Enhanced liquid crystal performance through alkoxybenzoic acid doping

ABSTRACT This study investigates the impact of doping agents, 4-octyloxybenzoic acid (8OBA) and 4-octyloxy-3-fluoro benzoic acid (8OBAF), on the thermodynamic and electro-optical properties of the nematic liquid crystal 5CB. The differential scanning calorimetry, dielectric, and polarising optical microscopic measurements revealed intriguing properties. The doped liquid crystal samples exhibit higher phase transition temperatures, increasing birefringence, and order parameters. Dielectric measurements disclose reduced free ion concentration with 8OBA and increased concentration with 8OBAF in the liquid crystal matrix. The threshold voltage, switching time, and rotational viscosity were also diminished in the 5CB + 8OBA mixture, whereas these parameters were increased in the 5CB + 8OBAF mixture due to the strong intermolecular interaction with liquid crystals. The results presented here demonstrate that 8OBA dopant can be used to achieve high thermal stability of the nematic liquid crystal phase with fast response times and low threshold. GRAPHICAL ABSTRACT


Introduction
Liquid crystals have a particular interest not only in fundamental sciences but also in engineering and manufacturing science, i.e. in liquid crystal display (LCD). ... They have been successfully applied in several areas including photonic applications, sensors, optical switches, and spatial light modulators [1][2][3][4][5][6][7].The advantage of such system lies in their anisotropic optic, dielectric, and electrical behaviour, allowing easy manipulation by an external magnetic or electric field.Different mesophases with a variety of behaviours have been reported based on the molecular structure and shape of the mesogen.With technological progress, the desired behaviour cannot be achieved by using pure liquid crystal (LCs) molecules.Strategies have been proposed to enhance the response time and electro-optic performances of LCs, such as employing a thin cell gap, polymer-dispersed liquid crystal (PDLC), and using LCs with low viscosity [8][9][10][11][12].Moreover, scientists have attempted to reduce the driving voltage of the LCs by incorporating nanoparticles [13][14][15][16][17][18][19][20].Nevertheless, doping LC by organic substances was used as another method to improve the physical properties of LC such as transition temperature, birefringence, dielectric properties, etc. which are relevant for various electro-optical applications [21][22][23][24][25][26][27].It was discovered that combining two or more LC constituents provides the desired properties and broadens the application of LCs in functional devices [27][28][29][30][31][32][33][34][35].For example, cholesteric liquid crystals (CLCs) with helical arrangement have been obtained by doping chiral additives into nematic liquid crystals (NLC) and are known to exhibit unique optical properties [28].Blue phase (BP) has been induced by doping some achiral bentshaped molecules into chiral NLC [29].For the BP, a wide temperature range has been revealed by doping CLCs host with bent-shaped molecules [30].Similarly, the effect of terminal-chain length on the bent-shaped molecule has been discussed.In addition, the eutectic mixture composed of the fluorinated chiral liquid crystals studied by Kumar et al. [31] has revealed a large stability of the anticlinic phase with low rotational viscosity.It has been shown that including some organic materials (hexane and water) as impurities in 5CB and 6CB LCs can increase the isotropic -nematic-phase transition temperature (T N-Iso ) and therefore increase the temperature range of the nematic phase [36].
Attempts have been made to comprehend the effect of carboxylic acids on the morphology of the nematicphase 5CB, as well as the isotropic-nematic transition temperature and viscoelastic properties [26].The inclusion of a rigid core contributes to an increase in T N-Iso , while the presence of flexible aliphatic chains produces the opposite effect.Furthermore, high rotational viscosity values have been obtained for these mixtures.Fluorine substitution in the aromatic core was also used to improve mesophase morphology, transition temperatures, viscoelastic, and dielectric properties [14,[32][33][34].
The purpose of this research is to investigate the effects of organic molecules with a rigid core and flexible aliphatic chains on the properties of nematic 5CB.For this reason, the host nematic 5CB was doped with 4-octyloxybenzoic acid (8OBA) and d 4-octyloxy-3-fluoro benzoic acid (8OBAF).The dielectric, electrooptical, and viscoelastic properties of the doped samples were evaluated and compared to the individual compounds.The molecular structures of the studied 5CB and organic dopants are illustrated in Table 1.

