Influence of Hydrogen Bonding Interactions Between New Pyridine Derivatives and 4-N-Alkyloxybenzoic Acids on Mesomorphism

1. Introduction

The organic or inorganic substances which show mesomorphism are known as Liquid Crystals (LCs). While studying the melting process in cholesterol derivatives systematically, double melting was observed by Friedrich Reinitzer in 1888, later further research on such substances by Otto Lehmann et al., coined them as Liquid Crystals. Since then, LCs have gained much attention due to their scientific and industrial applications. They possess unique characteristics such as anisotropy and optical birefringence of solids, isotropic properties of liquids etc. simultaneously [1,2,3,4]. By subjecting to heat [5,6,7] or dilution [4,8,9], the materials show the mesomorphism and are named as thermotropic and lyotropic LCs [1,10] respectively. The mesophases are neither crystalline nor liquid solely in nature but exhibit both the properties simultaneously. Thermotropic LCs are again classified into nematic and smectic [1,11] depending on the molecular arrangement and their interactions in inter and intra layers. In nematic phase, the LC molecules have long range orientational order while in the smectic phases possess partial positional order along with orientational order.

Liquid crystals (LCs) have gained significant attention due to their unique properties [12,13,14] and diverse applications [8,9,15,16,17,18] and considered as an exciting topic of interest due to their suitability in various fields [16,19,20] of research. The LCs have been exploited from flat panel displays to sensors to medical/biological fields. They could be tuned themselves in a unique way by application of heat, light, electric or magnetic fields [21,22,23,24].

Hydrogen bond (HB) interactions are considered [25,26,27] as the fifth fundamental type of chemical interactions [2] and are like tiny magnets between molecules. The interaction between a hydrogen atom and an electronegative atom such as oxygen or nitrogen or sulphur atom leads to the formation of HB. They have been exploited in recent years due to their powerful nature of self-assembly [28]. Because of the presence of both electron-donating and electron-withdrawing nature, Pyridine derivatives are potential candidates [29,30,31,32,33] for forming HBs with other molecules containing active Hydrogen atom. HB interactions generate supramolecular LCs [30,34,35], which provide a flexible platform for the design of materials with specific characteristics. Preparation of supramolecular complexes [25,29,36] through HB interactions using selective complementary functional groups is a fascinating approach and are exploring for diverse applications [37]. One of the potential techniques to realize mesomorphism in non-mesogenic molecules is through HB interactions. The chemical moieties with proton donor and proton acceptor capabilities are desirable [34] to form HB interactions and should have elongated length to show mesomorphism. It was reported that the mesomorphism is influenced by flexible alkyl chain length of the carboxylic acids through their electron donating ability [35], while Pyridine derivatives behave as proton acceptors [38,39,40]. HB interactions may induce or stabilize or quench or enhance the mesophase thermal spans viz., of Nematic (N), smectic-A (SmA), smectic-C (SmC) etc. depend on the substituents on proton donor or acceptor moieties. However, the parameters that dictate the evolution of mesomorphism are still mysterious. In our previous reports [30,32,33,34,37,41,42], Pyridine containing Schiff’s base with mono halogen substituent / flexible chain were used to form the HB interactions with nOBAs and observed that the smectic mesomorphism prevailed and nematic mesophase has been quenched. Interestingly, it was noticed that the non-mesogenic pyridine containing Schiff base and non-mesogenic halogen substituted benzoic acids showed smectic mesomorphism over large range of temperatures. The dihalogen substituted benzoic acids with the same pyridine derivatives showed the highest mesomorphism. However, the HB interactions between dihalogen substituted pyridine derivatives and benzoic acids are scarce in literature. In view of these observations, new pyridine containing Schiff’s base with dihalogen substituents viz., 3-chloro-4-fluoro-N-((pyridin-4-yl)methylene)benzenamine (4Py) are prepared and used as proton acceptors to form the HB interactions with 4-n-alkyloxybenzoic acids (ethyl to dodecyl) proton donors. This study may provide valuable insights in the development of new mesomorphic materials formed through HB interactions and may paved the way for their technical applications.

2. Results

2.1. Synthesis

The proton acceptor viz., 3-chloro-4-fluoro-N-((pyridin-4-yl)methylene)benzenamine (4Py) is prepared by condensation reaction [41,43,44] between 3-chloro-4-fluoro-aniline and 4-pyridinecarboxaldehyde as reported earlier. The ethanolic solutions of equimolar quantities (0.01M) of 3-chloro-4-fluoroaniline (1) and 4-pyridinecarboxaldehyde (2) are taken in a round bottom flask. About 2 drops of acetic acid are added and refluxed for an hour. The progress of the reaction is monitored by TLC. After completing the reaction, the contents are cooled, a white crystalline product (3, 4Py), is obtained. The final product is purified by recrystallization three times with ethanol. The synthetic route for the preparation of 4Py is depicted in Scheme 1.

