Triple Helical Molecular Braid and Parallel Packed Wavy Chain-Based

Article pubs.acs.org/crystal

Triple Helical Molecular Braid and Parallel Packed Wavy Chain-Based Supramolecular Organic Frameworks with Conformation- and Packing-Dependent Luminescent Properties Kai Xing,† Ruiqing Fan,*,† Jizhuang Fan,*,‡ Xi Du,† Yang Song,† Song Gao,† Ping Wang,† and Yulin Yang*,† †

MIIT Key Laboratory of Critical Materials Technology for New Energy Conversion and Storage, School of Chemistry and Chemical Engineering and ‡State Key Laboratory of Robotics and System, Harbin Institute of Technology, Herbin, P. R. China S Supporting Information *

ABSTRACT: Two three-dimensional supramolecular organic frameworks, [(H2bpdc)0.5(bipy)0.5] (1) and [(H3dpob)(bipy)·3H2O] (2), have been successfully constructed and structurally characterized (H2bpdc = 2,2′-biphenyldicarboxylic acid; H3dpob = 3-(2′,3′-dicarboxylphenoxy)benzonic acid; bipy = 4,4′-bipydine). 1 is driven by C−H···π hydrogenbonded packing to three dimension. 2 origins form parallel-packed wavy chains and show π−π stacking-directed structure. After the SOFs are formed via weak noncovalent interaction, 1 and 2 show an enhancement in thermostability. The different conformations of bipy induced by weak noncovalent interactions are responsible for the huge dissimilarity of structural and luminescence behavior between them. Under the excitation of 365 nm ultraviolet light, 1 and 2 show stronger blue-white and greenwhite luminescent emissions in the solid state, respectively. In comparison with 2, supramolecular organic framework 1 shows superior performance in aggregation-induced emission (AIE) activity, which is explained and elaborated via the simple model from the viewpoint of planarity (Ψr) and rotatability (θr). Moreover, stimuliresponses behaviors to mechanical force were evaluated to explore stability of AIE materials, luminescent shift (Δλem ≈ 36 nm) was observed in 2, whereas almost no change of 1 was found. Subsequently, 1 doped poly(methyl methacrylate) film demonstrates the comparable intensity with the solid state at concentrations of 2.0%, accompanied by the improvement of thermal stability.

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are weak and frail, they play key roles in conformation,13−15 molecular recognition,16−19 crystal packing,20 and enantioselective separation.21 Hence, great attention to SOF materials comes from the flexibility of molecular interactions, the control of modularity and functionality of the organic species. It is noteworthy that the appropriate hydrogen bonding and π−π stacking interactions could produce the aggregationinduced emission (AIE) phenomenon and influence the AIE activity in the assembly system, which was first discovered by Tang and co-workers in 2001.22 Unlike most molecules, a common feature of these materials is that they are weakly or nonemissive in solutions but have strong fluorescence in aggregated states, in which the weak noncovalent interaction restricts intramolecular rotations and forms highly ordered intermolecular packing.23,24 Because of the unique fluorescence property, it inspires us to further explore the valuable application in daily life. Generally, poly(methyl methacrylate) (PMMA) can be used as a popular polymer matrix for

INTRODUCTION Helical structures have been commonly recognized as a highly valuable shape and are ubiquitous in nature ranging from macroscopic phenomena to biomolecular systems.1−3 Motivated via aesthetical structures and biomimetic functions, an increasing contribution has been made by crystal engineers to design and synthesize molecular architectures that have capability to mimic biomacromolecules structurally and functionally.4−7 In general, helical structures usually are formations of multidentate organic strands with the assistance of a coordinate effect of the metal center. However, an overwhelming majority of helical chains in biomacromolecules are mainly constructed through the cooperative interaction of abundant and complex noncovalent interactions instead of metal coordination.8 Hence, for mimicking the biomacromolecules, the metal-free helices are more suitable models than the metal-containing ones. Moreover, the other extremely attractive subject of intense research in crystal engineering is supramolecular organic frameworks (SOFs), which are constructed from functional organic modules assembled via supramolecular interactions (e.g., hydrogen bonds, π−π stacking, C−H···π, and van der Waals interactions).9−12 Even though these interactions © XXXX American Chemical Society

