Halogenation in achiral liquid crystals: terminal and linking substitutions

Abstract

The eOE ect on the physico-chemical properties of achiral liquid crystals of introducing halogen atoms into their terminal and linking groups is discussed and rationalized in terms of existing theories; a comparison is made with the corresponding hydrogenated and other well-known groups.

Liquid-Crystalline Oxygen-Containing Heterocyclic Derivatives. 2 Six-Membered Rings Containing Oxygen as Structural Fragments in Liquid Crystals

“…(2) and the previous section it follows that compounds that have a large induced polarizability of their highly conjugated p-electron system (for example, 1,4-phenylene derivatives) exhibit an optical anisotropy D n ¼ n e -n o that is much larger than that of the corresponding saturated derivatives (compounds 3-1 to 3-5, 4-1 to 4-5; Tables 2 and 4). The fluorinated derivatives presented in Tables 2-4 and 6 show similar values of the optical anisotropy that have been observed earlier for other fluorinated derivatives [86]. The influence of the molecular structure of oxygen-containing liquid crystals on the optical anisotropy can be expressed by the following orders: …”

Synthesis and Mesomorphic Properties of New Fluorinated Schiff Base Liquid Crystals

“…As the ionic radius of the terminal substituent increases, the molecules tend to orientate in a parallel arrangement [22]. Thus, smectic polymorphism is not unusual for mesogens with a terminal halogen substituent, and this has been frequently observed as the length of the alkyl or alkoxy chains increases [20,23]. However, the present compound with fluoro substituent exhibited single mesophase.…”

Sulfur as a Structural Element in Calamitic Liquid Crystals. 2 Terminal, Linking, Axial and Lateral Substitutions. 3 Sulfur-Containing Rings

“…It is evident from Tables 2, 4-6, 11, 13 and the literature [4,52,135] that for the define molecular structure of liquid crystals, their dielectric anisotropy decreases approximately in the same sequence as the values of dipole moments for the terminal groups: SO 2 CF 3 , SO 2 CHF 2 , SOCHF 2 , SOCF 3 , SF 5 The total dipole moment and, consequently, the dielectric anisotropy of compounds presented in Tables 2, 4-6, 9-11, and 13 are strongly affected by values and directions of the dipole moments of the terminal, linking, lateral group, and molecular fragments (see Eq. (1)).…”

“…Though other sulfur and=or fluorine-containing phenylcyclohexanes are not mesomorphic (compounds 2-3 to 2-13, Table 2), the isothiocyanato derivatives 2-14 and 2-15 exhibit the nematic phases, which are formed mainly by their dimers [4]. The data presented in Tables 2-6 show that low thermal efficiency of the SF 5 derivatives (see, for example, compounds 2-3, 4-2, 5-1, 5-3, 6-1, [52]) can be increased by the introduction of the perfluoroalkyl connector between the terminal SF 5 group and molecular core of liquid crystals, which now show the nematic phase with moderate clearing point compared to those of the corresponding smectic halogen derivatives (compounds 3-1 to 3-5). It is interesting to note that the introduction of the SF 4 connector between the terminal CF 3 group and molecular core of three-ring derivatives produces only a nematic phase with the highest clearing and melting points among compounds 4-1 to 4-12 presented in Table 4.…”

“…(2) and the previous section, it follows that compounds that have a large induced polarizability of their highly conjugated p-electron system (for example, isothiocyanato derivatives) exhibit an optical anisotropy Dn ¼ n e À n o that is much larger than that of the corresponding halogen(s)-substituted derivatives (compounds 2-3 to 2-8, 2-11, 2-14; 5-1, 5-2; 6-12 to 6-16; Tables 2, 5, 6, [52]). A decrease in optical anisotropy for compounds with the halogenated terminal, linking, and lateral groups (Tables 2, 4 -6, 9-11, 13) can be explained in terms of a reduction in the effective conjugation of the p-electron system resulting in a shorter resonance wavelength of the UV absorption spectrum for the halogen(s)-substituted liquid crystals than for the corresponding parent compounds or having hydrogenated groups [52,[138][139][140][141]. The axial and lateral NCS substitutions do not contribute so much to the optical anisotropy of liquid crystals (Tables 10 and 11) and work as usual lateral substitutions [87].…”