# Influence of the chain length on the thermal and dielectric properties of thermotropic liquid crystals

Manel Ben Salah, Naoufel Ben Hamadi, Ahlem Guesmi, Taoufik Soltani, Yves Chevalier

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Manel Ben Salah, Naoufel Ben Hamadi, Ahlem Guesmi, Taoufik Soltani, Yves Chevalier. Influence of the chain length on the thermal and dielectric properties of thermotropic liquid crystals. Journal of Molecular Liquids, 2026, 441, pp.129010. 10.1016/j.molliq.2025.129010. hal-05384910

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# Influence of the chain length on the thermal and dielectric properties of thermotropic liquid crystals

Manel Ben Salah<sup>a</sup>, Naoufel Ben Hamadi<sup>b</sup>, Ahlem Guesmi<sup>b</sup>, Taoufik Soltani<sup>a,*</sup>, Yves Chevalier<sup>c,†</sup>

## ARTICLE INFO

### Keywords:

Nematic liquid crystal

Dielectric

Free ions

Electroconvection

## ABSTRACT

A novel series of liquid crystals was obtained using hydrogen-bonded 2:1 complexes of 4-*n*-alkoxy-2-fluorobenzoic acid (nOBAF) and succinic acid. The formation of these complexes was confirmed by infrared spectroscopy. Their thermal behavior was investigated by differential scanning calorimetry and polarized optical microscopy. All of the synthesized complexes exhibited smectic A (SmA) and nematic mesophases. The temperature range of the nematic phase was wider for the compounds with shorter alkoxy chain. The SmA phase became more prevalent as the alkoxy chain was lengthened. The complex dielectric permittivity in the frequency range from 1 Hz to 10 MHz showed a relaxation at 4 MHz coming from the rotations of molecules around their main axis and a low frequency relaxation due to the presence of ionic species. The latter was stronger for short-chained mesogens and caused a strong electroconvection phenomenon.

## 1. Introduction

The directional nature of hydrogen bonds enables the formation of a variety of hydrogen-bonded complex species with anisotropic properties. Supramolecular chemistry involving these anisotropic intermolecular interactions is a key area of interest in soft matter science, particularly in the field of liquid crystals. Hydrogen bonded liquid-crystalline materials (HBLCs) have attracted considerable attention as promising materials for next generations of displays because of their rich polymorphism [1,2]. Indeed, many works have disclosed new HBLCs forming nematic and smectic phases [3–13]. Two main molecular characteristics that influence the liquid crystalline phase behavior are the shape of the rigid part and the length of the flexible terminal chain [14]. The hydrogen-bonding interactions set the shape of the rigid part and therefore play a crucial role in the formation of bent-core liquid crystals from V-shaped supramolecular assemblies [14–17]. This has led to the development of several complexes that exhibit a nematic twist-bend (NTB) phase driven by hydrogen bonds. Lengthening the alkyl chain also introduces flexibility that is detrimental to the molecular organization. Thus, Patterson et al. [14] observed a decrease of the

$N_{TB}$–N and N–I transition temperatures as a function of length of the terminal chain in hydrogen-bonded liquid crystals. A slight odd-even effect was also observed. Walker et al. [15] showed that the length of the alkoxy chain in supramolecular liquid crystals influenced the phase behavior showing mostly nematic and smectic phases. Anisotropic liquid crystal materials, which are hydrogen-bonded, have been shown to display blue phases, featuring enhanced spontaneous polarization values and wide temperature ranges [18]. Furthermore, Chen et al. [19] explored the polar swapping current responses in the SmG phase of a hydrogen-bonded complex, which were generated by collective rotations along the long molecular axis. The thermal, optical and electrical properties of HBLCs are influenced by the type of pendant groups attached to the bent-core hydrogen-bonded structures [20,21]. Lin et al. [22] studied how the number of aromatic units and the position of hydrogen-bonding sites within the core affects the overall performance. Most investigations into structure-properties relationships were performed using hetero-associations by hydrogen bonds, that is, associations of a H-donor and a H-acceptor. In the present paper, the role of the alkyl chain length in determining the thermal and mesophase behavior is addressed in liquid crystals formed by hydrogen-bonded homo-

