Liquid crystalline hydrogen bonded oligo(p-phenylenevinylene)s

Abstract

The thermotropic behaviour of a series of hydrogen bonded chiral and achiral oligo(p-phenylenevinylenes) (OPVs) has been investigated. The liquid crystalline properties are studied as a function of their molecular design that is based on hydrogen bonding units, the lengths of the π-conjugated and flexible segments and the branching of the side chains. Two-dimensional wide-angle X-ray scattering experiments indicate self-assembly of all OPVs into columnar superstructures which is the result of hydrogen bonds formed by the diaminotriazine or ureidotriazine units and π-stacking interactions accompanied by a local phase separation between the alkoxy side chains and the aromatic rods. The phase behaviour of the compounds bearing the diaminotriazine functionality depends on the length of the OPV segment. OPV trimers are already liquid crystalline at ambient conditions, while longer OPV tetramers are in a well-organized crystalline state at that temperature. In the liquid crystalline phase the intracolumnar order decreases due to molecular motion as confirmed by solid-state NMR studies. Interestingly, the introduction of branched (S)-2-methylbutoxy side chains increases significantly the π-stacking distance while the dimensions of the hexagonal unit cell decrease. The hydrogen bonding ureidotriazine groups decrease the thermal stability and lower the degree of order in comparison with the diaminotriazine derivatives. This low degree of order is probably the result of the ureidotriazine units that form hydrogen bonded dimers while the diaminotriazine arrays form hydrogen bonded hexameric rosettes. In the latter case a more densely packed columnar structure is formed.