trans Tetra-arylethenes with Switchable Aggregation

Sep 22, 2017 - How to efficiently design and synthesize multifunctional molecules is particularly challenging. In this presentation, we devote to cons...
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Article Cite This: J. Org. Chem. 2017, 82, 10960-10967

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Hybrid cis/trans Tetra-arylethenes with Switchable AggregationInduced Emission (AIE) and Reversible Photochromism in the Solution, PMMA Film, Solid Powder, and Single Crystal Qianfu Luo, Fei Cao, Chaochao Xiong, Qingyu Dou, and Da-Hui Qu* Key Laboratory for Advanced Materials and Institute of Fine Chemicals, School of Chemistry and Molecular Engineering, East China University of Science & Technology, Shanghai 200237, China S Supporting Information *

ABSTRACT: How to efficiently design and synthesize multifunctional molecules is particularly challenging. In this presentation, we devote to constructing a kind of simple structures with composite functionalities through straightforward preparation. Starting from common commercially available materials, the titled cis/trans-tetraarylethenes can be conveniently obtained by a one-pot process under mild conditions. The different configurations were confirmed by 1H NMR and single crystal data analysis. The trans-tetraarylethenes could be converted into cis-forms not only by photoirradiation but also by microwave irradiation, which provided us a new choice for isomeric conversion, especially in relation to light sensitivity. Results show that all the hybrid cis/trans-isomers performed switchable fluorescence and reversible photochromism in solution, PMMA film, solid powder, and single crystal. Moreover, these hybrid tetraarylethenes could be utilized as photoswitchable media to tune the behavior of aggregation-induced emission (AIE) and aggregation-caused quenching (ACQ). These versatile properties are favorable for the potential applications in fluorescent photoswitches, nondestructive readout, and logic gates. We hope that our design strategy could provide a new protocol for constructing a kind of multifunctional molecules based some simple structure and convenient synthetic procedures.

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INTRODUCTION Phototriggered functional materials, especially photochromic diarylethenes, have received tremendous research effort because of their distinctive molecular architectures and unique properties.1,2 Since Irie and co-workers reported the first thermally irreversible photochromic diarylethene, a substituted cycloalkene bridged two aromatic heterocycle system with high fatigue resistance,3 plenty of such structures have been extensively investigated for their excellent properties and versatile applications in information storage, imaging devices, and molecular switches.4−20 Especially, many dithienylethene assisted fluorophores have been designed for various fluorescent photoswitches, nondestructive readout, and chemical/biosensors.21−26 However, most of them show significant fluorescence in solution but quenched in solid state or aggregate state, which limit their technological application, because many materials should be used in solid state or aggregate state. Conversely, tetraphenylethylene exhibits remarkable emission properties in solid state and aggregate state in contrast to near-quenching in solution because of the restriction of intramolecular rotation in solid state. From their structures, we can see that both diarylethene and tetraphenylethylene contain a core structural unit of ethylene bridged aromatic rings. But their functions as materials are completely different. The former is known as a prominent member of © 2017 American Chemical Society

organic photochromic materials. The latter is attractive for the aggregation-induced emission (AIE) effect.27−29 The extraordinary antiaggregation causing the quench effect enables tetraphenylethylenes to have potential application in diverse biolabels, bioprobes, fluorescence sensors, and photoelectronic devices.30−34In view of their complementary performances in fluorescent emission, several diarylethene derivatives connected tetraphenylethylene building blocks and tetrathienylethenes have been reported for multifunctional molecules in recent years.35−37 It is the most common protocol for the design of multifunctional molecules by bonding two or more building blocks with different functionalities together after various chemical or physical modifications. It often works, but sometimes the different units may interfere with each other in functions. In addition, the synthetic routes get inevitably longer and longer as the molecule structure becomes larger and larger. Based on the above-mentioned considerations, we strive to construct a simple structure with both photochromic performance and aggregation-induced emission. Then the titled hybrid tetraarylethenes cherishing five- and six-membered aromatic rings are developed (Scheme 1), in which the two aromatic Received: July 27, 2017 Published: September 22, 2017 10960

