1 Introduction

Liquid crystal exists between the solid and isotropic liquid phases [1], characterized by an orientational order, usually long-range and possible positional order partially. Technologically, liquid crystals have become a part of our lives, first appearing in wristwatches and calculators, but now being used in all types of flat panel displays [2, 3] of mobile phones, laptops, desktops, television screens, advanced instrumentation, thermometers, optical shutters, waveguides, spatial light modulators etc. Amongst the various types of liquid crystals [4,5,6,7], H-bonded Liquid Crystals (HBLCs) have gained momentum since Katoꞌs report [8]. The molecular structure of the chemical moieties employed to form HBLCs significantly tunes the properties required for various devices as evident from the literatures on applications of HBLCs [9,10,11]. Various desired properties are achievable by the proper combination of the donors and acceptors involved in developing HBLCs. Moreover, HBLCs exhibit liquid crystalline properties at ambient temperatures [7]. It is observed that both the proton donor and acceptors are mesogenic or not, or either of them is mesogenic[12]. It is stated [8] that the length of the rigid core and the other intermolecular interactions increases with intermolecular H-bond and stabilizes the LC phases. The phase stability of the HBLC is found to vary with the fresh interactions achieved/lost with H-bond formation. In addition the presence of hydrogen bonds in linear/lateral direction can also control the LCs phase range and induce new phases [13, 14].

Pyridine moieties and their derivatives are versatile materials as is evident from their numerous applications such as in pharmaceuticals [15], as catalysts [16], medicinal applications [17], in chemosensing applications [18], Recent developments in the synthesis and applications of pyridines is available in the literature [19]. Numerous liquid crystals have been synthesized with pyridine-based derivatives. Pyridines act as excellent hydrogen bond acceptors, which is helpful for the formation of liquid crystals [20,21,22,23,24].

Halogen atoms are electronegative, and their presence in a molecular structure adds to longitudinal or transverse dipole moments based on their position in the molecular structure [25, 26]. Amongst the halogen atoms, fluorine, with its small size and high electronegativity, is at the forefront in contributing to the physical and chemical properties of the molecules [27]. Fluorine containing compounds are found to influence the physiological activity of bioactive compounds [28].

In the field of liquid crystals, the low-temperature melting LC materials containing fluorine substitution is successfully obtained by Gray et al. [29] and other researchers recently [30, 31]. Fluorine-containing liquid crystals were found to be very stable materials with long lifetimes when exposed to light, and this paved the way for LCD-TFTs over the CRTs found in older televisions [32]. Liquid crystalline molecules generally possess unique physical and optical properties and exhibit strong nonlinear optical effects, which play an essential role in innovative applications.

Our research team has carried out studies on similar H-bonded liquid crystals [33,34,35,36,37,38,39], and the present report continues our work wherein a new Schiff base (4-pyridyl)-benzylidene-4ꞌ-fluoro aniline is synthesized, and H-bonded compounds are obtained through their interactions between alkyloxy benzoic acids (nOBAs). Propyloxy benzoic acid (3OBA) is exhibiting nematic phase with isotropic temperature of 158.8 °C and crystallization at 150.6 °C while dodecyloxy benzoic acid (12OBA) is exhibiting nematic and smectic C phases with isotropic temperature at 136.4 °C and crystallization at 94.4 °C. These acids are chosen as they exhibit different types of phases and possess variation in their chain length, isotropic and crystallization temperatures. The study is carried out to observe the effect of the new linear hydrogen bonding interactions on the above parameters of nOBs. The H-bonded compounds are observed to display a wide range of mesomorphism with a decreased isotropic and crystallization temperatures. The results on the effect of fluorine as a substituent are also discussed. The nonlinear optical properties of the representative compound is studied using DFT theoretical approach.

2 Experiment

2.1 Materials chosen and methods carried out

The commercially available compounds viz., 4-Pyridinecarboxaldehyde, 4-Fluoroaniline and alkyloxy benzoic acids, are bought from Aldrich, USA. Ethanol and Tetrahydrofuran (THF) of AR grade are acquired from E. Merck, India Ltd.

