Synthesis, crystal structures, and mechanochromic properties of bulky trialkylsilylacetylene-substituted aggregation-induced-emission-active 1,4-dihydropyridine derivatives

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Synthesis, crystal structures, and mechanochromic properties of bulky trialkylsilylacetylene-substituted aggregation-induced-emission-active 1,4-dihydropyridine derivatives Jingqing Peng , Yanze Liu , Mengzhu Wang , Shihao Huang , Miaochang Liu , Yunbing Zhou , Wenxia Gao , Xiaobo Huang *, Huayue Wu College of Chemistry and Materials Engineering, Wenzhou University, Wenzhou, 325035, PR China

A R T I C L E I N F O

A B S T R A C T

Keywords: Trialkylsilylacetylene-substituted 1,4dihydropyridines Mechanochromic properties Aggregation-induced emission Crystal structures Synthesis

Three new D-π-A 1,4-dihydropyridine compounds (DHP-TMSA, DHP-TBMSA, and DHP-TIPSA) containing bulky trialkylsilylacetylenes, such as trimethylsilylacetylene (TMSA), tert-butyldimethylsilylacetylene (TBMSA), and triisopropylsilylacetylene (TIPSA), were designed and synthesized. These compounds showed twisted molecular conformations confirmed by the X-ray crystallographic analyses and displayed aggregation-induced emission activities owing to the restriction of intramolecular rotation confirmed by the fluorescence and absorption spectra. Their crystalline structures exhibited orange or yellow fluorescence and could be destroyed by the grinding treatment owing to the looser packing modes, and were all converted to amorphous states. The crystalline-to-amorphous transition in morphology caused the solid-state fluorescence color changes, revealing mechanochromic (MC) properties. Furthermore, the red-shifted MC activities were confirmed to be ascribed to the increased molecular conjugations by the planarization of the molecular conformations. Compared with DHPTMSA, DHP-TBMSA and DHP-TIPSA were found to exhibit higher contrast MC activities because TBMSA and TIPSA units had larger steric hindrance effect than TMSA unit. This work broadens the structure types of 1,4dihydropyridine derivatives compounds and provides new information for the development of new 1,4-dihydro­ pyridine fluorescent materials containing ethynyl units with excellent solid-state emission stimuli-responsive properties.

1. Introduction Organic fluorescent molecules often exhibited two kinds of different fluorescence aggregation behaviors [1]. One of them was that fluores­ cent molecules with planar π-conjugated aromatic units showed very weak or no fluorescence in solid state because of the presence of an aggregation-caused quenching (ACQ) effect originating from strong π-π stacking interactions even if they could emit strong fluorescence in so­ lutions [2]. On the contrary, another kind of fluorescent molecules exhibited strong solid-state fluorescence, whereas very weak or no fluorescence in solutions, which was named as aggregation-induced emission (AIE) effect [3,4]. Unlike ACQ-active molecules commonly used in solutions, AIE-active molecules showed great application pros­ pects in the field of solid-state fluorescent materials [5–9]. AIE-active molecules often had twisted molecular conformations and the loose packing modes, which were easily influenced by the external stimuli,

and had the possibility of exhibiting solid-state emission stimuli-responsive behaviors based on the morphological changes, such as mechanochromic (MC), thermochromism, and vaporchromism properties [10–16]. It should be noted that not all AIE-active displayed MC activities, the proper crystallinity and adjustable morphologies play vital roles in the mechanochromism [17], however, they are often difficult to predict. Therefore, in view of a specific fluorescent structure unit, the appropriate structural modification is very important for achieving the MC activities and deserves further research [12]. 1,4-Dihydropyridine (DHP) has been demonstrated to have a large advantage in the design of D-π-A-type organic molecules with AIE properties owing to the prescence of nitrogen atom compared with the related compound 4H-pyran containing an oxygen atom [18–20]. N-Substituted groups are advantageous to enhance the distortion of molecular conformations and thus obtaining the DHP derivatives with AIE properties by the restriction of intramolecular rotation [18].

* Corresponding author. E-mail address: [email protected] (X. Huang). https://doi.org/10.1016/j.dyepig.2019.108094 Received 17 October 2019; Received in revised form 29 November 2019; Accepted 29 November 2019 Available online 30 November 2019 0143-7208/© 2019 Elsevier Ltd. All rights reserved.

Please cite this article as: Jingqing Peng, Dyes and Pigments, https://doi.org/10.1016/j.dyepig.2019.108094

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Scheme 1. Synthetic route towards the DHP derivatives.

