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Aggregation-Induced Emission and Light-Harvesting Function of Tetraphenylethene-based Tetracationic Dicyclophane Yawen Li, Yunhong Dong, Lin Cheng, Chunyan Qin, Hao Nian, Haiyang Zhang, Yang Yu, and Liping Cao J. Am. Chem. Soc., Just Accepted Manuscript • Publication Date (Web): 26 Apr 2019 Downloaded from http://pubs.acs.org on April 26, 2019

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Aggregation-Induced Emission and Light-Harvesting Function of Tetraphenylethene-based Tetracationic Dicyclophane Yawen Li,‡ Yunhong Dong,‡ Lin Cheng,‡ Chunyan Qin, Hao Nian, Haiyang Zhang, Yang Yu, and Liping Cao* Key Laboratory of Synthetic and Natural Functional Molecule Chemistry of the Ministry of Education, National Demonstration Center for Experimental Chemistry Education, College of Chemistry and Materials Science, Northwest University, Xi’an 710069, P. R. China. Supporting Information Placeholder ABSTRACT:

Here we report one-pot synthesis of tetraphenylethene-based tetracationic dicyclophane (1) and its self-assembly behaviors with aggregation-induced emission (AIE) and light-harvesting function. Confirmed by X-ray crystal structure and HR-TEM, this tetracationic dicyclophane can selfassemble into a 3D supramolecular framework to finally form crystalline nanospheres (2), which exhibits a strong emission (ΦF = 97.7%) via AIE effect in aqueous solution. Interestingly, AIE-active 2 as a single-molecule-based fluorescent supramolecular platform can encapsulate an organic dye (e.g. nile red) to further form light-harvesting nanospheres (3) with a large red-shift (Δλ = ~70 nm), highly efficient energy-transfer ability (ΦET = 77.5%), and high antenna effect (14.3).

Light, as an important source of food and energy, is essential to all life on the earth.1 Since ancient times, people started to utilize light from sun or fire for lightening, hunting, and cooking, which are partly companied with some kinds of energy transfers. Until now, scientists in chemistry, biology, and material science have paid increasing attention to construct photochemical/physical materials for highly efficient utilization of lights in varieties of applications, such as solar cells,2 white-light-emitting materials,3 bio-applications,4 and photodynamic/thermal therapy.5 For this purpose, some people focused on improving the photochemical/physical properties of optical materials to achieve broad color emission, strong intensity, high quantum yield, and long lifetime. Notably, Tang and co-workers discovered some propeller molecules are non-emissive or weakly emissive in solution but emit efficiently in the aggregated state, called aggregation-induced emission (AIE).6 On the other hand, some people preferred to mimic the light-harvesting function in photosynthesis process of plants or bacteria for improving the approach and efficiency of energy transfer from light to other forms of energy.7 Cyclophanes-type compounds as macrocyclic hosts are particularly intriguing in supramolecular chemistry.8 For example, Stoddart and co-workers have developed a class of cationic cyclophane—called “blue box”—and its derivatives, which exhibited excellent applications in supramolecular architectures, host-guest chemistry, catalysis, extraction and sequestration, and molecular electronics, etc.9 On the other hand, tetraphenylethene (TPE) as a typical AIE luminogen has been introduced to fabricate and construct fluorescent macrocycles or cages for supramolecular systems.10 Herein, we

designed and synthesized a tetraphenylethene-based tetracationic dicyclophane (1) bearing four cationic pyridinium units, one central TPE and two outer TPE moieties. Due to the AIE property of the TPE units, 1 can self-assemble into nanospheres (2) to achieve aggregation-induced emission with high quantum yield (ΦF = 97.7%) in aqueous solution. Although AIE can avoid the undesired aggregation-caused quenching (ACQ) effect of organic chromophores to enhance their emission property in an aqueous media, there is only few examples of AIE systems based on multiple-compound molecules for light-harvesting function.11 Therefore, AIE-active 2 as a single-molecule-based fluorescent supramolecular platform can further form light-harvesting nanospheres (3), when combined with nile red (NiR) via hydrophobic effect in aqueous solution.

