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 degrees C with heating and 116 degrees 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 (theta) and birefringence (Delta n) on the (001) face of the crystal transitionally; theta is rotated by +12 degrees, and Delta n 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 degrees C (T-end) and 68 degrees C (T-exo) ([AsF6-]:[PF6-] = 0.4:0.6).