MESOMORPHIC DONOR-ACCEPTOR TWIN MOLECULES WITH COVALENTLY-LINKED SHEET-LIKE PENTAALKYNE AND NITROFLUORENONE SUBUNITS

Charge-transfer (CT) twin mesogens are presented which consist of shee t-like pentayne donor moieties with different peripheral substituents chemically linked with nitro substituted fluorenone sub-units via flex ible alkyl spacers. A novel plastic rectangular columnar (Col(rp)) pha se is reported for the laterally unsubstituted member of the pentayne based CT twins. The phase is characterized by a three-dimensional crys tal-like correlation of ordered columns in a rectangular lattice, whil e the molecular sub-units are still able to perform relaxation motions . The attachment of peripheral substituents to the pentayne moieties r esults in a distortion of the three-dimensional positional order and t he five-fold alkyl substituted homologues exhibit a rectangular column ar ordered (Col(ro)) mesophase. The rectangular lattice symmetries ori ginate from the chemical linkage of the Bat donor and acceptor parts o f the molecules placed in an alternating manner within neighbouring co lumns, and the regular intracolumnar periodicity is enhanced by charge -transfer interactions. within the columns. A further structural modif ication consists of the incorporation of an asymmetric carbon into the spacer sequence resulting in pentayne based CT-twin mesogens displayi ng a nematic columnar mesophase with a helical twisting of the columns (N-col). Dielectric investigations reveal an unusual dynamic behavio ur of the donor-acceptor pentaynes. The glass relaxation process is ch aracterized by the occurrence of two relaxation modes, both following a WLF behaviour, a feature which has not previously been reported to o ur knowledge for columnar phase-forming disc-like liquid crystal mater ials.