Effect of Difluoro Substituents on the Formation of TGBA* Phase

• Main Authors: Jyun-Siang Jhang, 張俊翔
• Other Authors: Shune- Long Wu
• Format: Others
• Online Access: http://ndltd.ncl.edu.tw/handle/5c7yza

碩士 === 大同大學 === 化學工程學系(所) === 95 === From the results of recent papers, there are many factors affect the TGB phase formation such as steric, linking group and rigid core structure. The purpose of this research is apt for a better understanding the relationship between molecular structure and TGB phase formation. A homologous series of chiral materials derived from optically active methyl (S)-(-)-lactate has been successfully synthesized and the structures-property relationship investigated in the chiral liquid crystal system. The target compounds were modified independently by the effects of (i) the nonchiral peripheral alkyl chain length ‘m’, (ii) the lateral difluoro substituents in the phenyl ring of the core and (iii) the extending chiral peripheral alkyl chain length “n” on the TGB phase properties. The msomorphic phases and their corresponding transition temperatures for the chiral compounds were determined by polarizing microscopic textures and DSC thmograms. The results in compounds I(m, n, x; m=8-11, n=3, x=H) display SmA* and SmC* phases. Compounds II(m=8-12, 3, F) possess enantiotropic SmA* and SmC*phases. Among them, compounds II(m=8-9, 3, F) display additional the N* and TGBA*, suggesting that an introduction of difluoro-substituents could induce N* and TGBA* phases at the shorter achiral alkyl chain length of the molecules. In the series of compounds III(m, n, x), compounds III(m=8, n=2-5, F) and III(m=9, n=2-3, F) display a mesophase sequence of N*, TGBA*, SmA* and SmC* phases, while compounds III(m=9, n=4-5, F) suppress the TGBA* phase resulting in a phase sequence of N*, SmA* and SmC* phases. This result indicates that compounds with the shorter chiral alky chains are favorable of the formation of TGBA* phase. The magnitudes of spontaneous polarization and apparent title angle in the SmC* phase of the chiral compounds were also measured. The results show that compounds I(m=8-12) have the maximum PS values of about 70-110 nc/cm2, whereas compounds II(m=8-12) have the maximum PS values of about 32-81 nc/cm2. The results indicate that the compounds with lateral difluoro-substituents have lower PS values due to the fluoro-substituents have the transverse dipole opposite to the dipole associated with the carbonyl groups. Compounds I(m, n, x; m=8-12, n=3, x=H) have the maximum tilt angles of 27-32°, whereas compounds II(m, n, x; m=8-12, n=3, x=F) have the maximum tilt angles of 17-24°. These results indicate that the compounds with lateral difluoro-substituents have lower tilt angles values. In conclusion, the results show that addition of difluoro-substituents in the phenyl ring of the molecules induces the formation of TGBA* phase at shorter achiral and chiral alkyl chain length. The TGBA* phase is generated as a result of the competition between the stabilization of the smectic layered structure and the desire for the molecules to form a helix structure. Thus, it is strongly suggested that difluoro-substituents may reduce the stability of the layered structure; reduce the molecular interactions in the layer structure, thus, the TGBA* phase is generated. The results obtained from the measurements of physical properties indicate that the addition of difluoro-substituents on the phenyl ring of the molecules decreases the spontaneous polarization and apparent title angle. The decrease in spontaneous polarization in the difluoro-substituted compounds may be attributed to the difluoro-substituents point away from the direction of the chiral methyl group.