CC BY
7
112 AZERBAIJAN CHEMICAL JOURNAL № 3 2023 ISSN 2522-1841 (Online)
ISSN 0005-2531 (Print)
UDC 544.25
SYNTHESIS AND MESOGENIC PROPERTIES OF NEW AROMATIC SCHIFF BASE'S
ESTERS
Teck-Leong Lee1, Chelsea Mei-Ing Quek1, Sie-Tiong Ha1'*, Guan-Yeow Yeap2
1Faculty of Science, Universiti Tunku Abdul Rahman, Jln Universiti, Bandar Barat, 31900 Kampar,
Perak, Malaysia
Liquid Crystal Research Laboratory, School of Chemical Sciences, Universiti Sains Malaysia,
11800 Minden, Penang, Malaysia
[email protected] hast_utar@yahoo. com
Received 15.10.2022 Accepted 24.02.2023
A homologous series of new Schiff base's esters, 4-bromobenzylidene-4'-alkanoyloxyanilines were synthesized. The structure of the synthesized compounds was elucidated using IR and NMR spectroscopic techniques along with mass spectrometric method. Nine homologous members were prepared whereby the length of alkanoyloxy chain, Cn-1H2n.1COO is varied from n = 2, 4, 6, 8, 10, 12, 14, 16 and 18. Their thermotropic properties were analyzed using differential scanning calorimetry and polarized optical microscopy techniques. Whilst short chain members (number of carbons, n = 2, 4) are non-mesogens, member with n = 6 exhibited monotropic smectic A (SmA) phase. As the n increased to 8, this compound becomes the only member that exhibited SmA and SmB as enantiotropic phase. Varying from n = 10 to n = 16, these members displayed enantiotropic SmA and monotropic SmB phases. Longest chain member, n-octadecanoyloxy derivatives exhibited only single mesophase (monotropic SmA). The ester linkage and polar terminal (bromo) group in the present series are essential for the smectic polymorphism in Schiff bases.
Keywords: 4-bromobenzylidene-4'-alkanoyloxyanilines, imine, thermotropic, smectic A, smectic B.
doi.org/10.32737/0005-2531-2023-3-112-125
Introduction
Liquid crystals (LCs) are an attractive class of soft matter having properties somewhere between liquids and solid crystals. LCs have received an overwhelming response due to their very promising applications in various technological fields such as display devices, organic light emitting diodes, anisotropic networks, photoconductors, semiconductor materials, lubricating agents and LC sensors [1-5]. An effective way to develop new low-cost LC material with unique properties is the modification of molecular structure which has proven to be one of the finest strategies [6]. This has led to the synthesis of numerous mesogens, especially thermotropic type. Most thermotropic LCs are calamitic-shaped molecules having rigid core system which made of two or more phenyl rings and one/more flexible terminal alkyl/alkyloxy chains. The rigid core is connected by the linking unit. One of the most established linking units is Schiff base. It has a stepped core struc-
ture, but it can maintain linearity of the molecule. Hence, it provides higher stability and allow mesophase formation [6,7]. Much works has been emphasized on Schiff base system ever since the discovery of room temperature MBBA [8]. Previously, the works mainly concentrated on the Schiff bases having alkyl/alkyloxy chains. Current works has been shifting towards Schiff bases possessing alkanoyloxy chains owing to their interesting properties and substantial temperature range [9-20]. Besides application as liquid crystals, Schiff bases are also studied for their biological activities [21, 22] and also used as ligand for the coordination compounds [23].
Typical terminal substituents present in LC molecules are those with electronegative atoms, such as halogens. Type of phase and phase stability are mainly reliant on the dipole moment of the mesogenic part of the molecule, which varies based on the introduced polar groups and its steric factor [24]. Strong dipole moments exhibited by the polar substituents
possess the ability to enhance the formation of mesophase [25, 26]. When changing across the halogen series, the dipole moment of the halogen increases and this promotes the stability of the lattice and melting points [7]. Besides that, with the increased ionic radius of the halogen atoms at the terminal position, the molecules become readily to orientate in the parallel form [27]. Therefore, this type of mesogen can exhibit smectic polymorphism and this behavior has been regularly observed as the length of terminal chain increases [26, 28].
In the current work, alkanoyloxy chain at one end of the molecules and bromo substituent at the other end of the molecules are incorporated into Schiff base compounds. This has produced a new homologous series of LCs, 4-bromobenzylidene-4'-alkanoyloxyanilines (Scheme 1). FT-IR, NMR and EI-MS were used to confirm the molecular structure of the title compounds. Their mesomorphic properties were analyzed by DSC and POM techniques. Influence of varying the lengths of the alka-noyloxy chain on the mesomorphic behaviors are studied. In addition, the relationship between the molecular structure and LC properties by comparison with the related compounds is also discussed in this article.
