Synthesis and liquid crystalline properties of substituted 2,5-diaryl 1,3,4-oxadiazole derivatives without flexible chains

A series of new compounds based on aromatically 2,5-disubstituted 1,3,4-oxadiazoles without flexible chains, formulated as p-R-C6H4-(OC2N2)-(p-C6H4)2-R' with (i) R = CH3O, R' = CH3O, CH3S, F, H (Ia-Id), (ii) R = CH3S, R' = CH3O, CH3S, F, H (IIa-IId) and (iii) R = F, R' = CH3O, CH3S, F, H (IIIa-IIId) (p-C6H4 and OC2N2 represent a p-phenylene spacer and a 1,3,4-oxadiazole ring, respectively), were synthesised and characterised by 1H and 13C NMR, MS and HRMS techniques. Mesomorphic properties were investigated using differential scanning calorimetry and polarizing optical microscopy. All of the target compounds (except Id, IId, IIIc and IIId) exhibited an enantiotropic nematic mesophase with high melting temperatures. The liquid crystalline properties of these compounds were influenced greatly by polarity, steric factors and positions of the terminal groups. The effect of the terminal groups on the liquid crystal properties is discussed.