Mesomorphic Properties of p-Benzoquinone Derivatives

Abstract

Since their discovery, many varieties of discotic liquid crystals, whose molecules resemble flat, circular disks in shape, have been described. Among these compounds, Chandrasekhar et al. first reported the unstable discotic mesophase of tetrahydroxy-p-benzoquinone tetraoctanoate, i.e. 3,6-dioxo-1,4-cyclohexadiene-1,2,4,5-tetrayl tetraoctanoate. Then, Lillya and Thakur resumed this uncertain mesophase and concluded that 3,6-dioxo-1,4-cyclohexadiene-1,2,4,5-tetrayl tetraheptanoate (2a) and tetraoctanoate (2b) both exhibit a columnar mesophase with a narrow temperature range. Since 2,5-cyclohexadiene-1,4-dione has unique functionalities, we pay much attention to 2,5-cyclohexadiene-1,4-dione derivatives to apply them to discotic liquid crystals. In this study three series of 3,6-dioxo-1,4-cyclohexadiene-1,2,4,5-tetrayl tetrakis(4-alkoxybenzoate) 4, 2,3,5,6-tetraalkoxy-2,5-cyclohexadiene-1,4-dione 5, and 2,3,5,6-tetrakis(alkanoylamino)-2,5-cyclohexadiene-1,4-dione 7 were synthesized and their mesomorphic properties were examined. The benzoates 4 exhibited a simple crystal-isotropic liquid transition compared with the aliphatic esters 2. We concluded that the aromatic ring of benzoate 4 was not suitable for improving coplanarity and rigidity to the 2,5-cyclohexadiene-1,4-dione central core. We also obtained ethers 5 to improve chemical stability of esters 2, and found that 5b showed two endothermic peaks at 32 °C and 55 °C, respectively, by differential scanning calorimetry (DSC). However, only a vague texture was observed in the temperature range. Thus, we have not yet demonstrated that this mesophase was a liquid crystalline phase. Finally, we substituted an ester bond of 2 by an amide one to strengthen the inter-molecular interaction, and discovered that 7a displayed a dendric texture with thermal decomposition after being cooled from the isotropic liquid. We deduced this mesophase hexagonal columnar phase (Col_h) on the basis of the reported optical texture.