ATR Shimadzu-IR Spirit analyser is used to obtain the IR spectra. 1H NMR is recorded with 400 MHz Bruker spectrophotometer using CDCl3 as solvent, and the mass spectrum is recorded with a Thermo Scientific (Massachusetts, US) LC–MS with Dionex Ultimate 3000 liquid chromatograph interfaced with an LTQ XL linear ion trap mass spectrometer. The optical textures is confirmed by SDTECHS (India) Polarizing Optical Microscope (POM). Thermal data is collected from Shimadzu DSC-60.

2.2 Synthesis and characterization of schiff base and H-bonded compounds

2.2.1 Synthesis of (4-pyridyl)-benzylidene-4'-fluoro aniline

The proton acceptor, viz., (4-pyridyl)-benzylidene-4'-fluoro aniline (PyBF) is prepared by following the reported [26, 33,34,35,36] procedure (Scheme 1). Accurately weighed 4-fluoro aniline (0.01 mol) is dissolved in around 2 mL of ethanol in a 100-mL beaker. Similarly, 4-pyridinecarboxaldehyde (0.01 mol) is dissolved separately in 2 mL ethanol. The two solutions are mixed slowly and refluxed at 60–65 °C for six hours. On evaporation of ethanol, the beaker is allowed to cool to room temperature. A pale yellow product solidifies. It is then recrystallized using ethanol as the solvent. The yield is about 74%.

Scheme 1
scheme 1

Scheme of (4-pyridyl)-benzylidene-4'-fluoro aniline, (PyBF)

Full size image

2.2.2 Synthesis of PyBF:nOBAs

Equal amounts (0.01 mol each) of PyBF and 4-alkyloxybenzoic acid (nOBA; where n = 3, 12) is mixed thoroughly in the presence of 5 mL of THF as a solvent to get a homogeneous mixture. The solid product is formed once the THF evaporates. The H-bonded compounds are labelled as PyBF:3OBA and PyBF:12OBA. The reaction is given as Scheme 2. It is noticed that one of the compounds bearing the alkyloxy chain lengths of 12 is also reported by Sayed Z Mohammady [40] wherein the preparation and DFT study of the compounds are discussed while our present work describes the preparation, mesomorphic behavior of the compounds, nonlinear optical properties, influence of fluorine sustituent on mesomorphism and the comparison of crystallization temperatures of the new compounds with pure nOBA’s.

Scheme 2
scheme 2

H-bonded compounds of PyBF with 4-alkyloxybenzoic acids

Full size image

3 Results of the work carried and discussion

(4-pyridyl)-benzylidene-4′-fluoro aniline, (PyBF); IR (cm−1): 1627 (C=N str of Schiff base), 1592 (C=N str. Of pyridine moiety), 831 and 815 (Out of plane bending of C-H) (Fig SF1); PyBF:3OBA; 2972 (C-H str.), 2945 (-OH str.of H-bonding) (A-type fermi band), 2434 (B-type fermi band), 1926 (C-type fermi band), 1689 (C=O str.of acid), 1600 (C=N str. of pyridine), (Fig SF2); PyBF:12OBA; 2936 (C–H str.), 2917 (–OH str.of H-bonding), 2847 (A-type fermi band), 2469 (B-type fermi band), 1679 (C=O str.of acid), 1604 (C=N str. of pyridine), (Fig SF3); PyBF; 1H NMR (400 MHz, CDCl3, ppm): δ 7.06 (2H, Ar–H), 7.23 (2H, Ar–H), 7.68 -7.72 (2H, Ar–H), 8.41 (1H, Ar-N), 8.73 (2H, Ar–H) (Fig SF4); PyBF; MS (m/z of C12H9N2F): 200.98 (M-1) (Fig SF5).

In the FTIR spectrum SF1, -NH stretching peak above 3100 cm−1 is not observed, showing that free –NH2 of 4-fluoro aniline is absent in the product. Also, the peaks in the range of 1715–1695 cm−1, corresponding to the carbonyl stretching (i.e., C=O of 4-pyridine carboxaldehyde), are absent, indicating the involvement of aldehyde in the condensation product. Additionally, the peak at nearly 3073 cm−1 indicates that pyridine's C–H stretching is preserved unbroken with its aromatic ring. The peak at 1627 cm−1 implies the C=N stretch of the Schiffꞌs base. The peak at 1592 cm−1 indicates the pyridine moiety's C=N stretch, and the double peak at 831 cm−1 and 815 cm−1 implies the out-of-plane bending of C-H analogous to the pyridine moiety confirming the formation of imine group in PyBF. In the present series, the N-atom in the imine group of 4-pyridine carboxaldehyde is the acceptor, and the H-atom in the carboxyl group of nOBA (4-alkyloxybenzoicacids) is the proton donor.

