Novel donor-acceptor triple mesogens incorporating disc-like and rod-like molecular sub-units

Novel non-symmetric charge-transfer trimers are presented which consist of an electron-rich penta-alkynylbenzene group with different peripheral subst ituents and a flat nitrofluorenone acceptor moiety both of which are chemic ally linked to the terminal positions of a rod-like azobenzene unit. The la terally unsubstituted member of the pentayne based CT-triple compounds and the five-fold methyl modified homologue exhibit an enantiotropic mesophase which can be obtained as a frozen-in glassy state at room temperature. The X-ray investigations reveal a nematic columnar (N-Col) like molecular arran gement for the mesophases of both compounds. The columns are formed through an intercalated stacking of the flat donor and acceptor sub-units of diffe rent molecules. The rigid rod-shaped moieties are aligned with their long a xes orthogonal rather than parallel relative to the columns axis. A further elongation of the five lateral alkyl substituents of the donor group enhan ces the steric frustration between the disc-like and the rod-like molecular groups and the five-fold pentyl substituted pentayne donor-acceptor trimer no longer exhibits mesomorphic properties. Dielectric investigations confi rm the occurrence of a glass transition well below the clearing temperature and reveal the presence of thermally activated secondary relaxation proces ses in the low temperature range. For these trimers containing an azo group , an additional secondary relaxation (beta(2)-process) is observed, which i s probably caused by a local motion of the azo group within the spacer segm ent.