Construction of a Layer Hydrogen-Bonded Organic Framework

Article Cite This: J. Phys. Chem. C XXXX, XXX, XXX−XXX

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Construction of a Layered Hydrogen-Bonded Organic Framework Showing High-Contrast Mechanoresponsive Luminescence Turn-On Yu-Xin Peng,† Yi-Tao Gan,‡ Rong-Guang Shi,† Wei Huang,*,† and Tao Tao*,‡ †

J. Phys. Chem. C Downloaded from pubs.acs.org by UNIV OF SOUTH DAKOTA on 12/18/18. For personal use only.

State Key Laboratory of Coordination Chemistry, Nanjing National Laboratory of Microstructures, School of Chemistry and Chemical Engineering, Nanjing University, Nanjing 210093, Jiangsu Province, P. R. China ‡ Jiangsu Key Laboratory of Atmospheric Environment Monitoring and Pollution Control, Collaborative Innovation Center of Atmospheric Environment and Equipment Technology, School of Environmental Science and Engineering, Nanjing University of Information Science & Technology, Nanjing 210044, P. R. China S Supporting Information *

ABSTRACT: This paper probes the mechanoresponsive luminescence and aggregation-induced emission (AIE) properties of N-phenylcarbazole-based thiophene/dicyanovinyl (T−DCV) derivatives. In bis-T−DCV planar molecule 1, a layered hydrogen-bonded organic framework (HOF) structure is formed with the assistance of multiple C−H···N and strong interlayer interactions. The formation of such a layer packing HOF structure can quench the solid fluorescent emission when aggregated as nanosuspensions but guarantees the highcontrast luminescence on−off switching (1.2 × 103 fold) in the solid state. Moreover, the grinding-induced destruction of HOF 1 is reflected by powder X-ray diffraction measurements, in which the destruction of the α and β rings in the planar HOF stripe and the close interlayer stacking could be observed. This work achieves the successful construction of longwavelength emission AIE molecules (2 and 3) by introducing sufficient steric hindrance rotors to disturb the intermolecular π−π stacking and/or dipole−dipole interactions.

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INTRODUCTION Intermolecular interactions, such as hydrogen-bonding, π−π stacking, and noncovalent conformational locks, have a close relation to the photophysical properties of π-conjugated molecules, which plays very important roles in determining the final performance of organic optoelectronic devices for many advanced applications, such as organic light-emitting diodes (OLEDs), field-effect transistors, and photovoltaic cells.1−8 For example, the achievement of high carrier mobility is one of the major goals in the field of organic semiconductors. To fulfill this requirement, the molecule is generally designed to have a large defined length to render expected photo/electronic properties, in which strong intermolecular π−π stacking and dipole−dipole interactions are vital to the conducting pathways.9−11 In contrast, π−π interactions are detrimental to electroluminescence materials for OLEDs because of the formation of excimers and exciplexes in their condensed state.12 Therefore, fine-tuning of intermolecular interactions is of great significance for rational design of π-conjugated molecules. Since the concept of aggregation-induced emission (AIE) was pioneered by Tang and co-workers in 2001,13 a new perspective to develop functional luminescence materials has opened up. The AIE phenomenon, different from aggregation-caused quenching (ACQ), is very important and practical owing to its inherent application potential in the solid state, including high© XXXX American Chemical Society

tech applications in optoelectronics, chemosensors, and bioimaging.14−18 Recently, some AIE-based dual-function molecular materials have also been reported, which exhibit both semiconducting and emissive properties.19,20 More importantly, mechanoresponsive luminescence (MRL) materials, one of the smart responsive materials, have attracted considerable attention because of their interesting applications in sensors, memory chips, and smart inks.21−28 One obvious feature of MRL materials is that their absorption and/or emission of light can be modulated by mechanical forces, such as grinding, smearing, or hydrostatic pressure. However, most of the known MRL molecules give only luminescence dichromic behavior and reports on high-contrast MRL turn-on are still rare.29−33 Recently, a new strategy for constructing highperformance on−off AIE−MRL materials has been reported by combining nitrophenyl groups with traditional AIE molecules, where the twisted and propeller-like conformation of parent AIE core as well as the high crystallinity of resultant organic molecules plays important roles in the MRL process.34,35 Moreover, the ultrasensitive MRL properties of nitrosubstituted tetraphenylethene have been successfully applied into the Received: October 17, 2018 Revised: December 4, 2018 Published: December 5, 2018 A

