Polymorphisme Cristallin des Sept Premiers Termes de la Serie des 4,4′-Di-n-alcoxyazoxybenzenes—II—Etude des Modes Internes (Spectroscopies Infrarouge et Raman)

Abstract

A study of the infrared and Raman spectroscopic changes occurring in the frequency range 200–1600 cm⁻¹ at the solid-solid or solid-liquid crystal transitions of the homologous series of 4,4′-di-n-alkoxyazoxybenzenes (n – 1 to 1) has enabled us to determine their molecular configurations in the solid phases and the liquid crystal state. In the low temperature solid phase, the alkoxy tails of the n = 2,3, 5,7 compounds have an all-trans configuration whereas those of the n = 4 and 6 compounds show g^± or g^± tg^± defects. Some kind of configurational change appears at the crystal polymorphic transition for n = 3, 4, 5 and at the mesomorphic transition for n = 6 and 7 according to the relative importance of the enthalpic and entropic variations of the crystal-crystal or crystal-liquid crystal transition. As to the n = 1 and 2 compounds, it seems that the perturbations observed are related to a statistical disorder of the azoxy group rather than the two possible orientations of the methoxy or ethoxy groups around the ring-to-oxygen bond.