Synthesis, Mesomorphic Properties and Application of (R,S)-1-Methylpentyl 4′-Hydroxybiphenyl-4-carboxylate Derivatives

Abstract

Thirteen new liquid crystalline racemic mixtures were synthesized and investigated. For these racemic mixtures, the phase sequences and their changes were determined by polarizing optical microscopy (POM). The phase transition temperatures and transition enthalpies were checked by differential scanning calorimetry (DSC). All new racemates have an anticlinic smectic C_A phase in a broad temperature range. Three highly tilted antiferroelectric mixtures were doped with six racemates at a concentration of 20% by weight. The helical pitch of the prepared mixtures was measured by the spectrophotometry method. All doped mixtures have a longer helical pitch than the base mixtures.

1. Introduction

The racemic mixture, also called racemate, is the mixture of equal quantities of two enantiomers or substances that have dissymmetric molecular structures, i.e., mirror images of one another (Figure 1). Each enantiomer rotates the plane of polarization of plane-polarized light through a characteristic angle, but because the rotatory effect of each component exactly cancels that of the other, the racemic mixture is optically inactive [1,2].

The racemates can be used as dopants for the liquid crystalline mixtures, improving some of their properties, such as the antiferroelectric phase range, the helical pitch length, the reduction of the rotational viscosity, the decrease of the values of the spontaneous polarization, etc. [3,4,5]. Liquid crystalline mixtures are widely used in optical devices; therefore, new materials are constantly being developed for this purpose [6,7,8,9,10,11]. The use of the racemates also leads to a reduction in the preparation costs of the liquid crystalline mixtures. They can also be separated by chiral chromatography (HPLC, UPLC, SFC) [12,13,14,15,16,17,18,19].

Herein, thirteen new racemic mixtures with the acronym: n.(X₁X₂) (R,S) and the general formula shown in Figure 2a are synthesized, and their mesomorphic and thermodynamic properties are measured and discussed.

Six of the racemates are used as dopants for three highly tilted antiferroelectric liquid crystalline mixtures. The base mixtures are described in Refs. [20,21]. All the components of the base mixtures are (S) enantiomers belonging to the same homologous series. The aim of the work is to discuss the mesomorphic properties of the racemates with a wide-temperature anticlinic phase and to demonstrate the advisability of using the racemates as dopants for the antiferroelectric mixtures. If so, it would be possible to extend the range of the antiferroelectric phase and thus increase the helical pitch, which is quite sensitive to doping [3,4,22,23,24,25]. An analysis of the obtained results is presented.

2. Synthesis of the Racemic Mixtures

In this work, thirteen previously unpublished racemates were synthesized by treating (R,S) 4′-(1-methylpentyloxycarbonyl)biphenol with benzoic acid chloride, see Figure 2b. The synthesis of the racemates was carried out as described in Ref. [3]. The benzoic acids were synthesized using the method described in Ref. [26].

For the synthesis of (R,S) 4′-(1-methylpentyloxycarbonyl)biphenol, the method described in Ref. [27] was chosen. The commercially available (R,S)-2-hexanol was used for the synthesis.

The purity of the synthesized racemates was checked using a Shimadzu prominence chromatograph with an SPD-M20A diode array detector. The purity of the racemates was also monitored by thin-layer chromatography (silica gel on aluminum).

The purity of the racemates, determined by HPLC, is shown in

Table 1. The purity of the synthesized racemic mixtures.

The Acronym of the Racemic Mixture	Purity [%]
2.(HH) (R,S)	99.0
2.(HF) (R,S)	99.0
2.(FH) (R,S)	99.7
3.(HF) (R,S)	99.5
3.(FF) (R,S)	99.5
4.(HH) (R,S)	99.9
4.(HF) (R,S)	99.0
5.(HH) (R,S)	99.8
5.(FH) (R,S)	99.8
5.(FF) (R,S)	99.8
6.(FH) (R,S)	99.3
6.(FF) (R,S)	99.7
7.(HH) (R,S)	99.2

. About 1 g of the final racemates were synthesized in all cases. The purity is 99% or more.

The structure of the racemic mixtures was confirmed by ¹H NMR and ¹³C NMR nuclear magnetic resonance. The NMR spectra were acquired on a Bruker Avance III 500 MHz spectrometer. This device has a superconducting magnet, which generates a magnetic field at induction 11.75 T, and for the sample radiation effects at a frequency of 500 MHz for protons and 125 MHz for carbon nuclei. The deuterated chloroform (CDCl₃) as the solvent was used. The spectra of all of the samples were measured at 25 °C.

