Observation of ferroelectric behaviour in non-symmetrical cholesterol-based bent-shaped dimers†

Vidhika Punjani ^ab, Golam Mohiuddin ^ac, Susanta Chakraborty ^d, Priyanta Barman ^d, Anshika Baghla ^a, Madhu Babu Kanakala ^e, Malay Kumar Das ^d, Channabasaveshwar Yelamaggad ^efg and Santanu Kumar Pal *^a
aDepartment of Chemical Sciences, Indian Institute of Science Education and Research (IISER) Mohali, Sector 81, Knowledge City, Manauli 140306, India. E-mail: skpal@iisermohali.ac.in; santanupal.20@gmail.com
^bCentre of Molecular and Macromolecular Studies, Polish Academy of Sciences, 90-363 Łódź, Poland
^cDepartment of Chemistry, University of Science & Technology Meghalaya, Ri-Bhoi, Meghalaya 793101, India
^dDepartment of Physics, University of North Bengal, Siliguri 734 013, India
^eCentre for Nano and Soft Matter Sciences, Bengaluru 560013, India
^fDepartment of Chemistry, Manipal Institute of Technology, Manipal Academy of Higher Education, Manipal 576104, India
^gSJB Institute of Technology, Health & Education City, Kengeri, Bengaluru 560060, India

First published on 24th July 2024

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Introduction

Chirality plays a pivotal role in the dynamic self-assembly of liquid crystals (LCs) and has continued to attract the attention of researchers even after its discovery over a century ago.¹ The incorporation of chirality in LCs can lead to the nucleation of different macroscopic chiral structures such as BPs, N*, SmBPs, TGB, SmA/SmA*, SmC/SmC*, etc. which are important from the viewpoint of advanced technology as well as fundamental research.^2–9 These mesophases exhibit a vast variety of interesting properties such as thermochromism, ferroelectricity, antiferroelectricity, electro-clinism, pyroelectricity, etc. Therefore, many calamitic chiral LCs and cholesterol-based dimers are synthesized to realize these intriguing properties.¹⁰

Non-symmetrical cholesterol-based dimers are generally synthesized by linking the aromatic mesogenic core with a cholesterol moiety via a spacer of varying length and parity.¹⁰ A cholesterol moiety is employed widely in LCs because of its widespread commercial availability and natural properties. Furthermore, cholesterol plays an important role as a building block in numerous mesogenic systems and supramolecular assemblies. Moreover, cholesterol is inexpensive and has a rigid structure with eight chiral centers which is a prerequisite for chiral recognition. Cholesterol molecules can be derivatized easily and can lead to the formation of various aggregates, such as gels, monolayers and LCs.¹¹ Cholesterol-based dimers are significant research targets because they exhibit a variety of intriguing mesophases and serve as model compounds for main-chain polymers.¹² Besides, the spacer length and the length of the terminal alkyl chain have a significant impact on the structure–property relationship of cholesterol-based dimers.¹³ Hence, the odd–even effect is well-pronounced in such kinds of systems. Therefore, these cholesterol-based dimers are of great interest to researchers as they display exceptional liquid crystalline properties.^14–38

Herein, we synthesized a series of non-symmetrical dimers that consists of a salicylaldimine or N-(2-hydroxy-4-alkoxybenzylidene)aniline (SAN) unit in the aromatic core which is linked to the cholesterol unit via a spacer of varying length and parity. SANs have piqued the interest of researchers and technologists in recent years due to their unique physical and chemical properties.^39–44 SANs are o-hydroxy Schiff bases formed by the condensation of salicylaldehydes and primary amines in the presence of a catalytic amount of an acid.⁴⁵ Salicylaldimine-based non-symmetrical cholesterol-based dimers are advantageous over Schiff's base (or azomethine) due to the presence of the intramolecular hydrogen bonding between the hydroxyl group and the imine linkage. Additionally, these dimeric systems can be easily substituted with a variety of substituents and can coordinate with a range of metal ions to form metallomesogens.^42,46 Moreover, salicylaldimine-based materials form a six-membered ring owing to their stability towards heat and moisture. These materials exhibit reversible proton transfer in the solid state as well as the solution state and therefore show interesting properties such as thermochromism, photochromism, etc.⁴⁴ Since the SAN core is highly robust and functional, it has been used in a variety of thermotropic LCs, such as calamitic LCs,^39,40 bent-shaped LCs,⁴¹ metallomesogens,⁴²etc. SANs have also been used in a dimeric system where the pro-mesogenic moiety is tethered to the SAN core via a flexible spacer of varying lengths.^44–46 Furthermore, the SAN core has also been incorporated to realize ferroelectric materials.^39,47 Keeping all these advantageous properties of SANs and cholesterol-based dimers, we have synthesized a non-symmetrical cholesterol-based bent-shaped dimer incorporating SAN.