<footnote><sup>a</sup> Laboratoire de Physique de la Matière Molle et de la Physique des Fluides, Faculté des Sciences de Tunis, Université de Tunis El Manar, 2092 Tunis, Tunisia</footnote>

<footnote><sup>b</sup> Chemistry Department, College of Science, Imam Mohammad Ibn Saud Islamic University (IMSIU), P.O. Box 5701, Riyadh 11432, Saudi Arabia</footnote>

<footnote><sup>c</sup> Laboratoire d'Automatique, de Gènie des Procédés et de Gènie Pharmaceutique (LAGEPP), Université Claude Bernard Lyon 1, CNRS UMR 5007, 43 bd 11 Novembre, 69622 Villeurbanne Cedex, France</footnote>

<footnote>* Corresponding authors.</footnote>

<footnote>E-mail addresses: taoufik.soltani@fst.utm.tn (T. Soltani), yves.chevalier@univ-lyon1.fr (Y. Chevalier).</footnote>
associations of carboxylic acids. Moreover, the literature review points that the influence of terminal chain length on the phase behavior can follow one trend or the reverse, which may require some clarification. Thus, Paterson et al. [14] reported a decrease of the Sm–N<sub>TB</sub> and N<sub>TB</sub>–N transition temperatures with respect to the chain length, whereas Walker et al. reported increases of the SmA–N [15] and both SmC–N<sub>TB</sub> and SmA–N [17] transition temperatures.

Electroconvection (EC) is another phenomenon that has been widely observed and studied in recent years [23–28]. Various pattern morphologies induced by the electric field have been observed, and discussed focusing on their frequency and amplitude. Typically, this phenomenon occurs in ionized systems as well as liquid crystals that are doped with dyes, electrolytes, nanoparticles and nanotubes, all of which enhancing the convection effect [24–27]. Recent numerical simulations by Lee et al. [28] showed that electroconvection in a pure nematic phase may occur without the presence of free charges. Additionally, several theoretical models have been developed to analyze and gain a deeper understanding of the electroconvection mechanism in liquid crystals [27–29].

This study focuses on the effect of the alkoxy chain length of the phase behavior and electro-optical properties of hydrogen-bonded fluorinated liquid crystalline materials. We present the synthesis and characterization of a series of fluorinated liquid crystalline dimesogens, with terminal chain lengths ranging from 7 to 18 carbon atoms. The structure of these new supramolecules assembled by hydrogen bonds were analyzed using IR spectroscopy. The mesomorphic properties of these compounds were evaluated through differential scanning calorimetry (DSC) and polarized optical microscopy (POM). The dielectric and electro-optical behaviors of the liquid crystalline phases were investigated.

## 2. Experimental section

Succinic acid was purchased from Sigma-Aldrich. The 4-*n*-alkoxy-2-fluorobenzoic acids (nOBAF) with varying chain lengths (*n* = 7, 9, 14 and 16) were synthesized and characterized by NMR and IR spectroscopy following the procedure presented in [4]. The results of the analyses are given in the Supplementary Information. The hydrogen-bonded complexes (Fig. 1) were prepared by dissolving 2 mmol of nOBAF and 1 mmol of succinic acid (SA) in dichloromethane to create a homogeneous solution of the nOBAF/SA complex species with a 2:1 M ratio of nOBAF to succinic acid. The solvent was then left to slowly evaporate in open air at room temperature under the fume hood, and finally, the mixtures were dried in a vacuum oven at 50 °C for 10 h.