Experimental setups
The nematic 5CB and 4-octyloxybenzoic acid (8OBA) were procured from Sigma-Aldrich, and 4-octyloxy-3-fluoro benzoic acid (8OBAF) was synthesised based on our previous work [24].To create a mixture containing 5 wt% of benzoic and fluorobenzoic acids, an appropriate mass of each acid was dissolved in acetone.Subsequently, in a glass vial, a suitable mass of the solution was blended with a defined volume of the nematic 5CB.Then, the mixture was ultrasonicated at 90°C for 3 h to evaporate the solvent and obtain a homogeneous mixture.The transition temperatures were measured using differential scanning calorimetry (DSC) (Perkin-Elmer DSC7) with linear heating and cooling rates of 2°C/min and verified using an Olympus B×51polarizing optical microscope (POM) connected to a digital CCD camera.
To investigate the dielectric and electro-optic behaviours of the present systems, a commercially available planar cell (AWAT, Poland) with a cell gap of 6 µm was used.The liquid crystal cells were prepared using two rubbed indium-tin-oxide (ITO) coated glass plates.To promote a planar alignment of the liquid crystal molecule, the glass plates were assembled in an anti-parallel direction relative to the rubbing direction.Therefore, the cell was filled by each mixture in the isotropic phase and connected to an Agilent 33220A waveform function Table 2.The electrical parameters for the pure 5CB and 5CB doped with 8OBA and 8OBAF at 30°C.generator coupled with an amplifier (KH-KROHN-HITE MODEL 7500 Amplifier).The function was evaluated through sample-phase retardation (δ) [32].The input signal was a polychromatic light beam with an average wavelength of 648 nm.The impedance/gain phase analyser (Solartron SI1260) coupled to a 1296 dielectric interface was used to measure the real (ε') and imaginary (ε") elements of the complex permittivity (ε*) of 5CB and 5CB + organic dopants.The Perpendicular (ε ⊥ ) and parallel (ε ‖ ) permittivity were evaluated under the application of an AC voltage of 0.2 V and a DC voltage of 10 V, respectively.The dielectric anisotropy Δε = ε ‖ − ε ⊥ was measured at 10 kHz.The texture modification under the application of the electric field was used to determine the threshold voltage of the Fredericks transition (V th ).The response time (τ) was evaluated by using the square wave drive frequency f = 1 Hz with the amplitude 10 V.

Thermal properties
DSC and POM were used to explore the isotropicnematic (Iso-N) transition temperature.Figure 1 depicts the polarising optical micrographs of pure 5CB and doped samples in the nematic phase and at the T N-Iso .The plot of heat flow with temperature for pure and doped samples is shown in Figure 2. Cooling scans confirmed a well-defined exothermic peak at the Iso-N phase transition in all samples.
The DSC thermogram spectra revealed that the Iso-N transition retains its first-order character in doped samples (red and blue curve).In comparison to pure 5CB (black curve), the temperature range of the nematic phase was increased by 6.9°C for 5CB + 8OBA and by 4°C for 5CB + 8OBAF.It is worth to mentioning that the crystallisation and melting points were not influenced by these dopant compounds.Moreover, POM observations confirmed the phase transition temperatures.This behaviour indicates an increment in molecular order due to the strong interaction between 5CB and benzoic acid.Vanessa et al. have also observed that the T Iso-N increases when 5CB is doped with carboxylic acid and this increment was proportional to the molar fraction of the dopant [26].While the slight reduction in T N-Iso and the significant decrement in the rotational viscosity and the threshold voltage have been observed by Selvaraj et al. in the nematic E7 doped by organic materials [27].1) and ( 2) [37,38].
Where T is the ambient temperature, ∆n 0 is the birefringence of the LC mixture at 0 K, and β is a material parameter.Here, the aromatic structure of the 8OBA and 8OBAF materials induces the π-π strong interactions between 5CB and organic dopant molecules, increasing the polarisability (α).Notably, the organic dopants have a higher polarisability than the 5CB.As a result, the birefringence and the order parameter of the LC mixtures are increased by the present dopants.
In the case of benzoic acid, Martinez et al. [39] observed a dynamic equilibrium formed between closed dimers (1681 cm −1 ), open dimers (1700 cm −1 ), and monomeric species (>1730 cm −1 ).Moreover, the coplanarity of the cyclic dimers promotes short-range order in the plane parallel to the long axis of the dimer leading to the stabilisation of the nematic phase.In the present work, the extended nematic phase by the presence of the benzoic acid indicates the formation of cyclic dimers.This behaviour is proved by the IR spectrum of the doped sample 5CB + 8OBA (Figure 1 in the supplementary information).The absence of the intense band at ν > 3500 cm −1 associate with ν (O-H) stretching mode and the presence of the sharp band at the wavenumber ν = 1683 cm −1 of the C=O interaction suggests the closed dimer of the HBLC.

