Synthesis, Characterization and Study of Mesomorphic Behavior of New Bent and Liner Core Compounds Containing Heterocyclic

Abstract We described herein the synthesized and characterized of new bent and liner core compounds containing thiazolidin-4-one ring[XI-XIII] and [XIV-XVI] respectively. These compounds synthesized by sequence reactions starting from reaction resorcinol or hydroquinone with chloracetyl chloride to yield compounds [I] and [II] ,then the later compounds reactant with 4-hydroxybenzylaldehyde to product dialdehyde compounds [III] and [IV] .The Schiff bases compounds[V-VII] and [VIII-X] synthesized from reaction the compound [III] or [IV] with different aromatic amines, while the bent and liner core mesogens containing thiazolidin-4-one ring [XI-XIII] and [XIV-XVI] synthesized from reaction Schiff bases compounds[V-VII] or [VIII-X] with thioglycolic acid. In addition estrfecation of cinnamic acid with methanol to yield ester compound[XVII]which was converted to their acid hydrazid[XVIII] and the acid hydrazid reacted with glysine in phosphorous oxychloride to yield (5-styryl-1,3,4-oxadiazol-2-yl) methanamine [XIX]. Then reactant the1,3,4-oxadiazol compound[XIX] with dialdehyde compound [III] or [IV] to product new Schiff bases compounds [XX] and [XXI] , afterword added thioglycolic acid to give new thiazolidin-4-one derivatives [XXII] and [XXIII], respectively. The synthesized compounds were characterized by melting points , FTIR , 1 HNMR and mass spectroscopy (of some of them).The liquid crystalline properties were studied by hot stage polarized optical microscopy (POM) and differential scanning calorimetry(DSC).All the Schiff bases compounds [V-X] showed liquid crystals phases while thiazolidin-4-one derivatives[XI-XIII],[XIV-XVI]and [XXIII] didn’t show any liquid crystals properties except the compound [XXII] showed enantiotropic nematic phase.


Introduction
In recent years, there has been interest study of liquid crystals resulting in the discovery of many applications in modern technologies, particularly in display devices due to their unusual optical and electrical properties.The banana-shaped liquid crystals are also interested widely, due to its unique electro-optic and photo-induced characteristics [1,2]moreover, these materials are also of significant interest for command surfaces and sensors [3] Many workers synthesized bent or liner core liquid crystals compounds containing1,3disubstitutedphenylene or 1,4-disubstitutedphenylene from resorcinol or hydroquinone, respectively, as Ciobanu et.al. [4] synthesized and investigated liquid crystalline behavior of symmetrical non liner liquid crystal containing 1,3-disubstituted benzene ring as a central core also consisting of different linkage groups beside to terminal alkyloxy chain containing 6 to 10 carbon atoms (1).All compounds appeared enantiotropic mesophases (nematic and smectic phases).
Also, Huzum et al. [5]synthesized and studied mesomorphic behavior of two new series of bent core liquid crystalline compounds based on a 1,3-dihydroxybezene core and containing a cholesteryl 6-oxyhexanoate wing.While, Alaasar et al [6]synthesized and investigated bent-core mesogens derived from 4-methylresorcinol with azobenzene wings and terminal group as (OC n H 2n+1 ,n=6-22).The short homologues chain exhibit smectic phases and for the longes thomologue only a crystalline phase was observed.
In addition ,Yuvaraj et al. [7]investigated the liquid crystalline and light-induced properties of resorcinol spacer based dimericazobenzene(2),the compound [2]showed nematic and smecticA phases (2) One the other hand ,synthesized liner core liquid crystals compounds containing 1,4disubstitutedphenylene from hydroquinone as, synthesized three aromatic rigid mesogenicdiols(3) based on hydroquinone and 4-hydroxy benzoic acid as starting materials [8].The experimental results reveal that hydroquinone based rigid triad mesogenicdiol have higher thermal stability.In the literature, we did not found any studies of the effect of thiazolidin-4-one derivativeson the liquid crystals properties, therefore this paper presents the synthesis, structural characterization and mesomorphic properties of new bent and liner core liquid crystals containing thiazolidin-4-one unit as while as 1,3,4-oxadiazol ring.

Experimental Chemicals
All chemicals were supplied by Fluka, Merck and Aldrich chemicals Co.

Techniques
FT-IR spectra were recorded by 600 FT-IR spectrometer.

