A Binary Supramolecular Assembly with Intense Fluorescence Emission, High pH Stability, and Cation Selectivity: Supramolecular Assembly-Induced Emission Materials.

We construct a fluorescent supramolecular system (TPE-Q4 ⊂ DSP5) of excellent tolerance to a wide range of pH by the facile self-assembly of a new pillar[5]arene bearing disulfonated arms (DSP5) with an AIE-active tetraphenylethene-based tetratopic guest bearing four quaternary ammonium binding sites (TPE-Q4), which exhibits strong blue emission even in dilute aqueous solutions along with much higher quantum yield and longer fluorescence lifetime than TPE-Q4 itself. This appreciable property can be attributed to the supramolecular assembly-induced emission (SAIE) mechanism endowed by the host-guest inclusion complexation based on synthetic macrocycles. Remarkably, the enhanced fluorescence of the supramolecular assembly is quenched efficiently and exclusively by ferric ions in water with a high Stern-Volmer formula constant of 1.3 × 105  mol-1, demonstrating the excellent cation selectivity and visualized responsiveness in ion sensing and detection.

1. Introduction

Fluorescent supramolecular assemblies represent an extraordinarily attractive class of functional systems, due to their integrated functionalities, highly tunable fluorescence properties, and good responsiveness towards external stimuli [1]. The inherent features of their fluorescent entities that can be further mediated by the rational design of supramolecular building blocks endow them extensive potentials in smart materials and light-emitting devices [2]. In 2001, Tang and coworkers discovered some propeller-like molecules with π-conjugated structures that exhibited weak fluorescence in solution but bright emission in aggregated states in sharp contrast to traditional fluorescent molecules of aggregation caused quenching (ACQ) and creatively named this phenomenon aggregation-induced emission (AIE) [3]. Many fluorophores with AIE properties (AIEgens) have been discovered and extensively investigated for applications in chemistry and materials science [4, 5]. The mechanism of fluorescent emission enhancement of AIEgens in the aggregated states mainly lies in the restriction of intramolecular motion (RIM) and the blockade of nonradiation energy dissipation channel from the excited state to ground state [4, 5].

Inspired by the mechanism of AIE, researchers have successfully developed a supramolecular pathway, namely, supramolecular assembly-induced emission (SAIE), to achieve effective emission enhancement of AIEgens in the ordered assembly states no matter in dilute solutions or aggregated solid states [2, 6–19]. In 2014, Tang and coworkers covalently linked a cyclodextrin with tetraphenylethene (TPE) and utilized self-inclusion complexation for RIM to achieve enhanced emission in aqueous solutions, which represented the prototype SAIE study and paved the way of recruiting supramolecular macrocycles to manipulate the fluorescent properties of AIEgens [6]. Considerable effort has been made in the rapid development of SAIE recently, accompanied by the synthesis of functional supramolecular assemblies with tunable emission [7-19]. Various types of supramolecular fluorescent assemblies based on AIEgens and functional synthetic macrocycles have been reported for applications in cell imaging, [11] fluorescent supramolecular polymers and gels [7–10, 13, 14], room-temperature phosphorescence [15], controlled drug delivery [16], and other stimuli-responsive materials [17-19]. But nonetheless, challenges in their preparation and application remain.

On the other hand, pillarenes, as an important class of macrocyclic hosts in the realm of supramolecular chemistry and advanced functional materials, are attracting more and more attention due to the unique pillar-like rigid structure, electron-rich cavity, well-studied host-guest properties, and facile modification methodology [20-22]. Functionalized pillarenes have played important roles in ion detection [23], antibacterial therapeutics [24], supramolecular nanovalves [25-29], and so on [30]. Significantly, by elaborate functionalization, pillarenes have been utilized as an important candidate of synthetic macrocycles in the construction of stimuli-responsive fluorescent materials through the marriage with AIEgens [2].

Herein, we report on the fabrication of a pH-tolerant fluorescent system for specific cation recognition based on supramolecular self-assembly of disulfonated pillar[5]arene (DSP5) and a water-soluble TPE-based tetratopic guest (TPE-Q4) with four quaternary ammonium binding sites (Scheme 1). Because of the high binding affinity of DSP5 towards TPE-Q4, a supramolecular assembly (TPE-Q4⊂ DSP5) was fabricated in aqueous solution via host-guest interaction, exhibiting bright blue emission in a wide range of pH conditions. Meanwhile, this supramolecular assembly shows a specific response of fluorescence quenching to Fe3+ through the coordination between the sulfonate arms of DSP5 and Fe3+ ions, endowing the material with smart cation responsiveness.