The structural characterization of the materials was performed by means of ¹H, ¹⁹F and ¹³C NMR of nOBAF in solution. The NMR spectra were run on a Bruker AvL300 spectrometer working at 300 MHz Larmor frequency for ¹H NMR. Chemical shifts of ¹H and ¹³C NMR spectra are given in ppm from TMS and those of ¹⁹F NMR spectra are given in ppm from CFCI₃. Infrared spectra were recorded for dry powders using Attenuated Total Reflectance (ATR) mode on a Nicolet™ iS50 FTIR spectrometer. The spectra were collected by accumulation of 32 scans in the wavenumber range of 400–4000 cm⁻¹ with a spectral resolution of 4 cm⁻¹. The transition temperatures for the prepared series were determined using a Perkin-Elmer DSC7 differential scanning calorimeter,

<figure><img src="image_3.png" /><figcaption>**Fig. 1.** Structure of the nOBAF/SA 2:1 hydrogen-bonded complexes of nOBAF and succinic acid (*n* = 7, 9, 14 and 16).</figcaption></figure>

with heating and cooling rates of 1 °C·min⁻¹. Phase characterization was carried out using polarized optical microscopy (POM) with an Olympus BX51 microscope, equipped with a Sony digital CCD camera.

For investigations of the dielectric and electro-optical properties, commercial planar alignment cells (HG, INSTEC) were filled with the compounds via capillary suction at the temperature of the isotropic phase. The cell thickness of 15 µm was maintained using Mylar wedges; the active area was 0.3 cm². The cells were then slowly cooled from the isotropic to the nematic phase at 0.1 °C·min⁻¹. The dielectric behavior was measured using an impedance/gain phase analyzer (Solartron SI 1260) connected to a 1296 dielectric interface in the frequency range of 1 Hz–10 MHz. Electro-optical measurements were carried out using a polarizing microscope (Nikon Eclipse LV100N), along with a high-power amplifier and an Agilent 33220 A function generator to provide either a DC voltage or a sinusoidal signal with adjustable amplitude.

## 3. Results and discussion

### 3.1. IR analysis

The IR spectra of the nOBAF/SA complexes confirmed the structure of the 2:1 nOBAF/SA complexes as a representative example the IR spectra of the supramolecule 7OBAF/SA and its components 7OBAF and SA are presented in Fig. 2 (the full set of spectra is given in Fig. S13 of Supplementary Information). Indeed, the bands associated stretching vibrations of free O–H at wavenumbers around 3500 cm⁻¹ were not observed, indicating that the O–H groups of the carboxylic acids were involved in intermolecular hydrogen-bonding with C=O groups in all samples, 7OBAF, SA and nOBAF/SA. The ν(O–H) and ν(C=O) stretching modes of succinic acid appear as broad bands because the hydrogen bonds between the OH and C=O groups of SA assumed several different configurations. The broadening of the O–H and C=O bands in hydrogen bonded acids is quite a common phenomenon resulting from the combination of the unresolved bands of the several configurations [30]. Conversely, the band of the ν(C=O) mode observed at 1683 cm⁻¹ in 7OBAF and 7OBAF/SA was sharp because the hydrogen bonded complex assumed only one single configuration. This band is characteristic of the formation of cyclic hydrogen-bonded complexes between the carboxylic acids [31] in pure 7OBAF and between the carboxylic acid groups of 7OBAF and SA in 7OBAF/SA. As in aliphatic carboxylic acid dimers, a strong broad absorption band of the stretching vibration ν(O–H) might be expected around 3000 cm⁻¹ wavenumber [32]. It presently looked being missing for 7OBAF and the 7OBAF/SA complex. Only the two weak absorption bands at 2550 cm⁻¹ and 2650 cm⁻¹ were detected. They are corresponding of the B-type bands of H-bonded O–H vibrations [33,34]. Actually, the IR bands of O–H vibrations are weak in the cases of strong hydrogen bonds [35] and the O–H stretching band located around 3000 cm⁻¹ has approximately the