Recently, our group has been devoted to the construction of various solid-state stimuli-responsive fluorescent molecules by the effective structural modification based on the traditional fluorophores [12, 21–30]. Based on the DHP unit, the increase of the alkoxy chain length in the chemical structures of the AIE-active derivatives could lead to more outstanding MC phenomena [21], while a shorter length in the alkyl chains was more advantageous to increase the number of crystal polymorphs of the derivatives and obtain multi-coloured MC activities [22]. Different electron-withdrawing end groups in the D-π-A DHP de­ rivatives with AIE properties were found to result in different solid-state stimuli-responsive fluorescent behaviors under pressure stimulation [23], and the electron-donating group with bulky hindrance led to multi-stimulus-responsive solid-state fluorescent properties [24]. The halogen-substituted DHP derivatives exhibited higher contrast MC ac­ tivities compared with the derivative without a halogen atom because the halogen atoms could result in more twisted molecular confromations and looser packing modes [25]. Furthermore, the introduction of appropriate structure units to the DHP ring could obtain the MC-active derivatives with multiple crystalline polymorphs [26,27] or the acidochromic-active derivatives with multiple coloured emissions switching [28]. Even so, the structural types of the reported DHP de­ rivatives are still very limited. As we know, the DHP derivatives con­ taining electron-donating and electron-withdrawing groups linked by the ethynyl unit have not been reported. To further broaden the struc­ ture types of the DHP derivatives with AIE and MC activities, in this work, three new DHP derivatives with different steric hindrances, namely DHP-TMSA, DHP-TBMSA, and DHP-TIPSA containing trime­ thylsilylacetylene (TMSA), tert-butyldimethylsilylacetylene (TBMSA), and triisopropylsilylacetylene (TIPSA), respectively, were synthesized to investigate the influence of steric hindrances on their MC activities (Scheme 1). Because of the introduction of bulky trialkylsilylacetylene and N-butyl group, these compounds showed twisted molecular con­ formations and adopted loose packing arrangements, which were confirmed by the X-ray single-crystal analyses. These compounds were found to display AIE activities by the restriction of intramolecular rotation and showed orange or yellow fluorescence in the crystalline state. Upon grinding, their crystalline structures were destroyed and exhibited the solid-state fluorescence color changes, revealing MC ac­ tivities. Compared with DHP-TMSA, DHP-TBMSA and DHP-TIPSA displayed higher contrast MC properties because of more twisted

molecular conformations and looser packing arrangements. The photo­ physical properties of these compounds and the corresponding mecha­ nism were detailedly investigated. 2. Experimental 2.1. Measurements and materials The 1H and 13C NMR spectra were conducted on a Bruker AV 400/ 500 NMR spectrometer using CDCl3 as the solvent and tetramethylsilane as the reference. The mass spectra were obtained by using a Finnigan DCMDX-30000 LCQ DCMD mass spectrometer. The absorption spectra were measured on a Cary 500 spectrometer. The fluorescence spectra were conducted on a HITACHI F-4500 spectrophotometer. The timeresolved fluorescence decay parameters and absolute solid-state fluo­ rescence quantum yields (ФF) were conducted on a Fluoromax-4 spec­ trophotometer. The elemental analyses were measured on an Elementar Vario MICRO analyzer. The X-ray diffraction (XRD) measurements were measured on a Rigaku SmartLab (9 kW) X-ray diffractometer. Singlecrystal X-ray analyses were measured on a Bruker SMART II CCD area detector with graphite monochromatic MoKα radiation (λ ¼ 0.7173 Å). 1H-Indene-1,3(2H)-dione, 2,6-dimethyl-4H-pyran-4-one (1), 4-bromo­ benzaldehyde, ethynyltrimethylsilane, tert-butyl (ethynyl)dimethylsi­ lane, ethynyltriisopropylsilane, and n-butylamine were purchased from commercial suppliers and used directly. 2-(2,6-Dimethyl-4H-pyran-4ylidene)-1H-indene- 1,3(2H)-dione (2) was prepared from the Knoeve­ nagel condensation reaction of compound 1 with 1H-indene-1,3(2H)dione in acetic anhydride according to previously reported procedure [24]. 2.2. Synthesis of 2-(2,6-bis((E)-4-bromostyryl)-4H-pyran-4-ylidene)-1Hindene-1,3(2H)-dione (3) Compound 2 (252.1 mg, 1.0 mmol), 4-bromobenzaldehyde (560.9 mg, 3.0 mmol), piperidine (0.5 mL) were dissolved in acetonitrile (20 mL). The mixture was heated at 80 � C for 6 h under N2. After cooling, a large amount of solid precipitation separated out from the mixture. After vacuum filtration, the precipitate was washed three times using aceto­ nitrile and ethanol, respectively, and then dried to afford pure com­ pound 3. Orange solid (549.1 mg), 94% yield, m.p. 263–264 � C. 1H NMR 2

