Distinct Nanostructures and Organogel Driven by Reversible

Article Cite This: Chem. Mater. XXXX, XXX, XXX−XXX

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Distinct Nanostructures and Organogel Driven by Reversible Molecular Switching of a Tetraphenylethene-Involved Calix[4]areneBased Amphiphilic [2]Rotaxane Reguram Arumugaperumal,† Putikam Raghunath,‡ Ming-Chang Lin,‡ and Wen-Sheng Chung*,† †

Department of Applied Chemistry, National Chiao Tung University, Hsinchu 300, Taiwan Center for Interdisciplinary Molecular Science, Department of Applied Chemistry, National Chiao Tung University, Hsinchu 300, Taiwan

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‡

S Supporting Information *

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 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.

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multi stimuli-responsive supramolecular amphiphiles.22,23 Supramolecular amphiphiles that can self-assemble in aqueous solution into many well-defined nanostructures such as nanowires, nanofibers, nanotubes, nanorings, and hollow micro- and nanospheres have been reported.24−28 For example, Qu et al. have reported the synthesis of a reversibly switched [2]rotaxane that undergoes morphological transformations from spherical vesicles to worm-like micelles using acid/base stimuli.29 Indeed, few MIMs with bistable [2]rotaxane or ureabased [2]rotaxane can form organogels.30,31 Moreover, Jiang and co-workers successfully fabricated a fluorescent supramolecular gel from bodipy-stoppered pillar[5]arene-based [2]rotaxane via self-assembly in dimethyl sulfoxide solvent.32

INTRODUCTION Inspired by naturally occurring biomolecular machines,1 chemists have designed and developed a number of artificial molecular machines2,3 such as molecular switches,4,5 molecular elevators,6 and molecular muscles.7,8 Rotaxanes, well-known for their ability to experience the relative movements of their ring and dumbbell components with a unique functioning can be actuated by external stimuli modes include acid/base,9 redox,10,11 solvent,12 temperature13,14 and light.15,16 In most cases, acid/base has been considered the most promising tool to trigger the molecular machines and shuttles.17−19 Recently, mechanically interlocked molecules (MIMs) have been found to form supramolecular organogels20 via the self-assembly of hydrogen-bonding, electrostatic, π−π interaction, van der Waals force, and solute−solvent interactions.21 The strategy of noncovalent interactions not only activate the aggregation/ gelation in different solvent polarities but also construct the © XXXX American Chemical Society

Received: August 3, 2018 Revised: September 17, 2018

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DOI: 10.1021/acs.chemmater.8b03286 Chem. Mater. XXXX, XXX, XXX−XXX

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Chemistry of Materials Scheme 1. Chemical Structures of Acid/Base Switchable [2]Rotaxanes (a) R1 and R1-b and (b) R2 and R2-b

improvement in rotaxane field was made by the groups of Gaeta and Neri who developed an acid/base responsive calix[6]arene-based [3]rotaxane through stereoprogrammed direct synthesis.45 A particularly interesting class of artificial molecular machines is to further incorporate them with tetraphenylethene (TPE) as an active molecule for aggregation induced emission (AIE).46 In 2001, Tang’s groups introduced the concept of AIE, which has become a milestone in the field of optoelectronics.47 Since then, it has been widely developed in organic light emitting diodes (OLEDs),48,49 biomaterials,50,51 explosive detection,52,53 luminescent polymers54,55 and gels56,57 etc. In order to make use of the AIE in molecular machines, we have incorporated a TPE onto a rotaxane system so that it provides a novel strategy to either amplify or transduce mechanical motion into fluorescence signals. Using these methodologies, there have been some intelligent works on AIE active artificial molecular machines.58−60 To the best of our knowledge, calix[4]arene-based amphiphilic rotaxanes with multistimuli responsiveness, especially those possessing AIE functionalities, have not been reported thus far. In this work, amphiphilic [2]rotaxanes consist of either a hydrophobic t-butylcalix[4]arene or a calix[4]arene macrocycle have been synthesized to serve as a wheel to reside on the