The ¹H NMR and ¹³C NMR spectra of all racemates were added to the Supplementary Materials (Figures S1–S26). Details of the basic chemical characterization of the new racemates are presented below (the acronyms of the racemates are used).

2.(HH) (R,S)

¹H NMR [ppm]: 0.94 (t, -CH₃), 1.39 (d, -CH₂-); 1.66 (m, -CH₂-); 1.78 (m, -CH₂-); 4.06 (t, -CH₂-); 4.13 (t, -CH₂-); 4.06-4.27 (t, -CH₂-); 5.21 (sext, -CH-), 7.02 (d, C_ArH), 7.34 (d, C_ArH), 7.68 (q, C_ArH), 8.15 (dd, C_ArH).

¹³C {¹H} NMR [ppm]: 14.0 (s, CH₃); 20.1 (s, CH₃); 22.6 (s, CH₂), 27.6 (s, CH₂), 35.8 (s, CH₂), 67.6 (s, CH₂), 68.2 (t, CF), 71.2 (s, CH₂), 71.8 (s, CH), 114.4 (s, C_ArH), 122.2 (s, C_ArH), 122.3 (s, C_Ar), 126.9 (s, C_ArH), 128.3 (s, C_ArH), 129.8 (s, C_Ar), 130.1 (s, C_ArH), 132.4 (s, C_ArH), 137.8 (s, C_Ar), 144.6 (s, C_Ar), 151.1 (s, C_Ar), 162.9 (s, C_Ar), 164.8 (s, C_Ar), 166.1 (s, C_Ar).

2.(HF) (R,S)

¹H NMR [ppm]: 0.94 (t, -CH₃), 1.39 (d, -CH₂-); 1.61 (m, -CH₂-); 1.78 (m, -CH₂-); 4.06 (t, -CH₂-); 4.12 (t, -CF-); 4.25 (t, -CH₂-); 5.22 (sext, -CH-), 6.77 (dd, C_ArH), 7.35 (d, C_ArH), 7.68 (q, C_ArH), 8.09 (s, C_ArH), 8.11 (s, C_ArH), 8.13 (s, C_ArH), 8.14 (s, C_ArH).

¹³C {¹H} NMR [ppm]: 14.0 (s, CH₃); 20.1 (s, CH₃); 22.6 (s, CH₂), 27.6 (s, CH₂), 35.8 (s, CH₂), 67.9 (s, CH₂), 68.2 (t, CF), 71.0 (s, CH), 71.8 (s, CH), 103.1 (d, C_ArF), 110,6 (d, C_ArF), 110.8 (d, C_ArF), 122.2 (s, C_ArH), 126.9 (s, C_ArH), 128.3 (s, C_ArH), 129.8 (s, C_Ar), 130.1 (s, C_ArH), 134.0 (s, C_Ar), 137.9 (s, C_Ar), 144.6 (s, C_Ar), 150.8 (s, C_Ar), 162.3 (s, C_Ar), 162.4 (s, C_Ar), 164.0 (d, CF), 164.9 (s, C_Ar), 166.1 (s, C_Ar).

2.(FH) (R,S)

¹H NMR [ppm]: 0.94 (t, -CH₃); 1.38 (d, -CH₂-); 1.66 (m, -CH₂-); 1.78 (m, -CH₂-), 4.09 (t, -CH₂-), 4.16 (t, -CH₂-), 4.35 (t, -CH₂-), 5.22 (sext, -CH-); 7.09 (t, C_ArH), 7.32 (d, C_ArH), 7.69 (t, C_ArH), 7.95–8.02 (dd, C_ArH); 8.13 (d, C_ArH).

¹³C {¹H} NMR [ppm]: 13.9 (s, CH₃); 20.1 (s, CH₃), 22.6–35.8 (s, CH₂), 68.4 (t, CF); 69.0 (s, CH); 71.1 (s, CH); 71.8 (s, CH); 122.2 (s, C_ArH), 122.7 (d, C_ArF), 126.9 (s, C_ArH), 127.4 (d, C_ArF), 150.9 (d, C_ArF), 151.2 (d, C_ArF), 152.9 (s, C_Ar), 163.0 (d, C_ArF), 166.0 (s, C_Ar).