Dielectric properties
Figure 4 presents the real (ε') and imaginary (ε") parts of the permittivity of pure and doped samples over a frequency ranging from 1 Hz to 10 MHz.As seen, the host LC and organic dopants have differently to each other.When the nematic 5CB was doped with 8OBA, ε' increased and ε 00 decreased; whereas for 5CB + 8OBAF ε' decreased and ε 00 increased.
Additionally, the curve of ε 00 shows two frequency relaxation modes in the undoped and doped samples.The low-frequency mode related to space charge is approximately at f = 10 Hz for 5CB + 8OBA and 100 Hz for 5CB + 8OBAF.The high-frequency relaxation mode, known as Soft mode, was observed at f ≈ 3 10 6 Hz for 5CB + 8OBA and 4 10 6 Hz for 5CB + 8OBAF attributed to the reorientation of the director around the short axis.When compared to pure nematic 5CB, the relaxation  frequency (f r ) of both modes is shifted to higher values for 5CB + 8OBAF and lower values for 5CB + 8OBA.
According to the theoretical model demonstrated by Uemura [40], ε' and ε" are proportional to the number of free ions in the sample at low frequency.So, the analysis of the dielectric measurement provides a useful method to calculate the number of the space charges in a sample.The frequency dependence of ε' and ε" is expressed by the following Equations [40][41][42]: Where d is the cell gap, q is the electric charge, k B is the Boltzmann constant, ε 0 is the permittivity in free space, T is the absolute temperature, ε b ′ is the intrinsic dielectric constant of the LC bulk, and n is the bulk ionic concentration.The ion concentration of all samples was deduced by fitting the measured ε″ and ε' into Equations ( 3) and ( 4), respectively.In Figure S2 (in SI file), ε″(f), ε′ (f), and the loss tangent for 5CB + 8OBA and 5CB + 8OBAF, were depicted as an example.It was found that n = 14.4610 19 m −3 , 46.6310 19 m −3 , and 53.19 10 18 m −3 for 5CB, 5CB + 8OBAF and 5CB + 8OBAF, respectively.As expected, the ion concentration of 5CB + 8OBAF is higher than that of the pure one, and the lowest value is obtained in 5CB + 8OBA proving the ionic adsorption behaviour in this latter.The temperature dependence of dielectric anisotropy at 10 kHz of doped and pure samples is illustrated in Figure 5.
As shown in Figure 5, Δε is slightly increased when 8OBAF is dispersed in 5CB, but it decreases by 100% in the case of 8OBA with respect to 5CB.It is well known that the polarisability, dipole moment, and aspect ratio are important parameters that can influence dielectric behaviour.
In LC phases, the dielectric anisotropy was related to dipole moment, the order parameter, and polarisability [43]: Where N is the molecular number density, Δα is the polarisability anisotropy, F and h are the reaction field factor and the cavity factor, respectively, ε 0 is the vacuum permittivity, θ is the dipole moment orientation angle relative to the long principal axis, and K B is the Boltzmann constant.
Analysing the results of the dielectric anisotropy revealed that the small dipole moment of 8OBA is the principal reason for the decrease of Δε of 5CB + 8OBA.While in 5CB + 8OBAF, the high values of α and S compared to pure 5CB compensate for the small dipole moment, explaining the slight increment of Δε.In addition, for all cases, there is a sharp increase in Δε at the N-Iso phase transition generous value of T Iso-N , which is consistent with POM and DSC measurements.