Combining these intelligent strategies help us to develop an amphiphilic molecular machine with highly efficient molecular shuttling motion and gelation behavior. Last but not least, to develop a new and reliable amphiphilic rotaxane-based molecular shuttle with gelation property is still a highly desirable and challenging task. In order to form organogels in acid/base controllable molecular switches or molecular machines, urea moiety plays important roles not only as a station in the rotaxanes33 but also as a key component for intermolecular hydrogen bonding.34 In addition, supramolecular macrocyclic compounds such as cyclodextrins, 35,36 pillararenes, 37−39 calixarenes, 40 and cucurbit[n]urils41 have been incorporated to construct the rotaxane-based molecular machines. Among these macrocycles, calixarene wheels have constituted one of the most important families of supramolecular gels because their lower-rim hydroxyl groups can participate in intermolecular hydrogen bonding interactions, while their upper-rim t-butyl groups facilitate mutual van der Waals force interactions.42 Our group recently developed an azobenzene-bridged biscalix[4]arene as organogel, whose morphological change can be driven by light.43 The first calix[4]arene-based rotaxane host system for anion binding via anion templated synthetic methodology has been described by the group of Beer.44 Another important B


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Chemistry of Materials

Figure 1. 1H NMR spectra (400 MHz, 298 K, CD3CN) of (a) uncomplexed thread T1, (b) [2]rotaxane R1 and (c) t-butylcalix[4]arene macrocycle.

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RESULTS AND DISCUSSION Molecular Design and Synthesis. Switchable AIE active [2]rotaxane derivatives (R1, R2, R1-b and R2-b) were synthesized via a multistep synthetic pathway as fully detailed in the Supporting Information. AIE active [2]rotaxanes consist of t-butylcalix[4]arene or calix[4]arene macrocycle were locked into thread bearing two distinguishable stations either secondary ammonium or urea station. The macrocycle can be switched between the two different stations on the axle component by using acid/base stimuli. Theoretically, the ammonium of a secondary amine is a better binding site for macrocycle crown ether in the protonated state of [2]rotaxanes (R1 and R2), while the macrocycle preferred to stay at the urea station in the deprotonated state of [2]rotaxanes (R1-b and R2-b). In particular, TPE unit was introduced as a stopper at one end of the axle and aggregation was attained with different water fraction ( f w) of CH3CN/water cosolvent system. Notably, t-butylcalix[4]arene macrocycle could not only serve as a shuttling motion wheel but also as an excellent tool in controlling the aggregation and gelation behavior during the molecular motion. Similarly, one should note that urea unit serves not only as a second molecular station for tbutylcalix[4]arene or calix[4]arene macrocycle but also as a key component in gelation when the urea group is “revealed”. Initially, self-complexation of t-butylcalix[4]arene or calix[4]arene macrocycle S8 or S10 (Schemes S3 and S4, Supporting Information) with TPE secondary ammonium salt S4 (Scheme S1, Supporting Information) forms a pseudorotaxane in dry dichloromethane. Subsequently, triazole formation was proceeded by adding the urea functionalized azide group via copper-catalyzed azide−alkyne cycloaddition (CuAAC) reac-

hydrophilic stations of ammonium or urea of the axle. Moreover, the axle is functionalized with an AIE active TPE unit on one side of the arm; therefore, together they will form new types of dynamic amphiphilic [2]rotaxanes responding to local steric environment (see Scheme 1). Molecular switching between the two different stations of amphiphilic [2]rotaxanes R1 and R2 were well regulated by acid/base triggering strategy in CH3CN. Fluorescence enhancement of [2]rotaxanes R1 and R2 in CH3CN/water cosolvent system started to aggregate at 65% and 70% water fraction (f w), respectively. In contrast, 70% and 75% f w were required to initiate the aggregation of deprotonated [2]rotaxanes R1-b and R2-b, respectively. As a result, the distance between the TPE and the macrocycles, and with or without the t-butyl groups on the macrocycles could be important factors in controlling the radiative transitions of the TPE unit of the [2]rotaxanes in solution. Furthermore, several distinct nanostructures were obtained by the molecular shuttling motion of [2]rotaxanes in various water fractions of the CH3CN/water cosolvent system. An acid/base reversible organogel was successfully fabricated only from [2]rotaxane R1 in methanol solvent in contrast to all other [2]rotaxanes studied here. Therefore, the gelation behavior of [2]rotaxane R1 relies on the interplay of π−π stacking, intermolecular Hbonding interaction of “exposed” urea station, and van der Waals force interactions of t-butyl groups. Such self-assembly and dynamic nature of [2]rotaxane systems could be tunable for different potential applications such as drug delivery, optoelectronic devices, biological and biochemical areas.61,62 C


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Figure 2. 1H NMR spectra (400 MHz, CD3CN, 293 K) of [2]rotaxane R1 (a) in its primary state, (b) after the addition of one equiv of base and (c) after further addition of one equiv of TFA to the solution of panel b.

