Host–Guest Supramolecular Systems Containing AIE-Active Building Blocks

8.05 Host–Guest Supramolecular Systems Containing AIE-Active Building Blocks JZ Sun, W Bai, and Z Wang, Zhejiang University, Hangzhou, China BZ Tang, Zhejiang University, Hangzhou, China; South China University of Technology, Guangzhou, China; and The Hong Kong University of Science and Technology, Kowloon, Hong Kong, China Ó 2017 Elsevier Ltd. All rights reserved.

8.05.1 8.05.2 8.05.2.1 8.05.2.2 8.05.2.3 8.05.3 8.05.3.1 8.05.3.2 8.05.4 References

8.05.1

Introduction Host–Guest Interaction-Induced Emission Enhancement Interaction Between Cyclodextrins and AIE-Active Molecules Self-Assembling of AIE-Active Building Blocks Modified by Crown-Ether Moieties Interaction Between Pillar[n]arenes and AIE-Active Building Blocks Incorporation of AIE-Active Building Blocks into Metal-Organic Frameworks More Detailed Understanding of Mechanism for AIE Phenomenon Novel Multifunctional MOFs Containing AIE-Gens Conclusion and Outlook

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Introduction

According to the definition by Prof. J.-M. Lehn, “supramolecular chemistry” refers to the area of chemistry beyond that of molecules.1 In this sense, it is a study on molecular assemblies constructed from molecular and macromolecular building blocks rather than atomic ones in a noncovalent fashion. Utilizing this strategy, nature creates complex functional materials and systems by elegantly self-assembling simple molecular and macromolecular building blocks. The sophisticated functions of the precisely organized supramolecular structures observed for proteins, DNA, nucleic acids, and phospholipid membranes reveal the notion of “the whole is greater than the sum of its parts.” But this notion seems to be “untrue” in the case of conventional luminescent molecules. For example, perylene bisimide represents a class of luminescent molecules that are intensively investigated as active elements in a variety of photonic/electronic materials and devices. As shown in Fig. 1, perylene bisimide shows intense emission in dilute solution where they exist in an individual molecular state. When they form aggregates, their emission will be partially or even totally quenched. This photophysical phenomenon has frequently been referred to as aggregation-caused quenching (ACQ).2 An “aggregate”, as defined in the Oxford Dictionary and Wikipedia, is “a whole formed by combining several separate elements” and “a collection of items that are gathered together to form a total quantity”, respectively. The emission from a whole or aggregate is smaller than its parts. This concept had never been queried until an opposite photophysical phenomenon came into light. In 2001, Tang and colleagues reported the unique emission behavior of hexaphenylsilole (HPS).3 No detectable emission could be recorded when HPS was dissolved in good solvent (such as tetrahydrofuran (THF) and acetonitrile) to form dilute molecular solution. But intense fluorescence could be recorded when a high enough fraction of poor solvent (e.g., water and hexane) was introduced into the molecular solution. The addition of a poor solvent led to the formation of molecular aggregates, thus the emission turn-on was termed as “aggregation-induced emission” (AIE). For luminescent molecules with AIE characteristic, the whole is really more brilliant than the parts.4 The research field of AIE has been flourishing since the concept was presented. In the past 15 years, a variety of different molecular and macromolecular systems with AIE properties have been designed and synthesized. Meanwhile, the mechanistic understanding of the AIE phenomenon becomes deeper and broader. To date, three main types of AIE-active species have been discovered. As shown in Fig. 2 (top), the first type is represented by HPS and tetraphenylethene (TPE),3,5 which are composed of a conjugated central stator and several aromatic (mainly phenyl) rotators that are covalently linked to the stator via single bonds. This type of AIE-active molecules takes a propeller shape. In dilute solution or other free states, the phenyl rings can rotate or twist against the ethene stator. These active intramolecular rotations play the role of nonradiative decay channel for the molecule in an excited state to the ground state. In aggregates, the intramolecular rotations are restricted, thereby the nonradiative channels are blocked and the radiative relaxation channels are opened. The mechanism of AIE phenomenon observed for propeller-shaped molecules is ascribed to the restriction of intramolecular rotation (RIR).6 An elegant example of the second type of AIE-active molecules is 10,100 ,11,110 -tetrahydro-5,50 -bidibenzo[a,d][7]annulenylidene (THBA).7 As shown in Fig. 2 (middle), THBA can be divided into two parts that are concatenated with a bendable flexure. The flexible linkage allows the two parts to vibrate in a flip-flap fashion. Thus a THBA molecule takes a shell-like configuration. The intramolecular vibrations, just like the intramolecular rotations in propeller-shaped molecules, play the role of nonradiative decay channel to exhaust the energy in the excited state and quenched the emission in freely states. Once aggregates are formed, the intramolecular vibrations are restricted and the nonradiative relaxation channels are blocked and meanwhile the radiative

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Figure 1 Fluorescence images of solutions or suspensions of a perylene bisimide (upper) and hexaphenylsilole (HPS, lower) in THF/water mixtures with different water fractions (fW, % by volume). The perylene bisimide and HPS show typical aggregation-caused quenching (ACQ) and aggregationinduced emission (AIE) behaviors, respectively. Reproduced from Hong, Y.; Lam, J. W. Y.; Tang, B. Z. Chem. Soc. Rev. 2011, 40, 5361 with permission. Copyright 2011. The Royal Society of Chemistry.