Scheme 1

Chemical structures and cartoons of DSP5 and TPE-Q4 and the schematic presentation of their self-assembly into a fluorescent supramolecular system for the selective detection of Fe3+ ions.

2. Results

2.1. Host–Guest Complexation

DSP5 was successfully designed and synthesized for the first time via a Williamson ether-type synthetic approach (). To investigate the host-guest property of DSP5 toward TPE-Q4, the following experiments have been conducted. A TPE-based monotopic guest (TPE-Q) with single quaternary ammonium binding site was chosen as a model guest for the study of the host-guest binding between DSP5 and the guest linker moiety on TPE in deionized water at 298 K via fluorescence titration (). The stoichiometric ratio of DSP5 and the monotopic model guest TPE-Q was determined to be 1:1 and their association constant (K) was calculated to be (2.68 ± 0.24) × 105 M−1. The 2D NOESY spectrum (Figures and ) of the 4:1 mixture of DSP5 (4.0 mM) and TPE-Q4 (1.0 mM) in D2O showed obvious correlations between the phenyl protons Ha of DSP5 and the alkyl chains protons H1-H4 and methyl protons H9 of TPE-Q4, indicating the existence of strong host–guest interactions between DSP5 and TPE-Q4 and the inclusion of alkyl chains of TPE-Q4 into the cavities of DSP5 macrocycles. Furthermore, DOSY experiments were employed to investigate the formation of TPE-Q4⊂ DSP5 in water (see ). The measured weight-average diffusion coefficient (D) of TPE-Q4 decreased from 9.66 × 10−10 to 8.85 × 10−10 m2/s with the addition of 4 equiv. of DSP5, suggesting the formation of supramolecular assemblies. Experimental evidence given above has demonstrated the successful construction of the 1:4 supramolecular inclusion complex of TPE-Q4⊂ DSP5.

2.2. Fluorescent Properties and Morphology

Fluorescent changes of TPE-Q4 (1 μM) in water in the presence of different concentrations of DSP5 were monitored by fluorescence spectroscopy (Figure 1). The emission of TPE-Q4 solution is negligible; however upon addition of DSP5, the whole system exhibited green emission color and the fluorescence intensity dramatically increased, reaching the maxima when 4 eq. of DSP5 was added ([DSP5]/[TPE-Q4] = 4:1) (Figure 1(a)). The rational explanation is that the intramolecular rotation of phenyl rings of TPE-Q4 was restricted by DSP5 upon host-guest inclusion complexation and supramolecular assembly (SAIE). The maximum fluorescence enhancement at the host-guest molar ratio of 4:1 indicated a complete formation of TPE-Q4⊂ DSP5; thus this ratio was chosen for the following studies. Additional supporting data including UV-Vis absorption spectra have also been obtained (). It is worth mentioning that negligible changes (Figure 1(b)) could be observed in the fluorescence intensity of TPE-Q4⊂ DSP5 over a wide range of pH (3-13), proving its great pH tolerance and system robustness.

Figure 1

Fluorescence characterizations of TPE-Q4 and TPE-Q4⊂ DSP5. (a) Fluorescence spectra of TPE-Q4 (1 μM) in water with different concentrations of DSP5 (inset images: (A) DSP5; (B) TPE-Q4⊂ DSP5; (C) TPE-Q4 in water); (b) fluorescence spectra of TPE-Q4⊂ DSP5 in PBS of different pH values. Experimental conditions: λex = 330 nm; λem = 475 nm; slit widths: Ex. 5 nm, Em. 5 nm; 298 K; [TPE-Q4] = 1 μM, [DSP5] = 4 μM. (c) Fluorescence decay profiles and (d) absolute fluorescence quantum yields of TPE-Q4 and TPE-Q4⊂ DSP5. Experimental conditions: λex = 365 nm, 298 K, [TPE-Q4] = 1 μM, [DSP5] = 4 μM in water.

