Mesomorphic Behavior of Symmetrical and Unsymmetrical Azomethines with Two Imine Groups

Seven symmetrical azomethines with two imine groups (HC=N) were synthesized by condensation of the benzene-1,4-dicarboxaldehyde with five amines (first group: A1-A5) and of the 2,5-thiophenedicarboxaldehyde with two amines (second group: AT1-AT2). Additionally, two unsymmetrical azomethines were obtained by a two step condensation of benzene-1,4-dicarboxaldehyde with pyren-1-amine (1st step) (abbreviated hereinafter as AP1) and then AP1 was reacted with 4-dodecylaniline or 4-hexadecylaniline (2nd step) (third group: AP1A-AP1B). Liquid crystalline properties of the azomethines were studied by differential scanning calorimetry (DSC), polarizing optical microscopy (POM) and UV-vis spectroscopy in the function of temperature [UV-vis(T)]. The Wide-Angle X-ray Diffraction (WAXD) technique was used to probe the structural properties of the azomethines. Mesomorphic behavior was observed for symmetrical and unsymmetrical azomethines, obtained from the benzene-1,4-dicarboxaldehyde and symmetrical ones prepared from 2,5-thiophenedicarboxaldehyde and different amines having aliphatic chains. Based on the POM and DSC measurements the following mesophases were detected: nematic, smectic A, smectic C, smectic F (I), smectic G (J).


Introduction
The azomethines, also called Schiff bases, have been widely investigated as liquid crystalline (LC) materials. Special attention has been paid to the study of the LC properties of azomethine oligomers, prompted by their use as building blocks for polymer chains [1]. The liquid crystalline properties of the azomethines could be divided, depending on the number of HC=N bonds in the structure into: (1) Schiff bases having one azomethine (HC=N) bond [2][3][4][5][6][7][8] and (2) symmetrical and unsymmetrical azomethines with two HC=N bonds in the structure [9][10][11][12][13][14][15][16]. Moreover, numerous scientists have investigated the LC properties of symmetrical azomethines with two HC=N bonds in the structure. For example, Ribera et al. [9] studied dimeric reactive diglycidyl ethers containing imine mesogens and aliphatic spacers. The authors found that odd-member spacers led to the formation of smectic C mesophases, whereas even-membered spacers led to smectic A and/or nematic mesophases. Ribbera et al. also investigated a series of dimeric LC diglycidylester compounds containing imine groups [10]. They found that all compounds exhibited nematic mesophases. Henderson et al. [11] characterized a series of semiflexible LC tetramers containing four liquid crystal units (two H 3 CO-Ph-N=N-Ph and two -Ph-HC=N-Ph-) connected via three flexible alkyl spacers. The length of the outer spacers was varied from 3 to 12 units. All the tetramers exhibited an enantiotropic nematic phase. Kishikawa et al [12] synthesized one symmetrical azomethine having two imine groups and an OC 3 H 7 group as outer spacer. The compound exhibited Sm H, Sm G, Sm C, Sm A and nematic phases along with the temperature of isotropisation at about 255 °C. Naito et al. [13] found that twin dimers with 3-methylpentane spacers behaved as U-shaped molecules forming Sm A phases when the carbon number of the alkyl tail group was longer than 12. On the other hand these authors found that in the case of dimers with linear pentane spacers the antiferroelectric Sm CA was formed [13]. Sudhakar et al. [14] investigated the LC properties of the molecules constructed from 1,4-disubstituted benzene rings linked through ester and HC=N units. The outer-spacer was held via n = 2, 4, 6, 8, 10, 12, 14 or 16 aliphatic groups. The lower members of the serie (n = 2-8) were nematogenic, while the higher members (n = 10-14) exhibited both nematic and Sm A phases. The compound with 16 methylene groups presented only Sm A phase. Cozan et al. [15] investigated new azomethine sulfone macromers containing benzylideneaniline mesogens. A nematic texture was observed for macromers containing two benzylideneaniline mesogens either connected by etheric (-O-), methylenic links (-CH 2 -) or directly coupled. Mesogenic diols containing one ester group in the center and varying alkoxy spacer length (n = 2, 4, 6, 8, 10) were synthesized by Srinivasan et al. [16]. All the diols displayed nematic mesophase. Also a lot of works in the past were dedicated to the liquid crystalline properties of terephthalylidene-bis-4-n-alkylanilines [17][18][19][20][21][22][23] with aliphatic chains consisting of five to ten carbon atoms.