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(400 MHz, CDCl3) δ ppm: 8.42 (s, 2H), 7.74 (dd, 3J ¼ 5.6 Hz, 4J ¼ 2.8 Hz, 2H), 7.60 (dd, 3J ¼ 5.6 Hz, 4J ¼ 2.8 Hz, 2H), 7.53 (d, J ¼ 8.4 Hz, 4H), 7.40 (d, J ¼ 8.4 Hz, 2H), 7.37 (d, J ¼ 16.4 Hz, 2H), 6.84 (d, J ¼ 16.0 Hz, 2H). 2.3. Synthesis of PR-TMSA, PR-TBMSA, and PR-TIPSA General procedure: Compound 3 (876 mg, 1.5 mmol), Pd(PPh3)2Cl2 (105.2 mg, 0.15 mmol), CuI (28.7 mg, 0.15 mmol), PPh3 (39.3 mg, 0.15 mmol) and trimethylsilylacetylene/tert-butyldimethylsilylacetylene/ triisopropylsilylacetylene (9.00 mmol) was dissolved in the mixed sol­ vent of Et3N (30 mL) and THF (30 mL). The reaction mixture was heated at 75 � C for 8 h under a nitrogen atmosphere. After cooling, the solvent was removed and the residue was extracted with chloroform three times. The combined organic layers were washed with water and then dried with anhydrous Na2SO4. The solvent was evaporated in vacuum to dryness and the residue was purified by silica gel column chromatog­ raphy (petroleumether/ethyl acetate) (15:1, v/v) to afford pure 4Hpyran derivative. Characterization data for 2-(2,6-bis((E)-4-((trimethylsilyl)ethynyl) styryl)-4H-pyran-4-ylidene)-1H-indene-1,3(2H)-dione (PR-TMSA). Orange-red solid (890.1 mg), 87% yield, m.p. 204–205 � C. 1H NMR (500 MHz, CDCl3) δ ppm: 8.35 (s, 2H), 7.68–7.67 (m, 2H), 7.54–7.53 (m, 2H), 7.47–7.43 (m, 8H), 7.33 (d, J ¼ 16.0 Hz, 2H), 6.77 (d, J ¼ 16.0 Hz, 2H), 0.29 (s, 18H). 13C NMR (125 MHz, CDCl3) δ ppm: 192.3, 159.4, 147.3, 140.7, 135.2, 134.9, 133.3, 132.5, 127.4, 124.6, 121.4, 120.7, 109.6, 109.4, 104.6, 96.9, 29.7. MS (ESI, m/z): 621.22 [MþH]þ. Anal. Calcd for C40H36O3Si2: C, 77.38; H, 5.84. Found C, 77.69; H, 5.87. Characterization data for 2-(2,6-bis((E)-4-((tert-butyldimethylsilyl) ethynyl)styryl)-4H-pyran-4- ylidene)-1H-indene-1,3(2H)-dione (PRTBSA). Orange-red solid (965.1 mg), 91% yield, m. p. 250–251 � C. 1H NMR (400 MHz, CDCl3) δ ppm: 8.45 (s, 2H), 7.76 (dd, 3J ¼ 5.2 Hz, 4J ¼ 2.8 Hz, 2H), 7.61 (dd, 3J ¼ 5.6 Hz, 4J ¼ 2.8 Hz, 2H), 7.51 (s, 8H), 7.45 (d, J ¼ 16.0 Hz, 2H), 6.89 (d, J ¼ 16.0 Hz, 2H), 1.01 (s, 18H), 0.21 (s, 12H). 13 C NMR (125 MHz, CDCl3) δ ppm: 192.3, 159.4, 147.3, 140.7, 135.3, 134.9, 133.3, 132.5, 132.2, 129.0, 127.4, 124.7, 121.4, 120.7, 109.3, 105.3, 95.4, 26.2, 16.7, 4.6. MS (ESI, m/z): 705.31 [MþH]þ. Anal. Calcd for C46H48O3Si2: C, 78.36; H, 6.86. Found C, 77.98; H, 6.83. Characterization data for 2-(2,6-bis((E)-4-((triisopropylsilyl)ethy­ nyl)styryl)-4H-pyran-4- ylidene)-1H-indene-1,3(2H)-dione (PR-TIPSA). Orange-red solid (969.3 mg), 82% yield, m. p. 265–266 � C. 1H NMR (400 MHz, CDCl3) δ ppm: 8.46 (s, 2H), 7.76 (dd, 3J ¼ 5.2 Hz, 4J ¼ 2.8 Hz, 2H), 7.61 (dd, 3J ¼ 5.2 Hz, 4J ¼ 2.8 Hz, 2H), 7.52 (s, 8H), 7.46 (d, J ¼ 16.0 Hz, 2H), 6.89 (d, J ¼ 16.0 Hz, 2H), 1.15 (s, 42H). 13C NMR (125 MHz, CDCl3) δ ppm: 192.3, 159.5, 147.3, 140.8, 135.3, 134.8, 133.2, 132.6, 132.2, 129.0, 127.4, 125.0, 121.4, 120.6, 109.6, 109.3, 106.7, 93.5, 18.7, 18.4, 11.4. MS (ESI, m/z): 789.41 [MþH]þ. Anal. Calcd for C52H60O3Si2: C, 79.14; H, 7.66. Found C, 79.57; H, 7.62.