To further investigate the emissive characteristics of TPE-Q4⊂ DSP5, time-resolved fluorescence data of TPE-Q4 and TPE-Q4⊂ DSP5 were collected (Figures 1(c) and 1(d), ). The absolute fluorescence quantum yield of TPE-Q4⊂ DSP5 was 54.40%, which is much higher than that of bare TPE-Q4 (0.68%). The photoluminescence decay profiles of TPE-Q4 and TPE-Q4⊂ DSP5 suggested that the fluorescence lifetime of TPE-Q4 itself is 1.14 ns, while the excited state of TPE-Q4⊂ DSP5 relaxed mainly through a slow pathway with a lifetime of 5.64 ns. Interestingly, TPE-Q4⊂ DSP5 showed one order of magnitude smaller nonradiative decay rate constant (8.08 × 107 s−1) and ca. 16 times bigger radiative decay rate constant (9.64 × 107 s−1) as compared to bare TPE-Q4 (8.68 × 108 s−1 and 5.90 × 106 s−1, respectively) (). These experimental results further support the mechanism of SAIE in the system that the intramolecular motions of the TPE-Q4 units are restricted by DSP5 macrocycles, which block the nonradiative relaxation pathway and populated excitons that underwent radiative decay. The above results evidenced that TPE-Q4⊂ DSP5 can emit strong fluorescence even in a dilute solution and DSP5 synthetic macrocycle plays a crucial role in the fluorescence enhancement of this supramolecular assembled system.

Based on the aforementioned knowledge, we deduced that the amphiphilic nature of TPE-Q4 would lead to its self-assembly in aqueous solution, while the ensembles might undergo observable alteration by the host-guest complexation upon addition of DSP5. To obtain direct evidence, the size and morphology of the supramolecular assemblies were studied by scanning electron microscope (SEM) and dynamic light scattering (DLS). Interestingly, cube-shaped assemblies of TPE-Q4 with a diameter of ~300 nm and spherical micelles of DSP5 with a diameter of ~20 nm were observed, respectively (Figures 2(a1), 2(a2), 2(b1), and 2(b2)). However, as we envisioned, TPE-Q4⊂ DSP5 exhibited bulky cube-like morphology, yet it possessed a larger diameter of ~500 nm as compared with the nanostructures formed by TPE-Q4 or DSP5 alone (Figures 2(c1) and 2(c2)). The formation of supramolecular host-guest complex between DSP5 and TPE-Q4 affected the overall amphiphilic nature including the size of building blocks (TPE-Q4⊂ DSP5 versus TPE-Q4 or DSP5) and introduced the inter-building block electrostatic repulsions, simultaneously contributing to the increased size of the assembled architectures.

Figure 2

Morphology and size characterizations. SEM images and DLS measurements of (a1) and (a2) TPE-Q4, (b1) and (b2) DSP5, (c1) and (c2) TPE-Q4⊂ DSP5, and (d1) and (d2) TPE-Q4⊂ DSP5 with the addition of Fe3+ ([TPE-Q4] = 1 μM, [DSP5] = 4 μM).

2.3. Selective Detection of Fe Ion

Inspired by the above advances, fluorescence experiments were conducted to investigate the responsiveness of TPE-Q4⊂ DSP5 toward different analytes such as ions. Fluorescence quenching degrees and the Stern–Volmer formula constants of TPE-Q4⊂ DSP5 upon addition of different cations and anions at the same concentration were provided (Figures 3(a) and 3(b)), with Cl− and Na+ as the counterions, respectively. Surprisingly, only Fe3+ ions caused a remarkable fluorescence quenching (1 - I/I0 = 0.84) with a considerably high Stern–Volmer formula constant (K = 2.7 × 105 mol−1). To further confirm this ion sensing specificity, experiments on competitive cations were performed and the results showed that fluorescence quenching degrees are as high (~1 - I/I0 > 0.75) with or without competition cations (). The limit of detection (LOD) of TPE-Q4⊂ DSP5 to Fe3+ was calculated to be 8.6 × 10−7 M on the basis of 3δ/S, indicating that the material possesses a highly sensitive responsiveness to Fe3+ (). Considering that the material exhibits enhanced fluorescence with good pH stability, the detection of Fe3+ by TPE-Q4⊂ DSP5 was further testified under varied pH circumstances, and efficient fluorescence quenching was observed within the wide pH range in the presence of Fe3+ (), which denoted that the material can be applied for the selective detection of Fe3+ in aqueous conditions without the interference of pH levels.

