Influences of Central Units and Terminal Chains on the Banana-Shaped Liquid Crystals

Abstract

Azo-functionalized materials are one of the appealing groups of the functionalized materials owing to their photoswitching behaviour and have been explored for various potential applications viz., optical data storage, sensor, display devices, nonlinear materials and molecular switches. Recently, azo-functionalized bent-core liquid crystals (BCLCs) have gained significant attention because they have dual properties of BCLCs and azobenzene, which enables to generate new multifaceted functional and smart materials. In this report, the recently synthesized azobenzene containing bent-core mesogens and its subclass, the so-called hockey stick and V-shaped molecules are summarized. The mesomorphic behaviour of reported BCLCs affected by the type of central core unit, the nature, number and position of the lateral substituents and the type and length of the terminal chain are discussed. The photoisomerization process of these photoresponsive BCLCs in solid, solution and mesophase, as well as the impact of light on the chemical and electrical properties of them, are discussed.

1. Introduction

The bent-core molecules or banana liquid crystals (LCs) have attracted significant attention in the field of soft matter science due to their remarkable and unique properties. They exhibit macroscopic polar order and chirality in their mesophases, although the constituent molecules of these mesophases are achiral [1,2]. Historically, the first bent-core molecules were synthesized and reported by Vorländer without any impact for over 60 years [3,4]. The theoretical and technological interest towards the ferroelectricity in mesophases leads to the rebirth in interest for bent-core mesogens. Until the 1990s, the ferroelectric property was only discovered in tilted smectic phases of chiral calamitic and discotic molecules [5,6,7,8,9,10]. However, the synthesis of chiral rod and dis-shape molecules with tilted smectic phases is relatively expensive. Therefore, a great deal of theoretical and experimental efforts was put into finding ferroelectric mesophases using achiral molecules [11,12,13,14,15,16,17,18,19,20,21,22,23]. In 1996, a novel type of mesogenic material breathed new life into the liquid crystal field and brought new expectations: the so-called bent-core liquid crystals (BCLCs) or banana liquid crystals [1]. BCLCs exhibit a wide variety of bent-core mesophases such as B₁, B₂, …, B₈, bent-core nematics (BCN) and dark conglomerates (DC), in addition to the conventional nematic and smectic phases. A few previous studies have summarized the design and remarkable characteristics of BCLCs [6,24,25,26,27,28,29,30,31]. In general, eight different banana phases have been identified until today and are designated as B₁–B₈ phases based on the sequence of their discovery. Among the banana phases (B₁–B₈), the smectic B₂ phase, now designated as the SmCP phase, is the most widely studied phase due to its ferroelectric switching behaviour that can be used in fast switching electro-optical devices [24,32]. Nowadays, the primary concern of many research groups is to create advanced materials using BCLCs where their properties can be adjusted by different external stimuli, in particular, the light [33].

Podruczna et al. mentioned that azobenzene derivatives were among the first recognized groups of liquid crystalline molecules [34,35]. The significant interest in using the azobenzene as a fundamental component for LC molecules is based on two reasons. Firstly, it is a photosensitive chromophore that undergoes trans-cis isomerization upon light exposure, causing the configurational modifications that bring significant changes in physical properties [36,37,38,39,40,41]. When an azobenzene-containing molecule is exposed to UV light with λ = 365 nm, the energetically more stable trans configuration will change into the cis configuration and reverse back to the original configuration upon the illumination with visible light (within the range 400–500 nm) [42]. Cis-trans isomerization can also occur by keeping the excited trans isomers in the dark place. This spontaneous process is called thermal back relaxation, where the metastable cis isomers revert back to the thermodynamically more stable trans form [43]. This particular aspect of photonics, in which molecular configuration can be manipulated by light, is crucial especially in the research fields such as holographic media [44,45], photoalignment of LC system [46], optical storage [47,48,49] and photochemical molecular switches [45,50]. In the case of optical data storage devices, one of the most requirement for azobenzene to be utilized is long thermal back relaxation [51]. Secondly, molecules based on azobenzene have delocalized electronic charge distribution between acceptor and donor atom or group of atoms substituted with it. This feature may be used to design materials with good nonlinear optical (NLO) properties. In particular, for optical second-harmonic generation (SHG), the azobenzene moiety seems to be a good candidate to be incorporated in bent-core LCs [52].

In recent years, azo functionalized bent-core liquid crystals received great attention because they combine the photoswitching behaviour of azobenzene and liquid crystalline properties of BCLCs, creating potential new multi-functional and smart materials [53]. For this reason, azobenzene-containing BCLCs draw the attention of many researchers in the past few years. Many different azo functionalized BCLCs have been synthesized to understand their rich mesomorphism and multifaceted properties [52,54,55,56,57,58,59,60,61,62]. This paper focuses on reviewing the most recent development of azobenzene-containing BCLCs of different molecular structures understands the structure-property relationship. In the following sections, the observed mesophases along with the transition temperatures for each compound are shown below the chemical structure. It should be noted that some detailed information of mesophases observed in the discussed BCLCs will not be included in this paper. Therefore, readers are suggested to refer to the original publications for more details.

2. Bent-Core Liquid Crystals (BCLCs)

Banana/bent-shaped compounds can be classified into three categories: (i) molecules incorporating a rather rigid bent aromatic core, such as compound 1 derived from the rigid resorcinol core, (ii) mesogenic dimers consists of two mesogenic units linked via a flexible spacer, usually, an alkyl chain (compounds 2 and 3) [63], and (iii) hockey-stick molecules, where an alkyl chain is attached to the meta-position at one end of the aromatic core (type 1 hockey-stick molecule 4) [64,65,66] and type 2, where the aromatic ring is attached to the meta-position at one end of the aromatic core (compound 5). The molecular structure and carbon view of BCLCs are shown in Figure 1.

As mention earlier, BCLCs bearing with azo moiety exhibit remarkable properties because they possess the photochromic properties of azobenzene and the unusual physical properties of bent-core mesogens [57,70]. Alaasar [57] pointed out that it is crucial to note that Vorländer, the pioneering scientist in the field of LCs, had prepared the first azo-containing BCLC derived from the isophthalic acid as the bent-core unit (compound 6) in 1929 [3] although Prasad et al. initiated the research on BCLCs incorporating with azobenzene groups [36,71,72]. In 1932, Vorländer et al. reported another bent-core mesogen that consists of resorcinol as the central core unit and azoxy as the linker (compound 7) (see Figure 2) [4]. This mesogen exhibited an unidentified smectic phase which was later known as the B₆ phase after the further investigation conducted by Pelzl et al. [57,73].

2.1. Molecular Structure of Bent-Core Liquid Crystals

The existence of chirality and polar order in mesophases of BCLCs is strongly affected by the molecular structure and intermolecular interactions [67]. BCLCs are built up from three basic parts, which are the central bent unit (BU), linking groups and rod-like wings. The general structure of the BCLC and some of the reported central bent units are shown in Figure 3.

In the designing of BCLCs, choosing the right central bent unit (BU) is very important because the length and the bending angle of the BU are the most crucial parameters for the formation of polar ordered mesophases in BCLCs. The bending angle of the central BU has to be within the range between 105° and 140° is to show the layer polarity [67,74]. Modification of the bending angle can be done by introducing a five-membered heterocycles core unit [75,76], instead of a six-membered ring, or by attaching the substituents at the BU close to the linkages X and X′ [67,77,78]. Among the linking groups, the ester linkage (X, X′) is the most often linking group used to connect the central core unit to rod-like wings due to the enhanced chemical stability [57,79]. However, other linking groups such as azomethine (C=N) and double bond (C=C) are also employed as the linker between BU and rod-like wings [57]. Linking groups (Y, Y′) that can be inserted within the rod-like wings of BCLCs are Schiffs base, azo (N=N), double bond (C=C) and triple bond (C≡C) [67]. The function of linkers within the rod-like wings (Y, Y′) is to preserve a certain degree of flexibility of BCLCs to promote the mesomorphism and prevent crystallization. Besides, their direction also has a significant impact on the mesomorphic behaviour of BCLCs. Besides the BU and linkage groups, the mesogenic properties of BCLCs can also be tuned by the lateral substitution on the rod-like wings, the type and length of terminal chains as well as the number of aromatic units in the molecular structure [57,67]. In the following sections, the recent reported azobenzene-containing BCLCs are classified according to the central BU incorporated in its molecular structure.