macrocycle and the ammonium station of the axle component T1 (Figure S2, Supporting Information). The formation of [2]rotaxanes R1 and R2 was further confirmed by high resolution mass spectrometry, which shows the signal at m/z 1877.935 and 1653.683 corresponding to respective [M − PF6−]+ (Figures S36 and S39, Supporting Information). Additionally, 2D-ROESY NMR spectra of [2]rotaxanes R1 and R2 revealed a strong through-space correlation between tbutylcalix[4]arene or calix[4]arene macrocycle and the ammonium station as depicted in Figures S48 and S49 (SI). Dynamic nature of [2]rotaxanes R1 and R2 in CD3CN was investigated using 1H NMR spectroscopy by determining the oscillation of the macrocycle between primary (“off” state of secondary ammonium) and secondary binding sites (“off” state of urea). When [2]rotaxane R1 was deprotonated with NaOH, significant chemical shift changes were observed (see Figure 2). Obviously, signals of the protons Ha and Hb adjacent to the ammonium station were upfield shifted by 0.26 ppm and two of the aryl protons (Hd) were downfield shifted by 0.28 ppm, and the methylene protons He were downfield shifted by 0.10 ppm owing to the deshielding effect of the t-butylcalix[4]arene macrocycle. The results indicated that in the presence of one equiv of base t-butylcalix[4]arene macrocycle moved from the ammonium station to the urea station. On the other hand, signal of the urea proton Hk was downfield shifted by 0.60 ppm, whereas signal of the other urea proton Hl disappeared completely, suggesting that H-bonding interactions occurred between t-butylcalix[4]arene crown-ether and the urea group. However, the chemical shift of triazole proton Hf remains unchanged after deprotonation of the ammonium ion by base, indicating that t-butylcalix[4]arene macrocycle does not locate on the triazole. Interestingly, small changes were observed for signals of the protons Hm, Hn and Ho adjacent to urea station and the peak intensities were decreased and broadened due to

tion to obtain [2]rotaxanes R1 and R2 both with 28% yield (Schemes S6 and S7, Supporting Information). Thereafter, deprotonation of the [2]rotaxanes R1 and R2 with aqueous NaOH solution (0.1 M) yielded the desired [2]rotaxanes R1-b and R2-b each in 50% yield (Schemes S8 and S9, Supporting Information). All the [2]rotaxane compounds were fully characterized by 1H, 13C, 2D TOCSY and ROESY NMR spectroscopy, FTIR, and high-resolution mass spectrometry. Investigation of Mechanically Interlocked and Molecular Shuttling Motion of [2]Rotaxanes R1 and R2 by 1 H NMR Spectroscopy. A comparison of the 1H NMR spectra of mechanically interlocked [2]rotaxanes R1 and R2 with spectra of axle component (T1, Scheme S5, Supporting Information) and t-butylcalix[4]arene or calix[4]arene macrocycle precursors (S8 and S10) are shown in Figures 1 and S1. Significantly, signals of adjacent protons Ha and Hb to the secondary ammonium ion were downfield shifted (Δδ = 0.41 ppm), which could be attributed to strong hydrogen bonding interactions between the ammonium group of axle component T1 and the crown-ether oxygen atoms of t-butylcalix[4]arene macrocycle. Meanwhile, the proton signal of Ha and Hb changed from two singlets to a multiplet because of the complexation of the secondary ammonium ion with the calixcrown ether. However, the signals of benzyl aromatic protons Hc and Hd were downfield shifted by 0.10 ppm and upfield shifted by 0.32 ppm, respectively, owing to the anisotropic effect of the tbutylcalix[4]arene macrocycle. The results indicated that the macrocycle resided on the secondary ammonium station in [2]rotaxane R1. Noticeably, signals of the methylene bridge protons H5 of the calix[4]arene were split into two doublets, which are downfield shifted. All protons of the [2]rotaxane R2 were clearly assigned and chemical shifts changes were observed due to complexation between the calix[4]arene D


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Figure 3. Fluorescence spectra and emission photographs of [2]rotaxanes R1 and R1-b in CH3CN/water cosolvent system with different water fractions (λex = 340 nm), (a and b) fluorescence spectra of [2]rotaxanes R1 and R1-b (10 μM), respectively. Photos in panels c and d are the fluorescence emission of [2]rotaxanes R1 and R1-b at different water fraction f w (taken when irradiated at 365 nm).

Figure 4. Fluorescence spectra and emission photographs of [2]rotaxanes R2 and R2-b in CH3CN/water cosolvent with different water fractions (λex = 340 nm), (a and b) fluorescence spectra of [2]rotaxanes R2 and R2-b (all at 10 μM), respectively. Photos in panels c and d are the fluorescence emission of [2]rotaxanes R2 and R2-b at different water fraction f w (taken by UV lamp irradiation at 365 nm).