The symmetrical azomethines with two HC=N bonds and demonstrated banana-shapes were widely investigated as well [24][25][26][27][28][29][30]. For example, liquid crystalline trimers composed of banana-shaped and rodlike anisometric segments were synthesized and investigated by Yelamaggad et al. [24]. The LC azomethines consisted of two ester groups in the center and two cyanobiphenyl groups at the periphery of the compounds. Four alkylene spacers (6,7,8,10) were used as a flexible spacer. All of the trimers displayed an enantiotropic uniaxial nematic phase. Additionally, the compound possessing a heptamethylene (odd-parity) spacer exhibited also a metastable Sm phase.
In this paper we present two groups of symmetrical azomethines and one group of unsymmetrical azomethines with two azomethine bonds. Symmetrical and unsymmetrical azomethines, obtained from benzene-1,4-dicarboxaldehyde, having methyl-or methoxy-aliphatic chains presented polymorphism, while symmetrical azomethines containing carbonitrile-or azo-moiety did not exhibit LC behaviour. Symmetrical azomethines having thiophene ring in the centre also demonstrated mesomorphic behaviour.
Liquid crystal properties of the azomethines were studied by differential scanning calorimetry (DSC) and polarizing optical microscopy (POM). The structural characterization was performed by NMR and FTIR characteristic completed via X-ray diffraction measurements and optical investigations. The LC behavior as well as photoluminescence properties of the unsymmetrical azomethines were not investigated so far. To best of our knowledge the absorption properties of the azomethines in the function of temperature were first time analyzed hitherto.
The most remarkable feature of the azomethines that we have investigated was the incorporation of carbonitrile-, azo-pyrene or methyl groups in the symmetrical or unsymmetrical azomethines. It was shown that these groups dramatically influence the thermal and optical properties of the azomethines. Additionally, synthesis and investigation of new mesogens is one of the important and interesting fields for materials research community.

Results and Discussion
The azomethines synthesized were new, except for A2 and A3, described in [31,32], respectively, and used here as a references.

Synthesis and characterization
The synthesis of the new symmetrical azomethines with two azomethine bonds consisted of one step, while the synthesis route of the unsymmetrical azomethines consisted of two steps. Chemical structures of the symmetrical and unsymmetrical compounds and of the aldehyde are presented in Figure 1, whereas details of synthesis procedures and molecular characteristics along with proton NMR, FTIR, UV-vis, photoluminescence (PL) spectra and elemental analysis of the azomethines and the aldehyde are given in Experimental. The spectral data of all azomethines and aldehyde were found to be consistent with their molecular structure. The absence of the residual amino (NH 2 ) and aldehyde (CHO) groups together with the appearance of a typical for azomethine bond (HC=N) band was confirmed by FTIR and NMR spectra. The spectra confirmed both their purity and their molecular structures.
