Synthesis of Thiophene-Based Derivatives and the Effects of Their Molecular Structure on the Mesomorphic Behavior and Temperature Range of Liquid-Crystalline Blue Phases

: The development of blue-phase liquid crystal (BPLC) materials with a wide temperature range is of great signiﬁcance for practical applications in the optoelectronic ﬁeld. In the study, bent-core derivatives with a 3-hexyl-2,5-disubstituted thiophene central ring in the λ -shaped molecular structure were designed and synthesized. Their mesomorphic behavior and effect on the blue-phase (BP) temperature range were investigated. Interestingly, a BP was achieved both during the heating and cooling processes by doping with a proper concentration of chiral compound into the thiophene bent-shaped molecule with high rigidity, while derivatives with ﬂuorine atom substitution only exhibited cholesteric phase no matter how many chiral compounds were added. This result proved that BP is highly sensitive to the molecular structures of bent-shaped molecules. Moreover, the BP temperature range was broadened when adding these molecules into a BPLC host, which thus improved the BP temperature range from the initial value, no more than 4 ◦ C, to as much as 24 ◦ C. The experimental phenomena were reasonably explained through molecular simulation calculations. The study may provide some experimental basis and theoretical guidance for the design of novel bent-shaped molecules and BPLC material with a wide temperature range.


Introduction
Liquid-crystalline blue phases (BPs) have recently drawn vast and increasing attention due to their exotic fluid self-assembled three-dimensional periodic supernanostructures and the characteristic of selective reflection of circularly polarized light in the visible-light range [1]. BPs generally appear in the narrow temperature range close to the isotropic phase (Iso) in a liquid crystal (LC) material system with a high chiral dopant concentration. Unlike the one-dimensional helical twist structure of chiral nematic phase (Ch), the BPs' molecules twist along two different orientation axes, forming a double-twisted cylinder (DTC) arrangement spontaneously. As the double-twisted structure is unable to continuously occupy a three-dimensional space, BPs must coexist with defects. Since the generation of such a defect involves some energy, the entire system becomes unstable. This is the main reason for the poor thermodynamic stability performance of BPs. Depending on the difference in the three-dimensional long-range-ordered assembly structure of the DTCs, BPs are classified into three categories: BP I, BP II, and BP III. These three subphases generally appear in sequence with the increase in temperature or chiral content [2]. It has been revealed that BP II has a simple cubic unit cell of lattice defects and BP I is body-centered cubic, both with lattice parameters in the order of several hundred nanometers and resulting in light reflection in the visible spectrum [3,4]. BP III is amorphous and its symmetry is the The 1 H NMR spectrum of C6O-6Th, C5PP-6Th, C5PPF(2)-6Th and C5PPF(3)-6Th can found in supplementary materials ( Figures S1-S4). The temperatures and enthalpies the transition were investigated using differential scanning calorimetry (DSC) (Perk nElmer, DSC8000, Waltham, MA, USA) at a scanning rate of 10.0 °C min −1 under a d nitrogen purge. The initial phase assignments and corresponding transition temperatur for the final compounds were determined by using thermal optical microscopy with polarizing microscope (POM) (Carl Zeiss, AxioVision SE64, Oberkohen, German equipped with a hot stage calibrated to an accuracy of ±0.1 °C (Linkam LTS420, Salford United Kingdom). The LC cells used in the study were made with indium-tin-oxide (ITO coated glass substrates with no inner surface treatment or parallel treatment. The thic ness were controlled to be approximately 20 µm by poly(ethylene terephthalate) film. T BP textures were identified under the reflection (R)-mode POM. The reflection spect were measured by using a reflection-type optical fiber spectrometer (AvaSpec-ULS204 Avantes China, Beijing, China).