2.1.1. 1,3-Dihydroxybenzene/Resorcinol

Resorcinol is the most widely used central BU in designing bent-core liquid crystals (BCLCs) that display banana phases as well as smectic or nematic phases [57,78,80,81,82,83,84,85]. Compound 8 is formed by connecting the resorcinol central core to azobenzene sidearms with the double bond as the terminal chain via the ester linkage. When the fluorine atom is inserted to the periphery of the azobenzene sidearm in compound 8, compound 9 that shows the similar phase sequence with compound 8 is formed. However, it has the lower phase transition temperature compared to that of compound 8 [86].

Compound 10 is formed when there is an addition of a –O(CH₃)_n unit inserted between the azobenzene sidearm and the alkoxy double bond terminal chain. The homologues of monomers 10 (n = 4, 5 and 6) exhibit an enantiotropic intercalated smectic A phase, the so-called B₆ phase, in a broad temperature range [42].

Substituted 1,3-Dihydroxybenzene/Resorcinol

As mentioned in Section 2.1, the mesomorphic behaviour of BCLCs is significantly affected by the slight modification of the molecular structure. The lateral substitution at different positions in the central BU is one of the possible modifications. 4-substituted resorcinol represents a typical structure of this kind affecting the bend angle of the bent-core mesogen greatly as a result of the effects of the lateral group on the conformation of the adjacent ester group, especially in the case of 4-cyanoresorcinol [57,87]. For example, the 4-cyanoresorcinol core is a beneficial BU with a reduced bending angle that used to generate new mesophases and/or new phase sequences [57,88,89]. For simplicity, the chiral phase is given a symbol [*].

Alaasar et al. reported three series of BCLCs (11, 12 and 13) that consist of a 4-cyanoresorcinol as the central core unit combined with fluorinated rod-like azobenzene sidearms [90,91,92,93,94]. This research provides important clues to understand the development of polar order in lamellar LC phases of achiral bent-core molecules, depending on the chain length, temperature and position of fluorine substitution structure. The longest homologue of series 11 where n = 20, was reported previously [95]. Only the shorter homologues of compound 11 (n = 4 and 6) exhibit the cybotactic bent-core nematic phase constituted by non-tilted SmA clusters (N_cybA) (see

Table 1. Phase transition of homologues of compounds 11.

Compound	n	Phase Transition
11a	4	Cr 164 NcybA 136 Iso
11b	6	Cr 112 SmAPAR 101 SmAPR 117 NcybA 128 Iso
11c	8	Cr 112 SmCaPA 104 SmAPAR 110 SmAPR 137 Iso
11d	10	Cr 108 SmCaPA 108 SmCsPAR 112 SmAPR 143 Iso
11e	12	Cr 104 SmCaPA 98 SmCsPA 108 SmCsPAR ≈ 111 SmCsPR[*] 135 SmA 147 Iso
11f	14	Cr 101 SmCaPA 98 SmCsPA 107 SmCsPAR ≈ 112 SmCsPR[*] 134 SmCs[*] 142 SmA 147 Iso
11g	16	Cr 101 SmCaPA 93 SmCsPA 105 SmCsPAR ≈ 112 SmCsPR[*] 126 SmCs[*] 145 SmA 149 Iso
11h	18	Cr 102 SmCaPA 94 SmCsPA 101 SmCsPAR ≈ 108 SmCsPR[*] 121 SmCs[*] 143 SmA 148 Iso

). This type of bent-core nematic is rare compared to that of N_cybC phase, which is formed by the tilted SmC clusters [29,52,87,96]. Bent-core nematics (N_cybC/N_cybA) are very unusual compared to the conventional N phase of the calamitic LCs because they are responsible for possible bixiality [29,97,98,99], giant flexoelectricity [100] and ferroelectricity [101]. Unlike the conventional smectic and columnar phases, the existence of the nematic phase in BCLCs is not that often. As the length of alkyl chains increases, the macroscopic polar order developed continuously from the low-permittivity SmA phases (SmA) through a series of uniaxial (SmAP_R, SmAP_AR) and biaxial high-permittivity paraelectric smectic phases (SmC_sP_R^[*^], SmC_sP_AR) to the antiferroelectric polar smectic phases (SmC_sP_A and SmC_aP_A) (see Table 1). In fact, the overall phase sequence of homologues 11 is actually affected by the core-chain nanosegregation, a phenomenon that is controlled by the chain length [90,102].

When the fluorine is shifted from the peripheral 3-position to the inner 2-position, homologues of compound 12 (n = 8–16) are formed. The change in the position of the fluorine atom from the peripheral 3-position to the inner 2-position reduces the melting point and clearing temperature [90]. The B₂/SmCP phase is disappeared in series 12 due to the unfavourable steric hindrance of fluorine-substitution in this position, regardless of the alkyl chain length. This observation is in line with the previous observations made for rod-like mesogens [103,104,105].

Homologues of compound 13 are generated by substituting the fluorine atom at the ortho and meta positions with respect to the terminal alkoxy chain. The transition temperatures of this series are higher compared to series 12, but lower than the series 11. No nematic phase could be observed in homologues of compound 13, as it is exhibited by homologues in series 11 and 12. Although homologues 13 with n ≥ 12 exhibit the chiral synclinic tilted SmC_s^[*^] phases, no polarization current peaks could be observed in the triangular wave experiment. The formation of chiral conglomerates in homeotropic alignment indicates the presence of SmC_s^[*^] phase (see Figure 4) [90].

Figure 4. Textures of the chiral SmC_s^[*^] phase of compound 13e at T = 127 °C, (a,c) between slightly uncrossed polarizers and (b) between crossed polarisers, exhibiting dark and bright domains that indicate the presence of chiral domains with opposite handedness. (Reproduced with permission from ref. [90], copyright John Wiley and Sons 2017, https://doi.org/10.1002/chem.201606035).

Figure 4. Textures of the chiral SmC_s^[*^] phase of compound 13e at T = 127 °C, (a,c) between slightly uncrossed polarizers and (b) between crossed polarisers, exhibiting dark and bright domains that indicate the presence of chiral domains with opposite handedness. (Reproduced with permission from ref. [90], copyright John Wiley and Sons 2017, https://doi.org/10.1002/chem.201606035).

The replacement of the fluorine atom in compound 11h with less polar bromine atom results in the formation of compound 14 with the reduced phase transition temperature, although the Iso-SmA-SmC_s^[*^]-SmC_sP_AR phase sequence has remained. When the bromine atom in compound 14 is replaced by the non-polar methyl group with similar volume (compound 15), the stability of mesophases is further reduced and leave only a nonpolar and non-chiral SmC and nematic phases (N_cybC). This is mostly attributed to the unfavourable reduction in core-core interactions by the non-polar and electron-donating methyl group [90].

Recently, Alaasar et al. had reported asymmetric BCLCs derived from the 4-cyanoresorcinol BU connected to two different rod-like wings [91]. Homologues of compound 16 were prepared by connecting the 4-cyanoresorcinol to the azobenzene wing and the phenyl terephthalate wing with the –COO linkage inverting downward. All homologues of compound 16 (n = 8, 12, 16 and 20), except for the shortest homologue 16a, exhibit the same phase transition sequence of a non-polar SmA phase, a paraelectric SmC_s phase involving the SmC_s^[*^], SmC_sP_R^[*^] and SmC_sP_AR, the polar modulated Sm$\tilde{\mathrm{C}}$_sP_A and the non-modulated SmC_sP_A phases as the temperature decreases (see

Table 2. Phase transition of homologues of compounds 16 and their derivative 17.

