Photo-induced guest-host interactions produce grain boundaries between smectic blocks

Photo-induced isomerization of an azobenzene unit not only induces phase transition between two different phases but also brings marked changes of physical properties in solids. We prepared a symmetric trimer possessing an azobenzene unit, 4, 4’-bis{9-[4-(5-octylpyrimidin-2-yl)phenyloxy]nonyloxy}azobenzene ( I ), and investigated the phase transition behaviour of trimer I and that of its binary mixture with a host liquid crystal. Trimer I exhibited three crystalline phases on cooling. Photo-induced isothermal crystal– liquid transition occurred only in the highest temperature crystalline phase through the trans - cis isomerization of the azobenzene unit. A binary mixture of trimer I (20 wt%) and a host liquid crystal (80 wt%) was found to exhibit grain boundaries consisting of nematic liquid crystals between smectic C blocks during a cooling process under UV irradiation. We explain the phase separation in terms of guest-host interactions between the photoinduced cis -trimer and its surrounding host liquid-crystalline molecules. investigated dynamic behaviour of liquid-crystalline molecules in smecic phases using solid-state C-13 between adjacent layers. pictures make us possible to design various kinds of liquid-crystalline oligomers stabilizing a which two phenylpyrimidine units connected with optically active 3-methyladipic induces TGB phases in mixture with smectic If we design a photo-responsible molecule which intercalates to the adjacent


Introduction
Phase-separation phenomena in soft matter have attracted considerable attention because of its importance in the understanding of the molecular organization. [1][2][3][4] Phase separation observed for a liquid-crystalline mixture is coexistence of different phases which appear in the temperature sequence of the mixture. On the other hand, some chiral frustrated liquid-crystalline phases, i.e., blue phases and twist grain boundary (TGB) phases, have defects in which the molecular orientation is thought to be isotropic. Anisotropy and isotropy coexist in these frustrated phases. TGB phases were predicted by de Gennes 5 and later by Renn and Lubensky 6 to be a result of the competition between twist deformation and the desire for molecules to form a layer structure. The first experimental observation of a TGB phase was reported by Goodby et al. 7,8 It was shown to exhibit blocks of smectic A molecular order mediated by screw dislocations. Such competition is thought to produce a new type of phase separation in which a coexistence of smectic blocks and liquid-like regions. Recently, a twist-bend nematic phase (N TB ) showing domains of opposite handedness was observed for members of achiral -bis-4-(4'cyanobiphenyl)alkanes with flexible odd-numbered methylene spacers. 9 The molecules form a helix. The director is tilted with respect to the helical axis in the N TB phase. 10,11 Dozov proposed that inducing some twist can produce a spontaneous bend. Moreover, he predicted that the N TB phase can exist for bent molecules. 12 The N TB phase has been observed for liquid crystal dimers with an odd-numbered methylene spacer, 13 bent-core liquid crystals 14 and oligomers. 15,16 Furthermore, a heliconical tilted smectic C (SmC TB ) phase has been discovered for achiral asymmetric dimers with an odd number of atoms in the spacer. 17 However, to the best of our knowledge, a bend grain boundary phase has never been found so far. 18,19 If a bend deformation is induced in a smectic phase under an external field, a field-induced smectic phase accompanying grain boundaries can be formed.
We investigated dynamic behaviour of liquid-crystalline molecules in smecic phases using solid-state C-13 NMR. 20 The NMR studies revealed the following pictures: (1) cooperative motion for the core parts contributes to the orientational order of the molecules in each layer and (2) interlayer permeation of tails causes correlation between in adjacent layers. The pictures make us possible to design various kinds of liquidcrystalline oligomers stabilizing frustrated phases. 21,22 For example, we reported that a chiral dimer in which two phenylpyrimidine units are connected with optically active 3methyladipic acid induces TGB phases in the mixture with a smectic liquid crystal. 23 If we design a photo-responsible guest molecule which intercalates to the adjacent layers composed of host smectic molecules, the photo-induced conformation change of the guest molecule can cause a layer deformation due to the guest-host interactions (Fig. 1). In this report, we designed a symmetric trimer possessing an azobenzene unit and two phenylpyrimidine cores, and investigated the phase transition behaviour of the trimer and that of the mixture with a host liquid crystal. Photo-induced isomerization of an azobenzene unit is well known to cause a remarkable change in the physical properties of liquid crystals. [24][25][26][27][28] Recently, the photo-induced crystal-melt and crystal-crystal transitions have attracted much attention. [29][30][31] A photo-induced crystal-liquid transition was observed in many azobenzene-containing systems, which in many cases, means photochemical reactions. 32 However, that in the present study means a photo-induced isothermal phase transition. Photo-induced cis-azobenzene derivatives form a bent molecular shape with the twisted azobenzene unit. Phenylpyrimidyne is well known a rigid core to stabilize a smectic phase via core-core interactions. Guest-host interactions between the photo-induced bent-shaped trimer and the surrounding host molecules is expected to occur. We report here that: 1) the newly designed trimer exhibits a photoinduced isothermal crystal-liquid transition and 2) the mixture with a host liquid crystal exhibits a smectic C (SmC) phase accompanying grain boundaries under UV irradiation.
h Fig. 1 Design concept of the trimer for producing a bend deformation in a smectic phase.

