Solvent and Structure Effects on the Shuttling in Pillar[5]arene/Triazole Rotaxanes
2017
Wang, Shuangshuang | Shao, Xueguang | Cai, Wensheng
In the present contribution, a pillararene-based rotaxane, formed by a pillar[5]arene (P[5]) and a dumbbell-shaped thread composed by four 1,2,3-triazole moieties alternatively linked by three methylene moieties and thus leading to two kinds of stations (the C-ended and N-ended ones), was investigated at the atomic level. The effect of the linkers on shuttling in CHCl₃ was investigated by building four rotaxane models with different lengths of methylene groups. The free-energy profiles delineating the shuttling of the P[5] along the thread revealed that the shuttling rate varied regularly with the length (n) of the methylene moieties and exhibited the slowest value for the rotaxane (n = 5). Decomposition of the free-energy profiles into free-energy contributions suggested that electrostatic interactions constitute the main driving force responsible for shuttling. Moreover, the stability of C-ended station is found to be much lower than the N-ended station in each rotaxane, which can also be ascribed to the electrostatic interactions of P[5] with the stations. To investigate the effect of the solvent, the shuttling movement of the rotaxane (n = 4) in DMSO was also studied and compared to that in CHCl₃. The shuttling barrier in DMSO decreased significantly, which can be attributed to its higher polarity and the formation of H-bonds between DMSO and the triazole units. Therefore, the polarity of the solvent and its hydrogen-bond acceptor ability can affect the shuttling rates of the rotaxanes. The present results provide understanding of the shuttling mechanism of the molecules formed by pillararenes and triazole moieties and are expected to serve in the design of pillararene-based molecular machines.
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