Compound	X	n	Phase Transition
16a	H	8	Iso 164 SmA ~ 154 SmAPR 114 SmCaPAR 109 Sm$\tilde{\mathrm{C}}$sPA 82 SmCsPA 58 Cr
16b	H	16	Iso 163 SmA 156 SmCs[*] ~ 126 SmCsPR[*] 111 SmCsPAR 109 Sm$\tilde{\mathrm{C}}$sPA 78 SmCsPA 62 Cr
17	F	16	Iso 165 SmA 152 SmCsPR[*] 127 SmCsPAR 120 Sm$\tilde{\mathrm{C}}$sPA 107 SmCsPA 100 Cr

). It should be noted that SmC_s^[*^], SmC_sP_R^[*^] and SmC_sP_AR phases have distinct properties even they are considered in temperature ranges of the paraelectric SmC_s phase. The compound 16a behaves differently from the higher homologues due to the reduced tilt, leading to the formation of a non-tilted and achiral paraelectric SmAP_R phase followed by a transition to an anticlinic SmC_aP_AR (see Figure 5a). The discovery of new SmC_aP_AR phase in this study completes the presently known series of synclinic tilted SmC_sP_AR [106] and non-tilted SmAP_AR phases types [95,107]. Substituting the outer azobenzene ring with a lateral fluorine atom in compound 16b (n = 16) results in the formation of compound 17. The result indicated that there is almost no difference in phase sequence between compounds 16b and 17, except for the absence of SmC_s^[*^] phase in compound 17 (see Table 2). Besides, textures and polarization current curves of compound 17 are very similar to compound 16b. (Figure 5b,c) Therefore, it seems to be no significant effect of the monofluorination of a single benzene ring on the sequence and LC behaviour [91].

Replacing the phenyl terephthalate wing in homologues series of compound 16 with a benzoylated 4-hydroxybenzoate wing in the molecular structure leads to homologues series of compound 18. This series has the narrower mesophases, higher melting temperatures and a larger tilt compared to series 16. Except for the longest homologue 18b (n = 20), all homologues 18 show a skewed cybotactic nematic phase (N_CybC) instead of the SmA phase as it exhibited by compounds 16 (see

Table 3. Phase transition of homologues of compounds 18 and their derivative 19.

Compound	X	n	Phase Transition
18a	H	16	Iso 123 NCybC 121 SmCsPR[*] 96 SmCsPAR 80 Sm$\tilde{\mathrm{C}}$sPA 73 SmCs’PA 66 SmCsPA 48 B5 36 M
18b	H	20	Iso 130 SmCsPR[*] 98 SmCsPAR 75 Sm$\tilde{\mathrm{C}}$sPA 36 Cr
19	F	16	Iso 131 SmCsPR[*] 104 SmCsPAR 87 Sm$\tilde{\mathrm{C}}$sPA 84 SmCs’PA 60 Cr

). The N_CybC phase is wholly disappeared and replaced by the SmC_sP_R^[*^] phase for the longest homologue 18 [91]. The fluorinated compound 19 (n = 16) is formed by substituting the fluorine atom on the outer phenyl of the azobenzene wing. Unlike compound 18a (n = 16), it does not show N_CybC phase. The mesophases and the phase sequence of compound 19 are almost similar to the 18b (see Table 3), indicating that decreasing the electron density of the core by fluorination has the same impact as elongation of the alkyl chain [91].

Although a 4-chlororesorcinol is a commonly used BU in the bent-core system [82,108,109], only a relatively limited number of compounds employing both these motifs (4-chlororesorcinol and azo group) has been synthesized and studied. Šmahel et al. prepared two isomeric series of nonsymmetrical BCLCs (series 20 and 21), which possesses the 4-chlororesorcinol as the central unit and various linking groups with different rigidity (benzoate ester, biphenyl, cinnamate, Schiff base and stilbene) in the lengthening arm, aiming to tune the properties of the bent-core nematic (BCN) phase.

This study raises the question of how much of the rigidity of the lengthening arms can preserve the preferential formation of nematic to lamellar phases. Besides, the position of the lateral chlorine substituent in the central resorcinol unit respect to azobenzene will additionally affect the mesogenic properties of the studied compounds. The compound of series 20 with the lateral chlorine substituent is located next to the photosensitive azo side arm exhibit SmC-type of phases below the nematic phase (

Table 4. Phase transition of compounds of series 20.

Compound	X	L	Phase Transition
20a	H	OOC	Iso 92 N 64 SmCAPA 46 Cr
20b	H	– *	Iso 98 N 79 SmCX 71 Cr
20c	H	OOC–CH=CH	Iso 107 N 70 Cr
20d	OH	N=CH	Iso 105 SmCAPA 64 Cr
20e	H	CH=CH	Iso 113 N 95 SmCAPA 88 Cr

* Single bond.

). For compound 20b, the unidentified SmC_X phase was found below the nematic phase (Figure 6). In contrast, compounds of series 21 with the lateral chlorine substituent opposite to the photosensitive azo side arm form only the nematic phase or crystalline, except for the compound 21d (

Table 5. Phase transition of compounds of series 21.

Compound	X	L	Phase Transition
21a	H	OOC	Iso 97 Cr
21b	H	– *	Iso 94 N 78 Cr
21c	H	OOC–CH=CH	Iso 110 N 83 Cr
21d	OH	N=CH	Iso 113 N 110 SmCAPA 99 Cr
21e	H	CH=CH	Iso 123 Cr

* Single bond.

) [110].

Substituting the –Cl and –COOH groups are at position 5 of the central core in the compound 8 results in the formation of compounds 22. Compound 22a and 22b show the B₆ and crystalline phases, respectively (

Table 6. Phase transition of compounds 22 and 23.

Compound	X	Y	Phase Transition
22a	H	Cl	Iso 144.3 Smintercal 126.0 Cr
22b	H	COOH	Iso 168.9 Cr
23a	F	Cl	Iso 142.9 Smintercal 126.3 C
23b	F	COOH	Iso 165.0 Cr

). Their mesomorphic behaviour remained the same even there is the electronegative –F group attaching to the outer ring of azobenzene sidearm (compounds 23a and 23b). However, the presence of the fluorine atom at the peripheral of the azobenzene reduces the transition temperature of fluorinated compounds [86].

2.1.2. 3-Hydroxybenzaldehyde

3-Hydroxybenzaldehyde has been used to prepare different asymmetric five-ring containing azobenzene-based BCLCs. Compounds 24b and 24c are synthesized by lateral substituting X = H with the electronegative halogens (Cl and F) in the compound 24a, respectively. All of them exhibit an enantiotropic B₂ phase, regardless of the nature of lateral substituents. However, the transition temperatures were affected because the mesophase of compound 24b was found to supercool up to the room temperature [111].

Substituted 3-Hydroxybenzaldehyde

In 2018, Hedge et al. reported achiral bent-core salicylaldimine compounds derived from 2,5-dihydroxybenzaldehyde as the central core, and they are found to exhibit dark conglomerate (DC) and B₂ mesophases. There are three types of DC phases which are spongy, helical nano-crystalline (HNC) and helical nano-filament (HNF) mesophases [112,113], depending on the local structure and packing density of BCLCs. The DC* phase is characterized by the optically isotropic activity or has little birefringence. Therefore, conglomerates of macroscopically chiral domains with opposite handedness can be observed under the polarizing optical microscope (POM). BCLCs exhibiting DC* phases are of significant importance [92,114] because they may be considered as smart materials using in various applications, such as LC phase grating, fibre-optic core and optical waveguides. However, only the spongy phase is electrically switchable among the DC phases. All the homologues of compound 25 (n = 12, 14 and 16) exhibit enantiotropic sponge (DC₁*) and HNC (DC₂*) mesophases, where the existence of the DC* phase is proven by the presence of chiral domains (Figure 7a–f). When X = H is substituted with F and Cl in homologue 25a, compound 26 and 27 are formed. Compound 26 exhibits both the sponge (DC₁*) and HNC (DC₂*) mesophases as exhibited by homologues 25, whereas compound 27 shows only a sponge mesophase [115].