Scheme 1. Synthesis of tetraphenylethene-based tetracationic dicyclophane 1 and schematic illustration of AIE and light-harvesting function.

As shown in Scheme 1, 1 with PF6- as counterions was synthesized from tetrapyridyl TPE 4 as central cross linker with bis(bromomethyl) TPE 5 in the molar ratio of 1:2 via SN2 reaction in 15.3% yield (Figures S11-S12). 1 was characterized by electrospray ionization time-of-flight mass spectrometry (ESI-TOF-MS) as well as 1H and 13C NMR spectroscopy, which provided strong evidence for the molecular formula and timeaveraged D2h symmetry of 1.

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Figure 1. (a) Fluorescence spectra and (b) the 1931 CIE chromaticity coordinate changes of 1 (20 μM) in different solvents (1% MeCN). Inset: Fluorescence images of 1 (20 μM) under UV light (365 nm) in different solvents [H2O (A), THF (B), Acetone (C), DCM (D), MeOH (E), CHCl3 (F), MeCN (G), DMSO (H)]; (c) Fluorescence spectra and (d) plot of maximum emission intensity of 1 (20 μM) versus water fraction in MeCN-water mixture. Inset: fluorescence images of 1 (20 μM) in MeCN (left) and in water (right) under UV light (365 nm). λex = 410 nm, Ex/Em slit = 3 nm.

Due to the AIE property of the TPE units, UV/vis and fluorescence spectra of 1 showed different absorption, emission intensity, and fluorescent color in different solvent systems (Figures 1a-b and S13). The solution of 1 showed a weak orange fluorescence with ΦF value of only 19.7% centered at 595 nm in MeCN, which is a good solvent, and a strong yellow fluorescence with ΦF value of 97.7% centered at 580 nm in water, which is a poor solvent (Figures 1a and S14, and Table S1). When the water fraction was below 80% in MeCN-water mixture, the fluorescent intensities were gently increased with low ΦF values of 21.2%-42.6% (Figures 1c-d and S15, and Table S2). However, upon further increment to 90~99%, the fluorescent intensities were abruptly increased (Figure 1d). The fluorescence lifetime of 1 in water is longer than that in MeCN (Figure S17 and Table S4). Furthermore, temperature-dependent fluorescence experiment showed that the fluorescence intensity of 1 increased linearly as temperature gradually decreased from 60 oC to 5 oC, indicating the aggregation of AIE-active 1 at lower temperature (Figures S18-S19).12 And, 1 in MeCN has a low critical aggregation concentration of 13.3 μM (Figure S20). To better understand the luminescence properties of 1, we also synthesized two template compounds T1 and T2 (see the Supporting Information). Based on the results of UV/vis and fluorescence experiments, the luminescence unit of 1 is mainly the central TPE unit at the core of dicyclic structure (Figures S22-23). Compared with T1, 1 has approximate fluorescence lifetimes but higher quantum yield (Figure S24 and Tables S3-S4), because dicyclic structure restricts free rotating of the central TPE unit at the excited state.10c To explore the relationship between emission property and aggregation behavior in the AIE process, we performed scanning electron microscopy (SEM) and dynamic light scattering (DLS) for investigating self-assembly of 1 in different solvents. Compared with irregular morphology of 1 in other solvents, SEM images obtained from a solution of 1 (20 μM and 0.20 mM) in MeCN or H2O (1%/10% MeCN) showed nanospheres (2) with diameters ranging from 25 nm to 77 nm (Figures 2a and S26-S31). The results of DLS experiments showed that the average hydrodynamic diameter (DH) of 1 (0.20 mM) were ~30 nm (ΦF = 19.7%), ~60 nm (ΦF = 96.7 %),

Figure 2. SEM images of (a) 2 ([1] = 20 μM, in H2O with 10% MeCN) and (b) 3 ([1] = 20 μM and [NiR] = 90 μM in H2O with 10% MeCN). (c) DLS profiles of 1 in MeCN (black), H2O (1% MeCN) (red), H2O (10% MeCN) (blue), and 1+NiR (4.5 eq.) in H2O (10% MeCN) (purple) ([1] = 0.20 mM). (d) Schematic illustration of FRET process in 3. HR-TEM images of (e) 2 and (f) 3. Inset: SAED pattern and full view of nanospherical crystals.