DOI: 10.1021/acs.jpcc.8b10118 J. Phys. Chem. C XXXX, XXX, XXX−XXX

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The Journal of Physical Chemistry C Chart 1. Chemical Structures of PC−T−DCV-Based Compounds 1−3

(EI-TOF-MS) (m/z): calcd for [C34H17N5S2]+ 559.1 (100.0%), found 558.9 (100.0%). Anal. calcd for C34H17N5S2: C, 72.97; H, 3.06; N, 12.51%. Found: C, 72.73; H, 3.28; N, 12.39%. Drak red single crystals of 1 suitable for X-ray diffraction determination were grown from a solution of tetrahydrofuran (THF) by slow evaporation in air at 8−15 °C for 7 days. Compound 2. 2 was prepared according to a procedure similar to that for 1 except that 8 was used in place of 7. Compound 8 (0.10 g, 0.17 mmol), malononitrile (22.5 mg, 0.34 mmol), piperidine (two drops), CH3CN (20 mL), and CH2Cl2 (10 mL) were used. Red solid, yield: 89%. 1H NMR (500 MHz, CDCl3) δ: 8.58 (s, 2H), 8.22 (d, 2H, J = 7.7 Hz), 9.01 (d, 2H, J = 8.3 Hz), 7.82−7.78 (m, 3H), 7.73−7.68 (m, 5H), 7.64−7.61 (m, 2H), 7.58−7.54 (m, 5H), 7.50−7.45 (m, 3H), 7.37−7.32 (m, 2H). 13C NMR (125 MHz, CDCl3) δ: 154.8, 150.6, 142.2, 141.2, 140.9, 140.5, 136.8, 136.5, 133.5, 133.4, 130.2, 128.7, 128.3, 127.0, 126.0, 124.9, 124.3, 123.6, 123.5, 123.4, 120.4, 120.0, 119.3, 119.1, 114.6, 113.8, 110.9, 110.8, 109.9. EI-TOFMS (m/z): calcd for [C44H26N4S]+ 642.2 (100.0%), found 642.0 (100.0%). Anal. calcd for C44H26N4S: C, 82.22; H, 4.08; N, 8.72%. Found: C, 82.01; H, 4.30; N, 8.56%. Red single crystals of 2 suitable for X-ray diffraction determination were grown from a solution of CHCl3 by slow evaporation in air at 20−25 °C for 3 days. Compound 3. 3 was prepared according to a procedure similar to that for 1 except that 9 was used in place of 7. Compound 9 (0.10 g, 0.17 mmol), malononitrile (22.5 mg, 0.34 mmol), piperidine (two drops), CH3CN (20 mL), and CH2Cl2 (10 mL) were used. Red solid, yield: 85%. 1H NMR (500 MHz, CDCl3) δ: 8.56 (dd, 2H, J = 12.3, 2.7 Hz), 8.49 (d, 1H, J = 1.5 Hz), 8.29 (d, 1H, J = 2.8 Hz), 7.86−7.82 (m, 1H), 7.79−7.77 (m, 2H), 7.74−7.70 (m, 2H), 7.68−7.59 (m, 8H), 7.55−7.50 (m, 5H), 7.46−7.43 (m, 3H), 7.36−7.31 (m, 1H). EI-TOF-MS (m/z): calcd for [C44H26N4S]+ 642.2 (100.0%), found 642.0 (100.0%). Anal. calcd for C44H26N4S: C, 82.22; H, 4.08; N, 8.72%. Found: C, 81.97; H, 4.40; N, 8.44%. Red single crystals of 3 suitable for X-ray diffraction determination were grown from a solution of CHCl3 by slow evaporation in air at 10−15 °C for 2 days. Compound 5. In the absence of light, N-bromosuccinimide (NBS) (1.52 g, 8.54 mmol) was dissolved in CH2Cl2 (50 mL) and injected into a solution of compound 4 (3.00 g, 8.49 mmol) in CH2Cl2 (100 mL). The mixture was refluxed for 20 h, and the resulting slurry was cooled to 0 °C and filtered. The solid was rinsed with distilled water, recrystallized from CHCl3−hexane, and dried in vacuum to give pure compound 5 as a pale yellow solid (3.38 g, 92%). 1H NMR (500 MHz, CDCl3) δ: 9.92 (s, 1H), 8.41 (s, 1H), 8.33 (s, 1H), 7.80−7.75 (m, 2H), 7.68−7.65 (m, 2H), 7.56−7.55 (m, 4H), 7.49−7.48 (m, 1H), 7.44−7.42 (m, 1H), 7.31 (s, 1H). 13C NMR (125 MHz, CDCl3) δ: 182.6, 155.4, 141.7, 140.2, 137.7, 136.7, 130.2, 129.4, 128.2, 126.9,

