A heptamethyltrisiloxane liquid crystal (LC) exhibiting I-SmA^{*}-SmC^{*} phases has been characterized by calorimetry, polarizing microscopy, x-ray diffraction, electro-optics, and dielectric spectroscopy. Observations of a large electroclinic effect, a large increase in the birefringence (Δn) with electric field, a low shrinkage in the layer thickness (∼1.75%) at 20 °C below the SmA^{*}-SmC^{*} transition, and low values of the reduction factor (∼0.40) suggest that the SmA^{*} phase in this material is of the de Vries type. The reduction factor is a measure of the layer shrinkage in the SmC^{*} phase and it should be zero for an ideal de Vries. Moreover, a decrease in the magnitude of Δn with decreasing temperature indicates the presence of the temperature-dependent tilt angle in the SmA^{*} phase. The electro-optic behavior is explained by the generalized Langevin-Debye model as given by Shen et al. [Y. Shen et al., Phys. Rev. E 88, 062504 (2013)10.1103/PhysRevE.88.062504]. The soft-mode dielectric relaxation strength shows a critical behavior when the system goes from the SmA^{*} to the SmC^{*} phase.

Calamitic liquid crystals based on 5-phenyl-pyrimidine derivatives have been designed, synthesized, and characterized. The 5-phenyl pyrimidine core was functionalized with a chiral (R,R)-2,3-epoxyhexoxy chain on one side and either siloxane or perfluoro terminated chains on the opposite side. The one involving a perfluorinated chain shows SmA^{*} phase over a wide temperature range of 82 °C, whereas the siloxane analog exhibits both SmA^{*} and SmC^{*} phases over a broad range of temperatures, and a weak first-order SmA^{*}-SmC^{*} transition is observed. For the siloxane analog, the reduction factor for the layer shrinkage R (relative to its thickness at the SmA^{*}-SmC^{*} transition temperature, T_{AC}) is ∼0.373, and layer shrinkage is 1.7% at a temperature of 13 °C below the T_{AC}. This compound is considered to have "de Vries smectic" characteristics with the de Vries coefficient C_{deVries} of ∼0.86 on the scale of zero (maximum-layer shrinkage) to 1 (zero-layer shrinkage). A three-parameter mean-field model is introduced for the orientational distribution function (ODF) to reproduce the electro-optic properties. This model explains the experimental results and leads to the ODF, which exhibits a crossover from the sugar-loaf to diffuse-cone ODF some 3 °C above T_{AC}.

All reagents were purchased from Sigma Aldrich, Fluorochem, Alfa Aesar, ABCR and used without any further purification. Solvents were purchased from Sigma Aldrich, DMF was purchased pre-dried, THF was dried using a sodium/benzophenone still under N2. All reactions were generally carried out under argon using oven-dried glassware. TLC plates were performed on Merck silica gel 60 F254 and were visualized using a 254 nm light source. Flash column chromatography was performed on Fluorochem silica gel 60 (40-63 micron).1 H and 13 C spectra were recorded at 25 o C (CDCl3 as solvent and TMS as reference) using either a Bruker 400 MHz Ultrashield (Avance 400) or Bruker 300 MHz Ultrashield. HRMS spectra were recorded using a Waters -TOF Electrospray micromass LCT premier. Scheme 1: Reagents and conditions: a) K2CO3, 11-bromo-1-undecene, DMF, 80 o C; b) t BuLi, trimethylborate, THF,-78 o C; c) (S)-(+)-2-Octanol, Na, toluene, 55 o C; d) K2CO3, Pd(PPh3)4, toluene/MeOH/H2O, reflux; e) 1,1,1,3,3,5,5-heptamethyltrisiloxane, Karstedt's catalyst, THF.1, 1-bromo-4-(undec-10-en-1-yloxy)benzene 4-Bromophenol (3.46 g, 20.00 mmol) and potassium carbonate (4.15 g, 30.03 mmol) were added to a 100 mL round bottom flask with a condenser and put under nitrogen. 40 mL of dry DMF was added and the suspension stirred. Undec-10-enyl bromide (5.40 g, 23.16 mmol) was added to the solution drop-wise. The reaction was stirred for 8 hours at 80 o C. The suspension was filtered and DMF was evaporated in vacuo. 30 mL of water was added and the compound extracted with dichloromethane (3 x 30 mL). The organic phase was dried and solvent evaporated. The resulting oil was passed through a short silica column using hexane and concentrated to yield colourless oil (5.20 g, 16.04 mmol, 80 %).

Two approaches exist in the literature for describing the orientational distribution function (ODF) of the molecular directors in SmA* phase of liquid crystals, though several models are recently proposed in the literature for explaining the de Vries behaviour. These ODFs correspond to either the conventional unimodal arrangements of molecular directors arising from the mean field theory that leads to the broad or sugar-loaf like distribution or to the "diffuse-cone-shaped" type distribution proposed by de Vries. The hypothesis by de Vries provides for a realistic explanation as to how at a molecular level, a first-order SmA* to SmC* transition can occur where the uniform molecular director azimuthal distributions condense to values lying within a narrow range of angles; finally these condense to a single value while at the same time ensuring a little or no concomitant shrinkage in the layer spacing. The azimuthal distribution of the in-layer directors is probed using IR and polarized Raman spectroscopic techniques. The latter allows us to obtain the ODF and the various order parameters for the uniaxial and the biaxial phases. Based on the results of these measurements, we conclude that the "cone-shaped" (or volcano-shaped) de Vries type of distribution can most preferably describe SmA* where "a first-order phase transition from SmA* to SmC*" and a low layer shrinkage can both be easily explained.

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Two new ferroelectric liquid crystalline compounds using 5-phenyl-pyrimidine core terminated by achiral trisiloxane chain, and chiral (S) alkyl chain on the opposite ends of their respective mesogens are designed with better characteristics of switching speed and broader temperature range.

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