Phase Behavior and Liquid Crystalline Ordering of [2]Catenated Molecular Systems

Catenated molecule as one type of mechanically interlocked molecule (MIM) is formed by not only conventional covalent bonds but also mechanical bonds between molecular rings. The [2]catenane molecule consists of two mechanically interlocked macrocycles, which is a typical example combining nonconvex molecular rings and mechanical bonds. In this study, coarse-grained molecular dynamics simulation is performed to investigate the thermodynamics and phase behavior of [2]catenane for deepening our understanding of the role of the mechanical bond in determining the phase behavior of catenated molecular systems. We demonstrate that the mechanical bond in [2]catenane destabilizes the packing between molecular rings and also provides an additional internal degree-of-freedom for two interlocked macrocycles, which shows an entropy-driven molecular conformation transition from free-rolling to cross-stacking with respect to density increase. The [2]catenane molecule with a large ring size undergoes a transition from disorder to a smectic layering structure with high free volume. Different from conventional smectics, the smectic state in [2]catenane with large pore sizes shows cubatic ordering due to the interpenetration of macrocycles. The diffusion behavior of [2]catenane confirms the molecular fluidity in the smectic layer, and interlayer displacement behaves as subdiffusive in the smectic state of [2]catenane