Synthesis and characterization of triad based rigid mesogenic diols derived from hydroquinone and 4-hydroxybenzoic acid

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1 Indian Journal of Chemistry Vol. 53B, May 2014, pp Synthesis and characterization of triad based rigid mesogenic diols derived from hydroquinone and 4-hydroxybenzoic acid Khudbudin B Mulani a, Nitin V Ganjave b & Nayaku N Chavan* a a Polymer Science and Engineering Division, CSIR-National Chemical Laboratory, Pashan Road, Pune , India b Department of Chemistry, Fergusson College, F. C. Road, Pune , India nn.chavan@ncl.res.in Received 26 February 2013; accepted (revised) 18 March 2014 Triad based rigid mesogenic diols have been synthesized by four step synthesis method using protection-deprotection method. Hydroquinone and 4-hydroxy benzoic acid have been used as starting materials. Synthesized diols have been characterized by IR, 1 H and 13 C NMR, and mass spectroscopic methods. Thermal properties have been determined by thermo gravimetric analysis method and degree of crystallinity have been measured by wide angle X-ray technique. Substituted hydroquinones (methyl and chloro) have been used to study the effect of substitution on physical and thermal properties. Synthesis of rigid mesogenic diol monomer using p-hydroxy benzoic acid and hydroquinone is reported, which is a facile route. Hydrolysis of diacetate derivatives of rigid mesogenic diols is performed in good yields, even though two types of ester groups present in the same moiety, aromatic and aliphatic. The experimental results reveal that hydroquinone based rigid triad mesogenic diol have high thermal stability and degree of crystallinity as compared to methyl-and chlorosubstituted rigid triad mesogenic diols. Keywords: Mesogenic diols, degree of crystallinity, thermal properties, spectral properties, ester hydrolysis Liquid crystalline materials have large number of applications, particularly in display devices due to their unusual optical and electrical properties. The long thin film liquid crystal molecules cause light to travel at different speeds along the molecular axis and perpendicular to that axis. The long thin liquid crystal molecules also have a tendency to align parallel to an applied electrical field. This response and the optical properties of liquid crystals led to their application in various devices such as watches, calculators, computers, televisions, etc. The liquid crystal display has become common in watches due the low electrical power demands of the LCD panel 1. Thermotropic liquid crystalline materials consist of anisotropic molecules that self-assemble into phases with orientational order, but no positional order, in their simplest form. These phases exist between the crystalline phase and isotropic liquid phase. These molecules show a high degree of shape anisotropy, which manifests itself in many ways, such as dielectric and optical anisotropies. The dielectric anisotropies of liquid crystalline molecules make them susceptible to electric fields. This is the fundamental property that makes electro-optic liquid-crystal devices possible. Thermotropic liquid crystalline materials exhibit a phase transition into the LC phase as temperature is changed 2. The current interest is in polymers which display thermotropic mesomorphism due to the structure of the rigid-flexible main chain. In this respect, the triad based rigid mesogens are important which are responsible for the formation of thermotropic liquid crystalline phases 3-8. The synthesis of rigid mesogens required for thermotropic liquid crystalline phases are reported by several authors Krigbaum, et al. reported the synthesis