1. Introduction

Blue phases (BPs) are of particular interest for their optically exotic structure, exhibiting no birefringence but selective reflection of circularly polarized light, which is quite different from ordinary LCs with fluid mobility and solid anisotropic. In the cubic lattice of blue phases, the basic unit is the double-twist cylinder (DTC) in which the director is parallel to the axis at the centre, and rotates spatially about any radius, which is also different to the single twist structure of cholesteric phase (or chiral nematic) and no twist arrangement in nematic phase. On the other hand, BPLC-mode display is emerging with potential to become a next generation optoelectronic display because it exhibits some revolutionary features [1], such as insensitivity of cell gap, orientation-independent behavior, energy-saving and fast electro-optical response in sub-millisecond. Additionally, BPLC could be classified as blue phase I (BPI) with body-centered cubic structure, blue phase II (BPII) with simple cubic structure, and blue phase III (BPIII) with amorphous structure according to the double-twist cylinder packing structures respectively [2], therefore BPLCs with these highly fluid self-assembled three-dimensional (3D) cubic defect structures thus could be used as fast light modulators or tunable photonic crystals in the field of optoelectronics and photonics with a relatively simple fabrication process. For example, external temperature, the concentration of chiral dopant, external electric field, laser or UV irradiation and even a boundary condition of substrates, can be used to easily control the photonic band gaps. BPLCs have become increasingly important materials for information display and photonics applications.

The main obstacle to their potential applications is the narrow temperature range (usually less than a few Kelvin), which exclusively appears below the isotropic liquid phase in the high chirality systems. Many efforts had been made to stabilize BP structures over a broader thermal phase range. For example, many biaxial compounds with the moieties such as 1,3-disubstituted benzene [3], 1,2-disubstituted benzene, 2,5-disubstituted 1,3,4-oxadiazole [4], 2,5-disubstituted thiophene [5], 2,2’-disubstituted 1,10-binaphthalene [6] or 2,7-disubstituted naphthalene [7], had been synthesized as LC dopants with molecular biaxiality to induce wide BP in the chiral nematic LC (N*LC). BP could be observed in the chiral mesogen structures with large flexoelectricity [8], bent-shaped [9],T-shaped [10], and could be stabilized with polymer network [11], hydrogen-bond [12], nanoparticles [13]. Nevertheless, a more convenient procedure for stabilizing BPs without a conventional time-consuming and expensive chemical synthesis was needed for the applications in the fast light modulators or tunable photonic crystals. Fortunately, H-bonded self-assembly has been proven to be one of the fastest approaches to prepare materials or devices [14]. Benefitted from the facileness of self-assembly preparation and the convenience to adjust composition, wide BP range could be simply achieved in H-bonded complex of chiral fluoro-substituted benzoic acid and pyridine derivatives [12, 15 ] or in bent-shaped and T-shaped molecules [10].

2. Materials and experiments

Chemicals used in this work were commercially available and used as supplied without further purification. The molecular structures of H-bonded mesogens as well as the analogous C-bonded mesogens were shown in Fig. 1 and obtained by the similar procedures reported [12]. The thermal behaviors of H-bond precursors and complexes were investigated by using an Olympus BX-51 polarizing microscope equipped with Linkam Scientific LTS 350 heating / freezing stage, and a Perkin-Elmer DSC 6 instrument with scanning rate was 10 °C / min. The nematic LC matrixes in this study were SLC4 (Δn = 0.24, Δε = 29.6), S7011 (Δn = 0.15, Δε = 16.1) and S1717 (Δn = 0.20, Δε = 9.0) from Slichem. The helical twisting power (HTP) corresponding to the molecular chirality was calculated based on the following equation: $HTP=\overline{n}/\lambda X_c$, Where Xc is the weight concentration (wt%) of the chiral dopant dissolved in the host nematic LC (SLC-1717), ñ is the mean refractive index and λ is the reflected wavelength of the LC mixture. The HTP of H-bonded complexes and C-bonded compounds were labled in Fig. 1 and the Data File 1.

 

Fig. 1 The molecular structure of the H-bonded and C-bonded mesogens. (See Data File 1 for underlying values.)

