Mixed-Substituent Cyclophosphazenes with Calamitic and Polycatenar Mesogens

A synthetic strategy has been developed to prepare liquid crystalline cyclotriphosphazenes that have two different types of mesogenic units linked to the same phosphorus atom. Hexachlorocyclotriphosphazene, N₃P₃Cl₆, was reacted with 3 mol of the calamitic unit 4-cyano-4'-hydroxybiphenyl to give a mixture of compounds in which the nongem-trans-trisubstituted derivative N₃P₃Cl₃(OC₆H₄C₆H₄{CN}-p)₃ was the major product. The substitution of all three chlorine atoms in this nongeminal compound gave rise to the hydroxyl-functional phosphazenes, nongem-trans-N₃P₃(OC₆H₄C₆H₄{CN}-p)₃(OC₆H₄{OH}-p)₃ or nongem-trans-N₃P₃(OC₆H₄C₆H₄{CH₃}-p)₃(OC₆H₄{OH}-p)₃, from which the second mesogenic unit, a polycatenar one, was introduced. The chemical structure of the resulting materials, deduced from spectroscopic and MALDI-TOF techniques, was in accordance with monodisperse, fully functionalized cyclotriphosphazenes. Mesomorphism is highly dependent on the terminal group of the calamitic units, and liquid crystal phases were only detected on the cyano-derivatives. The calamitic or columnar nature of the mesophase depends on the number of alkyl chains of the polycatenar moieties.