of rigid mesogenic diol using p-hydroxy benzoic acid and hydroquinone, but this route is not facile 10. Luyen, et al. also reported the synthesis of rigid mesogenic diacetate from p-hydroxy benzoic acid and hydroquinone, but hydrolysis of diacetate derivative is not reported, which is a critical step in the synthesis of rigid mesogenic diols. This is because within the same molecule, two types of esters groups are present, aromatic and aliphatic. The objective of this research was to synthesize triad based mesogenic rigid diols using hydroquinone and p-hydroxybenzoic acid as starting materials to obtain thermotropic liquid crystalline polymers In this communication, the synthesis and characterization of triad based mesogenic rigid diols using p-hydroxy benzoic acid and hydroquinone/substituted hydroquinone as starting materials is reported. Particularly, physical and

2 592 INDIAN J. CHEM., SEC B, MAY 2014 thermal properties of these rigid mesogenic diols are reported in detail. Experimental Section Hydroquinone, 2-methyl hydroquinone, 2-chlorohydroquinone and 4-hydroxy benzoic acid were purchased from Aldrich. Hydroquinone and 4-hydroxy benzoic acid were purified by recrystallization from acetone and hot distilled water respectively, whereas 2-methyl hydroquinone and 2-chloro-hydroquinone were purified by recrystallization from toluene. Acetic anhydride, dichloromethane, methanol, pyridine, chloroform, and potassium hydroxide were procured from Merck and used as received. Melting points are uncorrected and were determined with a capillary melting point apparatus. 1 H NMR were recorded on 300 MHz Bruker spectrometer using TMS as an internal standard. IR spectra were obtained on Perkin-Elmer FTIR spectrometer using KBr pellet and were collected in the range of cm -1. Elemental analysis was performed on Flash EA 1112 series, ThermoFischer Scientific. Mass spectrometry study was carried out on AB Sciex, Triple TOF 5600 system. Thermogravimetric analysis was conducted on a Perkin-Elmer 7 applying a heating rate of 10 C/min in nitrogen atmosphere to determine thermal stability. Wide angle X-ray scattering (WAXS) curves were obtained by a Philips PW X-ray diffractometer equipped with 1830 generator and 1710 adjustment with Cu-Kα radiation and a scanning angle of Preparation of triad based mesogenic rigid diol Bis(4-hydroxybenzoyloxy)-1,4-benzene (DHBPPE), bis(4-hydroxybenzoyloxy)-2-methyl-1,4-benzene (M- DHBPPE), and bis(4-hydroxybenzoyloxy)-2-chloro- 1,4-benzene (C-DHBPPE) were prepared from p-hydroxy benzoic acid and hydroquinone/substituted hydroquinone as starting materials. The synthesis scheme of rigid mesogenic diols is depicted in Scheme I using protection/deprotection method 1, and general experimental procedure is presented below. HO H 4-Hydroxy Benzoic acid ( CO) 2 O H H 4-Acetoxy benzoic acid SOCl 2 Pyridine 80 o C 8 h DCE COCl Pyridine HO OH X OOC X KOH 0-5 o C Hydrolysis HO OOC OH X Where X = H,, or Cl Scheme I Synthesis of triad based mesogenic rigid diols

3 MULANI et al.: TRIAD BASED RIGID MESOGENIC DIOLS Acetoxy benzoic acid Into a 500 ml beaker, p-hydroxy benzoic acid (13.81 g, 0.1 mol) and distilled water (250 ml) were added and stirred to make uniform slurry. In another beaker, sodium hydroxide (4.4 g, 0.11 mol) was dissolved in distilled water (50 ml), added slowly to the slurry and the solution stirred till complete dissolution of 4-hydroxy benzoic acid to its sodium salt. To this solution, distilled acetic anhydride (11 ml, 0.11 mol) was added slowly and the mass stirred for 4 hr at RT to obtain a precipitate of 4- acetoxy benzoic acid. The solution was filtered, the residue washed successively with cold dilute hydrochloric acid and distilled water. The crude product was purified by recrystallization from methanol and dried under reduced pressure at 80 C for 8 hr. Yield: 80%; m.p C; IR (Nujol): 1730 cm -1 [C=O stretching]; 1 H NMR (DMSO-d 6 ) : δ 8.00 (d, 2H), 7.30 (d, 2H), 2.30 (s, 3H) and (s, 1H). 