Liquid crystalline materials containing pyrimidine rings

ABSTRACT

Liquid crystals with their unique ordered mesophases and anisotropic physical properties, such as optical, magnetic and dielectric anisotropy, have found wide opto-electronic applications, especially liquid crystal displays (LCDs) in digital electronic devices, such as smart phones, tablets, laptops, TVs, etc. Liquid crystals incorporating 2,5-disubstituted pyrimidine rings in their aromatic cores are often used in commercial LCDs. They are also critical building blocks of liquid crystalline organic semiconductors designed for use in novel organic electronic devices, such as organic light-emitting diodes (OLEDs), organic photovoltaics (OPVs), sensors, etc. This review will focus specifically on pyrimidine-based liquid crystals, such as calamitic, bent, discotic, taper-shaped, small molecules, oligomers and polymers. It will highlight the relationships between molecular structure and liquid crystalline behaviour, i.e. mesomorphism of relevance to organic and opto-electronic applications. It will highlight the development, optimisation and innovation of liquid crystalline materials containing pyrimidine rings, thereby stimulating the potential for new structural design and applications, such as sensors, flexible electronics, 4D printing and subsequently opening up completely new markets for these advanced functional materials.

GRAPHIC ABSTRACT

The electron-withdrawing pyrimidine units containing two nitrogen atoms at C-3 and C-5 positions in six-membered aromatic rings play a vital role in constructing various liquid crystalline materials, such as calamitic, bent, discotic, taper-shaped, photopolymerised, oligomer and polymer, due to the presence of characteristic mesophases and opto-electronic properties compared with the traditional 1,4-phenylene rings. The heterocyclic pyrimidine-based liquid crystals with a reduced-symmetry structure, highly conjugated π-deficient aromatic rings, and good metal coordinating ability are often used as functional mesogens for application in some promising opto-electronic devices such as ferroelectrics, organic semiconductors, and sensors.

KEYWORDS:

• Pyrimidine liquid crystals
• ferroelectrics
• organic semiconductors
• sensor

1. Introduction

Liquid crystals are currently an important phase of matter both scientifically and technologically. They are recognised as a stable phase exhibited by many compounds, thus putting them on equal footing to the solid, liquid, and gas phases. This situation is not a recent development and liquid crystals have been used in liquid crystalline displays (LCDs) for over 50 years [1,2]. Benefiting from the great commercial success of LCDs, liquid crystalline materials have attracted much attention from chemists and physicists in both academic and industrial fields. A variety of liquid crystal molecular structures have been designed and reported since the discovery of the first liquid crystal ‘cholesteryl benzoate’ by the Austrian botanist Friedrich Reinitzer in 1888 [3,4]. Undoubtedly, the major applications of liquid crystal materials are for displays. However, most of high-end active-matrix LCDs, such as TVs, computer screens, smartphones, pads, and so on, require nematic materials with low viscosity, medium birefringence (0.1), high resistivity, large dielectric anisotropy, and excellent stability [5]. To fulfil above-mentioned properties, mixed liquid crystals such as fluorinated LC compounds, phenyl LC compounds, heterocyclic LC compounds, are commonly employed. But in this article, we stressed pyrimidine compounds as the reviewing perspective to give a summary of liquid crystal molecular structure, properties and application. Figure 1 gives the schematic exhibition of the review article structure.

Figure 1. (Colour online) The schematic exhibition of the review article structure.

Among these molecular structures, pyrimidine units containing two nitrogen atoms at C-1 and C-3 positions in a heterocyclic, six-membered aromatic ring, play a vital role in defining the mesophases and opto-electronic properties of liquid crystalline materials. The presence of heteroatoms, such as nitrogen (N), sulphur (S) and oxygen (O), for example, in aromatic rings and/or flexible chains, in the molecular structures of liquid crystals co-determines their mesogenic behaviour and other physical properties. These properties may differ to a significant degree from those of their corresponding non-heterocyclic analogues due, for example, to differences in polarisability and geometric shapes [6–9]. Overall, the presence of mesophases is closely related to their intrinsic molecular structures, such as geometric shape, length-to-breadth ratio, nature and shape of their aromatic cores, nature, position and length of flexible chains, molecular polarity and polarisability, etc. The schematic exhibition is clearly shown in Figure 2.

Figure 2. (Colour online) The schematic exhibition of structures and mesophases for pyrimidine liquid crystals based on geometric shape, flexible chains, molecular polarity and polarizability, etc.

Upadhyay et al. [10] compared the physical properties and mesomorphism of nitrogen-substituted liquid crystals and those of their non-heterocyclic analogues. They compared the values for the degree of polarisability, electronegativity, conjugation (delocalisation), HOMO-LUMO energy gaps and mesomorphic properties of 4-cyano-4’-pentylbiphenyl (5CB) with those of its nitrogen-containing analogues (see Table 1). We can see clearly from Table 1 that the pyrimidine compounds do significantly increase the melting temperature, but the effect of pyrimidine compounds on the melting temperature and solubility in eutectic mixtures can vary depending on specific conditions and the nature of the compounds involved. Pyrimidine compounds, which are nitrogen-containing heterocyclic aromatic molecules, may form stronger intermolecular interactions, such as hydrogen bonding or π-π stacking, with other components in a mixture. These interactions can lead to an increase in the melting temperature of the resulting eutectic mixture. The enhanced bonding between pyrimidine compounds and other components can raise the energy required to break these bonds during the melting process, effectively elevating the melting temperature. The relationship between melting temperature and solubility is intricate. In general, an increase in melting temperature may lead to a decrease in solubility, especially if the components of the eutectic mixture are not able to form strong interactions with the surrounding solvent or matrix. Higher melting temperatures often imply stronger cohesive forces between molecules, making it more difficult for the solute to dissolve. But if the pyrimidine compounds could form strong interactions with the solvent, such as hydrogen bonding, they would improve the solubility. Besides, when some branched alkyl chains are designed to construct the pyrimidine compounds, it will also decrease the melting points and solubility. Therefore, it is crucial to note that the behaviour of pyrimidine compounds can vary based on their specific chemical structure, as well as the other components present in the mixture. The solvent or matrix used for formulation also plays a significant role. The presence of functional groups and the overall molecular geometry of the pyrimidine compound can influence its interactions with other components, affecting both the melting temperature and solubility. In summary, while there is a tendency for pyrimidine compounds to increase melting temperatures and potentially decrease solubility in eutectic mixtures, the actual impact depends on the specific chemical context. Careful consideration of the molecular interactions and formulation conditions is necessary to predict and control these effects accurately. Schilling et al. [11] reported a series of interesting boron liquid crystals containing heteroaryl rings, such as 2,5-disubstituted pyrimidine and pyridine rings (see Figure 3). Lai’s group [12,13] reported a new class of pyrimidine-based discotic liquid crystals exhibiting high clearing points and wide temperature ranges for their columnar mesophases (see Table 2). The authors elucidated their structure–property relationships, such as the effects of the nature and position of polar groups in terms of electron-withdrawing effects, for example. The presence of the nitrogen atoms of the 2,5-disubstituted pyrimidine rings is considered to be a key factor that determines the enhanced mesomorphic properties with respect to their non-heterocyclic analogues. In addition, the presence of heterocyclic aromatic rings, such as the 2,5-disubstituted pyrimidine moiety, in disk-shaped molecules appears to favour the formation of columnar phases. Majumdar et al. [14,15] for example, studied the mesogenic behaviour of pyrimidine-based liquid crystals incorporating a cholesteryl ester with a number of chiral centres (see Figure 4). The presence of cholesterol-based molecular units in a liquid crystal can help stabilise frustrated fluid phases, such as blue phases and twist grain boundary phases.

Figure 3. (Colour online) The chemical structures, mesophase transition temperatures and POM textures of boron pyrimidine liquid crystals [11].

Figure 4. (Colour online) The chemical structures, mesophase transition temperatures and POM textures of cholesteric liquid crystals.

Table 1. The chemical structures and mesophase transition temperatures (^oC) of nitrogen-substituted 5CB derivatives.

Ref.	No.	R1	R2	X	Y	Mesophase Transition Temperatures (^oC)
[10]	LC1 (5CB)	C5H11	CN	CH	CH	Cr 22.5 N 35 I
	LC2	C5H11	CN	N	N	Cr 71 N (52) I
	LC3	CN	C5H11	N	N	Cr 96 N 109 I

Table 2. The chemical structures and mesophase transition temperatures (^oC) of discotic pyrimidine liquid crystals [12,13].