The nOBAs are dimers at room temperature and hence a wide and intense absorption peak in the range of 2950–2547 cm−1, shows the involvement of -OH of nOBAs in intermolecular HB. The distinctive C=O stretching of the acid group is usually observed in 1675–1690 cm−1. The peaks at 1602 cm−1 and 1512 cm−1 attributes to the skeletal vibrations of the aromatic system [41].

The FTIR spectrum SF2, shows a peak at 2972 cm−1, which corresponds to the stretching of the C-H bond of pyridine with an aromatic ring. Broad peaks at 2950–2547 cm−1 affiliated with –OH stretching observed in pure nOBA are reduced to 2945 cm−1 along with B-type fermi band at 2434 cm−1 and C-type fermi band at 1926 cm−1 indicating the involvement of –OH in H-bonding. The 1689 cm−1 and 1600 cm−1 peaks imply the C=O stretch of an acid and the C=N stretch of the pyridine moiety, respectively.

Similarly in the FTIR spectrum SF3, peak at 2972 cm−1, corresponds to the stretching of the C–H bond of pyridine with an aromatic ring. The H-bonding –OH stretch is observed at 2917 cm−1 and 2847 cm−1 as A-type fermi band and 2469 cm−1 as B-type fermi band. The 1679 cm−1 and 1604 cm−1 peaks imply the C=O stretch of an acid and the C=N stretch of the pyridine moiety, respectively.

It may be observed that the fluorine substituent of the synthesized Schiff base can exhibit weak hydrogen bonding interaction with -COOH of the proton donor as compared to hydrogen bonding interactions of nitrogen atom of pyridine of the Schiff base. It may be due to the lone pair of electrons on the nitrogen atom of pyridine acting as a better acceptor. In the FTIR spectra of HBLCs, the weak hydrogen bond interactions of fluorine may be submerged in the –OH stretching region of 2940–2300 cm−1.

3.1 Phase transitions by POM and DSC

Microscopic slides are prepared to observe the LC phases of the H-bonded compounds. The sample slide is mounted on a heating block configured on a stage between crossed POM setup polarisers. The H-bonded compound PyBF:3OBA exhibited a nematic phase while the the H-bonded compound PyBF:12OBA smectic phase. The microscopic nematic droplets of PyBF:3OBA at 85 °C and focal conic textures of PyBF:12OBA at 72 °C is given in Fig. 1a, b respectively.

Fig. 1
figure 1

a nematic droplets exhibited by PyBF:3OBA at 85 °C (10×) b focal conic textures exhibited by PyBF:12OBA at 72 °C (10×)

Full size image

The temperatures at the phase transition and the corresponding changes in enthalpy are carried out by Shimadzu DSC60 instrument. A few milligrams of the sample is crimpled in a pre-weighed aluminium pan with lids and crimpled. The mass of the sample is noted and the sample is run at 5 °C/min. Multiple runs are carried out for the reproducibility of the data. The thermograms of PyBF:3OBA and by PyBF:12OBA is presented in Fig. 2a, b, respectively.

Fig. 2
figure 2

Thermograms of a PyBF:3OBA b PyBF:12OBA

Full size image

The H-bonded compound, PyBF:3OBA on heating, melted at 84.39 °C with a change in enthalpy of 57.9 J/g and converted into liquid at 111.96 °C with a change in enthalpy of 2.69 J/g. On cooling, it started to form a nematic phase (as noticed in POM studies) at 109.12 °C with a change in enthalpy of 2.9 J/g and converted into crystal at 64.4 °C with a change in enthalpy of 50.32 J/g. Since this phase is observed in the heating as well as cooling cycles, the transitions correspond to the enantiotropic phase.