First, we investigated the AIE behavior of axle component T1 with an incremental addition of water and we found that it started to aggregate and fluorescence emission enhanced when water fraction (f w) reached 75%. The fluorescence intensity of T1 reached a maximum value at 99% f w (Figure S3, Supporting Information). The results imply that the rotation of the phenyl rings of TPE was severely hindered at high water fraction rendering the system with high fluorescence intensity. The UV−vis absorption spectra of all [2]rotaxanes (namely R1, R2, R1-b and R2-b) were also examined in different water fraction. As expected, hyperchromic effect of all absorption bands of these [2]rotaxanes was observed at higher water fractions (see Figures S4 and S5, Supporting Information). Next, we examined the AIE behavior of [2]rotaxanes R1 and the deprotonated state R1-b in CH3CN/water cosolvent system (10 μM). As shown in Figure 3, [2]rotaxanes R1 and R1-b were nonfluorescent below 65% and 70% f w, respectively.

less tight complexation of the secondary station. Upon further protonation of solution b with TFA (Figure 2c), all the chemical shifts were restored to the original spectrum, which indicated that t-butylcalix[4]arene macrocycle has shuttled back to reside over the ammonium station. Similar acid/base induced dynamic change of [2]rotaxane R2 was observed by 1H NMR spectroscopy (see Figure S2, Supporting Information). 1H NMR spectroscopy directly showed the positions of where t-butylcalix[4]arene or calix[4]arene macrocycle located between the two different stations of the [2]rotaxanes R1 and R2 by the external chemical stimuli (acid/base). Investigation of Aggregation Induced Emission Behavior. To better understand the AIE behavior of axle T1, [2]rotaxanes R1 and R2, and their deprotonated states R1b and R2-b, their fluorescence spectra in various water fraction in CH3CN/water cosolvent system (10 μM) were studied. E


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Figure 5. Optimized chemical structures at the B3LYP/6-31G level of [2]rotaxane R1 at different states: (a) at the ammonium state and (b) the deprotonated state [2]rotaxane R1-b complex.

tigation of fluorescence emissions of axle T1, [2]rotaxanes R1, R2, R1-b and R2-b (10 μM) in CH3CN/water cosolvent system showed that they started to rise at 75%, 65%, 70%, 70% and 75% f w, respectively. Consequently, we could see a slight variation in the water fractions to initiate the fluorescence enhancement for all compounds. However, secondary ammonium salt and urea station worked as a hydrophilic part, which could be easily dissolved in higher water fraction for axle T1, and [2]rotaxanes R1 and R2.65 These potential hydrophilic moieties when complexed with the macrocycle wheels may start to hinder the intramolecular rotation process at lower water fraction through shorter distance between the TPE unit and the calix[4]arene macrocycles. On the contrary, when the secondary ammonium station was deprotonated, the urea station (hydrophilic) was blocked by the t-butylcalix[4]arene or calix[4]arene macrocycle, thus allowing to initiate the aggregation at 70%, and 75% f w for [2]rotaxanes R1-b and R2b, respectively. On the basis of these fluorescence spectroscopy studies, we found that not only the distance between the TPE unit and calix[4]arene macrocycles but also the availability of the urea station had remarkable influence on the aggregation processes of these [2]rotaxanes. The fluorescence intensity of [2]rotaxanes R1 and R2 decreased with a further increase of water fraction to >90% f w in contrast to those of axle T1, [2]rotaxanes R1-b and R2-b, which showed continuous increase of fluorescence until 99% f w. Such a difference is predicted to derive from the large amount of nano aggregates and self-assembled structures in the solution, which lead to the suppression of the emissive nature of AIEgens owing to strong interaction between TPE and macrocycle.66 The aggregation induced fluorescence of [2]rotaxane R1 was achieved with lower water fraction compared to that of [2]rotaxane R2, because the nonradiative decay channels were blocked and the radiative transitions were promoted with the help of interactions between t-butylcalix[4]arene macrocycle and TPE. The results indicate that the AIE behavior could be adjusted by triggering the molecular shuttling motion of the macrocycle component via distance dependent AIE effect. Theoretical Study. The shuttling motion and AIE effect of [2]rotaxanes R1 and R2 and their deprotonated sates R1-b and R2-b were further confirmed by density functional theory

