Thermal and Mesomorphic Investigations of 1:1 ...

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Thermal and Mesomorphic Investigations of 1:1 Supramolecular Assemblies of 4-[(4-(n-Alkoxy)phenylimino)methyl]benzoic Acids Having Symmetrical and Un-Symmetrical Terminal Chain Lengths

Fowzia S. Alamro 1, Hoda A. Ahmed 2,3,* , Ayman M. Mostafa 4,5,6 and Magdi M. Naoum 2

Citation: Alamro, F.S.; Ahmed, H.A.; Mostafa, A.M.; Naoum, M.M. Thermal and Mesomorphic Investigations of 1:1 Supramolecular Assemblies of 4-[(4-(nAlkoxy)phenylimino)methyl]benzoic Acids Having Symmetrical and Un-Symmetrical Terminal Chain Lengths. Symmetry 2021, 13, 1785. sym13101785

Academic Editors: Rui Tamura and Sergei D. Odintsov

Received: 5 August 2021 Accepted: 18 September 2021 Published: 25 September 2021

Publisher's Note: MDPI stays neutral with regard to jurisdictional claims in published maps and institutional affiliations.

Copyright: ? 2021 by the authors. Licensee MDPI, Basel, Switzerland. This article is an open access article distributed under the terms and conditions of the Creative Commons Attribution (CC BY) license (https:// licenses/by/ 4.0/).

1 Department of Chemistry, College of Science, Princess Nourah bint Abdulrahman University, Riyadh 11671, Saudi Arabia; fsalamro@pnu.edu.sa

2 Department of Chemistry, Faculty of Science, Cairo University, Cairo 12613, Egypt; magdinaoum@yahoo.co.uk

3 Chemistry Department, College of Sciences, Taibah University, Yanbu 30799, Saudi Arabia 4 National Research Centre, Spectroscopy Department, Physics Division, El-Buhouth St., Dokki,

Giza 12622, Egypt; aymanmdarwish@ 5 Laser Technology Unit, Centre of Excellence for Advanced Sciences, National Research Centre, Dokki,

Giza 12622, Egypt 6 Center for Imaging and Microscopy (CIM), Zewail City of Science and Technology, October Gardens,

6th of October, Giza 12578, Egypt * Correspondence: ahoda@sci.cu.edu.eg

Abstract: Thermal and mesomorphic properties of possible 1:1 supramolecular complexes (SMCs) (Im/In) designed from two members of 4-[(4-(n-alkoxy)phenylimino)methyl]benzoic acid with symmetrical or un-symmetrical alkoxy terminal flexible chains (carbons of m and n = 6, 8 and 16), were analyzed by differential scan-calorimetry (DSC), thermogravemetric (TG) analysis, and their mesophases identified by polarized optical microscopy (POM). The equimolecular mixtures of the two acids possess symmetrical and un-symmetrical terminal lengths. The mesomorphic properties of the binary mixtures were examined as a function of the total alkoxy chain length on both sides. Results revealed that the nematic mesophase temperature range increases as the total terminal length increases for all designed un-symmetrical mixtures. A comparison was constructed between the formed SMCs and of those of the previously prepared 4-n-alkoxyphenylazo benzoic acids as well as the 4-n-alkoxy benzoic acids, to examine the impact of mesogenic core on the mesomorphic properties. The comparison indicated that as the mesogenic portion lengthens the thermal mesophase stability exhibits higher values of phase transition temperatures; whereas, the azo and Schiff base moieties exhibited near thermal properties.

Keywords: symmetrical and un-symmetrical supramolecular-hydrogen bonding; intermolecular interactions; 4-[(4-(n-alkoxy)phenylimino)methyl]benzoic acid; thermal stability; phase transitions

1. Introduction Supramolecular interactions resulted from H-bonded systems have wide attention in

technological applications [1?6]. Liquid crystal (LC) materials have non-covalent interactions as well as potential applications for functional molecular shapes. The shape of the LC compounds depends on the molecular architecture [7?9] and thus has essential role for mesophase formation. One of the most important interactions in biological and chemical processes is the SMC of LCs, namely the association of molecules. Generally, the mesomorphic stability depends on the polarity and/or polarizability of the central mesogenes of the molecule. Numerous of investigated symmetrical and un-symmetrical mesomorphic H-bonding based on the calamitic intermolecular H-bonding interactions [10?15]. Another report has studied the formation of angular complexes through intermolecular H-bond

Symmetry 2021, 13, 1785.



