Aggregation-induced emission behavior of a pH-controlled molecular shuttle based on a tetraphenylethene moiety

Tetraphenylethene (TPE) with aggregation-induced emission (AIE) behavior as a popular backbone is applied widely in the construction of functional supramolecular systems. In this work, a TPE-based linear molecule having amide and amine units is synthesized. Its ammonium template is used to construct the N-hetero crown ether-based [2] rotaxane by the template-directed clipping approach. Their structures are well-characterized by NMR, MALDI-TOF-MS and elemental analysis. Owing to the existence of the amide unit, [2] rotaxane possesses the function of a molecular shuttle. The shuttling motion of the macrocycle component between the ammonium station and the amide station can be driven by external acid-base stimuli in solution, accompanied by changes in visual behavior. Investigation on their AIE behavior shows that (1) ammonium reaches the aggregation state almost in the presence of same water with the deprotonated form of ammonium; (2) the [2] rotaxane that the macrocycle component locates at the site of ammonium forms the aggregation state in the presence of less water than the deprotonated [2] rotaxane that the macrocycle component locates at the site of the amide, attributed to stronger interaction between the crown ether component and the TPE unit of the template component when the distance between the two is shorter. The result indicates that the shuttling motion of the macrocycle component can adjust the aggregation state of AIE molecules.