2H NMR studies of phase behaviour and molecular motions of doped discotic liquid-crystalline systems

Publisher Website
Google Scholar
Abstract
A discotic triphenylene monomer as well as a dimer and a main chain polymer, all substituted with heptyloxy side groups, were doped with an electron acceptor (2,4,7-trinitrofluorenone (TNF)) to give charge transfer complexes. These doped systems were aligned in a magnetic field, thus proving their liquid crystallinity. 2H NMR measurements show that the electron acceptor molecules are incorporated into the columns built of triphenylene cores. In the charge transfer complex with the triphenylene monomer almost all the electron acceptor molecules stack in the columns even close to the clearing temperature T 1, while for the dimer and especially for the polymer a significant fraction of the TNF molecules exhibits isotropic motion, which is attributed to their location in the region between the columns, already way below T 1. This isotropically distributed part increases on approaching T 1. Fast rotation of the discs around their column axes takes place in the monomer and is quenched in the dimer and the polymer, due to the interlinking of the columns by the spacer. The electron acceptor molecules, on the other hand, exhibit free rotation in all samples, even in the charge transfer complexes with the triphenylene dimer and polymer. In the side group labelled triphenylenes increased 2H NMR spectral narrowing is detected in the charge transfer complex samples compared with the corresponding pure triphenylenes. This indicates higher side group mobility in the doped systems, since the incorporation of TNF molecules increases the spacing between the discotic units.