Z Isomerization, and

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Facile Synthesis, Macroscopic Separation, E/Z Isomerization, and Distinct AIE properties of Pure Stereoisomers of an Oxetane-Substituted Tetraphenylethene Luminogen Xiaofeng Fang, Yu-Mo Zhang, Kaiwen Chang, Zhihe Liu, Xing Su, Haobin Chen, Sean Xiao-An Zhang, Yifei Liu, and Changfeng Wu Chem. Mater., Just Accepted Manuscript • DOI: 10.1021/acs.chemmater.6b02746 • Publication Date (Web): 06 Sep 2016 Downloaded from http://pubs.acs.org on September 7, 2016

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Chemistry of Materials

Xiaofeng Fang †, Yu-Mo Zhang ‡, Kaiwen Chang †, Zhihe Liu †, Xing Su ‡, Haobin Chen †, Sean XiaoAn Zhang ‡, Yifei Liu *‡ and Changfeng Wu *† †

State Key Laboratory on Integrated Optoelectronics, College of Electronic Science and Engineering, Jilin University,

Changchun, 130012, P. R. China. Email： [email protected] ‡

College of Chemistry, Jilin University, Changchun, 130012, P. R. China.

Email： [email protected] ABSTRACT: Control of stereochemistry plays a key role in medicinal chemistry, material and life science. As a prominent AIE luminogen, tetraphenylethene (TPE) derivatives have E/Z isomers which are challenging to separate even by HPLC. Herein, we designed oxetane-substituted TPE (TPE-2OXE) and separated pure isomers by simple column chromatography with high yields, as confirmed by mass spectrometry, IR and NMR spectroscopy. The isomerization of the two isomers can occur by photo- and thermo-activation. Importantly, Z-TPE-2OXE isomer solid shows bathochromic emission with a quantum yield 5 times higher than that of E-TPE-2OXE. The differences in emission wavelength and quantum yield are derived from distinct emission mechanisms of locally excited (LE) state emission of E-TPE-2OXE and charge transfer (CT) state emission of Z-TPE-2OXE. The two isomers are also good piezochromic luminescent materials, which have not only obvious emission color shift, but also significant enhanced luminescence brightness by external force. Besides, E-TPE-2OXE solids show self-healing ability, which can crystallize spontaneously from ground amorphous state. The higher brightness of E-TPE-2OXE can be retained in solution, so fluorescent AIE nanodots are prepared from the two isomers. Cell labeling experiments also show that Z-TPE-2OXE AIE dots have higher labelling brightness as compared to the E-TPE-2OXE isomer. The synthesis and distinct properties of E/Z isomers are beneficial to further development of new TPE derivatives for various applications.

Cis-trans isomerization is an important yet interesting phenomenon in chemistry, especially in medicinal applications. Isomers that have the same molecular formula but different geometrical positions in space can show entirely different properties.1,2 For example, dichlorodiamineplatinum(II) in the cis form is an efficient anti-cancer drug, the trans form is however noneffective.3,4 By controlling the cis-trans isomerization, a variety of applications can be achieved. For example, azobenzene and derivatives, as the most classical smart molecules with light responsive cistrans isomerization,5,6 have been demonstrated for widespread applications such as light-driven plasticmotor,7 reversible liquid crystalline elastomers,8 photomodulation and data storage. Anthradithiophenes (ADT) are utilized as key building blocks in organic field-effect transistors. Due to the different geometrical position of

sulphur atoms in space, the anti-isomer showed much higher charge mobility, thus improved device performance as compared to the syn-isomer.9,10 Because of these attractive properties and important applications, synthesis and separation techniques that can lead to a pure isomer, as well as the techniques that can control the reversible isomerization are highly intriguing. Tetraphenylethene (TPE) and its derivatives (usually E/Z isomers mixture) have attracted a great deal of attention on account of the aggregation-induced emission (AIE) effect.11 The AIE luminogens have overcome the aggregation caused quenching problem in many organic dyes.12 As efficient light emitters in solid state and aggregate state, TPE derivatives have shown applications for bioimaging,13 biosensing,14-16 mechanochromophores17-19 and optoelectronic devices.20-24 Especially, fluorescent AIE nanodots exhibit tunable emissions, high brightness,