The H-bonded compound, PyBF:12OBA melted at 81.28 °C with an enthalpy change of 61.11 J/g on heating and transformed into its isotropic liquid at 118.48 °C with an enthalpy change of 6.90 J/g. On cooling, it started started developing focal conic fans (as noticed in POM studies) at 116.02 °C with an enthalpy change of 8.38 J/g and on additional cooling, the transformation to crystal takes place at 49.80 °C with an enthalpy change of 40.28 J/g.

The temperatures at transition with their enthalpy changes (in parenthesis) for the H-bonded compounds is presented in Table 1.

Table 1 Temperatures (◦C) at transitions with change in enthalpy (J/g) for PyBF:nOBA compounds
Full size table

It is evident from various reports [42, 43] that several factors like polarity, intermolecular interactions, electronic factors etc. govern the thermal stability of the mesophase. The presently synthesized calamitic liquid crystals possess polarity in their molecular structure due to the presence of terminally substituted fluorine of the aniline. The aniline condensed with pyridine induces polarity in the resulting Schiff base. This Schiff base which is involved in the H-bond formation with alkyloxy benzoic acids alongside inducing polarity, also enhances the length of the molecule and induces longitudinal dipole moment. This creates a favourable environment for the stacking of molecules, which enhances the mesomorhism found in the present H-bonded compounds. In Yuan Chen et al.'s work [44] fluorinated liquid crystals were obtained by introducing cyclohexyl and bicyclohexyl moieties in the molecular core to get a low viscosity and enhanced dielectric anisotropy materials. The fluoro substituents are chosen owing to their high electronegativity and small size. Various liquid crystalline materials were obtained by considering mono, di and trifluoro substituted compounds and the effect of fluoro substitution on mesomorphism is discussed. In the work done by Sie Tiong Ha et al. [45], imine linkage and reverse imine linked Schiff bases are synthesized by considering terminal substituents as halogens. Halogens are chosen as they are polar, possess strong dipole moments, and can induce mesomorphism. It is observed that fluoro-substituted liquid crystals have lower transition temperatures than chloro, bromo and iodo-substituted moieties. In work done by Sie Tiong Ha et al. [46] various Schiff bases were synthesized. It involved an ester with an aldehyde counterpart, and a substituted hydroxyl group was used to obtain a Schiff base upon condensation with various laterally substituted anilines. The Schiff base with fluoro substituted aniline is found to exhibit the smectic mesomorphism over a short range of 6 °C from 80.17 °C up to the crystallization at 74.24 °C. It is discussed that the more electronegative fluorine atom reduces the degree of molecular order leading to a shorter range of mesomorphism. In our present work the synthesized H-bonded compounds involving a fluoro-substituted aniline exhibit a wide range of mesomorpism after the formation of the hydrogen bonding. Due to its dimeric nature, the nOBAs are inherently mesogenic (from propyl homologue onwards). Even though the lower homologues viz., methyl and ethyl homologues are dimers, they are found to be non-mesogenic as the length-to-breadth ratio is insufficient to exhibit mesomorphism. The dimers possess a complementary H-bond and are called as homosynthons. When these dimers are diluted and mixed with proton acceptors containing pyridyl units, they form stronger H-bond and are called heterosynthons. The mesomorphic thermal stabilities of nOBAs and their H-bonded compounds with PyBF are given in Table 2a. It is obvious that the mesomorphic thermal range is considerably increased after the formation of the heterosynthon. The H-bonds in nOBAs is complementary wherein the proton acceptor's highly electronegative oxygen atom is involved in forming hydrogen bonds with the other acid acting as the proton donor. In the present series of HBLCs, the nitrogen atom of the Schiff base is a proton acceptor, while nOBAs are the proton donors. This leads to linear H-bond replacing the complementary one that was initially present in the nOBAs. Also, the fluorine atom present in the rigid rod moiety of the H-bonded compound induces polarity and enhances the longitudinal dipole moment in the compounds. There are three rigid cores in the current series of H-bonded compounds, while the the dimeric nOBAs have only two. Also in comparison of the H-bonded compounds of the current series with the earlier reported [25, 26, 33, 35] HBLCs by the same group, the Schiff base possessed a long alkyl chain in place of fluorine atom while the proton donors were the substituted carboxylic acids.