Physical properties
The initial phase assignments and corresponding transition temperatures for the final compound was determined by thermal optical microscopy using a polarizing microscope we observed the electronic spectrum change at 360 nm at room temperature.

Trimer I
We prepared trimer I in which an azobenzene unit and two phenylpyrimidine units are connected with flexible odd-numbered methylene spacers. Although it has three mesogenic units within a molecule, it showed no liquid-crystalline properties. Fig. 2 shows the molecular structure of trimer I with the phase transition temperatures. Trimer I exhibited three crystalline phases, i.e., Cry1, Cry2 and Cry3, on cooling. The sample on a glass plate with a cover glass was exposed to UV irradiation in Cry1. Fig. 3 shows the optical textures of trimer I before and after 365 nm UV irradiation at 10 mW cm -2 in Cry1.
The crystal changed to the isotropic liquid in about 30 s with the UV irradiation at T-T Iso-Cry1 = -0.8 K ( Fig. 3(a) and (b)). This transformation is not attributed to photothermal heating as described in Experimental. When the light was switched off, Cry1 phase reappeared in 1s. However, the reappeared Cry1 consisted of smaller domains than the original Cry1 ( Fig. 3(c)). When the sample was irradiated with the UV light at T-T Iso-Cry1 = -1.5 K, the half of the sample changed to the isotropic liquid, whereas the other changed to an intermediate state ( Fig. 4(a) and (b)). When the UV light was turned off, the isotropic region changed to Cry1 consisting of the smaller domains, on the other hand, the intermediate state returned to the original Cry1 (Fig. 4(c)). To confirm reversibility, when the sample (Fig. 4(c)) was irradiated again, it changed to the same texture as shown in  As the irradiation intensity increased, an absorbance at 360 nm associated with the π-π* transition of the trans form decreased whereas an absorbance at 450 nm associated with the weak symmetry forbidden n-π* transition of the cis form increased, revealing that the trans-cis photoisomerization of the azobenzene unit occurs. In order to evaluate stability of the cis isomer, we measured the thermal cis-trans isomerization rate of trimer 1 in a chloroform solution at room temperature. The first order rate constant was found to be 8 x 10 -4 s -1 . On the other hand, the liquid to Cry1 transition of trimer I occurred immediately after the UV light was turned off. There is no marked temperature dependence of the transition rate. We used a polarized optical microscopy equipped with a halogen lamp to observe the transition phenomena. Not only the high temperature but also the visible light might accelerate the cis-trans isomerization.
The crystal to isotropic liquid transition of trimer I is explained in terms of the trans-cis photoisomerization of the azobenzene unit. The isotropic to crystal transition occurred through the cis-trans isomerization to produce the crystal consisting of the smaller domains. According to Barrett et al., 31 photochemical effect of the cis to trans isomerization of azobenzene involves a single crystal to polycrystal transformation due to the large structural change. As decreasing temperature in Cry1, the cis over trans ratio is thought to decrease, which might decrease the photo-induced transition area. As shown in Fig. 4, the photo-induced Cry1-Iso transition occurs via the intermediate state where the cis over trans ratio is too small to transform the isotropic liquid. The photo-induced transition between solid and liquid, and that in solid phases are of potential interest for applications. [33][34][35] Modification of the molecular structure is now in progress for obtaining materials exhibiting the photo-induced Cry-Iso phase transition with a wide temperature range.