DOI: 10.1021/acs.joc.7b01877 J. Org. Chem. 2017, 82, 10960−10967

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The Journal of Organic Chemistry Scheme 1. Hybrid tetra-arylethenes with Five- and SixMembered Aromatic Rings

thiophenes (components of photochromic dithienylethene) and two benzenes (constituent units of tetraphenylethylene with AIE) share the same ethylene bridge. The advantages of this novel design are that the two five-membered thiophenes together with the two six-membered benzenes constitute a new hybrid tetra-arylethene system with AIE, and the different steric configurations of the two benzenes assist the two thiophenes to form different photochromic units. Moreover, this design takes into account atom economy and greatly simplifies synthesis procedure. Starting from commercially available materials, the cis- and trans- tetra-arylethenes can be conveniently obtained by one pot synthesis under mild conditions.

Figure 1. Comparison of partial 1H NMR spectra of trans-1o and cis1o.

obvious differences between the cis-form and trans-form. In the high field, the signal of cis-1o located at 2.03 ppm ascribed to methyl protons. In the low field, a set of signals appearing at 6.81 ppm were attributed to protons in the 3-position of the thiophene ring. However, for trans-1o, the chemical shift of methyl protons up-shifted to 1.95 ppm. At the same time, the chemical shift of the 3-position protons in the thiophene ring shifted to lower field and appeared at 6.88 ppm. A similar phenomenon was observed between cis-1′o and trans-1′o, which was included in the Supporting Information (Figure S1). Single crystal data analysis further confirmed the two transform structures. The single crystals of trans-1o and 1′o were obtained via slow evaporation of the dichloromethane− methanol mixture solution and analyzed by X-ray diffraction crystallography. The CCDC deposition numbers of single crystal trans-1o and trans-1′o are 1540622 and 1540610, respectively. From the ORTEP drawings shown in Figure 2, we could see intuitively that the two thiophene rings distribute on different sides of the double bond. Their X-ray crystallographic analysis data were listed in the Supporting Information ((SI) Tables S2 and S3). The crystal system and space group are monoclinic and P21/n, Z = 4, and the unit cell volume is 2186(3) Å3. In the case of compound trans-1′o for example, the intramolecular distance between the two centric double-bond carbon atoms (C1 and C2) is 1.357 Å. The distances between the ethylene bridge carbon C1 or C2 to other four aromatic groups, C1−C4, C1−C9, C2−C16, and C2−C21, are 1.490, 1.492, 1.481, and 1.495 Å, respectively, which are all longer than the distance between the double bond carbon C1 and C2. The dihedral angles between the thiophene ring and phenyl ring on the same side of the double bond is 58.06°. Especially, the distance between the photoactive carbon atom C10 in the thiophene ring and the closest carbon atom in the phenyl ring (C15) is 3.201(1) Å, which is shorter than 4.2 Å, implying that trans-1′o might undergo photochromism in the crystal state.40−42 The following experimental results coincided with this speculation. [CCDC 1540622 and 1540610 contain the supplementary crystallographic data for this paper. These data can be obtained free of charge from The Cambridge Crystallographic Data Centre via www.ccdc.cam.ac.uk/data_ request/cif.] It is noteworthy that the trans-forms can be converted into cis-forms induced not only by 420 nm light but also by

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RESULTS AND DISCUSSION Synthesis and Characterization. Starting from common commercial available material, substituted benzene and thiophene, we first obtained intermediate 2 and 2′ by classic Friedel−Crafts reaction. With them in hand, the titled cis/trans tetra-arylethenes could be conveniently obtained in one pot through a common McMurry coupling reaction (Scheme 2). Scheme 2. Synthesis of the cis/trans-Tetraarylethenes and the Interconversion among the Different Isomers

The product was primarily purified by column chromatography on silica gel to obtain cis and trans mixtures, then via slow evaporation from a dichloromethane−methanol mixture solution to give pure cis-1o, 1′o and trans-1o, 1′o isomers (1: R = phenyl, 1′: R = methyl; o: open-ring isomer), respectively. From the 1H NMR spectra in Figure 1, we could see there were 10961