In this article, we report a dimeric system where 2-methyl-3-nitrobenzoic acid has been used as the central aromatic core, and the cholesterol unit is tethered to the central core through a spacer unit via an ester linkage. On the other side, SAN linkage has been formed by the reaction of the amine of the central core and 4-n-alkoxy-2-hydroxybenzaldehyde. Here, the 1,3-disubstituted core was used to introduce the bent molecular architecture. The molecular structure of the non-symmetrical cholesterol-based dimer is presented in Fig. 1. The molecular structure of this series (1/n) is derived from one of the earlier reports by our group where the cholesterol unit is directly attached to the central aromatic core.^48,49 The spacer moiety has been deliberately included to reduce the transition temperatures and to explore the extraordinary properties of the cholesterol-based dimers where the terminal alkyl chain is varied (1/12, 1/14, 1/16). This article investigates the LC properties of non-symmetrical cholesterol-based dimers with 5-bromovaleric acid as a flexible spacer, promoting long-range liquid crystalline behaviour. Alterations in the spacer length, both shorter and longer, led to short-range LC phases, which will be discussed further in a separate article. Additionally, the terminal flexible chains significantly affect the LC properties of these dimers. This study examines three homologues varying in the terminal chain length (n = 12, 14, 16), exhibiting a variety of mesophases, including N*, TGBA, SmA, and SmC*.

Two homologues of the series (1/12, 1/14) displayed a series of interesting mesophases consisting of N*, TGBA, SmA, and SmC* whereas the homologue 1/16 exhibited the SmA and SmC* phase. The mesomorphic behaviour of all the compounds has been studied in detail using polarizing optical microscopy (POM), differential scanning calorimetry (DSC), and small-angle/wide-angle X-ray scattering (SAXS/WAXS). Chiro-optical studies have been carried out to determine the chirality of the mesophase. Static permittivity measurements, dielectric spectroscopy studies, and electro-optical investigations for two homologues (1/14, 1/16) have been performed where both the materials display ferroelectric properties in the SmC* phase. Ferroelectric LC materials are important as these compounds find potential applications in the field of information devices, electro-optical devices, switchable non-linear optics, and spatial light modulators. These materials can be rapidly switched between the two different liquid crystalline states using an electric field which is a remarkable property of ferroelectric LCs.

Experimental methods

The detailed synthetic procedures and the chemical characterization of the chiral bent-shaped liquid crystalline dimers are provided in the ESI† (Scheme S1 and Fig. S1–S16). The mesomorphic properties of all the compounds (series 1/n) are studied explicitly via polarizing optical microscopy (POM), differential scanning calorimetry (DSC), and small-angle/wide-angle X-ray scattering (SAXS/WAXS). The bent-molecular architecture of the non-symmetrical dimer has been confirmed by density functional theory (DFT) calculations with the B3LYP functional and the 6-311G(d,p) basis set as shown in Fig. S17 (ESI†). Furthermore, the chiro-optical studies in the entire mesophase range were carried out via temperature-dependent circular dichroism (CD) studies. Moreover, we have extensively carried out physical studies of all the liquid crystalline compounds in the entire mesophase range by static permittivity, dielectric spectroscopy, and spontaneous polarization measurements. The two homologues 1/14 and 1/16 of the series 1/n exhibited ferroelectric behaviour in the SmC* mesophase.

Results and discussion

Thermal behaviour: polarizing optical microscopy (POM) and differential scanning calorimetry (DSC)

The mesomorphic behaviour of all the synthesized compounds (1/12, 1/14, 1/16) was studied by POM and DSC. The transition temperatures and their associated enthalpies attained by POM and DSC are summarized in Table 1. All the compounds displayed enantiotropic mesomorphism.