<figure><img src="image_4.png" /><figcaption>**Fig. 2.** IR spectra 7OBAF/SA and its pure components 7OBAF and SA.</figcaption></figure>
same intensity as the B-type bands as it has been observed in the IR spectrum of solid benzoic acid [36]. Owing to the observed weak absorption bands at 2550 cm⁻¹ and 2650 cm⁻¹, the intensity of the stretching vibration v(O—H) was not detected because it was much weaker than that of the C—H stretching vibrations located around the same wavenumber. The apparent absence of O—H vibration at 3000 cm⁻¹ in hydrogen-bonded liquid crystals made of benzoic acid derivatives has been reported by Song et al. [37] and Alaasar et al. [38].

## 3.2. Phase sequences

The phase sequences of the complexes were determined using polarized optical microscopy observations (POM) and differential scanning calorimetry (DSC). The DSC thermograms of nOBAF/SA are given in Fig. 3. DSC of 7OBAF/SA revealed three phase transitions both at heating and cooling. The two high-temperature transitions occurred at similar temperatures in the heating and cooling scans; the endothermic peak at heating was at 107.5 °C and exothermic peaks at cooling was at 105.3 °C. On the other hand, the low-temperature phase transition showed a large hysteresis with an endothermic peak at heating at 74.3 °C and an exothermic one at cooling at 53.1 °C. A solid crystalline phase (Cry) and a liquid isotropic phase (Iso) were observed at low and high temperatures, respectively. The intermediate liquid crystalline phases were assigned to smectic A (SmA) and nematic (N) based on POM observations discussed below.

At elevated temperatures, the isotropic and nematic phases were clearly identified by optical observations under crossed polarizers (Fig. 4

and 5). A new phase characterized by a fan-shaped texture appeared at 98 °C as the nematic phase was cooled, providing evidence for the occurrence of an intermediate SmA phase [39,40]. All the nOBAF/SA compounds displayed a sequence of SmA and nematic liquid crystalline phases, both upon heating and cooling (see Fig. 5).

All transitions were observed by both POM and DSC during cooling. The same phase transitions could be observed by both POM and DSC upon heating for 7OBAF/SA and 9OBAF/SA. DSC missed some of them for 14OBAF/SA and 16OBAF/SA, so that their transitions upon heating could only be observed by POM. The Cry-SmA and SmA-N transitions observed upon heating the samples correspond to the thermodynamic equilibrium. All of them were observed by POM upon heating, indicating that the SmA and N phases of all nOBAF/SA were thermodynamically stable. They are reported in Table 1 together with the transition temperatures upon cooling that are useful because metastable liquid crystal structures can be formed by cooling from the isotropic phase. The SmA and N phases of 7OBAF/SA were observed upon heating at 84.2 °C and 107.5 °C; and they formed upon cooling at 102.3 °C and 87.3 °C. The hysteresis was very small. Conversely, the Cry-SmA transition slowed a large hysteresis: 53.1 °C upon cooling against 74.3 °C upon heating. The SmA was present under a metastable state under cooling between these two temperatures. The same phase behavior was observed for all compounds of the series.

DSC upon heating apparently missed some transitions although the full phase sequence Cry-SmA-N-Iso was observed by POM. The SmA-N transitions of 9OBAF/SA and 14OBAF/SA could not be observed upon heating by DSC; but they were observed by POM. The transitions to the

<figure><img src="image_5.png" /><figcaption>**Fig. 3.** DSC thermograms of the nOBAF/SA (n = 7, 9, 14 and 16) complexes upon heating and subsequent cooling. Exothermic is up.</figcaption></figure>
<figure><img src="image_6.png" /><figcaption>**Fig. 4.** POM images of SmA and nematic phases observed upon cooling. 7OBAF/SA: SmA at 80 °C (a) and N at 96 °C (b); 9OBAF/SA: SmA at 70 °C (c) and N at 96 °C (d).</figcaption></figure>