Received: May 25, 2016 Revised: June 28, 2016

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DOI: 10.1021/acs.cgd.6b00791 Cryst. Growth Des. XXXX, XXX, XXX−XXX

Crystal Growth & Design

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luminescent SOFs.25 Compared with most common rare earth coordination polymers doped with PMMA, setting SOFs as dopants not only realizes a rich luminous color with little doping amount,26 but also is a convenient and easily prepared polymer with a remarkable optical quality. Hence, supramolecular organic frameworks with PMMA may open up a bright prospect in the field of farm plastic−film and organic fluorescent glass.27 On the basis of the in-depth analysies and studies to wellknown SOF materials, acid−pyridine, one of the common and effective supramolecular synthon,28 has been utilized. Meanwhile, aiming to obtain SOFs having helical characteristics and discover the domination of weak interaction, two kinds of carboxylic acid organic molecules with different structural rigidity, viz. rigid H2bpdc and semirigid H3dpob, were chosen to synthesize two SOFs, [(H2bpdc)0.5(bipy)0.5] (1) and [(H3dpob)(bipy)·3H2O] (2) (H2bpdc = 2,2′-biphenyldicarboxylic acid; H3dpob = 3-(2′,3′-dicarboxylphenoxy)benzonic acid; bipy = 4,4′-bipydine) under hydrothermal conditions. 1 features a rare 41 intertwining triple helical molecular braid consisting of two typical rigid organic molecules and constructs the well-defined three-dimensional (3D) architectures through C−H···π hydrogen-bonded packing, while 2 built from more flexible acid organic species processes parallel-packed wavy chains and forms a 3D framework via π−π stacking. The structure of 1 has been reported by Soleimannejad et al.,29 but they directly use ethanol rather than H2O as solvent. Herein, we mainly talked about the influence of conformation and structure induced by the C−H···π and π−π weak interaction under the same external hydrothermal synthesis method with 2. For the results, 1 and 2 with the naked eye show blue-white and green-white emission, respectively, and show stronger intensity in the solid state excited by the ultraviolet than these in tetrahydrofuran (THF) solvent. Interestingly, 1 shows superior performance in AIE activity and poorer response property to mechanical force than 2 because of the torsional conformations of bipy induced by noncovalent interaction and unchangeable molecular packing modes. Because of the greater mechanical stability of 1, the luminescent intensity and lifetime as well as the thermal stability of 1−PMMA are studied in detail.

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τ12A1% + τ22A 2 % τ1A1% + τ2A 2 %

(1)

. The luminescence quantum yields of SOFs were measured in THF at room temperature and cited relative to a reference solution of quinine sulfate (Φ = 0.546 in 0.5 mol dm−3 H2SO4) as a standard, and they were calculated according to the well-known equation (a):

ϕoverall ϕref

⎛ n ⎞2 A I = ⎜ ⎟ ref ⎝ nref ⎠ A Iref

(2)