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Different density of the stabilizing BP polymer were investigated by mixing the rigid monomer C6M (Merck) and the flexible monomers with different functionalities, that is, lauryl methlacrylate (12MA), 1,6-hexanediol diacrylate (HDDA) and trimethylolpropane trimethacrylate (TMPTMA). The mixture was then injected into IPS cells with electrode width 5.0 μm, cell gap 10.0 μm and electrode gap 5.0 μm, and then polymerized during the BP range by in situ 365 nm-UV irradiation (intensity ~20 mW/cm²) for 30 min. The voltage-dependent transmittance was then measured in the IPS cell with a He–Ne laser λ = 633 nm at BP state.

3. Results and discussions

Usually, there are two strong H-bond self-assembly processes, that is, the –O–H…N from S8HBA vs. PyBH3, and the –O–H…O = C– from S8HBA vs. S8HBA. Owing to the H bonds of –O–H…N being usually relatively stronger than those of -O–H…O = C–, the S8HBA molecules thus easily combine with PyPH3 molecules prior to the formation of carboxylic acid dimers. As the IR spectra of Fig. 2(a) shown, S8HBA showed an associated characteristic stretching band before assembly from 2540.0 cm⁻¹ to 3200.0 cm⁻¹ which were resulting from dimerized carboxylic acid, and a band of C = O stretching peak centered at about 1678 cm⁻¹. While in the IR spectra of the equimolar H-bonded complexes, the band of C = O stretching peak shifted to about 1688 cm⁻¹, and the broad band of dimerized carboxylic acid disappeared and two emerging stretching bands appeared at about 2471 cm⁻¹ and 1904 cm⁻¹,which were attributed to the intermolecular H-bonding between chiral carboxylic acid and pyridine unit [14]. The stability of the H-bond in the complexes could be directly confirmed by temperature-dependent IR spectra shown in Fig. 2(b). It was found that the appearance of two bands centered at 2471 cm⁻¹ and 1904 cm⁻¹ in S8HBA-PyBH3 changed little within the clear temperature of the complex.

 

Fig. 2 (a) Comparison of infrared spectra before and after H-bond assembly; (b) the temperature-dependent spectra of H-bonded complex S8HBA-PyBH3.

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The mesogenic behaviors of H-bonded complexes were summarized in Fig. 3(a) and the detailed phase transition data were listed in the Data File 2 and Data File 3. It was found that the proton donor with double cyclic-ring (S8NBA, CB15A and S8PBA) had LC properties, while the acceptor of pyridine derivatives and the single cyclic-ring acid donors did not show any mesophases before self-assembly. The PyBC5-derived H-bonded complex such as S8PBA-PyBC5 was found to have partial phase-separation during the H-bonded assembly process due to the low miscibility between the precursors. While all the PyBH3-derived complexes appeared LC behaviors without any phase-separation perhaps due to the flexible cyclo-hexane structure of PyBH3 had advantages of improving the solubility and increasing the molecular length-to-width ratio. In addition to the N* phase, smectic phase also could be observed in the PyBH3-derived complexes when bearing at least four cyclic-ring. It was found that longer H-bonded complexes such as CB15A-PyBH3, S8PBA-PyBH3 and S8NBA-PyBH3 with 5-cyclic-mesogen had relatively wider enantiotropic LC range and higher transition temperature than the shorter ones. While in the 3-cyclic-mesogen complexes, only narrow monotropic N* phase was found in the cooling cycle (see S5FBA-PyBC5 and S8HBA-PyBC5), and no mesogeic behavior in S8FBA-PyBC5 due to the small molecular length-to-width ratio. When the H-bonded mesogens changed from biphenyl ring to naphthalene ring, the smectic phase disappeared obviously and the phase transition would decrease even in the 5-cyclic-ring complex (S8NBA-PyBH3). Additionally, the length of terminal chian also had effect on the mesogenic behaviours. When the mesogen was terminated a shorter chain like 2-methylbutoxy or 2-methylbutyl terminals, the smectic phase could be obviously depressed, even in a 5-cyclic-ring complex (CB15A-PyBH3). Additionally, these complexes had a relatively wider N* phase and higher phase transition temperature than the logner 8-carbon alkoxyl terminated complexes. For example, the widest N* range in this study was found in CB15A-PyBH3, while relatively narrower N* phase and wider SmC* range were observed in S8PBA-PyBH3. The lateral fluoro-substituent group was found to help decrease the phase transition temperature and depress the smectic range of complexes. For example, S8FBA-PyBH3 and S8DBA-PyBH3 had lower phase transition temperature and narrower smectic range than the S8HBA-PyBH3 complex.