~80 nm (ΦF = 97.7 %) in MeCN including 0%, 90% and 99% H2O, respectively (Figure 2c). It confirmed that H2O as a poor solvent can increase DH of 1, indicating the aggregation behavior of 1 induces the enhancement of fluorescence intensity. High-resolution transmission electron microscope (HR-TEM) images of 2 showed crossed lattice fringes with average distance of (3.32±0.02) Å, indicating the highly ordered arrangement of 1 in 2. Selected area electron diffraction (SAED) pattern acquired from the nanospheres showed high hexagonal order and confirmed the crystalline nature of 2 (Figures 2e and S32).

Figure 3. X-ray crystal structure of 1: (a) Single molecule, (b) side view of the 1D dual nanotube from the b axis, (c) side view of 2D nanotubular layer from the b axis, and (d) top view of the 3D supramolecular framework from the a axis. Here, each 2D layers are colored by different colors, respectively.

We obtained X-ray-quality single crystals of 1 by slow vapour diffusion of diethyl ether into an MeCN/acetone solution of 1 at room temperature.13 1 possesses dual cavities formed mainly by twelve pyridinium/benzene rings (Figure 3a). In each cavity, the distances between the centers of two pyridinium rings range from 7.56 Å to 7.66 Å. Interestingly, a 3D supramolecular framework stacked by 1 molecule is revealed. Firstly, oblique 1 molecule are aligned parallelly to

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Journal of the American Chemical Society form a 1D dual nanotube, in which each 1 molecule are in register to one another to extend through a lattice translation (~14.4 Å repeat) along the a axis (Figure 3b). Secondly, neighboring 1D dual nanotubes with an opposite angle form a 2D nanotubular layer in the zigzag alternate pattern along the c axis (Figure 3c). Close inspection of this 2D nanotubular layer, two neighboring 1D dual nanotubes contact with each other with an angle of ~115o through CH···π (d = ~2.52 Å) and π···π interactions (d = ~3.52 Å) between one benzene ring of TPE unit as guest and one cavity of neighboring 1 as host (Figure S34). Finally, neighboring 2D nanotubular layers are stacked parallelly to form a 3D supramolecular framework (Figure 3d).

Figure 4. (a) Emission and absorption spectra of 1 and NiR, respectively. (b) Absorption spectra, (c) fluorescence spectra, and (d) the 1931 CIE chromaticity coordinate changes of 1 (20 μM) titrated by NiR (0-4.5 eq.). Inset: Photographs of 1 (left), 1+NiR (middle) and NiR (right) under sun light or upon UV irradiation (365 nm). λex = 410 nm, Ex/Em slit = 3 nm, solution: H2O/MeCN = 9/1 (v/v).