3.(HF) (R,S)

¹H NMR [ppm]: 0.94 (t, -CH₃); 1.38 (d, -CH₂-); 1.65 (m, -CH₂-); 1.78 (m, -CH₂-); 2.15 (m, -CH₂-); 3.83 (t, -CH₂-); 3.98 (t, -CH₂-); 4.17 (t, -CH₂-); 5.21 (sext, -CH-); 6.7–6.9 (dd, C_ArH); 7.34 (d, C_ArH), 7.68 (q, C_ArH), 8.09 (t, C_ArH), 8.13 (d, C_ArH).

¹³C {¹H} NMR [ppm]: 14.0 (s, CH₃); 20.1 (s, CH₃), 22.6–35.8 (s, CH₂); 64.9 (s, CH₂); 67.8 (t, CF), 69.1 (s, CH); 71.8 (s, CH); 102.9 (d, CF), 110.1 (d, C_ArF); 110.7 (d, C_ArF); 162.4 (d, C_ArF); 162.9 (s, C_Ar), 164.5 (d, C_ArF), 165.0 (s, C_Ar), 166.1 (s, C_Ar).

3.(FF) (R,S)

¹H NMR [ppm]: 0.94 (t, -CH₃), 1.39 (d, -CH₂-); 1.66 (m, -CH₂-); 1.78 (m, -CH₂-); 2.19 (sext, -CH-), 3.86 (t, -CH₂-), 3.99 (t, -CH₂-), 4.28 (t, -CH₂-), 5.21 (sext, -CH-), 6.88 (t, C_ArH); 7.34 (d, C_ArH); 7.69 (t, C_ArH); 7.90 (t, C_ArH); 8.13 (d, C_ArH).

¹³C {¹H} NMR [ppm]: 13.9 (s, CH₃); 20.1 (s, CH₃); 22.6–35.8 (s, CH₂), 66.0 (s, CH₂), 67.8 (t, CH), 68.8 (s, CH₂), 71.8 (s, CH), 108.5 (s, C_ArH), 122.2 (s, C_ArH), 126.9 (s, C_ArH), 127.1 (d, C_ArF), 128.4 (s, C_ArH), 129.9 (s, C_ArH), 130.1 (s, C_ArH), 138.1 (s, C_ArH), 140.4 (dd, C_ArF), 144.5 (s, C_Ar), 150.6 (s, C_Ar), 150.8 (d, C_ArF), 161.9 (s, C_Ar), 166.0 (s, C_Ar).

4.(HH) (R,S)

¹H NMR [ppm]: 0.94 (t, -CH₃), 1.39 (d, -CH₂-); 1.66 (m, -CH₂-); 1.78 (m, -CH₂-); 1.85 (m, -CH₂-); 1.96 (m, -CH₂-); 3.72 (t, -CH₂-); 3.97 (t, -CH₂-); 4.11 (t, -CH₂-); 5.21 (sext, -CH-), 7.00 (d, C_ArH), 7.33 (d, C_ArH), 7.69 (q, C_ArH), 8.13 (d, C_ArH), 8.18 (d, C_ArH).

¹³C {¹H} NMR [ppm]: 14.0 (s, CH₃); 20.1 (s, CH₃); 22.6 (s, CH₂), 25.7 (s, CH₂), 26.2 (s, CH₂), 27.6 (s, CH₂), 35.8 (s, CH₂), 67.8 (s, CH₂), 71.8 (s, CH), 72.6 (s, CH), 114.3 (s, C_ArH), 121.6 (s, C_Ar), 122.3 (s, C_ArH), 126.9 (s, C_ArH), 128.3 (s, C_ArH), 129.7 (s, C_Ar), 130.1 (s, C_ArH), 132.4 (s, C_ArH), 137.6 (s, C_Ar), 144.6 (s, C_Ar), 151.2 (s, C_Ar), 163.5 (s, C_Ar), 164.9 (s, C_Ar), 166.1 (s, C_Ar).