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On cooling the sample from the isotropic liquid in an untreated glass slide with a coverslip, compound 1/14 showed focal conic-like textures at 112.9 °C, characteristics of the N* phase (ΔH = 6.5 kJ mol⁻¹). These focal conic-like textures consist of helixes arranged in a disordered fashion as there is no alignment of the helixes under a bare (untreated) glass slide with a coverslip. Upon further cooling the sample, the filamentous-like textures of the TGBA phase started to develop at a temperature of 109.2 °C for a transient period. On further reducing the temperature, fan-shaped bâtonnets of the SmA mesophase appeared with a homeotropic background formed on further reducing the temperature (Fig. S18, ESI†).^50,51 Therefore, the transition at a temperature of 105.0 °C is characteristic of the SmA–SmC* transition.^50,51 The sample 1/14 crystallized at a temperature of 59.3 °C (ΔH = 21.4 kJ mol⁻¹). Similarly, in a 3.3 μm homeotropic cell, the samples showed the characteristic filamentous texture of the TGBA phase at a transition from the TGBA to the SmA phase, and homeotropicity was attained under crossed polarizers beneath TGBA. However, schlieren-like textures were obtained below 81.1 °C which confirmed the formation of the SmC* phase (Fig. S19, ESI† and Fig. 2a–c). The dark observance of the POM textures up to a temperature of 81.1 °C may be possible due to the pitch of the helix in the SmC* phase which is out of the range of visible light. Below 81.1 °C, there is an observation of schlieren-type texture of SmC* with a continuous change in the pitch which is reflected in the colorful POM micrographs of this series of compounds (Fig. 2a–c).

The characteristic filamentous textures of TGBA were observed in the homologue 1/14. The confirmation of the TGBA mesophase was also proven by recording the micrographs at a higher magnification of ×400 of compound 1/14 in the heating as well as the cooling cycle (Fig. S20, ESI†). The micrographic evidence for the crystal polymorphism is shown in Fig. S21 (ESI†). However, the homologue 1/16 showed only SmA and SmC* phases in the cooling cycle. On an untreated glass slide with a coverslip, compound 1/16 showed fan-shaped bâtonnets of SmA at 113.5 °C (ΔH = 7.9 kJ mol⁻¹). On further cooling of the sample, the homologue 1/16 showed SmC* at 107.0 °C and displayed a schlieren-like texture at a temperature of 78.0 °C (Fig. 2d–f) which was retained until the compound crystallized at a temperature of 50.2 °C (ΔH = 19.5 kJ mol⁻¹). The POM micrographs showing the change in the mesophase from SmA to SmC* are shown in Fig. S22 (ESI†). The transition from SmA to SmC* was observed using POM and no peak was detected in the DSC thermogram. DSC thermograms of all the compounds are well presented in Fig. S24 (ESI†).

The dark observance of the POM textures up to a temperature of 78.0 °C may be possible due to the pitch of the helix in the SmC* phase which is outside the range of visible light. Below 78.0 °C, there is an observation of schlieren type texture of SmC* with a continuous change in the pitch which is reflected in the colorful POM micrographs of this series of compounds (Fig. 2d–f). This observation is similar to that of compound 1/14. Compound 1/12 also showed similar mesomorphic behaviour to that of the homologue 1/14 and is shown in Table 1 (Fig. S23, S24, ESI†).

Small-angle/wide-angle X-ray scattering (SAXS/WAXS)