relaxation channels are opened. Analogously, the mechanism of AIE phenomenon observed for shell-shaped molecules is ascribed to the restriction of intramolecular vibrations (RIV).7,8 Integrating RIR and RIV together, a more general mechanistic description of AIE phenomenon is referred to as restriction of intramolecular motions (RIMs), which can account for the experimental observations and theoretical deductions in both of the propeller-shaped and shell-like molecules. The third type is called luminescent clusters, which are composed of a large ensemble of electron-rich heteroatoms (such as oxygen and nitrogen) with lone-pair electrons in their molecular or macromolecular structures. Poly[(maleic anhydride)-(vinyl acetate)] and vinyl polymers carrying succinic anhydride terminal groups are early examples.9,10 These polymers do not have any conventional large p-conjugated luminogens in their structures. However, they are efficiently emissive when their conformations are rigidified by covalent bonding, noncovalent interaction, and cryogenic cooling. An illustration is displayed as Fig. 2 (bottom), and the luminescence turn-on is tentatively associated with the formation of dense clusters of electron-rich moieties.11 This type of AIE-active materials have received much interest in more recent years because similar emission behaviors are observed for some biocompatible polymers and even natural biomacromolecules such as polysaccharides and starches,12–14 although the underlying mechanism has not been clearly addressed yet. Nowadays, AIE has become a highly interdisciplinary research field and is making a significant influence on the development of materials, chemical, and biological sciences. “Aggregation” itself is a process of the formation of molecular assembly. It is a spontaneous progress from aggregation or assembly to self-assembled entities and from AIE to supramolecular chemistry. Considering that there have been a series of tutorial and comprehensive reviews and books on the general topics of molecular recognition, chemical and biological sensing, bioimaging, and mechanochromism,15–24 this article will focus on host–guest interactions involving AIE-active molecules that have not been covered by the above-mentioned reviews and books. This work covers the most recent 5 years.

8.05.2

Host–Guest Interaction-Induced Emission Enhancement

8.05.2.1

Interaction Between Cyclodextrins and AIE-Active Molecules

Cyclodextrins (CDs) are cyclic oligosaccharides composed of six (a), seven (b), eight (g), or more glucopyranoside units. CDs are one of the most extensively studied hosts and have been widely applied across multiple fields including supramolecular chemistry, pharmaceutical, and biomedical science.25–30 In such fields, the host–guest interaction between the hydrophobic cavity of CDs, and

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Figure 2 Illustration of three types of AIE luminogens. (Top) Propeller-shaped luminogen of tetraphenylethene (TPE) is nonluminescent in free state but becomes emissive when TPE molecules form aggregate, due to the restriction of intramolecular rotation (RIR) of its phenyl rotors against its ethylene stator. (Middle) Shell-like luminogen of 10,100 ,11,110 -tetrahydro-5,50 -bidibenzo[a,d][7]annulenylidene (THBA) also shows AIE property, which is ascribed to the restriction of intramolecular vibration (RIV) of its bendable vibrators in the aggregate state. (Bottom) Emissive cluster formed by an unconventional luminogen system which is composed of a mass of electron-rich heteroatoms with lone-pair electrons in their molecular or macromolecular structures. Reproduced from Mei, J.; Hong, Y.; Lam, J. W. Y.; Qin, A.; Tang, Y. H.; Tang, B. Z. Adv. Mater. 2014, 26, 5429 with permission. Copyright 2014. Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim.

guest molecules with suitable shape and size, nearly dominates the entire self-assembling process. Besides offering a hydrophilic environment, the multiple hydroxyl groups on the rim of CDs are usually used as reactive sites for covalent modification but have seldom been considered as important structural factors affecting the host–guest interaction. In 2012, Liu et al. reported a comprehensive study on the cooperation of the hydrophobic host–guest interaction and dynamic covalent interactions between boronic acid and diols that forms cyclic esters in aqueous media by using AIE-active fluorescent probes.31 As shown in Fig. 3A, the fluorescence (FL) intensity of the diboronic acid-modified TPE (M1, inset of Fig. 3B) in carbonate buffer solution was unchanged at a molar ratio of < 1 for M1/b-CD. With increasing b-CD fraction, FL was progressively intensified and the highest intensity, about 10.4 times stronger than that of M1 in buffer solution, was recorded in the molar ratio of TPEDB/ b-CD of 500. Interestingly, addition of a-CD and g-CD could only lead to trivial enhancement of FL intensities of M1 in carbonate buffer solution (Fig. 3B). To explain these observations, working models 1–4 were proposed as depicted in Fig. 4. Model 2 is the one that can be easily conceived because b-CD may serve as a host to accommodate a hydrophobic guest phenyl group. The host–guest interaction partially restricts the rotations of the related phenyl groups on TPE core, thereby inducing enhanced emission. But this model was denied by a series of comparative experiments using different water-soluble TPE derivatives (Fig. 5). From M2 to M4, no matter positive or negatively charged, dicationic or monocationic, these AIE-active probes showed little fluorescent responses in the carbonate buffer solutions to the addition of three CD analogous (Fig. 5), indicating that the host–guest interaction between the hydrophobic cavity of CDs and the phenyl group of the probe molecules is not a dominate role. Model 1, or the formation of dynamic polymers through the reversible reaction between boronic acid and diols on the rims of CDs, may cause the FL enhancement in the M1 and b-CD system. This assumption was partially supported by the comparative study by using monoboronic acid-modified TPE derivative M5 as a fluorescent probe (Fig. 5). Without a pair of diols, the condensation reaction is unavailable, thus the dynamic polymer cannot be generated. This experiment suggested the necessity of the diboronic acid groups and the diols. However, this model is excluded because the condensation reaction between M1 and CDs is difficult

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Figure 3 (A) Fluorescence (FL) spectra of M1 (10 mM) in carbonate buffer (0.2 M, pH 10.5) containing 0.5 vol% dimethylsulfoxide (DMSO) in the presence of different amount of b-CD. (B) Variation in the FL intensity (I) of M1 (10 mM) with their peaked values as a function of the concentration of CDs. I0 presents the intensity in the absence of CDs. Inset of B: chemical structure of M1. Reproduced from Liu, Y.; Qin, A.; Chen, X.; Shen, X. Y.; Tong, L.; Hu, R.; Sun, J. Z.; Tang, B. Z. Chem.-Eur. J. 2011, 17, 14736 with permission. Copyright 2012. Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim.