Fig. 1. Crystal structures of DHP-TMSA (a) and DHP-TBMSA (b), and the diheral angles between different planes in the crystal structures. Hydrogen atoms are omitted for clarity.

ppm: 192.5, 149.1, 148.5, 140.3, 138.5, 135.1, 132.5, 131.8, 127.2, 124.4, 119.90, 119.87, 114.8, 104.6, 103.1, 96.5, 49.9, 31.5, 19.6, 13.4. FT-IR (KBr, cm 1): 2961, 2157, 1627, 1587, 1537, 1494, 1341, 1249, 1090, 977, 864, 843, 730. MS (ESI, m/z): 676.30 [MþH]þ. Anal. Calcd for C44H45NO2Si2: C, 78.18; H, 6.71. Found C, 78.51; H, 6.68. Characterization data for 2-(1-butyl-2,6-bis((E)-4-((tert-butyldime­ thylsilyl)ethynyl)styryl)pyridin-4(1H)-ylidene)-1H-indene-1,3(2H)dione (DHP-TBMSA). Yellow solid (364.4 mg), 32% yield, m.p. 316–317 � C. 1H NMR (400 MHz, CDCl3) δ ppm: 9.03 (s, 2H), 7.63 (dd, 3J ¼ 5.2 Hz, 4J ¼ 3.2 Hz, 2H), 7.52–7.45 (m, 10H), 7.36 (d, J ¼ 15.6 Hz, 2H), 7.02 (d, J ¼ 15.6 Hz, 2H), 4.13 (t, J ¼ 8.0 Hz, 2H), 1.87–1.79 (m, 2H), 1.48–1.42 (m, 2H), 1.02–0.97 (m, 21H), 0.21 (s, 12H). 13C NMR (125 MHz, CDCl3) δ ppm: 192.6, 149.3, 148.5, 140.4, 138.6, 135.2, 132.6, 131.9, 127.3, 124.5, 119.95, 119.92, 114.9, 105.3, 103.2, 95.1, 49.9, 31.6, 26.2, 19.7, 16.8, 13.4, 4.6. FT-IR (KBr, cm 1): 2928, 2153, 1620, 1588, 1486, 1355, 1338, 1312, 1090, 974, 863, 846, 832, 774. MS (ESI, m/z): 760.39 [MþH]þ. Anal. Calcd for C50H57NO2Si2: C, 79.00; H, 7.56. Found C, 79.35; H, 7.59. Characterization data for 2-(1-butyl-2,6-bis((E)-4-((triisopropylsilyl) ethynyl)styryl)pyridin-4(1H)-ylidene)-1H-indene-1,3(2H)-dione (DHPTIPSA). Yellow-green solid (455.3 mg), 36% yield, m.p. 334–335 � C. 1H NMR (400 MHz, CDCl3) δ ppm: 9.01 (s, 2H), 7.62–7.60 (m, 2H), 7.52–7.43 (m, 10H), 7.33 (d, J ¼ 15.6 Hz, 2H), 6.99 (d, J ¼ 15.6 Hz, 2H), 4.09 (t, J ¼ 8.0 Hz, 2H), 1.82–1.78 (m, 2H), 1.44–1.37 (m, 2H), 1.15 (s, 42H), 0.97 (t, J ¼ 7.2 Hz, 3H). 13C NMR (125 MHz, CDCl3) δ ppm: 192.8, 149.4, 148.4, 140.4, 138.8, 135.1, 132.6, 131.9, 127.2, 124.9, 120.0, 119.8, 114.9, 106.6, 103.4, 93.3, 49.9, 31.8, 19.8, 18.7, 13.5, 11.4. FTIR (KBr, cm 1): 2946, 2150, 1629, 1591, 1358, 1338, 1312, 1093, 994, 980, 865, 820, 732. MS (ESI, m/z): 844.49 [MþH]þ. Anal. Calcd for C56H69NO2Si2: C, 79.66; H, 8.24. Found C, 79.29; H, 8.20.

2.4. Synthesis of DHP-TMSA, DHP-TBMSA, and DHP-TIPSA General procedure: A mixture of compound PR-TMSA/PR-TMBSA/ PR-TIPSA (1.5 mmol) and n-butylamine (15 mL) in acetonitrile (25 mL) was heated at 50 � C for 0.5 h under a nitrogen atmosphere. Then, the heating was stopped and the mixture was stirred for 0.5 h. After cooling, a large amount of solid precipitation separated out from the mixture. After filtration, the precipitate was washed three times using acetonitrile and methanol, respectively, and then dried to afford pure DHP derivative. Characterization data for 2-(1-butyl-2,6-bis((E)-4-((trimethylsilyl) ethynyl)styryl)pyridin-4(1H)- ylidene)-1H-indene-1,3(2H)-dione (DHPTMSA). Orange solid (465.8 mg), 46% yield, m.p. 287–288 � C. 1H NMR (400 MHz, CDCl3) δ ppm: 9.02 (s, 2H), 7.61 (dd, 3J ¼ 5.6 Hz, 4J ¼ 3.2 Hz, 2H), 7.51–7.44 (m, 10H), 7.35 (d, J ¼ 15.6 Hz, 2H), 7.01 (d, J ¼ 15.6 Hz, 2H), 4.12 (t, J ¼ 7.6 Hz, 2H), 1.86–1.79 (m, 2H), 1.49–1.40 (m, 2H), 0.99 (d, J ¼ 7.2 Hz, 3H), 0.28 (s, 18H). 13C NMR (125 MHz, CDCl3) δ

3. Results and discussion 3.1. Synthesis and characterization The chemical structures and synthetic routes of DHP-TMSA, DHPTBMSA, and DHP-TIPSA are shown in Scheme 1. Firstly, the 3