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formation [16]. Several mesogenic derivatives containing the Schiff base as a central linking group have been reported [17?21].

On the other hand, symmetrical 4-n-alkoxyphenylimino benzoic acids [22] have been shown to possess broad ranges of mesomorphic temperatures compared to their corresponding 4-n-alkoxyphenylazo benzoic acids analogues [15] and simpler acids, 4-nalkoxybenzoic acids [14]. Other reports have also been interested on the H- bonded LC polymers [23].

The role of H-bonded interaction in the formation and stabilization of LC mesophases has been recognized in recent years [24], its role in the self-assembly of the phase has also been evaluated [25,26]. Thus, the intermolecular H-bond interactions have shown high potential in the thermotropic LC systems [27,28]. In addition, the dimerization of aromatic carboxylic acids is the first example of LC formation [29]. The behaviour of LC material is mainly dependent on the shape of the resulting SMC. Many SMCs are based on the carboxylic and pyridyl components, as proton-acceptor and proton-donor moieties, respectively [30,31].

LC materials for device displays are mostly mixtures because no single compound fulfills all the essential criteria. Therefore, the investigation of mixtures of LC components is a subject of considerable interest [32]. The most used compounds for the preparation of LC blends, via H-bonding interactions, are the benzoic acid derivatives [33]. Thus, the aim of our study is to investigate the possible formation of SMC via H-bonding interactions (Im/In) formed between pairs of 4-n-alkoxyazomethine benzoic acid derivatives having different terminal alkoxy chain length that varies between 6, 8 and16 carbons. The study also aims to investigate the phase behaviour of the 1:1 molar ratio binary mixtures of any two complimentary components. Furthermore, this study will evaluate the effect of the mesogenic cores as a function of total alkoxy chain length of designed complexes.

2. Experimental 2.1. Synthesis of 4-[(4-(n-Alkoxy)phenylimino)methyl] Benzoic Acid (Im & In)

Materials Im and In were prepared according the previously reported method [22].

2.2. Formation of SM H-Bonded Complexes (Im/In) Details given in supplementary data (see Scheme 1).

SSyymmmmeetrtryy22002211, ,1133, ,1x78F5OR PEER REVIEW H2m+1CmO H2n+1CnO

O HO

N

N

OH O

Im , m = 6, 8, 16

+

O HO

N

N

OH O

In , n = 6, 8, 16

33ooff1101 OCmH2m+1

OCnH2n+1

(1:1) molar ratio

H2m+1CmO

N

O HO OH O

N

OCnH2n+1

Im/In

SScchheemmee11.. SStteeppss ooffSSMMCCss((IImm//IInn))pprreeppaarraattiioonnss..

33.. RReessuullttss aanndd DDiissccuussssiioonn

3o3f..11S..MEEfCfffeescctt ooff TToottaallTTeerrmmininaallCChhaaininLLenenggththoonnMMeseosommoorprphhicicBBehehaavvioiouursrsofof11:1:1MMoloalrarRRataitoisos of

SMCs

The possible 1:1 molar binary mixtures that are formed from the three derivatives of

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pfrroempatrhede.thTrheeemdeersiovmatoivrpehs iocf apnredpPaOreMd.. TThhee mDeSsComthoerrpmhoic-

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uDnSdCercPuOrvMesaorfe Ig6i/vI8eanriendFeipguicrteed2.iFnoFritghuerein1d.ivRiedpuraelsaecnitdastiavsewteexltluarsetshoefirt1h:e1 mmeoslaorpShMasCes