Binary system
Then, we designed a binary system of trimer I and a host liquid crystal. We used compound II as a host liquid crystal. Compound II exhibiting N and SmC phases with wide temperatures is known to be a suitable component for preparing ferroelectric liquid crystal mixtures. 36 The molecular structure of compound II with the phase transition temperatures are shown in Fig. 5. We prepared some binary mixtures of trimer I and compound II with different composition ratios. We demonstrate transition properties of a mixture containing of 20 wt% of trimer I because it showed evident photo-induced effects. Fig. 6 shows a cooling DSC thermogram of the mixture. On cooling, the isotropic  Scale bars correspond to 500 µm.
The mixture exhibiting a coexistence of the N and X phases was irradiated with 365 nm UV light at a power of 20 mW cm -2 at 78.9 °C (Fig. 8(a)). The X phase changed to a new texture in 60 s ( Fig. 8(b)). When the UV light was turned off, the texture was spread gradually but the N phase remained (Fig. 8(c)). It took about five minutes to reach an equilibrium state after the UV light was turned off. The photo-induced texture change is irreversible. On further cooling, the remained N phase changed to the SmC phase at 70.8 °C. Also see Fig. S4. The photo-induced phase and the SmC phase were crystallized at 41.9 °C. When the sample exhibiting a coexistence of the SmC and Y phases were irradiated with the UV light, no marked change was observed for their textures. The photochemical effects were also observed for the mixture on a glass plate without surface treatment. small bright spots ( Fig. 10(a)). When the sample was rotated between crossed polarizers, the dark texture did not change. Therefore, the molecules are thought to be reoriented to align almost perpendicular to the glass substrate. Spherical nuclei were formed in the N phase at 65.8 °C and they were spread radially. Based on the observation of the sample confined in a homogeneously aligned cell, it might be the SmC phase in which the molecules are aligned homeotropically. Collision regions between different nuclei became blight gradually to form grain boundaries ( Fig. 10(b)). Also see Fig. S5. Grain boundaries are usually observed for polycrystalline materials. The domain brightness did not change neither by rotation of the sample between the crossed polarizers nor by that of an analyzer as shown in Fig. 11, which indicates that the molecules orient quasi isotropically without forming a helical structure. Interestingly, schlieren texture can be seen in the grain boundaries as shown in Fig. 10 (c), revealing that the nematic phase exists there. The texture as shown in Fig. 10(b) did not change when the UV light was turned off. The SmC phase accompanying grain boundaries consisting of nematic liquid crystals changed to the crystalline phase at 37.4 °C (Fig. 10(d)). We investigated how the external fields, i.e., UV irradiation and temperature, affect the formation of the grain boundaries. After the spherical nuclei began to form in the N phase, we observed the optical textures with the UV irradiation at 61.9 °C. They were spread without cooling as described above. However, when the UV light was turned off at 61.9 °C, the formed nuclei were not spread but many small nuclei appeared simultaneously to fill the space (Fig. S6). When the sample was cooled from the isotropic liquid with 365 nm UV irradiation at a power of 10 mW cm -2 , the N phase appeared at 154.8 °C. The SmC phase appeared at 69.1 °C. It is true that boundaries exist, but the boundary is not so distinct (Fig.12).

