Synthesis and Liquid Crystalline Study of 2 , 4-bis ( 4-n-heptyloxybenzoyloxy )-benzylidene-4-n-alkoxy aniline

Abstract The synthesis and study mesomorphic properties of a new series of 2,4-bis(4-nheptyloxybenzoyloxy)-benzylidene-4-n-alkoxy aniline (DC7A1-10) Have been reported. The structures of the prepared compounds were confirmed by FT-IR, H-NMR and elemental analysis (CHN). The mesomorphic properties were studied by differential scanning calorimetry (DSC) and polarizing optical microscopy (POM) measurements. All compounds of the series exhibited nematic (N) phases but the smectic phases are appear in (DC7A3-A10). The first two series (DC7A1, DC7A2) display only a nematic phases whereas the highest homologues (DC7A3A10) exhibit enanthiotropic dimorphism Nematic (N) and smectic (S) phases (the DC7A6 is only compound exhibit Sc and SA phases). The liquid crystalline behavior has been analyzed in terms of structural property relationship. The mesomorphic properties and thermal stabilities of the present series (DC7A1-10) were compared with those structurally related homologous series.

Figure 1 1+n2 The mesomorphic behavior of this Schiff base compound show the (N), (Sc) and (SI) mesophases.In this work, we describe the synthesis and mesomorphic properties of homologous series (DC7A1-10) (Figure 1I).

2-Instrumentations
Elemental analysis (HN) was carried out using a Perkin-Elmer model 2400instrument.The elemental analysis:

2-Mesomorphic properties:
The The persisting of nematic phase indicates that the net effect imparted due to the end-to-end cohesive forces in this series is similar as all the other molecular forces are the same.The increasing of carbon chain (n>2) leads to the formation of smectic mesophase due to enhances lateral attraction as compared with terminal attractions, which facilitates lamellar packing that required for the formation of the smectic mesophase [9].The appearance of SmA phase for compound (DC7A6) was understandable and that could be that the three alkoxy terminal groups with 7, 7 and 6 carbon atoms give rise to more planner and polarizable geometrical structure of DC5An [10].The plot of transition temperatures against the number of carbon atoms in the alkoxy chain (Figure 2).Showed the relation between carbon number (n) of alkoxy group and transition temp. of mesophases.The mesophases exhibited by compounds of series DC7A1-10 were identify according to their optical textures, which were observed by POM, using the classification systems reported by Demus and Richter [11] and Dierking [12].The optical photographs of DC7A3 are depicted in (Figure 3) as a representative illustration,  The typical SmA for DC7A6 was appeared by cooling (Figure 5) as a broken focal conic SmA.When the compound DC7A8 cooling from isotropic phase the thread like SmC appeared (Figure 6).  2.

Table 2 :
mesophases of the present series (DC7A1-10) were observed by polarized optical microscope (POM) during heating and cooling cycles.The thermal behaviors obtained by DSC are consistent with the data observed by (POM).The data obtained by DSC and POM analysis of the present series (DC7A1-10) are summarized in Table (2).Transition temperatures and total enthalpy changes of (DC7An) 121 N 126 (9.53)I cr=Crystal; N=Nematic; SmA=Smectic A; SmC=Smectic C; I=Isothipic transition.All the members of series DC7An exhibit enantiotropic mesophases.The DC7A1 and DC7A2 are homologues are purely enantiotropic mematogenic.With increasing length of Alkoxy chain of DC7A3 to DC7A10 the enantiotropic smectic (C) phase was observed and accompanied the nematic phase.The nematic phase start from n=1 and persists up to the last number of the series.

Figure 2 :
Figure 2: Dependence of transition temperature on the increasing number of carbon atom (n) in the terminal alkoxy chains for the DC7A1-10 series of compound

Figure 3 :
Figure 3: The Nematic droplet for the compound DC7A3

Figure 4 :
Figure 4: The Thread-like nematic of compound DC7A3

Figure 5 :
Figure 5: The Broken focal conic smectic A of compound DC7A6

Figure 6 :
Figure 6: The Thread-like smectic C of compound DC7A8 of 2,4-bis (4 --n-heptyloxybenzoyloxy)-benzylidene-4 = -n-alkoxy aniline (DC7An) have been prepared by changing the terminal alkoxy chain length (n=1-10).The chemical structures of the series were identified by IR, 1 H-NMR and elemental analysis.The results are in agreement with the considered molecular structure.The liquid crystal properties and optical textures of the compounds were studied by POM and DSC.The change in terminal alkoxy chain length has pronounced effect on the mesomorphic behavior.The first two homologues (DC7A1, DC7A2) exhibited only N phases but the others (DC7A3, DC7A10) display dimesomorphism N and SmC phases.But with compound (DC7A6) the other SmA phase was observed.The mesomorphic properties and thermal stabilities of the present series is compared with those of structurally related homologous series.

Table 1 :
Elemental analysis of all compounds

Table 3 :
Comparison between transition temperatures, mesomorphism and average thermal