Figure 7. The optical texture of DC₁* obtained for compound 25b at 170 °C: (a,c) between the uncrossed polarizers by an angle of about 8° and (b) between crossed polarisers; (d–f) textures of DC₂* obtained under the same conditions as observed (a–c), at 158 °C; (g–i) textures of Cr* obtained at the room temperature [115].

Figure 7. The optical texture of DC₁* obtained for compound 25b at 170 °C: (a,c) between the uncrossed polarizers by an angle of about 8° and (b) between crossed polarisers; (d–f) textures of DC₂* obtained under the same conditions as observed (a–c), at 158 °C; (g–i) textures of Cr* obtained at the room temperature [115].

Reversing two arms of compounds 25 results in the formation of compounds 28 (n = 12, 14 and 16), whereas replacing the ester linkage with azo linkage in one arm of compounds 28 results in the formation of compounds 31 (n = 12, 14 and 16). Both of the compounds (28 and 31) shows a similar trend in their mesomorphic behaviour where they display only the B₂ mesophase. The B₂ mesophase is changed to the DC₁* mesophase when X = H is replaced by F (29 and 32) and Cl (30 and 33) atoms. The DC₂* phase that presents in compounds 25 is completely removed in compounds 28 and 31. An interesting phenomenon has been found in series 25, 28 and 31, where the chirality observed in the mesophases is maintained even in their crystalline form (Cr* phase) (Figure 7g–i). Compared to compounds 36 with the salicylaldimine group is present in one of the side arms, series 25, 28 and 31 (the salicylaldimine group is present at the centre of BCLCs) tend to form DC* mesophase rather than the B₂ phase, regardless the type of lateral substituent. Besides, compounds of these three series have very high transition temperatures compared to that of 36 [115].

2.1.3. 3-Hydroxybenzoic Acid

Similar to 3-hydroxybenzaldehyde BU, 3-hydroxybenzoic acid is used to prepare the asymmetric photoresponsive BCLC. The first report about this kind of BCLCs was done by Prasad et al. in 2004 [116]. However, compounds derived from the 3-hydroxybenzoic acid as the BU have been studied much less compared to compounds based on resorcinol. Most of the reported compounds consist of one azo unit in their structures and exhibited a columnar B₁ phase, a lamellar B₂ phase or a modulated smectic B₇-typed mesophases. Compounds with two azo groups do not show the liquid crystalline phase [57,60].

Recently, Hedge et al. had synthesized two series of photosensitive BCLCs based on a slight modification of the previously reported compounds [116]. For both homologue series 34 and 35 (n = 4, 6, 8, 10 and 12), the length of the terminal chain at one end is remained constant as n = OC₁₀H₂₁, while the other end is either terminated with the alkyl (34) or alkoxy chain (35). Both of them exhibit a similar trend in which the lower homologues (n < 12) show the B₁ mesophase, whereas the higher homologues (n = 12) exhibit the B₂ mesophase. In the case of compound 35b, an unknown monotropic mesophase M was observed on cooling from the isotropic phase, in addition to B₁ mesophase [117]. The homologues of compounds 36 (n = 12, 14, 16), wherein one of the side arms of the BCLC bearing with the salicylaldimine group, show only B₂ mesophase (Figure 8). Similar to homologues 36, both fluorinated (37) and chlorinated (38) compounds exhibit the B₂ mesophase [115].

Figure 8. The B₂ mesophase observed under the POM for compound (a) 36c at T = 140 °C and (b) 36b at T = 134 °C [115].

Figure 8. The B₂ mesophase observed under the POM for compound (a) 36c at T = 140 °C and (b) 36b at T = 134 °C [115].

Substituted 3-Hydroxybenzoic Acid

Compounds 39 derived from a 4-methyl substituted 3-hydroxybenzoic acid was done by Kohout et al. in 2017. Compound 39a with the lateral –CH₃ on the BU opposite to the azo-bearing sidearm was found to have rich polymorphism. On cooling from the isotropic phase, the nematic phase followed by three synclinic smectic phases (SmC_s1, SmC_s2 and SmC_s3) and a DC phase had been identified (

Table 7. Phase transition of compounds 39.

Compound	X	Y	Phase Transition
39a	N=N	COO	Iso 106 N 105 SmCs1 92 SmCs2 82 SmCs3 80 DC
39b	OOC	N=N	Iso 108 Cr
39c	N=N	N=N	Iso 117 N 116 SmCs 107 Cr

and Figure 9). The X-ray diffraction (XRD) result revealed that the DC phase of 39a is different from those of B₄ phases and DC sponge phases, but very similar to the recently described new-type of a DC phase exhibited by the related 4-methylresorcinol derived BCLCs (Figure 10) [118]. In contrast, reversing the positions of the ester and azo groups in compound 39a causes compound 39b lost the mesomorphic behaviour (Table 7). When the ester linkage is replaced with the azo linkage in the compound 39a or 39b, a compound 39c with the nematic phase appeared above the SmC_s phase is formed (Table 7). Non-polar character is displayed in both compounds 39a and 39c, although the tilted SmC phase had been identified in their phase sequence [33].

Compounds of series 40, which are the analogues of compounds 39, are formed when the lateral methyl group is changed to position 6 of 3-hydroxybenzoic acid. They have mesomorphic properties that differ from compounds 39. The SmC_S phases exhibited by compounds 39 were replaced by either SmC_aP_A phases or crystalline phases in series 40. The formation of the SmC_aP_A phase in compound 40b is probably due to the preferred bent shape of the compound. For compound 40a and 40c, the non-mesomorphic behaviour can be explained by the limited flexibility of the azo-bearing side-chain next to the lateral substituent [33].

Replacing the methyl group at the central unit in the compound 39b with the fluorine atom leads to the formation of compound 41. Instead of using the longer alkoxy chain –OC₁₆H₃₃ as the terminal end chain, the shorter alkoxy chain where n = 8 is used. The DSC result shows the nematic phase and the low-temperature smectic phase (SmX) on the cooling cycle [119].

2.1.4. Substituted 3-Nitrobenzoic Acid

Several examples of asymmetric azobenzene based BCLCs with five rings derived from substituted 3-nitrobenzoic acid have been reported. In 2017, Prasad et al. reported some photoswitchable BCLCs with five rings incorporating 2-methyl-3-nitrobenzoic acid as the central unit. It was a continuation of the previous work [87] where the earlier synthesized compounds that consisted of –CH=N– linkages are found to have too high transition temperature, which is unsuitable for physical studies. The higher homologues of compounds 42, where n >10 exhibited enantiotropic nematic phase and two unidentified SmX₁ and SmX₂ phases (Figure 11). However, the transition temperatures of these compounds were still very high (

Table 8. Phase transition of compounds 42.

Compound	n	Phase Transition
42a	10	Cr 101.0 SmA 124.5 N 185.1 Iso
42b	12	Cr 98.4 SmX2 126.1 SmX1 130.8 SmA 132.2 N 180.7 Iso
42c	18	Cr 99.1 SmX2 117.0 SmX1 1135.5 SmA 145.0 N 167.8 Iso

) [120].

