Self-assembly of pseudorotaxane films with thermally reversible crystal phases and optical properties

Abstract

Highly ordered thin films of pseudorotaxane consisting of the host cyclic molecule, dibenzo[24]crown-8 (DB24C8), and the guest axle molecule, N-(xylylammonium)-methylferrocene, were prepared on solid substrates via solution casting followed by annealing. Collective evidence obtained from X-ray diffraction, differential scanning calorimetry, polarised optical microscopy (POM) and grazing incident wide-angle X-ray scattering (GIWAXS) experiments indicates that the specially architectured host and guest molecules can first form a highly ordered crystalline phase upon solvent evaporation. Annealing of this phase at temperatures above 130 °C leads to an irreversible transition of the crystal phase to a high-temperature (HT) crystalline phase with triclinic crystalline domain sizes of 50–100 μm. When cooled back to room temperature, the HT phase undergoes a solid–solid phase transition to a low-temperature (LT) crystalline phase of a similar triclinic form but favouring a tighter packing of axle molecules. Thermally reversible LT–HT phase transitions were consistently evidenced via in situ observations by POM and X-ray scattering and are attributed to a subtle ordering competition between the host cyclic molecules and guest axle molecules. This local structural transition, however, only slightly affects the stability of the layering frame structure of the pseudorotaxane film, in which the supermolecular complex adopts an edge-on orientation with the cyclic molecular plane being perpendicular to the substrate and is interlocked via interactions with the axle molecules. The results demonstrate the feasibility of self-assembled pseudorotaxane films with thermally driven phase transitions as a possible molecular switch.