visualization of stress/strain distributions on metal specimens by Tang and co-workers.36 Actually, the exploration of novel AIE and/or MRL molecules having new parent skeletons is a long-standing and challenging issue. Thiophene−dicyanovinyl (T−DCV) derivatives are considered as promising candidates for studies on organic semiconducting devices, where the unique intramolecular cyano-thiophene (CN···S) interactions could enforce the molecular planarity and promote the π−π stacking formation. At the same time, high polarization effect of the DCV group generally leads to strong intramolecular charge transfer (ICT) and low band gap energy.37,38 Therefore, incorporation of such a T−DCV group into a luminogen is expected to alter the photophysical behavior of the resultant molecule, particularly its color tenability. Nevertheless, the risk of ACQ may also increase. Actually, examples of T−DCV-based AIE and two-color MRL compounds are seldom mentioned in the literature.20,39 However, as far as we are aware, the observation of highcontrast MRL turn-on by using T−DCV groups is not documented. We report herein on N-phenylcarbazole− thiophene−dicyanovinyl (PC−T−DCV)-based luminogens 1−3 (Chart 1). The middle N-phenylcarbazole is used as an electron-donating (D) heterocyclic unit to connect the electron accepting (A) T−DCV group and allowed to further link different substituents at the 6-position (1−3) to finely tune the optical properties. Compared to the already known compounds exhibiting AIE and/or MRL behavior in the relative shortwavelength region, our new molecules have the distinguishing A−D−A and D−D−A structures and long-wavelength emissions would be expected. Furthermore, introduction of two T−DCV moieties in 1 tends to form larger planar conformation and the resulting molecular assemblies could have stronger dipole−dipole interactions, in which a metastable nonemissive state and high-contrast MRL on−off properties may be generated. Compared with 1, compounds 2 and 3 have isomeric N-phenylcarbazole units, in which the packing fashions would be loose because of their highly twisted molecular structures and the AIE property would be expected.

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EXPERIMENTAL SECTION Syntheses of Compounds. Compound 1. Compound 7 (0.10 g, 0.22 mmol), malononitrile (72.7 mg, 1.10 mmol), and a catalytic amount of piperidine (two drops) were dissolved in CH3CN (20 mL) and CH2Cl2 (10 mL). The mixture was refluxed for 5 h, and the resulting slurry was cooled to 0 °C and filtered. The solid was rinsed with distilled water, recrystallized from CHCl3−hexane (6:1), and dried in vacuum to give pure compound 1 as a red solid (0.91 g, 74%). 1H NMR (500 MHz, DMSO-d6) δ: 8.92 (s, 2H), 8.64 (s, 2H), 7.99 (d, 2H, J = 4.1 Hz), 7.92−7.88 (m, 4H), 7.75−7.61 (m, 5H), 7.45 (d, 2H, J = 8.6 Hz). Electron ionization time of flight mass spectroscopy B


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The Journal of Physical Chemistry C Scheme 1. Synthetic Routes for PC−T−DCV-Based Compounds 1−3

Figure 1. HOMO and LUMO spatial distributions calculated by B3LYP/6-31G* for compounds 1−3.