Figure 4

Proposed interaction patterns between TPEDB (M1) and b-CD.

to form due to the low affinity of boronic acid to trans-diols in CDs. In addition, a-, b-, and g-CDs are chemical analogous, and the diols on their rims don0 t have such remarkable difference in reactivity when they reacted with M1. Consequently, there must be other mechanism(s) underlying the specifically fluorescent response of b-CD to M1. Induced circular dichroism (ICD) is a powerful technique to study the host–guest interaction. CDs are chiral hosts and capable of inducing the circular dichroism signals of bound achiral guests. Meanwhile, a clear relationship between the sign, intensity of the ICD signal, and the spatial arrangement of guest molecules and CDs has been well documented.32,33 As shown in Fig. 6, M1 showed different ICD responses to these CDs. b-CD induced a negative ICD signal to M1 and signal intensity increased with the concentration of b-CD. These data suggested that there existed interaction between its phenyl rings and the b-CD cavity. In contrary, almost no ICD signals of M1 were recorded upon addition of a-CD, and weaker negative signals of M1 were recorded for g-CD, which became gradually stronger until reaching the plateau. Comparative experiments using M5 as probe gave similar results. These results confirmed the necessity of the interaction between the phenyl rings the probes and the b-CD cavity. Beside the above investigations, other control experiments had been carried out. Addition of D-glucose into the mixture of M1 and b-CD quenched

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Figure 5 FL responses of water-soluble TPE derivatives (10 mM) in carbonate buffer (0.2 M, pH 10.5) containing 0.5 vol% DMSO to CDs (5.0 mM). Inset: chemical structures of TPE derivatives of M2–M5; FL response of M5 (10 mM) in carbonate buffer containing 20 vol% DMSO to CDs (5.0 mM). Reproduced from Liu, Y.; Qin, A.; Chen, X.; Shen, X. Y.; Tong, L.; Hu, R.; Sun, J. Z.; Tang, B. Z. Chem.-Eur. J. 2011, 17, 14736 with permission. Copyright 2012. Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim.

Figure 6 (A) ICD spectra of TPEDB (10 mM) in carbonate buffer (0.2 M, pH 10.5) containing 0.5 vol% DMSO in the presence of different amount of b-CD. (B) Variation in the ellipticity values (q) with CD concentrations. Reproduced from Liu, Y.; Qin, A.; Chen, X.; Shen, X. Y.; Tong, L.; Hu, R.; Sun, J. Z.; Tang, B. Z. Chem.-Eur. J. 2011, 17, 14736 with permission. Copyright 2012. Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim.

the ICD signals and FL emission from M1. These observations were ascribed to the facile reaction of diboronic acids with the two pairs of cis-diols on D-glucose. Addition of 1-adamantecarboxylic acid, a much stronger binder of the cavity of b-CD into the mixture, also led to the quenching of the ICD signals and FL emission from M1, indicating efficient guest exchange. Theoretical simulation suggested that the distance between the two boronic acid groups matched better to the trans-diols on the wide rim of b-CD than to that of a- and g-CD. Putting all these data together, an unprecedented working mechanism was forwarded to explain the specific recognition of M1 to b-CD. Model 3 is a preferred binding model. In this case, the weak host–guest interaction (due to the size of a phenyl is not fully matched with the cavity of b-CD), the unfavorable reaction (due to the lower reactivity of a trans-diol than a cis-diol), and the matched distance between the reactive functionalities work cooperatively to fulfill a specific recognition process. Such a working scheme is frequently employed by nature, and it is of great significance in biological systems. Thanks to the AIE-active probes, and the unique RIR mechanism allows the understanding of the cooperative recognition event with fluorescent signaling technique.

8.05.2.2

Self-Assembling of AIE-Active Building Blocks Modified by Crown-Ether Moieties

Crown-ethers are another type of classical host molecules and the host–guest recognition of crown-ether to different guest species, ranging from potassium ions to ammonium and paraquat derivatives has received considerable investigation. In 2015, Bai et al. reported a concept-proof work by using TPE as the AIE-gen and the host–guest interaction between dibenzo[24]crown-8 and benzylamino moieties as the driving force of supramolecular assembly.34 The molecular structure of the building blocks and the host–guest self-assembly-induced emission process are depicted in Fig. 7.

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Figure 7 Molecular structure of M6, M7, and a schematic illustration of the reversible self-assembling and disassembling between M6 and M7 via host–guest interaction. Reproduced from Bai, W.; Wang, Z. Y.; Tong, J. Q.; Mei, J.; Qin, A.; Sun, J. Z.; Tang, B. Z. Chem. Commun. 2015, 51, 1089 with permission. Copyright 2015. The Royal Society of Chemistry.