Mesomorphic Behavior
The thermotropic liquid-crystalline behaviors were examined using POM and DS The DSC curves for the compounds in the initial cooling and subsequent heating process are presented in Figure S5. The phase transition temperatures and enthalpy changes f the compounds were derived from the DSC measurements and are shown in Table 1. T POM textures of this series of bent-shaped molecules during the cooling cycle at differe temperatures are shown in Figure 2. By combining the POM typical textures and DSC te results, the mesomorphic behaviors are summarized in Table 1. It can be seen that com pound C6OP-6Th showed no mesophases but, surprisingly, a low melting point of abo 59.4 °C. By comparison, compounds C5PP-6Th, C5PPF(2)-6Th, and C5PPF(3)-6Th exhi ited nematic phase with differences in the phase transition temperature. It could be su posed that the λ-shaped molecules exhibited the LC phase by increasing the rig

Mesomorphic Behavior
The thermotropic liquid-crystalline behaviors were examined using POM and DSC. The DSC curves for the compounds in the initial cooling and subsequent heating processes are presented in Figure S5. The phase transition temperatures and enthalpy changes for the compounds were derived from the DSC measurements and are shown in Table 1. The POM textures of this series of bent-shaped molecules during the cooling cycle at different temperatures are shown in Figure 2. By combining the POM typical textures and DSC test results, the mesomorphic behaviors are summarized in Table 1. It can be seen that compound C6OP-6Th showed no mesophases but, surprisingly, a low melting point of about 59.4 • C. By comparison, compounds C5PP-6Th, C5PPF(2)-6Th, and C5PPF(3)-6Th exhibited nematic phase with differences in the phase transition temperature. It could be supposed that the λ-shaped molecules exhibited the LC phase by increasing the rigid structures of the side substitution groups on the 3-hexyl-2,5-disubstituted thiophene central ring [38]. By the lateral F moiety situated around the ester linkage of the molecular skeleton, the phase transition temperatures from the crystal state to nematic phase were decreased. structures of the side substitution groups on the 3-hexyl-2,5-disubstituted thiophene central ring [38]. By the lateral F moiety situated around the ester linkage of the molecular skeleton, the phase transition temperatures from the crystal state to nematic phase were decreased. Moreover, the position of -F also mattered, especially its ortho-substitution, which dropped the clearing point temperature by more than 30 °C.

BP Behavior of Thiophene Bent-Shaped Molecules with Added Chiral Compounds
In previous studies, it was found that adding chiral additives to nonchiral bentshaped LC molecules could induce the formation of BPs [20]. Thus, chiral dopant R811 was mixed with the thiophene molecules in increasing concentration to investigate the BP

