Synthesis and study of mesomorphic properties of unsymmetrical cyclohexanone-derived bis-chalcones

ABSTRACT A series of (2E,6E)-2-(4-n-alkoxybenzylidene)-6-((benzo[d][1,3]dioxol-6-yl)methylene)cyclohexanones were synthesised by stepwise condensation reactions from cyclohexanone. The optical and thermal analysis of the series indicated the mesomorphic properties of some derivatives. The compounds with C7–C16 substituents showed the nematic phases in the range of 90–73°C. The single crystal X-ray analysis confirms the head to tail packing arrangement. GRAPHICAL ABSTRACT


Introduction
Design, synthesis and study of organic molecules with specific applications constitute an important aspect of modern chemistry. Innovation in the area of novel materials has contributed in shaping the life of modern society. Many devices with novel properties and specific applications are now available as a result of the research in material science. Liquid crystalline compounds are one such class of materials which has triggered massive research activity and has resulted in commercial development of many materials of interesting applications. Liquid crystals (LC) are recognised as orientationally ordered fluids formed by shape-anisotropic organic molecules [1][2][3][4][5][6]. For many years, aromatic compounds based on benzene, biphenyl or terphenyl core unit were the most commonly studied liquids crystals, partly because of their easy synthesis and partly due to the link between highly polarisable π-electron system and overemphasised mesogenicity [7][8][9][10]. As a result of further study, it became clear that saturated systems are also excellent units when placed appropriately in relation to other planer or high polarisable moieties; in some cases, the entire saturated systems are also key for this behaviour. Examples of such type of mesogenic, non-aromatic structures include trans-1,4-disubstituted cyclohexanes and 1,4-disubstituted bicyclo [2.2.2] octanes [11,12]. The presence of slightly flexible cyclohexane-based core offers a possibility of introducing an element of mobility in these compounds [13]. From the available literature on different types of liquid crystalline compounds, such types of compounds are scarcely known to show mesogenicity. This includes an early report on 2,5-bis(4-methoxybenzylidene)cyclopentanone and 2,6-bis(4-methoxybenzylidene)cyclohexanone and their mesogenic property, though without much details about the transition temperatures [14][15][16]. Matsunaga and Miyamoto [17] have reported synthesis and systematic study of the mesogenic property of symmetrically substituted derivatives of cyclic ketones. A series of alkoxy derivatives 2 of the basic compound 1 were reported with the details of the comparison of their mesogenic properties. Structurally similar liquid crystalline copolyarylidene ethers derived from 4-methyl cyclohexanones and 4-tert-butyl cyclohexanone [18] along with poly(azomethine ether)s [19] were also reported.
In the present study, we propose to modify the cyclohexanone-based compounds and prepare unsymmetrical derivatives of the type 3, where we expect the crystal packing to be different than the symmetrical compounds (Scheme 1). We report the synthesis of a series of such compounds, study the crystal packing and phase transition behaviour.
The design of the chalcone derivative 3 involves introduction of a benzo [1,3]dioxolyl unit as structurally flat component in the unsymmetrical molecule. These bis-chalcone units can be introduced by basemediated stepwise condensation reaction of cyclohexanone, first with piperonal followed by 4-n-alkoxy benzaldehyde.

Result and discussion
Synthesis of the title compound 3 is outlined in Scheme 2, where mono chalcone 6 of cyclohexanone 4 and piperonal 5 was prepared by base-mediated condensation reaction. 1 The intermediate mono chalcone 6 was separated and purified. The chalcone 6 was then subjected to second condensation reaction with appropriate 4-n-alkoxy benzaldehyde 7 with methanolic sodium hydroxide at slightly elevated temperature. The unsymmetrical bis-chalcones, 3a-3h, were isolated in good yields and were fully characterised.

Thermotropic liquid crystals
The higher homologues of the synthesised compounds are found to exhibit liquid crystalline phases (mesogens) during the heating-cooling cycle when observed under a polarising optical microscope (POM). Hence, they are thermotropic LCs. The mesogenic properties of these compounds were further investigated by differential scanning calorimetry (DSC). It is known that the thermotropic LCs are highly sensitive to their molecular constitution. Any alterations in the molecular core are expected to alter the mesogenic properties. Hence, the thermal stability and mesophase length can be correlated with the molecular constitution of these compounds.

Polarising optical microscopy
The mesogenic properties of eight compounds of the series (3a-3h) were investigated by using a POM fitted with a heating stage (Leica, dfc 295). Thin film of the samples was prepared by sandwiching a small amount  of each compound between a glass slide and a cover slip. As expected, lower members with smaller alkoxy chains (3a n = C4 and 3b n = C6) did not exhibit mesomorphic property. The compounds 3c (n = C7) to 3h (n = C16) upon heating though melted directly to isotropic liquid phase. However, during the cooling part of the cycle, the isotropic liquids produced tiny spherical droplets (Figure 1a) which coalesced in to the Schlieren texture (Figure 1b), which is characteristic of the nematic mesophase. Thus, these compounds are monotropic LCs. There was no fundamental difference in the texture in all the members of this series.