(100.0%). Anal. calcd for C27H17NOS2: C, 74.45; H, 3.93; N, 3.22%. Found: C, 74.21; H, 4.11; N, 3.01%. Compound 7. 6 (0.30 g, 0.69 mmol) was allowed to dissolve completely in 20 mL of DMF. POCl3 (0.19 mL, 112.8 mmol) was added dropwise to the mixture at 0 °C. The reaction mixture was then heated to 70 °C with stirring for 8 h. Upon cooling, the mixture was poured into an ice bath and neutralized with Na2CO3. The product was extracted with chloroform and purified by silica gel column chromatography using CHCl3 as an eluent. Yellow solid, yield: 32%. 1H NMR (500 MHz, DMSOd6) δ: 9.95 (s, 2H), 8.85 (s, 2H), 8.01(d, 2H, J = 3.9 Hz), 7.92 (dd, 2H, J = 8.6, 1.6 Hz), 7.78−7.73 (m, 4H), 7.70 (d, 2H, J = 7.5 Hz), 7.64−7.61 (m, 1H), 7.49 (d, 2H, J = 8.6 Hz). 13C NMR (125 MHz, CDCl3) δ: 128.5, 155.2, 141.8, 141.6, 137.7, 136.5, 130.1, 128.2, 126.8, 125.2, 123.2, 118.5, 110.7. EI-TOF-MS (m/ z): calcd for [C28H17NO2S2]+ 463.1 (100.0%), found 463.3 (100.0%). Anal. calcd for C28H17NO2S2: C, 72.55; H, 3.70; N, 3.02%. Found: C, 72.31; H, 3.94; N, 2.78%. Compound 8. 8 was prepared according to a procedure similar to that for compound 6 except that 4-(N-carbazolyl)phenylboronic acid was used in place of 2-thiophenylboronic acid. Yellow solid, yield: 82%. 1H NMR (500 MHz, CDCl3) δ: 9.93 (s, 1H), 8.56 (d, 2H, J = 8.3 Hz), 8.22 (d, 2H, J = 4.7 Hz), 8.00 (d, 2H, J = 4.8 Hz), 7.83−7.78 (m, 3H), 7.73−7.69 (m, 4H), 7.66 (d, 2H, J = 4.7 Hz), 7.59−7.52 (m, 5H), 7.50−7.46 (m, 3H), 7.36−7.33 (m, 2H). 13C NMR (125 MHz, CDCl3) δ:

125.6, 125.4, 124.7, 123.3, 123.3, 122.9, 118.5, 113.4, 111.7, 110.7. EI-TOF-MS (m/z): calcd for [C23H14BrNOS]+ 433.0 (100.0%), 431.0 (99.3%), found 433.0 (89.3%), 431.1 (100.0%). Anal. calcd for C23H14BrNOS: C, 63.90; H, 3.26; N, 3.24%. Found: C, 63.74; H, 3.51; N, 3.00%. Compound 6. A mixture of 5 (0.40 g, 0.93 mmol), 2thiophenylboronic acid (0.13 g, 1.02 mmol), Pd(PPh3)4 (0.06 g, 0.05 mmol), and Cs2CO3 (0.91 g, 2.79 mmol) was dissolved in a degassed mixture of dioxane (50 mL) and H2O (5 mL), put into a degassed three-necked flask, and refluxed under argon atmosphere for 10 h. After cooling to room temperature, the solution was added into 100 mL of CHCl3 and the organic layer was washed with water, dried over anhydrous Na2SO4, and concentrated in vacuum. The crude product was finally separated by silica gel column chromatography using CHCl3 as the eluent to give pure compound 6 as a yellow solid (0.30 g, 75%). 1H NMR (500 MHz, CDCl3) δ: 9.93 (s, 1H), 8.50 (s, 1H), 8.42 (s, 1H), 7.81 (d, 2H, J = 3.8 Hz), 7.77−7.73 (m, 2H), 7.69−7.66 (m, 2H), 7.60 (d, 2H, J = 7.7 Hz), 7.56−7.54 (m, 1H), 7.52 (d, 1H, J = 3.8 Hz), 7.46−7.41 (m, 3H), 7.33 (d, 1H, J = 5.0 Hz), 7.17−7.15 (m, 1H). 13C NMR (125 MHz, CDCl3) δ: 182.5, 155.7, 145.1, 141.8, 141.5, 141.0, 137.7, 137.0, 130.1, 128.1, 128.0, 127.5, 126.9, 125.4, 125.3, 125.0, 124.1, 123.9, 123.5, 123.2, 122.5, 118.6, 117.9, 110.6, 110.5. EI-TOF-MS (m/ z): calcd for [C27H17NOS2]+ 435.1 (100.0%), found 435.2 C


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The Journal of Physical Chemistry C

Figure 2. Photos of crystal and as-prepared samples of compounds 1−3 taken under room and UV light irradiation with their respective solid quantum yields.