Ref.	No.	X1	X2	X3	Y1	Y2	Y3	Z1	Z2	Z3	Mesophase Transition Temperatures (^oC)
[12]	LC8	OC8H17	OC8H17	OC8H17	OC8H17	OC8H17	OC8H17	H	H	H	Cr1 29.8 Cr2 38.9 ColL 90.5 I; I 87.8 ColL −0.36 Cr
	LC9	OC8H17	OC8H17	OC8H17	OC8H17	OC8H17	OC8H17	H	NHSO2CF3	H	Cr 30.0 ColL 148.4 I; I 143.7 ColL 14.2 Cr
	LC10	OC8H17	OC8H17	OC8H17	OC8H17	OC8H17	OC8H17	H	F	H	Cr1 9.96 Cr2 21.4 ColL 128.2 I; I 125.8 ColL −19.5 Cr
	LC11	OC8H17	OC8H17	OC8H17	OC8H17	OC8H17	OC8H17	H	Cl	H	Cr1 1.66 Cr2 13.9 ColL 140.9 I; I 138.6 ColL −35.2 Cr
	LC12	OC8H17	OC8H17	OC8H17	OC8H17	OC8H17	OC8H17	H	Br	H	Cr1 −3.00 Cr2 9.39 ColL 144.4 I; I 142.3 ColL −39.4 Cr
	LC13	OC8H17	OC8H17	OC8H17	OC8H17	OC8H17	OC8H17	H	I	H	Cr 7.23 ColL 138.0 I; I 136.2 ColL −39.8 Cr
	LC14	OC8H17	OC8H17	OC8H17	OC8H17	OC8H17	OC8H17	H	OCH3	H	Cr 36.4 ColL 104.5 I; I 102.2 ColL −37.1 Cr
	LC15	OC8H17	OC8H17	OC8H17	OC8H17	OC8H17	OC8H17	H	CH3	H	Cr −2.89 ColL 104.9 I; I 102.2 ColL
	LC16	OC8H17	OC8H17	OC8H17	OC8H17	OC8H17	OC8H17	H	C2H5	H	Cr 12.5 ColL 101.5 I; I 99.5 ColL −35.9 Cr
	NLC1	OC8H17	OC8H17	OC8H17	OC8H17	OC8H17	OC8H17	NHSO2CF3	H	H	ColL 101.2 I; I 91.3 ColL
	LC17	OC8H17	OC8H17	OC8H17	OC8H17	OC8H17	OC8H17	F	H	H	Cr 27.3 ColL 119.8 I; I 117.3 ColL −16.6 Cr
	LC18	OC8H17	OC8H17	OC8H17	OC8H17	OC8H17	OC8H17	Cl	H	H	Cr 5.23 ColL 119.0 I; I 116.1 ColL 1.94 Cr
	LC19	OC8H17	OC8H17	OC8H17	OC8H17	OC8H17	OC8H17	Br	H	H	Cr 19.4 ColL 117.0 I; I 114.5 ColL 13.1 Cr
	LC20	OC8H17	OC8H17	OC8H17	OC8H17	OC8H17	OC8H17	OCH3	H	H	Cr 18.2 ColL 111.5 I; I 109.3 ColL −39.2 Cr
	LC21	OC8H17	OC8H17	OC8H17	OC8H17	OC8H17	OC8H17	CH3	H	H	Cr1 −16.3 Cr2 13.3 ColL 88.9 I; I 86.8 ColL −22.4 Cr
[13]	NLC2	H	OC8H17	H	H	OC8H17	H	H	OC8H17	H	Cr 74.8 I; I 31.5 Cr
	NLC3	H	OC12H25	H	H	OC12H25	H	H	OC12H25	H	Cr 64.5 I; I 33.8 Cr
	NLC4	H	OC8H17	OC8H17	H	OC8H17	OC8H17	H	OC8H17	OC8H17	Cr 101.3 I; I 80.0 Cr
	NLC5	H	OC12H25	OC12H25	H	OC12H25	OC12H25	H	OC12H25	OC12H25	Cr 106.7 I; I 83.5 Cr
	LC22	H	OC10H21	OC10H21	H	OC10H21	OC10H21	OC10H21	OC10H21	OC10H21	Cr 59.7 Colhd 105.0 I; I 97.7 Colhd 34.6 Cr
	LC23	OC10H21	OC10H21	OC10H21	OC10H21	OC10H21	OC10H21	H	H	H	Cr 45.7 Colhd 82.9 I; I 73.6 Colhd 37.9 Cr
	LC24	OC10H21	OC10H21	OC10H21	OC10H21	OC10H21	OC10H21	H	OC10H21	H	Cr 56.5 Colhd 114.0 I; I 104.0 Colhd 41.5 Cr
	LC25	OC10H21	OC10H21	OC10H21	OC10H21	OC10H21	OC10H21	H	OC10H21	OC10H21	Cr 36.2 Colhd 131.4 I; I 124.3 Colhd 5.92 Cr
	LC26	OC10H21	OC10H21	OC10H21	OC10H21	OC10H21	OC10H21	OC10H21	OC10H21	OC10H21	Cr 62.1 Colhd 140.5 I; I 134.4 Colhd 34.0 Cr

The chiral nematic, smectic A and smectic C phases are important mesophases for the realisation and operation of various electro-optical devices. A wide range of liquid crystals incorporating aromatic, six-membered heterocyclic rings, especially 2,5-disubstituted pyrimidine and pyridine rings, have found broad application in ferroelectric LCDs, organic semiconductors, sensors, etc. Nishiyama et al. [16] reported a new class of laterally connected liquid crystals containing 2,5-disubstituted pyrimidine rings, which exhibit biaxiality in the nematic phase (see Figure 5). The pre-organisation effect on chirality, polarity and biaxiality in liquid crystals has been discussed in terms of their influence on the physical properties of mesogens incorporating these elements. Camerel’s group [17], for example, reported a series of bipyrimidine-based discotic liquid crystals with an unusual combination of luminescent and nonlinear optical properties (see Table 3). Detailed structure–property relationships were established using a combination of DFT calculations and solid-state temperature-dependent infrared spectroscopy [18]. However, one of the most important applications for liquid crystals incorporating 2,5-disubstituted pyrimidine rings is in commercial ferroelectric liquid crystal displays (FLCDs). FLCDs possess an advantageous combination of electrooptic properties, such as a very fast opto-electronic response, hysteresis switching behaviour, in-plane switching of the optic axis, high optical contrast, wide viewing angles, low power consumption, sequential colours and inherent bistability [19,20]. Ferroelectric liquid crystals with a reduced molecular symmetry due to the presence of a chiral centre exhibit a spontaneous polarisation (Ps) in the absence of an external field. However, on application of an electric field, the direction of polarisation can be reoriented [21]. Liquid crystals incorporating a 2,5-disubstituted pyrimidine unit have often been found to exhibit a smectic C mesophase. The presence of an optically active chiral centre in such molecular structures then gives rise to liquid crystals with chiral smectic C mesophases (SmC*) with macroscopic ferroelectric properties [19,22].

Figure 5. (Colour online) The chemical structures, mesophase transition temperatures and POM textures of laterally connected pyrimidine liquid crystals [16].

Table 3. The chemical structures and mesophase transition temperatures (^oC) of bipyrimidine discotic liquid crystals.

Ref.	No.	R1	R2	R3	Mesophase Transition Temperatures (^oC)
[17]	LC35	OC12H25	OC12H25	H	Cr 69.7 LamColrec 102.0 Colhex 126.8 I; I 120.5 Colhex 87.6 LamColrec 70.0 Cr
	LC36	OC16H33	OC16H33	H	Cr 43.0 Colhex 127.5 I; I 123.2 Colhex 29.6 Cr
	NLC6	OC12H25	H	OC12H25	G 13.7 I; I 3.9 G
	NLC7	OC16H33	H	OC16H33	Cr 24.7 I; I 14.7 Cr
	NLC8	OC12H25	OC12H25	OC12H25	Cr −18.6 I; I −20.3 Cr

One 2,5-disubstituted pyrimidine ring and usually several such heterocyclic rings are often present in highly conjugated organic semiconductors for application in plastic electronic devices, such as organic light-emitting diodes (OLEDs) [23–28], organic photovoltaics (OPVs) [29–31], organic field-effect transistors (OFETs), etc [32–34]. Such heterocyclic mesogens, with an extended conjugated backbone exhibit strong intermolecular π-π stacking with overlapping overlapping molecular orbitals, which affords the high charge-carrier mobility required for the operation of such plastic electronic devices. The polarised nature of 2,5-disubstituted pyrimidine rings, as a so-called electron-withdrawing unit, facilitates the design of highly conjugated ‘push-pull’ molecules with efficient intra-molecular charge transfer from the electron-donating molecular units with high-lying HOMO energy levels to electron-withdrawing units with low-lying LUMO energy levels. This donor-acceptor (D-A) molecular design promotes effective charge-transport mobility in oriented thin layers in a planar device configuration [35–38]. Such heterocyclic liquid crystalline semiconductors have been synthesised with a combination of physical properties tailored for various plastic electronic devices. These properties are a complex combination of mesophase types, transition temperatures, charge carrier mobility, electroluminescent properties, etc.

Pyrimidine, also known as 1,3-diazine, can be found in many naturally occurring products, such as thymine, cytosine and uracil. Such heterocyclic compounds often exhibit a high biological activity of relevance for their use as pharmaceuticals, herbicides, antimicrobial, antibiotics, anticancer drugs, etc [39–41]. The presence of two nitrogen atoms in the aromatic cores of such molecules affords the possibility of protonation, metal ion chelation and hydrogen bonding, etc. Heterocyclic liquid crystals are of significant potential importance due to the formation of thin, uniform layers of highly conjugated supramolecular assemblies. Such mesogens, especially incorporating at least one 2,5-disubstituted pyrimidine unit, have facilitated the development of a new generation of chemical and biological sensors in recent years [42–44].

LCDs have become a part of our daily life, from wristwatches and pocket calculators to displays in all kinds of instruments, including portable computers, tablets, mobile phones and flat-panel televisions. The advantages of LCDs comprise low power consumption, low-voltage operation, full-colour, images at video rate with wide viewing angles, high contrast, thinness, and low weight. LCDs have established themselves as the dominant flat panel display over the last five decades, primarily due to this advantageous combination of attractive properties, but combined with scalability to very large screens and being cost competitive with competing technologies. However, research and development into liquid crystal technologies have taken them way beyond just the field of flat-panel displays. Scientifically, liquid crystals play an essential role in interdisciplinary material science research, involving chemistry, physics, mathematics, engineering, biology, etc., for a wide range of applications, such as photochromics [45–47], organic semiconductors for use in opto-electronic devices [48–50], 3D/4D printing materials [51–54], etc. The aromatic 2,5-disubstituted pyrimidine ring also plays an essential role in the molecular structure of novel liquid crystalline materials for this wide range of applications. For example, Ryu et al. [55,56] used phenyl pyrimidines to study the relationship between thermoelastic anisotropy and liquid crystalline mesophases in terms of achieving structural stability and efficient heat dissipation, especially for novel high-performance LCDs incorporating 4D-printed liquid crystalline elastomers for high-frequency applications.

However, there are only a few recent review articles are available summarising the nature and properties of pyrimidine-based liquid crystals from the perspective of molecular structures, mesophases transition and their applications. Petrov [57] has published a review article on pyrimidine as a structural fragment in calamitic, i.e. rod-like, liquid crystals. The author studied the relationship between the presence of the 2,5-disubstituted pyrimidine ring in the molecular structure of a wide range of calamitic liquid crystals on the nature and temperature ranges of their mesophases and their physicochemical and electro-optical properties. The article mainly focused on calamitic liquid crystals with linear bi-aryl and tri-aryl structures. However, since the publication of this review in 2006, the family of pyrimidine-based liquid crystals with a wide range of molecular structures, such as bent, discotic, taper-shaped, polymers, highly conjugated poly-aryl calamitics, and metal coordinated mesogens, has been very significantly extended and expanded for a wide range of applications in the intervening years. Achelle and Plé [58] also briefly reviewed pyrimidine-based conjugated materials for opto-electronic applications in 2012 as part of a broad, general review of the synthesis of a wide range of 2,5-disubstituted pyrimidine derivatives. It proves the importance of pyrimidine rings in constructing organic semiconductors with highly conjugated aromatic cores for novel application in plastic electronics, such as OFET, OPV and OLED. Ghosh et al. [59] also published a review on heterocycle-based calamitics in 2017. The authors briefly introduced many kinds of heterocyclic calamitic liquid crystals including five- and six-membered rings as well as fused aromatic rings. Therefore, we consider that a comprehensive review article, focused specifically on heterocyclic pyrimidine-based liquid crystals, would address a very significant and important knowledge gap. It should summarise the complex relationships between the molecular structures and the nature of their mesophases and their transition temperatures of relevance to specific opto-electronic applications. We make the case in this review article that innovation in terms of the synthesis of novel liquid crystalline materials containing pyrimidine rings will promote the rapid development of developing applications, such as 3D/4D printing, and flexible electronics facilitated by these advanced functional materials.

