Liquid Crystalline Ionic Dendrimers Containing Luminescent Oxadiazole Moieties

Two novel series of dendrimers constituted by the ionic grafting of poly(propyleneimine) PPI-(NH2)(x) (x = 4, 8, 16, 32, 64) and poly(amidoamine) PAMAM-(NH2)(x) (x = 64) with carboxylic acids bearing an oxadiazole ring have been synthesized, and their liquid crystalline properties have been investigated. Series I is generated by the ionic attachment between the dendrimers and 1,3,4-oxadiazole-containing acids. Series H results from the ionic junction of the dendrimers to 1,2,4-oxadiazole-containing acids. The liquid crystalline behavior has been investigated by means of differential scanning calorimetry (DSC), polarizing optical microscopy (POM), and X-ray diffractometry (XRD). The liquid crystal properties are significantly improved in the dendrimers compared to the mesogenic precursors. The structural parameters determined by X-ray diffraction reflect the different supramolecular organization built by each kind of oxadiazole-containing acid introduced. On the basis of these experimental results, a packing model is proposed based on a microsegregation phenomenon and a variable degree of interdigitation between the mesogenic units. The absorption and emission properties of the compounds have also been studied. To our knowledge, these are the first dendrimers reported up to date combining oxadiazole units and liquid crystal properties.