Synthesis of Schiff base compounds [V-VII] and [VIII-X]
A mixture of compound [III] or [IV] ( 0.001 mol ) and different aromatic amines (0.002 mol) were dissolved in 5ml ethanol absolute, refluxed for 3-4 hrs .and cooled to room temperature .the solid product was to give new Schiff bases .The physical properties are listed in Table (1).

Synthesis of thiazolidin-4-onederivatives[XI-XIII] and [XIV-XVI]
A mixture of Schiff bases [V-VII] or [VIII-X] (0.001mol) and thioglycolic acid (0.002 mol) was refluxed in dry benzene (3 ml) for 12 hrs .The solvent was evaporated and residue was neutralized with sodium bicarbonate solution then, the precipitate was filtered.The physical properties are listed in Table (1).

Results and discussion
In order to obtain the compounds [I] and [II] ,we reacted the resorcinol or hydroquinone with two moles from chloroacyl chloride in pyridine and DMF.These compounds were identified by FTIR spectroscopy.
The FTIR spectra of these compounds showed disappearance bands of the υ OH groups, which were observed in the starting materials and showed new absorption stretching bands at 1774 cm -1 and 1764cm -1 that are attributed to the C=O of ester groups [12],respectively.Also, showed stretching band at 793cm -1 and 758cm -1 for CH 2 Cl,respectively.
The dialdehyde [III] and [IV] were prepared from reacted compound [I] or [II]with two moles from 4-hydroxybenzylaldehyde and fused sodium acetate in absolute ethanol .The FTIR spectra of these compounds showed new stretching bands at 1686cm -1 and1668 cm - 1 assigned to CHO groups , also showed new stretching bands at 1217cm -1 and1215cm -1 for ether groups( -CH 2 -O-),respectively.While the 1 HNMR spectrum (in DMSO-d 6 as a solvent) of compound [III], Figure (1 )showed: a singlet signal at δ9.77 ppm for two protons of CHO groups, many signals in the region δ (6.17-7.75)ppm of twelve aromatic protons of benzene rings, a singlet signal at δ 3.45 ppm due to four protons of two (OCH 2 CO)groups.
The Schiff bases [V-VII]and[VIII-X] were synthesized by condensation reaction of one mole from the dialdehyde compounds[III] or [IV]with different aromatic amine in ethanol.These Schiff bases were characterized by melting points and FTIR and 1 HNMR spectroscopy.
The characteristic FTIR spectra of these compounds showed the disappearance of absorption stretching bands due to NH 2 and CHO for starting material together with the appearance of stretching bandsat range (1651-1654) cm -1 and (1638-1645)cm -1 assignable to C=N groups, respectively .The characteristics FTIR absorption bands of new Schiff bases are listed in Table '(2).The 1 HNMR spectra (in DMSO-d 6 as a solvent) for compound [V] and [IX ]showed: a singlet signals at δ8.45 ppm and δ8.59 ppm for two protons of azomethine ( N=CH) groups, respectively , multiplet signals appear at δ (6.85-7.74)ppmand δ (6.54-7.75)ppmfor twenty aromatic protons of benzene rings , respectively.A sharp signal at δ3.75 ppmthat could be attributed to the four protons of two(OCH 2 CO) groups and six protons of two (CH 3 O) terminal groups for compound [V].while two protons of two(OCH 2 CO) groups and six protons of two (CH 3 ) terminal groups for compound [IX] appear at δ3.32 ppm and δ2.92 ppm, respectively, the 1 HNMR spectrum of compound [V] as Figure ( 2).
The thiazolidin-4-one derivatives[XI-XIII] and [XIV-XVI] were synthesized by the reaction of two moles of thioglycolic acid with one mole of Schiff bases [V-VII]and [VIII-X] in dry benzene ,respectively.These compounds were characterized by their melting points and FTIR spectroscopy.The FTIR spectra showed the disappearance of a stretching band of imine groups and the appearance of a stretching band of carbonyl group of thiazolidinone moiety [13] around (1662-1678) cm -1 and(1654-1685) cm -1 ,while (C-S) groups [14] around ( 950-993) cm -1 and ( 970-997) cm -1 ,respectively.The FTIR spectral data for these compounds are listed in Table (3). 1 HNMR spectra of compounds [XII] and [XIV] (in DMSO-d6 as a solvent): showed signals at δ 2.18 ppm and δ 3.65 ppm for six protons of the two (CH 3 ) and The ester compound [XVII] was prepared from reaction of cinnamic acid in methanol with conc.H 2 SO 4 (as a catalyst).The FTIR spectrum for compound [XVII] showed the disappearance of absorption stretching bands of O-H and C=O groups of (carboxylic moiety) in starting materials together with the appearance of a new stretching band at 1716cm - 1 assigned to C=O group of ester moiety.