Experimental part
Characterization. Electron ionization mass spectrum (EI-MS) was recorded using a Finnigan MAT95XL-T mass spectrometer operating at 70 eV ionizing energy. FT-IR data were acquired with a Perkin Elmer 2000-FTIR spectrophotometer in the frequency range of 4000-400 cm-1 with samples prepared as KBr pellets. 1H NMR (400 MHz), 13C NMR (100 MHz) and 2D HMQC spectra were recorded using a JEOL LA-400 MHz NMR spectrometer. CDCl3 is used as solvent for the NMR analysis.
Mettler Toledo DSC823e differential scanning calorimeter was used to obtain the phase transition temperatures and associated enthalpy changes at heating rate of 10 oC/min. Temperature dependent studies of the mesophase textures are carried out using a Carl Zeiss polarizing optical microscope attached to a Linkam hotstage. A video camera (Video
Master coomo20P) attached on the microscope was connected to a video capture card (Video Master coomo600), allowing real-time video capture and image saving. Samples were prepared as thin films sandwiched between a glass slide and a cover slip. The texture of liquid crystals exhibited by the compounds was observed using polarized light with crossed polarizers.
Synthesis. Acetic acid, butyric acid, hex-anoic acid, decanoic acid, dodecanoic acid, tetradecanoic acid, hexadecanoic acid, octade-canoic acid, 4-bromobenzaldehyde, 4-amino-phenol and 4-dimethylaminopyridine (DMAP) were obtained from Merck (Germany) without further purification. Octanoic acid and N,N'-dicyclohexylcarboiimide (DCC) were purchased from Acros Organics (USA).
Synthesis of 4-bromobenzylidene-4'-alkanoyloxyanilines (nBrBA). The synthetic route of 4-bromobenzylidene-4' -alkanoyloxy-anilines, nBrBA is depicted in Scheme 1. The newly synthesized esters, nBrBA were prepared at the Department of Chemical Science, Univer-siti Tunku Abdul Rahman according to the reported procedures [10, 11]. Equivalent mole (5 mmol) of 4-bromobenzaldehyde and 4-aminophenol were added into 30 mL of methanol and refluxed with stirring for three hours. The resulting mixture was cooled to room temperature and filtered. The yellow product (BBHA) was washed with methanol. Then, BBHA (0.8284 g, 3 mmol), 3 mmol of fatty acids (CnH2n-1COOH, where n = 2, 4, 6, 8, 10, 12, 14, 16 or 18), DMAP (0.1222 g, 1 mmol) and DCC (0.6190 g, 3 mmol) were mixed and stirred at room temperature for six hours in appropriate amount of THF. The solvent was removed by evaporation. Solid obtained was re-crystallized several times with hexane and etha-nol whereupon pure compound was isolated. The purity of all compounds was checked by thin layer chromatography (Merck 60 F254) and visualized under short-wave UV light. Percentage yields of all compounds are given as follows: 2BrBA (23%), 4BrBA (27%), 6BrBA (29%), 8BrBA (35%), 10BrBA (36%), 12BrBA (48%), 14BrBA (52%), 16BrBA (53%),
18BrBA (58%). The EI-MS, IR and NMR (1H, 13C) for the representative compound, 12BrBA, are summarized as follows.
EI-MS m/z (rel. int. %): 459.2 (3.1) [M+2]+, 457.2 (3.1) [M]+, 275.0 (100); IR Vmax (KBr, cm-1): 3051 (v C-H aromatic), 2950, 2921, 2849 (v C-H aliphatic), 1753 (v C=O ester), 1625 (v C=N), 1586 (v C=C aromatic), 1204, 1100 (v C-O); 1H NMR (400 MHz, CDCl3, 5ppm): 0.9 (t, 3H, J = 6.9 Hz, CH3-), 1.2-1.5 (m, 16H, CH3-(CH2)8-), 18 (quint, 2H, J = 7.5 Hz, -CH2-CH2-COO-), 2.6 (t, 2H, J = 7.6 Hz, -CH2-COO-), 7.1 (d, 2H, J = 8.7 Hz, Ar-H), 7.2 (d, 2H, J = 8.7 Hz, Ar-H), 7.6 (d, 2H, J = 8.7 Hz, Ar-H), 7.8 (d, 2H, J = 8.7 Hz, Ar-H), 8.4 (s, 1H, -CH=N-); 13C NMR (100 MHz, CDO3, 5ppm): 14.23 (CH3-), 22.79 (CH3CH2-), 25.05 (CH3CH2CH2-), 29.36, 29.44, 29.56, 29.70 for methylene carbons (CH3CH2CH2-(CH2)6-), 32.01 (-CH2CH2COO-), 34.50 (-CH2COO-), 122.86, 122.38, 126.08, 130.25, 132.15, 135.07,
149.18, 149.20 for aromatic carbons, 159.11 (-CH=N-), 172.55 (-COO-).