. In eq 2, n, A, and I denote the refractive index of solvent, the area of the emission spectrum, and the absorbance at the excitation wavelength, respectively, and φref represents the quantum yield of the standard quinine sulfate solution. The subscript ref denotes the reference, and the absence of a subscript implies an unknown sample. For the determination of the quantum yield, the excitation wavelength was chosen so that A < 0.05. Synthesis of [(H2bpdc)0.5(bipy)0.5] (1). A mixture of 2,2′biphenyldicarboxylic acid (H2bpdc) (24.0 mg, 0.1 mmol), 4,4′bipyridine (bipy) (16.0 mg, 0.1 mmol) were dissolved in distilled water (8.0 mL). After being stirred for 30 min in air, it was transferred into a 20 mL Teflon-lined stainless steel autoclave and heated at 120 °C for 3 days. After being cooled the room temperature, colorless block crystals were obtained in a 64% yield, based on H2bpdc. Anal. Calcd for 1 (%) (Mr: 398.40): C, 72.36; H, 4.56; N, 7.03. Found: C, 72.33; H, 4.58; N, 7.01. IR bands (KBr pellet, cm−1) (Figure S7): 3456 (m), 3064 (m), 2531 (m), 1701 (s), 1680 (s), 1598 (s), 1571 (m), 1476 (m), 1411 (w), 1304 (w), 1250 (w), 1213 (m), 1064 (m), 807 (m), 625 (m), 528 (w). Synthesis of [(H3dpob)(bipy)·3H2O] (2). A mixture of 3-(2′,3′dicarboxylphenoxy)benzonic acid (H3dpob) (30.2 mg, 0.1 mmol), 4,4′-bipyridine (bipy) (16.0 mg, 0.1 mmol), with a molar ratio of 1:1 was mixed in distilled water (8.0 mL). After being stirred for 30 min in air, it was transferred into a 20 mL Teflon-lined stainless steel autoclave and heated at 120 °C for 3 days. After being cooled to room temperature, the mixture was washed with distilled water, filtered off and dried, and yellow block crystals were obtained in a 60% yield, based on H3dpob. Anal. Calcd for 2 (%) (Mr: 512.46): C, 58.59; H, 4.72; N, 5.47. Found: C, 58.56; H, 4.75; N, 5.45. IR bands (KBr pellet, cm−1) (Figure S7): 3475 (m), 3101 (w), 3057 (w), 2931 (w), 1705 (s), 1605 (s), 1563 (m), 1545 (s), 1486 (s), 1434 (s), 1315 (m), 1254 (s), 1065 (w), 804 (m), 654 (m), 567 (w). Synthesis of 1-Doped PMMA Polymer Films. The PMMA polymer was doped with 1 in the proportions 0.4, 0.8, 1.2, 1.6, and 2.0% (w/w). The PMMA powder was dissolved in 6 mL of tetrahydrofuran (THF), followed by addition of the required amount of 1 in THF solution, and the resulting mixture was heated at 40 °C for 60 min. The polymer film was obtained after evaporation of excess solvent at 60 °C. X-ray Crystal Structure Determination. The X-ray diffraction data taken at room temperature for supramolecular organic frameworks 1 and 2 were collected on a Rigaku R-AXIS RAPID IP diffractometer equipped with graphite-monochromated Mo Kα radiation (λ = 0. 71073 Å). The structures of 1 and 2 were solved by direct methods and refined on F2 by the full-matrix least-squares using the SHELXTL-97 crystallographic software.30,31 Anisotropic thermal parameters were refined to all of the non-hydrogen atoms. The hydrogen atoms were held in calculated positions on carbon atoms and nitrogen atoms and that were directly included in the molecular formula on water molecules. The CCDC contain the crystallographic data 2 of this paper. These data can be obtained free of charge at www.ccdc.cam.ac.uk/deposit. Crystal structure data and details of the data collection and the structure refinement are listed as Table 1, and selected bond lengths and bond angles of supramolecular organic framework 2 are listed as Table S5.

EXPERIMENTAL SECTION

Materials and Methods. All reagents were commercially available and used without further purification. Fourier transform (FT)-IR spectra were obtained from KBr pellets, using a Nicolet Avatar-360 Infrared spectrometer in the 4000−400 cm−1 region. Elemental analyses were performed on a PerkinElmer 240c element analyzer. Powder X-ray diffraction (PXRD) patterns were recorded in the 2θ range of 5−50° using Cu Kα radiation by Shimadzu XRD-6000 X-ray diffractometer. The thermal analysis was performed on a ZRY-2P thermogravimetric analyzer from 25 to 650 °C with a heating rate of 10 °C min−1 under a flow of air. Luminescence analysis and luminescence lifetimes were recorded on an Edinburgh FLS920 luminescence spectrometer at 298 and 77 K. Lifetime studies were performed using photon-counting system with a microsecond pulse lamp as the excitation source. Data were analyzed through the nonlinear least-squares procedure in combination with an iterative convolution method. The emission decays were analyzed by the sum of exponential functions. The decay curve is well fitted into a double exponential function: I = I0 + A1 exp(−t/τ1) + A2 exp(−t/τ2), where I and I0 are the luminescent intensities at time t = t and t = 0, respectively, whereas τ1 and τ2 are defined as the luminescent lifetimes. The average lifetime was calculated according to the following equation: B