The homologues of compounds 43 and 44 (n = 10, 12, 14, 16, 18) were synthesized when one of the arms of compounds 42 is substituted with the lateral methyl group at ortho and meta position with respect to the –N=N– unit, respectively. The introduction of lateral substituent decreases the isotropic temperature of series 43 and 44 compared to series 42 (see Table 8 and

Table 9. Phase transition of compounds 43 and 44.

Compound	n	X	Y	Phase Transition
43a	12	CH3	H	Cr 131.4 N 156.7 Iso
43b	18	CH3	H	Cr 129.5 N 145.0 Iso
44a	12	H	CH3	Cr 111.7 N 158.3 Iso
44b	18	H	CH3	Cr 96.3 SmX1 106.0 SmA 107.5 N 148.0 Iso

). The presence of the lateral –CH₃ group at the ortho position in homologues of compounds 43 removed the Sm mesophases entirely. For the homologues of 44, the Sm mesophases are suppressed to some extent but not entirely (Table 9) [120].

Replacing the lateral –CH₃ group with –Cl group in compounds 43 and 44 results in the formation of compounds 45 and 46, respectively. The homologues of compounds 45, where n = 10, 12, 14, 16, 18, show the enantiotropic nematic phase, except for the highest homologue 45b that has an additional monotropic SmA phase below the nematic phase. In the case of compounds 46, the phase sequence during the cooling cycle is isotropic-nematic-SmA-SmX₁-SmX₂-crystalline phase (

Table 10. Phase transition of compounds 45 and 46.

Compound	n	X	Y	Phase Transition
45a	12	Cl	H	Iso 149.0 N 114.4 Cr
45b	18	Cl	H	Iso 139.0 N 115.4 SmA 112.3 Cr
46a	12	H	Cl	Iso 160.4 N 106.7 SmA 101.0 SmX2 93.0 Cr
46b	18	H	Cl	Iso 149.0 N 119.3 SmA 111.8 SmX1 96.4 SmX2 91.0 Cr

). It can be concluded that the lateral ortho substituents with respect to –N=N– stabilize the nematic phase, whereas the lateral meta substituents induce the formation of both the Sm phase and the nematic phase. However, none of these compounds are found to be electrically switchable although they exhibit the nematic phase and some hidden Sm phases [120].

When the Cl group in the shortest homologue of compound 45 (n = 10) is changed to the highly electronegative –F group, the BCLC 47 that exhibits ferroelectric N_cybC and ferrielectric SmC phases are formed. It is worth noting that both electrically switchable nematic and smectic mesophases are found to exist in the same compound for the first time. The formation of the field-induced ferrielectric smectic mesophase is proved by the texture observed under the POM (Figure 12) as well as the appearance of three polarization current peaks for each half cycle at high voltage (35 V_pp µm⁻¹) (Figure 13b). A proposed model for the occurrence of the ferrielectric state and possible switching mechanism in this state is displayed in Figure 13a. Such ferroelectric N_cybC and ferrielectric SmC mesophases are of significant interest as they could open a new route in electro-optical devices [121].

Figure 12. Changes in the texture of the smectic phase in compound 47 observed under a triangular wave at T = 110 °C with increasing voltage from (a) 0 V_pp µm⁻¹, (b) 30 V_pp µm⁻¹ to (c) 35 V_pp µm⁻¹. (Reproduced with permission from ref. [121], copyright American Chemical Society 2018, https://doi.org/10.1021/acs.jpcb.7b11733).

Figure 12. Changes in the texture of the smectic phase in compound 47 observed under a triangular wave at T = 110 °C with increasing voltage from (a) 0 V_pp µm⁻¹, (b) 30 V_pp µm⁻¹ to (c) 35 V_pp µm⁻¹. (Reproduced with permission from ref. [121], copyright American Chemical Society 2018, https://doi.org/10.1021/acs.jpcb.7b11733).

2.1.5. Other Aromatic Bent-Core Units

Some other aromatic bent-core units are also used to synthesized azobenzene containing BCLCs, such as 2,7-dihydroxynaphthalene and 4-chloro-1,3-dinitrobenzene. For example, naphthalene based BCLC monomers incorporated with azobenzene units terminated with double bonds had been investigated by Lutfor et al. All homologues of compounds 48 (n = 3–9) display the enantiotropic B₆ phases, except for shorter homologues with n = 1 and 2 that exhibit the nematic phase (Figure 14). The transition temperature decreases as the length of alkyl chains increases, which is in accordance with previous studies. Besides, the odd-even number of alkyl chains is also an effect on the isotropic temperature of compounds 48. Compounds 48 with the even number of alkyl carbon have a higher transition temperature than that of the odd number of the alkyl carbon chain [122,123,124]. Substituting the fluorine atom at the periphery of the material 48a is results in the formation of compound 49. The fluorinated compound has a similar phase sequence as the monomer 48a but with the reduced phase transition temperature [123].

Figure 14. Optical textures taken for (a) compound 48a in the nematic phase at T = 155 °C and (b) compound 48b in the B₆ phase at T = 142 °C (Reproduced with permission from ref. [122], copyright John Wiley and Sons 2014, https://doi.org/10.1002/jccs.201300191).

Figure 14. Optical textures taken for (a) compound 48a in the nematic phase at T = 155 °C and (b) compound 48b in the B₆ phase at T = 142 °C (Reproduced with permission from ref. [122], copyright John Wiley and Sons 2014, https://doi.org/10.1002/jccs.201300191).

Bajzikova et al. reported BCLCs possessing the 2,7-dihydroxynaphthalene as the central core unit and different aromatic units at one of the terminal ends, aiming to enhance nanosegregation. The results show that the alkyl chain length between the rigid core and the terminal aromatic moiety bring a remarkable effect on the mesomorphic behaviour. The shorter homologue 50a (n = 1) showed only the nematic phase, whereas the longer homologue 50b (n = 8) exhibited the B_1Rev phase in addition to the nematic phase [125]. B_1Rev phase is different from the conventional columnar phase (B₁) phase in terms of the electro-optical response, where the B_1Rev phase is usually responsive to an applied electric field [24]. When the phenyl ring in the compound 50b is replaced with a thiophene (Th) unit, compound 51 with the nematic phase is observed. Compounds end-capped with a reactive aromatic/appropriately substituted aromatic group can be used to prepare new types of metallic nanoparticles grafted with organometallic mesogenic ligands in the future [125,126,127,128].

The first study of BCLCs containing two azo units as direct linkages to connect the BU with sidearms was reported by in 2020 by Lu et al. [129]. Previous studies suggest that swapping the position of the azo linker within the sidearm with the ester group of BU does not bring meaningful impact on the mesophase. Moreover, bringing the azo unit close to the BU can even suppress the development of mesophases [69,130]. However, these studies only focused on the impact of changing one azo linkage with the ester linkage and did not examine the situation of connecting two azo units directly to the BU. Therefore, Lu et al. synthesized the homologues of compounds 52 (n = 5–12) by using the azo linkages to connect the central unit, a 4-chloro-1,3-dinitrobenzene, to the ester-containing sidearms terminated with alkyl groups. This study revealed that compounds 52 give the lower melting points and broader temperature range of the nematic phase compared to the majority of BCLCs that utilize azo and ester units as linkers within sidearms and direct linkers of BU, respectively. This phenomenon happened because the azo units presented at BU can greatly reduce the electrostatic interactions, which is a controlling factor of mesomorphic properties, especially for the nematic phase. Generally, the phase transition temperatures decreased as the alkyl chain length increased, except for the Sm-N transition [129].

2.1.6. Heterocyclic Core Unit

Azobenzene containing bent-core mesogens derived from a heterocyclic core unit is rare. Lutfor et al. reported BCLC monomers 53 with a 2,6-pyrimidinediol as the BU connecting with the azobenzene-based sidearms terminated with the polymerizable double bond. The shorter homologues 53 (n = 1 and 2) do not show mesomorphic behaviour, whereas the longer homologues with n = 3, 4, 5 and 6 display the enantiotropic B₆ phase. However, there is an additional Cr₂ phase above the Cr₁ phase for homologues with an even number of alkyl chains (n = 4 and 6) [131].