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and good biocompatibility, which are promising for biological applications.25-29 TPE and its derivatives are usually synthesized by the McMurry coupling reaction. Although the reaction is high-efficient and low-cost, the products are usually mixtures of E/Z isomers due to the unsymmetrical coupling.30 Separation of the TPE E/Z isomers is highly challenging even by HPLC. 31-32 Tang et al. first reported the synthesis and macroscopic separation of TPE E/Z isomers by attaching polar triazole and bulky alkyl chains to enlarge the shape and polarity difference.33 In a further study, electron donating (D) and accepting (A) groups were incorporated to facilitate the E/Z isomers separation by the D-A interactions.34 The separated isomers show different mechanochromic properties and distinct biological responses to nucleic acid and target enzymes.35-41 However, the introduction of bulky and polar groups usually involves complex synthesis. In addition, the bulky substituents in TPE are not optically active, thus reduce the per-particle absorption cross-sections for AIE nanodots. The bulky substituents can also affect the transport of charge carriers in optoelectronic devices. These issues highlight the importance of synthesizing new and separable E/Z isomers of TPE with small substituted groups. We are interested in a four-member ring oxetane group as it can be photo-crosslinked by cationic ring opening polymerization to yield an insoluble film after deposition and curing. 42 The cross-linked film can prevent interface mixing and erosion during deposition of the subsequent layers. Oxetane substituted organic semiconductors have been used for fabricating multilayer OLEDs by solvent-based fabrication technique, which is more desirable over the expensive vacuum-based deposition techniques.43,44 In this work, we synthesized small group oxetane substituted TPE (TPE-2OXE), demonstrated the E/Z isomerization and distinct AIE properties of pure isomers. Pure E and Z isomers were easily separated with high yields by the macroscopic technique of column chromatography. The Z-TPE-2OXE solid exhibits a fluorescence quantum yield that is 5-fold higher than that of the E isomer. To the best of our knowledge, there is no report of separable TPE E/Z isomers with so significant difference in quantum yield. The Z-TPE-2OXE AIE nanodots also show higher cellular labeling brightness than that of the E isomer. However, the E isomer showed selfhealing properties in morphology and optical spectra because of the close molecular packing and strong intermolecular interactions in the E isomer solid as compared to the Z isomer. This study provides an ideal system to investigate the E/Z isomerization of the TPE model isomers and their distinct properties, which may promote further development of functional TPE E/Z materials by rational molecular design. Materials. Acetone, acetonitrile, dichloromethane （DCM）, THF and K2CO3 were purchased from Beijing Chemical Plant. Zn dust and TiCl 4 were all purchased from Aladdin. Methyl iodide, ethyl iodide, sodium iodide and poly (styrene-co-maleic anhydride) (PSMA, cumene terminated, average MW ≈ 1700, styrene content 68%) were purchased from Sigma-aldrich. All the above chemicals were of analytical grade and used as received without further purification. Tetrahydrofuran (THF) was distilled

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in presence of sodium benzophenone under protection of dry nitrogen prior to use. Synthesis of TPE-2OH. TPE-2OH was synthesized according to the McMurry reaction. 30 TiCl4 (2.8 mL, 25 mmol) were added to anhydrous THF (50 mL) under nitrogen atmosphere at 0oC. Then Zn dust (3.45 g, 50 mmol) were added to the mixture immediately. The black suspension was refluxed for 2 h. 4-Hydroxybenzophenone (1.98 g, 10 mmol) was added into the above mixture under nitrogen atmosphere and then refluxed for 12h. The reaction was cooled to room temperature and quenched with aqueous K2CO3 (20 mL, 10%). The organic layer was separated and the aqueous suspension was extracted with dichloromethane (3×50 mL). The organic phase was dried with anhydrous Na 2SO4. The crude products were purified by recrystallization to get crystals with a yield about 74.8%. δH (500 MHz, DMSO) 9.28 (2 H, d, J 14.0), 7.09 (6 H, tt, J 13.2, 8.3), 7.00 – 6.88 (4 H, m), 6.72 (4 H, dd, J 18.3, 8.6), 6.50 (4 H, dd, J 21.4, 8.6) (Figure S1). Synthesis of OXE-OTs. p-Toluenesulfonyl chloride (11.44 g, 15 mmol) were dissolved into the 5 ml anhydrous DCM and 5 ml anhydrous Py mixture. 3-Methyl-3-oxetanemethanol (3.96 ml, 10 mmol) were dropwise added at 0 oC under nitrogen atmosphere. The reaction mixture was allowed to warm to room temperature with stirring for 12 hours. The reaction was quenched with saturated NaHCO3 aqueous solution. The mixture was extracted with dichloromethane (3×50 mL) and the combined organic layers were washed with a saturated brine solution and dried over anhydrous Na2SO4. The solvent was evaporated, large amounts of white solid were generated after adding hexane under ultrasound with a yield about 100%. δ H (500 MHz, CDCl3) 7.81 (2 H, d, J 8.2), 7.37 (2 H, d, J 8.1), 4.35 (4 H, q, J 6.2), 4.12 (2 H, s), 2.46 (3 H, s), 1.31 (3 H, s) (Figure S2). Synthesis of OXE-I. A solution of OXE-OTs (1.0 g, 3.9 mmol) and sodium iodide (1.46 g, 9.8 mmol) in acetone (40 mL) was refluxed for 3 h.45 The solvent was removed in vacuo, and the residue was dissolved in Et2O and water. The organic layer was washed with brine, dried over Na2SO4. The solvent was evaporated in vacuo to give 3 (0.65 g, 78.6%) as a yellowish oil. δ H (300 MHz, CDCl3) 4.38 (4 H, dd, J 13.6, 6.1), 3.50 (2 H, s), 1.44 (3 H, s) (Figure S3). Synthesis of TPE-2OXE. TPE-2OH (0.87 g, 1eq) and K2CO3 (1.98 g, 6eq) were added into a 100 ml round-bottom flask with 20 ml acetonitrile. After refluxing 30 minutes, OXE-I (1.52 g, 3eq) were added and then refluxed for another 12 h. The reaction was finished until TPE-2OH was completely consumed, as monitored by TLC. The resulting mixture were cooled to room temperature and filtered to obtain the solids. The solution were added into 100 ml DCM and washed with water. The organic layer was dried with anhydrous Na2SO4. After filtration and solvent evaporation, the crude products were purified by column chromatography using ethyl acetate/ hexane mixture (1:4 by volume ratio) as the eluent. The E-TPE-2OXE white solids were obtained 0.49 g in a yield of 38.7%. δH (500 MHz, CDCl3) 7.11 (6 H, dd, J 8.8, 7.0), 7.07 – 7.03 (4 H, m), 6.92 (4 H, d, J 8.7), 6.65 (4 H, d, J 8.7), 4.59 (4 H, d, J 5.9), 4.42 (4 H, d, J 5.9), 3.94 (4 H, s), 1.40 (6 H, s) (Figure S4). δC (126 MHz, CDCl3) 157.43, 144.25, 139.68, 136.70, 132.59, 131.39, 127.73, 126.26, 113.66, 79.84, 77.30, 77.05, 76.79, 72.61, 39.67, 21.33 (Figure S5).