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Table 1. Crystal Data and Structure Refinement Parameters of 2 identification code empirical formula formula mass crystal system space group a (Å) b (Å) c (Å) α (deg) β (deg) γ (deg) V (Å3) Z Dc/(g·cm−3) μ (Mo Kα)/mm−1 F(000) θ range (deg) limiting indices

data/restraints/parameters GOF on F2 final R indices [I > 2σ(I)] R1a wR2b R indices (all data) R1 wR2 CCDC

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RESULTS AND DISCUSSION

Self-Assembly of 1 and 2. The hydrothermal reaction between rigid or semirigid carboxylic acid and rigid pillared 4,4′-bipyridine (bipy) happened in distilled water. The autogenous pressure generated by sealing heat has accelerated the dissolvent degree and reactivity at the same time brought a special and powerful driving force for symmetry building and periodicity arranging. 1 and 2, as colorless and yellowish blockshaped single crystals, are self-assembled based on O−H···N hydrogen bond and major induced by C−H···π and π−π intermolecular noncovalent interactions, respectively, after being cooled to room temperature slowly, in other words, the same external reaction energy as precondition, but the results of self-assembled are different in these two systems. Crystal Structure of 1 and 2. [(H2bpdc)0.5(bipy)0.5] (1). In the crystal structure of 1, H2bpdc as hydrogen bond donor, due to the stereohindrance effect, two benzene rings are not coplanar. The bipy acts as the hydrogen bond acceptor; however, the dihedral angle formed between two pyridine is 33.48° (Figure S3). To the best of our knowledge, without the existence of a powerful chemical force, the potentially unique conformation of bipy caused by the freedom of rotation about the C−C σ-bond is relatively rare. It should be pointed out the structure of 1 has been briefly reported previously, but herein the deeper analysis and description are from conformation, whole packing structure, and assembly categories for comprehensive comparison. The bipy is bonded to its neighbor H2bpdc through one strong hydrogen bond of 1.863 Å (O1A−H···N1), resulting in an orthogonal arrangement in a different surface of the interactive H2bpdc, which leads to the formation of the single-stranded helix along the 41 screw axis with the 50.024 Å helical pitch (Figure S4a). For increasing the utilization to

2 C25H24N2O10 512.46 triclinic P1̅ 8.9202(6) 11.4197(8) 12.2222(8) 106.023(2) 97.485(2) 92.187(2) 1182.87(14) 2 1.439 0.113 536 3.08−25.39 −10 ≤ h ≤ 9 −13 ≤ k ≤ 13 −14 ≤ l ≤ 12 4242/0/334 1.033 0.0717 0.1876 0.1242 0.2271 1480850

R1 = ∑∥F0| − | Fc∥/ ∑|F0. bwR2 = [∑[ w (F02 − Fc2)2]/∑[ w (F02)2]]1/2.

a

Figure 1. (a) A triple helix structure along the c axis in 1. (b) Weak noncovalent interactions at the twisted node of a triple helix in 1. (c) The C− H···π interactions between triple helices in 1. (d) A 3D supramolecular structure of 1 (O−H···N hydrogen bonds are shown by black dash line). C



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Figure 2. (a) Stick view of the 1D wavelike chain incorporating C(4) Hydrogen bond motifs and π−π stacking between adjacent chains in 2. (b) It shows π−π stacking arrangement distance between the 2D layers of 2. (c) A 3D supramolecular structure of 2 (hydrogen bonds are shown by black dashed line).