4.(HF) (R,S)

¹H NMR [ppm]: 0.94 (t, -CH₃), 1.39 (d, -CH₂-); 1.66 (m, -CH₂-); 1.82 (m, -CH₂-), 1.96 1.82 (m, -CH₂-), 3.71 1.82 (t, -CH₂-), 3.96 1.82 (t, -CH₂-), 4.08 1.82 (m, -CH₂-), 5.20 (sext, -CH-), 6.70 (dd, C_ArH), 6.81 (dd, C_ArH), 7.33 (d, C_ArH), 7.67 (q, C_ArH), 8.08 (t, C_ArH), 8.13 (d, C_ArH).

¹³C {¹H} NMR [ppm]: 13.9 (s, CH₃); 20.0 (s, CH₃); 22.6 (s, CH₂); 25.6 (s, CH₂); 26.1 (s, CH₂); 27.6 (s, CH₂); 35.8 (s, CH₂); 67.6 (t, CF); 68.3 (s, CH₂); 71.8 (s, CH); 72.6 (s, CH); 102.8 (d, CF); 109.8 (d, CF); 110.8 (d, CF); 122.3 (s, C_ArH), 126.9 (s, C_ArH), 128.3 (s, C_ArH), 129.7 (s, C_Ar), 130.1 (s, C_ArH), 133.9 (s, C_Ar), 137.8 (s, C_Ar), 144.6 (s, C_Ar), 150.8 (s, C_Ar), 162.4 (d, C_ArF), 162.9 (s, C_Ar), 164.7 (d, C_ArF), 165.0 (s, C_Ar), 166.1 (s, C_Ar).

5.(HH) (R,S)

¹H NMR [ppm]: 0.94 (t, -CH₃), 1.39 (d, -CH₂-); 1.63 (m, -CH₂-); 1.74 (m, -CH₂-); 1.89 (sext, -CH-), 3.67 (t, -CH₂-); 3.95 (t, -CH₂-); 4.09 (t, -CH₂-); 5.20 (sext, -CH-), 7.00 (d, C_ArH), 7.32 (d, C_ArH), 7.68 (q, C_ArH), 8.13 (d, C_ArH), 8.18 (d, C_ArH).

¹³C {¹H} NMR [ppm]: 14.0 (s, CH₃); 20.1 (s, CH₃); 22.4 (s, CH₂), 22.6 (s, CH₂), 27.6 (s, CH₂), 28.8 (s, CH₂), 29.2 (s, CH₂), 35.8 (s, CH₂), 67.3 (t, CF), 68.0 (s, CH₂), 71.8 (s, CH), 72.9 (s, CH), 114.3 (s, C_ArH), 121.5 (s, C_Ar), 122.3 (s, C_ArH), 126.9 (s, C_ArH), 128.3 (s, C_ArH), 129.7 (s, C_Ar), 130.1 (s, C_ArH), 132.4 (s, C_ArH), 137.7 (s, C_Ar), 144.6 (s, C_Ar), 151.2 (s, C_Ar), 163.5 (s, C_Ar), 164.9 (s, C_Ar), 166.1 (s, C_Ar).

5.(FH) (R,S)

¹H NMR [ppm]: 0.94 (t, -CH₃), 1.39 (d, -CH₂-); 1.64 (m, -CH₂-); 1.74 (m, -CH₂-); 1.93 (m, -CH₂-); 3.67 (t, -CH₂-); 3.95 (t, -CF-); 4.16 (t, -CH₂-); 5.21 (sext, -CH-), 7.06 (t, C_ArH), 7.31 (d, C_ArH), 7.68 (t, C_ArH), 7.93 (dd, C_ArH), 8.00 (d, C_ArH), 8.13 (d, C_ArH).

¹³C {¹H} NMR [ppm]: 14.0 (s, CH₃); 20.1 (s, CH₃); 22.3 (s, CH₂), 22.6 (s, CH₂), 27.6 (s, CH₂), 28.7 (s, CH₂), 29.2 (s, CH₂), 35.8 (s, CH₂), 67.6 (t, CF), 69.2 (s, CH₂), 71.8 (s, CH), 72.9 (s, CH), 113.4 (s, C_Ar), 117.8 (d, C_ArF), 121.8 (s, C_ArF), 121.9 (s, C_ArH), 126.9 (s, C_ArH), 127.4 (d, C_ArF), 128.4 (s, C_ArH), 129.8 (s, C_Ar), 130.1 (s, C_ArH), 137.9 (s, C_Ar), 144.5 (s, C_Ar), 150.9 (s, C_ArH), 151.8 (d, C_ArF), 152.9 (s, C_Ar), 164.0 (d, C_Ar), 166.0 (s, C_Ar).

