Columnar mesophases of octa-alkyloxydibenzopyrenes and their charge transfer complexes: Synthesis, X-ray and NMR

Abstract

The mesomorphic properties of five homologues of the 1,2,5,6,8,9,12,13-octa-alkyloxydibenzopyrene series (DBPn, where n is the number of carbons in each of the side chains) and their charge transfer complexes with 2,4,7-trinitro-9-fluorenone (TNF) are described. The neat compounds exhibit a single mesophase which has been identified by X-ray diffractometry and optical microscopy as hexagonal columnar (D_hd). Two-dimensional unit cell parameters are reported for the various homologues as functions of temperature within the mesophase region. Deuterium NMR of labelled DBPn was used to determine the nature and rate of the molecular reorientation in the mesophase. Addition of the electron acceptor TNF to the donor DBPn mesogens increases the stability range of the mesophases, with a maximum stability for the equimolar donor/acceptor mixtures. X-ray and deuterium NMR measurements on these charge transfer stabilized mesophases are also reported. Besides being thermally more stable, they are more highly ordered and even exhibit well defined intramolecular stacking order, classifying them as D_ho. Under certain conditions, a diffraction peak at a position corresponding to twice the normal intracolumnar distance is also observed, indicating that the donor and acceptor molecules are stacked alternately in the columns. When alkanes are added to the 1:1 charge transfer complexes, a lyotropic nematic phase is induced. It is shown that these nematics are of the type N_c, with columnar structures as the mesogenic units.