The synthesized proton acceptor viz., 4Py and the proton donors viz., 4-n-alkyloxybenzoic acids (nOBA, n is the no. of carbons in alkyl chain) are taken in 1:1 molar ratio and dissolved in a common solvent, tetrahydrofuran (THF). The mixture is refluxed for 2 h followed by cooling and distilling off the solvent, a binary mixture is formed, which on further characterization revealed the self-assembly of the two moieties through HB interactions. The general molecular structure of the HB complexes is given in Figure 1.

2.2. Characterization by the Spectral Studies:

The synthesized Pyridine containing Schiff base is characterized by NMR and FTIR spectroscopy. The ¹H and ¹³C NMR spectra of 4Py are given in Figure 2 and Figure 3 respectively. The observed chemical shift (δ) values in the ¹H NMR spectrum of 4Py are: ¹H NMR (400MHz, DMSO): δ 8.78 (d, J = 3.04 Hz, 2H, 2 × CH of Pyridine ring), 8.73 (s, 1H, 1 × aldimine), δ 7.85 (d, J= 4.8 Hz, 2H, 2×CH of pyridine ring), δ 7.64 (d, J= 6.4 Hz, 1H, 1 ×CH of benzene ring), 7.50 (t, J₁ =8.8 Hz, J₂ =8.8 Hz, 1H × CH of benzene ring), 7.40-7.37 (m, 1H, 1 × CH of benzene ring). The observed ¹³C NMR peaks and the values of corresponding chemical shifts for 4Py are: δ 117.83 (1C, s), 120.59 (2C, s), 122.73 (1C, s), 123.06 (1C, s), 123.23 (1C, s), 142.05 (1C, s), 148.05 (1C, s), 150.65 (1C, s), 151.38 (1C, s), 155.29 (1C, s), 161.30(1C, s). The peaks obtained in the spectra are in concurrence with the number of protons and carbons present in the molecular structure of 4Py.

All the HB complexes, i.e., 4Py:nOBA (n = 2 - 12), are characterized by FTIR. The FTIR spectra of the proton acceptor (4Py) is given in Figure 4. The absorption bands beyond 3100 cm⁻¹ are absent in spectrum of the Schiff base (3), which confirms the absence of free -NH₂ group of 3-chloro-4-fluoroaniline. The absorption band corresponding to C=O stretching of pyridine carboxaldehyde in around 1700 cm⁻¹ is also absent, which indicates that the condensation reaction between the aniline and carboxaldehyde have been taken place. The peak at 1605 cm⁻¹ infers the C=N stretching of Schiff base.

The FTIR spectrum of the proton donor, 9OBA (a representative of nOBA) is given in Figure 5. The C=O stretching of carboxylic acid appeared at 1672 cm⁻¹, the aromatic and aliphatic C-H stretching of the 9OBA are observed at 2912 and 2847 cm⁻¹ respectively. The O-H stretching of acid is merged with C-H at around 2912 cm^-1. Peaks at 1429 and 945 cm⁻¹ are attributed to the O-H in-plane and out-of-plane bending modes respectively. A peak at 1300 cm⁻¹ is attributed to the C-O stretching mode.

The FTIR spectrum of the HB complex i.e., 4Py:9OBA is given in Figure 6.

From the FTIR spectrum of 4Py:9OBA, it is observed that additional absorption bands at 2439, 1918 cm⁻¹ confirms the presence of intermolecular HB interactions between lone pair of electrons of pyridine nitrogen and hydrogen of carboxylic acid group. These observations are in concurrence with the other similar HB complexes reported [30,41,42,44]. Similar observations are found in other binary mixtures of HB complexes i.e., 4Py:nOBA (n = 2-12) in the present study and are included in the supplementary information.

2.3. Phase Behavior of HB Complexes, 4Py:nOBA:

The phase sequence and the transition temperatures of the HB complexes are evaluated with the help of POM under crossed polarizers and in conjunction with a temperature controller. Only nematic mesophase is prevalently reported in the lower homologues of 4-n-alkyloxybenzoic acids, viz., 4-propyloxy to 4-hexyloxybenzoic acids. The intermediate and higher members of the series viz., 4-heptyloxy to 4-dodecyloxybenzoic acids are reported [44,45] to exhibit both nematic and smectic-C mesophases. A threaded marble texture is observed for nematic phase in the lower homologues, schlieren texture is observed for nematic phase in the middle and higher members of the nOBAs. Broken focal conic fan texture in homogeneous regions and schlieren texture is observed in homeotropic regions simultaneously for smectic-C mesophase. When the nOBAs are treated with 4Py in the present study, very interesting mesomorphism is observed. The lower member of the series viz., 4Py:2OBA is non-mesomorphic, while the other lower homologues viz., 4Py:3OBA, 4Py:4OBA exhibited nematic phase with Schlieren texture containing four brush disclinations. The intermediate members viz., 4Py:5OBA and 4Py:6OBA exhibited an induced smectic-A (SmA) mesophase with focal conic fan texture in homogeneous regions and pseudo isotropic texture in homeotropic regions simultaneously. It is also observed that the nematic phase (characteristic of 1-D orientational ordering) is quenched in higher homologues (viz., 4Py:7OBA to 4Py:12OBA) of the series. The different mesophases exhibited by the 4Py:nOBA complexes are shown in Figure 7.