2.2. V-Shaped BCLCs

Almost all of the above-discussed BCLCs have a bent angle of approximately 120° because such angle appears to be a fundamental condition to design the bent-core mesogens. Previously, it was reported that those with bending angles considerably smaller or larger than 120° form conventional calamitic mesophases such as nematic or smectic phases [111,132]. Later, such compounds are also reported to exhibit banana phases. For example, previous studies reported that some of the V-shaped mesogens derived from the 1,7-naphthalene central core display hexagonal columnar (Col_h) and B₄ [133,134], B₂ and B₇ [135], cubic [136] and polar switchable SmA mesophases [132].

Hedge et al. reported the first example of a V-shaped compound (54) displaying a conversion from the isotropic phase to a polar biaxial smectic A phase (SmAP_A) directly without having any other mesophase (

Table 11. Phase transition of V-shaped compounds 54–56.

Compound	X	Phase Transition
54	H	Cr 112.7 SmAPA 116.6 Iso
55	Cl	Cr 104.2 Iso
56	F	Cr 102.5 B5 108.1 Iso

). This compound is the first V-shaped compound where the SmAP_A phase has been characterized thoroughly, owing to the reasonably low transition temperature. Besides, Hedge et al. also presented the first example of the V-shaped mesogen (56) showing the enantiotropic B₅ phase. For compound 56, the XRD result shows three reflections in the small-angle region with layer spacings of 32.98, 16.46 and 10.97 Å and a diffuse peak in the wide-angle regions. Plus, two extra peaks emerged in the wide-angle region, overlapped by the broad diffuse maxima representing an additional in-plane order which is a characteristic feature of a B₅ phase, having no correlation between the layers (Figure 15). The chloro substituted V-shaped compound (55) exhibits the monotropic mesophase that is unable to be characterized due to its metastability [111].

2.3. Hockey-Sick Molecules

A borderline category of molecular structure between bent-core mesogens and linear rod-like (calamitic) molecules, which is termed as “hockey-stick” molecules, have gained considerable attention in recent years, to understand how spatial correlations of shape-anisometric molecules leading to the novel phase structure formation [137,138,139]. Hockey-stick liquid crystals (HSLCs) are the asymmetric molecules where the lengths of two side arms of the bent mesogen are not equal [93]. There are two types of HSLCs, as they have been introduced in Section 2. All of the discussed HSLCs in this section belong to the second type of hockey-stick molecules. Although the majority of the reported HSLCs form non-switchable nematic and smectic phases [69,140,141,142,143], some of them still can preserve traits of BCLCs, such as the response to the electric field [144,145,146]. Incorporating hockey-stick molecules with functionalized azobenzene unit is of great interest due to the photosensitivity of azobenzene.

Khan et al. reported BCLCs with four aromatic rings linked by imine, ester and azo units [147]. Inserting the lateral -OH group into the molecular structure along with the terminal polar end group (–F, –Cl and –NO₂) in one of the sidearms generated compounds 57 to 59. The fluoro-substituted compound 57 exhibits the enantiotropic nematic phase, whereas the chloro- (58a) and nitro-substituted (59a) compounds show the smectic phase as well as the nematic phase. The clearing temperature and mesomorphic range of these compounds are varied, relying on the electron-withdrawing ability of the polar terminus [147]. The homologue of chloro-substituted compounds (58b) is found to have the cybotactic nematic phase (N_Cyb) with no ferroelectric property, whereas the nitro-substituted compound 59b has a wide range of SmA phase, but no nematic phase is identified in the phase sequence [148]. According to the study of Khan et al., it is clear that the polar end terminus affects the elastic properties of BCLC, which is the crucial parameter to determine the threshold voltage and sensitivity of electro-optical devices. The compound 57 has the splay elastic constant (K₁₁) more significant than the bend elastic constant (K₃₃) as observed in most bent-core nematics, while the other two compounds have K₃₃ > K₁₁ which is similar to rod-like LCs [147]. The negative elastic anisotropy (K₃₃–K₁₁) of the compound 57a was later turned to positive by doping with the ferroelectric nanoparticle, BaTiO₃ [149].

Changing the polar moiety to non-polar methyl moiety in compound 58b leads to compound 60 that shows N_Cyb phases. Similar to compound 58b, the N_Cyb phase of this compound does not show the ferroelectricity [148]. This result is against the previous studies where the N_Cybc clusters show a ferroelectric-like response in the presence of an external stimulus [150,151].

When both terminal ends attached to the alkoxy chain, the compound 61 with SmA and N_cybC phases is formed. Singh et al. had studied the mechanism of phase transition at the molecular level via the combination of temperature-dependent Fourier transform infrared spectroscopy (FTIR) and density-functional theory (DFT) approach. The results obtained from the spectral analysis of certain functional groups’ vibrational bands (OH, CH₂/CH₃, C=O and C=N) reveal the occurrence of Cr to the SmA phase transition at 125 °C [152].

Lateral substituting the –Cl group on the azobenzene in compounds 58 leads to two series of compounds (62 and 63). All the compounds demonstrate the nematic phase, except for compound 63b that exhibits a short-range monotropic SmA phase in addition to the N phase (

Table 12. Phase transition of compounds 62 and 63.

Compound	n	X	Y	Phase Transition
62a	4	Cl	H	Iso 147.9 N 79.7 Cr
62b	12	Cl	H	Iso 110.6 N 86.0 Cr
63a	4	H	Cl	Iso 140.3 N 97.0 Cr
63b	12	H	Cl	Iso 113.6 N 95.4 SmA 83.4 Cr

). Compounds 63 are found to have supergelation ability which can be used for versatile applications such as photonics, electronic and optoelectronic devices [153].

In the same year, two series of four-ring BCLCs with methylated azobenzene wings have been prepared by Begum et al. These two series differ from each other in the relative position of the methyl substituent in respect of azo unit in the molecule. Both of them demonstrate the nematic phase during the cooling process from the isotopic state as evident by POM (Figure 16a,b). XRD profiles for both compounds 64 and 65 display the small/moderately sharp peak in the small-angle region and wide-angle Cl with d = 4.71 Å and 4.92 Å, respectively (Figure 16c,d). Although the molecular length of these two compounds was almost identical, the d-values given by each compound were different. The difference in the d-spacing values between them may occur due to the positional variation in the methyl unit neighbouring to the azo linkage [154].

Figure 16. Textures of the nematic phase observed under POM for (a) compound 64 at T = 104.1 °C and (b) compound 65 at T = 92.8 °C; XRD profiles of (c) compound 64 and (d) compound 65, along with their insets indicate 2D images at the specific temperature. (Reproduced with permission from ref. [154], copyright American Chemical Society 2020, https://doi.org/10.1021/acs.jpcc.9b09326).

Figure 16. Textures of the nematic phase observed under POM for (a) compound 64 at T = 104.1 °C and (b) compound 65 at T = 92.8 °C; XRD profiles of (c) compound 64 and (d) compound 65, along with their insets indicate 2D images at the specific temperature. (Reproduced with permission from ref. [154], copyright American Chemical Society 2020, https://doi.org/10.1021/acs.jpcc.9b09326).

Replacing the terminal –Cl group in compound 64 and 65 with the highly electronegative fluoro group is results in the formation of compounds 66 and 67, respectively. Compounds 66 and 67 are bearing the shorter alkoxy chain (OC₅H₁₁) on the shorter arm, instead of the longer OC₁₂H₂₅ chain. They exhibit monotropic nematic phases and their mesophases do not vary much from their analogues bearing with the terminal chloro group (64 and 65). Compound 68 is prepared by substituting the Y = CH₃ with –Cl group in compound 67. The nematic phase is also observed in this compound. Compared to the compound having the substitution on the meta-position, the compound with the substituent located at ortho-position to the ester linking group has a higher isotropic phase temperature [155].