The UV-vis absorbance spectra of the symmetrical azomethines in solution (CH 3 Cl), except A5, displayed two absorption maxima, in the 272-298 nm and 357-362 nm regions (4.56-4.16 eV and 3.47-3.43 eV), whereas the UV-vis spectrum of the A5 showed one absorption maximum at 384 nm (3.23 eV). In the absorbance spectrum of the unsymmetrical azomethine AP1A three main absorption maxima at 318, 348 and about 450 nm were observed ( Figure 2). For the same representative samples, the fluorescence (emission) spectra in chloroform solution (1.25 × 10 -3 M) at room temperature were recorded for the 400 or 450 nm excitation wavelength. For the excitation wavelengths 400 nm, the symmetrical azomethines A3 and A4 exhibited one emission band at 535 nm (2.32 eV) and 489 nm (2.54 eV) , respectively, while the unsymmetrical azomethine AP1A exhibited one emission band at 520 nm (2.38 eV) under 450 nm excitation wavelengths. The Stokes shift for AP1A was 70 nm, while for A3 and A4 it was 178 and 127 nm, respectively. It is apparent that a hypsochromic effect (blue effect) occurred with the introduction of carbonitrile (A4) or pyrene moiety (AP1A). These spectroscopic data ascertained the fluorescence properties of these unsymmetrical and symmetrical azomethines.
As mentioned earlier, several different azomethines have been described in the literature, but only a few azomethines demonstrated fluorescence together with liquid crystalline properties have been reported hitherto [31,32].

Mesomorphic behavior
Liquid crystal properties of these three groups of the azomethines were investigated mainly with the help of differential scanning calorimetry (DSC) and polarizing optical microscope (POM). As examples, DSC thermograms obtained during the first heating and cooling cycles for A1 and AP1A are shown in Figure 3. The tentative mesophases identifications and the scenario (sequence) of phase transitions related to the all compounds are based on the identification of textures appearing in two reference textbooks for liquid crystals [33][34] and on repeated POM and DSC experiments.  Upon DSC analysis the symmetrical azomethines A1-A3 exhibited a typical transition behavior. Two enantiotropic transitions, crystal-to-mesophase (Cr/M) and mesophase-to-isotropic (M/I), were observed. The clearing temperature decreased with increase the length of the side chains, i.e., 261.3 °C (A1) > 230.5 °C (A2) > 160.7 °C (A3) on cooling cycle ( Table 2). Introduction of oxygen atom into A1 caused increase of melting point and isotropic state of A1 in comparison with A2. The temperature range of the mesophases was decreased with increase the length of side chains (compare A2 and A3 in Table 2), as well. Compound A1 showed a crystal to isotropic liquid transition at 268.0 °C with a ΔH=2.9 J/g on the first heating cycle. On cooling from the isotropic state to room temperature, a small exothermic peak appeared at 261.3 °C with a ΔH= 1.7 J/g, corresponding to the transition from the isotropic liquid to the liquid crystalline phase. Compounds A2 and A3 have isotropisation at lover temperature (at 230.8 °C and at 160.9 °C) then A1 ( Table 2).
The DSC analysis of the compounds A1-A3 showed three, four or six endothermic clearing transitions on the first heating cycle. When cooled from the isotropic liquid state to room temperature, the compounds exhibited three or five exotherms corresponding to the transition from the isotropic liquid to crystalline phase (see Figure 3a). Symmetrical azomethines A4-A5 did not exhibit liquid crystal behavior.
In the POM and DSC study, the compounds A1 and A2 shown stable enantiotropic smectic C (SmC) and nematic (N) liquid crystal phases, while the compound A3 presented stable enantiotropic smectic A (SmA) and nematic liquid crystal phases (see Table 1). As a representative case, the microphotographs of the nematic and smectic phases obtained for compound A1 are shown in Figure 4.  The nematic phase of the symmetrical azomethines A1-A3 and SmC phase of A1 (see Figure 4) demonstrated Schlieren texture, while smectic C phase of the A2 exhibits sanded texture (data not presented here) [31,32].