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Table 1 Various intermolecular interactions in crystals DHP-TMSA and DHP-TBMSA. Crystal

intermolecular interaction

d/Å

DHP-TMSA

C22–H22⋅⋅⋅π (Ph) C24–H24⋅⋅⋅π (Ph) C26–H26⋅⋅⋅π (Ph) C49–H49A⋅⋅⋅π (1H-indene-1,3(2H)-dione)

3.269 3.209 3.022 3.212

C51–H51A⋅⋅⋅π (1H-indene-1,3(2H)-dione)

3.140, 2.570

C48–H48B⋅⋅⋅π (Pyridine) C21–H21⋅⋅⋅H43A C36–H36C⋅⋅⋅H44B C38–H38⋅⋅⋅π (Ph) C50–H50B⋅⋅⋅π (Ph) C47–H47A⋅⋅⋅π (1H-indene-1,3(2H)-dione)

3.496 2.349 2.313 3.170 3.589 3.018

C48–H48A⋅⋅⋅π (1H-indene-1,3(2H)-dione)

3.271

C47–H47A⋅⋅⋅π (Pyridine) C6–H6⋅⋅⋅O2 C22–H22⋅⋅⋅O1 C35–H35⋅⋅⋅H50B

3.496 2.433 2.698 2.328

DHP-TBMSA

Fig. 2. Packing mode and intermolecular interactions in DHP-TMSA: (a) Packing mode; (b) C–H⋅⋅⋅H and C–H⋅⋅⋅π interactions in the same column; (c) C–H⋅⋅⋅H interaction between different columns.

intermediate 2-(2,6-bis((E)-4-bromostyryl)-4H-pyran-4-ylidene)-1Hindene-1,3(2H)-dione (3) was prepared from the Knoevenagel reaction of 4-bromobenzaldehyde and compound 2 in the yield of 94%, which was demonstrated by the single-crystal X-ray analysis (Fig. S1, see Supporting Information). Secondly, the Sonogashira-Hagihara reaction of compound 3 with ethynyltrimethylsilane, tert-butyl (ethynyl)dime­ thylsilane, and ethynyltriisopropylsilane afforded PR-TMSA, PRTBMSA, and PR-TIPSA in good yields (82–91%), respectively. Finally, DHP-TMSA, DHP-TBMSA, and DHP-TIPSA were prepared from the nucleophile substitution reaction of n-butylamine with PR-TMSA, PRTBSA, and PR-TIPSA, respectively. All DHP derivatives compounds were demonstrated by 1H NMR and 13C NMR spectroscopy, elemental analysis, and mass spectrometry. Furthermore, the chemical structures of DHP-TMSA and DHP-TBMSA were demonstrated by the singlecrystal X-ray analysis. 3.2. Single-crystal X-ray diffraction analysis The single crystals of DHP-TMSA and DHP-TBMSA were both cultured from a slow diffusion of a CHCl3/MeOH (v/v ¼ 1:1) mixture. The crystal structures of DHP-TMSA and DHP-TBMSA are shown in Fig. 1. The respective crystallographic data and refinement parameters are shown in Table S1 (Supporting Information). The single crystals of DHP-TMSA and DHP-TBMSA both belong to a triclinic system in a P�ı space group. For DHP-TMSA, taking 1,4-dihydropyridine ring (C10, C11, C12, C13, C14, N1) as ring A and 1H-indene-1,3(2H)-dione as ring B, the diheral angle between ring A and ring B is determined to be 6.20� (Fig. 1). Moreover, the diheral angles between ring A and phenyl ring C (C17–C22) and phenyl ring D (C25–C30) are 33.43� and 41.10� , respectively. The results reveal that DHP-TMSA has a twisted molecular conformation. The distances of the centers of the adjacent 1,4-dihydro­ pyridine units in the same column are 4.179 and 4.415 Å (Fig. 2b), revaling the absence of strong π–π stacking interaction in the crystal structure. As displayed in Fig. 2 and Table 1, the adjacent molecules of DHP-TMSA in the same column are linked by one kind of C–H⋅⋅⋅H van der Waals force with a distance of 2.349 Å and multiple C–H⋅⋅⋅π bonds (2.570–3.496 Å) (Fig. 2b), while those in different columns are only linked by a C–H⋅⋅⋅H interaction (2.313 Å) (Fig. 2c). These multiple weak

Fig. 3. Packing mode and intermolecular interactions in DHP-TBMSA: (a) Packing mode; (b) C–H⋅⋅⋅O hydrogen bond and C–H⋅⋅⋅π interactions in the same column; (c) C–H⋅⋅⋅H interaction, C–H⋅⋅⋅π interaction, and C–H⋅⋅⋅O hydrogen bond between adjacent columns.

interactions cause that the DHP-TMSA molecules adopted a zigzagshape arrangement with a head-to-tail aggregation (Fig. 2a). In view of DHP-TBMSA, the diheral angles between the 1,4-dihydro­ pyridine unit (ring A) and 1H-indene-1,3(2H)-dione (ring B), phenyl ring C (C33–C38), and phenyl ring D (C17–C22) are 13.72� , 32.58� , and 43.18� , respectively. This result indicates that DHP-TBMSA has more twisted conformation than DHP-TMSA, which might be attributed that the TBMSA unit has a larger spatial steric hindrance compared with the TMSA unit. As displayed in Fig. 3 and Table 1, the adjacent molecules of 4

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Fig. 4. Fluorescence spectra (a), fluorescence intensity (b), and absorption spectra (c) in the THF-water mixtures of DHP-TMSA with different water fractions. Inset: Photographic images of the THF-water mixtures at fw ¼ 0 and 90% under irradiation at 365 nm. Concentration: 10.0 μmol/L.

