New mesomorphic benzothiazol derivatives: Synthesis and characterization

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Original

article

New

mesomorphic

benzothiazol

derivatives:

Synthesis

and

characterization

Teck-Ming

Koh

a

,

Sie-Tiong

Ha

b,c,

*

,

Guan-Yeow

Yeap

d

,

Hong-Cheu

Lin

e

a_Faculty_of_Engineering_&_Science,_Universiti_Tunku_Abdul_Rahman,_Jln_Genting_Klang,_Setapak,_Kuala_Lumpur_53300,_Malaysia b_Faculty_of_Science,_Universiti_Tunku_Abdul_Rahman,_Jln_Universiti,_Bandar_Barat,_Kampar_31900,_Malaysia

c

CentreforBiodiversityResearch,UniversitiTunkuAbdulRahman,JlnUniversiti,BandarBarat,Kampar31900,Malaysia

d

LiquidCrystalResearchLaboratory,SchoolofChemicalSciences,UniversitiSainsMalaysia,Minden11800,Malaysia

e

DepartmentofMaterialsScience&Engineering,NationalChiaoTungUniversity,Hsinchu300,Taiwan,China

1. Introduction

The electronicconductingbehavioursof liquid crystals(LCs) havereceivedtremendousattentionsincethediscoveryof high holemobilityinthehexagonalcolumnarphaseof hexapenthyloxy-triphenylene [1–5]. Recently, calamitic LCs became another potentialmaterial for applications in electronic devices due to thepresenceofsomeorderedmesophases,suchasSmA,SmC,SmB and SmE. The two-dimensional (2D) arrangement (layers) in smectics ismore tolerantwithregard todefectsthan the one-dimensional(1D)ordereddiscoticcolumns,thusresultinginan improvedperformance[6].Theelectronicconductingbehaviourof calamitic LCs was ﬁrst examined in the SmA phase of 2-phenylbenzothiazolederivatives.Existenceofalayer(ed)structure inallsmectic LCsinduces

p

–

p

stacking interactions whichcan facilitatethehoppingofchargecarriermobility[7–9].

In our previous studies, we successfully synthesized and isolated a series of homologues of smectic LCs containing the benzothiazolemoiety[10–13].Incorporationofbenzothiazolering intocalamiticLCs,asthemesogeniccore,enabledthemtoeasily exhibitmesophasetransitionsduetothemorepolarizablenature

of theheteroatoms, sulfurand nitrogen[14].In this paper, we report another new homologousseries of benzothiazole liquid crystals(Scheme1).Threecoreunits(onebenzothiazoleandtwo phenylrings)wereconnectedviaa Schiffbaseandesterlinkers whichcanincreasethemolecularbroadnessandanisotropy.

2. Experimental

Thesyntheticprotocoltowards(producing/creating) thetitle compounds, 6-ethoxy-2-[4-(4-alkyloxybenzoyloxy)benzylidena-mino]-benzothiazoles (C2H5O-nBSPEP, wheren=7,8, 9, 10,12,

14) is illustrated in Scheme 1. 2-Amino-6-ethoxybenzothiazole wascondensed with 4-hydroxybenzaldehyde upon refluxingin ethanolfor3htoyieldintermediateA[11].Etherificationof ethyl-4-hydroxybenzoateinacetonewiththeappropriate 1-bromoalk-aneandacidificationusingconcentratedHClproduced intermedi-ateB[15].IntermediatesAandBwerethensubjectedtoSteglich esterificationwiththeappropriatefattyacidinthepresenceofDCC and DMAPaccordingtopreviously reported procedures[16]to yieldtitlecompoundsC2H5O-nBSPEP.Allthecrudeproductswere

puriﬁedbyrepeatedrecrystallizationusingethanoluntilconstant meltingpointswereobtained.Theyieldsoftheﬁnalproductswere between29%and55%(Scheme1).Structuralelucidationofthetitle compoundswascarriedoutviaelementalanalysis,FT-IR,NMR, andEI-MSspectroscopictechniques[17].

ChineseChemicalLetters24(2013)926–928

ARTICLE INFO Articlehistory: Received12April2013

Receivedinrevisedform23May2013 Accepted29May2013

Availableonline2July2013 Keywords: Benzothiazole Nematic SmecticC Mesomorphic ABSTRACT

In this paper, the synthesis of new mesomorphic benzothiazolyl derivatives,

6-ethoxy-2-[4-(4-alkyloxybenzoyloxy)benzylidenamino]benzothiazoles, is presented. The structures of the title

compounds were elucidated using spectroscopic techniques, such as FT-IR, NMR (1_H _and 13_C),

elementalanalysisandEI-MS.Themesomorphicbehavioursofthesecompoundsweredeterminedby

differentialscanningcalorimetricandpolarizingopticalmicroscopictechniques.Compoundsexhibited

nematicandtiltedsmecticphasesuponheatingfromcrystalphase.Anobviousodd-eveneffectwas

observedinthishomologousseries.

ß2013Sie-TiongHa.PublishedbyElsevierB.V.onbehalfofChineseChemicalSociety.Allrights

reserved.

*Correspondingauthorat:FacultyofScience,UniversitiTunkuAbdulRahman, JlnUniversiti,BandarBarat,Kampar31900,Malaysia.

E-mailaddress:[email protected](S.-T.Ha).