SAXS/WAXS measurements were carried out in 0.7 mm Lindemann glass capillary to confirm the mesophases formed by three of the homologues (1/12, 1/14, 1/16) of the series 1/n. The XRD diffractogram of all the dimeric compounds is composed of two peaks in the small-angle region and a broad peak in the wide-angle region corresponding to the liquid-like characteristic.^52,53 Compound 1/12 displayed two peaks in the small-angle region at a temperature of 90 °C at d₁ = 46.79 Å and d₂ = 23.39 Å as depicted in Fig. 3a where d₁ corresponds to the layer spacing and d₂ signifies the formation of a lamellar type mesophase. The X-ray diffraction intensity profiles were obtained from a linear scan of the diffraction patterns of the non-oriented sample in the SmC* mesophase. The intensity profile reveals the presence of relatively sharp reflections at small angles indicating the layered structure of the SmC* phase. The diffuse outer scattering at wide angles corresponds to the average intermolecular distance (d₃) between the long axes of neighbouring parallel molecules. The ratio of the two peaks d₁/d₂ was found to be 2:1 which confirms the formation of the Sm-like mesophase.^52,53 However, the mesophase was designated to be SmC* as it shows the formation of colorful schlieren-type textures. The inset of Fig. 3a shows the X-ray diffractogram of the unaligned sample obtained in the SmC* phase at 90 °C of compound 1/12. In the wide-angle region, there was a peak at d₃ = 5.55 Å which corresponds to the mean distance between the cholesterol-based bent-shaped molecules giving rise to the liquid-like behaviour in the mesophase (Fig. 3b). The large values of d₃ in the wide-angle region is attributed to the bent-molecular architecture of the cholesterol-based dimers. The inset of Fig. 3b shows the 2D diffractogram in the wide-angle region at a temperature of 90 °C. The temperature dependent SAXS/WAXS measurements suggested that there was a formation of a higher ordered phase below the N* mesophase as there was an increase in the intensity of the (10) peak with a decrease in the temperature as the mesophase traverses from TGBA to SmA to SmC*. The zoomed version of the (10) and (20) peaks in the SAXS/WAXS measurements along with the 2D diffractograms for homologue 1/12 is shown in Fig. S26 (ESI†). Similarly, compound 1/14 displayed two peaks in the small-angle region and a broad peak in the wide-angle region. The two peaks in the small-angle region correspond to the reflections from the (10) and (20) planes. At a temperature of 90 °C, the homologue 1/14 exhibited two peaks at d₁ = 47.24 Å and d₂ = 23.67 Å in the small-angle region (Fig. S25a, ESI†). The ratio of d₁/d₂ was found to be 2:1 which confirmed the formation of the Sm-like mesophase. The inset of Fig. S25a (ESI†) shows the X-ray diffraction photographs of the unaligned sample obtained in the SmC* phase at 90 °C of compound 1/14. The pattern obtained shows the presence of relatively sharp inner rings and a diffuse outer ring typical for the non-oriented samples. In the wide-angle region, compound 1/14 showed a broad peak at d₃ = 5.60 Å at 95 °C (Fig. S25b, ESI†). The wide-angle peak corresponding to d₃ = 5.60 Å indicates the mean distance between the bulky cholesterol-containing bent-shaped molecules giving rise to liquid-like behaviour within the mesophase. The inset of Fig. S25b (ESI†) shows the 2D diffractogram of compound 1/14 at 95 °C. The temperature dependent SAXS/WAXS measurements of the homologue 1/14 are detailed in the ESI† (Fig. S27). The highest homologue (1/16) showed a very similar diffraction pattern to those of the homologues 1/12 and 1/14. At a temperature of 90 °C, compound 1/16 exhibited two peaks in the small-angle region with a d-spacing ratio of 2:1 confirming the formation of the SmC* phase (Fig. S25c, ESI†). A broad peak in the wide-angle region at d₃ = 5.49 Å confirms the liquid-like characteristic of the mesophase (Fig. S25d, ESI†). The temperature dependent SAXS/WAXS measurements of the homologue 1/16 are detailed in the ESI† (Fig. S28). Furthermore, the variation of d₁-spacing with temperature confirmed the formation of different mesophases in two of the homologues (1/14 and 1/16) as depicted in Fig. 3c and d. The homologue 1/14 showed an increase in the d₁ spacing as the mesophase converts to SmA from TGBA up to a temperature of 105 °C. Below 105 °C, the d₁-spacing continuously decreases throughout the SmC* mesophase. The variation in layer spacing and the tilt angle with temperature is well presented in Table S1 and Fig. S29 (ESI†).

Compound 1/16 showed the SmA and SmC* mesophases. The plot of d₁-spacing vs. temperature shows the increase in the layer spacing up to a temperature of 108 °C and falls below 108 °C till the homologue 1/16 crystallizes signifying the transition from the SmA to SmC* mesophases at 107 °C. The variation of d₁ and d₂ and tilt angle with temperature for the homologue 1/16 is well presented in Table S2 and Fig. S30 (ESI†).