The phase transition temperatures and the enthalpy changes across the phase transitions are determined by DSC. The DSC thermograms of 4Py:12OBA are given in Figure 8 as a representative of the series. The DSC thermograms of other HB complexes are given in supplementary information.

The phase transition temperatures and the enthalpy changes of the HB complexes observed in thermal studies are given in the following Table 1.

The quenching of nematic phase and formation of smectic phases (with and without tilt order) with layering order in higher homologues is due to the formation of a relatively strong HB interactions between the proton-donor and proton acceptor moieties with the increasing chain length. Due to inductive effect, the electron donating tendency of Oxygen gets reduced as the alkyloxy chain increases in a homologue and it leads to promote proton donating tendency of the carboxylic acid. Thus, increasing chain length in 4Py:nOBA series exhibits an increasing strength of HB interactions. Hence, increasing chain length results in a layered organization, which in turn promoted smectic polymorphism by quenching nematic polymorphism.

The mesomorphic thermal spans of the present series of HB complexes are compared with the mesogenic 4-n-alkyloxybenzoic acids and are given in Table 2.

It is observed that the lower members of 4Py complexes showed higher mesomorphic thermal spans than the corresponding nOBAs. The mesomorphic thermal spans of intermediate and higher members of 4Py complexes are on par with that of nOBAs and not much appreciable change. The dihalo substituents on lateral positions of the carboxylic acid (non-mesogen) moiety appreciably affected the mesomorphic thermal spans with non-mesogenic calamitic Schiff base containing pyridine moiety through intermolecular HB interactions, in our previous studies. But the mesomorphic thermal spans in the present case in which the dihalo substituents are on the pyridine moiety stabilized the mesomorphism of nOBAs without appreciable change. Interestingly, nematic, and smectic mesophases towards the ambient temperatures are observed in the present studies. The clearing temperatures are also decreased in case of 4Py complexes in comparison with nOBAs.

3. Materials and Methods

The starting materials viz., 3-chloro-4-fluoroaniline and 4-pyridine carboxaldehyde are procured from Aldrich and TCI respectively; 4-n-alkyloxybenzoic acids (ethyl to dodecyl homologues) are purchased from Frinton Laboratories; Acetic acid and Tetrahydrofuran (THF) are purchased from E. Merck, India. The chemicals were used as received from the suppliers without any further purification. Double distilled Ethanol is used as solvent in the synthesis of Schiff base. To monitor the progress / completion of reactions and to check the purity of the product, thin layer chromatography (TLC) was used. Zinc plates precoated with silica gel / neutral alumina TLC plates are procured from E. Merck, India. Shimadzu-8701 Fourier Transform Infrared (FTIR) spectrometer in attenuated total reflectance (ATR) mode is used for recording the vibrational spectra. The ¹H and ¹³C Nuclear Magnetic Resonance (NMR) spectra of the products are recorded using Bruker Avance 400 MHz NMR spectrometer with DMSO as a solvent and tetramethyl silane (TMS) as an internal standard. Chemical shifts (δ) are reported in parts per million (ppm) and coupling constants (J) are given in Hz. The LC behavior of the newly synthesized HB complexes was studied with the help of a Leica Polarizing optical microscope (POM) in conjunction with a Linkam hot stage and a digital camera. The phase transition temperatures and the corresponding enthalpy changes are determined by differential scanning calorimeter (Shimadzu, DSC-60). The heating and cooling rates employed for textural observations are 10 °C/min and the thermograms are recorded at 10 °C/min for both heating and cooling cycle by DSC.

4. Conclusions

The role of molecular architecture on mesomorphism is studied systematically by varying the alkyl chain length of the 4-n-alkyloxycarboxylic acids. It is observed that the nematic phase exhibited by nOBAs is quenched and induced SmA mesophase i.e., the HB interactions favoured the layering order and stabilized the mesomorphism in the present studies. The intermolecular HB interactions between a dihalo substituted benzoic acid (non-mesogen) and a Schiff base containing pyridine (non-mesogen) were found to possess higher mesomorphic thermal spans than the present case of same between a dihalo substituted Schiff base containing pyridine and mesomorphic nOBAs. It is imperative that lateral substituents on either of the proton donor or proton acceptor influence the mesomorphic thermal spans as well as their melting and clearing temperatures. Through systematic studies by changing the nature and size of the substituent on proton donor and proton acceptor, the melting and clearing temperatures as well as the mesomorphic thermal span can be tuned through intermolecular HB interactions. Research in this area could expand to show the influence of different functional groups or substituents or structural variations on the formation or stability of mesomorphism. Apart from the traditional LC technologies, there is also considerable scope to explore these supramolecular systems in other applications such as smart coatings, energy storage, biocompatible materials for medical devices and so on.