Results and Discussion
The purpose of this work is the synthesis of new homologous series of liquid crystalline compounds and the investigation of their structure-property relationships. The aromatic Schiff base esters were synthesized in two-steps reaction. First, the benzylideneaniline skeleton was obtained through Schiff base condensation reaction between 4-bromobenzaldehyde and 4-aminophenol. Second, the formation of alka-noyloxy chain at the terminal position of the benzylideneaniline moiety was attained through Steglich esterification by using DCC as coupling reagent and DMAP acts as catalyst. The length of the alkanoyloxy chain were varied by using a series of fatty acids. These reactions were summarized in Scheme.
HO
NH--
HO
+
O
Br
/r
N
V \
Br
BBHA
1. Cn_1H2n_1COOH
2. DCC, DMAP
3. THF
Br
nBrBA
where n= 2, 4, 6, 8, 10, 12, 14, 16, 18
Scheme
H
CH3OH
Cn-1 H2n-1C00
Molecular structure of nBrBA was confirmed by using a combination of spectroscopic techniques. EI mass spectrum is depicted in Figure 1. The prominent molecular ion peak at m/z 457 suggested molecular formula of C25H32BrNO2, which supported the proposed structure of 12BrBA.
FT-IR spectrum of 12BrBA is shown in Figure 2. The absorption peaks at 2950, 2921, 2849 cm-1 are belonged to aliphatic alkyl functional groups. It was found that the relative intensity of absorption bands of aliphatic group increased upon ascending the series. It is due to the increasing number of carbons in the alkanoyloxy chain. Imine (C=N) group showed its absorption peak at 1625 cm-1. Carbonyl (C=O) of ester group showed it absorption peak at 1753 cm-1.
In the 1H NMR spectral data of 12BrBA, two triplets at 5 0.9 and 5 2.6, were respectively belonged to the methyl and methylene (-CH2COO-Ar) protons, while the multiplet between 5 1.2-1.5 was attributed to the methylene protons of the long alkyl chain {-(CH2)8-}. Aromatic protons exhibited four distinct doublets at 5 7.1, 5 7.2, 5 7.6 and 5 7.8 with the integration of two protons for each signal. A singlet peak appeared at the most downfield region, 5 8.4, confirmed the presence of the azomethine proton (CH=N) [11].
The molecular structure of 12BrBA was
13
further confirmed by using C NMR spectrosco-py. Protonated carbons are assigned based on the 13C-1H HMQC experiment (Figure 3) that showed the cross peak between 13C and 1H signals. The peak at 5 14.23 was assigned to the methyl carbon while the exhibition of signals at 5 22.79-34.50 was due to the methylene carbons of the long al-kyl chain. Twelve aromatic carbons are resonated at the range of 5 122.86-149.20. The peaks at the respective 5 159.11 and 5 172.55 supported the presence of the azomethine (CH=N) and carbonyl (C=O) carbons in the molecule.
Mesophase observation was carefully examined by microscope during both heating and cooling scans. Optical photomicrographs of 8BrBA are shown in Figure 4 as the representative illustration. Microscopy observation were supported by the DSC data. The transition temperatures, associated enthalpy changes, and phase sequences are summarized in Table 1.
Phase identification was based on the optical textures, and the magnitude of isotropization on enthalpies is tally with the assignment of each mesophase type, using the classification systems reported in the literature [29, 30].
Focal conic fan-shaped textures of a smectic A (SmA) phase were observed during the cooling cycle (Figure 4a). Upon further cooling, the back of the fan-shaped domains developed a series of dark-lines (Fig. 4b), which is temporary in nature [7, 31]. After further cooled, the bands expanded, met and eventually coalesced to give a mosaic-like texture (Figure 4c) [29]. This phase is assigned as a SmB phase. The same characteristic was also observed for a reported compound, 4-butyloxybenzylidene-4-chloroaniline [32].