 

Fig. 3 (a) Mesogenic behaviours of the H-bonded and the C-bonded mesogens; (See Data File 2 and Data File 3for underlying values.) (b) the BP range of the N*LCs (compsed of 75 wt% SLC4 and 25 wt% S811) doped with the H-bonded complexes and the C-bonded compounds.

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The analogous C-bonded compounds was found to have a higher phase transition temperature and a wider LC range than the H-bonded complexes. For example, the 4-cyclic-rings C-bonded compound S8PBABH3 has a higher clear point and a wider mesogenic range than the H-bonded complexes even with 5-cyclic-rings. LC behavior was observed in the C-bonded compound S8FBBABC5 but absent in the S8FBA-PyBC5 complex, which implied H-bond self-assembly could help improve the the order degree of intermolecular arrangement. while the covalent bond could only enhance the intermolecular forces in the form of the phase transition temperature. It is well known that H-bond is an intermolecular force, and flexibler than the covalent bond. H-bonded complex thus could spontaneously adjust itself to the local lower-energy positions during the process of self-assembly, thereby reducing the disorder degree of molecules arrangement. This interesting phenomenon also can be found in the emergence of higher order mesophase SmA* phase in S8HBA-PyBH3 and only lower order N* phase in the analogous C-bonded S8PBABH3 as shown in Fig. 3(a).

Due to that the BP usually appeared in a narrow temperature range at the high temperature (far beyond room temperature) and the mutual solubility of H-bonded complexes were not well, the H-bonded complexes and the analogous C-bonded mesogens were doped into N*LC matrix to improve the mutual solubility and investigate the performance differences of the two mesogens in extending the BP range. Here the N*LC was composed of eutectic nematic (75 wt%, Slichem, SLC4) and Chiral dopant (25 wt%, Merk, S811) and showed a narrow BP range from 46 to 50 °C. The detailed compositions and the resulting BP ranges were shown in Fig. 3(b). The typical POM texture of BPs in N*LC doped with H-bonded assemblies and C-bonded compounds were showed in Fig. 4 . The H-bonded ones had a lower phase transition temperature and thus contributed to move the BP range to room temperature. The S8FBA-PyBC5 complex was found did not help extend the BP ranges due to lack of the mesogenic behaviors. When addition of H-bonded complexes of S8HBA-PyBH3 from 10 wt% to 30.0 wt%, the BP range of doped N*LC presented a trend of increasing first and then decreasing with the widest BP range about 8.0 °C. While the C-bonded S8PBABH3 doped mixture exhibited a relatively wider BP range than the H-bonded ones (the widest BP range about 12.9 °C) with a colourful mosaic plate texture. It was measured that the HTP of H-bonded ones was much smaller than that of the C-bonded ones as shown in Fig. 1, which could be attributed to using achiral H-bond acceptor and the weak ability of chiral transfer from chiral H-bonded mesogen to the host LCs. That was the reason for the induced BP range of H-bonded complex with achiral acceptor was relatively narrower than that of the C-bonded ones. Nevertheless, It could be speculated that the induced BP range would be widened if using a larger HTP H-bond complex, which was confirmed by doping the complex S8HBA-PyS8PBA (consisted of a chiral donor and a chiral acceptor) into the N*LC. Although the HTP value of S8HBA-PyS8PBA was lower than that of the C-bonded compounds, the induced BP ranges were much wider than that of mixture doped with the analogous compound S8FBABPh with two chiral centers, with the widest BP range about 20 °C. Due to the well known fact H-bond is an reversible and flexible bond, the H-bonded complex could spontaneously adjust itself to the local defects during the self-assembly, and thus reduce the total free energy as well as the defect volume occupied by the disclination lines which had been demonstrated could stabilize the BP structure [11].

 

Fig. 4 The textures of S8HBA-PyBH3 in (a) N* phase (planar texture), (b) SmA* phass (fan-shaped texture), and the BPs textures observed in the N*LC (with 75 wt% SLC4, 25 wt% S811) doped with: (c) 5%S8FBA-PyBC5, (d) 20%S8HBA-PyS8PBA, (e) 10% S8FBABPh, (f) 30 wt% S8PBABH3.