BP Behavior of Thiophene Bent-Shaped Molecules with Added Chiral Compounds
In previous studies, it was found that adding chiral additives to nonchiral bent-shaped LC molecules could induce the formation of BPs [20]. Thus, chiral dopant R811 was mixed with the thiophene molecules in increasing concentration to investigate the BP behavior. The chiral dopant R811 was added into the bent-shaped molecules in a definite proportion. A small amount of dichloromethane was added and stirred to mix the system homogeneously. Then, the dichloromethane was evaporated completely in an oven at 80 • C. The prepared mixtures were heated above the clear point and poured into the LC cells. The POM textures were observed during cooling or heating processes. For C5PP-6Th, the doping amount of R811 and the corresponding phase transition temperature of the mixture samples are shown in Table 2.  The POM textures of the samples at different temperatures are shown in Figures 3, S7 and S8. During the test, the heating or cooling rate was 0.5 • C min −1 . For sample A1, oily streaks of Ch were displayed when heating over the crystallization point ( Figure S7a). As the sample was heated further to the clearing temperature, the oily striped texture disappeared and the POM presented a dark field. Similarly, only oily streaks of Ch were observed during the cooling process. In addition, the reflection color exhibited a blue shift with the decrease in temperature, which may contribute to the temperature sensitivity of the helical twist force of R811. When increasing the R811 doping amount to 30 wt%, the reflection color of Ch for sample A2 shifted to a short wavelength when compared with that of sample A1, as shown in Figure S7b. Thus, when the doping amount of R811 was less than 30 wt%, reversible phase transitions of Cr-to-Ch and Ch-to-Iso were observed. For sample A3, the oily striped texture disappeared, with blue fog being exhibited when increasing the temperature to 128 • C ( Figure 3). Until the temperature rose to 160 • C, the blue fog texture changed to a cyan platelet texture. When further increasing the temperature to 174 • C, the phase transition to Iso accrued. During the cooling cycle, a local green platelet texture appeared at 170 • C and gradually expanded with the temperature decrease. A regular multidomain BP platelet texture was observed in the cyan reflection color at 162 • C, which then changed to a blue color at 160 • C. Further cooling to 150 • C, the reflection color of the POM texture changed to navy blue and oily streaks arose, indicating a phase transition from BP to Ch. Thus, BP existed in both heating and cooling processes, with corresponding temperature ranges of about 46 • C and 20 • C, respectively. For the POM textures of A3 observed in the LC cell with no parallel treatment ( Figure S8), the phase transition processes were Ch 122.0 • C BP 165.0 • C Iso during the heating cycle, and Iso 160.5 • C BP 139.0 • C during the cooling cycle. The phase transition temperature slightly changed due to the different anchoring effects of the glass substrate. No obvious platelet textures were observed, which might be due to the high material viscosity hindering the growth of typical platelet textures of the BP. To further distinguish the phase state, the reflection spectra of sample A3 in the LC cell with parallel treatment during the heating and cooling processes were measured. The results are shown in Figure 4. The Ch is in a one-dimensional helical structure, while BPs are in double twist cylinder (DTC) structures. Therefore, the reflection wavelength of the Ch phase is usually smaller than that of BPs in the same material system. Then, in the reflection spectra, the peak position around 370 nm was assigned to the particular peak position for the Ch phase, while the reflection peak in the range of the 430-500 nm band was assigned to BPs. During the heating process, the reflection peak of Ch was significantly weakened at 130 • C. Due to the structural incompletion, the reflection peak of the BP was not obvious at this temperature. When further increasing the temperature to 140 • C, the reflection peak at 380 nm further decreased, while the reflection peak at 435 nm increased simultaneously. Subsequently, the reflection peak at about 370 nm completely disappeared, indicating phase transition completion from Ch to BP. The reflection peak of BP red-shifted from 435 nm to 454 nm and then disappeared with the increasing temperature. During the cooling process, a reflection peak at about 491 nm appeared and increased with the wavelength being shifted to the blue side with the decrease in temperature. After the decrease in temperature to below 152 • C, the peak intensity decreased rapidly to zero and the reflection peak at 370 nm appeared and increased at the same time. The results of the reflection spectra were almost consistent with the texture changes observed with the POM. Due to the lack of a long-range-ordered periodic arrangement, the sample in BP III generally showed no obvious reflection peak. Moreover, the peak position of BP did not undergo a sudden change during the temperature-changing process, which helped to exclude the possibility of a phase transition from BP II to BP I [39]. Thus, the BP phase formed by C5PP-6Th doping with the chiral compound might be recognized as BP I. Moreover, it could be found that there was still a weak reflection peak of Ch during the heating process below 150 • C, indicating the coexistence of Ch and BPs, while during the cooling process at about 152 • C, the reflection peak of BP disappeared rapidly when the reflection peak of Ch appeared. Compared to the construction of a double-twisted DTC from a single-axis helical arrangement and the fabrication of long-range periodic supernanostructures, the DTC unwinding to form a single-axis helical arrangement might be easier. This might explain the different BP temperature ranges during the heating and cooling processes. For sample A4, only a blue fog texture was observed, and the clearing point decreased greatly. Moreover, crystallization occurred at 100 • C, which was caused by the excessive content of R811 that induced compatibility problems. Due to the biaxial nature of the molecules, the observed temperature range in this case was much wider than in traditional single uniaxial rod-like molecules mixed with chiral compounds. In addition, it was wider than the blue-phase temperature range observed in earlier biaxial molecular systems. Therefore, BP was achieved during the heating or cooling process by doping with the proper concentration of chiral compounds into the compound C5PP-6Th. Compared to the construction of a double-twisted DTC from a single-axis helical arrangement and the fabrication of long-range periodic supernanostructures, the DTC unwinding to form a single-axis helical arrangement might be easier. This might explain the different BP temperature ranges during the heating and cooling processes. For sample A4, only a blue fog texture was observed, and the clearing point decreased greatly. Moreover, crystallization occurred at 100 °C, which was caused by the excessive content of R811 that induced compatibility problems. Due to the biaxial nature of the molecules, the observed temperature range in this case was much wider than in traditional single uniaxial rod-like molecules mixed with chiral compounds. In addition, it was wider than the blue-phase temperature range observed in earlier biaxial molecular systems. Therefore, BP was achieved during the heating or cooling process by doping with the proper concentration of chiral compounds into the compound C5PP-6Th.      Compared to the construction of a double-twisted DTC from a single-axis helical arrangement and the fabrication of long-range periodic supernanostructures, the DTC unwinding to form a single-axis helical arrangement might be easier. This might explain the different BP temperature ranges during the heating and cooling processes. For sample A4, only a blue fog texture was observed, and the clearing point decreased greatly. Moreover, crystallization occurred at 100 °C, which was caused by the excessive content of R811 that induced compatibility problems. Due to the biaxial nature of the molecules, the observed temperature range in this case was much wider than in traditional single uniaxial rod-like molecules mixed with chiral compounds. In addition, it was wider than the blue-phase temperature range observed in earlier biaxial molecular systems. Therefore, BP was achieved during the heating or cooling process by doping with the proper concentration of chiral compounds into the compound C5PP-6Th.     For C5PPF(2)-6Th and C5PPF(3)-6Th, BP was not observed in either the heating or the cooling processes, regardless of the content of R811. Generally, a lateral fluoro-substituent is frequently employed in LC structures to modify the physical properties of LCs. As the Crystals 2023, 13, 916 7 of 13 fluoro-substituent is larger than hydrogen, it may cause a significant steric effect when combined with the high polarity [40,41]. Generally, the coplanar of the organic molecules may be destroyed when introducing -F, -CN, or -Cl as a lateral group, and the molecular width will be increased [42,43]. Thus, it is hard for the neighboring molecules to form a close stack. In addition, the increase in intermolecular distance leads to a decrease in intermolecular lateral attraction, so it is unable to form a double-twist structure. As a result, BPs were not observed in these two molecular material systems.