Conductivity analysis
The increment in ε 00 indicates that the conductivity of the nematic phase is increased by the presence of the fluorinated 8OBAF compound.The DC conductivity (σ) is calculated using the following expression [44,45]: The frequency dependence of the DC conductivity (σ) for the pure 5CB and doped samples is illustrated in Figure S3 (in SI file).As seen, σ for 5CB + 8OBA is smaller compared with pure 5CB, while the high value of σ is obtained for 5CB + 8OBAF.As expected, σ increases with increasing temperature due to the increment of the free ions concentration and the enhancement of the ion transport.This behaviour becomes more significant at higher temperatures, which is in good agreement with the literature [44,45].Furthermore, three regions can be distinguished: in the lower frequency region the conductivity (σ) increases slowly with increasing frequency, which describes the electrode polarisation.The second region is the plateau region, which represents the DC conductivity.Finally, at higher frequencies, the conductivity increases strongly with frequency.The frequency dependence of σ is expressed by the Jonscher power-law expression [45]: Where σ D is the DC conductivity, f c stands for the characteristic frequency, and α is a parameter defining the degree of interaction between the mobile ions and their surroundings (0 < α < 1).The DC conductivity of all samples was obtained by fitting the calculated σ' into Equation (8).It was found that σ D = 2 10 −7 , 510 −8 and 1.210 −6 for 5CB, 5CB + 8OBAF, and 5CB + 8OBAF, respectively.The value of σ D in 5CB is comparable to that obtained in nematic LCs [45].Higher values of conductivity in this doped sample compared to that for a pure 5CB may be due to the fact that 8OBAF introduced into the 5CB contained a certain amount of ions.In addition, this intermolecular interaction and the polar Fluor atom help the dilution of the ions in the sample volume.