Owing to the TPE moieties, both the dibenzo[24]crown-8 and benzylamino-modified TPE derivatives (M6 and M7) demonstrated AIE-characteristics. The secondary amines in TPE-DBA are protonated by adding acid into the solution, and the protonated amines are capable to be hosted by the crown ethers, thereby the ionic M6 and M7 can self-assemble into supramolecular macromolecules via a host–guest interaction. The noncovalent interaction of the host will lock on the rotations of guest benzylamine groups, which involve with the phenyl rotations of TPE units. According to the RIR mechanism, the emission from M6 will be turned on. Meanwhile, the emission from the TPE moiety in M7 can also be partially initiated, due to the damped intramolecular phenyl rotations caused by the formation of the supramolecular macromolecules. The acidification-induced assemblies will disassemble when treated with base, because deionized amines cannot be hosted by crown ethers and the restriction effect disappears accordingly. This is an example of self-assembling-induced emission (SAIE) system. The host–guest interaction initiated the “polymerization” of the TPE-containing building blocks and the RIR process and turned on the emission of the AIE-gen. The breaking of the noncovalent bonds eliminated the restriction and turned the emission off. The assembling/disassembling processes accompanied by FL turn-on/off are dynamic, reversible, and tuneable by treating the system with acid/base. Crown-ether interactions are only one of various host–guest interactions, and AIE-gens can be chemically modified to accommodate different situations, thus the concept proved here is helpful and useful to construct more supramolecular AIE systems and to fabricate novel chemo-/biosensors and stimuli-responsive materials.

8.05.2.3

Interaction Between Pillar[n]arenes and AIE-Active Building Blocks

Pillar[n]arenes are macrocyclic compounds that are linked by methylene groups at the para-positions of 2,5-dialkoxybenzene rings. Pillar[5]arenes and pillar[6]arenes are the two main members in the pillar[n]arenes family. Pillar[n]arenes are named because of their pillar-like architecture, and their pillar-like symmetrical structures and easy functionalization make them excellent candidates for host–guest chemistry. With suitable functionalization, a variety of host–guest recognition motifs of pillararenes have been built, and their responses to external stimuli have been explored.35–37 Recently, Huang and colleagues reported a series of interesting systems based on the combination of pillararenes with AIE-active fluorogens and the exploration of fluorescent pillararene-based host–guest systems and their ensembles.38–42 In their first attempt, a host–guest inclusion complex was constructed from a water-soluble pillar[6]arene (WP6) and a TPE derivative (M8) in water.38 The chemical structures of WP6, M8, other compounds used in the research, and their assembling scheme are shown in Fig. 8. Due to the four quaternary ammoniums, M6 is water soluble. The TPE core in M8 is a representative

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Figure 8 Molecular structure of AIE-active probe M8, reference compound M9, other units used for self-assembling, and the proposed assembling scheme. Reproduced from Wang, P.; Yan, X.; Huang, F. H. Chem. Commun. 2014, 50, 5017 with permission. Copyright 2014. The Royal Society of Chemistry.

Figure 9 (A) Fluorescence (FL) spectral changes of M8 (2.00 mM) upon addition of WP6 (0.00–16.0 equiv.) in water. The inset photographs show the corresponding fluorescence changes (left: 2.00 mM M8; right: 2.00 mM M8 and 32.0 mM WP6). (B) The quenching of the FL of a solution of WP6 (32.0 mM) and M8 (2.00 mM) upon the titration with paraquat (0.00–160 mM) in water. The inset photographs show the corresponding fluorescence changes (left: 2.00 mM M8 and 32.0 mM WP6; right: 2.00 mM M8, 32.0 mM WP6, and 160 mM paraquat). Photos were taken upon excitation at 365 nm using a UV lamp at 298 K. Reproduced from Wang, P.; Yan, X.; Huang, F. H. Chem. Commun. 2014, 50, 5017 with permission. Copyright 2014. The Royal Society of Chemistry.

AIE-gen (like in the case of liquid crystals, where rod-like molecules showing mesomorphism are named as mesogens), and M8 is an AIE-active molecule. When dissolved in water, it is nonemissive. Strong fluorescent emission is recorded when WP6 is introduced into the nonemissive aqueous solution (Fig. 8). According to the well-accepted RIM mechanism, the complexation between WP6 and M8 hampers the intramolecular rotation of the phenyl rings of M8 by the formation of [M8]pseudorotaxane units, thus the fluorescence out of TPE core is triggered. The complexation process has been readily confirmed by monitoring the chemical shifts of the respect protons on M6 (see the hydrogens marked in blue in Fig. 8). When 32.0 mM WP6 was added into M6 aqueous solution, about 20-fold fluorescence enhancement was recorded, and this fluorescence change was clearly witnessed by the naked eyes, as displayed by the inset photographs of Fig. 9A. WP6 is a host that can bind to different guests, and the guest–molecule binding may break the [M8]pseudorotaxane units and release M8 molecules into the aqueous solution, thereby the fluorescence of M8 molecules will be quenched. As shown in Fig. 9B, the significant fluorescence quenching was recorded upon the addition of paraquat. This observation is in agreement with the expectation of RIR mechanism and is understood on account of the much higher binding constant of WP6 to paraquat (1.02  108 M 1) than that of WP6 to reference compound M9.43 Fluorescence titration of the inclusion complex with paraquat and exclusion of M8 was carried out at room temperature in water, and the recorded spectral changes are shown in Fig. 9B. Moreover, the “turn-off” fluorescence change produced by the addition of paraquat was easily visualized by the naked eyes (photographs, inset of Fig. 9B). As an amphiphilic molecule, M8 molecules self-assembled into spherical micelles with a diameter of around 300 nm in an aqueous solution of a concentration of 1.00  10 4 mol L 1 (see the transmission electron microscopy (TEM) images in Fig. 10A–C). These micelles have a regular spherical shape. Wheel-like nanostructures with a diameter of about 200 nm have been observed after addition of 4.00 equiv. of WP6 into M8 solution (Fig. 10D and E). The authors conceived that the self-assembling of WP6 and M8 form a host–guest complex, which is a new amphiphilic building block. The aromatic stacking between the complexes and electrostatic interactions between the quaternary ammonium groups and carboxylate groups are account for the stability of the resulting architectures (Fig. 10F).

