Crystal Engineering of Hand-Twisted Helical Crystals

A strategy
is outlined for the design of hand-twisted helical crystals.
The starting point in the exercise is the one-dimensional (1D) plastic
crystal, 1,4-dibromobenzene, which is then changed to a 1D elastic
crystal, exemplified by 4-bromophenyl 4′-chlorobenzoate, by
introduction of a molecular synthon −O–CO– in
lieu of the supramolecular synthon Br···Br in the precursor.
The 1D elastic crystals are next modified to two-dimensional (2D)
elastic crystals, of the type 4-bromophenyl 4′-nitrobenzoate
where the halogen bonding and C–H···O hydrogen
bonding are well-matched. Finally, varying the interaction strengths
in these 2D elastic crystals gives plastic crystals with two pairs
of bendable faces but without slip planes. Typical examples are 4-chlorophenyl
and 4-bromophenyl 4′-nitrobenzoate. This type of 2D plasticity
represents a new type of bendable crystals in which plastic behavior
is seen with a fair degree of isotropic character in the crystal packing.
The presence of two sets of bendable faces, generally orthogonal to
each other, allows for the possibility of hand-twisting of the crystals
to give grossly helical morphologies. Accordingly, we propose the
name hand-twisted helical crystals for these substances.