EFFECT OF MOECULAR STRUCTURES ON THE MESOPHASES AND MESOMORPHIC PROPERTIES OF CHIRAL LIQUID CRYSTALS

• Main Authors: Wang, Kung-Jen, 王冠人
• Other Authors: Wu Shune-Long
• Format: Others
• Language: zh-TW
• Published: 1997
• Online Access: http://ndltd.ncl.edu.tw/handle/12605747432530241269

碩士 === 大同工學院 === 化學工程學系 === 85 === A homologous series of chiral materials derived from optically active, (R)- (+)-alkylmercapto-2-methylpropionic acids, were synthesized and studied. The effects of molecular structures on the mesophases and mesomorphic properties of the liquid crystals can be summarized into three parts. The first part is to synthesize a homologous series of materials, (R)-(+)-6 -(3-alkylmercapto-2-methylpropionyloxy)-2- naphthyl 4-alkoxybenzoates (R)nMMPNDB (n=2~5), in order to explore the influence of increasing the length of periphe- ral terminal aliphatic chain appende in chiral centre on the mesophases and mesomorphic properties. All materials are found to be mesomorphic, and have the phase sequences of Iso-BP- N*-TGBA*-TGBC*-Cr. In this series, all compounds display frustrated phases, (BP, TGBA*, and TGBC*), indicating that the molecu- lar structure of the materials possess high chirality. By ascending the length of the peripheral chiral aliphatic chain, it would be damping the rotational freedom of molecules. THereby, the temperature range of TGBA* phase is depre- ssed. In contract, the temperature range of TGBC* phase is increasing. The second part is to discuss the optical purity on phase behavior. Thus, the enantiomers of (R)- and (S)-6-(3-ethylmercapto-2-methylpropionyloxy)-2- naphthyl 4-alkoxybenzoates were synthesized for binary mixture investigation. The experiment results suggest that the stability of the frustrated phases, ( BP, TGBA*, and TGBC*), are strongly dependent on the optical purity. The third part is to synthesize a homologous series of compounds: 1.(R)2MMPNDB, 2.(R)2MMPNDBC, 3.(R)EMMPNFDB, 4.(R)EMMPNCDB, and 5.(R)EMMPNDB, in terms of various rigid core structures to comprehend the influence on the mesophases and mesomorphic properties. The corresponding phases amd phase sequ- ences are listed below: (R)2MMPNDB: Iso-BP-N*-TGBA*-TGBC*-Cr. (R)2MMPNDBC: Iso-BP-N*-TGBA*-SA*-SC*-Cr. (R)EMMPNFDB: Iso-BP-N*-Cr. (R)EMMPNCDB: Iso-Cr. (R)EMMPNDB: Iso-BP-N*-SA*-Cr. Comparing the mesophases of compound (R)2MMPNDBC to compound (R)2MMPNDB, indicates that three aromatic rings as a part of rigid core possesses higher thermal stability than two aromatic rings. This character is disadvantage for the formation of TGBA* phase and prompts the disappearance of the TGBC* phase. In order to understand the influence of halogen substituents on molecular extensity or steric hindrance in the first phenyl ring, two series of materi- als, (R)EMMPNFDB and (R)EMMPNCDB were synthesized. The results show that the halogen substituents (fluoro or chloro) which are introduced in 3-position of aromatic ring would destroy the linearity of the molecular structures and increase the width of the molecular packing relax each other which decrease the thermal stability of mesophases. Moreover, study on the compound (R)EMMPNDB which has the linking group -O- in compound (R)2MMPNDB replaced by -COO- between achiral peripheral alkyl chain and rigid core, shows that a more polar group raises the packing strength and generally results high clearing transition temperature. To sum up, all new chiral materials (R)nMMPNDB displayed wide temperature range of frustrated phases indicate possessing high chirality. The results from binary mixture study demonstrate the frustrated phases are strongly dependent on optical purity. In addition, the variation of rigid core make the frustrated phases tend to depress and disappear. Therefore, it can be concluded that chiral molecules consist of a straight peripheral chain, a naphthyl benzoate and a alkoxy group attached on the chiral center are beneficial for formation of TGB phases.