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Figure 10 TEM images: (A) TEM image of M8; (B) enlarged TEM image of (A); (C) cartoon representation of the structure of micelles formed by M8; (D) TEM image of the inclusion complex between WP6 and M8; (E) enlarged TEM image of (D); (F) cartoon representation of the formation of the superstructure from M8 and WP6. Reproduced from Wang, P.; Yan, X.; Huang, F. H. Chem. Commun. 2014, 50, 5017 with permission. Copyright 2014. The Royal Society of Chemistry.

In another attempt, Huang’s group designed and synthesized two four-armed TPE derivatives containing electron-rich naphthalene (TPE-NP) and electron-deficient paraquat (TPE-PQ) groups (see the structures in lower panel of Fig. 11).39 TPE-NP and TPE-PQ selfassembled into nanorods in a one-dimensional (1D) packing mode (Fig. 12A–C). The driving force is ascribed to charge-transfer (CT) interactions as shown by the schematic illustrated in Fig. 12D. The CT complexation process leads to the RIRs of the phenyl rings in TPE moieties, thus significant enhancement in fluorescence emission is recorded (Fig. 12E and F), and the process can be directly visualized by the naked eye (inset of Fig. 12F). The good AIE and CT properties observed for TPE-NP, TPE-PQ, and their complex suggest that they are potentially employed as fluorescent agents in bioimaging. Before experimental attempts, their toxicity must be estimated properly. A simple cytotoxicity evaluation of TPE-NP, TPE-PQ, TPE-PQ@TPE-NP at different concentrations against MCF-7 and HEK293 cells was carried out by using a 3-(40,50-dimethylthiazol-20-yl)-2,5-diphenyltetrazolium bromide (MTT) assay. The results indicated that TPE-NP has a small influence on cell viability and proliferation for both MCF-7 and HEK293 cells incubated for 4 and 24 h with the concentration ranging from 20 to 150 mg mL 1. But TPE-PQ exhibited high toxicity against MCF-7 and HEK293 cells; the addition of TPE-PQ led to a rapid decrease in relative cell viability. To better the biocompatibility and lower the cell toxicity, the host–guest complexation strategy was used in this research. As shown by the upper panel in Fig. 12, a sodium-carboxylate functionalized 1,4-bis(butoxy)pillar[6]arene as the host (H) of PQ unit was designed and prepared. The host–guest interaction resulted in the formation of stable complex of four H with one TPE-PQ, or the host–guest complex H4XTPE-PQ. In sharp contrast, the relative cell viability of H4XTPE-PQ was higher than that of TPE-PQ at the same condition, indicating the significantly reduced toxicity of TPE-PQ upon formation of the stable host–guest complex. The confocal images of live MCF-7 breast cancer cells after incubation with H4X TPE-PQ@TPE-NP for 2 h suggest that the combination of the AIE effect and host–guest supramolecular chemistry has great potential in biologically and pharmaceutically relevant fields, including biosensors, drug and gene delivery systems, and cell imaging. More recently, Huang and coworkers reported the preparation of nanoparticles with near-infrared (NIR) emission enhanced by host–guest complexation between a water-soluble pillar[5]arene (WP5) and a cyano-stilbene derivative (M10) in water media.42 The molecular and supramolecular structures, illustration of self-assembling process, and the proposed mechanism of particle formation are depicted in Fig. 13A–C, respectively. The host–guest complexation process was carefully studied with 1H NMR and 2D NOESY NMR spectroscopic techniques. The chemical shifts of the red-marked protons shown in reference compound 2 (R2) helped decipher the complexation. The obtained results indicated that the positively charged trimethylammonium head of R2 was threaded into the cavity of the cyclic host WP5 to form a pseudorotaxane. The association constant (Ka) of the host–guest complex between WP5 and R2 was measured to be 1.75 ( 0.21)  106 M 1 in 1:1 complexation mode by isothermal titration calorimetry experiments.

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Figure 11 Top: synthetic route to difunctional pillar[6]arene H (Host). Middle: chemical structures of model compounds PQ and TNP. Bottom: chemical structures and cartoon representations of TPE-PQ and TPE-NP. Reproduced from Yu, G. C.; Tang, G. P.; Huang, F. H. J. Mater. Chem. C 2014, 2, 6609 with permission. Copyright 2014. The Royal Society of Chemistry.

Since the WP5I R2 recognition motif has been established, the supramolecular entity was prepared by simply mixing WP5 with M10 in water. The formation of the supramolecular amphiphile was confirmed by UV/vis absorption and fluorescent emission measurements on M10 and WP5IM10 in aqueous phosphate-buffered saline buffer solution (APBSBS, 1.00  10 2 M PBS, pH 7.4), as shown in Fig. 14A and B. In APBSBS, M10 showed a weak and featureless emission band. Upon addition of equimolar WP5, an intense NIR emission band centered at 665 nm appeared in the fluorescent spectrum. In the aqueous solution of WP5IM10, threading of the trimethylammonium group of M10 into the cavity of WP5 evidently lowered the water solubility and caused the formation of supramolecular aggregation, thus resulted in the enhancement emission from the TPE moieties (Fig. 14B and inset). As shown in Fig. 14C and D, the supramolecule WP5I M10 showed pH responsive fluorescence. In fact, acid protonates the carboxylate groups to convert WP5 to WP5H and destroyed the complex WP5I M10 (Fig. 13). Due to the presence of a tertiary amine group, M10 can be readily protonated by acid and converts M10 into water-soluble salt M10H. Consequently, the fluorescence of the WP5IM10 system is heavily quenched in acid solution. The amphiphilic M10 can self-assemble into nanoribbons in its aqueous solution in a concentration higher than its critical aggregation concentration ( 1.00  10 4 M). The self-assembly behavior and the morphology of the nanostructures were investigated with scanning electron microscopy (SEM) and TEM techniques. The ribbon-like aggregates are about 20 mm in length and 100 nm in width. A Tyndall effect (Fig. 15C) was observed for a solution of WP5IM10, indicating the average diameter of the self-assemblies was larger than 100 nm. Light-scattering measurement results showed an average diameter of  200 nm with a narrow size distribution