DOI: 10.1021/acs.joc.7b01877 J. Org. Chem. 2017, 82, 10960−10967

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Figure 2. ORTEP drawings of single crystal trans-1o (left, the CCDC deposition number is 1540622) and trans-1′o (right, the CCDC deposition number is 1540610) (50% ellipsoid drawings of trans-1o and trans-1′o; H atoms are not shown).

arylethene (Figure 3). It indicated that trans-forms have transformed to cis-forms. To our best knowledge, it was the first report on the trans−cis isomerization induced by microwave, which provided us a new choice for isomeric conversion, especially for compounds which are sensitive to UV-irradiation. DFT calculations indicated the ground-state energy differences between the two cis-tetraarylethenes and their corresponding trans-isomers are very small (Table S5), which also means that the trans-isomers could isomerize easily to the cis-isomers. Photochromic Behaviors in Various States (Solution, PMMA Film, Solid State, and Single Crystal State). We first investigated the photochromic performances of the two cistetraarylethenes (cis-1o and cis-1′o) in solution at room temperature. Figure 4a and Figure S4a showed their photoinduced absorption spectra and corresponding color changes in THF solution. The solutions of the two open-ring isomers are colorless and do not show any optical absorption in the visible light region. After irradiation by UV light, the solutions of the two cis-tetraarylethenes turned purple and orange, respectively, which is attributed to the formation of the closed-ring isomers (cis-1c and cis-1′c) by a conrotatory mechanism of the two cisform thiophenes.43 The backward reaction took place upon irradiation with more than 500 nm light. The photocyclization ratios of cis-1o and cis-1′o were 34% and 18%, respectively, calculated by 1H NMR.

microwave. Before irradiation, there were two single peaks corresponding to protons of methyl (a) and thiophene (b) of trans-1o at 1.95 and 6.88 ppm (Figure 3). After irradiation with

Figure 3. Comparison of partial 1H NMR spectra of trans-1o after photoirradiation and the 1H NMR spectra of cis-1o and trans-1o before irradiation.

light (420 nm) or microwave (SI, Figure S3), two new single peaks appeared at 6.81 and 2.03 ppm which were ascribed to protons of methyl (a’) and thiophene (b′) of cis-tetra-

Figure 4. Changes of the UV−vis absorption spectra and colors (inserts) of the cis- and trans-1o in THF solution irradiated by UV light (2.0 × 10−5 M). 10962

DOI: 10.1021/acs.joc.7b01877 J. Org. Chem. 2017, 82, 10960−10967

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The Journal of Organic Chemistry The 1H NMR spectra further confirmed the existence of closed-ring isomers in the photocyclization reaction. Partial 1H NMR spectral comparison of cis-1o before irradiation and the photostationary state (PSS) was illustrated in Figure 5. Before

was achieved. A clear isosbestic point was observed at 333 nm, which showed the reversible two-component photochromic reaction.44,45 Moreover, the magenta solution of the photostationary could be bleached entirely when exposed to visible light. This reversible color changes could recycle more than five times without obvious decay in color. The photochromic behavior of trans-1′o (Figure S4b) was similar to that of trans-1o. We further invesigated their repeatabilities of photochromic reactions (Figure S26). Results indicated that the two compounds having benzenes at the 5-position of the thiophene rings (cis- and trans-1o) showed better behaviors in fatigue resistance than the other two compounds substituted by methyl groups at the corresponding position (cis- and trans-1′o). On the other hand, we also found that the closed-ring isomers, trans-1c and trans-1′c, were thermally reversible, which could transfer to their open forms in the dark. During the photostationary state, when we kept the THF solution of trans1c in the dark for several hours, the maximum absorption at 502 nm decayed to the original spectra (Figure S6a), accompanied by the magenta solution fading to being colorless. It meant that the closed-ring form, trans-1c, could recover to the open-ring form (trans-1o). However, the THF solution of cis-1o at the photostationary state did not show any changes in color and UV−vis spectra when kept in the dark for several weeks. Similarly, when we compared the photostationary state of trans-1′o and cis-1′o in the dark for several hours, the maximum absorbance band of trans-1′c at around 463 nm decreased slowly (Figure S6b) but that of the cis-1′c did not show any changes. The results indicated that the thermal stability between the benzene ring and thiophene ring were totally different. This might be due to the aromatic stabilization energy of benzene which was much higher than that of thiophene and cyclohexadiene.1 Next, we investigated their photochromic behaviors in the PMMA films. Upon irradiation with 254 nm light, the colorless PMMA films containing trans-1o, cis-1o, trans-1′o, and cis-1′o turned magenta, purple, yellow, and orange, respectively, followed by the appearances of a new absorption band at around 504, 543, 467, and 470 nm (Figures 6 and S7). The optical physical properties of the four tetra-arylethenes before and after the photochromism in THF solution and PMMA film are summarized in Table S1. Compared to the absorption maximum in solution, the maximum absorption peak of their