Representative DSC thermograms for 4BrBA, 12BrBA and 18BrBA upon heating and cooling are depicted in Figure 5. A plot of the transition temperatures against the number of carbons in the alkanoyloxy chain during the heating cycle is shown in Figure 6. Out of the nine members, the first two members (C2 and C4) did not exhibit mesophase and show direct transition of crystal to isotropic phase (Figure 5a). These molecules with short alkanoyloxy chains are too rigid, therefore have high melting points and subsequently suppress their liquid crystal properties [33]. As the length of the terminal chain increased to n = 6, the molecule becomes more flexible, hence promoting a monotropic (less stable) mesophase. The melting point (108.30C) was higher than the smectic clearing point (90.40C). Therefore, the mono-tropic SmA was only observed on cooling cycle. Further lengthening of the alkanoyloxy chain has led to formation of enantiotropic (more stable) SmA and SmB phases in C8 member. This is due to the increased of flexibility provided by the longer terminal chain which contributed a suitable balance between rigidity and flexibility of the molecule. It subsequently encourages the formation of enantiotropic SmA and SmB phases [34]. The higher members, C10-C16 members exhibited enantiotropic SmA and monotropic SmB phases (Figure 5b). As moving to the highest member (C18), the SmB phase was disappeared and only SmA phase was observed on cooling run (Figure 5c).
Fig. 3. 13C-1H HMQC spectrum of 12BrBA. Table 1. Transition temperatures and associated enthalpy changes of nBrBA
Compound Transition temperature, 0C (AH, kJ mol-1) Hea^ng r Cooling
2BrBA Cr 139.1 (27.2) I 1118.8 (28.0) Cr
4BrBA Cr 113.1 (29.9) I 186.9 (29.2) Cr
6BrBA Cr 108.3 (32.6) I 190.4 (10.2) SmA 78.4 (20.2) Cr
8BrBA Cr 96.2 (16.3) SmB 99.5 (1.28) SmA 106.3 (6.8) I 199.7 (7.4) SmA 93.1 (4.0) SmB 55.5 (26.3) Cr
10BrBA Cr 88.3 (31.9) SmA 108.8 (5.0) I 196.1 (5.6) SmA 79.4 (2.6) SmB 19.2 (22.5) Cr
12BrBA Cr 99.6 (40.1) SmA 109.9 (7.6) I 1108.3 (7.6) SmA 96.2 (3.4) SmB 63.5 (31.8) Cr
14BrBA Cr 98.9 (42.0) SmA 108.3 (8.0) I 1106.9 (8.2) SmA 94.1 (3.2) SmB 70.3 (35.3) Cr
16BrBA Cr 101.0 (38.9) SmA 104.9 (3.1) I 1102.7 (6.2) SmA 89.8 (3.0) SmB 82.07 (41.9) Cr
18BrBA Cr 102.0 (71.5) I 198.0 (9.3) SmA 86.6 (64.7) Cr
Note: Cr = crystal; SmA = smectic A; SmB = smectic B; I = isotropic.
(c)
Fig. 4. Optical photomicrograph of 8BrBA taken during the cooling cycle. (a) Optical photomicrograph showing fan-shaped textures of a SmA phase. (b) Optical photomicrograph exhibiting temporary transition bars during the phase transition of SmA to SmB. (c) Optical photomicrograph displaying mosaic textures of SmB phase.
mW a J I M f I
a 30 « so 70 ® 90 100 110 120 133 1« 'C
20 30 40 SO 60 X) SO 90 100 110 130 130 140 4C
Fig. 5. DSC thermograms of (a) 4BrBA, (b) 12BrBA and (c) 18BrBA during heating and cooling scans.
From the plotted graph (Figure 6), the melting point decreased as the length of the chain increased from C2 to C10 member. This resulted from the increase in the flexibility of the molecule owing to the longer terminal chain. The melting temperature showed ascending trend from the medium chain member onwards whereby the melting temperature increased from the C10 (Tm = 88.3 0C) to the C18 (Tm = 102.00C) derivative [35]. This could have been resulted from the increased of the Van der Waals attractive forces between the molecules [36]. Hence, suitable chain length is required for low melting point which agrees with those reported for alkyl-cyanobiphenyl homologues [37].
While the C8 to C12 members showed an increase in their transition temperature of SmA-to-I, the reverse trend was detected for the C12 to C16 members. The transition temperature of SmA-to-I is depending on the terminal intermolecular attractions. As the terminal chains getting longer (from C12 to C16), the smectic molecular order which is determined by the terminal intermolecular attractions become weaker, therefore, the partial interpenetration of the layers become easily occurred. This in turn depress the transition temperatures of SmA-to-I [36, 38].
