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

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Abstract
Novel non-symmetric charge-transfer trimers are presented which consist of an electron-rich penta-alkynylbenzene group with different peripheral substituents and a flat nitrofluorenone acceptor moiety both of which are chemically linked to the terminal positions of a rod-like azobenzene unit. The laterally 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 (NCol) like molecular arrangement for the mesophases of both compounds. The columns are formed through an intercalated stacking of the flat donor and acceptor sub-units of different molecules. The rigid rod-shaped moieties are aligned with their long axes orthogonal rather than parallel relative to the columns axis. A further elongation of the five lateral alkyl substituents of the donor group enhances 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 confirm the occurrence of a glass transition well below the clearing temperature and reveal the presence of thermally activated secondary relaxation processes in the low temperature range. For these trimers containing an azo group, an additional secondary relaxation (beta2-process) is observed, which is probably caused by a local motion of the azo group within the spacer segment.