Pyridinium and imidazolium 1,3,4-oxadiazole ionic liquid crystals: a thermal and photophysical systematic investigation

Abstract

In this study we present the synthesis and complete structural characterization of twenty five 1,3,4-oxadiazole molecules (ionic and non-ionic) with some systematic structural variations, such as number of counterions, number of alkoxy chains, alkyl chain size, type of counterion and its position on the molecule. The thermal and photophysical properties for all compounds were investigated allowing a complete and interesting simultaneous study of the effect of structural changes on these properties. We show that every parameter studied affected both the thermal and photophysical properties. For these bent core molecules, the liquid crystalline behavior was observed only for compounds containing one counterion, SmA (monolayer or bilayer) being the predominant phase. The neutral (non-ionic) compounds and the imidazolium derivatives showed emission at around 360 nm and a high quantum yield, while most of the pyridinium derivatives showed emission at around 530 nm, a low quantum yield and a complex photophysical behavior. In all cases, the quantum yield for the different counterions follows the sequence ClO₄⁻ > BF₄⁻ > DS⁻ > NO₃⁻ > Br⁻(I⁻).