DHP-TBMSA in the same column are linked by a C–H⋅⋅⋅O hydrogen bond (2.698 Å) and multiple C–H⋅⋅⋅π bonds (3.018–3.496 Å) (Fig. 3b and Table 1). In different columns, the adjacent molecules are stabilized by a C–H⋅⋅⋅π interaction (3.589 Å), C–H⋅⋅⋅H interaction (2.328 Å), and C–H⋅⋅⋅O bond (2.433 Å) (Fig. 3c and Table 1). Similar to DHP-TMSA, the DHP-TBMSA molecules also adopted a head-to-tail zigzag aggregation (Fig. 3a). The difference is that the columns in DHP-TMSA develop along the long axis, while those in DHP-TBMSA develop along the short axis. As mentioned above, it can be concluded that DHP-TMSA and DHPTBMSA showed highly twisted molecular conformations. Moreover, the molecules of these compounds are mainly stabilized by some weak intermolecular interactions rather than π–π stacking interactions even if the existence of several aromatic rings, and adopt loose packing modes. Although attempts to obtain single crystals of DHP-TIPSA were unsuc­ cessful, it can be speculated that DHP-TIPSA should also have a twisted structure and adopt a loose packing mode because the TIPS unit has a larger spatial steric hindrance than the TBMSA and TMSA units.

could be found that the wavelengths of the absorption maximum of these compounds show different degree of blue shifts with the increase of the solvent polarity. Different from common D-π-A-type fluorescent molecules, the increased solvent polarity could not result in the red shifts of the emission wavelengths in their fluorescence spectra. This result indicated that the DHP derivatives did not exhibit obvious intra­ molecular charge transfer in the excited state even if they had D-π-Atype structures, which might be ascribed to their twisted molecular conformations confirmed by the X-ray crystallographic analyses. Considering that the DHP derivatives showed highly twisted molecular conformations and loose stacking arrangements, the AIE activities were first investigated. These compounds are soluble in THF but not in water. Their absorption and fluorescence spectra in THF-water mixtures by varying the volume percentage of water (fw) were conducted. Taking DHP-TMS as an example, the THF-water mixtures of this compound (10.0 μmol/L) emitted weak yellow fluorescence when fw � 60% (Fig. 4a and Fig. 4b). The phenomena could be explained as follows. When the water contents were low, the molecules were dispersed and their emissions were reduced by the non-radiative relaxations originating from the intramolecular free rotations [3,18]. When the fw value was in the range of 70–90%, the absorption spectra of the mixtures exhibited outstanding level-off tails in the long-wavelength region, which were caused by the light-scattering effects, revealing that the aggregates were generated (Fig. 4c) [31,32]. At fw ¼ 70%, the molecules began to aggregate and the emission of the mixtures was enhanced sharply. The emission intensity was up to the maximum at fw ¼ 90%, which was 10-fold to that of the pure THF solution. The results suggested that the aggregation of the molecules inhibited the intramolecular free rotations and prevented the non-radiative relaxations, and thus resulted in the enhancement of the emissions, revealing the AIE characteristic of DHP-TMSA [3,33,34]. Similarly, DHP-TBMSA (Fig. S6, see Supporting Information) and DHP-TIPSA (Fig. S7, see Supporting Information) both exhibited obvious AIE activities. The strongest fluorescence intensity was observed at fw ¼ 90% for DHP-TBMSA and DHP-TIPSA. It should be

3.3. AIE properties The DHP derivatives exhibited almost identical UV–vis absorption spectra with two absorption peaks at 341 and 452 nm, and fluorescence spectra with the emission wavelength (λem) of about 570 nm in tetra­ hydrofuran (THF) solution even if they had different tri­ alkylsilylacetylene units (Fig. S2, see Supporting Information). These results revealed that the different trialkylsilylacetylene units had no obvious influences on the excited states and ground states of these molecules. Furthermore, the absorption and fluorescence spectra of DHP-TMSA, DHP-TBMSA, and DHP-TIPSA (Figs. R1-R3, see Support­ ing Information) in various solvents with different polarity, such as toluene, CHCl3, dimethyl sulfoxide (DMSO), N,N-dimethylformamide (DMF), and CH3CN, were investigated and the corresponding absorption and emission data were listed in Table S2 (Supporting Information). It 5