Figure 5. Comparison of partial 1H NMR spectra of cis-1o before irradiation and at the PSS.

irradiation, there were two signals located at 2.03 and 6.81 ppm which were assigned to the methyl protons (a) and the protons (b) in the 3-position of the thiophene ring, respectively. After irradiation of the solution under 254 nm UV light, two sets of new proton signals appeared at 6.44 and 2.40 ppm. The signal corresponding to protons (b) up-shifted from 6.81 to 6.44 ppm (b′), and the methyl protons (a) down-shifted to 2.40 ppm (a′), due to the formation of the closed-ring isomer cis-1c. The 1 H NMR spectral change of cis-1′o is similar to that for cis-1o (Figure S5). Interestingly, besides cis-forms, the two trans-1o and trans1′o also showed noteworthy photochromic features. Upon irradiation with UV light (254 nm), the THF solution of trans1o turned magenta quickly. Meanwhile, a new absorption peak at 502 nm appeared along with the decrease of the absorption band at around 286 nm (Figure 4b). This phenomenon was attributed to the formation of the closed-ring isomer trans-1c by the cyclization between the thiophene and benzene at the same side. After irradiation for 70 s, the photostationary state

Figure 6. Changes of UV−vis absorption spectra and color in PMMA film (inserts) (w(cis‑1o)/w(PMMA) = 10 mg/1 g; w(trans‑1o)/w(PMMA) = 12 mg/1 g) irradiated by UV light. 10963

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The Journal of Organic Chemistry closed-ring isomers in PMMA film all red-shifted slightly on account of the polar effect of the polymer matrix.46,47 Just as the case in solution, the colored films could also be bleached quickly by irradiation with visible light and this reversible change could be repeated many cycles. Moreover, these compounds also showed prominent photochromism at solid state and single crystal. In Figure 7, we could

could be converted into cis-isomers induced by light or microwave. Interconversion pathways of these hybrid tetraarylethenes have been depicted in Scheme 2. This property is useful for the logic gates or logic keypad lock. Fluorescent Behaviors in Net THF Solution, H2O−THF Mixed Solution, Solid Powders, and PMMA Films. The fluorescent emission behaviors of the titled compounds in mixed solution containing water and THF with different proportions were first investigated. All four tetra-arylethenes were soluble in THF solvent but insoluble in water. Results showed their fluorescent emissions were very weak in the net THF solution, but gradually increased with the increase of the water fraction (Figure 8 and S9). Here we choose compound trans-1o as an example to show their fluorescent performances. From Figure 8 we could see that the fluorescence intensity was close to zero in the THF solution. Once we added some water into the solution, the fluorescence intensity increased markedly. When the H2O/THF ratio increased from 20:80 to 60:40, the fluorescence intensity increased, accordingly. When the ratio of H2O/THF reached 80:20, the fluorescence intensity increased dramatically. The SEM image of trans-1o in the H2O/THF mixed solution (H2O/THF = 80/20) is shown in the Supporting Information (Figure S10). It was found that regular shaped particles were formed in the mixed solution (the diameter of the formed aggregate particle is about hundreds of nanometers). The other three compounds also exhibited similar aggregation-induced emission (AIE).48 The optical properties including fluorescence quantum efficiency and the cyclization rate of the four tetra-arylethenes are summarized in Table S1. Results indicated that the cis- and trans-1o showed a higher fluorescence quantum efficiency and cyclization rate than cisand trans-1′o, which might be due to the substituent effect of the benzenes in the 5-position of the thiophene rings. Meanwhile, significant fluorescent emissions were also observed in the PMMA films and solid powders of the four hybrids. Alternately irradiated with UV and visible light, they showed reversible fluorescence switching properties in both solid powder and PMMA films. Figure 9 showed fluorescence changes of cis/trans 1′o by photoirradiation in PMMA film (λex = 320 nm). Before irradiation with UV light, the films containing the two compounds showed significant fluorescence (the insets of Figure 9). Upon irradiation with 254 nm light, the emission intensities declined dramatically. In contrast, their fluorescence rose again after irradiation with visible light. The fluorescence changes are more obvious in the film containing