Among the adjacent 1D wavelike hydrogen-bonded chains exist the π−π stacking arrangement in two-dimensional directions, resulting in a 3D supramolecular organic framework (Figure 2c). The π−π stacking distance between benzene ring and pyridine ring is 3.538 and 3.560 Å, respectively. Within the π-system layers of longer distance, the complex multi H-bonds interactions origin from six lattice water molecules stabilize the packing of these wavelike chains. In other words, the 3D supramolecular organic framework of 2, based on the 1D wavelike hydrogen-bonded chains, is a π−π stacking-directed structure. The crystal structure analysis and packing diagrams of 1 and 2 suggest that the basic construction of the title supramolecular organic frameworks are intrinsic O−H···N hydrogen bonded 1D chain oriented from carboxyl acid and 4,4′-bipyridine building blocks; however, the final structures feature the triplehelix molecular braid and the parallel-packed wavy chains. Between 1 and 2, the most obvious difference is concentrated on the rigidity of carboxylic acid. Generally speaking, the supramolecular frameworks may show greater tortuosity with the flexibility of building molecule increasing. Nevertheless, the helical structure of 1 which consists of two typical rigidity molecules may be beyond our original expectation. To look deep inside, this could be contributed by the conformation differences of 4,4′-bipyridine caused via the major intermolecular noncovalent interactions or the driving force in 1 and 2. The main driving force for 1 and 2 is the formation of intermolecular C−H···π hydrogen bonds and π−π stacking interaction, respectively. The assembly process of the 3D structure in 1 can be divided into three categories. First, a onedimensional (1D) single helix chain is constructed through a typical strong hydrogen bond between a pyridyl nitrogen atom and the HO group of a carboxylic acid (dH···N = 1.863 Å). Second, three single helices twist around each other with the help of 3.490 Å of C−H···π interaction into a triple helix, and together with weak π−π stacking of the pyridine ring, the triangle shape combination of weak noncovalent interactions

unoccupied void in space, every three concentric chains with the same type of chirality wind with the other to generate a triple helix column (Figure S4b). Under a close inspection of the inner of the triple helix, the twisted node shows that 3.490 Å of C−H···π and weak π−stacking (3.679 Å) interact with each other and form a triangle shape (Figure 1b), the masterly combination of weak noncovalent interactions in space make the triple helix connected to each other steadily. More deeply, the surrounding of a triple helix is four opposite chirality triple helices which leads to the loss of the chirality of the whole assembly system of 1. The connection between adjacent column is confirmed by strong C−H···π (2.687 Å) (Figure 1c). On the basis of that, 1 shows a 3D supramolecular organic framework in space (Figure 1d) (detailed weak intermolecular interactions are listed in Figures S5 and S6 and Table S1). As far as we know, this is a relatively less common example of rigid aromatic ligands composed of such triple-helix structures of opposite chirality, without the metal ion attendance in a crystal supramolecular system. [(H3dpob)(bipy)·3H2O] (2). When the character of hydrogen bond donor was changed from rigid to semirigid in the same synthesis procedure, we obtained 2 as a 1D wavelike hydrogenbonded chain with 1:1 stoichiometry of H3dpob and bipy. Single-crystal X-ray diffraction analysis reveals that different from 1, crystal 2 crystallizes in the triclinic P1̅ space group, and one H3dpob, one bipy, and three lattice water molecules exist in the asymmetric unit of 2 (Figure S2). The H3dpob is hydrogen bonded to bipy, yielding a D(2) motif32 to result in a 1D wavelike chain (Figure 2a). Owing to the twist of “center O atom” in H3dpob, the complexity of conformation may increase in 2. Furthermore, the dihedral angle formed between two benzene rings is 75.24°. As a hydrogen bond acceptor, the bipy lies on the inversion center, and its two pyridine rings are roughly coplanar with the deviation only about 0.29° with better coplanarity. The different conformations play an important role in avoiding the construction of helical structure and formation of C−H···π interactions in 2. D



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Figure 3. (a) Emission spectra of 1 and 2 at 298 K in the solid state and THF solvent; inset: emission spectra of 1 and 2 at 298 K in THF solvent. (b) The photos of 1 and 2 solid crystals under 365 nm UV-light and corresponding color coordinate diagram of emission (square and circle symbol represent 1 and 2, respectively).

