掌性超分子與掌性嵌段共聚物自組裝之掌性訊息傳遞

Chiral Information Transfer in Self-assembly of Chiral Supramolecules and Chiral Block Copolymers

林士傑(Shih-Chieh Lin)

指導教授 ： 何榮銘

國立清華大學/工學院/化學工程學系/博士(2011年)

https://doi.org/10.6843/NTHU.2011.00215

摘要

In this study, we aim to examine the mechanism of chiral information transfer by using chiral supramolecules and chiral block copolymers for self-assembly so as to achieve the helicity control of helical architectures at different length scales. First, a typical kind of achiral bent-core molecule, 1,3-phenylene bis[4-(4-n-heptyloxybenzoyloxy) benzoates] (BC7), and a linear shaped molecule, 1,4-phenylene bis[4-(4-n- heptyloxybenzoyloxy) benzoates] (LC7), were synthesized for self-assembly. Specific banana phase with chirality has been identified from the BC7 molecules. Systematic studies with respect to the mechanism for the formation of helical superstructures from molecular level were thus carried out. Interestingly, intense CD appeared after the formation of BC7 aggregates with helical superstructures resulting from self-assembly in solution whereas CD is silent and only lamellar crystal can be observed for LC7 aggregates, demonstrating the formation of helical superstructures from molecular chirality due to specific molecular geometry. Nevertheless, the helicity control in those helical morphologies remains challenging. To achieve the helicity control via homochirality transfer from configurational chirality into superstructural chirality, asymmetric chiral bent-core molecules, 3-[(4-{[4-(heptyloxy)benzoyl]oxy} benzoyl)oxy] phenyl 4-[(4-{[(1R)-1-methylheptyl]oxy}benzoyl)oxy] benzoate (BC7R) and 3-[(4-{[4-(heptyloxy)benzoyl]oxy}benzoyl)oxy] phenyl 4-[(4-{[(1S)-1-methylheptyl]oxy}benzoyl)oxy] benzoate (BC7S), by introducing chiral entities to the chain end, have been synthesized for self-assembly. Mirror-imaged CD spectra with split-type Cotton effect can be observed after the formation of self-assembled aggregates of the BC7R and BC7S, suggesting the formation of intermolecular exciton couplet with opposite optical activities for the BC7R and BC7S. Both twisted and helical ribbons with single handedness corresponding to the twisting character of intermolecular exciton couplet can be found in the aggregates. As a result, the mechanism of chiral information transfer for the self-assembly of chiral bent-core molecules to achieve the helicity control of helical architectures at different length scales was suggested.
In contrast to the small molecules or oligomers, macromolecules give rise to another dimension for the self-assembling behavior because of the varieties of chain conformations, in particular helical conformations. Polylactides is a chiral polymer which can be easily obtained by polymerization of L-lactide and D-lactide so as to form enantiomerically pure chiral polymers, poly(L-lactide) (PLLA) and poly(D-lactide) (PDLA), respectively. To determine the type of chiral entities (i.e., configurational chirality) on the polymer backbone, circular dichroism (CD) experiments were executed by observation of carboxylic and carboxylate electric transitions of lactic acids and polylactides, respectively. Positive Cotton effect can be observed in L-lactic acid whereas negative Cotton effect is found in D-lactic acid, suggesting that the UV absorption with respect to carboxylic electric transition of lactic acid are indeed affected by the chiral center. Similar results can also be found in PLLA and PDLA, and the CD intensity of polylactide is three times larger than that of lactic acid. We hypothesize that the enhancement in intensity is attributed to the presence of the helical conformation in chiral polylactide. To further examine the suggested helical conformation and to identify corresponding helicity (i.e., conformational chirality), vibrational CD (VCD) experiments were conducted by observation of IR absorptions with respect to carbonyl stretching motion. Mirror-imaged VCD spectra with split-type Cotton effect can be observed in PLLA and PDLA because of the intramolecular exciton couplet with opposite chirality whereas no specific VCD signal appears in lactic acids due to the absence of chiral effect on corresponding IR absorptions. The VCD results indicate that PLLA and PDLA adopt left- and right-handed helical conformations in solution and also in solid state. Accordingly, a block copolymer (BCP) with a chiral block (i.e., helical conformation) is deigned and referred as chiral block copolymer (BCP*) for self-assembly to fabricate helical nanostructures. A helical phase (H* phase) can be formed in polystyrene-b-poly(L-lactide) (PS-PLLA) and polystyrene-b- poly(D-lactide) (PS-PDLA) whereas no such phase can be obtained in achiral polystyrene-b-poly(L,D-lactide) (PS-PLA), reflecting the chiral effect on BCP self-assembly. However, the handedness of H* phase (i.e., phase chirality) is not able to be directly determined from TEM because of two-dimensional projection problem. To directly visualize real space morphologies, TEM tomography (i.e., three-dimensional TEM) was conducted; opposite handedness of helical microdomains can be found in PS-PLLA and PS-PDLA. As a result, the evolution of chiral information transfer in different levels can be examined so as to give complementary information for homochiral transfer mechanisms in nature.

關鍵字

自組裝 ； 超分子 ； 嵌段共聚物 ； 掌性 ； 旋性 ； 螺旋結構

並列摘要

無資料

並列關鍵字

self-assembly ； supramolecule ； block copolymer ； chirality ； helicity ； helical structure

參考文獻

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