5.(FF) (R,S)

¹H NMR [ppm]: 0.94 (t, -CH₃), 1.39 (d, -CH₂-); 1.62 (m, -CH₂-); 1.75 (m, -CH₂-); 1.94 (m, -CH₂-); 3.67 (t, -CH₂-); 3.83 (t, -CH₂-); 4.18 (t, -CH₂-), 5.21 (sext, -CH-); 6.85 (t, C_ArH); 7.36 (d, C_ArH), 7.69 (t, C_ArH); 7.89 (t, C_ArH), 8.13 (d, C_ArH).

¹³C {¹H} NMR [ppm]: 14.0 (s, CH₃); 20.1 (s, CH₃), 22.3–35.8 (s, CH₂); 67.6 (t, CF), 69.7 (s, CH); 71.8 (s, CH); 72.9 (s, CH); 108.4 (s, CH), 111.3 (d, C_ArF), 122.2 (s, C_ArH), 126.9 (s, C_ArH), 127.0 (d, C_ArF), 129.9 (s, C_ArH), 130.1 (s, C_ArH), 138.0 (s, C_Ar), 144.5 (s, C_Ar), 150.6 (s, C_Ar).

6.(FH) (R,S)

¹H NMR [ppm]: 0.94 (t, -CH₃), 1.39 (d, -CH₂-); 1.50 (m, -CH₂-); 1.56 (m, -CH₂-), 1.67 (m, -CH₂-); 1.78 (m, -CH₂-); 1.91 (m, -CH₂-); 3.64 (t, -CH₂-); 3.94 (t, -CH₂-); 4.15 (t, -CH₂-); 5.20 (sext, -CH-), 7.06 (t, C_ArH), 7.31 (d, C_ArH), 7.69 (q, C_ArH), 7.93 (dd, C_ArH), 7.99 (d, C_ArH), 8.13 (d, C_ArH).

¹³C {¹H} NMR [ppm]: 13.9 (s, CH₃); 20.1 (s, CH₃); 22.6 (s, CH₂); 25.5 (d, CF); 27.6 (s, CH₂); 28.9 (s, CH₂); 29.3 (s, CH₂); 35.8 (s, CH₂), 67.6 (t, CF), 69.2 (s, CH₂), 71.8 (s, CH), 73.0 (s, CH), 113.5 (s, C_Ar), 117.7 (d, C_ArF), 121.7 (s, C_ArF), 122.2 (s, C_ArH), 126.9 (s, C_ArH), 127.4 (s, C_Ar), 128.4 (s, C_ArH), 129.8 (s, C_Ar), 130.1 (s, C_ArH), 137.9 (s, C_Ar), 144.5 (s, C_Ar), 150.9 (d, CF), 151.9 (d, CF), 152.9 (s, C_Ar), 164.0 (d, CF), 166.0 (s, C_Ar).

6.(FF) (R,S)

¹H NMR [ppm]: 0.94 (t, -CH₃), 1.39 (d, -CH₂-); 1.48 (m, -CH₂-); 1.55 (m, -CH₂-), 1.66 (m, -CH₂-), 1.78 (m, -CH₂-), 1.91 (quin, -CH₂-); 3.64 (t, -CH₂-); 3.94 (t, -CH₂-); 4.16 (t, -CH₂-); 5.20 (sext, -CH-), 6.86 (t, C_ArH), 7.3 (d, C_ArH), 7.69 (t, C_ArH), 7.88 (t, C_ArH), 8.13 (d, C_ArH).

¹³C {¹H} NMR [ppm]: 13.9 (s, CH₃); 20.1 (s, CH₃); 22.6 (s, CH₂); 25.5 (d, CF); 27.6 (s, CH₂); 28.8 (s, CH₂); 29.3 (s, CH₂); 35.8 (s, CH₂); 67.6 (t, CF); 69.8 (s, CH₂); 71.8 (s, CH), 72.9 (s, CH), 108.5 (s, C), 111.2 (d, CF), 122.1 (s, C_ArH), 126.9 (s, C_ArH), 127.04 (s, C_Ar), 128.4 (s, C_ArH),129.9 (s, C_Ar), 130.1 (s, C_ArH), 138.0 (s, C_Ar), 140.5 (d, CF), 142.5 (d, C_ArF), 144.5 (s, C_Ar), 150.7 (s, C_Ar), 153.1 (s, C_Ar), 161.9 (s, C_Ar), 166.0 (s, C_Ar).