2. Pyrimidine-based liquid crystals for ferroelectric liquid crystal displays (FLCDs)

Figure 6 shows the schematic exhibition of pyrimidine-based liquid crystalline structures and mesophases for ferroelectric application in this section where it gives an overview of the structural development of pyrimidine-based liquid crystals specifically designed to exhibit chiral smectic C phases (SmC*) with ferroelectric properties for application in commercial FLCDs. This summary should help readers better understand the structure–property relationships of pyrimidine-based liquid crystals and stimulate the design and synthesis of novel liquid crystals for FLCDs with improved performance and also for new applications of ferroelectric mesogens.

Figure 6. (Colour online) The schematic exhibition of pyrimidine-based liquid crystalline structures and mesophases for ferroelectric application.

Liquid crystal displays (LCDs) have dominated the market of flat panel display (FDP) for many years due to their low cost, high resolution, long life, sophisticated processing technology and material system. However, current commercial LCDs using nematic liquid crystal mixtures still exist with potential issues of latency, image sticking and energy efficiency [60]. Although organic light emitting diode (OLEDs) displays exhibit many advantages due to the absence of backlight and are taking market share from LCDs slowly, the limitations of lifetime, large size, relatively high power consumption, flaw and process yields, etc., have presented some challenges in terms of market acceptability and market share in the flat panel display industry. FLCDs represent a promising alternative to nematic LCDs in meeting current challenges in flat panel display market due to their unique characteristics, such as high optical contrast ratio, high response, memory effect, large birefringence, wide viewing angle, low threshold voltage, low power consumption, and inherent bistability [20,60–62].

Meyer et al. [63] synthesised the liquid crystal 4-decyloxybenzylidene-4’-amino-2-methylbutylcinnamate in 1975 and reported the ferroelectric properties of a SmC* phase for the first time. Clark and Largerwall [64] reported the first FLCD using a liquid crystal mixture in the SmC* phase in 1980. These developments stimulated significant research interest and activities in the design and synthesis of SmC liquid crystals (hosts), optically active dopants (guests) and (guest/host) SmC*mixtures for the development of prototype FLCDs. The presence of an heteroatom, such as nitrogen, sulphur and oxygen, in one or more of the aromatic rings in the molecular core of these mesogens was found to promote the formation of a SmC phase often with a low viscosity [21]. Such guest/host mixtures were designed to manifest a mesophase sequence of Iso−N*−SmA*−SmC* on cooling features required for good defect-free alignment in FLCDs [21]. One of the most commonly encountered heterocyclic rings in the aromatic core of such mesogens are 2,5-disubstituted pyrimidine and pyridine rings. Suitably substituted pyrimidine-based liquid crystals often have fairly low melting points and reasonably high SmC transition temperatures, i.e. a reasonably broad SmC mesophase. The design of novel organic materials with both SmA and SmC phases required for application in FLCDs mainly focussed on the modifications of the rigid aromatic cores and flexible alkyl chains because they can affect lamellar (smectic) ordering, van der Waals interactions and thus mesogenic properties [65,66]. Two- and three-ring phenyl-pyrimidine structures were mostly studied for use as ferroelectric liquid crystals due to the presence of appropriate mesophases and transition temperatures. Tykarska et al. [67] reported a series of six ferroelectric liquid crystals containing phenyl pyrimidine cores and alkyl/alkoxy chains with different lengths as constituent components of liquid crystalline mixtures with the desired mesophases and temperature ranges (see Table 4). These smectic liquid crystals were also utilised to prepare different ferroelectric mixtures with dispersed nano-sized copper oxide for improved dielectric and electro-optical parameters [73]. Tykarska et al. reported that a longer alkoxy terminal chain attached to the pyrimidine ring or the presence of two alkoxy chains increases the tilt angle in smectic layers and produces high transition temperatures, e.g. SmC-SmA and SmA-I, for their requisite smectic mesophases. A value of 22.5° for title angle of the director of a smectic C phase is required for FLCDs and other applications in this article. Debnath et al. made use of related optically active, but non-mesogenic, dopants for preparing room-temperature ferroelectric liquid crystal mixtures suitable for FLCD applications [74]. Chakraborty et al. [75] used one of these materials to investigate the critical behaviour at mesophase transitions by exploiting high-resolution optical birefringence (Δn) measurements with a high degree of accuracy. LCDs based on nematic LCs require a birefringence value of between 0.05 and 0.10 in order to minimise variations in the chromatic retardance [76]. The birefringence of liquid crystals is determined by their anisotropy of polarisability, which depends on a high degree of aromatic conjugation within the aromatic core. Pozhidaev et al. [76] introduced trans-1,4-disubstituted cyclohexane rings into achiral liquid crystals designed for application in a range of different types of ferroelectric liquid crystal devices to study this effect. These authors used fluorinated phenylpyrimidine derivatives as chiral dopants in these studies in order to adjust the electro-optical parameters, such as the spontaneous polarisation (Ps) and the pitch of the smectic C* helix [76,77]. Such liquid crystal mixtures with a low value of the birefringence are suitable for most ferroelectric applications, such as surface-stabilised ferroelectric liquid crystal displays (SSFLCDs), deformed helix ferroelectric displays (DHFDs) and electrically suppressed helix displays (ESHDs).

Table 4. The chemical structures and mesophase transition temperatures (^oC) of phenyl pyrimidine liquid crystals.

Ref.	No.	R1	R2	R3	R4	Mesophase Transition Temperatures (^oC)
[67]	LC37	OC9H19	OC7H15	H	H	Cr 57.4 SmC 95.3 SmA 98.4 I
[67]	LC38	OC9H19	OC9H19	H	H	Cr 61.5 SmC 95.5 SmA 99.2 I
[67]	LC39	OC8H17	OC8H17	H	H	Cr 50.1 SmC 91.1 SmA 89.3–100.3 I
[67]	LC40	OC7H15	OC9H19	H	H	Cr 57.0 SmC 79.7 SmA 91.2 N 94.7 I
[67]	LC41	C8H17	OC6H13	H	H	Cr 27.5 SmC 46.3 SmA 57.4 N 65.3 I
[67]	LC42	C8H17	OC8H17	H	H	Cr SmX 68.1 SmC 151.7 N 174.0 I
[68]	LC43	OC8H17	OC6H13	H	H	I 98 N 95.7 SmA 88 SmC 43 Cr
[20]	LC44	OC9H19	OCHFC6H13	H	H	Cr 64.2 SmC* 92.1 SmA 96.3 I
[21]	LC45	C8H17	OC10H21	H	H	Cr 32 SmC 60 SmA 66 N 70 I
[21]	LC46	C9H19	OC8H17	H	H	Cr 33 SmC 60 SmA 75 I
[21]	LC47	C10H21	OC10H21	H	H	Cr 36 SmC 73 SmA 77 I
[21]	LC48	C9H19	OC8H17	F	F	Cr 43 SmC 54 I
[21]	LC49	OC9H19	OC9H19	F	F	Cr 48 SmC 78 I
[21]	LC50	OC9H19	OC9H19	F	H	Cr 45 SmC 62 I
[21]	LC51	OC8H17	OCH2C6F13	F	F	Cr 65 SmC (64) SmA 138 I
[21]	LC52	OCH2C6F13	OC8H17	F	F	Cr 77 SmC 127 I
[21]	LC53	C7H15	OC8H17	H	H	Cr 51 SmC (45) N 72 I
[21]	LC54	C10H21	OCH2CHFC6H13	H	H	Cr 60 SmC (58) SmA* 68 I
[69]	LC55	OC8H17	OC4H9	H	H	Cr 58 SmC 85 SmA 95 N 98 I
[70]	LC56	C8H17	OCH2CF2OC2F4OC4F9	H	H	I 70 SmA 49 SmC 8 Cr
[71]	LC57	C6H13	OCH2C7F15	H	H	I 133 SmA 56 SmC 50 Cr
[71]	LC58	C7H15	OCH2C7F15	H	H	I 125 SmA 67 SmC 54 Cr
[71]	LC59	C8H17	OCH2C7F15	H	H	I 117 SmA 80 SmC 71 Cr
[71]	LC60	C9H19	OCH2C7F15	H	H	I 112 SmA 85 SmC 71 Cr
[71]	LC61	C10H21	OCH2C7F15	H	H	I 104 SmA 87 SmC 75 Cr
[71]	LC62	C10H21	OCH2C5F11	H	H	I 84 SmA 73 SmC 47 Cr
[71]	LC63	C10H21	OCH2C3F7	H	H	I 64 SmA 52 SmC 36 Cr
[72]	LC64	OC8H17	OCH2CHFC6H13	H	H	I 94.15 SmA 78.49 SmC 44.94 Cr

Pramanik et al. [68] reported three binary types of liquid crystal mixtures created by mixing an achiral phenyl pyrimidine smectic compound with three different orthoconic anti-ferroelectric (SmC_A*) analogues exhibiting a mesophase sequence of SmA*-SmC*-SmC_A* (see Table 4). FLCDs based on orthoconic anti-ferroelectric liquid crystals have attracted much attention due to their fast response times, wide viewing angles, ideal dark state, broad grey scale and absence of substrate surface treatment. However, these mixtures exhibit high viscosity values for the SmC* and SmC_A* mesophases, which leads to unacceptably long response times for commercially viable SSFLCDs. Therefore, pyrimidine-based achiral liquid crystals with a low viscosity are often used to dope chiral orthoconic anti-ferroelectric liquid crystals, which possess high Ps values, in order to produce the desired electro-optical properties of the resulting mixtures. For example, this doping procedure often produces mixtures with a low spontaneous polarisation, long pitch and very short response times for use in high-speed devices. However, low Ps values and poor mechanical stability result in a quite challenging task in terms of optimisation of these liquid crystal mixtures for commercial FLCDs. Generally, the presence of a higher number of aromatic rings in the aromatic cores of liquid crystalline compounds results in higher viscosity values as well as higher liquid crystal transition temperatures. Lapanik et al. [20] recently reported new achiral smectic compounds with four or five aromatic rings in the aromatic core to induce shock-stable FLCDs with improved electro-optical properties (see Table 4). However, in order to reduce the viscosity of such host SmC mixtures for fast switching FLCDs, the authors utilised additional low-viscosity achiral phenylpyrimidine derivatives and doped them with chiral dopants with a high Ps value.