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MS (MALDI-TOF): m/z calcd. [M+H] 533.26 found 533.44 (Figure 1b). The Z-TPE-2OXE white solids were obtained 0.51g) in a yield of 45.3%. δH (500 MHz, CDCl3) 7.10 – 7.06 (6 H, m), 7.02 – 6.98 (4 H, m), 6.96 (4 H, d, J 8.7), 6.69 (4 H, d, J 8.7), 4.60 (4 H, d, J 5.9), 4.44 (4 H, d, J 5.9), 3.97 (4 H, s), 1.42 (6 H, s) (Figure S6). δC (126 MHz, CDCl3) 157.45, 144.18, 139.70, 136.77, 132.57, 131.41, 127.60, 126.25, 113.79, 79.85, 77.31, 77.05, 76.80, 72.66, 39.70, 21.34 (Figure S7). MS (MALDI-TOF): m/z calcd. [M+H] 533.26 found 533.49 (Figure 1b). Synthesis of TPE-4OXE. TPE-4OH was synthesized according to the literature.46 TPE-4OH (0.2 g, 1eq) and K2CO3 (0.85 g, 12eq) were added into a 100 ml round-bottom flask with 20 ml acetonitrile. After refluxing 30 minutes, OXE-I (0.65 g, 6eq) were added and then refluxed for another 12 h. The resulting mixture were cooled to room temperature and filtered out the solids. The solution were added into 100 ml dichloromethane and washed with water (3×50 ml). The organic layer was dried with anhydrous Na2SO4. After filtration and solvent evaporation, the crude products were purified by column chromatography using ethyl acetate/ hexane mixture (1:2 by volume ratio) as the eluent. The TPE-4OXE white solids were obtained and recrystallized in DCM (0.17 g) in a yield of 45.9%. 1H NMR (500 MHz, CDCl3) δ (ppm): 6.95 (d, J=8.6, 8H), 6.68 (d, J=8.6, 8H), 4.60 (d, J=5.9, 8H), 4.43 (d, J=5.9, 8H), 3.97 (s, 8H), 1.42 (s, 12H). 13C NMR (126 MHz, CDCl3) δ (ppm): 157.33, 138.42, 137.18, 132.59, 113.76, 79.83, 72.68, 39.70, 21.32. MS (MALDI-TOF): m/z calcd. [M]+ 732.37 found 732.62. Preparation of AIE dots. The TPE isomer was dissolved in THF to make the stock solution Ⅰ (1000 μg/ml). Using the similar methods make the PSMA stock solution Ⅱ (1000 μg/ml). Then, the two stock solutions were mixed with equal volume to produce a solution mixture. 1 mL of the solution mixture was quickly added to 5 mL MilliQ water under sonication. After the THF was completely removed by nitrogen blowing, the AIE dots were filtered by 0.22 μm filter. Photo-crosslinking in Z-TPE-2OXE thin films and AIE dots. Three letters “JLU” were patterned on a substrate of filter paper by using the solution of Z-TPE-2OXE in dichloromethane, then heated on a hot plate to remove the solvent. Next, the filter paper was immersed in a water solution of photoinitiator (triarylsulfonium hexafluorophosphate salts, 3 wt %), and irradiated with a UV lamp (high-pressure mercury lamp, 250 W) for 30 s at room temperature. The filter paper was repeatedly cleaned by acetone and dichloromethane (DCM) and immersed in the chloroform for 5 minutes. For the photocrosslinking in AIE dots, the Z-TPE-2OXE dots were prepared by mixing the solution of Z-TPE-2OXE in THF with water. In presence of photoinitiator in the solution, the AIE dots were irradiated by UV-light for 20 seconds. Next, aceton was added into the water under sonication to examine the stability of the AIE dots. Cell Culture. MCF-7 cells were ordered from American Type Culture Collection (ATCC, Manassas, VA, USA). Cells were cultured in an atmosphere of 5% CO2 at 37 oC, by using Dulbecco’s modified Eagle Medium (DMEM) with 10% Fetal Bovine Serum (FBS), 50 U/mL penicillin and 50 μg/mL streptomycin.

