Thermally-Induced Phase Transition of Pseudorotaxane Crystals: Changes in Conformation and Interaction of the Molecules and Optical Properties of the Crystals

This paper presents a pseudorotaxane that acts as a thermally
driven
molecular switch in the single-crystal state. Crystals of the cationic
pseudorotaxane consisting of dibenzo[24]­crown-8 (DB24C8) and N-(xylylammonium)-methylferrocene as the cyclic and axle
component molecules, respectively, undergo crystalline-phase transition
at 128 °C with heating and 116 °C with cooling, according
to differential-scanning-calorimetry measurements. X-ray crystallographic
analyses revealed that the phase transition was accompanied by rotation
of the 4-methylphenyl group of the axle component molecule and a simultaneous
shift in the position of the PF6– counteranion.
Crystalline phase transition changes the conformation and position
of the DB24C8 molecule relative to the ammonium cation partially;
the interaction between the cyclic component and the PF6– anion in the crystal changes to a greater extent.
Moreover, there are changes in the vibration angle (θ) and birefringence
(Δn) on the (001) face of the crystal transitionally;
θ is rotated by +12°, and Δn is
decreased from 0.070 to 0.059 upon heating across the phase transition
temperature. The phase transition and accompanying change in the optical
properties of the crystal occur reversibly and repeatedly upon heating
and cooling processes. The switching rotation of the aromatic plane
of the molecule induces a change in the optical anisotropy of the
crystal, which is regarded as a demonstration of a new type of optical
crystal. Partial replacement of the PF6– anion with the bulkier AsF6– anion
forms crystals with similar crystallographic parameters. An increase
in the AsF6– content decreases the reversible-phase-transition
temperature gradually down to 99 °C (Tend) and 68 °C (Texo) ([AsF6–]:[PF6–] = 0.4:0.6).