7.(HH) (R,S)

¹H NMR [ppm]: 0.94 (t, -CH₃), 1.39 (d, -CH₂-); 1.52 (m, -CH₂-); 1.66 (m, -CH₂-); 1.78 (m, -CH₂-); 1.85 (sext, -CH-), 3.62 (t, -CH₂-); 3.94 (t, -CH₂-); 4.07 (t, -CH₂-); 5.21 (sext, -CH-), 7.00 (d, C_ArH), 7.32 (d, C_ArH),7.69 (q, C_ArH), 1.18 (d, C_ArH), 8.19 (d, C_ArH).

¹³C {¹H} NMR [ppm]: 14.0 (s, CH₃); 20.1 (s, CH₃); 22.6 (s, CH₂), 25.7 (s, CH₂), 25.9 (s, CH₂), 27.6 (s, CH₂), 29.0 (d, CF), 29.4 (s, CH₂), 35.8 (s, CH₂), 67.6 (t, CF), 68.2 (s, CF), 71.8 (s, CH), 73.1 (s, CH), 114.3 (s, C_ArH), 121.4 114.4 (s, C_Ar), 122.3 (s, C_ArH), 128.3 (s, C_ArH), 129.8 (s, C_Ar), 130.1 (s, C_ArH), 132.3 (s, C_ArH), 137.7 (s, C_Ar), 144.6 (s, C_Ar), 151.2 (s, C_Ar), 163.6 (s, C_Ar), 164.9 (s, C_Ar), 166.0 (s, C_Ar).

3. Measurements

The mesomorphic properties of the racemates and the prepared mixtures were investigated using a polarizing optical microscopy technique. An OLYMPUS BX51 polarizing optical microscope equipped with a Linkam THMS-600 hot stage and a temperature controller TMS-93 were used. The phase sequences and the phase transition temperatures were determined by observation of the characteristic textures and their changes. In addition, differential scanning calorimetry (DSC) measurements were carried out using a Netzsch DSC 204 F1 Phoenix calorimeter. These measurements were performed in a nitrogen atmosphere under 2 °C·min⁻¹ heating and cooling rate.

The helical pitch (p) value measurements were based on the phenomenon of wavelength-dependent selective reflection of light [28]. The selective reflection wavelength was determined in the temperature range of the tilted chiral smectic phases on samples placed on a glass plate coated with a surfactant-promoting homeotropic orientation of the molecular director on the substrate glass plate. The other surface of the smectic slab was left free, which also assures the homeotropic orientation at the air-LC contact boundary. The experimental details have been reported elsewhere [29,30]. The helical pitch was calculated according to Equations (1) and (2) for the antiferroelectric and ferroelectric phases, respectively.

$${\lambda }_{\max }=n\cdot p$$

(1)

$${\lambda }_{\max }=2n\cdot p$$

(2)

Here λ_max is the wavelength at the center of the selective reflection waveband. The value of n = 1.5 was taken for the calculation [31,32]. The transmission spectra were acquired using a Shimadzu UV-VIS-NIR spectrometer UV-3600 at the wavelength range of 360–3000 nm. All the observations of the selective reflection spectra were carried out during the cooling cycle. An MLWU7 temperature controller with a Peltier element was used to drive the temperature within the range of 2–110 °C, with an accuracy of ±0.1 °C.

4. Mesomorphic Properties of the Racemic Mixtures

For the synthesized racemic mixtures, the phase sequences and the phase transition temperatures are summarized in

Table 2. The phase transition temperatures [°C] and enthalpies [J·g⁻¹] of the synthesized racemic mixtures. (“*” means that the phase is observed and “-” means that it is not observed; The enthalpy is indicated by the italics).