Figure 3

Highly selective detection of Fe3+ by TPE-Q4⊂ DSP5. (a) Fluorescence quenching degrees of TPE-Q4⊂ DSP5 in the presence of different cations and anions in water. (b) The Stern–Volmer formula constants of TPE-Q4⊂ DSP5 upon interacting with different cations and anions in water. (c) The pictures of (A) TPE-Q4, (B) TPE-Q4⊂ DSP5, (C) TPE-Q4⊂ DSP5 + Fe3+, and (D) DSP5 + Fe3+ in water. Experimental conditions: λex = 330 nm; λem = 475 nm; slit widths: Ex. 5 nm, Em. 5 nm; 298 K; [TPE-Q4] = 1 μM, [DSP5] = 4 μM, [Fe3+] = 20 μM, [Ions] = 20 μM.

2.4. Investigation of the Quenching Mechanism

To further explore the quenching mechanism, a series of measurements were carried out. SEM image of TPE-Q4⊂ DSP5 in the presence of Fe3+ showed irregular morphology, which was different from the assembly structure of TPE-Q4⊂ DSP5 itself (Figures 2(d1) and 2(d2)); thus we consider that a complexation between DSP5 and Fe3+ occurred. In order to exclude the influence of the electron-rich cavity of DSP5, monomer molecule (DSNa) with sulfonate groups was synthesized. SEM study demonstrated that Fe3+ could coordinate with the sulfonate groups of DSNa (). Moreover, 1H NMR experiments were carried out to confirm the coordination of DSP5 and Fe3+ (). Because TPE-Q4⊂ DSP5 precipitated from D2O in the NMR tube, no peaks could be observed in the 1H NMR spectrum. However, the proton peaks of TPE-Q4 reappeared after the addition of Fe3+ but the proton peaks of DSP5 were still absent in the spectrum, suggesting that DSP5 coordinated with Fe3+, forming water-insoluble coordination polymers. Fluorescent images were also obtained to further confirm the existence of TPE-Q4 in the aqueous media, where the fluorescence recovered when DSP5 was added after the precipitation ().

The ion-responsive property of TPE-Q4⊂ DSP5 and the interactions of DSP5 and Fe3+ were described in a more intuitive way in Figure 3(c). The TPE-Q4⊂ DSP5 water solution (B) was clear and emitted intensely under 365 nm irradiation, yet precipitation and quenching occurred after adding Fe3+ (C) at a higher concentration. After centrifugation, the precipitates were separated from the solutions (). However, A and B solutions still showed Tyndall effect but C and D solutions did not, indicating larger supramolecular assemblies in C and D than in A and B solutions. Importantly, the precipitation in D further confirmed that DSP5 interacted with Fe3+, leaving TPE-Q4 free in water, where the fluorescence was greatly quenched.

To prove the coordination effect between sulfonate groups of DSP5 and Fe3+, we used X-ray photoelectron spectroscopy (XPS) to record the valence state changes of DSP5 before and after the addition of Fe3+ (Figures 4(a)–4(d)). The XPS spectra demonstrated that the binding energies of the C1s peaks at 284.7 eV, 290.0 eV, and 286.2 eV underwent no obvious changes. But the binding energy of the S2p peak at 168.4 eV, attributing to the sulfonate group, changed to 168.8 eV upon the addition of Fe3+ [31]. Moreover, the binding energy of Fe2p (712.2 eV) is different from the ferric chloride (711.5 eV) according to previous literature [32]. Besides, solid-state CP-MAS 13C NMR of the DSP5 and DSP5+Fe3+ powder also provided convincing evidences for the coordination of DSP5 and Fe3+ (Figure 4(e)).

Figure 4

Coordination effect between DSP5 and Fe3+. (a) C1s, (b) O1s, and (c) S2p regions of XPS spectra of DSP5 before (up) and after (down) the addition of Fe3+, and (d) Fe2p regions of XPS spectrum of DSP5+Fe3+ powder. (e) Solid-state CP-MAS 13C NMR characterization of DSP5 and DSP5 + Fe3+ powder.

3. Discussion

In conclusion, we have successfully synthesized a novel water-soluble disulfonate-functionalized pillar[5]arene (DSP5), based on which a pH-stable fluorescent material (TPE-Q4⊂ DSP5) was facilely designed and constructed, exhibiting SAIE properties. The as-prepared supramolecular material showed excellent photoluminescent characteristics of emitting switchable bright blue fluorescence even in dilute aqueous solution. In addition, the supramolecular assembly possessed superior responsiveness to Fe3+ in water with high selectivity and sensitivity, since the enhanced fluorescence arising from TPE-Q4⊂ DSP5 could be efficiently quenched by the coordination effect of DSP5 and Fe3+. We envision that this new type of supramolecular fluorescent materials will find great applications in various fields, including fluorescent probing, cell imaging, and visualized monitoring of metal ions.