The condensation of one mole of ester compound [XVII] with excess of 80% hydrazine hydrate in ethanol produced the acid hydrazide [XVIII].The compound was characterized by FTIR spectroscopy ,the FTIR spectrum for this compound showed stretching vibration of (NH, NH 2 ) groups in the region (3280-3188)cm -1 as well as stretching absorption at 1664cm -1 of υ C=O (amide) and disappearance of absorption stretching band due to υ C=O of ester moiety.
The acid hydrazide[XVIII] and glysine compound could be smoothly cyclidehydrated by boiling in phosphorus oxychloride to produce the 1,3,4-oxadiazol compound[XIX].The FTIR spectrum for this compound showed absorption stretching band at 1637cm -1 due to υ C=N group for oxadiazol ring.The mechanism [15] of dehydration in the presence of POCl 3 is depicted in the following steps: (Scheme 2) Then, reactant the1,3,4-oxadiazol compound[XIX] with dialdehyde compound [III] or [IV] to synthesize new Schiff base compounds [XX] and [XXI] , respectively.These Schiff bases were characterized by melting points and FTIRand 1 HNMR spectroscopy.
The characteristic FTIR spectra of these compounds showed the disappearance of absorption bands due to NH 2 and CHO for stretching of starting material together with the appearance of stretching bandsat 1645cm -1 and1651cm -1 assignable to C=N groups,respectively .groups ,multiplet signals appear at δ (6.40-7.75)ppmfor twenty two aromatic protons of benzene rings, signals at δ(5.41-6.33)ppm that could be attributed to the four protons of two(-CH=CH-)groups.Also singlet signal appears at δ 3.51 ppm due to four protons of two (OCH 2 CO) groups and signal appears at δ2.08 ppm for four protons of two(CH 2 N) groups,.
In addition to, synthesizing new thiazolidin-4-one derivatives [XXII] and [XXIII] by adding two moles of thioglycolic acid with one mole of Schiff bases [XX] and [XXI] in dry benzene as a solvent.The FTIR spectra for these compounds showed the disappearance of a stretching band of imine groups and the appearance of a stretching band of carbonyl group of thiazolidinone moiety at 1674cm -1 and 1655 cm -1 also C-S groups at 997 cm -1 and 968cm - 1 ,respectively.The mass spectrum of compound [XXIII] that molecular weight =948 , Figure (5) showed base peak at m/z=110 .Also the spectrum showed peak attributed to the presence of the oxadiazol ring (m/z=81 and 53) and peak at (m/z=149)of the thiazolidin-4one ring ,as while as peaks at m/z= 77and 65 for aromaticity of this compound.The fragments of this compound were illustrated in Scheme (3).

Liquid crystalline properties
The liquid crystalline properties were investigated by polarized optical microscopy (POM) in association with differential scanning calorimetry(DSC).
All the schiff bases compounds [V-X] showed liquid crystals phases.The compound [V]showed monotropic droplets nematic texture under POM because the methoxy groups have a higher polarizability, this could be explained in term of terminal/lateral (t/l) interaction forces ratio, because the methoxy chain led to a higher ratio of t/l interaction forces and when this ratio is high, compound tend to give less ordered mesophases (nematic mesophase) [16] as Figure 6(a),while Figure (7) showed DSC thermogram for this compound.

Figure ( 1 )FigureFigure ( 4 )Figure ( 5 )
Figure (1 ):The 1 HNMR spectrum of compound [III] DMSO was used as a solvent with TMS as an internal standard, measurements were made at Central lab ,Tehran University.Massspectrum was determined by scan110(1.257min):directprob-013777.d,atCentral lab ,Tehran University.uncorrectedmelting points were determined by using Hot-Stage, Gallen Kamp melting point apparatus.The transition temperatures and textures of the mesophases were determined using polarized opticalmicroscope model Leica DM2500 Mand DSC measurements were conducted with STAPT-1000LINSIS.