Distinct Nanostructures and Organogel Driven by Reversible Molecular Switching of a Tetraphenylethene-Involved Calix[4]arene-Based Amphiphilic [2]Rotaxane

Aggregation induced emission (AIE) active and acid/base controllable amphiphilic [2]­rotaxanes R1 and R2 were successfully constructed with tetraphenylethene (TPE) as a stopper and t-butylcalix­[4]­arene or calix[4]­arene macrocycle as a wheel over the axle component. The AIE effect of [2]­rotaxanes R1 and R2 was greatly affected by the molecular shuttling of t-butylcalix­[4]­arene or calix[4]­arene macrocycle, which was triggered by the acid/base strategy. In the case of [2]­rotaxane R1, aggregation was achieved in the presence of less amount of water compared with those of [2]­rotaxane R2, and the deprotonated [2]­rotaxanes R1-b and R2-b, owing to the stronger interaction between the TPE and t-butylcalix­[4]­arene macrocycle and restricted intramolecular rotation (RIR), thus making it responses in less quantity of water along with highly fluorescent emission. [2]­Rotaxane R1-b started to aggregate at 70% water fraction (f_w), while [2]­rotaxane R2-b started to aggregate at 75% f_w which allowed them to morph into hollow nanospheres, whereas they formed only nanospheres at 99% f_w in CH₃CN/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 [2]­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 [2]­rotaxanes studied here, R1 possessed a narrower HOMO–LUMO band gap compared to those others, as confirmed by computational study. Furthermore, only [2]­rotaxane R1 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.