Czerwiński et al. [19] reported new ferroelectric liquid crystal mixtures of achiral two-ring pyrimidine compounds as parent mixture and chiral dopants. These mixtures show a stable enantiotropic ferroelectric phase with very good alignment in surface-stabilised FLCDs. The authors discussed the dependence of the ferroelectric properties, such as dipolar and multipolar interactions, of these materials on their molecular parameters. The study results show that the limited dipolar interactions in favour of quadrupolar ones are a feature observed previously for materials comprising chiral dopant with two chiral centres what is in contrast to mixtures with one chiral centre, where a linear relationship is observed. The structure–property relationship is still as complex as the influence of molecular parameters on the dipolar and multipolar interactions within FLC. Authors proved that an extended analysis of structure–property relation is an excellent tool to improve the knowledge of molecular interactions in LC media and is an advanced tool for LC media materials engineering. This group also used chiral dopants with pyrimidine rings in order to investigate the influence of the dopant structures on ferroelectric properties, such as title angle, spontaneous polarisation and layer spacing [78]. However, some challenges still remain in terms of commercial SSFLCDs in terms of defect-free alignment, zero or a minimal layer shrinkage, wide mesophase temperature ranges, etc [79]. An important development is the potential use of ‘de Vries’ smectic liquid crystals, which exhibit a maximum layer contraction of ≤ 1% upon transition from the SmA* phase to the titled SmC* phase and within the whole temperature range of the SmC* phase [79]. Pyrimidine-based liquid crystals possess intrinsic structural advantages for such ‘de Vries’ smectic liquid crystals with a small layer shrinkage and wide mesophase temperatures [21]. The Vij group reported such a series of ‘de Vries’ liquid crystals containing pyrimidine rings for ferroelectric applications with a small layer shrinkage and a fast electro-optical switching time [60,79–81]. They designed and synthesised two 5-phenyl pyrimidine epoxy derivatives with different either trisiloxane (LC65) or perfluorinated (LC66) terminal chains [79], see Table 5. The trisiloxane (LC65) was found to be a good ‘de Vries’ smectic liquid crystal as it exhibited both SmA* and SmC* phases with broad mesophase temperature ranges, a small layer shrinkage with 1.7% at 13°C below the SmA*-SmC* transition and a reduction factor with 0.37 [79]. The group further synthesised two new pyrimidine-based de Vries smectic liquid crystals with siloxane-terminal chains, LC74 and LC75 (see Table 6), exhibiting wide temperature ranges for the SmA* and SmC* phases and fast electro-optical switching [60]. These two compounds exhibit a low layer shrinkage of ca. 1.5% in the SmC* phase, extremely fast electronic response and a large, field-induced birefringence in the SmA* phase. Compound LC75 possesses a wide mesophase temperature range, thus making it highly suitable for FLCDs [60,80]. Furthermore, the group also synthesised LCs with carbosilane sidechains instead of siloxane terminal groups to study their de Vries properties [81]. The phenylpyrimidine LCs, LC76 and LC77, with tri- and tetra-carbosilane terminal groups, respectively, exhibit good ‘de Vries’ characteristics with a set of phase-transitions of Iso-SmA*-SmC*-SmX-Cr (see Table 6). Compound LC77, with a tetra-carbosilane chain, exhibits a smaller layer shrinkage (0.9%) compared with that (1.9%) of LC76 with a shorter tri-carbosilane chain. Therefore, the authors postulate that an even longer carbosilane chain could lead to a further reduction or even to zero-layer shrinkage. In addition, the authors replaced the methyl group in the terminal chains of compounds LC76 and LC77 with a perfluoromethyl group to give two new phenylpyrimidine compounds (NLC9 and NLC10). Unfortunately, these new materials do not exhibit any mesophases (see Table 6). These results demonstrate once more that heteroatoms present in molecular structures of LCs can significantly influence the molecular polarity and thus the exhibition of liquid crystal mesophases.

Table 5. The chemical structures and mesophase transition temperatures (^oC) of pyrimidine-based liquid crystals with epoxy [79], branched tricarbosilane [82], bent-core dimers [83], taper-shaped trimers [84].

Ref.	No.	Structures	Mesophase Transition Temperatures (^oC)
[79]	LC65 (adpc042)		Cr −5 SmC* 58 SmA* 82 I
[79]	LC66 (DR257)		Cr 60 SmA* 152 Iso
[82]	LC67 (QL30–6)		Cr 45 SmC 55 SmA 70 I
[82]	LC68 (QL29–6)		Cr 54 SmC 60 SmA 67 I
[83]	LC69		Cr 77 SmCA 81 SmA 115 N 131 I
[83]	LC70		Cr 78 SmCA 92 SmA 115 N 128 I
[84]	LC71		Cr 70 SmC 83.8 SmCanti 87.1 SmA 95.7 N 104.2 I
[84]	LC72		Cr 107.4 (SmX 97.0 SmCanti 101.9) SmA 113.4 I
[84]	LC73		Cr 105.1 SmC 105.2 N 110.2 I

Table 6. The chemical structures and mesophase transition temperatures (^oC) of biphenyl pyrimidine compounds with siloxane/carbosilane terminal chains.

Ref.	No.	X	m	n	A	R	Mesophase Transition Temperatures (^oC)
[60]	LC74 (DR118)	O	2	11		OCH(CH3)C6H13	Cr 60 SmC* 95 SmA* 113 I
[60]	LC75 (DR133)	O	2	11		OCH(CH3)C6H13	Cr 11 SmC* 94 SmA* 102 I
[81]	NLC9 (DR253)	CH2	2	11		OCH(CF3)C6H13	Cr 50.2 I
[81]	NLC10 (ADPD003)	CH2	2	11		OCH(CF3)C6H13	Cr 52 I
[81]	LC76 (DR276)	CH2	2	11		OCH(CH3)C6H13	Cr 14 SmX 48 SmC* 78.5 SmA* 87 I
[81]	LC77 (DR277)	CH2	3	11		OCH(CH3)C6H13	Cr 6 SmX 35 SmC* 65.5 SmA* 77 I

The Lemieux group has reported a wide variety of de Vries smectic liquid crystals with phenylpyrimidine cores and siloxane/carbosilane terminal chains for many years [69,85–88]. The design of these new materials is based on de Vries characteristics requiring the presence of both SmA* and SmC* phases, a small layer shrinkage at their transition, and a broad mesophase temperature range. An organosiloxane- or carbosilane-terminated chain promotes the exhibition of a SmC phase and a phenylpyrimidine core often induces the formation of a SmA phase [69,85,86]. Lemieux’s group used the presence of organosiloxane terminal chains in compounds designed to exhibit de Vries smectic mesophases (see Table 7). The compounds 3(n) (see Figure 7) exhibit a maximum layer contractions ranging from 0.5% to 1.4% [85]. These values are comparable to those of the classic de Vries LCs 3 M 8422 and TSiKN65 with layer contractions of 0.4% and 0.65%, respectively. In addition, the authors also studied the effect of halogen end-groups on promoting the formation of a SmA phase in phenylpyrimidine LCs [87] (see Table 8). Fluorinated phenylpyrimidine derivatives were widely studied in the 1990s in terms of smectic layer contraction and related ferroelectric properties [70,71,77,91,92] with some ongoing research interest [93,94]. Lemieux’s group mainly focuses on LCs with chloro-groups in terminal positions in the expectation of promoting the presence of an SmA phase due to the electron-withdrawing nature of chloro end-groups reducing electrostatic repulsion between the alkoxyl chains and increasing van der Waals interactions between the aromatic cores [87]. Therefore, the authors designed and synthesised a series of phenylpyrimidine de Vries LCs containing organosiloxane-terminated chains and chloro-substituted alkoxyl chains (see Table 7). These novel compounds exhibit the desired SmA* and SmC* phases with excellent alignment and electro-optical properties as ferroelectric liquid crystals [88]. However, in order to avoid the hydrolytic instability of siloxane structures [95], chemically inert carbosilane-terminated groups were introduced into similar molecular structures to provide a novel series of de Vries liquid crystals [82,89,90,95–99] (see Tables 7–Table 9). The compounds LC81 and LC91 (see Table 7) exhibit broad SmC mesophases and reduction factors R of 0.17 and 0.18, respectively, for potential use as chevron-free ferroelectric liquid crystals [89]. These results illustrate the fact that the presence of carbosilane-terminal chains do not dramatically change the mesomorphism and de Vries properties compared to those of analogous compounds with siloxane-terminated chains [89]. The title angle of these materials could be controlled by modifying the length of carbosilane chains without compromising ‘de Vries’ properties. The analogous compounds LC85-LC90 (see Table 7) with an inverted orientation of the nitrogen atoms in the pyrimidine rings cause a suppression of de Vries properties possibly due to non-covalent, core–core interactions in the intercalated smectic bilayers [89]. The group also synthesised de Vries liquid crystals, such as LC67 and LC68 (see Table 5), with branched carbosilane end groups [82]. These compounds exhibit a comparable mesomorphic behaviour and de Vries properties to those of the analogous compounds with linear tricarbosilane chains [82]. However, the presence of a mono-carbosilane chain, rather than a tri-carbosilane chain, results in significantly lower de Vries properties with an increase in the reduction factor R possibly due to a reduction in the electroclinic effect (ECE), nano-segregation and the quality of the lamellar ordering [90,97]. The chiral compound LC94 (see Table 7) with a long tricarbosilane-terminated chain exhibits a maximum layer contraction of only 0.2% and shows no appreciable optical stripe defects due to horizontal chevron formation [90,99]. In addition to flexible siloxane/carbosilane-terminated chains, the group also investigated the influence of rigid phenoxy end-groups in a terminal position of LCs on their mesomorphic behaviour [65]. These studies show that the presence of the phenoxy end group (LC148-LC160, NLC13-NLC17) causes a significant increase in melting point and inhibits, at least partially, the mesomorphism of these materials (see Table 10). In most cases, the broad enantiotropic SmC phase exhibited by the parent isomers is suppressed in the analogous compounds with a phenoxy end group (LC148-LC160, NLC13-NLC17) [65]. In addition, the group also studied the influence of the ethynyl spacer between aromatic rings on de Vries properties [96] (see Table 9). These studies confirm the assumption that an ethynyl spacer would create more free volume in the core sublayer and this would consequently reduce the orientational order parameter S2 in the SmA mesophase of such compounds [96]. On the other hand, the ethynyl spacer could be unstable under UV exposure and the process is known as UV-induced or photochemical polymerisation [100]. The carbon–carbon triple bond is composed of one σ bond and two π bonds. The presence of multiple π bonds makes alkynes reactive and susceptible to undergoing polymerisation reactions. Ultraviolet light provides the energy needed to break the π bonds in the alkyne. The energy from UV light is absorbed by the π electrons, promoting them to higher energy levels and leading to the weakening or breaking of the π bond. It is important to note that the specific conditions, such as initiators, temperatures and UV wavelength (energy), are required to initiate or prevent the polymerisation of the ethynyl spacers.