The length of terminal chain can also influence the phase width of liquid crystals. Figure 7 shows the plot of phase width against the number of carbons in the alkanoyloxy chain. Out of nine members, only C8 member possessed both SmA and SmB as enantiotropic phases. However, the SmB phase width (ASmB = 3.30C) is very narrow and appeared only for a while during heating cycle. Therefore, as the alkanoyloxy chain length increased to C10, the SmB phase disappeared during heating cycle and only appeared on the cooling cycle. The similar behavior persisted till C16 member.
As for the SmA phase width, it increased from C8 (ASmA = 6.80C) and reached the maximum SmA phase width at C10 (ASmA = 20.50C). Thereafter, it started to decrease until C16 (ASmA = 3.90C). At the highest member, C18, the SmA phase no longer appeared during the heating cycle and it was only observed on the cooling cycle. Therefore, for the current core system, it can be concluded that the suita-
ble terminal chain length to obtain the widest SmA phase is C10.
Molecular structure of organic compounds and their liquid crystalline properties are closely correlated. Table 2 lists the transition temperatures and mesomorphic behaviors of 8BrBA and structurally related compounds [25, 39-41].
Polar terminal substituents can change the polarizability and the dipole moment of the whole molecular structure to an extent, depending on its location and orientation in the molecule [42]. Halogen (F, Cl, Br) groups at the terminal position showed strong impact on the mesomorphic properties of a molecule. The melting and transitions temperatures of current compound, 8BrBA are higher than those of 8FBA [39] and 8ClBA [40]. The size of bromine atom is larger in comparison with fluorine and chlorine atoms and the electrons on the bromine atom are loosely held and far from the nucleus. Therefore, larger size atom is more easily polarized. Higher molecular polarizability contributed by the larger bromine atom led to increased melting and transitions temperatures in 8BrBA.
Secondly, the phase stability of liquid crystals was also controlled by the polarizability of halogen terminal groups. Lower polarizability chlorine atom gave rise to the less stable monotropic SmB phase in 8ClBA compared to the current compound (8BrBA) that exhibited enantiotropic SmB phase. Moreover, low polarizability molecule with the fluoro substituent has less tendency to exhibit mesophase and so, there is absence of mesophase in 8FBA.
Difference in the linking groups (ester group in 8BrBA and ether group in 8OBA-Br) at the terminal position can also influence the mesomorphic behavior. Besides that, the present Schiff base 8BrBA is different from previous 8BABr in the orientation of C=N linkage. The orientation of C=N of compound 8BABr is more proper than 8BrBA for resonance. Greater linearity of molecular structure provided by the ether linking group and better resonance caused by the C=N orientation has resulted a larger phase width of SmB and SmA phases in 8OBA-Br (ASmB= 150C; ASmA = 9.0 0C) when compared to 8BrBA (ASmB= 3.30C, ASmA = 6.80C).
Fig. 6. Graph of phase transition temperature against the number of carbon atoms (n) in alkanoyloxy chain.
Fig. 7. Plot of mesophase width against the number of carbons in alkanoyloxy chain.
Table 2. Comparison of present series with reported liquid crystalline compounds
Compound Structure Phase transition (0C) Phase width (0C)
8FBA [39] /=\ fVi F C7H15COO ^ h—W N-' Cr 75.8 I
8ClBA [40] C7H15COO—^ h—W \-/ Cr 93.0 SmA 98.1 I monotropic SmB, ASmA = 5.1
8BrBA (current study) C7H15COO—<\ h—W x—' Cr 96.2 SmB 99.5 SmA 106.3 I ASmB = 3.3, ASmA = 6.8
8BABr [41] c^CO^yJ^y™ Cr 89.6 SmB 94.4 SmA 110.6 I ASmB = 4.8, ASmA = 16.2
8OBA-Br [25] Cr 89.0 SmB 104.0 SmA 113.0 I ASmB = 15.0, ASmA = 9.0
Conclusions
In this article, a new homologues series of 4-bromobenzylidene-4' -alkanoyloxyanilines is reported. The first two members, «-ethanoyloxy and «-butanoyloxy derivatives were non-me-sogens. The SmA phase started to appear in the next member, «-hexanoyloxy derivative. When the number of carbons at the alkanoyloxy chain, n changed to 8, the compound exhibited both enan-tiotropic SmB and SmA phases. From n = 10 to 16, these compounds displayed enantiotropic SmA and monotropic SmB phases. The last member of the series, n-octadecanoyloxy displayed SmA phase only. Comparison of current series with the other structurally related compounds revealed that compounds with terminal bromo substituent possessed better mesomorphic behavior.
Acknowledgements
The authors would like to thank Univer-siti Tunku Abdul Rahman for the research facilities.