X-ray diffraction studies of the symmetrical azomethines A1-A5 powders revealed their crystalline nature. The WAXD patterns for the compounds showed very sharp diffraction peaks with several weak diffractions in smaller angles (2Θ scanning), indicating that a highly ordered crystalline structure exists in the azomethines. The wide-angle X-ray diffraction patterns of the A4 and A5 over the 2θ range of 5° -60°, as example, are shown in Figure 5a. The reflection peaks (2θ) and d spacing from WAXS diffraction diagram of the azomethines A1-A5 are collected in Table 3. The symmetrical compounds A1 and A2 exhibited one main reflection at 2θ = 6.45° and 20.56°, respectively (d spacing 13.69 Å and 4.31 Å). In the X-ray diffraction pattern of the azomethines A4 and

Thermal behavior of the symmetrical azomethines AT1-AT2
The symmetrical azomethines AT1 and AT2 were synthesized by replaced phenyl ring via thiophene one (Fig 1a). The thermal behavior of these compounds was investigated via DSC and POM techniques. Compounds were investigated by DSC during different heating and cooling cycles. The results of the phase transition (heating scan) are presented below: The numbers in square brackets under the transition temperatures designate the transition enthalpies in kJ/mol. The symbol Sm X is used here for an unidentified order smectic phase. DSC curves of the compounds AT1 and AT2 during different heating and cooling cycles are presented in Figure 6 a-d. Also presented in Figure 6 are the DSC curves of A3, for comparison of the thermal behaviors of AT1 and A3.   On cooling the isotropic liquid of AT1 and AT2 the mesophase appeared as a mosaic texture under the microscope characteristic for the Sm X phase. Moreover, a kind of batonnets texture could be obtained for AT1 (Figure 7a, left side). Additionally, nice crystals under POM investigations were observed for both samples (Figure 7b). In contrary to the AT1 optical micrographs of the AT2 evidence showed "spherulite-like" domains ( Figure 7b, right side).
To account for differences between the thermal behavior of A3 and AT1 a preliminary geometry optimalization (ACD/ChemSketch, 3D Structure Optimization) of these compounds was carried out. The average C-C bond length between the C=N linkage and phenylene ring in A3 was 1.502 Å, while that between the C=N linkage and thiophene ring in AT1 was 1.545 Å. The N-C bond length in A3 was 1.512 Å, while in the case of AT1 it was 1.552 Å. The C=N bond lengths of A3 and AT1 were 1.312 Å and 1.319 Å, respectively. Hence, bond length alternation between C-C and C=N was larger in AT1 (0.226 Å) than that in A3 (0.19 Å). As shown in Figure 1c, the valence angles between the C=N linkage and the adjacent phenylene ring in A3 and thiophene in AT1 were 122.55° and 124.75°, respectively. Also, a significant difference was shown on the valence angle between the C=N linkage and the adjacent N-phenylene ring. AT1 had a larger valence angle (125.45°), as compared with that (122.40°) of A3 by 3.05°, which suggests that A3 had a more planar conformation than AT1 (see Figure 1c).
The wide-angle X-ray diffraction patterns of the compounds AT1 and AT2 over the 2θ range of 5° -60° are shown in Figure 5b. The diffraction arising from the crystallites is observed which confirmed crystalline pattern of both compounds. The WAXD patterns of the compounds AT1-AT2 showed very sharp diffraction peaks with several weak diffractions in smaller angles (2Θ scanning), indicating that a highly ordered crystalline structure exists in the azomethines, especially in the AT1. The compound AT1 exhibited three main reflections at 2θ = 5.47°, 12.99° and 16.50° (d spacing 16.14, 6.81 and 5.37 Å, respectively). On the other hand the diffractions arising from the crystallites were observed for AT2 at 6.61°, 8 (Figure 5b).  Bent-shaped molecules (so-called banana molecules) have attracted great attention in the field of innovative liquid crystal research. Their unique properties of spontaneous chiral symmetry breaking and decoupling of polar and tilt order have been extensively studied for the last decade [24][25][26][27][28][29][30]. There are three main groups of bent-core liquid crystals: (1) molecules incorporating a rather rigid bent aromatic core (i.e. resorcinol derived compounds), (2) mesogenic dimers with an odd-number flexible spacer unit and (3) hockey-stick molecules, where an alkyl chain is attached to the meta-position at one end of an aromatic core [30]. In this paper two bent-shaped azomethines AT1 and AT2 were synthesized and investigated via DSC, POM and WAXD techniques. In X-ray diffraction measurements, a lot of sharp diffraction peaks were observed at wide angles and additionally several sharp peaks at small angles, as shown in Figure 5b. From these results the azomethines AT1 and AT2 probably exhibited B3 phase. This part of our work needs more investigations.