The results imply that the free rotation of the phenyl rings of TPE is still active below 65% and 70% f w.63 However, the [2]rotaxanes R1 and R1-b started to aggregate and fluorescence emission bands (λmax of 470−477 nm) were observed when the water fraction admixed with CH3CN reached 65% and 70% f w, respectively. Moreover, sequential increase of water fraction to 90−99% f w led to the increase of fluorescence intensities of [2]rotaxanes R1 and R1-b and their emission bands were slightly blue-shifted to λmax of 463−470 nm, respectively. In the aggregation states, intramolecular rotation of the phenyl rings of TPE was physically inhibited thus making it highly fluorescent via radiative decay pathways.64 These observations revealed that different water fraction (f w) was required to initiate the AIE effect of [2]rotaxanes R1 and R1-b. Compared to other [2]rotaxanes, [2]rotaxane R1 required a less amount of water (65% f w) to initiate the aggregation of TPE because t-butylcalix[4]arene macrocycle was located on the ammonium station and was very close to the TPE unit. Once aggregation started, the phenyl groups of TPE unit became highly hindered for rotation and strong interactions between the t-butylcalix[4]arene macrocycle and the TPE unit would be functioning. The distance between TPE and the calixarene macrocycle and the bulkiness of t-butylcalix[4]arene macrocycle play important roles in restricting the intramolecular rotation of phenyl groups, which subsequently enhanced the radiative transitions of the TPE unit in solution. In contrast, TPE unit of the deprotonated state of [2]rotaxane R1-b started to aggregate at a slightly higher water fraction (70% f w) because the t-butylcalix[4]arene macrocycle was resided on the urea station and was quite far away from the TPE unit. As a result, a larger amount of water was required for [2]rotaxane R1-b to initiate the aggregation compared to R1 and the aggregation restricted the intramolecular rotation of TPE unit in the solution, thus enhanced the fluorescence. For comparison, we also studied the AIE behavior of [2]rotaxanes R2 and R2-b by varying the water fraction in the CH3CN/water cosolvent system (Figure 4). The results showed that about 70% f w was required to start the fluorescence enhancement of [2]rotaxane R2, whereas about 75% f w was needed to initiate the fluorescence enhancement of the deprotonated state R2-b. Evidently, comparative invesF


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Figure 6. Band gaps of the frontier molecular orbitals of [2]rotaxanes R1 and R1-b at the B3LYP/6-31G level.

cycles prefer to encircle on the urea station via hydrogen bond formation. The bond lengths of hydrogen bonding of N−H··· O in [2]rotaxanes R1-b and R2-b were calculated to be 2.23/ 1.89 Å and 2.28/1.89 Å, respectively (Figures 5b and S6b). To understand the nature of the charge transfer of the electronic transition changes within the molecules, we also calculated energies of the highest occupied molecular orbital (HOMO) and lowest unoccupied molecular orbital (LUMO) along with distribution of frontier molecular orbitals at the B3LYP/6-31G level. The electron clouds of [2]rotaxanes R1 and R1-b are partially localized over the phenyl rings of calix[4]arene at HOMO level. However, the electron clouds of LUMO are fully localized on the TPE units (Figure 6). The band gap between the HOMO and LUMO energy levels of [2]rotaxanes R1 and R1-b was calculated to be 3.718 and 3.906 eV, respectively. The results suggested that the interactions between the TPE and the calix[4]arene macrocycle were stronger in [2]rotaxane R1 compared to that in [2]rotaxane R1-b. Similar results in the energies of HOMOs and LUMOs as well as the distribution of their electron clouds could be found in [2]rotaxanes R2 and R2-b (Figure S7). The band gaps between the HOMO and LUMO energy levels of [2]rotaxanes R2 and R2-b were found to be 3.899 and 4.073

(DFT) calculations. Geometric optimization using DFTB3LYP67,68 with 6-31G basis set was carried out for all the molecules. All calculations of electronic structures were performed with the Gaussian 09 software.69 The geometries of axle T1 and all [2]rotaxanes R1, R2, R1-b and R2-b were fully optimized as shown in Figures 5 and S6. The binding energies of [2]rotaxanes R1 and R2 were calculated to be 51.3 and 50.3 kcal/mol, respectively. Whereas, the binding energies of [2]rotaxanes R1-b and R2-b were found to be 30.1 and 30.0 kcal/mol, respectively. Therefore, thess observations revealed that secondary ammonium is a better binding site for tbutylcalix[4]arene or calix[4]arene macrocycle than the urea station. In addition, the bond lengths of all hydrogen bonding were estimated between the “O” atom of calixarene macrocycles and N−H of the secondary ammonium station or N−H of the urea station. In the protonated state, the secondary ammonium (R2NH2+) is the better station for t-butyl-calix[4]arene or calix[4]arene macrocycle. That is, in the urea “on” [2]rotaxanes (R1 and R2), the bond lengths of hydrogen bonding of N−H···O were estimated to be (2.07/1.83 Å, and 2.04/1.83 Å, respectively (Figures 5a and S6a). Whereas in the urea “off” state of [2]rotaxanes R1-b and R2-b, calix[4]arene macroG