<figure><img src="image_7.png" /><figcaption>**Fig. 5.** POM images of SmA and nematic phases and of the phase coexistence at their transition temperature. 14OBAF/SA: SmA (a), transition from N to SmA (b), N (c), transition from Iso to N (d); 16OBAF/SA: SmA (e), transition from N to SmA (f), N (g), transition from Iso to N (h).</figcaption></figure>

isotropic phase were observed by DSC and POM. This apparently single transition observed as a single endothermic peak by DSC upon heating was actually corresponding to the two unresolved SmA–N and N–Iso transitions. Owing to the small difference of these transition temperatures, DSC did not enable the separation into two well-resolved peaks. The temperatures of the SmA/N–Iso transition of 14OBAF/SA was close to that of the Cry–SmA transition, so that the small peak of the SmA/N–Iso transition appeared at the foot of the peak of the Cry–SmA transition. This trend was more marked in the case of 16OBAF/SA for which

the Cry–SmA, SmA–N and N–Iso transitions were merged into a single endothermic peak in the DSC measurements upon heating. All these transitions were clearly observed by POM however, and they were easily observed upon cooling. The missing transitions in DSC were definitely caused by a lack of resolution of the peaks due to the too narrow temperature domain of the phases. The temperature domains of the two liquid crystalline phases were extended upon cooling because of the strong hysteresis of the crystallization that shifted the SmA–Cry transition to much lower temperatures.
<table><caption>**Table 1**

Transition temperatures (°C) and enthalpies (J·g⁻¹, shown in brackets) of the nOBAF/SA complexes upon heating and cooling the compounds.</caption><thead><tr><th>Substances</th><th>Transition temperatures (°C) and enthalpies (J·g<sup>-1</sup>)</th></tr></thead><tbody><tr><td rowspan="2">7OBAF/SA</td><td>On heating: Cry-74.3(49.7)-SmA-84.2(40.6)-N-107.5(6.2)-Iso</td></tr><tr><td>On cooling: Cry-53.1(53.3)-SmA-87.4(26.3)-N-102.3(13.3)-Iso</td></tr><tr><td rowspan="2">9OBAF/SA</td><td>On heating: Cry-83.5(94.9)-SmA-105.0(-)-N-109.2(3.8)-Iso</td></tr><tr><td>On cooling: Cry-63.7(47.9)-SmA-83.2(40.1)-N-108.0(4.3)-Iso</td></tr><tr><td rowspan="2">14OBAF/SA</td><td>On heating: Cry-89.6(44.1)-SmA-97.0(-)-N-100.4(4.3)-Iso</td></tr><tr><td>On cooling: Cry-59.7(46.6)-SmA-93.2(44.1)-N-98.5(4.7)-Iso</td></tr><tr><td rowspan="2">16OBAF/SA</td><td>On heating: Cry-94.4(56.2)-SmA-103.0(-)-N-104.5(-)-Iso</td></tr><tr><td>On cooling: Cry-70.3(33.6)-SmA-94.2(56.2)-N-101.2(12.3)-Iso</td></tr></tbody></table>