Figure 7. (Colour online) The chemical structures, mesophase transition temperatures (^oC) and POM textures of 2(n) and 3(n) series of liquid crystals [85].

Table 7. The chemical structures and mesophase transition temperatures (^oC) of phenyl-pyrimidine liquid crystals with siloxane/carbosilane terminal chains.

Ref.	No.	X	m	n	P	Q	Y	Z	R	Mesophase Transition Temperatures (^oC)
[69]	LC78	O	2	11	CH	CH	CH	N	OC8H17	Cr 66 SmC 76 SmA 80 I
[89]	LC79 (QL16–4)	CH2	2	11	CH	CH	CH	N	OC4H9	Cr 43 SmC 44 SmA 68 I
[89]	LC80 (QL16–5)	CH2	2	11	CH	CH	CH	N	OC5H11	Cr 34 Cr’ 40 SmC 55 SmA 66 I
[89]	LC81 (QL16–6)	CH2	2	11	CH	CH	CH	N	OC6H13	Cr 33 SmC 64 SmA 69 I
[89]	LC82 (QL16–7)	CH2	2	11	CH	CH	CH	N	OC7H15	Cr 64 SmC 65 SmA 67 I
[89]	LC83 (QL16–8)	CH2	2	11	CH	CH	CH	N	OC8H17	Cr 58 SmC 66 SmA 68 I
[89]	LC84 (QL16–9)	CH2	2	11	CH	CH	CH	N	OC9H19	Cr 67 SmC (65) SmA (66) I
[89]	LC85 (QL17–4)	CH2	2	11	N	CH	CH	CH	OC4H9	Cr 39 SmC 42 SmA 68 I
[89]	LC86 (QL17–5)	CH2	2	11	N	CH	CH	CH	OC5H11	Cr 44 SmC 48 SmA 66 I
[89]	LC87 (QL17–6)	CH2	2	11	N	CH	CH	CH	OC6H13	Cr 48 SmC 59 SmA 72 I
[89]	LC88 (QL17–7)	CH2	2	11	N	CH	CH	CH	OC7H15	Cr 51 SmC 61 SmA 70 I
[89]	LC89 (QL17–8)	CH2	2	11	N	CH	CH	CH	OC8H17	Cr 56 SmC 63 SmA 70 I
[89]	LC90 (QL17–9)	CH2	2	11	N	CH	CH	CH	OC9H19	Cr 57 Cr’ 60 SmC 63 SmA 69 I
[89]	LC91 (QL24–6)	CH2	1	11	CH	CH	CH	N	OC6H13	Cr 50 SmF (42) SmC 78 SmA 81 I
[82]	LC92 (QL26–6)	CH2	0	11	CH	CH	CH	N	OC6H13	Cr 68 SmX (54) SmC 70 SmA 93 I
[82]	LC93 (QL25–6)	CH2	1	11	N	CH	CH	CH	OC6H13	Cr 60 SmA 83 I
[90]	LC94 (QL32–6)	CH2	2	11	CH	CH	CH	N	OCH2(CHF)2C3H7	Cr 53 SmC* 62 SmA* 77 I
[90]	LC95	CH2	1	11	CH	CH	CH	N	OCH2(CHF)2C3H7	Cr 45 SmC* 55 SmA* 84 I
[90]	LC96	CH2	0	11	CH	CH	CH	N	OCH2(CHF)2C3H7	Cr 56 SmX (42) SmC* (52) SmA* 93 I
[86]	LC97	O	2	11	CH	CH	N	CH	O(CH2)7Cl	Cr 26 SmC 84 SmA 98 I
[88]	LC98	O	2	11	CH	CH	N	CH	O(CH2)7CH2Cl	Cr 48 SmC 89 SmA 98 I
[88]	LC99	O	2	6	CH	CH	N	CH	O(CH2)7CH2Cl	Cr 45 SmC 76 SmA 85 I
[88]	LC100	O	2	11	CH	CH	N	CH	OCH2(CHF)2C5H11	Cr 67 SmC* 97 SmA* 99 I
[88]	LC101	O	2	6	CH	CH	N	CH	OCH2(CHF)2C5H11	Cr 61 Cr’ 64 SmC* 80 SmA* 88 I
[87]	LC102	O	2	11	CH	CH	N	CH	OC8H17	Cr 42 SmC 92 I
[87]	LC103	O	2	11	CH	N	CH	CH	OC8H17	Cr 35 SmC 82 Iso
[87]	LC104	O	2	11	CH	N	CH	CH	OC8H16Cl	Cr 56 SmC 83 Iso

Table 8. Effect of halogen end-groups with fluoro-, bromo-, iodo-substitution on mesophases and their transition temperatures for phenyl-pyrimidine liquid crystals.

Ref.	No.	X	Y	R1	R2	Mesophase Transition Temperatures (^oC)
[87]	LC119	CH	N	OC12H25	O(CH2)4Cl	Cr 72 N 111 I
	LC120	CH	N	OC11H23	O(CH2)5Cl	Cr 73 N 112 I
	LC121	CH	N	OC10H21	O(CH2)6Cl	Cr 56 N 103 I
	LC122	CH	N	OC9H19	O(CH2)7Cl	Cr 51 N 107 I
	LC123	CH	N	OC8H17	O(CH2)8Cl	Cr 67 N 105 I
	LC124	CH	N	OC7H15	O(CH2)9Cl	Cr 48 N 98 I
	LC125	CH	N	OC6H13	O(CH2)10Cl	Cr 67 N 95 I
	LC126	CH	N	OC5H11	O(CH2)11Cl	Cr 93 SmA (85) I
	LC127	N	CH	OC12H25	O(CH2)4Cl	Cr 59 N 100 I
	LC128	N	CH	OC11H23	O(CH2)5Cl	Cr 65 N 101 I
	LC129	N	CH	OC10H21	O(CH2)6Cl	Cr 61 N 97 I
	LC130	N	CH	OC9H19	O(CH2)7Cl	Cr 66 N 94 I
	LC131	N	CH	OC8H17	O(CH2)8Cl	Cr 76 N 95 I
	LC132	N	CH	OC7H15	O(CH2)9Cl	Cr 66 SmA 82 N 85 I
	LC133	N	CH	OC6H13	O(CH2)10Cl	Cr 72 SmA 76 N 85 I
	LC134	N	CH	OC5H11	O(CH2)11Cl	Cr 65 SmA 61 N 76 I
	LC135	CH	N	OC8H17	O(CH2)8F	Cr 47 N 101 I
	LC136	CH	N	OC8H17	O(CH2)8Br	Cr 52 N 100 I
	LC137	CH	N	OC8H17	O(CH2)8I	Cr 49 N 97 I

Table 9. The chemical structures and mesophase transition temperatures (^oC) of tricarbosilane-terminated phenylpyrimidine with ethynyl spacer.

Ref.	No.	X	Y	R	Mesophase Transition Temperatures (^oC)
[96]	LC138	CH	N	OC4H9	Cr 31 SmC 61 SmA 64 I
	LC139	CH	N	OC5H11	Cr 45 SmC 60 SmA 62 I
	LC140	CH	N	OC6H13	Cr 55 SmC 65 I
	LC141	CH	N	OC7H15	Cr 69 SmC (62) I
	LC142	CH	N	OC8H17	Cr 74 SmA (64) I
	LC143	CH	N	OC9H19	Cr 79 Cr’ 51 SmC 47 SmA 63 I
	LC144	N	CH	OC4H9	Cr 41 Cr’ 52 SmC 58 SmA 69 I
	LC145	N	CH	OC5H11	Cr 35 SmC 63 SmA 68 I
	LC146	N	CH	OC6H13	Cr 42 SmC 67 SmA 71 I
	LC147	N	CH	OC7H15	Cr 11 SmC 68 SmA 71 I
	NLC11	N	CH	OC8H17	Cr 48 I
	NLC12	N	CH	OC9H19	Cr 64 I

Table 10. The chemical structures and mesophase transition temperatures (^oC) of pyrimidine-based liquid crystals with phenoxy end groups.

Ref.	No.	X	Y	n	R	Mesophase Transition Temperatures (^oC)
[65]	LC148 (QL11–4/12)	CH	N	12	OC4H9	Cr 83 SmA 78 I
	LC149 (QL11–5/11)	CH	N	11	OC5H11	Cr 74 SmA 70 I
	LC150 (QL11–6/10)	CH	N	10	OC6H13	Cr 86 SmA 85 I
	LC151 (QL11–7/9)	CH	N	9	OC7H15	Cr 72 SmC 61 SmA 76 I
	LC152 (QL11–8/8)	CH	N	8	OC8H17	Cr 79 SmC 80 SmA 95 I
	LC153 (QL11–9/7)	CH	N	7	OC9H19	Cr 81 SmC 75 SmA 84 I
	LC154 (QL11–10/6)	CH	N	6	OC10H21	Cr 85 Cr’ 95 SmC 83 SmA 106 I
	LC155 (QL11–11/5)	CH	N	5	OC11H23	Cr 80 SmA 86 I
	LC156 (QL11–12/4)	CH	N	4	OC12H25	Cr 127 SmA 116 I
	NLC13 (QL12–4/12)	N	CH	12	OC4H9	Cr 83 Cr’ 88 I
	NLC14 (QL12–5/11)	N	CH	11	OC5H11	Cr 80 I
	NLC15 (QL12–6/10)	N	CH	10	OC6H13	Cr 92 I
	NLC16 (QL12–7/9)	N	CH	9	OC7H15	Cr 79 Cr’ 82 I
	LC157 (QL12–8/8)	N	CH	8	OC8H17	Cr 93 SmA 81 N 86 I
	NLC17 (QL12–9/7)	N	CH	7	OC9H19	Cr 72 I
	LC158 (QL12–10/6)	N	CH	6	OC10H21	Cr 87 SmC 84 SmA 93 I
	LC159 (QL12–11/5)	N	CH	5	OC11H23	Cr 80 SmA 76 I
	LC160 (QL12–12/4)	N	CH	4	OC12H25	Cr 96 SmC 100 SmA 101 I

Nanocomposites consisting of liquid crystals and nanoparticles have attracted a great deal of attention in recent years due to the combination of liquid crystal properties, such as excellent electro-optical behaviour, and quantum-confined properties of nanoparticles [101–104]. Romero-Hasler et al. [72] reported that a nanocomposite consisting of a ferroelectric liquid crystal and magnetic FeCo nanoparticles exhibits a high magnetic response to a very small applied electric field. The authors chose a phenylpyrimidine-based fluorinated compound with a chiral smectic phase (SmC*) (see Table 4) as the ferroelectric material. More detailed studies are still needed to systematically investigate the relationship between the physical properties of the liquid crystalline (host) compounds and the optical, electrical, magnetic properties of the (guest) nanoparticles on the resultant (guest/host) nanocomposite.