References
1. Lim S.K.J., Rahamathullah R., Sarih N.M., Khairul W.M. Tailoring tail-free nematogen of
ethynylated-schiff base and its evaluation as solu-tion-processable OLED emitting material. J Lumin. 2018. V. 201, P. 397-401.
2. Petti L., Rippa M., Fiore A., Manna L., Mormile P. Optically induced light modulation in an hybrid nanocomposite system of inorganic CdSe/CdS nanorods and nematic liquid crystals. Opt Mater. 2010. V. 32, P. 1011-1016.
3. Shurpo N.A., Vakshtein M.S., Kamanina N.V. Effect of CdSe/ZnS semiconductor quantum dots on the dynamic properties of nematic liquid-crystalline medium. Tech Phys Lett. 2010. V. 36. P. 319-321.
4. Wazynska B., Tykarska M., Okowiak-Chinalska J. New application of thermotropic liquid crystals. Mol Cryst Liq Cryst. 2011. V. 542. P. 213-220.
5. Popov N., Honaker L.W., Popova M., Usol'tseva N., Mann E.K., Jakli A., Popov P. Thermotropic Liquid Crystal-Assisted Chemical and Biological Sensors. Materials. 2018. V. 11(1). P. 20.
6. Zakaria M.A., Alazmi M., Katariya K.D., El Ki-lany Y., El Ashry E.S.H., Jaremko M., Hagar M., Mohammady S.Z. Mesomorphic Behaviour and DFT Insight of Arylidene Schiff Base Liquid Crystals and Their Pyridine Impact Investigation. Crystals. 2021. V. 11(8). P. 978.
7. Jamain Z., Khairuddean M., Zulbaharen N.N., Chung T.K. Synthesis, characterization and determination of mesophase transition of azo-azomethine derivatives with different terminal
chain lengths. Malaysian J. Chem. 2019. V. 21(3). P.73-85.
8. Kelker H., Scheurle B. A liquid-crystalline (nematic) phase with a particularly low solidification point. Angew. Chem. Int. Edn. 1969. V. 8. P. 884-885.
9. Eran B.E., Nesrullajev A., Canli N.Y. Characterization and investigation of the mesogenic, thermo-morphologic and thermotropic properties of new chiral (S)-5-octyloxy-2-[{4-(2-methylbuthoxy)-phenylimino(methyl]phenol liquid crystalline compound. Mat. Chem. Phys. 2008. V. 111. P. 555-558.
10. Selvarasu C., Kannan P. Effect of azo and ester linkages on rod shaped Schiff base liquid crystals and their photophysical investigations. J. Mol. Struct. 2016. V. 1125. P. 234-240.
11. Sharma V.S., Patel R.B. The effect of position of tert-butyl tail group on the formation of liquid crystal in Schiff base ester based homologous series. Mol. Cryst. Liq. Cryst. 2017. V. 648(1). P. 53-65.
12. Nafee S.S., Hagar M., Ahmed H.A., El-Shishtawy R.M., Raffah B.M. The synthesis of new thermal stable Schiff base/ester liquid crystals: a computational, mesomorphic and optical study. Molecules. 2019. V. 24(17). P. 3032.
13. Peng W., Li T., Yao D. Synthesis and mesophase behavior of Schiff base liquid crystals with long-chain aliphatic acid. IOP Ser.: Mater. Sci. Eng. 2019. V. 585. P. 012033.
14. Karanlik G., Ocak H., Eran B.B. Imine based chi-ral liquid crystals: effect of varying the terminal substituent and orientation of ester linking unit. J. Mol. Liq. 2019. V. 275. P. 567-577.
15. Ahmed H.A., Hagar N., Saad G.R. Impact of the proportionation of dialkoxy chain length on the mesophase behavior of Schiff base/ester liquid crystals; experimental and theoretical study. Liq Cryst. 2019. V. 46(11). P. 1611-1620.
16. Hagar M., Ahmed H.A., Saad G.R. Synthesis and mesophase behavior of Schiff base/ester 4-(arylideneamino)phenyl-4" -alkoxybenzoates and their binary mixtures. J. Mol. Liq. 2019. V. 273. P. 266-273.
17. Nakum K.J., Katariya K.D., Jadeja R.N., Prajapati A.K. Schiff base of 4-n-alkoxy-2-hydroxybenzaldehyde with 4-aminoacetophenone and their Cu(II) complexes: synthesis, characterization and mesomorphic behavior. Mol. Cryst. Liq. Cryst. 2019. V. 691(1). P. 1-13.
