Effect of Equipolar Endgroup of a Molecule on Mesomorphic-Isotropic Transition Curve

: A Homologous series:RO-C 6 H 4 -CH=CH-COO-C 6 H 4 -CO-CH=CH-C 6 H 4 -OC 18 H 37 (n) (para) of chalconyl novel ester of dual character as thermotropic and lyotropically bioactive is synthesized and studied with a view to understand and establish the relation between mesomorphism and molecular structure on the basis of molecular rigidity and flexibitity through thermotropic liquid crystalline(LC) state. Novel series consists of thirteen homologues of which four homologues (C 1 to C 4 ) are non liquid crystal (NLC) and the rest of the homologues (C 5 to C 18 ) are thermotropically enantiotropic nematic without exihibition of smectic property. Thermometric transition temperatures and textures of nematogenic phase are determined by an optical polarizing microscopy equipped with a heating stage (POM).Textures of nematic phase are threaded or schilieren. Transition temperatures of novel homologues are relatively lower than the transition temperature of corresponding dimeric trans n-alkoxy cinnamic acid. Cr-N/I and N-I transition curves behaves in normal manner with negligible abnormality at C 18 homologue in phase diagram of a series.Analytical and spectral data conforms the molecular structure of homologues. Thermal stability for nematic is 121.88°C whose degree of mesomorphism vary minimum from 4.0°C to a maximum of 34.0 °C, and hence novel series is of middle ordered melting type, Group efficiency order for -OC 18 H 37 tail


INTRODUCTION
The study of LC [1 ] property of a substance have played an important role from the point of view of utility in LC devices and pharmaceutical preparations. Thus, chalconyl derivatives of thermotropic LC may be used for the manufacture of LC devices which are to be operated at room temperature or at desired temperature [2,3,4] and their bioactive nature can be exploited against pharmaceutical preparation as antibacterial ,antimalarial, anticancer etc [5,6,7]. Hence, present investigation is planned to synthesize novel chalconyl derivatives through novel homologous series and to characterize homologues by thermometric, elemental analysis, IR and H 1 NMR spectra, LC properties like thermal stability, degree of mesomorphism etc. with a view to understand the effect of molecular structure on LC properties [8,9,10,11].The novel chalconyl derivatives may be further studied by the groups of researchers working with bioactivity of chalconyl ester derivatives, because present planned work will include only the study of thermotropic LC derivatives of chacone as novel series. Number of homologous series of ester including azoester, chalconyl ester, carboxy and vinyl carboxy ester have been reported till the date [12,13,14,15,16,17,18,19,20] .The results of novel thermotropic LC derivatives will be discussed and interpreted in terms of molecular rigidity and flexibility [21,22,23,24,25] and the LC behaviours of present study will be compared with other analogous or structurally similar series for deriving group efficiency order for nematic.

RESULTS AND DISCUSSION
Homologous series of present investigation is formed by The condensation of n-alkoxy cinnamic acids and the α -4-Hydroxy benzoyl β-4 ' -octadecyloxy phenyl ethylene (M.P 74°C,Yield 72%).The transition temperatures of novel chalconly ester derivatives of a series are nematogenic LC from and beyond Pentyloxy homologue (C 5 to C 18 ) with absence of smectic property. C 1 to C 4 homologue are nonliquidcrystal.Transition temperatures (table-2) as determined from an optical polarizing microscopy were plotted versus the number of carbon atoms present in n-alkyl chain bonded to first phenyl ring through oxygen atom (-OR). A phase diagram is obtained from Cr-N/I and N-I transition curves by linking like or related transition points .Cr-N/I transition curve adopted zigzag path of rising and falling with descending tendency and behave in normal manner. Homologous series: α-4-(4'-n-alkoxycinnamoyloxy) benzoyl-β-4''-octadecyloxyphenyl ethylenes

Compound
No.
n-Alkyl chain C n H 2n+1 Transition Temperatures in(˚C)