The presence of heteroatoms in the aromatic cores of LCs exerts a strong influence on their mesogenic properties and other physical properties, such as birefringence, dielectric anisotropy, etc [105–108]. Wilson’s group [105,109–111] synthesised heterocyclic ferroelectric LCs with a 2,5-disubstituted thiophene and a 2,5-disubstituted pyrimidine ring in the aromatic molecular core (see Table 11–Table 12). The 2,5-disubstituted thiophene ring produces a non-linear, i.e. bent aromatic core of LCs with, for example, an anti-ferroelectric SmC phase, low melting points and broad mesophase transition temperatures. It should be noted that the compounds with benzyloxy end-groups were first designed as intermediate materials for the synthesis of a variety of dialkyl thiophene-pyrimidine liquid crystalline materials by removing the protecting group, but the intermediates also exhibit mesophases [109] (see Table 11). Yoshizawa’s group [83] also reported bent liquid crystal dimers consisting of two mesogenic moieties joined by an aliphatic linkage (see Table 5). These compounds also exhibit antiferroelectric SmC* mesophases as well as a nematic phase. This combination is favourable for fabricating antiferroelectric LCDs without defects, which are often produced on the direct-phase transition from an isotropic liquid to the SmC* phase [83]. In order to identify the relationship of molecular structures and antiferroelectric mesophases, Yoshizawa’s group [84] synthesised a series of taper-shaped LC trimers containing 2,5-disubstituted pyrimidine rings, which exhibit antiferroelectric SmC phases (see Table 5). The taper shape is similar to a combination of linear and bent molecular structures. These taper-shaped molecules can form an intercalated structure due to the intermolecular core–core interactions. The intercalated head-to-tail dipole–dipole interactions between phenyl pyrimidine cores help stablise the SmA mesophases. However, the competition between intercalated dipole–dipole interactions and mono-layer packing entropy effects appears to promote the presence of a SmC phase. The transition of the antiferroelectric SmC* phase to the chiral SmC* phase might be driven by conformational change and the molecular reorganisation caused by packing entropy factors due to the bent molecular shape of these compounds [84]. In addition, the group also studied the effect of the presence of two chiral centres in phenylpyrimidine LCs on their mesomorphic behaviour and electro-optical parameters from the perspective of structure–property relationships [112–114] (see Table 13).

Table 11. The chemical structures and mesophase transition temperatures (^oC) of thienyl-phenyl-pyrimidine compounds.

Ref.	No.	R1	R2	Mesophase Transition Temperatures (^oC)
[110]	LC161	OC6H13	H	Cr 115.1 N (107.8) I
[110]	LC162	OC7H15	H	Cr 104.9 N (100.5) I
[110]	LC163	OC8H17	H	Cr 110.8 N (107.4) I
[110]	LC164	OC9H19	H	Cr 112.2 N (102.5) I
[110]	LC165	OC10H21	H	Cr 107.7 N (104.9) I
[110]	LC166	OC12H25	H	Cr 106.4 N (106.2) I
[110]	LC167	OC6H13	OC6H13	Cr 75.0 SmC 145.4 SmA 149.7 N 153.7 I
[110]	LC168	OC6H13	OC7H15	Cr 69.5 SmC 149.1 SmA 154.7 I
[110]	LC169	OC6H13	OC8H17	Cr 88.9 SmI 98.9 SmC 154.6 I
[110]	LC170	OC6H13	OC9H19	Cr 61.2 J/G 64.2 SmI 104.9 SmC 154.7 I
[110]	LC171	OC6H13	OC10H21	Cr 68.2 J/G 78.1 SmI 109.0 SmC 152.6 I
[110]	LC172	OC6H13	OC12H25	Cr 74.6 J/G 95.9 SmI 117.4 SmC 150.9 I
[110]	LC173	OC7H15	OC6H13	Cr 76.2 SmC 141.7 SmA 149.3 N 150.2 I
[110]	LC174	OC7H15	OC7H15	Cr 66.2 SmCalt 88.1 SmC 148.9 SmA 153.6 I
[110]	LC175	OC7H15	OC8H17	Cr 78.1 SmI 100.4 SmC 152.2 I
[110]	LC176	OC7H15	OC9H19	Cr 63.9 SmI 105.2 SmC 153.5 I
[110]	LC177	OC7H15	OC10H21	Cr 74.6 J/G 75.1 SmI 110.8 SmC 151.3 I
[110]	LC178	OC7H15	OC12H25	Cr 64.6 J/G 93.4 SmI 115.3 SmC 147.9 I
[110]	LC179	OC8H17	OC6H13	Cr 83.3 SmC 145.0 SmA 150.3 N 152.0 I
[110]	LC180	OC8H17	OC7H15	Cr 86.1 SmCalt 88.9 SmC 150.2 SmA 152.7 I
[110]	LC181	OC8H17	OC8H17	Cr 78.0 SmF 71.6 SmCalt 97.9 SmC 153.5 I
[110]	LC182	OC8H17	OC9H19	Cr 68.1 J/G 91.9 SmF 92.3 SmI 106.0 SmC 153.1 I
[110]	LC183	OC8H17	OC10H21	Cr 79.3 J/G 91.7 SmF 92.9 SmI 110.8 SmC 151.5 I
[110]	LC184	OC8H17	OC12H25	Cr 72.6 J/G 94.6 SmI 118.4 SmC 149.6 I
[110]	LC185	OC9H19	OC6H13	Cr 65.6 SmC 138.2 SmA 147.1 N 147.3 I
[110]	LC186	OC9H19	OC7H15	Cr 84.2 SmCalt 86.1 SmC 147.7 SmA 151.4 I
[110]	LC187	OC9H19	OC8H17	Cr 81.0 SmF 99.9 SmC 150.6 I
[110]	LC188	OC9H19	OC9H19	Cr 51.4 SmF 105.6 SmC 150.8 I
[110]	LC189	OC9H19	OC10H21	Cr 67.7 SmF 111.5 SmC 150.3 I
[110]	LC190	OC9H19	OC12H25	Cr 71.6 SmF 117.3 SmC 147.7 I
[110]	LC191	OC10H21	OC6H13	Cr 68.1 SmC 136.5 SmA 145.8 I
[110]	LC192	OC10H21	OC7H15	Cr 82.2 SmCalt 84.7 SmC 145.6 SmA 150.8 I
[110]	LC193	OC10H21	OC8H17	Cr 79.5 SmI 98.6 SmC 149.4 I
[110]	LC194	OC10H21	OC9H19	Cr 60.1 SmI 105.3 SmC 148.7 I
[110]	LC195	OC10H21	OC10H21	Cr 74.5 SmI 111.3 SmC 149.0 I
[110]	LC196	OC10H21	OC12H25	Cr 73.5 SmI 117.9 SmC 146.7 I
[110]	LC197	OC12H25	OC6H13	Cr 68.5 SmC 135.2 SmA 145.2 I
[110]	LC198	OC12H25	OC7H15	Cr 64.9 SmCalt 80.1 SmC 137.4 SmA 146.7 I
[110]	LC199	OC12H25	OC8H17	Cr 81.9 SmI 95.4 SmC 146.9 I
[110]	LC200	OC12H25	OC9H19	Cr 66.5 SmI 104.5 SmC 146.2 I
[110]	LC201	OC12H25	OC10H21	Cr 74.4 SmI 111.2 SmC 147.0 I
[110]	LC202	OC12H25	OC12H25	Cr 62.0 SmI 118.0 SmC 144.2 I
[105]	LC203	OC6H13		Cr 75.2 SmCA* 98.9 SmC* 99.8 I
[105]	LC204	OC7H15		Cr1 57.3 Cr2 73.4 SmCA* 97.2 SmC* 98.2 I
[105]	LC205	OC8H17		Cr1 64.1 Cr2 69.3 SmCA* 98.8 SmC* 99.8 I
[105]	LC206	OC9H19		Cr1 68.8 Cr2 77.2 SmCA* 86.5 SmC* 97.8 I
[105]	LC207	OC10H21		Cr1 72.4 Cr2 79.0 SmCA* 87.1 SmC* 97.6 I
[105]	LC208	OC11H23		Cr1 78.9 Cr2 84.7 SmC* 96.7 I
[105]	LC209	OC12H25		Cr1 80.7 Cr2 85.6 SmC* 95.3 I
[109]	NLC18		H	Cr 185.6 I
[109]	LC210		OC5H11	Cr 133.5 SmC 154.7 N 169.6 I
[109]	LC211		OC6H13	Cr 122.5 SmX 107.0 SmC 160.7 N 165.8 I
[109]	LC212		OC7H15	Cr 132.9 SmCalt 111.6 SmX 118.6 SmC 165.8 N 166.7 I
[109]	LC213		OC8H17	Cr 123.2 SmCalt 102.1 SmX 125.9 SmC 166.5 I
[109]	LC214		OC9H19	Cr 121.3 SmCalt 121.9 SmX 131.3 SmC 168.2 I
[109]	LC215		OC10H21	Cr 118.2 SmCalt 121.3 SmX 133.2 SmC 166.4 I
[109]	LC216		OC12H25	Cr 107.1 SmCalt 119.5 SmX 138.6 SmC 166.3 I

Table 12. The chemical structures and mesophase transition temperatures (^oC) of thienyl-pyrimidine liquid crystals.