The temperatures of the equilibrium phase transitions are those observed upon heating. Hysteresis upon cooling shifts the transition temperatures, so that the liquid crystalline phases observed upon cooling are often out-of-equilibrium. The equilibrium (T-n) phase diagram shown in Fig. 6 relied on transitions measured by the POM observations. The phase diagram shows the role played by the terminal alkoxy chain length in the thermal stability of mesophases. The temperature range of the nematic phase decreased as the length of the alkoxy chain increased. So, longer molecules reduced the temperature range of the nematic phase and favored the formation of the SmA phase. The influence of the alkoxy chain length on the transition temperatures is quite a complex matter because both increase and decrease of transition temperatures have been observed. The general framework for the formation of nematic phases by anisotropic particles by Onsager [41] has been extended to molecules by introducing the “length-to-breadth” ratio X [42] as a descriptor of the anisotropic shape of the molecules. This early work predicted that the N-Iso transition temperature may increase or decrease with respect to X, depending on the intermolecular interaction energies. An increase of the N-Iso transition temperature against X was given by Demus [43,44], based on experimental data. General trends may be extracted from the review of experimental data by Gray [45]. Gray showed that the temperature of the N-Iso transition decreases when transitions occur at high temperature but increases for transition occurring at low temperatures, the cross-over temperature being around 100C. In accordance with this trend, the present N-Iso transitions are close to 100 °C and the dependence on alkoxy chain length is quite weak. In most cases reported by Gray, the SmA phase is stabilized at the expense of the nematic phase by longer chain lengths (the SmA-N transition temperature increases with respect to the chain length), as it is the present case. The same trends have been reported by Blair et al. [46] in the specific case of mesogen molecules having two terminal alkyl chains. This trend is consistent with findings reported by Song et al. [37] and Alaasar et al. [38], who observed that increasing the length of alkyl chains in hydrogen-bonded liquid crystals, specifically those formed between 4-hexyloxybenzoic acid and azopyridine, led to a broader temperature range of the smectic phase. This behavior is at variance

<figure><img src="image_8.png" /><figcaption>**Fig. 6.** Equilibrium phase diagram of nOBAF/SA complexes determined from POM observations upon heating.</figcaption></figure>

with the observations by Patterson et al. [14] showing a decrease of the N-I, N<sub>TB</sub>-N and Sm-N<sub>TB</sub> transition temperatures with respect to chain length, and the occurrence of smectic phases for the shortest chains only.

### 3.3. Dielectric and electro-optic properties

Electric field-induced electroconvection (which will be discussed later) was observed for the nOBAF/SA with short alkoxy chain lengths (n = 7 and 9), therefore preventing any measurement of the dielectric anisotropy and threshold voltage. In contrast, the EC phenomenon was not observed for complexes with n ≥ 14, allowing their dielectric and electro-optic behaviors to be measured.

Fig. 7 shows the real (ε') and the imaginary (ε") parts of the dielectric permittivity of the 7OBAF/SA material in planar configuration in the temperature ranges of the nematic and smectic A phases as a function of the frequency. At lower frequency, the ionic contribution was clearly seen, which was in agreement with dielectric investigation previously reported by Missaoui et al. [47]. The relaxation observed at higher frequency (~4 MHz) was due to the molecular orientation around the short axis (Fig. 7b). The dielectric properties were similar in the isotropic, nematic and smectic A phases. The real part ε' slightly decreased upon cooling and there was no visible discontinuity of ε' at the N-SmA transition. Conversely, the ε' at low frequency jumped down by a factor of 4-5 at the transition from the isotropic to the nematic phase, reflecting a high ionic mobility in the isotropic phase. The temperature dependences of ε' and ε" were quite weak at high frequency and the relaxation frequency at 4 MHz did not significantly depend on temperature, showing that the molecular rotations were the same as all the liquid phases.

Similar behaviors were observed for other nOBAF/SA nematic phases (Fig. 8). Both 7OBAF/SA and 9OBAF/SA showed strong ionic contributions to both ε' and ε" at low frequency. The latter was moderate for 14OBAF/SA and 16OBAF/SA. This indicated a larger concentration or a higher mobility of ionic species in the short-chained compounds [48]. In addition, it has been observed that shorter alkyl chain LCs exhibit better ionic conductivity performance and higher power conversion efficiencies in photovoltaic applications. This implies that increasing the alkyl chain length suppresses ionic conductivity, potentially due to tighter packing [49].