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Figure 7. FE-SEM images of [2]rotaxane R1 (10 μM): (a) in CH3CN only, (b) at 65% f w of CH3CN/water cosolvent and (c) at 90% f w of CH3CN/water cosolvent. Scale bar was 1 μm for panels a, b and c.

eV, respectively. The results also indicated that the interactions between the TPE and the calix[4]arene macrocycles were stronger in R2 compared to that in [2]rotaxane R2-b. On the basis of theoretical results, the HOMO−LUMO band gap of [2]rotaxane R1 was lower than that of [2]rotaxane R2 owing to the stronger interactions of calix[4]arene macrocycle and TPE in the former. Thus, theoretical calculations provide good rationale for the results in fluorescence spectroscopy. Self-Assembled Morphological Transformations Controlled by Molecular Shuttling. To better understand the morphological changes of these [2]rotaxanes and their size distribution in the CH3CN/water cosolvent, we also studied the scanning electron microscopy (SEM) and dynamic light scattering (DLS) of a series of [2]rotaxanes at different concentrations. In [2]rotaxanes R1 and R2 (urea “on”), tbutylcalix[4]arene or calix[4]arene macrocycle mainly resides on the secondary ammonium station, thus, both the hydrophobic groups (TPE and calixarene macrocycle) are located very closely (see Figure 6). Typical microspheres were formed for [2]rotaxanes R1 and R2 in pure CH3CN (Figures 7a and S8a); however, nanospheres were dominated when the water fraction of CH3CN/water cosolvent reached 65% and 70% f w, respectively (Figures 7b and S8b). After the water fraction reached 90% f w, maximum fluorescence intensity and nanospheres were formed completely with increased degree of aggregation in the solution as shown in Figures 7c and S8c. DLS studies on the nanoparticles of [2]rotaxanes R1 and R2 showed that the average size of the microspheres remarkably decreased into nanospheres as the water fraction increased in the CH3CN/water cosolvent system (Figures S9 and S10). In the deprotonated [2]rotaxanes R1-b and R2-b, the tbutylcalix[4]arene or calix[4]arene macrocycle preferred to move toward the urea station (urea “off”). The self-assembly of [2]rotaxanes R1-b and R2-b into microspheres were found in pure CH3CN solvent (Figure S11a,b). However, after increasing the water fraction to 70% and 75% for [2]rotaxanes R1-b and R2-b, respectively, significant changes in morphology were observed along with few nanospheres started to show hollow structure on their surface and became hollow nanospheres with an average diameter of 120 and 220 nm, respectively (Figures 8, S13 and S14). Further increase the water fraction to 99% in the CH3CN/water cosolvent system, [2]rotaxanes R1-b and R2-b almost completely aggregated to form nanospheres as displayed in Figures S12a,b. The size distribution of self-assembled nanosphere structures for all [2]rotaxanes were further monitored by DLS method as shown in Figures S13 and S14. These results are in good agreement with the above-mentioned aggregation induced fluorescence properties as well as morphological transformations. In general, hollow spheres are originated from amphiphilic and solvophobic interactions of the gelators or polymers.70 In

Figure 8. FE-SEM images of [2]rotaxanes R1-b and R2-b in CH3CN/water (10 μM) cosolvent derived hollow nanospheres: (a) 70% f w and (b) 75% f w, respectively. Panels c and d are partially expanded plots of panels a and b, respectively. Inset in panel a shows a magnified portion of the hollow nanospheres. Inset in panel c shows the diameter of a hollow nanosphere. Scale bar was 1 μm for panels a and b, while it was 100 nm for panels c and d.

