SYNTHESIS, SINGLE CRYSTAL STRUCTURES, AND LIQUID CRYSTAL PROPERTY OF 2,5-DIPHENYL-1,3,4-OXADIAZOLES/1,3,4-THIADIAZOLES

A series of 2,5-diphenyl-1,3,4-oxadiazoles and 1,3,4-thiadiazole analogs, abbreviated as [p-CnH2n+1OC6H4 (XC2N2)-p-C6H4-R]: (i) X = O, n = 7, 4 and R = CH3O, CN, NO2 (1a-1f) and (ii) X = S, n = 7, 4 and R = CH3O, CN, NO2 (2a-2f) (where p-C6H4 and XC2N2 represents a p-phenylene spacer and a heterocyclic ring containing a C-X-C moiety, respectively), were synthesized and characterized by 1H NMR, MS and elemental analysis. The X-ray crystal structures of 1e, 2c, and 2f revealed that these compounds individually formed lamellar arrangements in the crystalline solids. The thermotropic liquid crystal behaviors were determined using polarized optical microscopy and differential scanning calorimetry (DSC). Among the oxadiazoles 1a-1f, only 1b displayed an enantiotropic smectic A phase, the others exhibited no liquid crystal properties. All the thiadiazoles 2a-2f exhibited various calamitic mesophases with wide mesomorphic temperature ranges. The relationship between the structure and mesomorphic properties was discussed in context of the geometrical configuration of heterocyclic rings, the polarity of terminal groups, and the length of the alkoxy chain.