Mesomorphic donor-acceptor twin molecules with covalently linked sheet-like pentaalkyne and nitrofluorenone sub-units

Abstract

Charge-transfer (CT) twin mesogens are presented which consist of sheet-like pentayne donor moieties with different peripheral substituents chemically linked with nitro substituted fluorenone sub-units via flexible alkyl spacers. A novel plastic rectangular columnar (Colrp) phase is reported for the laterally unsubstituted member of the pentayne based CT twins. The phase is characterized by a three-dimensional crystal-like correlation of ordered columns in a rectangular lattice, while the molecular sub-units are still able to perform relaxation motions. The attachment of peripheral substituents to the pentayne moieties results in a distortion of the three-dimensional positional order and the five-fold alkyl substituted homologues exhibit a rectangular columnar ordered (Colro) mesophase. The rectangular lattice symmetries originate from the chemical linkage of the flat donor and acceptor parts of the molecules placed in an alternating manner within neighbouring columns, and the regular intracolumnar periodicity is enhanced by charge-transfer interactions within the columns. A further structural modification consists of the incorporation of an asymmetric carbon into the spacer sequence resulting in pentayne based CT-twin mesogens displaying a nematic columnar mesophase with a helical twisting of the columns (N*col). Dielectric investigations reveal an unusual dynamic behaviour of the donor-acceptor pentaynes. The glass relaxation process is characterized by the occurrence of two relaxation modes, both following a WLF behaviour, a feature which has not previously been reported to our knowledge for columnar phase-forming disc-like liquid crystal materials.