A series of copper(bis-β-diketonate) complexes are reported
which display columnar liquid-crystal phases with a hexagonal disordered structure
(D
hd). The complexes do not have
the
disc-shape characteristic of most D
hd materials,
but produce a disc shape by forming dimers
with 90o rotations between nearest neighbors. In the
liquid-crystalline state this dimerized
nature produces short-range rotational correlations. Three
side-chain copper bis-β-diketonates with a single phenyl substituent, 2, are not liquid
crystalline, and it was found that
an extension of the mesogenic core is necessary to introduce liquid
crystallinity. The simple
phenyl analogues, 3a, are monotropic, and the addition of
electron-withdrawing substituents
to the phenyl moiety, 3b
−
d, results
in a stabilization of the mesophase. These
substituents
produce favorable dipolar interactions which stabilize the mesophase.
Consistent with this
explanation, electron-donating substituents are not effective at
stabilizing the mesophase.
Substitution of the complex with thiophene groups rather than
phenyls, 4, produces stable
mesophases with greatly lowered melting and clearing points. This
latter result indicates
that thiophene substitution provides dispersive forces which
destabilize the crystal phase.
Thiophene substitution may provide a general method for reducing
transition temperatures
in metallomesogens.