Distinct Nanostructures and Organogel Driven by Reversible Molecular Switching of a Tetraphenylethene-Involved Calixarene-Based Amphiphilic Rotaxane
2018-10-03T16:21:58Z (GMT) by
Aggregation induced emission (AIE) active and acid/base controllable amphiphilic rotaxanes <b>R1</b> and <b>R2</b> were successfully constructed with tetraphenylethene (TPE) as a stopper and <i>t</i>-butylcalixarene or calixarene macrocycle as a wheel over the axle component. The AIE effect of rotaxanes <b>R1</b> and <b>R2</b> was greatly affected by the molecular shuttling of <i>t</i>-butylcalixarene or calixarene macrocycle, which was triggered by the acid/base strategy. In the case of rotaxane <b>R1</b>, aggregation was achieved in the presence of less amount of water compared with those of rotaxane <b>R2</b>, and the deprotonated rotaxanes <b>R1-b</b> and <b>R2-b</b>, owing to the stronger interaction between the TPE and <i>t</i>-butylcalixarene macrocycle and restricted intramolecular rotation (RIR), thus making it responses in less quantity of water along with highly fluorescent emission. Rotaxane <b>R1-b</b> started to aggregate at 70% water fraction (<i>f</i><sub>w</sub>), while rotaxane <b>R2-b</b> started to aggregate at 75% <i>f</i><sub>w</sub> which allowed them to morph into hollow nanospheres, whereas they formed only nanospheres at 99% <i>f</i><sub>w</sub> in CH<sub>3</sub>CN/water cosolvent system due to the higher degree of aggregation in aqueous media. To our delight, controllable morphology of self-assembled structures was indeed formed from these rotaxanes. Interestingly, by the interplay of a wide range of multi-self-assembly driving forces, the slack stacking of rotaxane unit forms a hollow on the surface of nanospheres to become hollow nanospheres. Among the four rotaxanes studied here, <b>R1</b> possessed a narrower HOMO–LUMO band gap compared to those others, as confirmed by computational study. Furthermore, only rotaxane <b>R1</b> formed organogel in methanol solvent and its reversible gel–sol transition could be achieved by the addition of acid and base. This implies that the formation of dumbbell shape cross-linked 3D network structures were mainly governed by π–π stacking, van der Waals force, and intermolecular H-bonding interactions during the gelation processes.