Table 1. Spectroscopic Data for Compounds 1−3 solution in THF λabs/nm

λem/nm

470 459 467

554 566 601

a

1 2 3

b

solution in THF/water Φsc

λabs/nm

λem/nmb

Φs,aggrc (%)

473/f w/80% 498/f w/90%

609/f w/80% 633/f w/90%

18.43 15.51

a

(%)

22.21 9.32 0.77

a

Wavelength of maximum absorption. bWavelength of maximum emission. cFluorescence quantum yield was determined in ethanol using rhodamine B as a standard.

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RESULTS AND DISCUSSION Syntheses and Photophysical Property. Compounds 1− 3 were prepared through Knoevenagel condensation between malonodinitrile and related carbazole-thiophene-formyl compounds 8−9 (Scheme 1). Compound 5 was synthesized as a key intermediate through a bromination reaction between 740 and NBS in a high yield of 92%. Then, 5 was allowed to further react with various boronic acid reagents via Pd-catalyzed carbon− carbon bond cross-coupling reactions to afford 6, 8, and 9 smoothly. With precursor 6 in hand, compound 7 was prepared via the Vilsmeier formylation. The intermediates and final products were spectroscopically characterized with satisfactory data. The photophysical properties of compounds 1−3 in different organic solutions were investigated first. As can be seen in Figures S1−S3, the absorption spectra of three compounds in various solvents show similar absorption maxima and profiles, which could be ascribed to the little change in the dipole moment in the ground state.33,41 Nevertheless, the emission spectra of 1−3 display strong solvent polarity dependence and the emission bands are bathochromically shifted to the longer wavelength regions to different extents when the solvent polarity is increased from toluene to acetonitrile. In addition, their

182.6, 155.7, 141.8, 141.4, 141.1, 140.8, 140.6,. 137.8, 136.9, 136.3, 133.1, 130.1, 128.5, 128.0, 127.3, 126.9, 126.0, 125.9, 125.4, 124.9, 123.3, 123.1, 120.3, 119.9, 119.0, 118.4, 110.6, 109.8. EI-TOF-MS (m/z): calcd for [C41H26N2OS]+ 594.2 (100.0%), found 594.2 (100.0%). Anal. calcd for C41H26N2OS: C, 82.80; H, 4.41; N, 4.71%. Found: C, 82.56; H, 4.66; N, 4.47%. Compound 9. 9 was prepared to according to a procedure similar to that for compound 6 except that 9-phenyl-3carbazoleboronic acid was used in place of 2-thiophenylboronic acid. Yellow solid, yield: 85%. 1H NMR (500 MHz, CDCl3) δ: 9.93 (s, 1H), 8.57 (s, 1H), 8.53 (d, 2H, J = 13.4 Hz), 8.29 (d, 1H, J = 7.7 Hz), 7.86−7.76 (m, 4H), 7.71−7.63 (m, 8H), 7.57−7.52 (m, 5H), 7.48−7.47 (m, 3H), 7.37−7.35 (m, 1H). 13C NMR (125 MHz, CDCl3) δ: 182.5, 155.9, 141.7, 141.3, 140.5, 140.0, 137.8, 137.6, 137.1, 134.9, 133.8, 130.0, 129.9, 127.8, 127.4, 127.0, 126.8, 126.5, 126.1, 125.6, 125.1, 124.7, 124.1, 124.0, 123.6, 123.5, 123.0, 120.4, 120.0, 118.9, 118.8, 118.5, 110.4, 110.3, 110.0, 109.9. EI-TOF-MS (m/z): calcd for [C41H26N2OS]+ 594.2 (100.0%), found 594.2 (100.0%). Anal. calcd for C41H26N2OS: C, 82.80; H, 4.41; N, 4.71%. Found: C, 82.53; H, 4.56; N, 4.33%. D