Acronym	Cr		SmCA		SmC		SmA		Iso
		74.3		138.6
2.(HH) (R,S)	*	4.9	*	140.3	-		-		*
		34.9		9.9
		54.5		119.8
2.(HF) (R,S)	*	-	*	118.7	-		-		*
		30.1		9.1
		76.3		127.7		130.6
2.(FH) (R,S)	*	-	*	126.8	*	129.6	-		*
		35.5		2.0		6.4
		48.9		104.8
3.(HF) (R,S)	*	-	*	103.3	-		-		*
		22.2		8.2
		56.8		111.2		113.9		115.1
3.(FF) (R,S)	*	30.3	*	104.7	*	113.0	*	114.2	*
		23.9		0.07		1.15		5.5
		52.3; 56.7		144.4				148.4
4.(HH) (R,S)	*	-	*	143.7	-		*	147.4	*
		4.5; 20.2		1.7				6.8
		49.6; 60.2		122.5
4.(HF) (R,S)	*	-	*	122.6	-		-		*
		25.9; 3.2		6.2
		60.4		136.5				143.1
5.(HH) (R,S)	*	-	*	135.9	-		*	142.2	*
		35.6		1.1				6.9
		69.8		108.1		119.6		129.2
5.(FH) (R,S)	*	-	*	95.6	*	118.8	*	127.8	*
		33.6		0.04		1.1		6.3
		72.6		110.9		119.8		126.9
5.(FF) (R,S)	*	14.3	*	101.6	*	118.9	*	125.7	*
		25.2		0.03		0.9		5.6
		59.8		120.8				131.5
6.(FH) (R,S)	*	-	*	120.3	-		*	130.5	*
		25.9		0.75				6.5
		68.4		121.3				129.1
6.(FF) (R,S)	*	40.4	*	120.4	-		*	127.6	*
		26.9		0.76				5.9
		36.0		114.3		131.2		140.1
7.(HH) (R,S)	*	-	*	105.5	*	130.3	*	139.5	*
		13.25		0.03		0.8		6.8

First row—temperatures from DSC measurements obtained in the heating cycle; second row—temperatures from DSC measurements obtained in the cooling cycle; third row—transition enthalpies.

and visualized in Figure 3. The textures for the observed phases (the smectic A phase, the smectic C phase and the smectic C_A phase) are shown in Figure 4a–c.

All racemates have the anticlinic smectic phase in a broad temperature range. The broadest range of this phase is observed for the unsubstituted racemate with the oligomethylene spacer equal to 4 (above 87 °C). For four of the racemates, the direct transition from the SmC_A phase to the isotropic phase is observed. Such properties occur for the racemates with a substitution of the (HF) and (HH) type and a short oligomethylene spacer (n = 2, 3, 4). It has been found that the direct SmC_A-Iso phase transition in the liquid crystalline materials is beneficial for improving the alignment in the electro-optical cell [33,34,35]. The SmC and SmA phases are observed in a short or medium temperature range. The unsubstituted racemates have the highest clearing points. The melting points change irregularly; the lowest temperature is observed for the racemate with the longest oligomethylene spacer (36 °C).

5. Mixtures Compositions and Their Properties

Six racemic mixtures were used as dopants to formulate new multicomponent antiferroelectric mixtures. For the six prepared doped mixtures, the temperatures and enthalpies of the phase transitions, as well as the helical pitch, were examined. The three base mixtures selected for the study, with the acronyms W-458, W-459 and W-460, differ in the number of each doping components. All these mixtures have already been examined for their mesomorphic and thermodynamic properties, as well as the helical pitch [20,21]; therefore, it was possible to analyze the influence of the racemic mixtures on their properties. The base mixtures are characterized by very high values of the tilt angle of the molecules above 40° at lower temperatures. Six new mixtures with the racemic dopants at 20 wt% concentration were prepared (see

Table 3. The compositions of the six prepared mixtures.

Acronyms of the Doped Mixtures	Base Mixtures	Acronyms of the Dopants
W-458A	W-458	2.(FH) (R,S)
W-458B	W-458	3.(HF) (R,S)
W-459A	W-459	5.(FF) (R,S)
W-459B	W-459	7.(HH) (R,S)
W-460A	W-460	4.(HF) (R,S)
W-460B	W-460	6.(FH) (R,S)

).

The phase transition temperatures for the prepared mixtures are summarized in

Table 4. The mesomorphic and thermodynamic properties of the prepared mixtures. (“*” means that the phase is observed and “-” means that it is not observed; The enthalpy is indicated by the italics).

