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

Abstract

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 tbutylcalix[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 CH 3 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 [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.

Aggregation‐Induced Emission‐Active Gels: Fabrications, Functions, and Applications

“…The typical AIE‐active LMWGs employed in these studies were summarized according to the type of AIE moiety (i.e., TPE, tetraphenylsilole [TPS], and CNS), as outlined in Figure . More specifically, typical AIE‐active LMWGs based on TPE are 43 and 44 , [ 67 ] 45 , [ 68 ] 46 , [ 69 ] 47 , [ 70 ] 48 , [ 71 ] 49 , [ 72 ] 50 , [ 73 ] 51 , [ 74 ] 52 , [ 75 ] 53 , [ 76 ] 54 , [ 77 ] 55 , [ 78 ] 56 , [ 79 ] 57 , [ 80 ] 58 and 59 , [ 81 ] and 60 , [ 82 ] while typical AIE‐active LMWGs based on TPS are 61 and 62 , [ 83 ] and 63 , [ 62 ] and typical AIE‐active LMWGs based on CNS are 7 , [ 31,35–37 ] 64 , [ 84 ] 65 , [ 85 ] 66 and 67 , [ 86 ] 68 , 69 and 70 , [ 87 ] 71 , 72 and 73 , [ 88 ] 74 , [ 89 ] 75 and 76 , [ 90 ] 77 and 78 , [ 91 ] 79 , [ 92 ] 80 , [ 93 ] 81 , [ 94 ] 82 , [ 95 ] 83 and 84 , [ 96 ] 85 and 86 , [ 97 ] 87 , 88 and 89 , [ 98 ] 90 , [ 99 ] 91 , [ 100 ] 92 , [ 101 ] 93 , [ 102 ] and 94 , 95 and 96 . [ 103 ] Information of the AIE‐active supramolecular gels from the above LMWGs based on TPE, TPS, and CNS is also summarized in Table 2 .…”

Diversiform Nanostructures Constructed from Tetraphenylethene and Pyrene-Based Acid/Base Controllable Molecular Switching Amphiphilic [2]Rotaxanes with Tunable Aggregation-Induced Static Excimers

“…To synthesize the target rotaxanes P1 and P2 , we started with the synthesis of the half dumbbell alkyne ( S4 ) in four steps from known compounds (see Schemes and S1, Supporting Information). Next, the macrocycle S7 , featured as a benzocrown-8-ether in the lower-rim of t -butylcalix­[4]­arene, was obtained in 60% yield by the reaction of t -butylcalix­[4]­arene with compound S6 from the reported literature . On the other side of the axle, Py-U , a pyrene-urea linked with an azide functional group for the cycloaddition, was formed from the reaction of 1-aminopyrene with compound S5 in 61% yield (Scheme S3, Supporting Information).…”

“…Fascinated by this intriguing phenomenon, a large number of AIEgens, with diversified architectures and versatile properties, have been generated and applied in organic light-emitting devices, biological imaging, organelles imaging, and drug delivery monitoring. − In 2019, Yang and co-workers reported that in the presence of Hg 2+ , the RIR will be effected by host–guest interaction between the TPE bridge and the pillar[6]­arene functionalized arms . However, to the best of our knowledge, to date, only few examples of MIM-based AIE active reversible molecular switching have been reported to be operational in aqueous media. − In our previous work, we described that TPE–calix[4]­arene-functionalized MIM-based reversible molecular switching supramolecular amphiphiles manifested their fluorescence emission, distinct nanostructures, and gelation properties could be induced by the multinoncovalent interactions during the self-assembly processes . Combining the advantages of AIE/pyrene excimer emission with MIM-based supramolecular amphiphiles would provide novel and state-of-the-art approaches for biomedical application and molecular machines.…”

From Molecular Machines to Stimuli‐Responsive Materials

“…Upon addition of a base or a strongly binding anion, disruption of the gel phase occurs as a result of the shuttling of the macrocyclic unit over the urea moiety, which thus alters their hydrogen‐bonds networks. More recently, the group of Chung reported a similar strategy to achieve a sol‐gel transition in organic solvents using an amphiphilic calix[4]arene‐based [2]rotaxanes . However, in this case, SEM imaging suggests that a 3D arrangement of nanosphere aggregates obtained by a combination of various supramolecular interactions (π–π, H‐bonds, hydrophobic) leads to the formation of gelified structures, which can be disrupted in a reversible manner upon addition of base.…”