dilute solution, the ICD value became constant at over 60% ee (Fig. 11.12A). Direct evidence for

Fig. 11.12 (A) Changes in the cholesteric pitch and ICD intensity of 39-HCl versus the % ee of 47 (S rich) in concentrated (20 wt%) and dilute (inset, 1 mg mL_1) water solutions. (B, C) Polarized optical micrographs of cholesteric liquid crystalline phases of 39-HCl (20 wt%) in the presence of 0.001 equivalent of (S)-47 and 5% ee (S rich) of 47 (0.1 equivalent) in water. (D) Plots of the calculated % ee of helical sense-excess values of 39-HCl a chiral dopant ((S)-47 (red) and 47 (S rich) (blue) in the cholesteric LC state

Fig. 11.12 (A) Changes in the cholesteric pitch and ICD intensity of 39-HCl versus the % ee of 47 (S rich) in concentrated (20 wt%) and dilute (inset, 1 mg mL_1) water solutions. (B, C) Polarized optical micrographs of cholesteric liquid crystalline phases of 39-HCl (20 wt%) in the presence of 0.001 equivalent of (S)-47 and 5% ee (S rich) of 47 (0.1 equivalent) in water. (D) Plots of the calculated % ee of helical sense-excess values of 39-HCl a chiral dopant ((S)-47 (red) and 47 (S rich) (blue) in the cholesteric LC state versus those in dilute water. The helical sense-excess values of 39-HCl in dilute and concentrated LC water solutions were calculated using the maximum Ae2nd and qc values of —17.2 and 1.55 as the base values, respectively; qc = (2p/cholesteric pitch). (E) Schematic illustration of hierarchical chiral amplification in macromolecular helicity of 39-HCl in dilute solution and LC state. (Reproduced with permission from Ref. 78. Copyright 2006 American Chemical Society.)

the hierarchical amplification process of the helical sense excess of 39-HCl during the cholesteric LC formation was demonstrated by direct comparison of the excess of the one helical sense of the polymer in dilute solution with that in the cholesteric LC state (Fig. 11.12D) [78]. In the LC state, the population of the helical reversals between the interconverting right- and left-handed helical segments of the polymer may be reduced when compared to that in dilute solution, because the kinked helical polymer chain could likely interfere with the close parallel packing of helical polymer chains in the LC state (Fig. 11.12E) as observed in the LC polyisocyanates by Green et al. [79]. On the basis of the X-ray analyses of the LC samples, the most plausible helical structure of 39-HCl was proposed to be a 23 unit/10 turn (23/10) helix [78].

11.3.2.3 Other Induced Helical Polymers

Taking advantage of the helicity induction concept, a preferred helical conformation has also been induced in other dynamic racemic, chromophoric polymers or oligomers as a result of noncovalent binding of the nonracemic guests (Fig. 11.13). Aliphatic polyacetylenes (48-50) [80-82], polyisocyanates (19, 51 -53) [44, 83-85], poly- and oligosilanes (54-56) [86-89], poly(phenyl isocyanide) (57) [90],

Helicity induction

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