Ref.	No.	R	Mesophase Transition Temperatures (^oC)
[111]	LC217	C5H11	Cr 72.8 (G 46.5) SmA 109.0 I
	LC218	C6H13	Cr 50.1 SmA 108.0 I
	LC219	C7H15	Cr 61.6 SmA 114.7 I
	LC220	C8H17	Cr 56.4 (G 35.7) (SmX1 51.2) (SmX2 58.7) SmC 111.6 I
	LC221	C9H19	Cr 68.7 (G 32.9) (SmX1 45.6) (SmX2 66.5) SmB 75.5 SmC 116.3 I
	LC222	C10H21	Cr 60.0 (G 37.2) (SmX1 67.0) SmB 90.9 SmC 115.9 I
	LC223	C12H25	Cr 71.8 (G 42.4) (SmX1 70.4) B 107.6 SmC 114.5 I

Table 13. The chemical structures and mesophase transition temperatures (^oC) of phenyl pyrimidine liquid crystals with chiral centres.

Ref.	No.	X	Y	Z	R1	R2	Mesophase Transition Temperatures (^oC)
[112]	LC224	N	CH	CH			Cr 102 SmX 122 SmC* 128 I
	LC225	CH	N	CH			Cr 98 SmC* 102 SmA 136 I
	LC226	CH	CH	N			Cr 102 SmX 122 SmC* 128 I
	LC227	N	CH	CH			Cr 148 (SmX 112) SmC* 151 I
	LC228	N	CH	CH			Cr 115 SmC* 149 I
	LC229	N	CH	CH			Cr 95 IsoX 127 I
	LC230	N	CH	CH			Cr 129 SmX 139 SmC* 191 SmA 216 I
	LC231	N	CH	CH			Cr 109 SmC* 130 I
	LC232	N	CH	CH			Cr 86 SmC* 126 I

Although recent decades have witnessed significant progress in the synthesis and study of novel ferroelectric liquid crystals with optimised mesomorphic and other physical properties, FLCDs have not been mass-produced on a very large scale [21]. However, the ever-increasing requirement for future display technologies with very much greater information density combined with very fast response rates renders FLCDs of ongoing commercial interest in terms of R&D. This is especially the case for related fast switching electro-optical devices, such as spatial light modulators (SLMs), telecommunications switching, optical recognition, optical computing, etc [21]. Therefore, one of the main goals of this review article is to promote the rapid development of novel ferroelectric LCs and, especially, those based on phenylpyrimidine-based derivatives.

3. Pyrimidine-based liquid crystals for organic semiconductors

Figure 8 gives the schematic exhibition of pyrimidine-based polycatenar oligomers, photopolymerisable liquid crystals and copolymers for application in organic semiconductors. Organic molecules are usually regarded as insulators, but compounds with highly conjugated aromatic molecular cores possess a conjugated electron cloud with a high degree of delocalisation, and may exhibit semiconductor properties. However, for most of the organic semiconductors used for OLEDs, OPVs and OFETs, the hole mobility exhibits several orders of higher than electron mobility, so that the unbalanced electron-hole recombination in the active layer often results in poor quantum efficiency [115]. So in order to create more efficient devices, the organic materials with high electron mobility are demanded. Therefore, many highly conjugated heteroaromatic compounds have been studied as organic semiconductors due to a combination of relatively high electron mobility and appropriate morphology, such as the glassy state at and above room temperature [22,115–117].

Figure 8. (Colour online) The schematic exhibition of pyrimidine-based polycatenar oligomers, photopolymerisable liquid crystals and copolymers for application in organic semiconductors.

Liquid crystalline semiconductors have attracted much attention due to a favourable combination of mesophases, such as nematic, smectic and columnar (discotic), spontaneous thin-film formation with homogeneous alignment on planar surfaces, high charge transport, efficient photoluminescence and electroluminescence, etc. Columnar liquid crystals first attracted interest as ordered organic semiconductors with high charge carrier mobility attributable to their highly conjugated discotic aromatic cores being arranged above each other, with overlapping π-electron orbitals, in fluid columns of ordered discs that facilitate charge mobility [118–120]. Smectic liquid crystals also have an ordered structure comprised of layers of molecules with a rod-like, calamitic structure, so they have also often been studied as potential organic semiconductors for use in plastic electronics [121–123]. Nematic liquid crystals possess orientational order and no positional order, so they exhibit the least ordered structure and low viscosity among these mesophases [124–127]. However, their relatively low viscosity facilitates their macroscopical alignment as thin films at room temperature for application in OLEDs, OPVs and OFETs. Hence, a wide range of different liquid crystalline semiconductors exhibiting nematic, smectic or columnar phases have been widely reported for use as charge-transporting and/or luminescent materials in recent years [128–131]. However, liquid crystalline semiconductors containing 2,5-disubstituted pyrimidine rings have rarely been reported for application in OLEDs, OPVs or OFETs. Liquid crystals for FLCDs usually consist of two or three aromatic rings in their molecular core. In contrast organic semiconductors require larger aromatic cores to facilitate a high degree of electron delocalisation. Therefore, aromatic oligomers and polymers with large aromatic cores have been designed for use as organic semiconductors. Tong et al. [132] reported copolymers containing pyrimidine-based aromatic cores (see Table 14). Such compounds can exhibit good optical transparency, thermal stability and mechanical properties, e.g. for use as alignment layers with enhanced anchoring energy in LCDs. The Goto group at the University of Tsukuba reported a series of highly conjugated liquid crystalline oligomers and polymers containing pyrimidine rings. The group reported a low-bandgap liquid crystalline conjugated polymer (see Table 14) for potential use as a polarising emission material in the near-infrared spectroscopy (NIR) range [133]. The liquid crystal polymer incorporated phenyl pyrimidine-based moieties as side chains to generate mesophases and heterocyclic thiophene derivatives in the main chain to afford high conjugation and low bandgap. However, although the characteristic mesophase textures from string-like to large fan-shaped domain upon cooling have been observed by polarised optical microscopy (POM), DSC analysis did not give a clear transition signal [133]. Other liquid crystalline polymers consisting of a highly conjugated phenylene-thiophene main chain and pyrimidine-based side chains have also been reported by the group [134,135] (see Table 14). Interestingly, the group also studied the influence of a chiral nematic liquid crystal as a reaction medium in terms of induction of chiral properties in the polymers [136,138] (see Table 14). In addition, the azobenzene unit was also introduced to the main chain of the liquid crystalline polymers to replace the thiophene core, giving rise to photoisomerisation of the polymers [137] (see Table 14).

Table 14. The chemical structures of pyrimidine-based copolymers.

Ref.	No.	Structures
[132]	LC233
[133]	LC234
[134]	LC235 (n=1); LC236 (n=2); LC237 (n=3)
[135]	LC238 (m=1); LC239 (m=2); LC240 (m=3)
[136]	LC241 (m=1); LC242 (m=2);
[137]	LC243
[137]	LC244

Polymerisable liquid crystalline monomers have attracted much attention for application in some opto-electronic devices, such as augmented reality and virtual reality including AR/VR displays [139], due to the combined advantages of polymers and low-molecular-weight materials [140–144]. Lee’s group designed two calamitic liquid crystal monomers containing photopolymerisable acrylate groups and pyrimidine rings (see Figure 9) for application as host materials in guest-host polarisers [145]. Rahman et al. reported a series of pyrimidine-based bent-core liquid crystal monomers (see Table 15) for use as photo-switchable devices [45]. The Kelly and O’Neill group at the University of Hull reported many photopolymerisable liquid crystals for application in OLEDs, OPVs and OFETs [130]. They studied a series of pyrimidine-based reactive mesogens exhibiting a SmC phase as electron transporting materials. Different photopolymerisable end groups were also investigated in terms of their effect on the mesomorphic behaviour, transition temperatures and cross-linking efficiency [146–148] (see Table 16). Kelly and co-workers also studied a wide series of the phenylpyrimidine-based compounds with polymerisable vinyl groups at Roche in the 1990s [146]. They mainly investigated the influence of the positions and length of C-C double bonds on their mesomorphic behaviour [149]. They also reported polycatenar oligomers containing pyrimidine rings (see Table 17) with columnar phases for use as high charge carriers [150].

Figure 9. (Colour online) The chemical structures, mesophase transition temperatures (^oC) and POM textures of photopolymerisable pyrimidine liquid crystals [145].

Table 15. The chemical structures and mesophase transition temperatures (^oC) of azobenzene pyrimidine compounds.

Ref.	No.	n	Mesophase Transition Temperatures (^oC)
[45]	NLC19	1	Cr 98.7 I; I 93.9 Cr
	NLC20	2	Cr 91.1 I; I 88.7 Cr
	LC247	3	Cr 71.1 SmA 96.5 I; I 92.7 SmA 67.1 Cr
	LC248	4	Cr1 64.0 Cr2 72.3 SmA 102.8 I; I 101.3 SmA 68.2 Cr
	LC249	5	Cr 65.7 SmA 100.4 I; I 98.2 SmA 62.8 Cr
	LC250	6	Cr1 62.1 Cr2 100.6 SmA 113.8 I; I 111.2 SmA 81.8 Cr

Table 16. The chemical structures and mesophase transition temperatures (^oC) of pyrimidine-based photopolymerisable liquid crystals.

Ref.	No.	R	Mesophase Transition Temperatures (^oC)
[146]	LC251		Cr 25 SmC 124 I
	LC252		Cr 58 SmC 175 I
	LC253		Cr 56 SmC 126 I
	LC254		Cr 73 SmC 108 I
	LC255		Cr 52 SmC 116 I
	LC256		Cr 78 SmC 123 I
	LC257		Cr 82 SmC 150 I
	LC258		Cr 55 SmC 123 I
	LC259		Cr 97 SmC 141 I

Table 17. The chemical structures and mesophase transition temperatures (^oC) of pyrimidine-based polycatenary oligomers.

Ref.	No.	Structures	Mesophase Transition Temperatures (^oC)
[150]	NLC21		Cr 180 I
	LC260		Cr 16 Col 53 I
	LC261		Cr 181 Col 250 I

Organic semiconductors have experienced rapid development in terms of their opto-electronic properties in recent years. For instance, OLEDs are taking market share from LCDs due to a favourable combination of low power consumption without requiring backlight, high brightness, low switch-on and operating voltages, wide viewing angles, etc [129,151]. However, commercial OLEDs often use a vacuum-evaporated film-forming process and solution-processed OLEDs suitable for roll-to-roll printing and large-scale displays offer significant potential for future progress.