4-Acetoxy benzoyl chloride Into a single necked 100 ml round bottom flask, equipped with reflux condenser, 4-acetoxy benzoic acid (9.00 g, 0.05 mol) was placed. To this flask, thionyl chloride (5 ml, 0.07 mol) was added dropwise and the reaction mixture refluxed for 8 hr. The initial heterogeneous mass was homogenized. Excess thionyl chloride was removed by distillation. The crude product was purified by distillation under reduced pressure at 130 C. Yield: 90%; m.p C and b.p. 132 C; IR (Nujol): 1780 (COCl), 1730 (C=O stretching) and 1370 cm -1 (-CH, bending); 1 H NMR (CDCl 3 ): δ 2.1 (s, 3H), 7.1 (d, 2H) and 8.0 (d, 2H). Bis(4-acetoxybenzoyloxy)-1,4-benzene Into a 500 ml three necked round bottom flask, equipped with magnetic stirrer, nitrogen inlet and calcium chloride guard tube, hydroquinone (4.4 g, 0.04 mol) and pyridine (25 ml, 0.3 mol) were placed and the mass stirred till dissolution. 4-Acetoxy benzoyl chloride (23.82 g, 0.12 mol) in dry 1,2-dichloroethane (300 ml) was taken in another flask and this solution was added to the hydroquinone solution. The reaction mixture was stirred under nitrogen blanket for 48 hr at RT and the mixture was washed successively with 5% sodium carbonate solution, 5% hydrochloric acid and distilled water. Dichloroethane layer was concentrated to dryness on a rotary evaporator. The crude product was purified by recrystallization from chloroform/petroleum ether (60-80 C) solvent mixture. The solution was filtered and the obtained solid mass was dried under reduced pressure at 80 C for 4 hr. Yield: 90%; m.p C; IR (Nujol): 1723 cm -1 (C=0 stretching); 1 H NMR (CDCl 3 ): δ 2.2 (s, 6H), 7.1 (d, 8H) and 8.1 (d, 4H). Bis(4-hydroxy benzoyloxy)-1,4-benzene (DHBPPE) Into a single necked 250 ml round bottom flask, equipped with magnetic stirrer and ice bath, potassium hydroxide (1.32 g, 0.02 mol) and methanol (100 ml) were placed, stirred till dissolution and cooled to 0-5 C. To this solution, bis(4-acetoxybenzoyloxy)-1,4-benzene (4.34 g, 0.01 mol) was added and the mixture stirred at 0-5 C for 4 hr. The reaction mixture was then acidified with 4N hydrochloric acid solution. Methanol was removed under reduced pressure. The crude product was dissolved in 400 ml ethyl acetate and the solution was washed 3 times with distilled water. The ethyl acetate was then distilled off and the solid was extracted with chloroform. Chloroform was removed on a rotary evaporator. The solid product thus obtained was washed with methanol and dried under reduced pressure at 80 C for 4 hr. Yield: 80%; m.p C; MS: m/z 350 (DHBPPE), 364 (M-DHBPPE) and 386 (C-DHBPPE). Results and Discussion Synthesis It is well known that the thermal properties of the liquid crystalline monomers are dependent on the structure of the mesogenic unit. A long rigid mesogen shows high temperature thermal properties 17. Synthesis of rigid mesogenic diols with chloro-or methyl substitution on central hydroquinone moiety are studied for physical and thermal properties. Synthesis of bis(4-hydroxybenzoyloxy)-1,4- benzene (DHBPPE) is reported in literature either by melt or solution condensation method, but these routes are not facile 14,15, The alternative route has now been developed which is slightly lengthy but simple and straight forward. This route is started from commercially available 4-hydroxy benzoic acid. The four steps are required to get the desired product, and yields are more than 90% except step 4. US patents , , and describe the synthesis of triad based mesogenic rigid diols derived from substituted hydroquionone and 4- methoxy benzoyl chloride to study the liquid crystalline properties 14,16. Van Lueyn et al. reported the synthesis of diacetoxy derivative of triad based mesogenic diol prepared from the 4-hydroxy benzoic