Smectic
Nematic N-I transition curves for odd (C 5 , C 7 ) and even (C 6 …C 8 ) members are matching into each other at the C 10 homologue and than gradually ascended up to C 18 homologue as a single transition curve from and beyond C 10 homologue. Thus , it exhibits odd-even effect and deviated negligebly from and beyond C 12 homologue from normal descending tendency.Textures of nematogenic homologues are threaded or schlieren as determined by miscibility method. Analytical and spectral data confirms the molecular structures of homologues. Odd member`s N-I transition curve occupied higher position as compared to even member´s transition curve. The mesogenic (LC) properties from homologue to homologue cause variations keeping -OC 18 H 37 tail end constant and changing -OR tail group at the left. Thermal stability for nematic is 121.88 °C, with low degree of mesomorphism, which varied from 4.0°C to 34.0°C. Disappearance of dimerization of aromatic acids and the lowering of transition temperatures of resultant chalconly esters derivatives are attributed to the breaking of hydrogen bonding between acid molecules by way of esterification process. The inexihibition of any sort of mesophase formation by C 1 to C 4 homologues is attributed to their inability to resist exposed thermal vibration due to unsuitable magnitudes of intermolecular anisotropic end to end or/and lateral forces of cohesion and closeness as imerging from low dispersion forces and low dipole-dipole interactions, which induces high crystallizing tendency in a substance to transform into isotropic liquid sharply from crystalline state on heating and on cooling the same from isotropic liquid to directly solid crystalline state without passing through an intermediate state of existence called as LC state. However, the molecules of C 5 to C 18 homologues 156 ILCPA Volume 60 disalign at an angle less than ninety degree as a consequence of favorable molecular flexibility,which resisted exposed thermal vibrations due to suitable magnitudes of anisotropic end to end forces of intermolecular attractions, and causes to arrange the molecules in statiscally parallel orientational order under floated condition on surface, depending upon enthalpy (H) value of LC state within definite range of temperature, called as mesophase length of a homologue. Thus, nematogenic mesophase formation occurs excluding the possibility of smectic mesophase formation which is supposed to form prior to nematic mesophase occurrence. however due to insufficient magnitude of intermolecular lateral forces of attractions as required for lamellar packing of molecules in their crystal lattices of rigid crystalline state is deficient and hence, smectogenic mesophase formation is missing through out to series under dictions Odd-even effect observed for N-I transition curve of phase diagram is attributed to the odd and even number of carbon atoms present in n-alkyl (R) chain of the left n-alkoxy group. The disappearance of odd-even effect from and beyond C 10 homologue of the longer n-alkyl chain is attributed to the unusual status of the an alkyl chain, which may coil or bend or flex or couple to lie with the major axies of the core structure of molecules, resulting into end to end attraction and intermolecular distance almost equivalent. Hence a single transition curve appears with diminishing tendancy of odd-even effect.The changing trend in mesomorphic properties from homologue to homologue in same series is attributed to the seqrrantial addition of methylene units at the left n-alkoxy terminal (-OR) end groups which alters molecular length, molecular polarity and polarizability, molecular flexibility, permanent dipole moment across the long molecular axis, dipole dipole interaction, magnitude of intermolecular dispersion forces of cohesion and closness etc. including uncertainty in the status of an alkyl chain(R) of -OR group. The LC properties of present novel series are compared with structurally analogous series as mentioned below in figure-2.
Homologous series 1, X and Y are identical with respect to three phenyl rings, central bridge -CO-CH=CH and left n-alkoxy terminal end group for the same homologue from series to series. Homologous series 1 and X are identical with respect to first central bridge -CH=CH-COOincluding all above other , respect except right side flexible tail end group -OC 18 H 37 and-OC 14 H 29 respectively, which differes in molecular flexibility for the same homologue from series to series.Homologous series 1 and Y are identical in all respect for the same homologue except a central bridge linking first and middle phenyl rings viz. -CH=CH-COO-and -COO-respectively contributing to the total molecular rigidity. Thus, the difference of LC properties and the degree of mesomorphism can be correlated with differing features and differing magnitudes of molecular rigidity and/or flexible of molecular structure among the series 1, X and Y respectively for the same homologue from series to series and homologue to homologue in the same series. Some thermotropic LC properties are mentioned below for the comparative study in table-3  it commence from C 6 homologue of a series-X.  Thermal stability of series-1 and Y are almost nearer(21.88≈22.0) to each other ,but it is lower for series-X than the a series-1 and Y.  Degree of mesomorphism of series 1 and Y are almost equivalent but it is lower for a series-X than series 1 and Y.  N-I transition curves deviates from normal descending behavior in a negligible manner at the homologue C 18 , C 14 and C 18 in which both terminal end groups are equipolar and polarizable for series 1, X and Y respectively. Exhibition of only nematic property in case of series.1 and X is depended upon the difference of polarities of flexible tail end group -OR, -OC 14 H 29 . Hence magnitude of flexibility difference occurred due to n-alkoxy (-OR` where R`=C 18 H 37 and C 14 H 29 ) tail end group for the same homologue from series-1 to series-X which reduces nematic thermal stability and the degree of mesomorphism. The extent of molecular noncoplanarity difference as occurred due to unusual status of n-alkyl chain of tail end groups -OC 18 H 37 and -OC 14 H 29 have operated early (C 5 ) and late (C 6 ) commencement of series-1 and X respectively. Now on comparing the molecular structure of series-1 and Y in which the central bridge linking first and middle phenyl ring -CH=CH-COO-and -COO-differs keeping rest of the molecular parts including both terminal tail end groups(-OR and -OC 18 H 37 ) unaltered. Therefore, central bridge -CH=CH-COO-of series-1 is replaced by -COO-of series-Y which contributes partly to the difference of molecular rigidity. The vinyl carboxylate central group of present novel series-1 is relatively longer than carboxylate central.bridge of series-Y, which causes more noncoplanarity but the presence of another comman central bridge -CO-CH=CH-which bears conjugated double bond and maintains almost equal noncoplanarity of molecules of both series-1 andY under comparison. Thus, the effect of vinyl carboxylate group against carboxylate group toward the extent of noncoplanary of the molecular operate equally for commencement of LC phase from C 5 homologue of series 1 and Y.The negligible difference of thermal stability (121.88 and 122.0°C) is also attributed to the negligible difference of molecular rigidity due to presence of common central group-CO.CH=CH-which minimize the difference of effectivity of lengths between -CH=CH-COO-and -COO-group as a consequence of minor difference molecular rigidity, due to replacement of central ester group. In all these homologous series under comparative study, the molecules of mesogenic homologues exhibited nematogenic mesophase formation is attributed to the disalignment of molecules at an angle less than ninety degree which resists exposed thermal vibrations and arrange molecules to float on the surface with statistically parallel orientational order ,But the angle of molecular disalignement with floating surface which vary from series to series and homologue to homologue as a consequence of the magnitudes of changing molecular rigidity and /or flexibility of different magnitudes which affects