Effect of Thiophene Bent-Shaped Molecules on BP Temperature Range
In order to investigate the effect of these bent-shaped compounds on the BP range, these molecules were doped into the BPLC host as testing samples and observed with a POM. The nematic liquid crystal mixture (SLC-4, cleaning point T NI = 83.0 • C, ∆ε = 29.6 (at 25 • C), ∆n= 0.235 (at 25 • C)) obtained from Shijiazhuang Slichem Display Material Co., Ltd., Shijiazhuang, China was used to prepare the host BPLC material. The prepared LC-1 host contained 89 % SLC-4 and 11 wt% Iso-(6OBA) 2 , exhibiting a BP from 64.5 • C to 61.0 • C during the cooling process. By adding thiophene bent-shaped molecules into the LC-1 system with different concentrations, the BP temperature range was investigated. The samples were settled on the hot stage and heated from the Ch to the isotropic state; then, they were cooled from the Iso to Ch state with the same rate of 0.5 • C min −1 . The composition and phase transition temperature of the samples doped with C6OP-6Th are shown in Table 3. After the cooling process, every sample showed the typical BP texture and the microscopic pictures of sample 4 are presented in Figure 5 as a representative. ∆T is the blue-phase temperature range during cooling cycle.
the fluoro-substituent is larger than hydrogen, it may cause a significant steric effect when combined with the high polarity [40,41]. Generally, the coplanar of the organic molecules may be destroyed when introducing -F, -CN, or -Cl as a lateral group, and the molecular width will be increased [42,43]. Thus, it is hard for the neighboring molecules to form a close stack. In addition, the increase in intermolecular distance leads to a decrease in intermolecular lateral attraction, so it is unable to form a double-twist structure. As a result, BPs were not observed in these two molecular material systems.