ContentslistsavailableatSciVerseScienceDirect

Chinese

Chemical

Letters

j our na l ho me p a ge : w ww . e l se v i e r . com / l oc a te / c cl e t

1001-8417/$–seefrontmatterß2013Sie-TiongHa.PublishedbyElsevierB.V.onbehalfofChineseChemicalSociety.Allrightsreserved.

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3. Resultsanddiscussion

The liquid crystalline textures of the title compounds were observedbypolarizingopticalmicroscopeandphaseidentiﬁcation wasmadebycomparingtheobservedtextureswiththosereported intheliterature[18,19].

Transitiontemperaturesandcorrespondingenthalpychanges weredeterminedusingadifferentialscanningcalorimeterandthe datatabulatedinTable1.

ReferringtoTable1,allsynthesizedcompoundsexhibitedboth liquid crystal phase and nematic phase. A higherordered SmC phase emerged from dodecyloxy derivatives, which contain a relatively longer ﬂexible chain. Due to the thermally unstable

propertiesofthesecompounds,allthetransitiontemperaturescan beonlyobtainedfromtheheatingcycleinDSCanalysis.Thisisdue tothepartialdecompositionofthecompoundsduringheating,a phenomenonalsoreportedintheworkofWeietal.[20].

Representative optical photomicrographs of C2H5O-12BSPEP

areshowninFig.1.ThepresenceofthenematicphasesinC2H5

O-12BSPEPisevidencedbyatypicalmarble-liketextureuponcooling the compound from the isotropic liquid phase. Upon further coolingthenematicphasetoalowertemperature,ahigherordered SmCphaseformedandexhibitedthetransitionbartextureswhich in turnconﬁrmedtheconversionfromthenematic totheSmC phase(Fig.1b)[21].

Aplotoftransitiontemperaturesagainstthenumberofcarbons in thealkoxychainduringtheheatingcycleisshownin Fig.2. Firstly,itisevidentthatbothmelting(Cr-SmC/N)andclearing(N-I) pointsweregreatlyinﬂuencedbythelengthofalkoxychaininthe compounds. The ﬂuctuation of melting and clearing points observed in C7 to C10 compounds was due to the odd–even

Scheme1.Syntheticrouteforthetargetcompounds.(i)EtOH,(ii)CnH2n+1Br,K2CO3,

CH3COCH3,(iii)H2O:EtOH(1:1),KOH,conc.HCl,(iv)DCC,DMAP,DCM,DMF.Yield

of C2H5O-7BSPEP (29%), C2H5O-8BSPEP (33%), C2H5O-9BSPEP (37%), C2H5

O-10BSPEP(41%),C2H5O-12BSPEP(48%),andC2H5O-14BSPEP(55%).

Table1

PhasetransitionandtransitionenthalpychangesforC2H5O-nBSPEPuponheating.

Compound Phasetransition,8C(corresponding enthalpychanges,kJ/mol)

C2H5O-7BSPEP Cr168.0(9.1)Cr2126.9(40.9)N256.2(2.1)I C2H5O-8BSPEP Cr137.8(38.8)N248.5(2.3)I C2H5O-9BSPEP Cr124.8(42.3)N250.6(1.5)I C2H5O-10BSPEP Cr138.3(44.4)N250.2(1.9)I C2H5O-12BSPEP Cr91.0(51.2)SmC158.5(0.5)N223.2(1.7)I C2H5O-14BSPEP Cr169.9(24.1)Cr296.9(26.3)SmC172.1 (0.4)N223.6(1.6)

Cr1,crystal1;Cr2,crystal2;N,nematic;SmC,smecticC;I,isotropic.

Fig.1.Opticalphotomicrographs(100)of(a)nematicphaseshowingmarble-liketextureand(b)SmCphaseexhibitingtypicaltransitionbartextureinC2H5O-12BSPEP.

Fig.2.PhasebehaviourofC2H5O-nBSPEP.

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effect.Theodd–eveneffectcanbeexplainedbythefactthatthe extra carbon causes the chain to deviation from the linear structure,and this deviationfromthelinearityof thestructure causesareducedmeltingorclearingpoint.Thisﬂuctuationwas commonlyinvestigatedforshortchainderivativesina homolo-gousseries[22–25].Secondly,a higherorderedSmCphase was observed only in the homologues with relatively longer chain members,C12andC14inthiscase.Itcanbeunderstood thata longerterminalalkoxychainmayfavourthelamellararrangement in the smectic layer structure due to higher Van der Waals interactionsandthepossibilitiesofintertwiningbetweenalkoxy chains.Lastly,thenematicphaserange(

D

N)wasreducedandthe

SmC phase range (

D

SmA) was increased as the chain length

increased.TheincreasingVanderWaalsattractiveforcesprovide strongerlateralattractiveinteractions,whichinturn,resultedin formationoflayerpacking.

4. Conclusion

Conclusively,allthesynthesizedcompoundsexhibitedliquid crystal behaviour in which nematic and SmC phases were observed.Thelengthoftheterminalalkoxychaingreatlyaffects properties,suchasmeltingandclearingtemperatures,andliquid crystalphaseranges.Inourfuturework,molecularmodiﬁcations onthishomologousserieswillbeplannedinordertoyieldliquid crystalcompoundswithhighthermalstability.

Acknowledgment

The authors would like to thank Universiti Tunku Abdul Rahmanfortheﬁnancialsupportsandresearchfacilities. References

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