our case, different aggregation state of amphiphilic [2]rotaxanes R1, R2, R1-b and R2-b and their hydrophobic and hydrophilic unit in CH3CN/water cosolvent renders a welldefined dimensions and morphology of aggregated structures. Nanospheres were formed at different water fraction of [2]rotaxanes R1 and R2 in CH3CN/water cosolvent in contrast to the hollow nanospheres observed in [2]rotaxanes R1-b and R2-b. To our delight, the “exposed” state of urea, the strong RIR effect of TPE in [2]rotaxanes R1 and R2, and the shorter distance between the macrocycles and the AIE active TPE unit, allowed them to self-assemble into nanospheres. The rotaxane units are arranged exteriorly via C−H π interactions and π−π stacking of the TPE unit, whereas the alkyl chains are adapted interiorly owing to their hydrophobic nature. Furthermore, intermolecular hydrogen bonding interactions between the urea groups (N−H···O) allowed them to form nanospheres.71 On the contrary, the hallow nanospheres were originated from the loose packing of [2]rotaxanes R1-b and R2-b respectively at 70% and 75% water fraction of CH3CN/ water cosolvent system, owing to the forbidden of intermolecular hydrogen bonding interactions between the neighboring urea groups (N−H···O) and the minimized RIR effect by increasing the distance between the TPE and macrocycles.72 It is prominent to note that controllable morphological changes of [2]rotaxanes R1-b and R2-b could H


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Upon adding acid into the solution, the organogel was restored again. Certainly, the reversible manner of the organogel and solution was achieved by the addition of acid and/or base. To provide further information regarding the role of hydrogen bonding played in the gelation process of [2]rotaxane R1, FTIR spectroscopy was studied.73 As shown in Figure S15 (SI), the characteristic bands of [2]rotaxane R1 in CH3CN solution (free state) were observed at 3385/2932/ 1700 cm−1 corresponding to OH/NH stretching, CH stretching, and CO vibration, respectively. It should be noted that after gelation of [2]rotaxane R1, the broad OH/ NH stretching vibrational band shifted from 3385 to 3445 cm−1. Moreover, the characteristic C−H stretching band of the t-butyl group became a very broad peak under gelation conditions. The carbonyl (CO) peak was broadened and shifted to 1735 cm−1. The results indicated that various noncovalent interactions including intermolecular H-bonding and van der Waals interactions coexisted in the gelation networks. SEM images of the controlled self-assembly structures of [2]rotaxane R1 in dilution and gelation state were studied. Spherical structures were formed from the gelation networks at low concentration of R1 (1 × 10−3 M in MeOH) as shown in Figure 12a. Interestingly, a drastic change in the self-assembled structures were observed, where the spherical structures started to self-assembled into dumbbell shape 3D cross-linked network as the concentration of [2]rotaxane R1 increased (5 × 10−3 M in MeOH) as displayed in Figure 12b,c. Therefore, the gelation behavior of [2]rotaxane R1 could be promoted via multi-selfassembly driving forces and steric immobilization by the MeOH, which worked together to form dumbbell shape crosslinked 3D network structures. The gelation ability of [2]rotaxane R1 could be driven by the “on” state of the urea station, which mainly contributed to intermolecular hydrogen bonding interactions, moreover, it was also driven by the van der Waals force interaction between the t-butylcalix[4]arenes that brought two calix[4]arene molecules together. In contrast, the nongelative behavior of [2]rotaxanes R2, R1-b and R2-b could be due to the lack of either the van der Waals force interactions of the t-butylcalix[4]arenes or the intermolecular H-bonding interactions of the urea groups, indicating that both the t-butylcalix[4]arene and the free urea station are important factors for gelation to occur in these [2]rotaxanes. On the basis of all evidence, we presumed that the urea group of [2]rotaxane R1 allowed intermolecular H-bonding interactions to form between two neighboring urea units and prone to self-assemble into bimolecular-layered structure in tail-to-tail fashion, whereas the TPE unit of [2]rotaxane R1 were stacked via C−H···π and π−π interactions, and the upper-rim t-butyl groups are mainly involved in van der Waals forces interactions to form dumbbell and shape into 3D gelation networks as sown in Figure 13.

be achieved at 99% of water fraction in CH3CN/water cosolvent, in which no hollow nanosphere could be found and only nanospheres were observed due to the increased degree of aggregation in the aqueous solution. Hollow spheres through the nanoself-assembly of [2]rotaxanes R1-b and R2-b (urea station “off”) were attained only at 70% and 75% f w, respectively. Therefore, morphological transformations could be driven by the molecular shuttling motion as well as different water fractions (f w) of CH3CN/water cosolvent system. On the basis of these observations, hollow nanospheres were obtainable under two conditions: (1) when the degree of aggregation in solution was modest and (2) the urea group was occupied to minimize the intermolecular H-bonding interaction between the neighboring urea groups. Finally, two possible mechanisms for the self-assembly of these [2]rotaxanes into nanospheres and hollow nanospheres are proposed in Figures 9 and 10.28

Figure 9. Graphical representation of plausible hierarchies of the supramolecular self-assembly of [2]rotaxanes R1 and R2 into nanospheres driven by aggregation processes in aqueous medium.