Paul et al. had been designed hockey stick molecule 69 comprising the short arm attached with a reactive double bond, while the azobenzene based lengthening arm consists of methoxy terminus. When the lateral methyl group is substituted at the inner ring of the azobenzene sidearm in compound 69, compounds 70 and 71 are formed. All of them exhibit a similar phase sequence which is Cr → N → Iso. Based on the DFT calculations, variation in the position of the –CH₃ substituent does not alter the electronic and spectroscopic properties in a great scale although there is a slight shift of a strong peak in UV-vis absorption spectra. The melting and clearing temperature decrease distinctly when the lateral methyl group presented on the lengthening arm of the mesogen [156].

In 2017, Alaasar et al. reported six rings HSLCs derived from 4-cyanoresorcinol with azobenzene side arm opposite to the direction of the –CN group. Depending on the alkoxy chain length at the shorter (m = 6 and 12) and longer sidearm (n = 2, 6 and 20), various types of enantiotropic mesophases with phase sequences are produced. Compound 72b with very short ethoxy chain shows the SmA phase and N_CybA phase, while compounds 72a and 72c additionally form synclinic SmC phase (SmC_s) and antichlinic SmC phase (SmC_a). The transition between SmC_s and SmC_a phase is a typical characteristic of hockey-stick mesogens [66,141,143,157,158,159], as shown in Figure 17. This transition is accompanied by nucleation of domain walls with opposite tilt of the director in the SmC_s phase and disappearance of the Meyer domains at the transition to the anticlinic SmCa phase. When the alkoxy chain length attached at the shorter arm increases, the temperature range of the SmC_s phase is broadened (72a → 72c). On the other hand, the chain elongation occurs at a longer side arm causes the emergence of SmC_s phase (72b → 72c). It can be concluded that the chain elongation occurs at both shorter and longer sidearms favour the synclinic interlayer correlation, and therefore, the development of SmC_s phase is preferential. This result is in line with the previous report [158]. However, further increasing the alkoxy chain length at the elongated sidearm (72d) causes the nematic and SmC_a phases to be removed entirely [93].

Figure 17. POM images of a freely suspended film of compound 72a (a) in the SmA-SmC_s transition at T = 170 °C, showing the Meyer domains growing from the layer steps and (b) in the SmC_a phase at T = 147 °C. (Reproduced with permission from ref. [93], copyright The Royal Society of Chemistry 2017, https://doi.org/10.1039/C7TC01816A).

Figure 17. POM images of a freely suspended film of compound 72a (a) in the SmA-SmC_s transition at T = 170 °C, showing the Meyer domains growing from the layer steps and (b) in the SmC_a phase at T = 147 °C. (Reproduced with permission from ref. [93], copyright The Royal Society of Chemistry 2017, https://doi.org/10.1039/C7TC01816A).

Changing the orientation of the CN group at the resorcinol core in compound 72c is results in the formation of isomeric compound 73 with the reduced melting and clearing temperatures. Besides, the SmA and SmC phases are eliminated in this compound when the position of the CN group is altered. The nematic phase is the only mesophase observed for this compound [93].

2.4. Heterocyclic Aromatic Core

The hockey-stick molecules with 6-oxoverdazyl radical as the BU and the end of the short arm are terminated with either –CH₃, CF₃ and CN groups were reported in 2017 by Szymon et al. The results revealed that only the CN derivative 74 showed the mesogenic behaviour, that is the nematic phase [160].

3. Photoisomerization

Incorporation of photoswitches, molecules that undergo structural changes in response to light illumination, in the liquid crystal network has emerged a promising and exciting class of materials for achieving optical to mechanical energy conversion [62,161,162,163,164]. The azobenzene represents a classic example of photoswitchable material that finds tremendous applications in different fields, especially in the photonics, as it is mentioned in the introduction section. Therefore, most of the photoswitchable LC materials incorporate the azobenzene in their molecular structure substantially responsible for exhibiting photoresponsive behaviour. The azobenzene undergoes a reversible trans-cis photoisomerization under the appropriate UV-light illumination, as shown in Figure 18. This phenomenon will later bring about a geometrical change at the molecular level that disrupt liquid crystallinity and effectively promote disorder, causing a significant change in the physical property of azobenzene based materials [62]. As mentioned earlier, the cis-trans isomerization can occur in two ways, either by removing the source of irradiation or keeping the excited trans isomers in the dark condition, which is known as thermal back relaxation. The relative thermal stability of the two isomers can differ significantly among the different system. The cis/Z isomer of some compounds can alter quickly into their trans/E isomers, while the cis/Z isomer can have appreciable thermal stability in other systems. The longer thermal back relaxation (higher thermal stability) is very crucial when considering applications in technologies such as optical data and real-time information transfer [165]. Photoisomerization (trans → cis) and thermal isomerization (cis → trans) of the azobenzene are well-separated in time because the former process takes place considerably faster than the latter one. The photoisomerization behaviour of certain compounds that are mentioned in the previous sections will be discussed in this section.

Many research groups have studied the photoisomerization behaviour of azobenzene-containing BCLCs either in solution, solid, LC or mixture state under the UV-visible irradiation. The photoisomerization and thermal back relaxation of BCLCs are performed by measuring the UV-vis absorption spectra. For example, BCLC 71 is dissolved in the chloroform solution, and the changes in UV-vis absorption spectra were measured at three different circumstances: freshly prepared sample, the sample was irradiated with the light source with λ = 366 nm and the sample was kept in the dark. Upon the UV exposure, the absorption peak at λ ~ 346 nm, which is corresponding to π-π* transition of energetically more stable trans-isomers decreases. On the other hand, the intensity of the absorption peak at λ ~ 438 nm corresponding to n-π* transition of cis-isomers gradually increases (Figure 19). This phenomenon indicates the conversion of stable trans isomers into the thermodynamically unstable cis isomers at the photo equilibrium state during the photoisomerization process. The unstable cis isomers are isomerized back to the stable trans isomers when it is placed in dark condition, which defines the thermal back relaxation (Figure 19) [57,156]. The other simple method to detect the photoisomerization process is observing the number of spots coming from the thermally equilibrated sample or irradiation with UV light on the thin-layer chromatography (TLC) [14].

Although the photoswitching of azobenzene mesogens in solution is essential, the photoswitching effect in the solid-state is vital for practical applications, especially for the field in rewritable printing and imaging solid-state bistable compounds [166,167]. The photochromism of compound 63b in the solid-state was performed with UV-vis spectroscopic studies through the reflectance mode in KBr medium (Figure 20c). A prominent shift in the spectral behaviour has been observed upon illumination of light (λ = 365 nm), revealing the solid-state isomerization. Upon the light illumination, the π-π* band in the thermally equilibrated sample is shifted from 351 nm to the lower wavelength of λ = 320 nm and the intensity of n-π* band increases. The compound 63b was also dissolved in chloroform to reinforce the photochromism in the solid-state. The sample was passed under UV light for ~4 h followed by evaporating and drying of the solvent under vacuum. The colour of the compound was changed from a yellowish-orange to red (Figure 20d). The photoswitching effect in the solid-state can be further confirmed by performing XRD studies. Without the light irradiation, the XRD spectra in the thermally equilibrated sample showed many narrow peaks in small- and wide-angle regions (Figure 20a). Upon the illumination of light, the spectra still exhibited many narrow peaks in small- and wide-angle regions, but these peaks were found to be broader, as shown in Figure 20b [153].