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Figure 3. UV−vis absorption (a, c, and e for 1−3) and fluorescence emission spectra (b, d, and f for 1−3) in THF/water mixtures with the same starting concentration of 1.0 × 10−5 mol L−1 and different water volume fractions (f w). λex = 460 nm.

related Stokes Shift values are increased, which could be assigned as typical intramolecular charge-transfer (ICT) effects. Moreover, the ICT characteristics of the compounds are further supported by the density functional theory calculation results

(Figure 1), in which the highest occupied molecular orbitals (HOMOs) are mainly localized on the electron-donating units (D) and the lowest unoccupied molecular orbitals (LOMOs) are mostly distributed on the electron-withdrawing units (A). E


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solid emission behavior is decided by their respective packing modes, which will be discussed in the following crystal structure parts. Aggregation-Induced Emission Behavior. On the basis of the fact that the solid samples of 2 and 3 show stronger PL intensity than those in solution and hydrogen-bonded organic framework (HOF) 1 displays much stronger brown-yellow fluorescence in solution than its powder sample with weak darkred emission, the aggregation behavior of compounds 1−3 has been investigated for further comparisons (Table 1). A starting concentration of 1.0 × 10−5 mol L−1 in THF with the absorption close to 1 is used in the optical studies for the best sensitivity. As shown in Figure 3a,c,e, with the enhancement of water volume fractions (f w), the UV−vis absorption bands of compounds 1−3 are shifted to longer wavelength regions, indicative of the formation of aggregates. In the case of 1, the solution fluorescence is quenched step by step, with the increase of f w values exhibiting typical ACQ effects (Figures 3b and S5a). Upon going from the 100% THF solution to the 4:6 THF/H2O mixture, compound 2 shows polarity-induced fluorescence quenching. On further increasing the water fraction, the PL intensity is increased obviously together with the observation of insoluble aggregates. The highest fluorescence intensity of Φs,aggr = 18.43% could be recorded when f w is 80%, therefore testifying the AIE nature of this compound (Figures 3d and S5b). Similar to 2, compound 3 also displays AIE activity and the 90% water fraction THF/water mixture solution exhibits the highest PL intensity with Φs,aggr = 15.51%, which is several tenfold higher in comparison with that of their THF solution (Figures 3f and S5c). Mechanoresponsive Luminescence Properties. With the morphology-dependent results in hand, the possible MRL characteristics of 1−3 have been checked. As anticipated, nonemissive crystalline solid 1 becomes emissive upon grinding with the emergence of an obvious blueshift of 46 nm, as shown in Figure 4. At the same time, the solid-state luminescence quantum yield increases from 0.02 to 23.01% (1.2 × 103 fold) after grinding, indicating high-contrast MRL turn-on characteristics. In contrast, the as-prepared sample of 1 also exhibits a 5.8fold PL intensity before and after grinding accompanied by a lesser blueshift of 39 nm. This discrepancy could be ascribed to the presence of crystalline state in the as-prepared solid sample

Figure 4. Emission spectra of crystal, as-prepared, ground crystal, and ground samples for 1 and corresponding photographs (insets) taken under UV illumination; λex = 468 nm.

This ICT fluorescence is associated with the large dipole moment changes between the excited state and the ground state (Table S3). When the charge is transferred from the D parts to the A parts in a more polar solvent, the dipole moment in the excited state is increased more obviously in comparison to that in a less polar solvent. To match the enhanced dipole moment, the polarized excited state is stabilized via the relocation of the polar solvent, which results in the lower energy level and the redshifted photoluminescence (PL) maximum. Morphology-dependent photophysical properties for compounds 1−3 are then studied (Figure 2). The as-prepared solid samples (micro/nanocrystals or amorphous powders) are obtained by reprecipitation, in which a concentrated solution of related compounds in chloroform is poured into exceed petroleum ether. In contrast, all crystalline samples are prepared via slow evaporation (see the Experimental Section). It is noted that crystal 1 is nearly nonemissive with an ultralow quantum yield (Φa, 0.02%) and the luminescence contrast ratio between the as-prepared and crystalline samples has reached to 221. This result reveals that the solid-state luminescence of 1 is strongly related to the morphology, which is critical to the on−off ratios of MRL materials. Nevertheless, no obvious morphologydependent phenomenon could be observed for 2 and 3 with bright red fluorescence emission for both as-prepared and crystalline samples. The distinguishing morphology-dependent