Mesomorphic behavior of the unsymmetrical azomethines AP1A-AP1B
All phase transition parameters of the unsymmetrical AP1A were determined by the DSC and POM technique and are presented in Tables 1 and 2. Typical DSC traces of AP1A are shown in Figure 3b. This unsymmetrical azomethine exhibited very rich smectic polymorphism, as is shown in Tables 1 and 2 and in Figure 3b. The POM and DSC observations revealed that AP1A is an enantiotropic liquid crystal exhibiting four phases (SmG, SmF, SmC, SmA). In contrast, the unsymmetrical AP1B exhibited two enantiotropic smectic phases, which implies a significant effect of the length of the aliphatic chain on the mesomorphic properties. The identification of the smectic phases was confirmed by POM. Microphotographs of the AP1A are presented in Figure 8.  (Figure 8d at 170 °C).
It is very difficult to find changes between SmF and SmI or SmG and SmJ. The only change which occurs at the transition from SmF to SmI is one of tilt direction (SmF: tilt direction towards an edge of the hexagon while SmI: tilt direction towards an apex of the hexagon) [34]. Both SmF and SmI are tilted biaxial (2D) smectic mesophase in which the mesogenic molecules are hexagonally packed. They both share short range hexagonal order with no long-range correlations in between their smectic layers.
From a structural organization point of view, a transition from a SmI mesophase to a SmJ one (or from a SmF to a SmG mesophase) would imply the transition from an organization with titled 2D hexagonally packed LC molecules towards the one of titled 3D pseudo-hexagonally packed LC molecules. Put it differently a SmI > SmJ (or a SmF > SmG) transition results in a net gain of positional order of the LC molecules as well as by a transition from a 2D to 3D mesophase. When the SmG phase is formed by cooling a SmF phase the tilt direction with respect to the hexagonal net is retained it means that till towards the edge of the hexagon [34].
It is quite difficult to differentiate SmF and SmI mesophases unless they are present in a sequence of mesophases for a specific LC. Nevertheless detailed X-Ray diffraction measurements, neutron scattering studies and POM investigations could be used to highlight such a tiny difference. Additionally, DSC studies could be of great help in the case of LCs compounds show a mesophases SmF and SmI, which have usually enthalpy value in the range of 0.1-0.2 kJ/mol. Table 2 reveals that the compound AP1A exhibited a clearing point temperature lower than AP1B. On the other hand the compound AP1B had the highest clearing point temperature in comparison with another azomethines (Table 2). Also the melting temperature of AP1B was the highest in comparison with the other azomethines. The following order was observed: 69 °C (A2) < 72.8 °C (AP1A) < 87.9 °C (A1) < 88.4 °C (A3) < 99.8 °C (AP1B).
Additionally, the phase transitions were analysed based on their entropy values ( Figure 9). The melting process has the highest entropy values. The entropy values decreased from 129.1 J/mol K for A3 to the minimal values 19.5 and 20.3 J/mol K for AP1B and A1, respectively. The parameters of the heating and cooling process were more or less similar (Figure 9c-d).
The X-ray diffraction pattern obtained from AP1A is presented in Figure 5c. A broad halo and a sharp reflection at wide angles are shown in Figure 5c. One broad diffraction peak of diffusion type centered at 23.33° (2θ) and one sharp reflection at 13.82° was observed for AP1A.