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Figure 12 TEM images: (A) TPE-PQ@TPE-NP; (B) enlarged image of (A) (scale bar ¼ 500 nm); (C) enlarged image of (A) (scale bar ¼ 100 nm); (D) schematic representation of the self-assembly process of TPE-PQ and TPE-NP driven by charge-transfer interactions. (E) Fluorescence spectra of TPE-PQ@TPE-NP at different TPE concentrations in water at room temperature. (F) Plot of emission intensity at 491 nm vs. the TPE concentration. The inset in (F) is a fluorescent photo of TPE-PQ@TPE-NP in water at different TPE concentrations. Reproduced from Yu, G. C.; Tang, G. P.; Huang, F. H. J. Mater. Chem. C 2014, 2, 6609 with permission. Copyright 2014. The Royal Society of Chemistry.

at different concentrations and SEM image indicated around 200 nm in diameter of the spherical assemblies. When pH of the aqueous solution was lower than 4.00, the particles disappeared and irregular aggregates were formed due to disassembling of the complex WP5IM10.

8.05.3

Incorporation of AIE-Active Building Blocks into Metal-Organic Frameworks

8.05.3.1

More Detailed Understanding of Mechanism for AIE Phenomenon

Metal-organic frameworks (MOFs) are representative supramolecular architectures which are built up by coordination of inorganic metal-containing nodes and organic frames. Hollow structures, huge internal surface areas, and exquisite constructions make MOFs to be promising hosts for selectively molecular storage, catalyst loading, and gas separation matrix. Thus in recent research fields, MOFs have been continuously one of the hottest topics.44–48 The pioneer work of introducing AIE-gen into MOFs is reported by Dinca et al. in 2011.43 It was found that the coordination of tetrakis(4-carboxyphenyl)ethene (M11) to d10 metal cations produced luminescent MOFs, in which the TPE cores are not in van der Waals contact due to the spatial isolation (Fig. 16A). But the TPE-cores emit strong fluorescence and the fluorescence lifetimes are similar to those measured for molecular aggregates. Coordinative immobilization of functionalized TPE within the pores of MOFs turned on fluorescence nonemissive TPE core. The authors proposed a matrix coordination-induced emission effect (MCIE) to explain the emission behavior, which is in good agreement with the RIM mechanism. The designable porous structures of MOFs allow researchers to carefully study and understand more details about the mechanism of fluorescence quenching in AIE chromophores. To this end, Dinca and colleagues fabricated a perdeuterated TPE-based MOF.49 By using solid-state 2H and 13C NMR experiments, the phenyl ring dynamics of TPE cores that are coordinatively trapped inside a MOF

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Figure 13 (A) Structures and cartoon representations of M10, M10H, R2, WP5, and WP5H. (B) Cartoon representation of self-assemblies of M10 and WP5IM10 and pH responses of nanoparticles prepared from WP5IM10. (C) Mechanism of the aggregation of M10 and WP5IM10 and pH responses of WP5IM10 in water. Reproduced from Shi, B.; Jie, K. C.; Zhou, Y. J.; Zhou, J.; Xia, D. Y.; Huang, F. H. J. Am. Chem. Soc. 2016, 138, 80 with permission. Copyright 2016. American Chemical Society.

was investigated in an unprecedented detail and found a phenyl ring flipping energy barrier of 43(6) kJ mol 1. Density functional theory (DFT) calculation are then used to deconvolute the electronic and steric contributions to this flipping barrier. Integrating the NMR and DFT studies together with variable-temperature X-ray diffraction experiments, the authors proposed that both the ethylenic C]C bond twist and the torsion of the phenyl rings are important for quenching emission in TPE, but that the former may gate the latter (Fig. 16B).

8.05.3.2

Novel Multifunctional MOFs Containing AIE-Gens

These mechanistic understandings are helpful to establish the design criteria for the development of tunable turn-on porous sensors constructed from AIE-type molecules. Based on their newly findings, Dinca et al. proceeded their investigations on the design of new multifunctional MOFs. For example, when fluorescent molecule M11, or tetrakis(4-carboxyphenyl)ethene was incorporated as ligands in rigid, porous MOF of Zn2(M11) was fabricated.50 The fluorescence response kept unchanged to a much higher temperature

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Figure 14 (A) Absorption spectra of M10 (5.00  10 5 M) in aqueous phosphate-buffered saline buffer solution (APBSBS) without and with WP5. Inset: Photograph showing the color change from M10 to WP5I M10 in APBSBS. (B) Fluorescence (FL) spectral response of M10 (5.00  10 5 M) in APBSBS upon addition of 1.0 equiv. of WP5. Inset: Photograph of M10 and WP5IM10 (5.00  10 5 M) under a UV-lamp. (C) Influence of pH on FL of WP5IM10 in APBSBS. (D) Solution pH dependence of the FL intensity of WP5IM10 in APBSBS at 665 nm. Reproduced from Shi, B.; Jie, K. C.; Zhou, Y. J.; Zhou, J.; Xia, D. Y.; Huang, F. H. J. Am. Chem. Soc. 2016, 138, 80 with permission. Copyright 2016. American Chemical Society.