Figure 5. (A) ORTEP drawing, (B) C−H···N interactions, a = 2.656, b = 2.784, and c = 2.771 Å, (C) Perspective view of the crystal packing of compound 2. F


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Figure 6. (A) ORTEP drawing, (B) C−H···N interactions: a = 2.817, b = 2.671, and c = 2.598 Å, (C) Perspective view of the crystal packing of compound 3 (π−π distance: d = 3.641, e = 3.443 Å).

Figure 7. (A) ORTEP drawing of 1 (top view and side view), (B) Planar HOF stripe with α and β rings formed via multiple C−H···N interactions (a = 2.607, b = 2.442, c = 2.937, and d = 2.461 Å), in the HOF structure of 1. (C) The layer packing structure of HOF 1.

of 1, which has been extensively discussed by Tang’ group.34 As for compounds 2 and 3, the PL variations are not remarkable for as-prepared and ground samples (Figure S4). Namely, only subtle blueshifts in the emission maxima (λem) and enhancement of the fluorescence intensity are observed for 2 before and after grinding, whereas slight redshifts in λem and decrease in emission intensity are found in 3. Single-Crystal Structures. Single-crystal structures of 1−3 were obtained to reveal the relationship between the molecular structures of PC−T−DCV derivatives and their AIE and MRL properties. As can be seen in Figures 5A, 6A, and 7A, the DCV parts and their adjacent thiophene rings are nearly coplanar in compounds 1−3 with the assistance of intramolecular CN···S interactions, which are also known as one type of noncovalent conformational locks.5 The related C···S distances are 3.336 and 3.338 Å (1), 3.321 Å (2), and 3.441 Å (3). The dihedral angle

between the thiophene ring and its linked N-phenylcarbazole unit in 2 is only 0.77° and that in 3 is 4.44°, displaying good planarity of these special T−DCV parts. However, the whole molecules of 2 and 3 adopt the highly twisted conformation because of the existence of large dihedral angles between two nonplanar N-phenylcarbazole units (Figure S9). It should be mentioned that the AIE isomeric pair 2 and 3 has distinct packing structure compared to that of MRL turn-on compound 1. A one-dimensional wavelike chain was formed in 2, where the DCV parts of neighboring molecules are linked by three kinds of C−H···N interactions (Figure 5) between the cyano groups and N-phenylcarbazole, thiophene, and vinyl protons. Moreover, the packing between neighboring molecules is loose in 2 and no π−π stacking is found. As shown in Figure 6, every molecule of 3 is connected with three adjacent molecules via the C−H···N and G


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shifted emission and MRL turn-off when suffering mechanical stimuli. To further determine the mechanism of the MRL turn-on for HOF 1, powder X-ray diffraction (PXRD) measurements have been carried out (Figure 8). After grinding, the sharp peaks become evidently weak or even diminished, suggesting the destruction of crystal lattice through the mechanical force. In detail, the peaks at 2θ = 7.44° (11.97 Å) and 11.34° (7.85 Å) are significantly weakened after grinding, which are consistent with the distances of C8−C27 (11.95 Å) and S2−C3 (7.76 Å) belonging to two connected molecules in the single-crystal structure (Figures S5 and S6). This result indicates that the α and β rings shown in Figure 7B are destroyed to a great extent. Moreover, the sharp peak at 26.71° (3.33 Å) associated with the distance between two adjacent HOF stripe turns out to be broad and weak at the same time, suggesting that the regular and close interlayer stacking in the HOF (Figure S8) is broken and the blue-shifted MRL turn-on is observed.

Figure 8. PXRD spectra of 1 in different states: crystal simulated (blue), HOF state (black), ground HOF state (red).