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Fig. 5. Normalized emission spectra of the original samples of DHP-TMSA (a), DHP-TBMSA (b), and DHP-TIPSA (c) upon grinding and fuming (EA vapor). Inset: Photographic images of the solid-state samples under irradiation at 365 nm.

noted that the fluorescence wavelengths and intensities of DHP-TMSA (Fig. 4b), DHP-TBMSA (Fig. S6b, see Supporting Information), and DHP-TIPSA (Fig. S7b, see Supporting Information) with the increase of the water contents exhibited complex changes. The phenomena were often observed in the previously reported organic fluorescent molecules with AIE properties, which were thought to be attributed that the dispersed molecules could aggregate into different nanoaggregates with different morphologies at higher water fractions [35–38]. 3.4. MC properties Considering that the twisted molecular conformations and obvious AIE propertiers, the solid-state emission stimuli-responsive behaviors of these compounds were further investigated. The original sample of DHP-TMSA exhibited orange fluorescene with the λem value of 592 nm (Fig. 5a), while those of DHP-TBMSA (Fig. 5b) and DHP-TIPSA (Fig. 5c) both emitted yellow fluorescene with the λem value of 562 nm, which should be attributed that the molecules of DHP-TBMSA and DHP-TIPSA had more twisted molecular confromations than that of DHP-TMSA, and thus blue-shifted the fluorecence spectra. After being ground in a mortar with a pestle, the DHP-TMSA original sample was transformed into redemitting solid (λem ¼ 609 nm) and displayed 17-nm red-shift in the fluorescence spectrum. After grinding, the original samples of DHPTBMSA and DHP-TIPSA were both transformed into red-emitting solids with the λem values of 608 and 602 nm, and exhibited 46 and 40-nm redshift in the fluorescence spectra, respectively. Considering that the ground samples of these compounds showed almost identical fluores­ cence wavelengths, the ground samples should have very similar mor­ phologies even if they had different substituents. Furthermore, it could be concluded that the introduction of the substituents with larger spatial steric hindrance was advantageous to achieve higher contrast MC activities. To understand the causes of MC activities, the XRD curves (Fig. 6) and solid-state absorption spectra (Fig. S8, see Supporting Information) of the DHP derivatives before and after grinding were investigated. As shown in Fig. 6, the original samples of DHP-TMSA, DHP-TBMSA and

Fig. 6. XRD patterns of the solid samples of the DHP derivatives: (a) original sample of DHP-TMSA; (b) the ground sample of DHP-TMSA; (c) fumed sample of DHP-TMSA; (d) the original sample of DHP-TBMSA; (e) ground sample of DHP-TBMSA; (f) the fumed sample of DHP-TBMSA; (g) original sample of DHP-TIPSA; (h) the ground sample of DHP-TIPSA; (i) fumed sample of DHP-TIPSA.

the the the the the

DHP-TIPSA displayed some sharp and intense reflection peaks, which implied that they were in crystalline states. Furthermore, for DHPTMSA and DHP-TBMSA, the fluorescence spectra of their single crystals were investigated (Fig. S9, see Supporting Information). The results showed that these fluorescence spectra were in accordance with those of the corresponding original samples, which further confirmed the crys­ talline structures of the original samples of DHP-TMSA and DHPTBMSA. After grinding, the sharp and intense reflection peaks almost disappeared or widened markedly, revealing the amorphous character­ istics of the ground samples. These results indicated that grinding destroyed the crystalline structures of the DHP derivatives and caused 6

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Table 2 Fluorescence parameters of the solid samples of DHP-TMSA, DHP-TBMSA, and DHP-TIPSA. Compound

Type

λem (nm)

τ1 (ns)

τ2 (ns)

A1

A2

<τ> (ns)

