Influence of Mesogenic Properties of Cruciform-Shaped Liquid Crystals by Incorporating Side-Arms with a Laterally-Substituted-Fluorine

Fluoro substitution in thermotropic liquid crystals provides a general way of modifying the properties of a parent system. Transition temperatures, mesophase types and other physical properties can be affected by fluoro substitution, so that frequently the behaviors of the parent compound can be manipulated and improved in a predictable manner. This paper discusses the effects of a fluoro substitution in each side-arm of 1,2,4,5-tetrakis((4-(alkoxy)phenyl)ethynyl)benzenes on the resulting mesomorphic properties characterized by optical polarizing microscopy and differential scanning calorimetry. Without any fluoro-substituted side-arms, longer chain-length leads to a wider nematic temperature range on cooling. Incorporation of a fluoro substitution in each side-arm induces the formation of a lamellar suprastructure, lowers transition temperatures and results in a wider mesophase temperature range on cooling.


Introduction
Liquid crystals have many fascinating and useful properties that make them ideally suited for use in a wide range of technical devices.In a crystal, molecules occupy specific sites in a lattice and point their molecular axes in specific directions.Molecular shape concludes to a large extent the type of mesophase observed for thermotropic liquid crystals.Generally, rod-like molecules give rise to smectic and/or nematic mesophases, while disc-like molecules self-organize to furnish columnar mesophases.A wide range of molecular geometries, board-like (sanidic) molecules [1][2][3], polycatenar compounds resulting from calamitic compounds modified with several terminal chains [4,5], disc-like molecules where the number of substituents was reduced [3], rod-disc dimers [6], disc-disc dimers [7], different arrangements of disc-rod-disc and rod-half-disc dimers [8] were prepared and investigated.Among them, cruciform-shaped molecules exhibit spatial separation of their frontier molecular orbitals, building 1,2,4,5-tetraalkoxyphenylethynylbenzene (TPEB), and are excellent candidates for a variety of advanced materials applications, such as organic light-emitting diodes, solar cells and optical storage devices [9][10][11][12][13][14][15][16].Carbon-rich cruciforms already possess demonstrated potential as nonlinear optical/two-photon absorption [17,18] and fluorescent sensing materials [19][20][21].Changes in the substituents, substitution pattern, electronic structure and conjugation can provide highly variable photo physical properties for such materials.Therefore, it is expected that detailed study on the structure-property relationships for these materials will provide valuable information for molecular design with high performance.The cross-like TPEB (A), shown in Figure 1, was originally designed for achieving biaxial nematic behavior; however, nematic textures were only observed monotropically upon cooling at 74 °C on small regions, following clearing at 96 °C [22].The mesophase stability, as well as the small nematic temperature range hampered further biaxial investigations.Recently, based on the structure shown in Figure 1, enantiotropic nematic behaviors were reported by replacing the peripheral phenyls with biphenyls to stabilize the nematic phase [23].Nevertheless, compound 1's low clearing temperature can be of advantage in practical applications.
Fluorine's value as a lateral substituent in liquid crystalline materials lies in the fact that it is of small size and high electronegativity, and these two characteristics lead to a subtle modification of physical properties, often reducing melting points without totally eliminating liquid crystal phases [22].Moreover, the great strength of the C-F bond confers chemical stability on fluoro-substituted compounds.However, a terminal fluoro substitution in calamitic mesogens tends to induce the formation of smectic phases [24].
Current work explores the influence of attaching lateral fluorine groups on incorporating a lateral fluoro substituent ortho to the alkoxy chain onto each peripheral phenyls of the cruciform shaped compound 1.Though still monotropic, the fluoro-substituted compounds exhibit lower transition temperatures advantageous in practical applications.Moreover, the fluoro groups induce and promote the formation lamellar mesophase for the cruciform-shaped mesogens.

Results and Discussion
In this study, two novel cruciform-shaped liquid crystals, one with lateral-fluoro side-arms and the other with non-fluoro side-arms, are disclosed.The liquid crystalline properties of the synthesized compounds were investigated by polarizing optical microscopy (POM) and differential scanning calorimetry (DSC).

Liquid Crystalline Properties
The mesophase behaviors of all the new compounds were initially examined by polarizing optical microscopy (POM).The enthalpy change of different transitions was determined by differential scanning calorimetry (DSC) scans.Phase transition temperatures and enthalpies of compounds 7, 9 and 10 are summarized in Table 1.Compound 7 was synthesized for the realization of the influence of chain length on the transition temperatures of the nematic phase.Similar to compound 1, compound 7 exhibited a monotropic nematic phase with the characteristic two-and four-brush Schlieren texture (Figure 3a).However, for compound 7, the clearing temperature is significantly lower, and the nematic texture can be detected throughout the sample region within a reasonable temperature range on cooling.Herein, the balance of chain-length and conjugated core is crucial for obtaining wider mesophase ranges of cruciform-shaped TPEB.To evaluate the role of fluorine atoms on the properties of the TPEB system, additionally, two compounds with lateral fluoro substituents have been synthesized.Based on the superior mesogenic properties of compound 7, incorporating a lateral-F group onto three side arms as in compound 9 leads to the formation of a smectic phase in addition to a nematic phase, both on cooling, indicating that the introduction of a fluorine atom onto each of three peripheral phenyls induces the lamellar mesophase in addition to the original nematic phase.When all four side-arms are lateral-F-containing, as in compound 10, the nematic phase disappeared, and a monotropic lamellar phase, SmC, as indicated by the blurred Schlieren texture shown in Figure 3b, emerged as the sole mesophase.The incorporation of a lateral fluorine group on each side-arm results in enhanced intermolecular lateral interactions to promote the lamellar suprastructure and to result in a wider mesophase temperature range on cooling.

X-ray Diffraction Studies
The two mesophases exhibited by the TPEB derivative of compound 9 were probed by X-ray diffraction (XRD) studies, and the results are shown in Figure 4.The XRD pattern of compound 9 at 130 °C upon cooling showed two diffuse halos at d = 31.6and 4.7 Å. Upon further cooling, the XRD pattern at 55 °C exhibited a sharp signal at 41.1 Å and a diffuse halo at 4.5 Å.At both temperatures, the diffuse halo in the wide angle regime is ascribed to the liquid-like alkyl chains, indicating mesophase formation.The diffraction pattern of two diffuse halos at higher temperature is typical for a nematic phase.At 55 °C, the sharp small angle peak indicates the formation of a lamellar supra-structure, and the d spacing does not change significantly upon further cooling, which is typical for SmC mesophases.The XRD data are in accordance with the mesophase assignments by POM observations.In conclusion, the incorporation of fluorine atoms laterally onto side-arms of TPEB induced and promoted the formation of a SmC phase.Though the obtained nematic and smectic phases are only kinetically stable, the wider mesophase temperature ranges on cooling do provide a better window for mesophase characterization.A new smectic phase was achieved for the cruciform mesogens.

Conclusions
A systematic study of the influence of fluorine substituents on the liquid crystalline properties of cruciform shaped compounds based on 1,2,4,5-tetra(alkoxyphenylethynyl)benzene has been carried out.The incorporation of lateral-F-containing side-arms widens the mesophase temperature range and, at the same time, results in enhanced lateral interactions to induce and promote a smectic C phase.

Figure 4 .
Figure 4. XRD patterns of trifluoro compound 9: (a) the N phase at 130 °C and (b) the smectic C (SmC) phase at 55 °C.