Models
Let us discuss the photo-induced phase transitions of the binary system. According to semi-empirical calculations using MOPAC-6/PM3, we estimate the molecular shapes of trans-and cis-isomers of trimer I. Fig. 13 shows their MOPAC models with molecular length (l) and width (d). Based on the models, the l/d ratios for trans-and cis-isomers are 10.3 and 3.6, respectively. It should be noticed that the cis-form of azobenzene is twisted.
The photo-induced transition between nematic and isotropic liquid, and that in nematic phases for the binary system are explained in terms of the l/d ratios as follows. The photoinduced isomerization from trans-trimer I to cis-trimer I decreases the l/d ratio, which decreases the order parameter in the N phase of the binary system. The appearance of the X phase without UV irradiation is attributed to the trans-isomer. The photo-induced trans-cis isomerization of trimer I is thought to cause irreversible influence on interactions between trimer I and compound II, producing the phase separation of the N phase and the photo-induced phase. It seems to be realistic that the photo-irradiation causes guest-host interactions such as "Command Surface" 28,29 or "Sergeants and Soldiers effects". 37,38 Fig. 14 shows a schematic model for the photo-induced transition.  Then we discuss how the grain boundaries occur. According to the textures in Fig. 9, the lower temperature phase below the N phase is a synclinic SmC phase. However, no phase separation occurs for the mixture confined in a homogeneously aligned cell. Without surface treatment, the alignment of host liquid-crystalline molecules is influenced by the shape of a large guest molecule though the guest-host interactions. In the N phase of the mixture with UV irradiation, cis-trimer I is thought to have a twist-bent shape, which can induce a bend deformation in the N phase through the guest-host interactions. Generally, a bend deformation in a smectic phase is forbidden because it prohibits the formation of azobenzene unit. The photo-induced cis-isomer forms a bent molecular shape with the twisted azobenzene unit. Therefore, the twist-bent form of the cis-azobenzene unit is synchronized via cooperative motion for the core parts, which can preserve the bend deformation even in the SmC phase. The quasi isotropic texture without helicity as shown in Fig. 9 indicates that bend and/or partially twist deformations without producing a helical structure exist in the SmC phase. On the other hand, liquid-crystalline molecules prefer to fill the space uniformly. Thus, the competition between the bend deformation and the desire for molecules to form a layer structure leads grain boundaries between smectic blocks as predicted by de Gennes. 5 Some host liquid-crystalline molecules are recognized as impurities in the SmC phase and they are driven from the smectic blocks to the grain boundaries. They form a nematic organization in the grain boundaries. With the surface treatment, the alignment layer affects the orientation of the host liquidcrystalline molecules more strongly than trimer I. Therefore, the bend deformation is not induced in the homogeneously aligned SmC phase. Fig. 15 shows a schematic model for the formation of the grain boundaries. Although we do not have an experimental proof that the bend deformation exists in the SmC phase at present, we can say that the photoinduced bent-shaped trimer produces the SmC phase accompanying grain boundaries without chirality. and compound II (80 wt%) during a cooling process with UV irradiation.

Conclusions
The newly designed symmetric trimer possessing an azobenzene unit exhibits the photoinduced isothermal crystal to liquid transition through the trans-cis isomerization of azobenzene. The binary mixture with a host liquid crystal without surface treatment produces a SmC phase with grain boundaries in which nemtaic liquid-crystalline molecules exist during a cooling process under UV irradiation. The photo-induced cistrimer affects the orientation of the host molecules in the SmC phase to produce deformations. As a consequence, grain boundaries consisting of nematic liquid crystals appear between the SmC blocks.

Conflicts of interest
There is no conflicts to declare.