Chiro-optical studies

The newly synthesized cholesterol-based dimers were investigated for their chiro-optical characteristics in the mesophase as well as in the isotropic liquid by using circular dichroism (CD) spectroscopy. CD spectroscopy is light absorption spectroscopy that measures the difference in absorption of the left and right circularly polarised light by a molecule consisting of chiral groups. The chiral group present in these kinds of mesogens is a bulky cholesterol moiety.

This spectroscopic technique has been carried out to determine if the cholesterol moiety that accounts for the molecular chirality led to the induction of the chirality of the mesophase.

Generally, the CD spectrum is accompanied by artifacts such as linear dichroism (LD) and linear birefringence (LB). Nevertheless, the presence of small domains in the liquid crystalline mesophase is known to overturn such artifacts.^54–57

Two non-symmetrical cholesterol-based bent-shaped dimers (1/14 and 1/16) were investigated for the chiro-optical measurements. Minute quantities (∼1 mg) of these chosen samples were independently placed between the two clean quartz plates and heated to their isotropic liquid state. At this (clearing) temperature, the top quartz plate of the cell was mechanically sheared and pressed hard repeatedly; this process ensures the uniform spreading of the sample and the elimination of air bubbles.

After leaving the samples unperturbed for a while, CD measurements were performed on the isotropic liquid thin films of the two samples 1/14 and 1/16. As can be seen in Fig. 4a and b, the CD signal in the isotropic liquid is completely absent. This result of CD spectra in the isotropic liquid implies that the chromophores are not affected by the presence of chirality of the bulky and stereogenic cholesterol moieties. On cooling the sample from the isotropic liquid, the CD spectra for N*, TGBA, and SmA phases of compound 1/14 could not show any signal as the mesophases are short-lived. On further cooling of the sample, the SmC* phases appear. The CD signals arise due to the helicoidal structure of the SmC* phase as shown in Fig. 4c. The CD spectra of compound 1/14 in the SmC* phase were recorded as a function of temperature, as presented in Fig. 4c (Table S3, ESI†). The spectra consisted of a strong positive band at 398 nm and another band at 340 nm. Furthermore, the intensity of the peak at 398 nm increases progressively as the temperature decreases, implying that the proximity of the chromophore and the cholesterol moiety varies with temperature.

However, the position of the CD curve, wavelength, and sign remain unaffected, implying that the mesophase preferentially absorbs either left or right circularly polarised light rather than scattering it (Table S3, ESI†). The blue traces as observed in Fig. 4a do not display any signature due to LD activity as anticipated. Compound 1/16 displayed SmC* as an underlying mesophase. Interestingly, in this chiral LC, the CD signals are due to the helicoidal as well as the selective reflection of the SmC* phase.^58–62 The CD spectra of compound 1/16 in the SmC* phase recorded as a function of temperature are presented in Fig. 4d (Table S3, ESI†). The spectra revealed a bisignate curve with the 1st positive cotton effect at a longer wavelength. The blue shift of the peak, along with the increased intensity, is caused by significant variation in the face-to-face aggregation of the chromophore in chiral environments. The 2nd and the 3rd negative cotton effects appear at shorter wavelengths with a crossover point closer to the chromophore absorption maxima.

The intensity of the two bands in the lower wavelength region decreases with a decrease in the temperature. The appearance of the bisignate CD curves is due to chiral excitonic coupling caused by the aggregation of chromophores with transition dipoles organized in a helical manner.^63–65 Thus, temperature-dependent CD measurements yielded strong CD signals in the SmC*, implying that the non-symmetrical cholesterol-based dimer organizes itself in the helical phase.

Static permittivity

The measurement of static permittivity has been carried out for two samples (1/14, 1/16) filled in ITO-coated homogeneous and homeotropic anchoring commercial cells (AWAT, Poland) of spacing ∼5 μm. The LC cell has been placed in a heating/cooling stage HCS 302 (Instec, USA) connected to a programmable temperature controller mK1000 (Instec, USA) which controls and measures temperature with an accuracy of 0.1 °C min⁻¹. The permittivity values (ε_‖ and ε_⊥) were measured using a precession impedance analyzer (Agilent 4294A) with the variation of temperature at a frequency of 1 kHz and a signal voltage of 0.5 V.^66,67