Cytotoxicity Studies. The cytotoxicity of E-dots and Z-dot were assessed by MTT assay. MCF-7 cells were seeded in a 96-well plate (1×104 cells/well) and cultured in 100 μL growth medium at 37 oC and 5% CO2 for 24 h. Cell culture medium in each well was then replaced by 100 μL cell growth medium containing AIE dots at a series of concentrations ranging (0 μg/ml, 5 μg/ml, 10 μg/ml, 20 μg/ml, 40 μg/ml, 80 μg/ml). After incubation for 6 h, 20 μL of MTT (5 mg/mL in PBS solution) was added to each well and cells were incubated further for 4 h at 37 oC. The growth medium was removed gently and 150 μL of DMSO was then added to each well to dissolve the formazan crystals completely. The absorbance at the wavelength of 570 nm was measured by a Bio-Tek Cytation 3 cell muti-mode reader, and each data point represents a mean±SD from triplicate wells. Analysis of variance (ANOVA) and Student’s t-tests were used for MTT assays and statistical significance was reported for p< 0.05. Fluorescence Imaging. For cell fluorescence imaging, 1 × 105 MCF-7 cells were plated onto a 6-well plate and allowed to grow overnight. E-dots and Z-dots dispersions were added to the cell culture media at the same weight concentration of ∼40 μg/ml and allowed to incubate for 4 h. The cells were then washed with warm PBS buffer before viewing on a fluorescence microscope. Instruments. 1H NMR spectra were recorded on a Varian300 EX spectrometer using CDCl3 or DMSO-d6 as solvent and tetramethylsilane (TMS) as an internal standard (d= 0.00 ppm). 13C NMR spectra were recorded on a Bruker Avance 500 MHz spectrometer using CDCl3 as solvent and CDCl3 as an internal standard (d = 77.00 ppm). UV-Vis spectra were measured on a Shimadzu UV-2550 spectrophotometer. Photoluminescence (PL) spectra were measured with a RF-5301PC spectrofluorometer. Powder XRD patterns were obtained from a PANalytical B.V.Empyrean X-ray diffractomer with Cu-Ka radiation (l = 1.5418 Å) at 25 oC (scan range: 4–50 u). The fluorescence lifetime and PL quantum yields measurements were performed on an integrating sphere, with a 280 nm Edinburgh Instruments Ltd light emitting diode as the excitation source. Instrument model (FLS920). The differential scanning calorimeter (DSC) analysis was determined using a NETZSCH (DSC-204) instrument at 10 oC/ min under nitrogen flush. MTT are tested in a Bio-Tek Cytation 3 plate reader, and each data point represents a mean ± SD from triplicate wells. Fluorescence imaging was performed on an inverted fluorescence microscope (Olympus IX71) with a 0.95 NA UPLSAPO 20× objective. The excitation light was provided with a Mercury Lamp, and filtered by a band-pass filter (Semrock FF01−377/45). Fluorescence signal was filtered by a long-pass filter (Semrock FF01−460/60), and imaged on an Andor iXon3 frame-transfer EMCCD (Andor Technology, UK). Design, Synthesis, and Characterization of TPE-2OXE. While the McMurry coupling reaction is versatile and powerful for TPE synthesis, the products are usually mixtures of E/Z isomers. Oxetane is a four-member heterocyclic structure, which has a rigid space volume as compared to linear structures. We expect the simple oxetane modification can enlarge the difference of two isomers to realize macroscopic separation of the E/Z isomers. The E/Z isomers were synthesized by one-pot nucleophilic