4 594 INDIAN J. CHEM., SEC B, MAY 2014 acid and hydroquinone 9. Ignatious et al. reported the synthesis of mesogenic diol prepared from the condensation of 4-acetoxy benzoic acid and hexane diol to study the liquid crystalline properties 21. US patent illustrates the preparation of mesogenic diol based on p-hydroxybenzoate and aliphatic diol 15. Cai et al. describes the synthesis of mesogenic diol prepared from p-hydroxy benzoic acid and substituted 4,4 -biphenol Hydroxybenzoic acid and hydroquinone/ substituted hydroquinones are used as starting materials to obtain rigid mesogenic diols, because these diols are fully aromatic and have para-para orientation, which is one of the important parameters of rigidity. Basically, liquid crystalline phase formation is dependent on axial ratio. Minimum requirement of axial ratio for the stabilization of the nematic mesophase is 4.3 and for the smectic mesophase is in the range of 4.5 to 7.3 (Ref. 22). 4-Hydroxybenzoic acid contains two functional groups, viz. phenolic OH and carboxyl-h. The direct condensation of hydroquinone with 4-hydroxy benzoic acid is not possible to obtain the desired product, because self polymerization of 4-hydroxy benzoic acid would take place and yield of the desired product is extremely low. Thus, protection and deprotection strategy is applied to get the desired product in high yield 23. Protection of hydroxyl group of 4-hydroxy benzoic acid was performed with acetic anhydride using sodium hydroxide as a base catalyst at RT Acetoxybenzoyl chloride was prepared by treating 4-acetoxy benzoic acid with thionyl chloride and was purified by vacuum distillation 25. The condensation of 4-acetoxy benzoyl chloride with hydroquinone/substituted hydroquinone was performed using pyridine as catalyst under nitrogen blanket at RT 9. Diacetoxy derivative of rigid mesogeinc diol was purified by recrystallization from chloroform/pet ether solvent mixture. The hydrolysis of diacetoxy derivative of rigid mesogenic diol is an important step, because rigid diol contains two ester groups, aromatic and aliphatic. In general, hydrolysis of ester can be catalysed by acid, base or lipase. In the present system, alkaline hydrolysis was preferred to control the hydrolysis of aromatic esters, because aromatic esters are more stable than aliphatic esters 26. Base catalysed hydrolysis of rigid mesogenic diol was carried out using potassium hydroxide in 95% ethanol at lower temperature. The final product was purified with methanol to remove the hydrolysed diol impurities such as hydroquinone and 4-hydroxy phenyl 4-hydroxy benzoate 10, 27. Elemental analysis Elemental analysis of DHBPPE, M-DHBPPE and C-DHBPPE is presented in Table I. Calculated values of percentage of C, H and Cl are presented in parenthesis, whereas open values indicate actual percentages obtained. Elemental analysis reveals that the experimental and calculated values are close to each other. Spectroscopic Data DHBPPE. IR (KBr): 3332 (OH stretching), 3060 (C-H stretching), 1907 (anti symmetric C=C stretching), 1703 (C=O stretching), 1609 and 1594 (aromatic C=C stretching), 1504 (C-H bending), 1449 (symmetric C=C stretching), 1279, 1166 and 1078 ( C-O-C stretching), and 847 cm -1 (para-substitution); 1 H NMR (DMSO-d 6 ): δ (dd, 4H), 7.34 (d, 4H), (dd, 4H), (s, 2H); 13 C NMR (DMSO-d 6 ): δ (aromatic C, ortho to-oh), (aromatic C, ortho to-oh, central HQ), (aromatic C, meta to OH, outer phenyl ring), (aromatic C, central HQ), (aromatic C, para to OH, outer phenyl ring), (aromatic C, C-OH, outer phenyl ring), (C=O). M-DHBPPE. IR (KBr): 3382 (OH stretching), 3023 (C-H stretching), 1688 (C=O stretching), 1608 and 1591 (aromatic C=C stretching), 1512 (CH bending), 1448 (symmetric C=C-stretching), 1279, 1225, 1166 and 1078 (symmetric C-O-C-stretching), 847 (parasubstitution) and 765 cm -1 (, substitution); 1 H NMR (DMSO-d 6 ): δ 2.19 (d, 3H), (d, 4H), 7.25 (d and dd, 3H), (dd, 4H), 10.61(s, 2H); 13 C NMR (DMSO-d 6 ): δ (aliphatic substitution, ), (aromatic C, ortho to-oh), (aromatic C, central HQ), (aromatic C, meta to-oh, outer phenyl ring), (aromatic C, para to OH, outer phenyl ring), Table I Elemental analysis of triad based mesogenic rigid diols Sample Code DHBPPE M-DHBPPE C-DHBPPE ND = Not detected Found (Calcd) % C ( ( (62.17 H ) ) Cl ND ND )

5 MULANI et al.: TRIAD BASED RIGID MESOGENIC DIOLS (aromatic C, C-OH, outer phenyl ring), (C=O). C-DHBPPE. IR (KBr): 3393 (OH stretching), 3072 (C-H stretching), 1907 (anti symmetric C=C stretching), 1708 (C=O stretching), 1609 and 1591 (aromatic C=C stretching), 1512 (CH bending), 1447 (symmetric C=C stretching), 1276, 1162 (symmetric C-O-C-stretching), 1066, 762 (C-Cl substitution) and 846 cm -1 (para-substitution); 1 H NMR (DMSO-d 6 ): δ (dd, 4H), (dd, 3H), 7.96 (S, 1H), (dd, 4H), 10.64(s, 2H) respectively; 13 C NMR (DMSO-d 6 ): δ (aromatic C, ortho to OH, outer phenyl ring), (aromatic C, central HQ ring), (aromatic C, near chlorine, central HQ ring), (aromatic C, meta to-oh, outer phenyl ring), (aromatic C, aromatic C, para to-oh, outer phenyl ring), (aromatic C, C-OH, outer phenyl ring), (C=O). Above three 1 H and 13 C NMR spectra reveal that the proton and 13 C signals of rigid mesogenic diols are matched to chemical structures of triad mesogenic diols. Thermal analysis Decomposition temperatures of triad based mesogenic rigid diols are presented in Table II, and comparative thermo gravimetric analysis (TGA) curves of rigid diols are depicted in Figure 1 for DHBPPE, M-DHBPPE and C-DHBPPE. Two T max (maximum decomposition temperature) peaks were observed at and C for DHBPPE. Out Table II Decomposition temperature of triad based mesogenic rigid diols Sr. No. Polymer Code T max1 ( C) T max2 ( C) 1 DHBPPE M-DHBPPE C-DHBPPE Figure 1 Comparative TGA curves of mesogenic rigid diols DHBPPE, M-DHBPPE and C-DHBPPE of these, the peak observed at C is strong and indicates that maximum decomposition took place at that temperature, whereas the peak appearing at C is a small and indicates that only a small amount of decomposition took place at that temperature. M-DHBPPE shows three decomposition peaks observed at , and C. Out of these three peaks, the peak observed at C is strong and reveals that maximum decomposition took place at that temperature whereas the rest of the peaks showed comparatively lesser decomposition. C- DHBPPE exhibits three peaks at , and C. Out of these three peaks, the peak which appeared at C is strong and indicates that maximum decomposition took place at that temperature whereas the peak at C shows comparatively lesser decomposition and peak at C indicates an intermediate extent of decomposition. Thermal analysis study reveals that triad based mesogenic rigid diols are thermally highly stable and show two step degradation. Decreasing order of thermal stability is as below: DHBPPE > M-DHBPPE > C-DHBPPE. It indicates that substitution reduces thermal stability because substitution frustrates the chain packing, reduces chain packing efficiency and results