Table 2 Comparison of a. Mesomorphic thermal range b. crystallization temperatures of PyBF:nOBA with pure
Full size table

In the present series of H-bonded compounds a, relatively large ∆H values are observed Isotropic—SmA transition which may be due to the improved self-assembly of the pyridyl and acid moieties facilitated by soft-covalent interactions. The crystallization temperatures have drastically decreased compared to pure nOBAs, as shown in Table 2b. The H-bonded compounds are also found to have lower clearing temperatures than nOBA moieties, and mesomorphism is realized towards ambient temperatures.

The collective contribution of the linear hydrogen bond and the electronegative fluorine atom inducing polarity and net dipole moment in the molecule enhances the thermal mesomorphic range in the synthesized H-bonded compounds.

A comparison of molecular structure of the present series of the compounds is done with molecular structure containing flexible alkyl chain of sixteen carbon atoms in the place of fluorine of the present study [36]. The molecular structures of the compounds is given as Fig. 3.

Fig. 3
figure 3

Molecular structures of H-bonded LCs

Full size image

It is observed that the phases exhibited by the present H-bonded compounds are nematic and smectic A, while that in the compounds of sixteen carbon chain are smectic C and F. It can be observed that the alkyl chains are donating electrons inductively into the benzene ring while the fluorine atoms of the present series possess the electron withdrawing nature. It is discussed that the longer flexible chains induce steric intrusions into the adjacent smectic layers and creates an orientational disorder and induces the tilted phases of smectic C and F. The small fluorine atoms of the present study does not exhibit steric intrusion but helps in stacking of the molecules due to its polar nature leading to the arrangement of the molecules with orientational and positional order and exhibiting smectic A phases.

3.2 Computational details

All the DFT calculations were carried out using Gaussian 09 software. The liquid crystalline compound PyBFA:12OBA is studied as a representative compound. The compound is optimized by using DFT with the conventional basis set 6-311G (d, p) paired with the Lee, Yang, and Parr (B3LYP) hybrid functional for the correlation component and the Becke three hybrid density functionals for the exchange part [47,48,49,50].

3.2.1 Nonlinear optical properties (NLO)

NLO properties play an important role in innovative applications, including optical computing, optical storage, signal processing and among others. Liquid crystalline molecules generally show strong nonlinear optical effects due to their long molecular structure and property of aligning in the applied electric field. The compound PyBF:12OBA is optimized (Fig. 4a) and the total dipole moment, total polarizability, asymmetry parameters, anisotropic polarizability, and first order hyperpolarizability is calculated (Table 3) using DFT with B3LYP functional and 6-311G (d.p) standard basis set. Table 3 displays the components of polarizability and hyperpolarizability tensors, including isotropic polarizability (αiso), anisotropic polarizability (Δα), asymmetric parameters(ƞ), total molecular dipole moment (μtotal), total polarizability (αtotal), first order hyperpolarizability (ꞵ0). It is shown that components of dipole moment, polarizability, and hyperpolarizability have greater magnitudes which is validated by the compound’s smaller energy gaps [51,52,53]. The molecule shows improved NLO characteristics, which are determined by following formulas:

$$\mu_{total} = \sqrt {\mu_{x}^{2} + \mu_{y}^{2} + \mu_{z}^{2} }$$
(1)
$$\alpha_{total} = \frac{1}{\sqrt 2 }\sqrt[{}]{{\left( {\alpha_{xx} - \alpha_{xy} } \right)^{2} + \left( {\alpha_{yy} - \alpha_{zz} } \right)^{2} + \left( {\alpha_{zz} - \alpha_{xx} } \right)_{{}}^{2} + 6\alpha_{xy}^{2} + 6\alpha_{xz}^{2} + 6\alpha_{yz}^{2} }}$$
(2)
$$\eta = \frac{{\alpha_{xx} - \alpha_{zz} }}{{\alpha_{xx} - \alpha_{iso} }}$$
(3)
$$\Delta \alpha = \alpha_{xx} - \frac{{\alpha_{yy} + \alpha_{zz} }}{2}$$
(4)
$$\alpha^{iso} = \frac{{\alpha_{xx} + \alpha_{yy} + \alpha_{zz} }}{3}$$
(5)
$$\beta_{x} = \beta_{xxx} + \beta_{xyy} + \beta_{xzz}$$
(6)
$$\beta_{y} = \beta_{yyy} + \beta_{xxy} + \beta_{yzz}$$
(7)
$$\beta_{z} = \beta_{zzz} + \beta_{xxz} + \beta_{yyz}$$
(8)
$$\beta_{0} = \sqrt {\beta_{x}^{2} + \beta_{y}^{2} + \beta_{z}^{2} }$$
(9)
Fig. 4
figure 4