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substitution reaction between TPE-2OH and oxetane substituted by iodine (Figure 1a). During the synthesis, two new compounds with ref value 0.33 and 0.21 were identified in thin layer chromatography (TLC) using ethyl acetate and hexane mixture (1: 2 by volume ratio) as the developing agent (Figure 1c). The products can be easily separated by simple column chromatography. Surprisingly, the two compounds have the same molecular weight that is consistent with the target molecule, as evidenced by mass spectrometry (Figure 1b). This observation suggests the two compounds are E/Z isomers of TPE-2OXE. Further characterizations by NMR spectroscopy and IR spectroscopy indicate the new two compounds are indeed E/Z isomers of the TPE-2OXE molecule (Figure S4-S8). For comparison, we also synthesized three TPE molecules with small substituted groups. It is worth noting that the E/Z isomers of TPE-2OH, TPE-2OMe and TPE-2OEt have uniform ref value and cannot be separated in TLC (Figure 1c and S9-S10). Therefore, the oxetane substitution is vitally important for the separation of the related isomers. On basis of the polarity, we infer the compound with smaller ref value (Ref=0.21) as the Z-TPE-2OXE isomer and the other one (Ref=0.33) as the E-TPE-2OXE isomer.

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ance at δ=6.91 and 6.93 ppm are H7, H8, H20 and H21, δ=7.04 and 7.06 ppm are H9, H10, H22 and H23. As seen in Figure 2a, the cross-peaks in the blue circles indicate clear correlations between H9 / H10 / H22 / H23 and H7 / H8 /H20 /H21, which indicated that ring A and ring C are close in space, that is, on the same side of the central double bond. However, there are no correlations for those hydrogens in COSY NMR spectroscopy (Figure S12-S13). At the same time, we cannot find clear correlations between H9 / H10 / H22 / H23 and H7 / H8 /H20 /H21 in Figure 2b; because ring A/C are far from ring B/D in Z isomer (Figure S14-S16). Taken together, the NOESY and COSY NMR spectra confirmed the geometrical structures of the E/Z isomers of TPE-2OXE molecule.

Figure 2. NOESY-NMR of (a) E-TPE-2OXE and (b) Z-TPE2OXE (enlarged).

Figure 1. (a) Synthesis routes of E / Z-TPE-2OXE and fluorescent pictures of two isomers solid. (b) The mass spectrometry of two isomers. (c) The structures of TPE-2OMe and TPE-2OEt, the TLC picture of four TPE derivates.

The geometry structures of E/Z isomers were further confirmed by NOESY and COSY NMR spectroscopy, which are widely applicable to those structures that cannot easily grow into single crystals. 47 For the E-TPE2OXE isomer, the resonance at δ=1.40 ppm corresponds to the H1 and H14 protons, and the resonance at δ=3.94 ppm is due to the H4 and H17 protons. Follow this assignment and the clear correlations in the NOESY spectra (Figure S11), the protons with δ=4.42 and 4.43 ppm are attributed to H2 and H15, respectively, because of their strong correlation with H1 and H14; δ=4.58 and 4.60 ppm are H3 and H16, because of their strong correlation with H2, H4, H15 and H17. The resonance at δ= 6.64 ppm and 6.66 ppm are assigned to H5, H6, H18 and H19 because of their strong correlation with H4 and H17. The reson-

AIE and photo-crosslinking properties. The AIE properties were examined in a water-THF mixture with different water fractions. The spectra were measured immediately after the samples were prepared. As shown in Figure 3a, there is no emission as the amount of water in the aqueous mixture reaches a volume fraction 80%. When water fraction is increased to 90%, the fluorescence is turned on, showing the maximum emission wavelength at 478 nm. As indicated in the UV-Vis absorption spectra (Figure S17a), the spectral tails implied the formation of nanosized aggregates when water fraction is up to 90%. Similar AIE effect was observed for the Z isomer. The formation of nano-sized aggregates was also confirmed by dynamic light scattering (DLS) measurements (Figure S17), the aggregates sizes of two isomers are approximately equal at the same water/THF fractions. The E/Z isomers show similar absorption spectra. However, the fluorescence intensity of Z-TPE-2OXE is significantly higher than that of E- TPE-2OXE with the same conditions (Figure 3). The photo-crosslinking properties were demonstrated in thin films and AIE dots using Z-TPE-2OXE isomer. After photocrosslinking, the Z-TPE-2OXE molecules were polymerized and formed intermolecular network on the filter paper by covalent bonds. Although the filter paper was repeatedly cleaned by acetone and DCM, the “JLU” fluorescence pattern was clearly visible on the filter paper, while the one without photocrosslinking vanished (Figure S18a-b). This property indicates the oxetane group has potential applications for the fabrication of thin film photonic devices by solvent-based methods. In addition, the photo-crosslinking is also of benefit to