Equivalent electrical circuit analysis
A suitable equivalent electrical circuit (EEC) model that can analyse the experimental measurements was used to investigate the impact of 8OBA and 8OBAF on the impedance and electrical behaviour of the nematic phase.Several EECs was proposed in order to investigate and reproduce the impedance spectra of nematic LCs [46,47].Firstly, the most basic EEC model was used to analyse the impedance response of an LC material formed by the electrode resistance R CR connected in series with a set of the bulk LC resistance R LC ð Þ in parallel with the bulk LC capacitance C LC ð Þ.The previous mentioned EEC was then extended to create a model describing ion drift and diffusion by incorporating the electrical double-layer capacitance, which describes the space charges near the electrodes, and the finite diffusion Warburg element W ð Þ [46].Herein, the equivalent circuit used is the extended model, as seen in Figure S4 (in SI file).The generalised finite-length Warburg impedance Z W ð Þ in this case can be described by the following expression [46,47]: The parameters W Sr and W Sc are related to the surface concentration ion and diffusion constant as given: where ω is the angular frequency, N A is the Avogadro's number, R is the gas constant, T is the temperature, A is the surface area, D is the diffusion coefficient of the mobile ions, n S is the surface concentration of the ions, F is the Faraday constant, and δ N is the thickness of the Nernst diffusion layer.The electrical parameters of the EEC were evaluated by fitting the experimental data (Figure S4 in SI file) and are tabulated in Table 2.The resistivity values of the connectors and electrodes R CR of the pure 5CB and doped samples are practically the same within the experimental error bar.However, the C LC presents a small changes indicating that these both parameters are not influenced by the presence of the NPs.Interestingly, significant changes were observed for R LC and C DL values.The introduction of 7OBAF lowers the R LC value, indicating an increase in conductivity, whereas the R LC of 5CB + 8OBA is lower than that of pure 5CB.On the other hand, the value of C DL increases from 3:9nF for the undoped sample to 6:8nF for the one doped by 8OBAF and 8:8nF for the one doped by 8OBA.The formation of an electrical double layer as a result of ion accumulation near the electrodes may account for the increase in C DL values in doped samples.This behavior is consistent with the findings of Urbanski et al. [46].The electrical parameters procured from pure 5CB are lower than those of 5CB doped with gold nanoparticles [46].In addition to the previously mentioned ionic behaviors, we also addressed on mobile and diffusion charge carriers moving through the confined spaces of the LCs.From measurement of impedance spectroscopy spectra, we can calculate the mobility (µ) and diffusion coefficient (D).Ions travers the distance (d) between the electrodes during a transit period t tr , corresponding to half a cycle of the field.
The diffusion coefficient is deduced from mobility of singly charged ions [43].
Where μ is the ion mobility, V 0 is the effective voltage across the cell's electrodes, and α ¼ C DL C LC is a coefficient that indicates the degree of attenuation of the voltage inside the cell.The obtained parameters are tabulated in Table 3.The results for the 8OBA compound showed an increase in these parameters for the 8OBAF material and a decrease in the case of 8OBA compared to those in 5CB.This behavior is consistent with what was observed in [47] and can be attributed to the σ D evolution.According to the following Equations [47], σ D was proportional to the ion density (n) and the ion mobility (µ) or the diffusion coefficient (D): From Equation ( 14), σ D is related to D as: Therefore, we can conclude that the increase in D and µ in the 5CB + 8OBAF is caused by an increase of σ D in this sample.These parameters have the same order as that obtained in 5CB doped with CdSe/ZnS quantum dots [48].

Electro-optic properties
Figure 6 illustrates the temperature dependence of the threshold voltage (V th ) for pure 5CB and doped samples.It is evident from this figure that the threshold voltage increases with rising temperature for all samples.Thus, the presence of benzoic acid has a significant impact on V th .The threshold voltage is expressed by the following Equation [27,49]: Here K 11 is the splay elastic constant.V th is inversely proportional to Δε. Herein, doping 5CB by 8OBAF or 8OBA leads to an increase V th for the first mixture and a decrease V th for the second one.By analysing Equation ( 16), this behaviour is unexpected.In view of the fact that doping 5CB by 8OBAF or 8OBA provide the highest value of ∆ε for 5CB + 8OBA mixture and deprive the lowest value of ∆ε for 5CB + 8OBAF mixture.This unexpected behaviour, however, can be explained using the dielectric results.The decay of the total quantity of free charges is the key to dropping the threshold voltage V th .Hsiao et al. investigated the ion effect extensively [50].They demonstrated that the decrease of ionic concentration and the generation of electrical double layers by ion accumulation near the electrodes induces a repulsion interaction and reduces the conductivity, which leads to the decline of the effective electric field across the cell.
Figure 7 illustrates the response time of 5CB, 5CB + 8OBA, and 5CB + 8OBAF doped LC cells.All samples present classical temperature dependence.The increment in τ when 5CB is doped by the organic compounds is attributed to the increment in the rotational.This result is expected because, according to the POM and DSC measurements, the order of doped samples is greater than in pure 5CB.As the response time and the rotational viscosity are proportional to the parameter order, the response time in doped samples would be higher than in pure 5CB.