Figure 7. Photochromism of the tetra-arylethenes in single crystal state.

see that the single crystal of trans-1o was colorless before irradiation with UV light; however, its color turned purple immediately when exposed to UV light for several seconds. Conversely, the colored single crystal could be bleached quickly by irradiation with visible light. The single crystal of trans-1′o exhibited the same performances as that of trans-1o. A similar phenomenon was observed in the solid-state powders (Figure S8). The above-mentioned experimental results showed that not only the cis-tetraarylethenes but also their trans-forms exhibited versatile photochromic behaviors in solution, PMMA film, solid state powders, and single crystal. Moreover, the trans-isomers

Figure 8. Different changes of the emission spectra in THF/H2O mixed solution with different water fraction: (a) the trans-1o (c = 2.86 × 10−4 M); (b) cis-1o (c = 3.02 × 10−4 M). 10964

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Figure 9. Changes of emission spectra in PMMA film when irradiated by UV and visible light.

cis/trans-1′o whose thiophene rings were substituted by methyl groups in the 5, 5′ positions than those of cis/trans-1o whose thiophene rings connected to benzenes (Figure S11). This performance is suitable for potential use as fluorescent photoswitches in optical memory media. Figure 10 illustrated the color and the corresponding fluorescence changes of compound trans-1′o in solid powders

their mixed solution (H2O−THF, 80% water content) before photoirradiation and after the photostationary state were compared in the Supprorting Information (Figure S13). We could see that all their fluorescence intensity weakened after photoirradiation. It might be because the noncoplanar openring isomers transferred into the coplanar closed-ring isomer after the photocyclization reaction, and the coplanar closed-ring isomers had the aggregation-caused quenching (ACQ) property.49 The behavior of the aggregation-induced emission and the fluorescence intensity could be recovered by irradiation with visible light. It means that these hybrid tetra-arylethenes might be utilized as photoswitchable media to tune the performance of AIE and ACQ. This performance is desirable for practical application in photoswitchable fluorescence switches.

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CONCLUSIONS We present four hybrid cis/trans tetra-arylethenes sharing an ethylene bridge. The novel design ensures a simple structure with two independent functionalities. The cis- and trans-isomers can be conveniently obtained by one-pot synthesis from commercially available materials. The structures of the two trans-forms were further confirmed by X-ray single crystal diffraction, and they could be converted into cis-forms not only by photoirradiation but also by microwave irradiation, which provides us a new and convenient choice for isomeric conversion, especially in relation to sensitivity to UV light. Moreover, we found that both the cis- and trans-isomers exhibited reversible photochromism in solution, PMMA film, solid state, and single crystal state. The aggregation-induced emissions (AIEs) of these compounds before photoirradiation and after photocyclization were further investigated. Results showed that these hybrid tetra-arylethenes could be utilized as photoswitchable media to tune the behavior of AIE and ACQ (aggregation-caused quenching). These versatile properties are favorable for potential applications in fluorescent photoswitches, nondestructive readout, and logic gates. We hope our design strategy could provide a new protocol for constructing a kind of multifunctional molecules based on a simple structure and convenient synthesis.