Iumn/dIne,rmPeOsMomaorrepghiivcetnrainnsiFtiigonurteem2.pFeorrattuhreesinadnidvitdhueairl aascsiodcsiaatsewd eelnltahsaltphyeiarr1e:1summomlaarrSizMedC iInmT/Ianb, lme 1e.soPmhaosrepthraicnstirtaionnsitteiomnpteermatpuerersa,tuasredsriavnedn ftrhoemirDaSssComciaetaesdureenmthenatlps,yaraerde isspulmaymeda-

irnizFeidguinreT3aab?lce, a1s. Pahfausnecttiroannsoiftitohne tteomtapl eterarmtuirneasl, laesndgtrhiv(e?n=frmom+ Dn)SfCormmeaasnudrenmcehnatns,gaedre

bdeitswpleaeyned6,in8 Faingdur1e63caa?rcb, oasnsa, fiunnoctridoenrotfothinevteosttailgtaetremthinealeflfeencgttohf(?to=taml t+ernm) ifnoarlmalaknodxyn

cchhaainngleedngbtehtsw(meen+ 6n,) 8onanthde1m6 ecsaorpbhoansse, ibnehoarvdieorutrooifnSvMesCtisg.aFterotmheTeafbfleect1 oafndtoFtaigl uterrem3ain?acl

sahlkoowxythcaht aainnilrernegguthlasr(cmha+nnge) oonf mtheeltimngestorpanhsaisteiobneshaasvtihoeuralokfoxSyMcChsa.inFrloenmgtThaibnlcere1aasnesd

fFoirgaulrlep3reap?carsehdomwixtthuartesa.nMirorreegouvlaerr, cahllaSnMgeCosfhomweletdintgo terxahnisbititioennsanatsiothtreopailckomxoyncoh-aoirn

dlei-nmgothrpinhiccredaespeesnfdoirnagllopnrtehpeatreerdmminiaxltluernegst.hMs oofrecohvaienr,malal nSdMnC. Tshhoewdeesdigtnoeedxshuibpirtaemnoalnetci--

uoltarroHpi-cbomnodninog- ocormdip-lmexoersp(hIimc/Idne)peexnhdibinitgmoensotphheatseersmoifntahlelremnaglthstsaboiflicthieasin26m2.8a, n25d2.n0.aTnhde 2d4e6s.i8gnCedupsuonprhaematoilnegcufolarrcoHm-bpolenxdeisngI6/cIo8m, Ip6/lIe1x6easn(dImI/8I/nI)16e, xrhesibpietctmiveesloy.pAhadsdeistionf atlhlye,rmthael ssttaabbiilliittyieosf 2th6e2.N8, p2h52a.s0e,aTnNd-I2, 4d6e.c8re?aCseuspwointhhmea;twinhgerfeoarsctohme SpmleCxesstaIb6/iIl8i,tyI6i/nI1c6reaansdesIw8/Ii1t6h, rme-. Fsoprecmtiv=e6ly(.FAigdudrieti3oan)a, lalyll,StMheCsst,aIb6i/lIit8yaonfdthI6e/IN16 pinhcalsued, iTnNg-I,thdeecsryemasmesetwriictahl mdi;mwehr eIr6e/Ia6satrhee

dSimmCorsptahbicilpitoysisnecsrseinasgesmweictthicmC. (FSomr Cm) =an6d(Fniegmuraetic3a()N, )alml SeMsoCphs,aIs6e/sI8. Tanhde cIo6/mI16pilnecxluI6d/Iin16g sthoewsysmthme ehtirgichaelstdrimanegreIo6/fI6naermeadticmpohrapsheicnepaorslsye7ss7i.n3gCsmwehcitliec tChe(ScmomCp) laenxdI6n/Ie8meaxthicib(iNts) nmemesaotpohgaesneisc.rTahnegecoambopultex2.6I6/IC16. sThhoewsesrtehseulhtisgahreesitnranggreeemofennetmwaithicpprheavsioeunseraerplyor7t7s.[33?4C]

while the complex I6/I8 exhibits nematogenic range about 2.6 ?C. These results are in

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SSyymmmmeettrryy 22002211,, 1133,, 1x7F8O5 R PEER REVIEW

