Browsing by Author "Li, Wan-Sheung"

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Configuration Effects of H-Bonded Sites and Rigid Core Lengths on H-Bonded Banana-Shaped Liquid Crystalline Supramolecules Consisting of Symmetric Trimers and Asymmetric Heterodimers
(2009-11-05)
Wang, Ling-Yung
;
Chiang, I-Hung
;
Yang, Po-Jen
;
Li, Wan-Sheung
;
Chao, I-To
Several series of novel banana-shaped H-bonded symmetric trimers (with two H-bonds) and asymmetric heterodimers (with one H-bond) were self-assembled by appropriate molar ratios of proton donors (H-donors) and acceptors (H-acceptors). The influences of H-bonded linking positions and aromatic ring numbers (4-8 aromatic rings in the rigid cores) as well as the chain lengths (n, m = 12 or 16, respectively, in the flexible parts) on the mesomorphism and the switching behavior of the bent-core supramolecules were evaluated and theoretically analyzed. Except for the supramolecular structures with longer rigid cores or shorter flexible chains possessing the rectangular columnar (Col(r) or Bl) phase, the SmC(A)P(A) phase was revealed in most supramolecular asymmetric heterodimers and switched to the SmC(S)P(F) phase by applying electric fields. The polar smectic C phase was dominated for those with H-bonded sites apart from the core center. Interestingly, the SmA and nematic phases were observed in H-bonded asymmetric dimers with H-bonded sites close to the core center, which theoretically proved that the polar smectic C phase was disfavored due to an unfavorable bend angle (smaller than the lower limit of 110 degrees) in the lowest-energy H-bonded conformer. Compared with the fully covalently bonded analogue, lower transition temperatures and lower threshold voltages were developed in the H-bonded asymmetric dimers with the polar smectic C phase. On the basis of the theoretical calculations of molecular modeling, the existence of polar switching behavior in the polar smectic C phase of asymmetric heterodimers; was proven to be associated with their configurations with higher dipole moments and suitable bend angles. Furthermore, the lack of polar switching behavior in supramolecular symmetric trimers, which exhibited the regular SmC phase with weak electrical stabilities, was related to their configurations with smaller dipole moments and confirmed by theoretical calculations.
Regioselectivity in the 1,3-dipolar cycloaddition of adamantylidenefulvene and its modification by inclusion in cyclodextrins' solutions
(2006-07-31)
Chu, Jean-Ho
;
Li, Wan-Sheung
;
Chao, Ito
;
Lee, Gene-Hsiang
;
Chung, Wen-Sheng
The 1,3-dipolar cycloaddition of adamantylidenefulvene (1) with 2 equiv of nitrile oxides 2a-d gave 1/1 cycloadducts, 3a-d and 4a-d, as the major products, and four other 1/2 minor cycloadducts 5-8a,b. The ratios of 1/1 cycloadducts 3a-d to 4a-d in THF solution were about 1/1 in the four different nitrile oxides 2a-d studied and microwave was found to accelerate the reactions and enhance their yields. It is noteworthy that the regioselectivity of 3a/4a was enhanced to 71/29 in beta-cyclodextrin (beta-CD) aqueous solution compared to that of 40/60 in the absence of P-CD. The regioselectivity of 3b/4b was further enhanced to 99/1 when 4-tert-butylphenyl hydroximinoyl chloride (9b) was complexed with beta-CD and then proceeded to react with 1; this is in sharp contrast with that of 33/67 in the absence of beta-CD. The binding constant of 1 center dot beta-CD in acetone-d(6)/D2O (1/1) was determined to be 188 +/- 9 M-1 by H-1 NMR titration experiments. The binding mode of 1 center dot beta-CD was further determined by ROESY experiment. Furthermore, molecular dynamic simulations were carried out to provide information of the complexation modes of 1 center dot beta-CD, 3a center dot beta-CD, 4a center dot beta-CD, 9a center dot beta-CD, and 9b center dot beta-CD. It was found that both steric and electrostatic effects play important roles in determining the regio- and stereochemistry of 1,3-dipolar cycloaddition of 1. Finally, beta-CD is shown to serve as a chiral shift reagent to differentiate the enantiomers of 4a in H-1 NMR. (c) 2006 Elsevier Ltd. All rights reserved.