Figure 10. Color and the corresponding fluorescence changes of compound trans-1′o in solid powders before and after photoirradiation. (a) Color before UV-irradiation; (b) color after UVirradiation; (c) fluorescence before photoirradiation; (d) fluorescence after UV-irradiation.

before and after photochromism. The white solid (Figure 10a) exhibited strong blue fluorescence (λex = 320 nm, Figure 10c) before irradiation with UV light. After irradiation with 254 nm UV light for several minutes, the color changed from white (Figure 10a) to pink (Figure 10b), accompanied by the obvious fluorescence quenching shown in Figure 10d. Upon irradiation with visible light, the pink powders reverted to white, and the remarkably fluorescent emission could be observed again. The other three compounds also performed switchable fluorescence (SI, Figure S12). Besides the aggregation-induced emission (AIE) before photoirradiation, we further investigated the different behaviors in AIE after photochromism. The fluorescence behaviors of

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EXPERIMENTAL SECTION

All chemicals were obtained commercially and used as received unless otherwise mentioned. 1H NMR spectra were recorded at 400 MHz, and 13C NMR spectra were recorded at 101 MHz using CDCl3 as a solvent and tetramethylsilane as an internal standard. The UV light for 10965

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525.1702. trans-1o: 1H NMR (400 MHz, CDCl3, ppm): δ = 7.46−7.44 (m, 4H), 7.32−7.28 (m, 4H), 7.22−7.08 (m, 12H), 6.88 (s, 2H), 1.95 (s, 6H). 13C NMR (101 MHz, CDCl3, ppm): δ = 142.7, 140.6, 139.6, 136.3, 136.0, 134.5, 130.0, 128.9, 127.9, 127.1, 126.9, 126.6, 125.4, 14.6. HRMS (ESI-TOF) m/z: [M + H]+ calcd for C36H29S2 525.1711; found 525.1708.