4. Pyrimidine-based liquid crystals for sensors

Figure 10 gives the schematic exhibition of pyrimidine-based liquid crystals coordinated with palladium or platinum as metallomesogens and non-conventional functional mesomorphic materials for application in sensors. Stimuli-responsive materials possess wide applications in sensors, 4D printing, soft robotics, artificial muscles, drug delivery, etc [152–154]. Liquid crystals exhibiting unique ordered mesophases and optical anisotropy provide a highly promising class of responsive materials [51,155–157]. Phenyl pyrimidine-based liquid crystals particularly have attracted a lot of attention for use as chem/bio-sensors [158–160]. Compounds containing N-heterocyclic rings, such as pyrimidine, are often used as sensing materials due to their electrophilic nature and tendency to form coordination complexes with metal ions and form hydrogen bonds [161–164]. Tschierske’s group [163] reported six series of pyrimidine-based calamitic/discotic liquid crystals coordinated with palladium and platinum as metallomesogens and non-conventional functional mesomorphic materials (see Table 18). Pyrimidine-based sensors have been widely studied for selective and sensitive detection in solution as well as in the vapour phase for many applications, such as in water treatment and in other chemical, medical and biological fields, for example [161].

Figure 10. (Colour online) The schematic exhibition of pyrimidine-based liquid crystals coordinated with palladium and platinum as metallomesogens and non-conventional functional mesomorphic materials for application in sensors.

Table 18. The chemical structures and mesophase transition temperatures (^oC) of pyrimidine-based organometallic calamitic/discotic liquid crystals.

Ref.	No.	M	R1	R2	R3	R4	R5	R6	Mesophase Transition Temperatures (^oC)
[163]	LC262	Pd	H	H	H	H	C7H15	OC10H21	Cr1 64 Cr2 113 SmC 117 SmA 133 I
	LC263	Pd	OC10H21	H	H	H	C7H15	OC10H21	Cr 117 (SmA 99 N 101) I
	NLC22	Pd	OC10H21	H	OC10H21	H	C7H15	OC10H21	Cr 115 I
	LC264	Pd	OC10H21	OC10H21	H	H	C7H15	OC10H21	Cr1 38 Cr2 59 Colh 76 I
	LC265	Pd	OC10H21	OC10H21	OC10H21	H	C7H15	OC10H21	Cr 72 Colh 134 I
	LC266	Pd	OC10H21	OC10H21	OC10H21	OC10H21	C7H15	OC10H21	Cr 79 Colh 163 I
	LC267	Pd	H	H	H	H	C10H21	OC8H17	Cr 129 SmA 136 I
	LC268	Pd	OC10H21	H	H	H	C10H21	OC8H17	Cr 114 (SmA 103 N 105) I
	NLC23	Pd	OC10H21	H	OC10H21	H	C10H21	OC8H17	Cr 117 I
	LC269	Pd	OC10H21	OC10H21	H	H	C10H21	OC8H17	Cr 61 Colh 74 NCol 76 X 79 I
	LC270	Pd	OC10H21	OC10H21	OC10H21	H	C10H21	OC8H17	Cr 78 Colh 146 I
	LC271	Pd	OC10H21	OC10H21	OC10H21	OC10H21	C10H21	OC8H17	Cr 79 Colh 169 I
	LC272	Pd	H	H	H	H		C9H19	Cr 178 SmA 196 I
	LC273	Pd	OC10H21	H	H	H		C9H19	Cr 113 SmC 122 SmA 140 N 147 I
	LC274	Pd	OC10H21	H	OC10H21	H		C9H19	Cr 131 (SmC 111 SmA 112) I
	LC275	Pd	OC10H21	OC10H21	H	H		C9H19	Cr 108 Colh 115 I
	LC276	Pd	OC10H21	OC10H21	OC10H21	H		C9H19	Cr 82 ColX 102 Colh 103 I
	LC277	Pd	OC10H21	OC10H21	OC10H21	OC10H21		C9H19	Cr 125 Colh 166 I
	LC278	Pt	H	H	H	H	C7H15	OC10H21	Cr 137 SmA 145 I
	LC279	Pt	OC10H21	H	H	H	C7H15	OC10H21	Cr 126 (SmA 107 N 109) I
	NLC24	Pt	OC10H21	H	OC10H21	H	C7H15	OC10H21	Cr 117 I
	LC280	Pt	OC10H21	OC10H21	H	H	C7H15	OC10H21	Cr 62 Colh 75 I
	LC281	Pt	OC10H21	OC10H21	OC10H21	H	C7H15	OC10H21	Cr 74 Colh 151 I
	LC282	Pt	OC10H21	OC10H21	OC10H21	OC10H21	C7H15	OC10H21	Cr 76 Colh 170 I

Abbott’s group makes use of computational chemistry models to design a series of novel chemo-responsive LCs with improved chemical selectivity [158–160]. In particular, the authors focused on selectivity against water, as water is one of the most common substances in chemical sensing. Interestingly, LCs containing pyrimidine cores in particular were found to exhibit a good tolerance to humidity and selectivity when responding to a target analyte [158]. This behaviour could be attributable to the fact that binding of pyrimidine is stronger than that of a cyano-biphenyl mesogen, for example, but weaker than that of pyridine because modest binding forces can be achieved due to dynamic sensing by an anchoring and de-anchoring process. The result has been confirmed by computational chemistry and experiments [158]. However, cyanobiphenyl LCs exhibit highly sensitive orientational response to target oxidising gases such as O₃ and Cl₂ [160]. These examples suggest that the choice of mesogens coordinated with metal ions is particularly important for selective and sensitive detection, but they have not yet been deeply explored. Although many pyrimidine-based compounds have been studied for coordination with metal ions, such as palladium (see Table 19) and alkali metal ions, for example, they were not used in sensing fields [165,166] or did not exhibit liquid crystalline mesophases [161]. Most LC sensors are still based on a limited number commercial LCs, commonly used in opto-electronic displays, but which have not been optimised for use in sensors, for example. Therefore, it is intended that this review could inspire more researchers to design and synthesise new liquid crystals for application in highly sensitive and selective sensors.

Table 19. The chemical structures and mesophase transition temperatures (^oC) of binuclear cyclopalladated phenylpyrimidine compounds.

Ref.	No.	X	R	R’	Mesophase Transition Temperatures (^oC)
[165]	NLC25	Cl	C6H13	CH3	Cr 215.0 I
	LC283	Cl	C9H19	CH3	Cr 100.0 Cr’ 128.9 SmA 137.2 SmX 168.3 I
	LC284	Cl	C6H13	C11H23	Cr 100.0 Cr’ 177.2 SmA 218.8 I
	NLC26	Cl	C9H19	C9H19	Cr 207.0 I
	NLC27	Br	C6H13	CH3	Cr 202.0 I
	LC285	Br	C9H19	CH3	Cr 115.8 Cr’ 129.8 SmA 174.5 I
	LC286	Br	C6H13	C11H23	Cr 112.0 Cr’ 171.3 SmA 202.1 I
	LC287	Br	C9H19	C9H19	Cr 105.9 SmA 196.6 I
	NLC28	I	C6H13	CH3	Cr 205.0 I
	NLC29	I	C9H19	CH3	Cr 195.0 I
	LC288	I	C6H13	C11H23	Cr 109.7 Cr’ 148.6 SmA 192.9 I
	LC289	I	C9H19	C9H19	Cr 105.1 SmA 128.1 SmX 202.5 I
	NLC30	OAc	C6H13	CH3	Cr 183.0 I
	NLC31	OAc	C9H19	CH3	Cr 128.0 I
	NLC32	OAc	C6H13	C11H23	Cr 139.6 I
	NLC33	OAc	C9H19	C9H19	Cr 135.0 I

5. Conclusion and perspective

In this review, we summarise the use of pyrimidine-based liquid crystals in ferroelectric display devices, organic semiconductors, and highly selective and sensitive detection, for example. A wide variety of electron-deficient pyrimidine-based LCs with different types of structures including calamitic, bent, discotic, taper-shaped, photopolymerizable monomers, as well as oligomers and polymers, for example, have been discussed for use as ferroelectric LCs, organic semiconductors and sensors. Although many generations of novel LCs have been developed for flat panel displays (LCDs) of ever-increasing size, complexity and image quality, the design of novel LCs for some fields of technological applications, such as LC sensors, flexible electronics, 3D/4D printing, etc., have received less attention. Many studies have used a small number of standard, commercial LCs, such as RM82, RM257, 5CB, etc., which have excellent properties in LCDs but not optimised properties for LC sensors or 3D/4D printing, for example. Therefore, we also point out the current challenge for the design of novel pyrimidine-based liquid crystalline structures and mesophases, and the study of the complex relationship between molecular structures, mesophases and application modes and mechanisms of operation. We hope that this review will promote the rapid invention and development of novel heterocyclic LCs in general and phenylpyrimidies in particular, for many high-tech applications requiring new advanced functional materials to realise their potential.

Abbreviation

Cr, Cr’, Cr1, Cr2:	=	crystalline phase
G:	=	glass state
N:	=	nematic phase
Sm:	=	smectic phase
SmA1:	=	a monolayer smectic A phase
SmAd:	=	a partial bilayer smectic A phase
Col:	=	columnar phase
ColL:	=	lamellar columnar phase
Colh, Colhex:	=	hexagonal columnar phase
Colhd:	=	disordered hexagonal columnar phase
NCol:	=	columnar nematic phase
LamColrec =	=	lamellas-columnar phase with rectangular symmetry
Ch:	=	cholesteric phase
I:	=	isotropic liquid state
IsoX:	=	isotropic X phase
TGB:	=	a filament texture similar to the so-called fingerprint texture
SmCanti:	=	antiferroelectric smectic C phase
SmCalt:	=	alternating smectic C phase
*:	=	chiral mesophase
LC:	=	liquid crystal
NLC:	=	non-liquid crystal

Disclosure statement

No potential conflict of interest was reported by the author(s).

Additional information

Funding

The work was supported by the National Natural Science Foundation of China [62204093 and 22205155], Guangdong Basic and Applied Basic Research Foundation [2021A1515111166], The Natural Science Foundation of Jiangsu Higher Education Institutions of China [22KJB510014 and 22KJB150011], Natural Science Foundation of Jiangsu Province [BK20220640], Huai’An Talent Funding for Outstanding Doctors in Universities [Z302J22518], Natural Science Foundation of Huai’An City [HAB202149], and PhD Research Start-up Fund of Huaiyin Institute of Technology [Z301B19582].