Viscoelastic properties
The electro-optic property was influenced by the viscoelastic performance.Thus, the change of the response time and threshold voltage of 5CB doped with the benzoic acids indicates that the viscoelastic behaviour was also affected.Rotational viscosity γ and elastic constant (K 11 ) are the principal parameters that describe the viscoelastic behaviour of a liquid crystal.The splay elastic constant K 11 was calculated using the measured values of Δε and v th and Equation (8). Figure 8 illustrates the behaviour of the splay elastic constant (K 11 ) as a function of the temperature for cells filled with pure and doped nematic 5CB.
The obtained values of K 11 for the pure 5CB are in good agreement with the previous results [49].It is clear from Figure 8 that the presence of the benzoic acid in the nematic matrix increases considerably the splay elastic constant mostly for 8OBAF.Particularly, the value of K 11 for 5CB + 8OBA is about 73% lower than that of the pure 5CB at 28°C.This decrease can be attributed to the reduction of the anisotropic dielectric since this parameter is proportional to Δε (Equation ( 16)).The strong increase in K 11 for the case of 5CB + 8OBAF indicates a strong interaction between organic dopant and the host liquid crystal.
Moreover, γ is related to the response time by the following expression [27,49]: Using Equation ( 17), γ was calculated and illustrated in Figure 9.As the temperature increased, the rotational viscosity of the samples was reduced, and hence the response time tends to decrease.Importantly, γ of 5CB + 8OBA is lower than that of 5CB, which can be explained by the reduction of the concentration of free charges.The high value of γ in the case of 8OBAF can be explained by two reasons: the first one is the increment in the space charges and the second one may be the additional for induced by the intermolecular interaction due to the polar fluorine atoms.Moreover, in the liquid crystal system, rotational viscosity obeys the Arrhenius law [35]: Where E a is the activation energy for molecular rotation, k B is Boltzmann's constant, and T is the absolute temperature.These curves for all the samples give a straight line indicating that all samples present the Arrhenius-type behaviour.From the slope (Figure 10), E a for the samples are deduced, and it was found that E a   = 0.41, 0.35, and 0.51 for host nematic, 5CB + 8OBA, and 5CB + 8OBAF, respectively.The lower activation energy for 5CB + 8OBA permits the molecules to rotate easily.

Conclusion
In this study, the impact of alkoxybenzoic and fluorinated alkoxybenzoic acid on the properties of NLC (5CB) was investigated using complementary methods.Our investigation revealed a significant increase in the nematic isotropic transition temperatures, indicating an increase in the order parameter within doped samples.Electro-optic measurements coupled with dielectric spectroscopy are used to investigate the dielectric and viscoelastic behaviour of pure and doped samples.A key novelty of our results lies in this substantial enhancement, offering the potential to elevate liquid crystal materials' threshold voltage, rotational viscosity, and response time through the use of the organic dopant 8OBA, which captures free ions.These findings hold promise of practical applications in advancing of organic material design, particularly in areas such as advanced displays and electro-optic devices.

Disclosure statement
No potential conflict of interest was reported by the author(s).

Figure 3
Figure3depicts an analysis of the thermal behaviour using temperature dependencies of birefringence.The transition temperature T N-Iso is linearly proportional to the order parameter S and the birefringence ∆n, according to Equations (1) and (2)[37,38].

Figure 5 .
Figure 5. (Colour online) Variation of the dielectric anisotropy as a function of the temperature for pure 5CB, and doped samples at 10 kHz.

Figure 6 .
Figure 6.(Colour online) Temperature dependence of the threshold voltage for the pure for pure 5CB, and doped samples.

Figure 8 .
Figure 8. (Colour online) Temperature dependence of K 11 for the pure 5CB, and doped samples.

Figure 9 .
Figure 9. (Colour online) Temperature dependence of the rotational viscosity for the pure 5CB, and doped samples.

Figure 10 .
Figure 10.(Colour online) Arrhenius plots for the rotational viscosity of the pure 5CB, and doped samples.

Table 1 .
The

Table 3 .
Ion diffusion coefficient, mobility and concentration of nematic LC + 8OBA and 8OBAF at 30°C.