44 ooff 1101

agreement with previous reports [34] that is, the higher difference between the flexible wings of the mixed components, the wide N phase range will observed. For m = 8 (Figure 3athbga)r,et eaisml,l teShnMet hCwisgithIh8e/rIpndrheifvafevioreeuntshcreebspaeotmrwteseet[rn3e4tn]hdethoflafetxIi6is/b,Ilnet.hwMe iohnriggesohovefretrdh, eitfhfmeerirexneNdcecsotbamebtipwliotenieeensndttshe,ectrhfeleaewxseiibdilnee oo7ow3INI867brlroi//dd.)npII3g,ee8nghuarra.asalenMttlnsoosodedSfoI22Mrt71rI66ah6e81/00CneI/o..I.0ngs22vm1e,(,6e?IF,i22Cr8wxi/,r55gI,eeti88nulhdsr..lep0h0erocseieaaaobrpcv3nnmseNteceiddcv)prtt,svei11hovtl7e7aenhyed55b.eels..yinI88alSnF,tim??MtsowicCC,ereaCthshsmfftisooerdeleerrIe=own1IIctf6d888h/ri//tIdeIIeh(6oa66eFe,,afdsiIINneghi88Im//du6oiII/pnr88ImeIhenaar1o.oa6nn3r/MIslIbdddo1e86)eog/,IIsrIrru88aha1//e6tlenIIoool11sigw66vsS,,2eIMe1r6rpwrw6ee0u,C/ss.iIirt2ppdslnhel,eeeIle(o2cyc8Fin5btt/riiiIn8esgvvnNem.euee0rmhllrvayysaaetetan..voad3tIIdegbonnc.egi)t1Fl,hccnei7otataeni5ihrcssei.seeecm8sra,aSoomdpn=MffCeeogtt8cChhseftro(sreeesFneerahhiInsesIg1sooade86uirmm/n/IoirlIngy6e6-f-, SoamlnodgCuI1me6s/eIIs81o6s/pIhnhoa(wFsiegwuwidrieteh3ncse)tm,atbhaiteloitSgyMentCeicmsrpIa1en6/rgIa6etuannreedar2Il13y69/I7.787s.?3hCoawnadnwd7i1dr.a0enngCeem, r8ae2ts.o7pge?ecCnti.ivcAerllayln, rwgeeshunilleetastrholyef m7d7iem.3soearmnIdo16r/7pI1h1.6i0cis?bCpe,uhrraeevlsyipoenucertmirveaevtloeyga, elwendihcit,lhepaottshstehesedsiimmngiexrSinmIg16C/oIm1f6 oeisnsoeppudhreaerslieyvawntietvmheaswttaoibgthielinthyice,tepomtohpseserersadstoiunerges nS2mo39tC.d7imsCruesapontpdahrraraasnenggewe8mit2he.7nsttCaob.f iAmliltolylreetcseuumlletpsseowrfaimttuherisenot2mh3eo9r.s7pmh?eiCcctbiacenhodarvrNiaonupgrheares8ve2se..7ale?dC.thAatllthreesmulitxsinogf mofeosonme doreprihviactbiveehawviitohutrhreevoethaelerddtoheast nthoet dmisixruinpgt aorfroannegedmereinvtatoivf emwoliethcutlhees owthitehrindothees nsmotedctisicruoprtNarprhanasgeesm. ent of molecules within the smectic or N phases.

Figure 1. DSC thermograms of SMC I6/I8 mesured at a rate of ?5 ?C min-1 from the rounds of second heating and

cFoigoulirneg1. . Figure 1.

DSC DSC

thermograms of thermograms

oSfMSCMIC6/II86/mI8emsuerseudreadt aartatae

of ?5 C min-1 from rate of ?5 ?C min-1

the rounds of second from the rounds of

heating second

and cooling. heating and

cooling.

FFiigguurree 22.. PPOOMM tteexxttuurreessooff mmeessoopphhaasseess uuppoonn hheeaattiinngg ffoorrSSMMCCII66//II1166((aa))SSmmCCpphhaassee aatt 116677..00 ?CC aanndd (F(bbig)) uNNrepph2h.aaPsseeOaaMtt 22t22e00x..t00u?rCCes..of mesophases upon heating for SMC I6/I16 (a) SmC phase at 167.0 ?C and (b) NTphhaesremaatl22s0ta.0b?iClit.ies of the prepared SMCs, Im/In, also were confirmed by thermogravimetric (TG) analysis (Figure 4). The TGA measurements revealed that, the thermal

degradation takes place via one step with the maximum rate loss (Tmax) at ca. 330, 341 and 350 C for I6/I8, I6/I16 and I8/I16, respectively. This shows high thermal stabilities for all formed complexes above their isotropic transitions.