a Optimized structure of PyBF:12OBA b ESP of PyBF:12OBA c HOMO of PyBF:12OBA d LUMO of PyBF:12OBA

Full size image
Table 3 Nonlinear optical (NLO) properties of PyBF:12OBA
Full size table

3.2.2 Electro static potential (ESP)

DFT studies have been performed to provide additional insight into the molecule to more precisely comprehend its molecular behavior and structural conformation. The compound's electrostatic potential distribution can be more precisely studied with the use of the DFT technique. The compounds' electrostatic potential surface (Fig. 4b) illustrates the molecule's electrophilic and nucleophilic characteristics and is a crucial tool for researching the reactivity of compounds. The light blue color in the ESP graphs shows the largest amount of the positive zone, where the nucleophilic reaction occurs. In contrast, the reddish region represents the negative region, where the electrophilic reaction occurs. The compounds' electrostatic potential surfaces are found to be sterically bulky. Furthermore, it is noted that the compound's fluorinated terminal chain had a low electrostatic potential, as evidenced by the dull reddish color in the accompanying image. The dark red color in the picture represents the compound's greatest electronegativity potential, and most of the electrophilic reaction occurs in the N and O atoms. The color sky-blue symbolizes the compound's positive electrostatic potential. Most of the compound's positive electro potential is found in the alkyl terminal chain and core.

The total electrostatic potential (ESP) analysis shows that electron density is spread homogeneously almost throughout the compound, which increases the application of the LCs. Different colors signify different values of the compound's electrostatic potential surface, and the potential increases in the following order: red, orange, yellow, green, and blue.

3.2.3 HOMO–LUMO and reactivity parameters

Studies on the highest occupied molecular orbitals (HOMO) and lowest unoccupied molecular orbitals (LUMO) are critical for understanding the optical and electrical properties of liquid crystalline substances. The HOMO and LUMO of PyBF:12OBA is given as Fig. 4c, d respectively. The energy gap is essential for understanding the stability and reactivity of liquid crystalline materials. A larger energy gap indicates an inability to interact with other molecules and lower reactivity, whereas a lower energy gap indicates easy contact with other molecules and polarization. The molecule in this work has a reduced energy gap and exhibits unusual optoelectrical behavior due to its easily polarising nature [51, 52, 54,55,56].

In this study the standard basis set 6-311G (d, p) with B3LYP functional was used to calculate the HOMO LUMO, energy gaps and other reactivity parameters. Chemical reactivity parameters play an important role in understand liquid crystalline compounds’ molecular behaviour. The chemical reactivity parameters such as Ionization potential (I), Electron affinity (A), Chemical hardness (ƞ), Chemical potential (µ), electrophilicity index (ω) are calculated using DFT with B3LYP functionals (Table 4). These parameters are calculated using the given formulas:

$${\upmu } = { }\frac{ - I + A}{2}$$
(10)
$$\eta = {\text{I}} - {\text{A}}$$
(11)
$${\upomega } = { }\frac{{{\upmu }^{2} }}{{2\eta { }}}$$
(12)
$$I = - E_{HOMO}$$
(13)
$$A = - E_{LUMO}$$
(14)
Table 4 Chemical reactivity of PyBF:12OBA
Full size table

4 Conclusion

The H-bonded compounds of (4-pyridyl)-benzylidene-4'-fluoro aniline (PyBF) with 4-alkyloxy benzoic acids (nOBAs) are successfully synthesized which are found to exhibit wide range of mesomorphism in comparison with nOBAs alone. The clearing and crystallization temperatures have drastically decreased, and the mesomorphism is obtained at convenient working temperatures. Nonlinear optical properties of the compound demonstrated that these thermotropic materials are conducive to use in formulations with other LC materials for various technological applications.