4. Materials and Methods

4.1. Materials and Characterization

All reagents were commercially available and used as supplied without further purification. 1H NMR spectra were collected on a temperature-controlled 300 MHz spectrometer. 13C NMR spectra were recorded on a DMX-500 spectrometer. Solid-state cross-polarization magic angle spinning (CP/MAS) 13C NMR spectra of polymers were measured on a Bruker Digital Avance III HD 400 WB (400 MHz) NMR spectrometer at ambient temperature with a magic angle spinning rate of 7.0 kHz internal reference. 2D diffusion-ordered NMR spectroscopy (DOSY) and 2D nuclear Overhauser effect NMR spectroscopy (NOESY) were recorded on a Bruker Avance DMX 600 spectrophotometer. High-resolution electrospray ionization mass spectra (HRESI-MS) were obtained on a Bruker 7-Tesla FT-ICR mass spectrometer equipped with an electrospray source (Billerica, MA, USA). The UV-vis spectra were collected via UV-vis spectroscopy on a Shimadzu UV-1800 spectrophotometer. Deionized water, purifying by Experimental Water System (Lab-UV-20), was used in relevant experiments. Scanning electron microscope (SEM) investigations were carried out on a HITACHI-SU8020 instrument. Dynamic light scattering (DLS) results were obtained on a Zetasizer Nano ZS instrument. The fluorescence experiments were conducted on a RF-5301 spectrofluorometer (Shimadzu Corporation, Japan). The surface compositions of polymers were determined by X-ray photoelectron spectroscopy (XPS) measured on a PREVAC XPS/UPS System using monochromatic Al K (1486.7 eV) radiation. The time-resolved fluorescence decay curves and absolute fluorescence quantum yields were obtained on a FLS920 instrument (Edinburgh Instrument) with the excitation of 330 nm.

4.2. Syntheses of Compound

The detailed synthesis procedures and characterizations of the studied compound (DSP5, TPE-Q4, TPE-Q, and DSNa) can be found in the Supplementary Materials.

4.3. Detection of Fe

Experiments were carried out to examine the potential for detecting cations and anions. Firstly, we prepared the DSP5 and TPE-Q4⊂ DSP5 aqueous solution ([TPE-Q4] = 4 μM, [DSP5] = 1 μM). 40 μL M(Cl)x aqueous solution (Mx+ = Na+, Fe3+, Fe2+, Co2+, Ni2+, Cu2+, Cu+, Zn2+, Cd2+, Hg2+, Mg2+, K+, Ca2+, NH4+, Pr3+, Nd3+, Sm3+, Eu3+, Gd3+, Tb3+, Dy3+, Ho3+, Er3+, Tm3+, Yb3+, Lu3+, and Al3+, [Mx+] = 2 mM) and 40 μL (Na)yN aqueous solution (Ny- = Cl−, F−, OH−, ClO−, CH3COO−, NO2−, HPO42−, H2PO4−, PO43−, S2O32−, HSO3−, SO42−, S2O42−, HCO3−, CO32−, [Ny-] = 2 mM) were added into the above TPE-Q4⊂ DSP5 solution (3.96 mL), respectively. Then, the fluorescence of each solution was recorded immediately. Similarly, interference metal ion with the same concentration as Fe3+ was added into the TPE-Q4⊂ DSP5 solution containing Fe3+ to further investigate the fluorescence recognition of Fe3+. The fluorescence quenching induced by Fe3+ was investigated to confirm whether the developed TPE-Q4⊂ DSP5 could be used to quantitative detection of Fe3+. Thus, different volumes of Fe3+ solutions (1 mM, 0~28 μL) were added into TPE-Q4⊂ DSP5 solution (1 μM) and the fluorescence intensities were recorded, respectively.

4.4. Preparation of DSP5 + Fe Powder

FeCl3 aqueous solution (1 mL, 0.1 mM) was added to DSP5 aqueous solution (0.1 mM, 8 mL) and the precipitates were generated immediately. Let the mixture stood for about 24 h and the precipitates were isolated by centrifuge at 10000 rpm. Then the product was washed with deionized water for three times to remove the FeCl3 residue and dried under high vacuum oven to get a brown powder.