Figure 10. Graphical representation of plausible hierarchies of the supramolecular self-assembly of [2]rotaxanes R1-b and R2-b into hollow nanospheres driven by aggregation process in water fraction (a) 70% and (b) 75%, respectively, in the CH3CN/water cosolvent system.

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CONCLUSIONS A rare example of reversible molecular switching amphiphilic [2]rotaxanes R1 and R2 composed of an axle with two distinguishable recognition stations and one side of the arm terminated with AIE active TPE unit and a mechanically bonded t-butylcalix[4]arene or calix[4]-arene macrocycles as the wheel were successfully synthesized. The aggregation behavior with tunable morphologies could be adjusted by triggering the molecular shuttling motion of the macrocycle component via distance dependent AIE effect. Different water

Gelation Transitions Controlled by Molecular Shuttling. The gelation behavior of amphiphilic [2]rotaxanes R1, R2, R1-b and R2-b were investigated in a range of solvents (Table S1, Supporting Information). Among them, only [2]rotaxane R1 formed organogels in MeOH with a minimum gelation concentration (MGC) of 2.5 w/v% (Figure 11). Besides sonication and heating/cooling treatment of the solutions, mechanical agitation significantly activated the gelation process. Adding base into the organogel system, the organogel slowly collapsed and eventually became solution. I


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Figure 11. Reversible supramolecular gel formation and degradation in MeOH driven by acid/base external stimuli.

Figure 12. FT-SEM images of the gel formation of [2]rotaxane R1 in MeOH at different concentrations (a) 1 × 10−3 M, (b) 5 × 10−3 M and (c) magnified image of panel b. Scale bar was 1 μm for panels a, b and c.

Figure 13. Chemical structures and schematic representations of plausible dumbbell shape cross-linked supramolecular network structures of [2]rotaxane R1 during the gelation process via multi-self-assembly driving forces.

organogel in MeOH solvent with a MGC of 2.5 w/v%. The urea group can serve as a dual-functional station: (1) served as a secondary molecular station for macrocycle under deprotonated state and (2) involved in intermolecular H-bonding interaction in CH3CN/water cosolvent and MeOH when the urea station is “revealed”. Importantly, the formation of nanospheres, hollow nanospheres, and organogels could be driven by the multiself-assembly forces such as intermolecular H-bonding, C−H···π and π−π interactions, hydrophobic interactions and van der Waals forces. We postulate that the development of AIE behavior amphiphilic [2]rotaxanes and

fraction (f w) was required to start the aggregation induced fluorescence enhancement for all [2]rotaxanes. Note that the disparity of aggregation process mainly induced by virtue of hydrophobic nature of upper-rim t-butyl groups and the location of the t-butylcalix[4]arene or calix[4]arene macrocycles on the molecular station (either “ammonium” or “urea”). Interestingly, hollow nanospheres were obtained from the [2]rotaxanes R1-b (f w = 70%) and R2-b (f w = 75%); however, nanospheres were formed for [2]rotaxanes R1 and R2 in the CH3CN/water cosolvent system. Among the four [2]rotaxanes studied in this work, only R1 formed J


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mechanically driven organogels are urgently desired for the implementation of molecular machines in the fields of biological, biomedical and material science.

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.chemmater.8b03286.

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Experimental section, synthesis overview and procedures; 1H, 13C NMR, mass, and IR spectra of axle T1, [2]rotaxanes R1, R2, R1-b and R2-b; 2D-TOCSY and ROESY spectra of [2]rotaxanes R1 and R2; fluorescence spectra of axle T1 and UV−vis spectra of [2]rotaxanes R1, R2, R1-b and R2-b in different water fractions (f w); optimized molecular structures of [2]rotaxanes R2 and R2-b; SEM images of all [2]rotaxanes in various conditions (PDF)

AUTHOR INFORMATION

Corresponding Author

*Prof. Wen-Sheng Chung. Tel: +886-3-5131517. Fax: +886-35723764. E-mail: [email protected]. ORCID

Ming-Chang Lin: 0000-0003-3963-6017 Wen-Sheng Chung: 0000-0003-0134-9725 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS We thank the Ministry of Science and Technology, Taiwan, ROC for providing us following four research projects: MOST107-2113-M-009-014, MOST-107-2113-M-009-019, MOST106-2113-M-009-003 and MOST-106-2811-M-009-062.

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Distinct Nanostructures and Organogel Driven by Reversible

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