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form stable AIE dots. As shown in Figure S18c, the photocrosslinked AIE dots show stable and strong fluorescence even in presence of acetone solvent. The newly generated covalent framework by photo-crosslinking restricted the intramolecular motion and decreased the solubility of the TPE molecule in acetone, so the fluorescence is preserved. However, the AIE dots without photocrosslinking were destroyed and the fluorescence disappeared after addition of the solvent. These results demonstrate the importance of oxetane group for forming stable AIE dots for further applications.

Figure 4. 1H NMR spectra of (a) E-TPE-2OXE, (b) Z-TPE-2OXE exposed to UV lamp (365 nm, 0.6 mW/cm2) for 10 min in CDCl3, (c) Z-TPE-2OXE heated at 200 oC for 15 min, (d) Z-TPE-2OXE.

Figure 3. Fluorescent spectra of (a) E-TPE-2OXE and (b) Z-TPE-2OXE (1×10-5 M， λex= 320 nm) in THF and water mixtures with different water fractions (inset: fluorescent images with different water fractions).

Photo- and thermo-activated conformation changes. Photoisomerization is an important phenomenon that has been used for molecular devices, such as molecular switches, molecular motors, and molecular electronics.7,8 We explore the photo-induced E/Z isomerization (EZI) processes of the pure TPE-2OXE isomers by NMR spectroscopy (Figure 4a and 4d). The chloroform-d solution of the Z isomer was irradiated by a UV lamp (365 nm, 0.6 mW/cm2) for 10 minutes. The resonance peaks appeared in the high-field regions, which are consistent with the resonance peaks of the E-TPE-2OXE (Figure 4d). This indicates that UV irradiation readily converts the Z isomer to the E isomer. Similarly, because the E isomer and Z isomer have nearly identical UV-Vis absorption, the EZI also occurred in the E isomer under the same condition (Figure S19). The conversion ratio under the same light illumination can be calculated by the 1H NMR spectra (Figure S19e). It is worth noting that the E to Z isomerization rate is lower than the Z to E process, indicating higher energy barrier of isomerization from the E isomer as compared to the Z isomer. It is known that EZI process can also be induced by thermal activation. We found the EZI process from the Z isomer occurred at 200 oC, which is about 30 oC higher than the melting temperature of the Z isomer (Figure 4c). However, no change was observed in the NMR spectrum when the E isomer was heated to 200 oC. Instead, when the temperature was increased to about 220 oC, the EZI process was clearly observed by 1H NMR spectrum (Figure S19c). The high isomerization temperature for the E isomer further indicates a lower energy state for the E isomer as compared to the Z isomer. The photo- and thermo-activated isomerization of the TPE-2OXE molecule holds potential for their applications as stimuli-responsive sensors and actuators.

Photophysical properties of the TPE isomer solids. We examined the mechanofluorochromic properties of the E/Z isomer solids. TPE and derivatives possess propeller-like shapes. This distinct structural feature leads to the fact the packing structure as well as the optical properties are susceptible to external stimulus such as mechanical force, heating and solvent vapor.48,49 The E isomer is a white solid with a blue emission (439 nm) at the pristine state, the emission wavelength is red-shifted to 475 nm by a complete grinding (Figure 5a). After grinding, the diffraction peaks disappeared or significantly decreased in the XRD profile, indicating that the mechanochromic properties are indeed associated with the morphology transformations between the crystalline and amorphous phases (Figure S20a).50-52 The amorphous molecules take a more planar conformation with a red-shifted emission. The emission of the ground E isomer quickly turns back to the initial blue emission as the sample was fumed by DCM, owing to the recrystallization induced by DCM vapor. The color change can be easily distinguished by naked dyes under UV light (Figure 5c). The Z isomer at the pristine state show an emission peak at 451 nm (Figure 5b), which is red-shifted as compared to the E isomer (439 nm). The grinding shifts the emission peak of the Z isomer solid from 451 to 477 nm. It also can return to initial state fumed by DCM. We noticed the color change of the E isomer solid that requires a much hard grinding. This is likely due to more regular molecular packing and stronger intermolecular interactions in the E isomer solid than those in the Z isomer.