SymmetSryym2m02e1tr,y132,012718, 513, x FOR PEER REVIEW

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Table 1T. aMbleeso1p. hMaesseotpehmapsertaetmurpeesr(atCu)r,eesn(?thCa)l,peynt(hkaJ/lpmyo(lk)Ja/mndole)natnrodpeynotrfotpraynosfittiroannfsoitriothnefoSMr tChes,SIMm/CIns,. Im/In.

System System?

TC?r-SmC TCHr-SCmrC-SmC HTCSrm-SCm-CI

THSmSCm-IC-I HTSSmmCC--IN TSHmCS-mNC-N HTSNm-CI-N

THN-NI -I HNS-I/R S/R

I6/I6 I6/I8 I6/I16 I8/I8 I8/I16

I6/I6 12 I6/I8 14 I6/I16

22

I8/I8 I8/I1616 I16/I1624

11829.0 18955.0.7

55.7

11745.4 17540.4.1

40.1

21528.9 15839.9.6

39.6

16

182.0

49.8

21842.0 15849.2.8

36.3

31528.2 15736.0.3

56.9

239.7

257.4 260.2 174.7 258.0

31.775.8

2572..47

2603..22 1742..74 258.0 1752..81

2.6

226.75.9

236.22.8 225.42.0 2.1 226.61.0

246.8

2615..89

2612..98 2512..70 261.0 2416..98

1.1

1.80.40 0.40

1.90.43 0.43 1.70.39 0.39 1.9 0.43 1.10.43 0.25

0.25 0.87

I16/I16 Abbre3v2iations: C1r5-S7m.C0= crysta5l6t.o9 smectic23C9.p7hase; SmC3-I.7= smectic C to isotropic liquid phase; SmC-N = smectic C to0n.8e7matic

Abbrevpiahtaiosnes;:NC-Ir-=SmnCem= cartyicsttaol tiososmtroecptiicc Cliqpuhiadsep; hSmaCs-eI ;=?sm= Secutmic Coftoteisromtrionpailcclihqauiind lpehnagsteh; SmmC+-Nn=. smectic C to nematic phase; N-I =

nematic to isotropic liquid phase; ? = Sum of terminal chain length m + n.

Figure 3. DSC transitions for the total length of alkoxy terminals of 1:1 molar SMCs, Im/In; where ? = m + n. Figure 3. DSC transitions for the total length of alkoxy terminals of 1:1 molar SMCs, Im/In; where ? = m + n.

The transition normalized entropy changes (S/R) were estimated for all SMCs and tabulated TinhTeramblael1s.tDabeiplietinedseonfcethoef epnretrpoapryedchSaMngCess, wImi/tIhn,thaelstootwalelreencgothnfoirfmcoemd pblyextehsermoare regprraevseimnteetdrigcr(aTpGh)icaanllaylyinsisFi(gFuigreur5e. F4r).omThtehTe GdaAtamineaTsaubrleem2eanntds rFeigvuearele5d, tthheate,nthtreopthyermal changdeesg(raSd/aRti)osnhotawkeeds pslliagchetvdieacorenme setnetpvwaliuthesthweitmh athxeiminucmrearaseteolfotshse(Ttomtaaxl) aaltkcoax.y33c0h,a3in41 and length35?0 u?Cp tfor?I6=/I82,4I6c/aI1r6baonds (I8/SI1/6R, r=e0sp.2e5c)titvheelny.juTmhips stoho(wSs/hRig=h0t.h8e7r)matal?s=tab3i2l.itiTeshefor all specififocrimnteedraccotmiopnlsebxetswaebeonvethteheuinr-ilsiokterompeiscotgraensitlieoandss. to the production of the smectic

mesophase in un-symmetric complexes [35,36]. The irregular relation may be attributed to the different type of mesophases of complexes from ? = 12 to ? = 24. S/R of ? = 24 is lower

than that of ? = 22 which may be attributed to the different biaxiality of their mesogenic

groups arrangement [37,38]. The N mesophases covered all of the SMC mixtures, while

for ? = 32 only the SmC mesophase was observed. Thus, the increase of the end?to-end

associations of molecules with increasing the total length of the molecule is a result of the

production of wide range of N phase up to ? = 24. Additionally, the relatively small values

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