Mixtures	Cr		SmCA*		SmC*		SmA*		Iso
				70.9–73.6		95.0–95.6		100.5–101.7
				48.2–49.6		94.5–94.9		99.8–101.3
W-458A	*	-	*	71.0	*	93.8	*	99.4	*
				58.5		94.1		99.9
				0.08		1.98		4.3
				74.8–76.0		92.8–93.2		97.0–97.7
				57.2–57.8		92.5–92.8		96.4–97.1
W-458B	*	-	*	75.0	*	91.8	*	96.2	*
				63.8		92.0		96.3
				0.15		2.15		4.4
				70.5–71.7		85.9–90.4
				55.4–56.3		84.5–87.6
W-459A	*	-	*	72.2	*	85.1	-		*
				60.5		86.5
				0.08		7.5
				72.5–73.6		86.4–92.4
				61.7–62.5		84.8–89.8
W-459B	*	-	*	73.1	*	84.9	-		*
				64.9		88.2
				0.75		6.1
				78.5–79.2		86.3–91.5
				71.2–72.0		83.8–88.4
W-460A	*	-	*	78.0	*	84.2	-		*
				72.0		87.2
				0.17		6.4
				72.0–73.1		84.0–84.7		88.7–93.2
				63.1–61.9		83.2–84.0		87.3–90.6
W-460B	*	-	*	71.3	*	82.3	*	87.2	*
				63.8		83.4		89.6
				0.07		1.63		4.1

First row—temperatures from POM measurements obtained in the heating cycle; Second row—temperatures from POM measurements obtained in the cooling cycle; Third row—temperatures from DSC measurements obtained in the heating cycle; Fourth row—temperatures from DSC measurements obtained in the cooling cycle; Fifth row—transition enthalpies [J·g⁻¹].

and visualized in Figure 5 (comparing them with the base mixtures).

All doped mixtures have the antiferroelectric phase (SmC_A*) in a very broad temperature range. The antiferroelectric phase range exceeds 70 °C for each of the doped mixtures. The broadest range of this phase is shown by the mixtures W-458B and W-460A doped with the racemic mixtures with the direct SmC_A-Iso phase transition with the acronyms 3.HF (R,S) and 4.HF (R,S). The ferroelectric phase (SmC*) is present in all doped mixtures in a medium or a broad temperature range. The broadest range of this phase is shown by the mixtures W-458A and W-458B (23 and 17 degrees Celsius, respectively). The smectic A* phase occurs in a short temperature range in the mixtures W-458A, W-458B and W-460B.

All doped mixtures have higher clearing points than the base mixtures. None of the doped mixtures crystallized, and the DSC measurements were carried out from −15 °C.

The comparison of the helical pitch versus temperature for the base mixtures and the doped mixtures is shown in Figure 6a–c.

As could be expected (and of benefit), the doped mixtures are characterized by a longer helical pitch than the base mixture in both chiral phases. In the antiferroelectric phase, the helical pitch increases with increasing temperature, while in the ferroelectric phase, the helical pitch practically does not change upon heating, and it is very short (below 250 nm). The helical pitch in the SmC_A* phase reaches maximum values above 1400 nm for the mixtures W-460A and W-460B, while for the mixture W-460B, it occurs at a lower temperature.

6. Conclusions

The newly synthesized racemates exhibit a wide temperature range for the anticlinic smectic C_A phase and are promising components of the liquid crystalline mixtures. The synclinic smectic C phase and smectic A phase in these racemates depend on the length of the oligomethylene spacer and the substitution of the benzene ring. The widest temperature range of the SmC_A phase is observed for the unsubstituted racemic mixtures.

The direct SmC_A-Iso transition is observed for the racemic mixtures with a short oligomethylene spacer: 2.(HH) (R,S), 2.(HF) (R,S), 3.(HF) (R,S) and 4.(HF) (R,S). The SmC and SmA phases occur in a short or medium temperature range. New racemic mixtures can also be used as dopants which increase the helical pitch values and extend the temperature range of the SmC_A* phase in the highly tilted antiferroelectric mixtures.

The mixtures doped with the racemates have favorable physical and electro-optical properties, as previously shown in Refs. [3,4,5]. In the next step, the measurements of the tilt angle of the molecules and the spontaneous polarization for the doped mixtures will be carried out.

It is also planned to develop the compositions of the other antiferroelectric liquid crystalline mixtures, based on the synthesized racemates, and separate the racemates into enantiomers by chiral liquid chromatography [36,37,38,39,40].