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CONCLUSIONS In summary, three long-wavelength emissive N-phenylcarbazole/thiophene/dicyanovinyl luminogens 1−3 have been designed and synthesized. The successful construction of a bis-T−DCV-based planar HOF compound 1 with the short interlayer separation of 3.33 Å guarantees the high-contrast luminescence on−off switching (1.2 × 103 fold) in the solid state. Furthermore, the grinding-induced destruction of HOF 1 could be identified by the PXRD analyses, where the α and β rings in the planar HOF stripe and the close interlayer stacking are broken obviously, which is responsible for the blue-shifted MRL turn-on phenomenon. Moreover, the implantation of rotors with sufficient steric hindrance, i.e., two isomeric Nphenylcarbazole, can weaken the π−π overlaps and generate AIE activity in the presence of the same T−DCV stator. This work is suggested to provide new insight into the rational design of longwavelength AIE-active molecules compared with the traditional AIE luminogens. We also anticipate that this study will inspire the development of a new class of high-contrast MRL turn-on materials by using the layer HOF-involved molecules.

π−π interactions to assemble a three-dimensional (3D) network. With regard to 1, two planar T−DCV fragments and their adjacent carbazole rings adopt cis/trans configurations with the dihedral angles of 4.25/4.89° and the whole molecule except the N-phenyl unit is essentially coplanar, with the mean deviation from the least squares plane of 0.0693(5) Å. Owing to the unique cis/trans configurations of planar T−DCV fragments of 1, a planar HOF stripe containing two types of hydrogenbonded rings (α and β) is built up with the assistance of four types of C−H···N interactions among four adjacent molecules only involving the thiophene and vinyl protons (Figure 7B). The planar layers are further assembled into a three-dimensional HOF structure, with the interlayer separation of 3.33 Å (Figure 7C). AIE and MRL Mechanisms. On the basis of the abovementioned structural analyses, it comes to the conclusion that the T−DCV group can work as a stator to connect with its adjacent molecules via multiple C−H···N interactions, which will rigidify the molecular conformation of 1−3. In unsymmetrical compounds 2 and 3, the twisted conformation of two N-phenylcarbazole units helps to prevent the formation of excimers and exciplexes in their condensed state. When aggregated as nanosuspensions, intramolecular rotation of its aryl rotors in 2 and 3 is restricted, thus leading to the observed AIE effects. It is also noted that the absence and presence of π−π stacking in the isomeric pair of 2 and 3 could result in the different solid fluorescence (Φc 20.12 vs 13.21%) in the premise of the analogous affixation of T−DCV and rotation restriction of bis-N-phenylcarbazole units, because π−π interactions are believed to significantly quench the fluorescent emission. As for 1, the formation of a closely packed HOF structure in 1 will facilitate the interlayer dipole−dipole interactions and quench the solid emission, which is responsible for the ACQ behavior during the aggregation process. On the other hand, the metastable HOF structure of 1 may be easily destroyed by the mechanical stimuli and lead to the recovery of solid PL emission (MRL turn-on). In contrast, unsymmetrical 2 and 3 have the same T−DCV moiety but different twisted N-phenylcarbazole units in comparison with 1, which favors the formation of twisted molecular stacking. Such twisted molecular stacking is beneficial to AIE but hampers the interlayer slipping and MRL turn-on. Moreover, the 3D packing in 3 has much more effective interlayer interactions than 2, which would result in the red-

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ASSOCIATED CONTENT

* Supporting Information S

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.jpcc.8b10118. CCDC reference numbers 1834673−1834675 (CIF) (CIF)

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Materials and measurements, details of synthetic characterization data (NMR and MS spectra), absorption and emission spectra in different organic solutions, and crystal data; X-ray single-crystal diffraction data of HOF 1 grown from a mixture of CHCl3 and CH3OH is attached for verifying the reproducibility (PDF)

AUTHOR INFORMATION

Corresponding Authors

*E-mail: [email protected] (W.H.). *E-mail: [email protected] (T.T.). ORCID

Wei Huang: 0000-0002-1071-1055 Tao Tao: 0000-0001-8686-988X H


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The Journal of Physical Chemistry C Notes

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The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was financially supported by the National Natural Science Foundation of China (Nos 21871133 and 21501097), the Natural Science Foundation of Jiangsu Province (Nos BK20171334 and BK20150890), and the Natural Science Foundation of the Jiangsu Higher Education Institutions of China (No. 15KJB150021) for financial aids.

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