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the quantum yield [ = kr/(kr+knr) ] and fluorescence lifetime results [ = (kr+knr)-1 ]. The radiative decay rate constant (kr) of Z isomer is determined to be 1.8×108 s-1, similar to that of the E isomer (1.3×10 8 s-1). However, the nonradiative decay rate constant (knr) of the Z isomer is 2.0×108 s-1, which is much lower than that of the E isomer (11.9×108 s-1). This indicates that the molecular packing of Z isomer might effectively suppress the energy loss through nonradiative decay processes. After grinding, the quantum yields and lifetimes of both isomers increase, resulting in the enhanced luminescence brightness. As seen from Table 1, both radiative and nonradiative rate constants were decreased by the grinding. However, the decrease in nonradiative rate constants was more significant than that in radiative rate constants. The molecules after grinding likely adopt planar conformations, thus reducing the vibrational or rotational relaxations. The ground samples were fumed by DCM solvent, leading to rapid recovery of the morphology and emission color. The simultaneous color shift and intensity change stimulated by external force have potential applications in highcontrast optical storage materials.57-61

Name

Figure 5. Fluorescent spectra of (a) E-TPE-2OXE and (b) Z-TPE2OXE: pristine, ground and fumed by DCM; (c) and (d) are fluorescent images with different treatments: I. Pristine; II. Grinding at one side; III. Grinding all; IV. Fumed by DCM; Molecular orbital amplitude plots of HOMO and LUMO energy levels of E (e) and Z(f) isomers calculated using B3LYP/6-31G(d) by Gaussian 09.

We calculated the HOMO and LUMO energy levels of two isomers using B3LYP/6-31G (d) by Gaussian 09 (Figure 5e and 5f).53 For the E isomer, the electron density of HOMO and LUMO are similar, which indicate that the emission of E isomer may be assigned to the locally excited (LE) state. However, the electron density of HOMO in the Z isomer is mainly distributed in the double bond, while LUMO is distributed in the benzene rings. Therefore, the emission of Z isomer may be assigned to the charge transfer (CT) state emission which is usually bathochromic.54-56 The HOMO/LUMO calculations are consistent with the difference in isomer configuration and experimental observation of a higher fluorescence brightness of the Z isomer solid as compared to the E isomer. Fluorescence quantum yield and lifetime measurements were further performed to characterize the optical properties of the E/Z isomer solids. The quantum yield of Z isomer in crystalline state was determined to be 48%, which is about 5-fold that of the E isomer (Table. 1). Similarly, the fluorescence lifetime (2.65 ns) of Z isomer in crystalline state is about 3-fold that of the E isomer (0.76 ns). The fluorescence radiative rate constant kr and nonradiative rate constant knr were estimated by combining

ETPE2OXE

ZTPE2OXE

ɸf

τ

kr/108

knr/108

(%)

(ns)

(s−1)

(s−1)

pristine

10

0.76

1.3

11.9

ground

14

5.82

0.2

1.5

fumed by DCM

10

0.75

1.3

12.0

pristine

48

2.65

1.8

2.0

ground

66

7.15

0.9

0.5

fumed by DCM

42

2.77

1.5

2.1

Treatment

Table. 1 Photophysical parameters of TPE-2OXE solids after different treatments. We observed a distinctive feature for the E isomer solid that exhibits self-healing ability in morphology and optical property. The red-shifted emission of the ground E isomer was blue-shifted spontaneously at room temperature under the dark (Figure 6a). Within 5 hours, the emission turns back to 450 nm that is close to the crystal emission. Obvious diffraction peaks were observed in the XRD profile at 5 hours after grinding (Figure 6c), which is also similar to the diffraction peaks of the pristine sample. However, the ground Z isomer didn’t show noticeable changes during this time period (Figure 6b, 6d), indicating that the Z isomer is stable in the amorphous phase. The self-healing property of the E isomer reveals strong intermolecular interactions as compared to the Z isomer, which are consistent with the E/Z isomers difference in melting temperatures, as determined by DSC curves (Figure 6e,


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6f). It is difficult to get absolutely amorphous phase of the E isomer by grinding. We believe that the ground E isomer solids consist of a small amount of microcrystals as a few diffraction peaks were observed in the XRD profile of the ground E isomer. The presence of microcrystals can stimulate the ground solid to crystallize. The DSC thermogram of the ground E isomer shows an exothermic peak at ∼68 oC (Figure 6e), which is much lower than that of the ground Z isomer (127 oC). This further confirmed that the E isomer tends to crystallize as compared to the Z isomer. The spontaneous recovery of the morphology and fluorescence makes the E isomer a promising material for functional actuators.62,63

ble without noticeable leaching from the nanoparticles. Zeta potential measurements show that the E-dots and Z-dots have negatively charged surface with zeta potentials of −46.7mV and -30.8 mV, respectively. The resulting AIE dots show negligible fluorescence change over time (Figure 7d). The quantum yields of the E and Z AIE dots were determined to be 15% and 27%, respectively. These results indicate the high brightness of the Z isomer can be retained in the AIE dots, which are promising for cell imaging applications.