The temperature-dependent static permittivity components as well as the dielectric anisotropy (Δε = ε_‖ − ε_⊥) and average permittivity {ε_avg = 1/3(2ε_‖ + ε_⊥)} for the compounds 1/14 and 1/16 are shown in Fig. 5a and b, respectively. Upon decreasing the temperature from the isotropic phase, the perpendicular permittivity component ε_⊥ monotonically increases for both compounds while the parallel component (ε_‖) initially decreases. Effectively, the dielectric anisotropy (Δε) has been found to be negative throughout the mesomorphic range for a frequency of 1 kHz. However, the Δε values are almost similar for both compounds. The gradual increase of ε_⊥ in these compounds reveals that there is a continuous strengthening of polar correlation upon decreasing the temperature.^66,68

Dielectric spectroscopy measurements

Broadband dielectric spectroscopy was carried out in the frequency range of 40 Hz to 25 MHz using a computer-controlled Agilent 4294A impedance analyzer. On application of a driving voltage of 0.5 V (rms), measurements of the real and imaginary parts (ε′ and ε′′) of the complex permittivity for the sample (1/14, 1/16) filled in a planar aligned sample were carried out during cooling from the isotropic liquid.^66,68

In order to shed light on the relaxation processes, the obtained profiles of ε′′ were fitted with the Havriliak–Negami (H–N) equation containing the low-frequency conductivity term. The temperature and frequency variation of ε′′ for compound 1/14 is shown in Fig. 6a. Two different relaxation modes, namely mode 1 and mode 2, are observed in the entire mesomorphic range. The extracted values of relaxation frequency (f_R) and dielectric strength (δε) are plotted in Fig. 6b and c, respectively. The low-frequency mode 1 appeared just after the transition from the SmA to SmC* phase at around 368 Hz and proceeded to a further low-frequency region with a decrease in temperature. However, the δε₁ value sharply increases to a maximum value of ∼7.5 upon lowering the temperature. This low-frequency mode is a signature of Goldstone-like mode, and this is supported by the observation of the suppression effect of relaxation mode over the application of DC bias voltage. Fig. 6d and the associated inset reveal that this mode is completely suppressed at around 10 V. Due to the presence of the SmC* phase in this compound, the collective rotation of the molecular polarization vector induces such a polar mode. On the other hand, relaxation mode 2 appeared from the isotropic phase in the high-frequency region (∼290 kHz) with a dielectric strength of ∼4. By lowering the temperature, the value of f_R slightly approaches to a lower frequency region, while the δε₂ value has been found to increase gradually.

Additionally, this mode remains almost invariant with increasing the DC voltage. Therefore, this mode can be attributed to the non-collective molecular mode that arises due to the rotation around the short axis.

A similar type of both collective and non-collective modes is reported earlier in the bent-shaped systems including dimeric mixtures.⁶⁹ Similar to compound 1/14, compound 1/16 also possesses two relaxation modes, mode 1 and mode 2, and mode 1 appears at ∼240 Hz in the SmC* phase as shown in Fig. 7a. On the other hand, mode 2 has been found in the entire mesomorphic region in the high-frequency region initiated from the isotropic state (see Fig. 7b). With decreasing the temperature, the relaxation frequency (f_R) of both relaxation processes was observed to be decreased gradually.

However, the dielectric strength δε₁ revealed a comparatively higher value of mode 1 than that of mode 2 in the SmC* phase as observed in Fig. 7c. Again, on applying the bias voltage at a particular temperature, mode 1 was completely suppressed without affecting mode 2 as seen in Fig. 7d. The inset of Fig. 7d represents that the value of δε₁ approached to almost null value at a bias voltage of ∼10 V, while δε₂ remained invariant upon increasing the bias voltage up to 20 V. Therefore, mode 1 is assigned to a collective mode, while mode 2 resembles a molecular mode like the previously investigated compound 1/14.

Spontaneous polarization measurements

The electric field-induced polarization measurement was carried out by using a field reversal technique^68,70–72 in which an external AC electric field was applied on the sample-filled ITO-coated cell of thickness ∼5 μm with a proper resistive circuit. The polarization-switching measurements were carried out at each temperature by stabilizing them for 5 minutes. During cooling from the isotropic state, no polarization peak was observed in the N*, TGBA and SmA phases for compound 1/14. However, a single peak was detected in the switching current under a triangular voltage (60 V_pp-peak to peak, 20 Hz) below 10 °C from the TGBA–SmA phase transition point (Fig. 8a).