ФF

DHP-TMSA

Original Ground Fumed Original Ground Fumed Original Ground Fumed

592 609 589 562 608 564 562 602 562

0.26 0.40 0.37 0.53 0.57 0.61 0.47 0.41 0.36

2.69 3.11 2.65 3.77 3.34 3.75 4.52 3.64 5.15

0.92 0.86 0.93 0.85 0.89 0.87 0.81 0.88 0.72

0.08 0.14 0.07 0.15 0.11 0.13 0.19 0.12 0.28

0.45 0.44 0.53 1.02 0.87 1.02 1.24 0.80 1.7

2.5% 1.8% 2.6% 7.8% 1.9% 8.4% 5.8% 2.3% 3.5%

DHP-TBMSA DHP-TIPSA

the transformations from crystalline states to amorphous states in the morphologies [37,39–41]. According to the results of the X-ray crys­ tallographic analyses, the DHP derivatives displayed twisted molecular conformations and adopted loose packing arrangements, which pro­ vided considerable space to facilitate the planarization of the molecular conformations when the crystalline samples were ground. Generally, the planarization of the molecular conformations easily resulted in the formation of the eximers caused by the enhanced π-π stacking in­ teractions. Therefore, the red-shifted fluorescence spectrum might arise from the eximer formation or the planarization of the molecular conformation, in which the absorption maximum or onset was almost unchanged for the former, while showed outstanding change for the latter [42,43]. For DHP-TMSA, DHP-TBMSA, and DHP-TIPSA, as des­ picted in Fig. S8 (Supporting Information), the onsets of the solid-state absorption spectra of the ground samples were obviously larger than those of the corresponding original samples. This result indicated that although the grinding treatment could bring the molecules of the ground samples closer, the strong π-π stacking interactions were not formed because of the existence of the large steric hindrance groups. The red-shifted emissions of these compounds upon grinding should be mainly ascribed to the increased molecular conjugations by the plana­ rization of the molecular conformations. Considering that the different trialkylsilylacetylene units had no obvious effects on the intramolecular charge transfer of the excited states for the similar molecular confor­ mations (Fig. S2b, see the Supporting Information), the ground samples of the DHP derivatives showed almost identical emission wavelengths (Fig. 5). Compared with DHP-TMSA, DHP-TBMSA and DHP-TIPSA were found to exhibit higher contrast MC activities, which should be attributed that they had more twisted molecular conformations than the former. For these DHP derivatives, when their ground samples were fumed by ethyl acetate (EA) vapor, the diffraction peaks recovered and the λem values blue-shifted to 589 (DHP-TMSA), 564 (DHP-TBMSA), and 562 nm (DHP-TIPSA), which were almost identical to that of the respective original sample. This result suggested the fuming treatment could lead to the recovery of fluorescence and these compounds exhibited reversible MC activities. The solid-state time-resolved emission decay behaviors of the DHP derivatives before and after grinding/fuming were further investigated (Table 2). The weighted mean lifetime <τ> could be determined from the following equation: <τ> ¼ (A1τ1þA2τ2)/(A1þA2). Herein, τ1 and τ2 are the lifetimes of the excited molecules decaying by the fast and slow channels, respectively, and A2 and A1 are their respective fractions, and these values are obtained from I ¼ A1exp (-t/τ1)þA2exp (-t/τ2) [44]. As despicted in Table 2, the <τ> values of the original samples of DHP-TMSA, DHP-TBMSA, and DHP-TIPSA were 0.45, 1.02, and 1.24 ns, while those of the corresponding ground samples were 0.44, 0.87, and 0.80 ns. For DHP-TMSA, the <τ> values before and after grinding were similar, revealing that the molecular conformations did not change much, which was consistent with the change of emission wavelength. However, with regard to DHP-TBMSA and DHP-TIPSA with high contrast MC activities, grinding decreased the <τ> values significantly, which was similar to previously reported results [21,37,45]. In accor­ dance with this, the original samples of DHP-TBMSA and DHP-TIPSA

showed greater changes in the ФF values than that of DHP-TMSA. The decrease of fluorescence quantum yield might be due to the fact that grinding destroyed the intermolecular interactions in crystal structures and enhanced the non-radiative relaxations. 4. Conclusions In this work, three D-π-A DHP derivatives containing tri­ alkylsilylacetylenes were synthesized to investigate the influence of steric hindrance on the MC activities. The X-ray crystallographic ana­ lyses revealed that these compounds had twisted molecular conforma­ tions. The different trialkylsilylacetylenes were found to have no effect on the fluorescence and absorption spectra of these compounds in THF solutions. Compared with orange-emitting DHP-TMSA (λem ¼ 562 nm), DHP-TBMSA and DHP-TIPSA in the crystalline state both emitted yel­ low fluorescene and blue-shifted fluorescence spectra (λem ¼ 592 nm) owing to more twisted molecular confromations caused by the larger steric hindrances. Furthermore, the loose packing modes in the DHP derivatives, as revealed by the X-ray crystallographic analyses, caused that their crystalline structures were easily destroyed, resulting in MC properties owing to the crystalline-to-amorphous transformation. Upon grinding, DHP-TMSA showed a change of solid-state fluorescence color from orange to red, while DHP-TBMSA and DHP-TIPSA both exhibited a color change from yellow to red, which revealed that the introduction of the substituents with larger spatial steric hindrances was favorable for achieving higher contrast MC activities. The red shifts of the fluores­ cence spectra caused by grinding were confirmed to be mainly ascribed to the increased molecular conjugations by the planarization of the molecular conformations rather than the formation of strong π-π stack­ ing interactions. Our results provide useful information for the design and synthesis of new DHP derivatives containing ethynyl units with excellent MC properties. Declaration of competing interest There are no conflicts to declare. CRediT authorship contribution statement Jingqing Peng: Methodology, Investigation. Yanze Liu: Investiga­ tion. Mengzhu Wang: Investigation, Validation. Shihao Huang: Soft­ ware, Validation. Miaochang Liu: Data curation, Resources. Yunbing Zhou: Formal analysis. Wenxia Gao: Visualization. Xiaobo Huang: Conceptualization, Funding acquisition, Writing - review & editing. Huayue Wu: Supervision, Funding acquisition. Acknowledgments The authors gratefully acknowledge the financial support from the National Natural Science Foundation of China (Nos 21572165 and 21472140) and the Graduate Innovation Foundation of Wenzhou Uni­ versity in China (No 3162018035).

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Appendix A. Supplementary data [24]

Supplementary data to this article can be found online at https://doi. org/10.1016/j.dyepig.2019.108094.

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