Given supramolecular framework with cavities/interspaces in 2, organic dye molecules could be dispersed in nanospheres to avoid self-aggregation and be located at a proper distance with donor to achieve energy transfer (Figure 2d). To investigate the feasibility of such an energy transfer, we chose nile red (NiR) as the acceptor, the absorption band of which matches well with the AIE-based fluorescence emission of 2 formed from 1 in H2O (10% MeCN), which is favorable for the Förster resonance energy transfer (FRET) process (Figure 4a). To add NiR into 2 in H2O (10% MeCN), the cationic and hydrophobic cavities/interspaces of supramolecular framework of 2 encapsulated NiR molecules to give nanospheres (3) with only an increase of average DH (~140 nm) (Figures 2b-d). Compared with 2, 3 exhibited spherical shape with larger diameters ranging from 35 nm to 83 nm (Figure 2b). HR-TEM images of nanospherical crystals of 3 also revealed hexagonal order of SAED pattern and periodic arrangement with crossed lattice fringes with average distance of (6.38±0.02) Å (Figures 2f and S33), which is belonged to a dspacing of stacking distances of NiR molecules (Figure S35), indicating that the interspaces within 2 may be almost filled with NiR via a highly ordered co-assembly to finally form 3. Therefore, nanospherical crystals 2, similar to nonporous adaptive (nano)crystals (Figure S36),14 could be inserted by NiR molecules into the order arrangement in between 1 molecules to form nanospherical crystals 3. The zeta-potential experiments showed that 3 (34.1±0.6 mV) has a higher positive zeta potential than 2 (16.5±0.7 mV), indicating 3 has higher stability due to higher repulsive forces among these nanospheres (Figures S37-S38).11c Additionally, the local molecular arrangements for 2 and 3 were further confirmed using powder X-ray diffraction (PXDR) measurements (Figure

S35). The d-spacings corresponding to the intermolecular stacking is observed at 0.46 and 0.39 nm, in agreement with the π-π stacking distance between benzene rings of 1 in 2.15 Compared with d-spacings of stacking distances in alone NiR (0.69 nm) and 2, 3 exhibited a series of shorter d-spacings of 0.66 nm for the intermolecular stacking of NiR and 0.30~0.37 nm for the stacking of 1, respectively, indicating that ordered co-assembly of 2 and NiR in 3 enable donor and acceptor to close each other for achieving FRET. As showed in Figure 4b, both UV/vis and 1H NMR titrations showed that the absorption and chemical shifts of 1 stayed nearly constant while only the absorption intensity and the chemical integrals of NiR increased gradually upon addtion of NiR, which suggested the absence of any ground-state interactions between 1 and NiR molecules (Figure S41).11 Moreover, fluorescence titration of a solution of 1 in H2O (10% MeCN) showed that the emission intensity of 1 as donor at 580 nm gradually decreased while the emission intensity of NiR as acceptor at 650 nm gradually increased, accompaning with a fluorescence color change from yellow to red under UV light (Figure 4c). The CIE chromaticity diagram also confirmed a linear track of the fluorescence color change, followed by the titration of NiR (Figure 4d). The energy-transfer efficiency (ΦET) and the antenna effect of this fluorescent supramolecular system were calculated as 77.5% and 14.3 at a donor/acceptor ratio of 1:4.5 in H2O (10% MeCN), respectively (Figure S42). In control experiment, free NiR was non-emitted upon excitation at 410 nm, which ruled out the possibility for direct excitation of the acceptor (Figure S43). In summary, we have synthesized a tetraphenylethenebased tetracationic dicyclophane with an excellent aggregation-induced emission (ΦF = 97.7%) when it selfassembles into crystalline nanospheres in aqueous solution. These AIE-active nanospheres with well-defined supramolecular framework structure can be loaded by organic dyes (e.g. nile red) to achieve a FRET (ΦET = 77.5%) with a high antenna effect (14.3) in aqueous environment. This AIE and light-harvesting supramolecular system as fluorescent nanomaterials has potential applications for cancer cell imaging, and even cancer diagnosis/photodynamic therapy in near future. Therefore, this water-compatible supramolecular system shows a simple approach to address a challenge how to design and fabricate highly-efficient energy transfer system in biological systems.

ASSOCIATED CONTENT Supporting Information The Supporting Information is available free of charge on the ACS Publications website. The crystal of 1 (CIF) Synthetic and experimental details, NMR, UV/vis, fluorescence, DLS, SEM, TEM, PXRD, and BET data (PDF)

AUTHOR INFORMATION Corresponding Author *[email protected]

Author Contributions ‡These authors contributed equally.

Notes The authors declare no competing financial interests.

ACKNOWLEDGMENT

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We thank the National Natural Science Foundation of China (21771145 and 21472149). L.C. thanks Prof. Kaiqiang Liu (Shaanxi Normal University) for performing HR-TEM experiments.

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