Figure 15 SEM (A) and TEM (B) images of the nanoribbon aggregates of M10 (1.00  10 3 M). (C) Dynamic light scattering (DLS) study and Tyndall effect of the host–guest complex WP5IM10 assemblies (1.00  10 3 M). (D) and (E) SEM and TEM images of the nanoparticles of WP5I M10 (1.00  10 3 M). (F) TEM image of WP5I M10 complex after the addition of Hþ in pure water (1.00  10 3 M). Reproduced from Shi, B.; Jie, K. C.; Zhou, Y. J.; Zhou, J.; Xia, D. Y.; Huang, F. H. J. Am. Chem. Soc. 2016, 138, 80 with permission. Copyright 2016. American Chemical Society.

than in molecular crystals as shown in Fig. 17A. The remarkable high-temperature ligand-based fluorescence demonstrated with TPEbased linkers offers the potentiality to selective and rapid detection of analytes in the gas phase. At room temperature, MOF of Zn2(M11) showed no NH3 selectivity, but at 100 C the MOF was shown to be a selective sensor for ammonia (Fig. 17B). The preferential analyte binding was studied by variable-temperature diffuse-reflectance infrared spectroscopy, fluorescence spectroscopy, X-ray crystallography and by the help of the density-functional calculations to interrogate the temperature-dependent guest– framework interactions. The derived results demonstrated an unrecognized, but potentially general property of many rigid, fluorescent MOFs. 4,40 -((Z,Z)-1,4-diphenylbuta-1,3-diene-1,4-diyl)dibenzoic acid (TABD-COOH, M12) is an AIE-active molecule.51 Using M12 as organic linker, Wang and colleagues reported the synthesis of three MOFs, where the metal ions were Mg2 þ, Ni2 þ, and Co2 þ, and

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Figure 16 (A) Turn-on fluorescence in a TPE rotor by aggregation (AIE) and by coordination in a rigid MOF matrix (MCIE). (B) An illustration showing that both the ethylenic C]C bond twist and the torsion of the phenyl rings are important for quenching emission in TPE. Reproduced from Shustova, N. B.; McCarthy, B. D.; Dinca, M. J. Am. Chem. Soc. 2011, 133, 20126 and Shustova, N. B.; Ong, T.-C.; Cozzolino, A. F.; Michaelis, V. K.; Griffin, R. G.; Dinca, M. J. Am. Chem. Soc. 2012, 134, 15061 with permission. Copyright 2011 and 2012. American Chemical Society.

Figure 17 (A) Temperature-dependent fluorescence decay profiles of MOF of Zn2(M11) (1, squares) and TPE (circles), where I0 is the fluorescence intensity at room temperature (rt). Heating and cooling cycles are represented as filled and open symbols, respectively. The inset shows PXRD patterns of activated 1 (MOF of Zn2(M11)) and after heating at 350 C in air. The optical micrographs show fluorescent 1 (MOF of Zn2(M11)) (lex ¼ 350 nm) upon heating at various temperatures in air. (B) Illustration of the fluorescence shift, turn-on mechanism of chemical sensing, where detection monitoring at lf will reveal an intensity increase upon interaction with an analyte. (C, D) In situ normalized emission spectra (lex ¼ 350 nm) of MOF of Zn2(M11) exposed to various analytes at rt and 100 C, respectively. Reproduced from Shustova, N. B.; Cozzolino, A. F.; Reineke, S.; Baldo, M.; Dinca, M. J. Am. Chem. Soc. 2013, 135, 13326 with permission. Copyright 2013. American Chemical Society.

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Figure 18 (A) Second building units (SBUs), (B) 2D sheets or 1D chains, and (C) crystal structures of TABD-MOFs. C, dark blue; O, red; N, green; Mg, sky blue; Ni, yellow; Co, orange. H atoms have been omitted for clarity. The photographs in the lower panel show bright-field images and fluorescence images of (left) TABD-MOF-1, (middle) TABD-MOF-2, and (right) TABD-MOF-3. Reproduced from Guo, Y.; Feng, X.; Han, T.; Wang, S.; Lin, Z. G.; Dong, Y. P.; Wang, B. J. Am. Chem. Soc. 2014, 136, 15485 with permission. Copyright 2014. American Chemical Society.

