New Liquid Crystalline Materials Based on Two Generations of Dendronised Cyclophosphazenes

A divergent approach was used for the synthesis of dendritic structures based on a cyclotriphosphazene core with 12 or 24 hydroxyl groups, by starting from [N₃P₃(OC₆H₄OH‐4)₆] and using an acetal‐protected 2,2‐di(hydroxymethyl)propionic anhydride as the acylating agent. Hydroxyl groups in these first‐ and second‐generation dendrimers, G1‐(OH)₁₂ or G2‐(OH)₂₄ , were then condensed in turn with mono‐ or polycatenar pro‐mesogenic acids to study their ability to promote self‐assembly into liquid crystalline structures. Reactions were monitored by using ³¹P{¹H} and ¹H NMR spectroscopy and the chemical structure of the resulting materials was confirmed by using different spectroscopic techniques and mass spectrometry (MALDI‐TOF MS). The results were in accordance with monodisperse, fully functionalised cyclotriphosphazene dendrimers. Thermal and liquid crystalline properties were studied by using optical microscopy, differential scanning calorimetry and X‐ray diffraction. The dendrimer with 12 4‐pentylbiphenyl mesogenic units gives rise to columnar rectangular organisation, whereas the one with 24 pentylbiphenyl units does not exhibit mesomorphic behaviour. In the case of materials that contain polycatenar pro‐mesogenic units with two aromatic rings ( A4 vs. A5 ), the incorporation of a short flexible spacer connected to the periphery of the dendron (acid A5 ) was needed to achieve mesomorphic organisation. In this case, both dendrimer generations G1 A5 and G2 A5 exhibit a hexagonal columnar mesophase.