irradiation was generated by LED sources, and the lamp power is 5 W. The lamp power monochromatic 420 nm light is 50 W. All mass spectrometric analyses were performed on ThermoStar mass spectrometer. Absorption and fluorescence spectra were recorded on a Varian Cary 500 and a Varian Cary Eclipse using a quartz cuvette at room temperature, respectively. For the absorption spectral measurements, optical cells with 1 cm light path lengths were used for the absorption spectral measurement of the solutions. The X-ray experiment of the single crystal was performed on a Bruker SMART APEX2 CCD area-detector equipped with graphite monochromatized Mo Kα radiation at room temperature (290 ± 2 K). All the DFT calculations were carried out using the GAUSSIAN 09 package. Geometry optimization and energy calculations were performed with B3LYP. The 6-31G basis set was used for all atoms. All the reactants and products are calculated with no imaginary frequency, and transition state structures have only one imaginary frequency. Synthesis of cis/trans-1,2-Bis(2,5-dimethylthiophen-3-yl)1,2-diphenylethene (cis-1′o; trans-1′o). To a suspension of zinc powder (1.07 g, 16 mmol) in 20 mL of anhydrous THF at −10 °C under argon in the absence of light, the titanium tetrachloride (0.86 mL, 8 mmol) was added slowly by syringe. The mixture warmed to reflux for 3 h and then cooled to room temperature. The 10 mL THF solution of (2,5-dimethylthiophen-3-yl)(phenyl)methanone (2′)38(1.2 g, 5.6 mmol) was dropped into the mixture above. Afterward, the reaction mixture was refluxed for another 3 h. The reaction was quenched by a saturated sodium carbonate solution and then extracted with ethyl acetate (3 × 30 mL); the combined organic phase was washed with water (30 mL) and brine (30 mL), dried over Na2SO4, and filtered. After the solvents were evaporated, the residue was purified by flash silica-gel column chromatography using petroleum ether as the eluent to afford a cis-isomer and trans-isomer mixture (220 mg) as a white solid. The mixture was filtered after recrystallization via slow evaporation of the dichloromethane−methanol mixture solution. Drying the filter cake afforded trans-1′o: 120 mg, yield 10.7%, melting point 165.0−166.1 °C. Removal of the solvent from the filtrate afforded cis-1′o: 100 mg, yield 8.9%. cis-1′o: 1H NMR (400 MHz, CDCl3, ppm): δ = 7.10−7.07 (m, 6H), 7.00−6.98 (m, 4H), 6.15 (s, 2H), 2.27 (s, 6H), 1.90 (s, 6H). 13C NMR (101 MHz, CDCl3, ppm): δ = 142.9, 139.7, 136.5, 134.2, 134.1, 131.0, 128.3, 127.7, 126.4, 15.3, 14.3. HRMS (ESI-TOF) m/z: [M + H]+ calcd for C26H25S2 401.1398; found 401.1397. trans-1′o: 1H NMR (400 MHz, CDCl3, ppm): δ = 7.14−7.08 (m, 6H), 7.01−6.99 (m, 4H), 6.25 (s, 2H), 2.29 (s, 6H), 1.83 (s, 6H). 13C NMR (101 MHz, CDCl3, ppm): δ = 143.2, 139.2, 136.2, 135.0, 133.6, 129.9, 128.5, 127.6, 126.4, 15.3, 14.2. HRMS (ESITOF) m/z: [M + H]+ calcd for C26H25S2 401.1398; found 401.1384. Synthesis of cis/trans-1,2-Bis(2-methyl-5-phenylthiophen-3yl)-1,2-diphenylethene (cis-1o; trans-1o). The titanium tetrachloride (0.86 mL, 8 mmol) was added slowly by syringe to a suspension of zinc powder (1.07 g, 16 mmol) in 20 mL of anhydrous THF at −10 °C under argon in the absence of light. The mixture was warmed to reflux for 3 h and then cooled to room temperature. The 10 mL THF solution of (2-methyl-5-phenylthiophen-3-yl)(phenyl)methanone (2)39 (1.5 g, 5.4 mmol) was dropped into the mixture above. The reaction mixture was refluxed for another 3 h. Then the reaction was quenched by a saturated sodium carbonate solution and extracted with ethyl acetate (3 × 25 mL); the combined organic phase was washed with water (2 × 50 mL) and brine (50 mL), dried over Na2SO4, and filtered, and the solvent was removed in vacuum. The residue was purified by flash silica-gel column chromatography using petroleum ether as the eluent to afford a cis-isomer and trans-isomer mixture (280 mg) as a white solid. The mixture was filtered after recrystallizzation via slow evaporation of the dichloromethane− methanol mixture solution. Drying the filter cake afforded trans-1o: 150 mg, yield 10.6%, melting point 155.4−157.1 °C. Removal of the solvent from the filtrate afforded cis-1o: 130 mg, yield 9.2%. cis-1o: 1H NMR (400 MHz, CDCl3, ppm): δ = 7.39−7.38 (m, 4H), 7.28 (s, 1H), 7.24 (s, 1H), 7.20−7.07 (m, 14H), 6.81 (s, 2H), 2.03 (s, 6H). 13C NMR (101 MHz, CDCl3, ppm): δ = 142.3, 141.0, 139.0, 136.8, 136.7, 134.6, 131.0, 128.8, 127.9, 127.0, 126.7, 126.5, 125.5, 14.6. HRMS (ESI-TOF) m/z: [M + H]+ calcd for C36H29S2 525.1711; found

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.joc.7b01877. Copies of 1H NMR, 13C NMR and HRMS spectra of all new compounds and the single crystal information on compound trans-1o and trans-1′o (PDF) Crystallographic data for trans-1′o (CIF) Crystallographic data for trans-1o (CIF)

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. ORCID

Da-Hui Qu: 0000-0002-2039-3564 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS We thank the support of NSFC/China (21672060), the Fundamental Research Funds for the Central Universities (WJ1616011, WJ1213007, 222201717003), the Programme of Introducing Talents of Discipline to Universities (B16017). We are grateful to Dr. Xiaochuan Li and Dr. Xin Zheng from Henan Normal University for the measurement of single crystal, our colleagues Dr. Dengyuan Li and Dr. Shuang Chen for helping the calculation of ground-state energy difference. Special thanks to Prof. He Tian and Weihong Zhu from East China University of Science & Technology for his valuable support on this project.

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DOI: 10.1021/acs.joc.7b01877 J. Org. Chem. 2017, 82, 10960−10967