18. Das N., Borah D., Acharya H., Choudhury S., Prasad S.K., Rao D.S.S., Bhattacharjee C.R. Grafting a mesomorphic Schiff base onto gold nanoparticle via ester link-photoluminescence, mesomorphism, electrical conductivity and antioxidant activity. Liq Cryst. 2019. V. 46(4). P. 609-617.
19. Nafee S.S., Hagar M., Ahmed H.A., Alhaddad O.A., El-Shishtawy R.M., Rafiah B.M. New two rings Schiff base liquid crystals; ball mill synthesis, mesomorphic, Hammett and DFT studies. J. Mol. Liq. 2020. V. 299. P. 112161.
20. Ahmed H.A., Hagar M., Alhaddad O.A. Mesomorphic and geometrical orientation study of the relative position of fluorine atom in some themotropic liquid crystal systems. Liq Cryst. 2020. V. 47(3). P. 404-413.
21. Al-Muamin T., Al-lami N., Rahman S., Ali R. Synthesis, characterization and antimicrobial activity of new nucleoside analogues from benzotri-azole. Chem. Chem. Technol. 2016. V. 10(3). P. 271-278.
22. Yeromina H., Demchenko N., Kiz O., Ieromina Z., Demchenko S. The synthesis and antimicrobial properties of new 2-(R-phenylimino)-1,3-thiazoline derivatives containing the N-methylpiperazine moiety. Chem. Chem. Technol. 2019. V. 13(2). P. 150-156.
23. Hassan Z.M., Alattar R.A., Abass S.K., Mihsen H.H., Abbas Z.F., Hussain K.A. Synthesis, characterization and biological activity of mixed ligand (imine of benzidine and 1,10-phenanthroline) complexes with Fe(II), Co(II), Ni(II) and Cu(II) ions. Chem. Chem. Technol. 2022. V. 16(1). P. 15-24.
24. Naoum M.M., Fahmi A.A., Ahmed N.H., Saad G.R. The effect of lateral methyl substitution on the mesophase behavior of aryl 4-alkoxyphenylazo benzoates. Liq Cryst. 2015. V. 42. P. 1627-1637.
25. Galewski Z. Liquid crystalline properties of 4-halogenobenzylidene-4' -alkoxy-anilines. Proc SPIE. 1993. V. 1845. P. 452-458.
26. Galewski Z., Coles H.J. Liquid crystalline properties and phase situations in 4-chlorobenzylidene-4'-alkylanilines. J. Mol. Liq. 1999. V. 79. P. 77-87.
27. Dave J.S., Menon M. Azomesogens with a heterocyclic moiety. Bull. Mater. Sci. 2000. V. 23. P. 237-238.
28. Petrov V.F. Halogenation in achiral liquid crystals. II terminal and linking substitutions. Liq Cryst. 2010. V. 517(1). P. 27-42.
29. Sackmann H., Demus D. The polymorphism of liquid crystals, Mol. Cryst. Liq. Cryst. 1966. V. 2, P. 81-102.
30. Gray G.W., Goodby J.W. Smectic Liquid Crystals: Textures and Structures. Leonard Hill; London: 1984.
31. Goodby J.W., Gray G.W. Tilted smectic B phase or smectic H phase? J. de Physique. 1979. V. 40(C3), P. 363-370.
32. Cozan V., Avadanei M., Perju E., Timpu D. FTIR investigations of phase transitions in an asymmetric azomethine liquid crystal. Phase Trans. 2009. V. 82(8). P. 607-619.
33. Kumar S. Liquid Crystals: Experimental Study of Physical Properties and Phase Transitions. Cambridge University Press; Cambridge: 2001.
34. Demus D., Richter L. Textures of Liquid Crystals. Verlag Chemie; New York: 1978.
35. Kelker H., Hatz R. Handbook of Liquid Crystals. Verlag Chemie; Florida: 1980.
36. Gray G.W. Molecular structure and properties of liquid crystals. Academic Press; London: 1962.
37. Wang K.L., Szilvasi T., Gold J., Yu H.Z., Bao N., Rai P., Mavrikakis M., Abbott N.L., Twieg R.T. New room temperature nematogens by cyano tail termination of alkoxy and alkylcyanobiphenyls and their anchoring behavior on metal salt-decorated surface. Liq Cryst. 2020. V. 47(4). P. 540-556.
38. Prajapati A.K., Bonde N.L. Mesogenic benzothia-zole derivatives with a polar nitro substituent. Mol. Cryst. Liq. Cryst. 2009. V. 501. P. 72-85.