UV-vis(T) Studies
The absorption properties of the azomethines in the function of temperature were first time analyzed hitherto. Figure 10 shows the temperature dependence of the UV-vis spectra of A2, as an example, in a temperature range from room temperature to the clearing point in the heating process. Using UV-vis spectroscopy to monitor the changes in spectroscopic properties of A2 during heating, three changes in the position of the isosbestic point were observed, indicating that the differential transitions proceeds not randomly but stepwise. An isosbestic point appears when a compound is quantitatively transformed from crystal to one mesophase and later into another during heating, so the three different isosbestic points observed suggest that three different transitions were successively formed. The spectra of the A2 gradually changed, with an isosbestic point at 406 nm due to the increase the tempararture from 25 °C to 70 °C ( Figure 10,   It should be mentioned that such LC compounds which exhibit a mesophase or a sequence of mesophases over a broad temperature range could be used in: (i) color information technology-based applications [35] for thermally stable glassy derived from cholesterol LC molecules and (ii) organic electronic devices (organic light-emitting devices, organic solar cells). Obtained compounds could be utilized in devices which operating temperature is much higher than room temperature. Moreover, such types of LCs compounds have been used in mixture with other LCs for LC displays. Particularly important are twisted nematic (TN) cells which rely on calamitic mesogens, and usually exhibit high clearing temperatures.
The phase transitions and mesogenicity were studied by differential scanning calorimetry (DSC) and polarizing microscope observations (POM). DSC were measured on a TA-DSC 2010 apparatus using sealed aluminium pans under nitrogen atmosphere at a heating/cooling rate 0.5 °C/min in a temperature range from -20 °C to over the clearing point.
The temperature dependence of the UV-vis spectra was measured for thin film on the glass (film cast from dichloroethane) by JASCO V-570 UV-Vis-NIR spectrometer using a temperature-controlled optical cell in a temperature range from the room temperature to clearing point in the heating process.

Synthesis of symmetrical azomethines
The symmetrical azomethines were prepared using the synthetic routes shown in Scheme 1a, and identical to those used to prepare the terephthalylidene-bis-4-n-alkylanilines [31][32]. Generally, a solution of dialdehyde (1.0 mmol) in DMA (5 mL) was added to a solution of amine (2.0 mmol) in DMA (5 mL) with 0,06 g of p-toluenesulfonic acid. The mixture was refluxed with stirring for 10 hours. Then the compound was filtered, washed with ethanol and later with acetone and dried at 60 °C under vacuum for 12 hours.Azomethines A2 and A3 were synthesized and described previously [31][32] and were used as references in this paper.

Conclusions
In summary, we have reported the synthesis and characterization of new symmetrical and unsymmetrical series of the calamitic azomethines with aromatic central core based on 2,5thiophenedicarboxaldehyde and benzene-1,4-dicarboxaldehyde. The one group of the symmetrical compounds (i.e. the three benzene-1,4-dicarboxaldehyde derivatives) consisted of molecules containing two liquid crystal units with different length of the outer spacers, i.e. the length of the outer spacer is varied from 5 to 16 methylene groups. The second symmetrical group (i.e. the two 2,5thiophenedicarboxaldehyde derivatives) consisted of molecules containing also two liquid crystal units with the 16 methylene or 16 methoxy groups as an outer spacers. The third group consist of unsymmetrical molecules containing three LC units with different length of the outer spacers (12 or 16 methylene groups). At an ambient temperature all of the azomethines were crystalline substances which melting point ranged from 92 °C to 282 °C. The mesomorphic properties of the compounds depend on both the length of the outer flexible spacers, the connection of the azomethine group with the rod, i.e. via phenyl or thiophene ring and symmetry of the molecule. The symmetrical compounds exhibited smectic and/or nematic phases whereas the unsymmetrical azomethines displayed rich polymorphism.
The obtained unsymmetrical and symmetrical azomethines exhibit mesophases over a broad temperature range and can be used potentially in optoelectronics. For example, such LC compounds could be applied in organic light-emitting devices, whose operating temperatures could be higher than room temperature. Additionally, the obtained azomethines could be used in mixtures with other LCs for LC displays.