Upon decreasing the temperature, this peak continued and became sharper at the SmC* phase as shown in Fig. 8b. In addition to this, under the application of a square wave voltage, a clear hump appeared at a 1 ms timescale in the switching output profiles (inset of Fig. 8a). Such a single polarization peak is the characteristic signature of the ferroelectric response of the medium. The contribution of ionic impurities can be ruled out because of the appearance of the delayed current peak in the triangular wave and current hump within the 1 ms scale.⁷² The temperature variation of polarization depicted in Fig. 8b states that a maximum polarization (P_s) value of ∼132 nC cm⁻² was obtained in the SmC* phase. Additionally, the voltage variation of P_s in Fig. 8c showed that the polarization value increased with the applied voltage and attained a saturation level beyond 70 V_pp.

Moreover, we have observed the switching behaviour up to 500 Hz as depicted in the frequency variation of spontaneous polarization in the inset of Fig. 8c. On the other hand, the electro-optical behaviour for the homologous compound 1/16 appears similar to that of compound 1/14. In this case, a single polarization peak has been observed just after the SmA phase. The obtained value of P_s was found to increase throughout the SmC* phases (without any discontinuity) with a decrease in the temperature and attains a maximum of about 149 nC cm⁻² (see Fig. 9a). The voltage-dependent polarization shows a saturation level as well beyond 80 V_pp and the switching mechanism has been detected up to 600 Hz frequency as given in Fig. 9b.

Conclusions

In summary, the mesomorphic behaviour of the selected non-symmetrical cholesterol-based bent-shaped dimers (1/12, 1/14 and 1/16) has been studied. A series of interesting mesophases were found consisting of N*, TGBA, SmA, and SmC* in two of the homologues (1/12 and 1/14). However, the higher homologue of the series exhibited the SmA phase along with the underlying helical SmC* mesophase. The mesophases have been thoroughly investigated via POM, DSC, and SAXS/WAXS measurements. The chiro-optical characteristic of the mesophases was determined by carrying out temperature-dependent CD studies throughout the mesophase range of 1/14 and 1/16. The CD signal could not be recorded in the N*, TGBA and SmA phases as these mesophases are short-lived. The underlying SmC* showed CD signals in both the homologues (1/14 and 1/16). Compound 1/14 showed a strong positive CD signal at a wavelength of 398 nm whereas the homologue 1/16 showed a bisignate curve. Physical aspects such as static dielectric permittivity, dielectric spectroscopy, and electro-optical measurements were also carried out. Dielectric anisotropy was found to be negative for both the homologues. Broadband dielectric spectroscopy measurements suggested the exhibition of a low-frequency molecular mode and a high-frequency collective mode. Electro-optical measurements displayed the ferroelectric behaviour and polarization in the SmC* mesophase of compounds 1/14 and 1/16. The maximum value of P_s in the homologues 1/14 and 1/16 was found to be 132 nC cm⁻² and 149 nC cm⁻², respectively.

Author contributions

The manuscript was written through the contributions of all authors. All authors have given approval to the final version of the manuscript.

Data availability

The data supporting this article have been included as part of the ESI.†

Conflicts of interest

There are no conflicts to declare.

Acknowledgements

VP thanks CSIR for fellowship with file no. (09/947(0253)/2020-EMR-I) for funding. SKP acknowledges project file no. CRG/2019/000901/OC from the DST-SERB project. We thank NMR, SAXS/WAXS, HRMS facility, the liquid nitrogen facility, and all other departmental facilities at IISER Mohali. GM acknowledges the SERB-SIRE grant SIR/2022/000175, University of Science & Technology Meghalaya (USTM), India, and the University of York, UK. This work is a part of the PhD thesis work by Dr Vidhika Punjani at IISER Mohali.⁷³

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Footnote

† Electronic supplementary information (ESI) available: Detailed synthesis of all the new materials and their characterization by 1H NMR, 13C NMR, ESI, IR, UV-vis, POM, DSC, SAXS/WAXS studies and CD studies. See DOI: https://doi.org/10.1039/d4sm00496e

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