in decrease in decomposition temperature. X-ray diffraction The wide angle X-ray diffractogram of DHBPPE exhibits relatively sharp and strong absorption peaks at 2θ of 18 and 24, whereas peaks of intermediate intensity are observed at 2θ of 13 and 28 and peaks of weak intensity are observed at 2θ of 16, 32, and 34. The wide angle X-ray diffractogram of M-DHBPPE shows relatively sharp and strong absorption peaks at 2θ of 14, 25 and 26, whereas peaks of medium intensity are observed at 2θ of 12, 14, 18, and 20 and peaks of weak intensity observed at 2θ of 15, 23, and 28. The wide angle X-ray diffractogram of C-DHBPPE displays relatively sharp and strong absorption peaks at 2θ of 19 and 24, whereas absorption peaks of medium intensity are observed at 2θ of 13 and peaks of weak intensity are observed at 2θ of 15, 26, 27, 28 and 30. On the basis of rough estimation of degree of crystallinity, the decreasing order is as follow: DHBPPE > M-DHBPPE > C-DHBPPE. Conclusion Three aromatic rigid mesogenic diols based on hydroquinone and 4-hydroxy benzoic acid as starting materials were synthesized. Methyl hydroquinone and chlorohydroquinone were used to study the effect of

6 596 INDIAN J. CHEM., SEC B, MAY 2014 substitution on physical, thermal and optical properties. The experimental results reveal that hydroquinone based rigid triad mesogenic diol have higher thermal stability, degree of crystallinity than methyl-and chloro-substituted rigid triad mesogenic diols. References 1 Schadt M, Ann Rev Mater Sci, 27, 1997, Brown G H, Doane J W & Neff V D, A Review of the Structure and Physical Properties of Liquid Crystals (Butterworth, London), 1971, p Chavan N N, Material Sciences and Applications, 10, 2011, Chavan N N, Mathew J, Ghadage R S, Rath A K & Ponrathnam S, Eighth Annual Meeting of Polymer Processing Society (PPS-8), held at New Delhi, 1992, p Chavan N N & Ponrathnam S, International Symposium POLYMERS' 94, held at Indian Petrochemicals Corporation Limited, Vadodara, India, Polymer Science, Recent Advances, 2, 1994, Chavan N N, Ghadage R S & Ponrathnam S, International Symposium on Polymers Beyond AD 2000, held at Indian Institute of Technology, New Delhi (India) in January, 1999, p Chavan N N, Hargude S S & Anasane V P, International Symposium on Frontiers of Polymer Science and Engineering, Macro 2002, held at Indian Institute of Technology, Kharagpur, India, December, 2002, P-1. 8 Chavan N N, Ph D Thesis, Shivaji University, Kolhapur, India, December, van Luyen D & Strzelecki L, Eur Polym J, 16, 1980, Krigbaum W R, Kotek R, Ishikawa T, Hakemi H & Preston J, Eur Polym J, 20, 1984, Ignatious F, Ward W J, Lenz R W & Kantor S W, Polymer, 36, 1995, Chai Z, Sun J, Zhou Q & Xu J, J Polym Sci Polym Chem Ed, 45, 2007, Jin J I, Chang J & Jo B, Polym Bull, 20, 1988, Wellinghoff S T & Honson D P, US Patent, 2004, ; (b) ibid, 2006, 7041; (c) ibid, 2007, Kim K, Dashevsky S & Palmaska S W, US Patent, 1995, Sugimoto H, Obbe Y & Hayatsu K, US Patent, 1992, Castell P, Serra A & Galia M, J Polym Sci Polym Chem Edn, 42, 2004, Jin J I & Chang J, Macromolecules, 22, 1988, Matyus E & Fodor-Csorba K, Liq Cryst, 30, 2003, Cai Z, Sun J, Zhou Q & Xu J, J Polym Sci Polym Chem Ed, 45, 2007, Ignatious F, Ward W J, Lenz R W & Kantor S W, Polymer, 38, 1995, Twieg R J, Chu V, Nguyen C, Dannels C M & Viney C, Liq Cryst, 20, 1996, Sartori G, Ballini R, Biggi F, Bosica G, Maggi R & Righi P, Chem Rev, 104, 2004, Garcia T, Carreon-Castro M D P, Gelpver-Santiago A, Ponce P, Romero M & Rivera E, Designed Monomers and Polymers, 15, 2012, Zuffanti S, J Chem Edu, 25, 1998, Broxton T J & Duddy N W, J Org Chem, 46, 1981, Khurana J M, Chauhan S & Bansal G, Monat Chem, 135, 2004, 83.