the MOFs were named as TABD-MOF-1, -2, and -3, respectively.52 The crystal structures were determined by single-crystal X-ray diffraction. TABD-MOF-1 is a two-dimensional MOF, and TABD-MOF-2 and TABD-MOF-3 are one-dimensional MOFs (Fig. 18A–C). The fluorescence of these three MOFs was tuned from highly emissive to completely nonemissive via ligand-to-metal charge transfer by rational alteration of the metal ion. Coordination of M12 with Mg2 þ resulted in an efficiently luminescent MOF (TABD-MOF-1, FF ¼ 38.5%). Replacing Mg2 þ by Ni2 þ and Co2 þ with incomplete d subshells yielded the barely fluorescent and completely nonfluorescent MOFs TABD-MOF-2 and TABD-MOF-3, respectively (Fig. 18). The heavily quenched fluorescence can be attributed to the stronger ligand-to-metal charge transfer effect. Weakening or eliminating the ligand-to-metal charge transfer will allow a fluorescence turn-on detection of guest molecules. These three TABD-MOFs with different inherent emissions were further used for selective sensing of five-membered-ring energetic heterocyclic compounds, the detection of which is ever urgent but has not been realized by a fluorescent approach. As shown in Fig. 19, these MOFs are capable of detecting explosives such as five-membered ring energetic heterocyclic compounds in a highly sensitive and selective way in a few seconds through emission shift and/or turn-on. Remarkably, TABD-MOF-3 can selectively sense the powerful explosive 5-nitro-2,4-dihydro-3H-1,2,4-triazole-3-one with high sensitivity discernible by the naked eye (detection limit ¼ 6.5 ng on a 1 cm2 testing strip) and possess per billion-scale sensitivity by spectroscopy via pronounced fluorescence emission. Due to the propeller- or shell-like shape, the AIE-active molecules are usually composed of movable parts, such as phenyl rotors and aromatic vibrators. Such kinds of molecular structures bestow the solids of AIE-active molecules with pronounced mechano fluorochromic or piezofluorochromic effect. Can this property be kept in the rigid MOFs? Recently, Su and Zhou demonstrated a piezofluorochromic Zr-MOF, which is constructed from H4ETTC and eight-connected Zr6 clusters, denoted as PCN-128W (PCN means porous coordination network).53 PCN-128W was obtained as white crystalline powder under solvothermal conditions with trifluoroacetic acid as the competing reagent. When compressed with a glass slide or treated with 6.0 M HCl solution, PCN-128W changed its color from white to yellow. The yellow powder PCN-128Y (Y means yellow) showed similar but distinct PXRD pattern compared to PCN-128W. High-resolution synchrotron-based PXRD characterizations were also carried out for both PCN-128W and PCN-128Y to further evaluate the differences. In comparison with PCN-128W, the diffraction peaks of PCN-128Y in the low angle region (< 2 degrees) shifted to smaller angles and the peaks in region 2.5–5.0 degrees showed evident differences (Fig. 20). Further experiments revealed that the luminescence of PCN-128W was bathochromically shifted from 470 to 538 nm to form PCN-128Y by change of the exterior (physical) or interior (chemical) pressures (Fig. 21A). The process was fully reversible by treating PCN-128Y with trifluoroacetic acid in N,N-dimethylformamide (DMF) at elevated temperature (Fig. 21B). These results demonstrated a piezofluorochromic MOF, PCN-128W, an unprecedented example of reversible three-dimensional (3D) piezofluorochromic MOF.

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Figure 19 (A) Photographs of TABD-MOF-3-deposited paper strips upon addition of tetrahydrofuran (THF) solution of NTO at different concentrations under UV light. (B) Fluorescence spectra of TABD-MOF-3 in THF upon addition of NTO solution at different concentrations followed by addition of hexane. (C) Fluorescence enhancement efficiencies ((I  I0)/I0) obtained from different analytes by TABD-MOF-3. Excitation wavelength: 360 nm. Reproduced from Guo, Y.; Feng, X.; Han, T.; Wang, S.; Lin, Z. G.; Dong, Y. P.; Wang, B. J. Am. Chem. Soc. 2014, 136, 15485 with permission. Copyright 2014. American Chemical Society.

Figure 20 (Upper) (A) Construction of PCN-128W from the ETTC linker and Zr6 cluster. (B) Two types of 1D channels in PCN-128W, the hexagonal channel (purple pillar) and the triangular channel (red pillar). (Lower) (A) Synchrotron-based PXRD patterns of PCN-128W (blue) and PCN-128Y (red). (B) Comparison of ETTC linkers in PCN-128W (red) and PCN-128Y (green). Reproduced from Zhang, Q.; Su, J.; Feng, D.; Wei, Z.; Zou, X. D.; Zhou, H.-C. J. Am. Chem. Soc. 2015, 137, 10064 with permission. Copyright 2015 American Chemical Society.

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Figure 21 (A) Diffuse reflectance spectra of PCN-128W (dashed blue line), PCN-128Y (dashed green line), and H4ETTC (dashed red line) and fluorescent spectra of PCN-128W (solid blue line), PCN-128Y (solid green line), and H4ETTC (solid red line) at room temperature. (B) The color of PCN-128W and PCN-128Y under room light (left) and under UV light (right). Reproduced from Zhang, Q.; Su, J.; Feng, D.; Wei, Z.; Zou, X. D.; Zhou, H.-C. J. Am. Chem. Soc. 2015, 137, 10064 with permission. Copyright 2015 American Chemical Society.

8.05.4

Conclusion and Outlook

In this article, the cross-disciplinary research on host–guest supramolecular interaction and AIE-active compounds in the most recent 5 years is briefly summarized. Due to the fast development of these two flourishing research areas, some relevant work may be missed in this brief summary. Host entities are ubiquitous existing both in the natural world and synthetic chemistry. Therefore, the application of host–guest interactions to the AIE-concept, or on a more general platform, the application of supramolecular science to optoelectronic materials, still has huge space for growth. Proteins, for instance, often have hydrophobic pockets or domains to fulfill special biological functions. These domains can be considered as “naturally designed” hosts. Taking the advantage of the hydrophobic/hydrophilic interaction to detect and analyze proteins with fluorescent technique has been a significant research topic, and introducing AIE element into this topic has witnessed much progress. This work is summarized in some tutorial reviews.4,16,22,24 Therefore, this part has been excluded from this article. But for future research consideration, most of the AIE-active molecules have a propeller configuration and a bulky size, thus are unsuitable for analyzing some functional proteins and enzymes possess small pockets and functional domains. We cannot change the domain size and shape, thus we can only change our probes. It is a challenging work to design novel AIE-active fluorescent probes to match with these practical requirements. DNA can also serve as a host due to its major and minor grooves, which are hydrophobic domains with unique spatial, steric, dimensional, and base-pair sequence-based structures. Although the detection and analysis of DNA using AIE-active probes have gained great success in recent years, as summarized in a few recent reviews and book chapters,4,16,22,24 AIE-active probes aimed at a specific DNA sequence or a target site is still elusive. For synthetic hosts, to date, research only includes CDs, crown ethers, and pillar[n]arenes. Calixarenes, cucurbiturils, and cavitands, for example, are also common hosts that have received great research attention. The interaction of AIE-active molecules with these untouched hosts will lead to interesting results.

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