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The H-bonded complex doped LCs were found to be irreversibly switched at BP state without adding a certain amount of polymer, which could attribute to the relatively higher viscosity in the mixture. The C-bonded compound doped LCs were also found to be irreversibly switched at BP state when the external electric field exceeded 7.0 Vμm⁻¹, which may be caused by the electric field induced phase transition. Here, the dopants such as the H-bonded complex, chiral content, and monomers were orthogonally investigated to explore the effect on the electro-optical performance of BPLC. The detailed compositions and the corresponding BP ranges before polymerized were shown in the Data File 4. After polymerization, all the samples could be switched reversibly under an electric field (~20.0 Vμm⁻¹) with the respond time (rise time) about 1 ~4 ms as Fig. 5(a) shown. However, a hysteresis loop was observed which may be resulted from the lattice deformation by the electrostriction effect.

 

Fig. 5 (a) The VT curves of the IPS cell of BPLC doped with 20% H-bonded complex as a function of an AC field at a frequency of 60 Hz; (See Data File 4 for underlying values.) (b) Variation of induced birefringence with square of an applied electric field with or without H-bonded complex; (c) Orthogonal analysis of the impact of various factors on Kerr constants; (d) Orthogonal analysis of the impact of various factors on hysteresis.

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It is well known that BPLC is a macroscopically isotropic when there is no external electric field (E). As E increases, the BPLC becomes anisotropic along the electric field direction. The induced birefringence ∆n_induced is related to E, wavelength λ, and Kerr constant K. The Kerr constant as known can be approximated by the following equation [1]:

Figure 5(b) calculated the relationship between the electric birefringence of BP and external electric field strength in sample No.5 (with 20 wt% H-bonded complex, See Data File 4) and sample No.0 (without H-bonded complex). It was found that both electric birefringence were approximately proportional to the square of the electric field (~100 V²/μm²), which meant that the equation for the Kerr law (Eq. (1) was nearly followed in the weak electric field and the slope was the Kerr constant. After calculation, the Kerr constant in the polymerized systems changed from 3 to 12 × 10⁻¹¹ m/V² with the doping content of H-bonded complexes, and was little lower than the sample without H-bonded complex (~14.4 × 10⁻¹¹ m/V²). Kerr constant of BPLC doped with H-bonded complex was nearly comparable with the previous polymer-stabilized BPLC of C-bonded mesogens (4 ~11 × 10⁻¹¹ m/V²) [9] and about 10 times larger than the well-known nitrobenzene (K = 2.4 × 10⁻¹² m/V²). It was found in Fig. 5(c) that the Kerr constant presented a trend of increasing first and then decreasing due to the H-bonded mesogen had a smaller elastic constant but relatively lower physical parameters (ΔnΔε value) than the C-bonded mesogens. Unfortunately, there were obvious electro-optical hysteresis in the samples, especially in the sample using S7011 (ΔnΔε = 1.80) and S1717 (ΔnΔε = 2.42) as matrix. Figure 5(d) orthogonally analyzed the impact of factors on the electro-optical hysteresis. It was found that hysteresis could became serious with increase of the doping amounts of S811 and the H-bonded complex, which could be resulted from the induced viscosity and lower ΔnΔε parameters caused by doping. Therefore, it can be speculated that the degree of hysteresis would be gradually reduced if the physical parameters of ΔnΔε was improved to a higher leverl (ΔnΔε = 7.11, SLC4). On the other hand, the hysteresis also could be reduced with the increase of polymer network density by using multi-functional monomers such as TMPTMA. In general, H-bonded mesogens possessed relatively lower phase transition temperature, relatively higher viscosity and smaller ΔnΔε parameters than the C-bonded mesogens, which implied the H-bonded complexes should be suitable for use in the non-display field, but the H-bonded assembly had advantage in the fast preparing wide BP materials which could help optimize the needed C-bonded structure.

4. Conclusion

A series of chiral H-bonded LCs and the analogous C-bonded LCs with similar structures were synthesized to investigate the effect on the mesogenic behaviours and the performance of blue phases (BPs). It was found that the H-bonded ones had a relatively lower phase transition temperature, narrower LC range, relative lower HTP than the C-bonded ones. H-bond self-assembly could help improve the the arrangement order degree and extend BP range like a relative higher HTP C-bonded compound, especially by using chiral donor and chiral acceptor. The bigger length-to-width ratio of H-bonded mesogen and the complex with longer terminal chain had relatively wider enantiotropic LC range (especially smectic phase) and higher transition temperature than the shorter ones. The polymerized H-bonded complex doped BPLC could be reversibly switched under the electric field of ~20.0 Vμm⁻¹ with relatively lower electro-optical speed (with rise time about 1~4 ms) and lower Kerr constant (3 ~12 × 10⁻¹¹ m/V²) than the sample without H-bonded complex doped.