Figure 7. TEM image and DLS data (inset) of the (a) E-dots and (b) Z-dots; the zeta potential (c) and fluorescence change with time (d) of the E-dots and Z-dots.

Figure 6. Fluorescent spectra (a) E-TPE-2OXE and (b) ZTPE-2OXE solid placed different time after fully grinding; XRD patterns of (c) E-TPE-2OXE and (d) Z-TPE-2OXE of different states; DSC curves of E-TPE-2OXE (e): pristine, fully ground, placed 5 h after grinding at room temperature under dark and fume by DCM and Z-TPE-2OXE (f): pristine, fully ground, placed 5 h after grinding at room temperature under dark and fumed by DCM.

Cell imaging of the TPE isomers. We used an amphiphilic polymer poly (styrene-co-maleic anhydride) (PSMA) to prepare stable AIE dots for cellular toxicity and imaging studies.64-68 The mixture of AIE molecules and PSMA polymer in THF solution was quickly added into water to prepare the AIE dots. With the PSMA functionalization, we prepared uniform AIE dots with good brightness and stability. The sizes of the two types of AIE dots were measured by dynamic light scattering (DLS) (Figure 7a, 7b inset), which show narrow particle size distributions around 16 nm for both the two isomers. The morphology of the AIE dots were also further characterized by transmission electron microscopy (TEM) (Figure 7a, 7b), consistent with the DLS results. The AIE dots are colloidally sta-

To further explore the application of E-dots and Z-dots as bioprobes for cellular imaging, the cytotoxicity of the two types of AIE dots to MCF-7 cancer cells were first evaluated using the MTT assays. The MCF-7 cells grew normally in culture medium containing E-dots and Z-dots in the concentration range of 5-80 μg/ml for 6 hours (Figure 8a, 8b), indicating low cytotoxicity of the E and Z-dots. We next used the same weight concentration (∼40 μg/ml) and the same size (∼16 nm) of E-dots and Z-dots to incubate with MCF-7 cells for four hours, respectively. Fluorescence imaging (Figure 8c, 8d) showed obvious AIE dots internalization, the E and Z dots are preferentially stained in the cytoplasmic regions of MCF-7 cells rather than in their nucleic parts. As expected, the MCF-7 cells stained by the Z-dots showed higher labelling brightness than those stained by the E-dots under identical experimental conditions. This observation is consistent with the higher brightness of the Z isomer in solid and nanoparticle forms. The cellular imaging also indicates the Z isomer is more suitable for cell imaging, as the higher brightness improve the imaging quality.



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crosslinking of Z-TPE-2OXE in solid thin films and AIE dots. This material is available free of charge via the Internet at http://pubs.acs.org.

*E-mail: [email protected] *E-mail: [email protected]

The authors declare no competing financial interest.

Changfeng Wu acknowledges financial support from “Thousand Young Talents Program” and the National Science Foundation of China (Grant No. 61335001, 61222508). Yifei Liu acknowledges financial support from the National Science Foundation of China (Grant No. 51303063).

Figure 8. Metabolic viability of MCF-7 cells after incubation with E-dots (a) and Z-dots (b) at different concentrations for 6h. Fluorescence imaging of MCF-7 cells incubated with 40 μg/ml E dots (c) and Z-dots (d). From left to right are bright field images, fluorescence images, and combined bright field and fluorescence images. Scale bar represents 20 μm.

In summary, we have synthesized and purified the E/Z isomers of oxetane-substituted TPE by the macroscopic technique of column chromatography. Their geometry structures were confirmed by NOESY and COSY NMR spectroscopy. Combined the theoretical calculation with optical spectroscopy, we investigated the distinct properties of the E/Z isomers in terms of the photoluminescence and emission mechanism. The Z isomer with CT state emission shows bathochromic emission with a higher quantum yield and longer lifetime as compared to those of the E isomer. The E/Z isomers present different mechanochromic behaviour as the E isomer exhibits self-healing ability in morphology and optical property. Finally, the AIE dots of the Z isomer also show higher brightness and enhanced cell labelling brightness than the E isomer under the same conditions. These results improve our understanding of the E/Z isomers of TPE derivatives, which are beneficial to the design and synthesis of functional TPE molecules for various applications.

Supporting Information NMR spectra, IR spectra and NOESY and COSY NMR spectra of two isomers. NMR spectra changes of E isomer induced by UVlight and heating. Powder X-ray diffraction patterns of two isomers with different treatments. UV-Vis absorption spectra and DLS results of two isomers with different water fractions. Photo-

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