Effect of Thiophene Bent-Shaped Molecules on BP Temperature Range
In order to investigate the effect of these bent-shaped compounds on the BP range, these molecules were doped into the BPLC host as testing samples and observed with a POM. The nematic liquid crystal mixture (SLC-4, cleaning point TNI = 83.0 °C, Δε = 29.6 (at 25 °C), Δn= 0.235 (at 25 °C)) obtained from Shijiazhuang Slichem Display Material Co., Ltd., Hebei, China was used to prepare the host BPLC material. The prepared LC-1 host contained 89 % SLC-4 and 11 wt% Iso-(6OBA)2, exhibiting a BP from 64.5 °C to 61.0 °C during the cooling process. By adding thiophene bent-shaped molecules into the LC-1 system with different concentrations, the BP temperature range was investigated. The samples were settled on the hot stage and heated from the Ch to the isotropic state; then, they were cooled from the Iso to Ch state with the same rate of 0.5 °C min −1 . The composition and phase transition temperature of the samples doped with C6OP-6Th are shown in Table 3. After the cooling process, every sample showed the typical BP texture and the microscopic pictures of sample 4 are presented in Figure 5 as a representative. ΔT is the blue-phase temperature range during cooling cycle. It can be seen from Table 3 that the addition of bent-shaped molecules was beneficial to the widening of the BP temperature range, which exhibited a tendency of first increasing and then decreasing with the increase in doping amount. The widest BP temperature range was about 13.2 °C at 2.0 wt% added amount. When the doping content was further increased, undissolved or precipitated crystals appeared locally in the sample. Because of the low melting point, the phase transition temperature of the samples decreased with the increase in the added amount of C6OP-6Th. It can be seen from Table 3 that the addition of bent-shaped molecules was beneficial to the widening of the BP temperature range, which exhibited a tendency of first increasing and then decreasing with the increase in doping amount. The widest BP temperature range was about 13.2 • C at 2.0 wt% added amount. When the doping content was further increased, undissolved or precipitated crystals appeared locally in the sample. Because of the low melting point, the phase transition temperature of the samples decreased with the increase in the added amount of C6OP-6Th.
According to the above method, the BP temperature range was measured for the samples doped with the other three molecules in different doping amounts, as shown in Figure 6. Due to the enhancement of the rigid molecular structure, the solubility of the corresponding derivatives in LC-1 was limited. When the added amount exceeded 2 wt%, obvious precipitates or insoluble substances appeared. In addition, the phase transition temperature of the doped samples increased slightly with the increase in bent-shaped molecular addition, as did the high liquid-crystalline phase temperature. For these threthe e molecules, the optimal doping amount in LC-1 was different. For C5PP-6Th, the widest BP temperature range of 18.4 • C was obtained at 1.0 wt% added amount. For C5PPF(2)-6Th and C5PPF(3)-6Th, the optimal amount was the same, 1.5 wt%, broadening the BP to 24.1 • C and 20.5 • C, respectively. When the content exceeded a certain level, bent-core molecules may have increased the energy of the double-twisted cylinder arrangement of the rod-like LC molecules and, thus, decreased the stability of the BP. Through a comparison of the experimental results, the lateral F moiety substitute definitely enhanced the BP broadening effect. The solubility improvement in LC-1 benefiting from -F might also take effect. Moreover, as its ortho-substitution around the ester linkage of the molecular skeleton decreased the clearing point temperature dramatically, the position of -F also made a difference [41]. According to the above method, the BP temperature range was measured for the samples doped with the other three molecules in different doping amounts, as shown in Figure 6. Due to the enhancement of the rigid molecular structure, the solubility of the corresponding derivatives in LC-1 was limited. When the added amount exceeded 2 wt%, obvious precipitates or insoluble substances appeared. In addition, the phase transition temperature of the doped samples increased slightly with the increase in bent-shaped molecular addition, as did the high liquid-crystalline phase temperature. For these threthe e molecules, the optimal doping amount in LC-1 was different. For C5PP-6Th, the widest BP temperature range of 18.4 °C was obtained at 1.0 wt% added amount. For C5PPF(2)-6Th and C5PPF(3)-6Th, the optimal amount was the same, 1.5 wt%, broadening the BP to 24.1 °C and 20.5 °C, respectively. When the content exceeded a certain level, bent-core molecules may have increased the energy of the double-twisted cylinder arrangement of the rod-like LC molecules and, thus, decreased the stability of the BP. Through a comparison of the experimental results, the lateral F moiety substitute definitely enhanced the BP broadening effect. The solubility improvement in LC-1 benefiting from -F might also take effect. Moreover, as its ortho-substitution around the ester linkage of the molecular skeleton decreased the clearing point temperature dramatically, the position of -F also made a difference [41]. To study the influence of molecular structure on their mesogenic behavior and BP thermodynamic stability, the bent-shaped molecules were calculated with density functional theory (DFT) using the GAUSSIAN 09 program, in which the FOPT calculation type with the B3LYP method was employed for the nonlocal correlation and the 6-31G basis set was used for geometry optimizations [38]. The equilibrium geometries corresponding to To study the influence of molecular structure on their mesogenic behavior and BP thermodynamic stability, the bent-shaped molecules were calculated with density functional theory (DFT) using the GAUSSIAN 09 program, in which the FOPT calculation type with the B3LYP method was employed for the nonlocal correlation and the 6-31G basis set was used for geometry optimizations [38]. The equilibrium geometries corresponding to the global minimum optimized structure of each compound are shown in Figure 7. The optimized structural parameters of all the molecules are summarized in Table 4. As is known, one of the major structural requirements for organic compounds exhibiting mesophase behavior is that the molecules must have a large length-to-width ratio (L/W Crystals 2023, 13, 916 9 of 13 ratio), usually greater than 4. Based on the calculated L/W ratios, we can easily understand that C6O-6Th exhibits no mesogenic properties as the L/W ratio is less than 4.   As has been reported previously, bent-shaped molecules can induce the double-twisted helical arrangement of LC molecules due to their special molecular twist configuration and biaxiality, which is conducive to the transfer of chirality [22]. Thus, compared with C6OP-6Th, the molecules C5PP-6Th, C5PPF(2)-6Th, and C5PPF(3)-6Th with strong biaxiality exhibited a significant effect on stabilizing the BP at low concentrations. From the simulation calculation results, it can be seen that the presence and position of fluorine atoms have a significant impact on the value of molecular dipole moments. In accordance with the experiments, the BP broadening effect was improved with the increase in the dipole moments. Thus, it could be supposed that the molecular dipole moment might be a key factor affecting the structural stability of the BP. Moreover, the addition of bent-shaped molecules was proven to modify the elastic constant and decrease the ratio of K 33 and K 11 of LC materials. Thus, the free energy of the system declines and the DTC nanostructure of the BP can be stabilized [28]. Theoretical research shows that K 33 /K 11 is closely related to the value of the length/diameter ratio, which reduces with the decrease in aspect ratio [44]. In the case of other identical structures, the introduction of fluorine atom side-substituting groups at different positions could reduce the molecular effective aspect ratio slightly, as summarized in Table 4. Therefore, BPLC materials with a temperature range greater than 20 • C were obtained by adding a small amount of C5PP(2)-6Th to a BPLC host.