The dielectric anisotropy Δε = ε<sub>‖</sub> - ε<sub>⊥</sub> of 16OBAF/SA was also measured and its temperature dependence is given in Fig. 9a. From the latter, Δε remained constant in the nematic phase and sharply dropped down to zero at the transition to the isotropic phase. This behavior is similar to that obtained in 8CB near the N-I phase transition [47]. It should be mentioned that the value of Δε in the present complexes was higher than that in dimeric compounds nOBAF (about 0.6), indicating that the incorporation of the succinic acid enhanced this parameter. This phenomenon may be due to the higher polarizability of the mixed complexes nOBAF/SA compared to the dimeric compounds of nOBAF. This is in agreement with the previous report that the high polarizability of the 9OBAF/5CB complex was the origin of the enhancement of Δε [13].

Since lower voltages suggest reduced energy consumption and help at keeping the thermal stability of the nematic phase, a lower threshold voltage (V<sub>th</sub>) is desirable for applications. V<sub>th</sub> decreased from 4 V at 96 °C to 3 V near the transition to the isotropic phase (Fig. 9b). As expected, the V<sub>th</sub> values of the nOBAF/SA complexes were lower than those of dimeric complexes of pure nOBAF, because of their enhanced dielectric anisotropy (Δε).

### 3.4. Electroconvection

The EC phenomenon has been experimentally observed and theoretically studied in nematic phases, either made of pure materials or doped with ionic species [50–52]. In the present case, this phenomenon occurred in undoped nematic phase of short-chained 7OBAF and
<figure><img src="image_9.png" /><figcaption>Fig. 7. Frequency dependence of ε' (a) and ε'' (b) for 7OBAF/SA in the N and SmA phases upon cooling from the isotropic phase.</figcaption></figure>

<figure><img src="image_10.png" /><figcaption>Fig. 8. Frequency dependence of ε' (a) and ε'' (b) for the nOBAF/SA nematic phases at 96 °C.</figcaption></figure>

<figure><img src="image_11.png" /><figcaption>Fig. 9. Evolution of (a) the dielectric anisotropy Δε and (b) the threshold Vth of 16OBAF/SA as a function of temperature.</figcaption></figure>

9OBAF. However, it is difficult to ascertain the total absence of ionic impurities in these compounds. When the applied voltage exceeded the threshold, regular EC patterns appeared (Fig. 10a), indicating the onset of the EC phenomenon. The patterns became irregular as the voltage further increased (Fig. 10b), eventually switching to a chaotic dynamic scattering mode state (Fig. 10c). In the present case, there was no significant change in the threshold voltage with temperature; however, it strongly depended on the frequency.

The evolutions of the threshold voltage ($V_1$) and the voltage required

for transition into the chaotic state ($V_2$) as a function of frequency are shown in Fig. 11. $V_1$ and $V_2$ increased from 2.6 V and 12 V at 3 Hz to 5.6 V and 17.5 V at 3 kHz, respectively. This behavior is consistent with Garbovskiy and Glushchenko [53] who observed an increase in threshold voltage with frequency. Indeed, at low frequencies, the dielectric constant is strongly frequency-dependent (Fig. 7), affecting the charge relaxation time; and consequently, the onset voltage. In contrast, at higher frequencies, the dielectric constant becomes less frequency-dependent, making the charge relaxation time more constant,
<figure><img src="image_12.png" /><figcaption>**Fig. 10.** Experimental EC patterns observed with the nematic phase of 7OBAF/SA (a, b and c) and 9OBAF/SA (d, e and f) at 96 °C at 100 Hz for increasing voltages: 5 V (a, d), 10 V (b, e) and 18 V (c, f).</figcaption></figure>

<figure><img src="image_13.png" /><figcaption>**Fig. 11.** Threshold voltage $V_{th}$ against frequency at 96 °C.</figcaption></figure>

and resulting in a higher voltage requirement at onset of electroconvection.

In this case, ions responsible for electroconvection are typically impurities that are either intrinsic to the liquid crystal material itself, introduced from the environment, or generated through the chemical processes.

