The effect of a lateral alkyloxy chain on the ferroelectric nematic phase

The synthesis and characterisation of two series of low molar mass liquid crystals, the 4-[(4-nitrophenoxy)carbonyl]phenyl 2-alkoxy-4-methoxybenzoates (series 5-m) and the 4-[(3-fluoro-4-nitrophenoxy)carbonyl]phenyl 2-alkoxy-4-methoxybenzoates (series 6-m) are reported in order to explore the effects of a lateral alkyloxy chain on the formation and stability of the recently discovered ferroelectric nematic phase. In both series m, the number of carbon atoms in the lateral chain, is varied from one to nine. The two series differ by the addition of a fluorine substituent in the 6-m series. 5-1 is the extensively studied ferroelectric nematogen RM734. All the members of the 5-m series exhibited both a conventional nematic, N, and ferroelectric nematic, NF, phase, whereas all the members of the 6-m series exhibit a direct NF–I transition with the exception of 6-1 that also exhibits a N phase. The replacement of a hydrogen atom by a fluorine atom reduces the nematic–isotropic transition temperature, TNI, whereas the ferroelectric nematic–nematic, or isotropic, transition temperature, TNFN/I, increases. This is interpreted in terms of the reduced structural anisotropy associated with the larger fluorine atom whereas the increase in the stability of the NF phase reflects changes in polarity and polarizability. The dependence of TNI and TNFN/I on m in both series is similar, and these initially decrease on increasing m but converge to limiting values on further increasing m. This suggests that the lateral alkyloxy chain may adopt conformations in which it lies along the major axis of the mesogenic unit.


4-(2-Alkoxy-4-methoxybenzoyloxy)benzoic acids (4)
To a pre-dried flask flushed with argon, Compound 3 (1 eq) was dissolved in a mixture of dichloromethane and ethanol and stirred. The mixture was sparged with argon and 5 % Pd/C catalyst was added. The argon atmosphere was evacuated under vacuum and replaced by hydrogen gas. The quantities of the reagents used in each reaction are listed in Table 4. The reaction was allowed to proceed for 4 h at room temperature, with the extent of the reaction monitored by TLC using an appropriate solvent system (RF values quoted in the product data). The hydrogen gas, after the reaction was completed, was evacuated under vacuum and the flask was purged using argon. The mixture was filtered through Celite, and the collected solvent was evaporated under vacuum to leave a solid which was carried forwards without any further purification.  2995,2836,1705,1683,1604,1573,1505,1429,1388,1315,1301,1268,1249,1213,1176,1139,1110,1074,1044,1033,926,883,843,822,781,764,752,738,687,654,586,551,505,459

4-[(4-Nitrophenoxy)carbonyl]phenyl 2-alkoxy-4-methoxybenzoates (5-m)
To a pre-dried flask flushed with argon and kept in an ice bath in order to maintain the temperature at 0°C, Compound 4 (1 eq), 4-nitrophenol (1.3 eq, 0.91 eq for m = 4, 6) and 4dimethylaminopyridine (0.15 eq, 0.1 eq for m = 4, 6) were added. The solids were solubilised with dichloromethane (30 mL) and stirred for 10 min before N-ethyl-N′-(3dimethylaminopropyl)carbodiimide hydrochloride (1.5 eq) (N,N'-dicyclohexylcarbodiimide (1.2 eq) for m = 4, 6) was added to the flask. The quantities of the reagents used in each reaction are listed in Table 5. The temperature of the reaction mixture was increased to room temperature and the reaction was allowed to proceed overnight. For m = 4, 6, the white precipitate which formed was removed by vacuum filtration and the filtrate collected. The solvent was removed under vacuum and the crude product was purified using a silica gel column with an appropriate solvent system (RF values quoted in product data). The eluent fractions of interest were evaporated under vacuum to leave a white solid which was recrystallised from hot ethanol (50 mL).

4-[(3-Fluoro-4-nitrophenoxy)carbonyl]phenyl 2-alkoxy-4-methoxybenzoates (6)
To a pre-dried flask flushed with argon and kept in an ice bath in order to maintain the temperature at 0°C, Compound 4 (1 eq), 3-fluoro-4-nitrophenol (1.3 eq, 0.91 eq for m = 6) and 4-dimethylaminopyridine (0.15 eq, 0.1 eq for m = 6) were added. The solids were solubilised with dichloromethane (30 mL) and stirred for 10 min before N-ethyl-N′-(3dimethylaminopropyl)carbodiimide hydrochloride (1.5 eq) (N,N'-dicyclohexylcarbodiimide (1.2 eq) for m = 6) was added to the flask. The quantities of the reagents used in each reaction are listed in Table 6. The temperature of the reaction mixture was increased to room temperature and the reaction was allowed to proceed overnight. For m = 6, the off-white precipitate which formed was removed by vacuum filtration and the filtrate collected. The solvent was removed under vacuum and the crude product was purified using a silica gel column with an appropriate solvent system (RF values quoted in product data). The eluent fractions of interest were evaporated under vacuum to leave a white solid which was recrystallised from hot ethanol (50 mL).

Reagents
All reagents and solvents that were available commercially were purchased from Sigma Aldrich, Fisher Scientific or Fluorochem and were used without further purification unless otherwise stated.

Thin Layer Chromatography
Reactions were monitored using thin layer chromatography, and the appropriate solvent system, using aluminium-backed plates with a coating of Merck Kieselgel 60 F254 silica which were purchased from Merck KGaA. The spots on the plate were visualised by UV light (254 nm).

Column Chromatography
For normal phase column chromatography, the separations were carried out using silica gel grade 60 Å, 40-63 µm particle size, purchased from Fluorochem and using an appropriate solvent system.

Structure Characterisation
All final products and intermediates that were synthesised were characterised using 1 H NMR, 19 F NMR, 13 C NMR and infrared spectroscopies. The NMR spectra were recorded on a 400 MHz Bruker Avance III HD NMR spectrometer. The infrared spectra were recorded on a Thermal Scientific Nicolet IR100 FTIR spectrometer with an ATR diamond cell.

Purity Analysis
In order to determine the purity of the final products, high-resolution mass spectrometry was carried out using a Waters XEVO G2 Q-Tof mass spectrometer by Dr. Morag Douglas at the University of Aberdeen.