39. Ha S.T., Lee T.L. Synthesis and Mesomorphic Properties of New Fluorinated Schiff Base Liquid Crystals. ISRN Mat. Sci. 2014. Article ID 904657.
40. Ha S.T., Ong L.K., Sivasothy Y., Yeap G.Y., Bo-ey P.L., Lin H.C. New Mesogenic Schiff Base Esters with Polar Chloro Substituent: Synthesis, Thermotropic Properties and X-Ray Diffraction Studies. Am. J. Appl. Sci. 2010. V. 7(2). P. 214220.
41. Ha S.T., Lee T.L., Lin H.C., Ito M.M., Abe K., Kubo K., Sastry S.S. Mesogenic Schiff base with bromo end group: synthesis and thermotropic properties. Asian J. Chem. 2012. V. 24(8). P. 3679-3684.
42. El-atawy M.A., Naoum M.M., Al-Zahrani S.A., Ahmed H.A. New Nitro-Laterally Substituted Az-omethine Derivatives; Synthesis, Mesomorphic and Computational Characterizations. Molecules. 2021. V. 26(7). P. 1927.
YENi AROMATiK §iFF EFIrLORMN SiNTEZi УЭ MEZOGEN XASSOLORl
Teck-Leong Lee, Chelsea Mei-Ing Quek, Sie-Tiong Ha, Guan-Yeow Yeap
§iff asaslarinin yeni efirlarinin homoloji seriyasi, 4-bromobenziliden-4'-alkanoil-hidroksianilinlar sintez edilmi§dir. Sintez edilmi§ birla§malarin strukturu iQ va NMR spektroskopiyasi, hamginin kütlavi spektrometriya ila müayyan edilmi§dir. Cn-1H2n-1COO alkanoiloksi zancirinin uzunlugu n = 2, 4, 6, 8, 10, 12, 14, 16 va 18 arasinda dayman doqquz homoloji üzv alda edildi. Onlarin termotropik xüsusiyyatlari diferensial skan kalorimetriya va polyar optik mikroskopiya ila tahlil edildi. Qisa zancir üzvlari (karbonlarin sayi, n = 2, 4) mezogen deyilsa da, n = 6 olan üzv monotrop smektik A (SmA) fazasini nümayi§ etdirir. (O vaxtdan) n 8-a qadar artdigindan, bu birla§ma enantiotrop faza kimi SmA va SmB nümayi§ etdiran yegana üzv olur. n = 10 ila n = 16 araliginda bu terminlar enantiotrop SmA va monotrop SmB fazalarini nümayi§ etdirir. On uzun zancirli n-oktadekanoiloksi töramalari yalniz bir mezofaza (monotrop SmA) göstarir. Bu siradaki efir bagi va qütb terminali (brom) qrupu §iff asaslarinda smektik polimorfizm ügün vacibdir.
Agar sözlzr: 4-bromobenziliden-4'-alkanoiloksianilinl3r, imin, termotrop, smektik A, smektik B.
СИНТЕЗ И МЕЗОГЕННЫЕ СВОЙСТВА НОВЫХ АРОМАТИЧЕСКИХ ЭФИРОВ ШИФФА
Тэк-Лонг Ли, Челси Мей-Инг Кек, Си-Тионг Ха, Гуан Йов Йап
Синтезирован гомологический ряд новых эфиров оснований Шиффа, 4-бромбензилиден-4'-алканоил-оксианилинов. Строение синтезированных соединений установлено методами ИК- и ЯМР-спектроскопии, а также масс-спектрометрическим методом. Было получено девять гомологичных членов, при этом длина алканоилоксицепи Cn-1H2n-1COO варьировалась от n = 2, 4, 6, 8, 10, 12, 14, 16 и 18. Их термотропные свойства анализировали с помощью дифференциальной сканирующей калориметрии и методом поляризованной оптической микроскопии. В то время как члены с короткой цепью (количество атомов углерода, n = 2, 4) не являются мезогенами, член с n = 6 проявляет монотропную смектическую фазу A (SmA). Поскольку (так как) n увеличилось до 8, это соединение становится единственным членом, который проявляет SmA и SmB в качестве энантиотропной фазы. В диапазоне от n = 10доп = 16 эти члены проявляют энантиотропную SmA и монотропную SmB фазы. Производные н-октадеканоилокси с самой длинной цепью демонстрируют только одну мезофазу (монотропный SmA). Эфирная связь и полярная концевая (бромная) группа в данной серии существенны для смектического полиморфизма в основаниях Шиффа.
Ключевые слова: 4-бромбензилиден-4'-алканоилоксианилины, имин, термотропный, смектик А, смектик Б.