Electroconvection is often caused by the presence of ionic species within the liquid crystal material. The presence of free charged species is not mandatory however [28,50–52]. It is very difficult to know from experimental measurements whether ionic contamination is involved because the detection of very low concentrations of ions is challenging. Therefore, most studies claiming for EC in ion-free nematics rely on simulations and theoretical calculations. In the present case, the occurrence of electroconvection in short-length liquid crystals, but not in long-length ones, is consistent with the observation of a strong dielectric relaxation at low frequencies that indicated the presence of a

rather high concentration of mobile ionic species (Fig. 8). The same strong relaxation was also observed in the isotropic phase, showing that it was not specific of the nematic orientation of molecules. Therefore, the present EC behavior of the short-chained 7OBAF/SA and 9OBAF/SA complexes was tentatively ascribed to ionic contaminations and explained by differences in their viscosity, dielectric anisotropy and charge carrier mobility compared to the longer-chained homologues. Indeed, increasing the alkyl chain length leads to a higher viscosity, which slows down charge transport and suppresses the fluid flow necessary for EC pattern formation. Additionally, dielectric measurements reveal that both the dielectric permittivity and the ionic conductivity decrease with increasing chain length, allowing the reduced dielectric response and ionic mobility in long-chain compounds effectively inhibit the development of EC instabilities, while liquid crystals with short chain length molecules have lower viscosity, allowing ions or charge carriers to move easier when an electric field is applied. Thus, the presence of EC patterns in short-chained liquid crystals is a direct consequence of the higher conductivity observed in dielectric measurements. This is consistent with observations by Yoshioa et al. [54] and Kobayashi et al. [55]. The concentration of ionic species responsible for EC are very low. Estimates of such concentrations have been inferred in insulating oils from measurements of their electrical conductivity and their viscosity. Thus, Bartnikas [56] gave a concentration range of $10^{-14}–2 \times 10^{-13} \text{ g} \cdot \text{cm}^{-3}$, the upper bound causing an increase of $\epsilon''$ by a factor of 10 over one decade of frequency. In such a case, “the charge carriers are supplied either by the dissociating molecules of the liquid itself or the electrolytic impurities contained within the liquid” [56].

## 4. Conclusion

We report the study of new systems formed by intermolecular hydrogen bonds. Our investigation by differential scanning calorimetry and polarizing optical microscopy revealed that they have mesomorphic properties. Longer chains promote the formation of smectic phases,
while shorter chains often favor nematic phases. The incorporation of succinic acid into fluorinated alkoxybenzoic acid led to an enhancement of dielectric anisotropy and threshold voltage. In addition, the present study showed that a high concentration of free charges may induce electroconvection phenomena in cells containing short-chained liquid crystal, which is consistent with dielectric measurements. Furthermore, the frequency dependence of the threshold values exhibits a gradual increase, in agreement with findings reported in the literature.

### CRediT authorship contribution statement

**Manel Ben Salah:** Writing – original draft, Methodology, Investigation. **Naoufel Ben Hamadi:** Resources, Methodology, Investigation, Funding acquisition. **Ahlem Guesmi:** Resources, Investigation, Funding acquisition. **Taoufik Soltani:** Writing – review & editing, Writing – original draft, Validation, Supervision, Project administration, Methodology, Funding acquisition, Data curation, Conceptualization. **Yves Chevalier:** Writing – review & editing, Validation, Supervision, Methodology, Formal analysis.

### Funding

This work was supported and funded by the Deanship of Scientific Research at Imam Mohammad Ibn Saud Islamic University (IMSIU) (grant number IMSIU-DDRSP2503).

### Declaration of competing interest

The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

### Appendix A. Supplementary data

Supplementary data to this article can be found online at https://doi.org/10.1016/j.molliq.2025.129010.

### Data availability

Data will be made available on request.

### References

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