Differential scanning calorimetry
DSC is a useful method for the detection of phase transition. It yields quantitative results; therefore, conclusions may be drawn concerning the nature of the phases which participate in the transition. In the present study, enthalpies of all the liquid crystalline unsymmetrical bis-chalcones 3c-3h were measured by DSC (cryo-DSC model DSC 822 e , Mettler Toledo, Switzerland, having Star e software).
The phase transition behaviour of the mesogenic bis-chalcones reported in this article was investigated over the heating-cooling cycle between ambient temperature and 140°C and back to 30°C at a rate of 5°C min −1 . The transition temperatures recorded from DSC studies and the enthalpy values associated with the phase transitions are compiled in Table 1. All the compounds exhibited clear-cut transition temperatures in the DSC thermograms.
A plot of transition temperature against the number of carbon atoms in alkoxy chain is presented in Figure 2. The trend shows initial rising tendency from n-heptyloxy (3c) to n-octyloxy derivative (3d) which may be due to the usual odd-even effect. This was followed by a steady drop in the nematic isotropic (N-Iso) transition temperatures for all the synthesised derivatives, 3e-3h.
DSC thermograms of all the compounds showed one endotherm for the crystal (Cr) to isotropic phase (Iso) transition (i.e. isotropisation) peaks during the heating scan. The compounds 3d and 3h also showed one additional endotherm for the crystal (Cr 1 ) to crystal (Cr) transition (modification) during the heating scan.
During the cooling part of the cycle, all the compounds showed two exotherms. The first exotherm  corresponds to the isotropic liquid phase (Iso) to nematic mesophase (N) and the second exotherm corresponds to the nematic mesophase to crystallisation phase (Cr). In addition to the above, compound 3d also showed a third exotherm, this possibly corresponds to the crystal (Cr) to crystal (Cr1) modification on cooling. The central unit increases the molecular biaxiality of the compound and this has the effect of reducing the clearing enthalpy. This is described in a range of materials as can be seen in many recent references [20][21][22][23]. As the chain length is increased, this effect is reduced and hence more typical enthalpies are observed.
The enthalpy values associated with the phase transitions also agree well with those in the literature [24,25], providing confirmations of the type of mesophases observed in the microscopic study.

X-ray diffraction study
One of the compounds 3d, with octyloxy substituent, was further studied by single crystal X-ray diffraction analysis. 2 The crystal packing indicated the presence of two molecules in the unit cell arranged in head-tail manner as shown in Figure 3. Further, it is observed that the crystal is packed in the ordered pattern where the average interlayer distance is observed to be 3.505 Å (Figure 4).

Comparison with reference compounds
All the homologues of the present series exhibit only monotropic nematic mesophase. However, the methoxy to n-decyloxy homologues of structurally related series [17] of 2,4-bis(4-n-alkoxybenzylidene)cyclohexanones 2 exhibit nematic phase, but the higher members exhibit a smectic mesophase. In addition, the   n-nonyloxy and n-decyloxy derivatives exhibit smectic A phase also. This type of observation is expected from the nature of packing of molecules in the solid state. During the transition from mesophase to solid, only orientational order of the molecules is likely in the former, whereas in the latter, in addition to the possibility of maintaining orientational order, higher members can maintain positional order also, because of symmetric nature of the molecules.
The comparison of molecular structure, transition temperatures and energy minimised ball and stick model (MM2 models) with length and breadth of the n-octyloxy derivative of the present series 3d and structurally related compound 2, taken from reference [17], is presented in Figure 5.
Unsymmetrical compound 3d exhibits monotropic nematic mesophase while symmetrical compound 2 (n = C8) exhibits enantiotropic nematic mesophase. The nematic mesophase length and thermal stability (N-Iso transition temperatures) of compound 3d is lower than the reference compound 2 (n = C8) by 12°C and 41°C, respectively. These molecules have the same central cyclohexanone unit and differ only at the side arms. The compound 3d has piperanalidene moiety as one arm and the 4-n-octyloxybenzylidene as other arm, whereas compound 2 has the identical substituents on both sides. From Figure 5, it can be seen that the length/breadth ratio (L/B) of compound 3d (5.43) is quite lower than that of the corresponding derivative of 2 (6.78). According to Gray [7], decrease in the L/B ratio of the molecules reduces both the smectic and nematic thermal stabilities. This, in fact, partly accounts for lower thermal stability, the elimination of smectogenic tendencies and monotropic nature of the mesophase of compound 3d [26].