When the azobenzene moiety is applied in the LC environment, it plays a vital function in modifying the phase structure. It has been witnessed that the compound 20d showed the transformation from the SmC_AP_A phase to the isotropic upon the illumination with UV light. Without the light illumination, a sharp maximum in a region of small scattering angles is detected, indicating the long-range of the lamellar structure in the smectic phase. When the compound 20d is exposed to UV light, the peak intensity is reduced by several orders of magnitude, which revealed the melting of the smectic structure into the liquid (Figure 21) [110]. For compound 63b, the photoswitchable ability in the LC state was performed in situ-observed by POM. It was fixed in the middle of a glass plate and a coverslip before heating it to the isotropic phase. During the experiment, the red filter was fixed on the POM to eliminate the UV component in the visible light released from the tungsten-halogen incandescent lamp of the POM. The Schlieren texture feature of the N phase (Figure 22a) emerged upon the cooling process from the isotropic state. This feature vanished immediately under the UV light illumination and leaving the dark image (Figure 22b) that represents the order-nematic phase is tuned to the disorder-isotropic phase. When the UV light is removed, the Schlieren texture reappeared slowly (Figure 22c). The same condition is observed when the compound 63b is further cooling from the N phase to the SmA phase (Figure 22d–f) [153].

The impact of light on the phase transition temperature of BCLCs based on azobenzene has been studied. When the compound 41 is exposed to UV light at λ = 365 nm in the nematic phase region (T_sm < T_N < T_iso), the transition temperature of isotropic, T_iso, decreases gradually owing to the growing number of cis-form isomers. When the concentration of cis-form isomers reaches the threshold level, T_iso (125 °C) becomes lower than the transition temperature of nematics, T_N (128 °C) (Figure 23a), causing a nematic-to-isotropic isothermal phase transition in the irradiated area (Figure 23e). Similarly, the nematic phase can be recovered faster upon cooling from the isotropic phase when the sample is illuminated with visible light (λ = 457 nm), as shown in Figure 23c. The application of visible light on the sample in the isotropic phase facilitates the oriented liquid crystalline state. Thus, it induces an isotropic-to-nematic phase transition in the illuminated region (Figure 23g) [119].

Besides, the photoisomerization brings effect to the electric properties upon light exposure. In the case of compound 65, the threshold value (U_th) for Frѐedericksz transition (FT) to occur decreases as the intensity of UV light (I₃₆₅) increases (Figure 24a). From a practical view, a high U_th restrict the application of BCNs, and therefore, decreasing U_th is highly desirable. Previously, Frѐedericksz transition, which is the most fundamental mechanism for applying nematics as display and information processing components [168], are investigated by the electric stimuli only. In the study of Begun et al., the FT effect of BCN material (compound 65) is explored additionally by the light stimuli. The lower U_th value is attributed by the photoisomerization of compound 65, where the compound 65 changes the shape from trans isomer (unsymmetrical V-shape) to distorted Z-shaped cis isomer (Figure 24b). When the UV light is removed, it changes back to the original structure that causes the recovery of U_th value [154]. Similarly, the behaviour of flexoelectric domains (FDs) in the bent-core nematic of compound 41 can also be modulated by different wavelengths and intensity of light. The electric threshold and periodicity of FDs can be adjusted by changing the ratio of trans-cis isomers upon the illumination of light. So far, most of the research studied on FDs highlight the electrical characteristics of FDs (threshold voltage and voltage-dependent periodicity) [169,170,171], and the possibility of tuning the properties of FDs by light irradiation has not yet been reported. Therefore, the novel phenomenon of contactless light-control tunability found in compound 41 serves as a guideline to improve the performance of certain traditional electric-field-induced photonic devices by additional light fields [119].

The first case of the photoinduced alteration of the switching mode under the electric field was found in the compound 18a due to the existence of the azobenzene unit in its molecular structure. The compound 18a was placed in a 6 µm ITO cell on a temperature-controlled heating stage and irradiated with light (λ = 405 nm) at T = 101 °C, which is close to the SmC_sP_AR-SmC_sP_R^[*^] transition temperature, for 2 s. As shown in Figure 25f–g, the AF-like double peaks switching is changed into the FE-like single peak switching upon the light illumination. When the light source is off, the FE-like single peak switching is relaxed back to the AF-like mode. Similarly, the reversible photoinduced on-off-switching of chirality was demonstrated for the first time by the compound 16b (n = 16). Upon the light irradiation with λ = 405 nm, the chiral SmC_s^[*^] phase of the compound 16b switched to the achiral SmA phase (Figure 25a–c). The chirality was restored after switching off the light, but the positions of the chiral domains change (Figure 25c–e). The occurrence of the photoinduced on-off -switching of chirality (16b) and the photoinduced change of switching mode between the ferroelectric-like and antiferroelectric-like (18a) is attributed to the trans-cis photoisomerization. During the trans-cis photoisomerization, compounds 16b and 18a change from a well-defined bent-shape to a more non-specific one. This process causes the reduction in packing density and coherence length of polar order below a critical value that eventually changes the mode of polar switching and photoinduced on-off switching of chirality in compounds 18a and 16b, respectively [91].

4. Potential Applications and Challenges

As the discovery of cis-trans photoisomerization, azobenzene functionalized BCLCs have been exploited extensively for potential applications in enantioselective syntheses, photonics and nonlinear optics (NLO). However, numerous scientific issues concerning these photoresponsive BCLCs still need to be examined. One of the crucial aspects that still represent a difficulty in the use of these photosensitive BCLCs concerns bulk phase alignment. It is challenging to obtain a proper alignment in the case of BCLCs since the procedures that generally used for the calamitic LCs are not applicable in the BCLC system [27,57]. It is vital to obtain a breakthrough in this aspect because the proper sample alignment is a target not only for applications but also for basic research purpose. In developing new applications based on the photosensitive BCLCs with a wide range of chiral or polar stabilized mesophases at room temperature, the mesomorphic behaviour of bent-core mesogens should be maintained at room temperature or as close to room temperature as possible. Third, it is difficult to predict an ideal criterion to design the biaxial nematic phase (N_b), a potential candidate for the application in the next generation of ultrafast and wide-angle display devices due to the fast switching response, compared to the uniaxial nematic phase. The nature of the reported N_b in azobenzene containing BCLCs still open the question to be answered. Fouth, although there is a massive amount of reported BCLCs, the requirement for more BCLCs with enhanced NLO properties is still high because they might yield a broad range of applications [57]. Despite the promising effects and possible applications are given by the reported azobenzene based BCLCs, most of them are only used for basic and theoretical research as the problems mentioned earlier limit the practical applications. Therefore, further explorations on the possible practical applications of BCLCs should be intensified in the future.

5. Conclusions

A review of the recently designed photoresponsive azobenzene based BCLCs exhibiting numerous types of mesophases including bent-core nematics with cybotactic clusters (N_cybC and N_cybA), polar and non-polar Sm phases and chiral sponge and HNC phases (DC₁* and DC₂*) as well as the conventional nematic and smectic phases has been provided. It was concluded that the mesogenic properties of BCLCs could be altered by minor structural change, for instance, type of BU unit, the overall length of the molecular structure, types and orientation of linking groups and the lateral substitution on BU or sidearms in addition to types and length of terminal chains. The substitution of the BU unit, especially in position 4, has been proved to be beneficial in producing the new mesophases, such as the SmC_aP_AR phase generated from the 4-cyanoresorcinol. Although BCLCs are presently at the interesting research stage, the field looks sufficiently mature that more ambitious projects can be proposed for these materials owing to their remarkable and unique properties. They can certainly be put forward as novel tools to be used in developing new, smart and often with multifunctional for electronic, magnetic and photonic devices. Therefore, further investigations should be undertaken to achieve practical applications based on BCLCs in the future. Hopefully, the different examples of discussed BCLCs in this paper can provide insights to researchers to conduct their own studies.