||There are many factors affect the properties of ferroelectric chiral smectic C phase such as chiral group, core structure, linking group, terminal chain length and optical purity in rod-like chiral liquid crystals. The purpose of this research is looking for a better understanding the relationship between molecular structure and mesomorphic properties by changing the molecular architecture from rod-like to hockey-stick.
Thus, new non-fluorinated and semi-fluorinated chrial alcohols, (S)-1-butyloxy-2-propanol, (S)-1-(2,2,3,3,4,4,4-heptafluorobutyloxy)-1-propanol, and (S)-1-(2,2,2- trifluoroethoxy)-2-propanol, were designed and synthesized by the reaction of (S)-(-)-propylene oxide with alcohols under the base condition. These alcohols were then connected to the meta-position of phenyl ring in the compounds by the ester linkage to form compounds I(m,CH2CH2CH3), II(m, CF2CF2CF3) and III(m, CF3). (S)-1-(2,2,3,3,4,4,4-Heptafluorobutyloxy)-1-propanol was connected to the meta-position of phenyl ring in the compounds by the ether linkage to produce compounds IV(m, CF2CF2CF3).
The mesomorphic phases and their corresponding phase transition temperatures were determined by the polarizing microscopic texture and DSC thermograms. The results show the non-fluorinated liquid crystal materials posses SmA* and SmC* phases, and the semi-fluorinated liquid crystal materials enhance the thermal stability of smectic phases. In addition, as the semi-fluorinated chiral alkyl chain at chiral tail extending from -CF3 to -CF2CF2CF3 favors the formation and thermal stability of SmC* phase.
The results also show that compounds having ether linkage (IV(m=8-12)) at m=12 posses SmA* and unidentified SmX1* phases, and at m=8-11 posses SmA* and unidentified SmX1* and SmX2* phases. The electric field driven activities show that compounds IV(m) can be switched in SmX1* and SmX2* phases by the dc field, but the measurements of switching behavior and dielectric constants ε′ can not provide evidence for the existing ferroelectricity or antiferroelectricity in SmX1* and SmX2* phases.
The physical properties such as switching current, dielectric constant, spontaneous polarization, and optical tilt angle in the ferroelectric SmC* phase of chiral compounds were measured. The maximum magnitudes of the spontaneous polarization for compounds I(m=8-12), II(m=8-12), and III(m=7,10,12) are approximately in a range of 12.02-18.30 nC/cm2. Comparing rod-like compounds to hockey-stick compounds that possessing the same non-fluorinated chiral group show that there have nearly the same magnitudes of the maximum Ps value. But when comparing rod-like compounds to hockey-stick compounds that possessing the same semi-fluorinated chiral group show that the hokey-stick compounds have much lower magnitudes of the maximum Ps value. Changing the linking group from ester to ether, the Ps value was suppressed.
The maximum optical tilt angle of compounds I(m=8-12), II(m=8-12), and III(m=7,10,12) are approximately in a range of 26-38°. The results show that compounds in non-fluorinated and semi-fluorinated chiral group have nearly the same maximum optical tilt angle of chiral group. Comparing rod-like compounds to hockey-stick compounds show that the maximum apparent tilt angle of hockey-stick compounds are larger than that of rod-like compounds.
In conclusion, our results indicate that the hockey-stick chiral compounds having fluorinated chiral alkyl chain enhance the thermal stability of smectic phase, especially SmC* phase. It is interesting to find that two chiral smectic phases: SmX1* and SmX2* phases in compounds IV(m=8-12). There has no evidence to prove the existence of ferroelectricity or antiferroelectricity in SmX1* and SmX2* phase.