Conclusions
In summary, four thiophene bent-shaped molecules with different molecular long-axis lengths or lateral substituent structures were prepared. The effect of molecular structure on the phase transition behavior was investigated. LC phases were achieved for the molecules with more benzene rings on the long axis. By the lateral F moiety situated around the ester linkage of the molecular skeleton, the phase transition temperatures from crystal state to nematic phase were decreased. The clearing point temperature reduced by 30 • C due to F-ortho-substitution. By doping C5PP-6Th with R811 in 35 wt%, BPs were achieved during both heating and cooling processes, with corresponding temperature ranges of about 46 • C and 20 • C, respectively. In contrast, for C5PPF(2)-6Th and C5PPF(3)-6Th, only Ch was observed no matter how many chiral compounds were added, which might have contributed to the steric effect of the lateral F substitution that inhibited the formation of DTCs of the BP. The effect of these bent-shaped compounds on the BP range was studied by doping them into a BPLC host. The BP temperature range first widened and then narrowed with the increase in the doping concentration due to the limited solubility. The sample with 1.5 wt% proportion of C5PPF(2)-6Th exhibited the widest BP temperature range up to 24 • C, and the transition temperature was close to room temperature. Through the theoretical calculation of density functional theory, the optimized structural parameters of the molecules were obtained, which explained their mesogenic properties using the calculated length-to-width ratio. Moreover, through the analysis of experimental and computational results, the molecular dipole moment might be a key factor affecting the structural stability of the BP in a bent-shaped-molecule-doped LC material system. The study may provide new insights for the molecular design of bent-shaped molecules and has excellent application prospects in novel BPLC materials.

Data Availability Statement:
The raw/processed data required to reproduce these findings cannot be shared at this time as the data also forms part of an ongoing study.

Conflicts of Interest:
The authors declare no conflict of interest.