Figure 4. Emission spectra of 1 (a) and 2 (c) in THF/water mixtures with different water fractions ( f w) at 298 K under excitation at 365 nm and the plot of the changes in the luminescent peak intensity with different water fractions in 1 (b) and 2 (d). Inset: photos of 1 and 2 in THF/water mixtures ( f w = 0, 100%) under 365 nm UV lamp illumination and the hydrogen bonding dimers formed in crystals.

The purity of the bulk 1 and 2 samples was confirmed by powder XRD and thermogravimetric analysis (TGA) studies. The powder XRD pattern of 1 and 2 at room temperature matches well with the simulated XRD pattern based on the crystal structure, in terms of the peak positions, confirming that the powder samples are single phase. The TGA curves of 1 and 2 are shown in Figure S9. The framework of 1 is stable up to 200 °C under air atmosphere. 2 shows the first weight loss of 10.97% from 100 to 266 °C, which is consistent with the removal of three water molecules (calcd 10.53%), and then the whole structure starts to collapse. Compared to each single organic molecule, after assembly of supramolecular organic framework via noncovalent interactions, whole structural thermostability of 1 and 2 has an enhancement in quality.

gives the reliable reason for stabilization. Third, the 3D networks are obtained through the formation of stronger C− H···π interaction (2.687 Å).33,34 Obviously, the stronger noncovalent interactions is a benefit to lower assembly category; nevertheless, the whole framework is induced or controlled by the weaker. The 3D supramolecular organic framework of 2 is determined via two types of π−π stacking interactions formed between benzene and pyridine rings. The π−π stacking arrangement not only increases the overlap level of aromatic system and is the main driving force but also makes an effective contribution to stabilize the whole supramolecular framework. The tiny differences of building blocks caused by the individual driving force may make a huge impact in structure and specific properties. E



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Luminescent Properties. The solid-state emission spectra of 1 and 2 in this work were measured at room temperature, results of which are given in Figure 3a. When excited at 365 nm, the strong luminescent emissions of 1 and 2 are centered at 452 and 515 nm respectively with broad region displaying naked eye blue-white and green-white light, and commission Internacionale d’Eclairage (CIE) coordinates codes are (0.26, 0.29) for 1 and (0.31, 0.39) for 2 (Figure 3b). Compared with 1, the obvious red shift of the maximum luminescent emission peak (53 nm) of 2 has been observed (Figure S11), which should be due to the coplanarity and parallel packing mode.35 In 2, the smaller dihedral angle between two pyridine rings of bipy (0.29°) means that the π-electron conjugation degree is higher than that in 1 (33.48°). Therefore, the red shift of the maximum emission of 2 compared to that of 1 should be mainly attributed to the molecular conformational difference between them. Additionally, we measured the emission response of 1 and 2, together with all the free organic ligands, in dilute tetrahydrofuran (THF) solvent with the concentration of 1.0 × 10−5 mol/L at 298 K. The only weak or almost no luminescence is observed by the naked eye for 1 and 2. Namely, the two title SOFs both exhibit drastic changes in the luminescent intensity from the strongly luminescent solid state to the weakly luminescent dilute THF solution state, with the luminescent intensity decreasing by several 10-fold under the same test conditions. Thus, the two SOFs exhibit a typical AIE phenomenon. Aggregation-Induced Emission (AIE) Properties. We have further investigated the AIE behaviors of 1 and 2 through a solvent-poor-solvent luminescence test, which is commonly used in the literature.23,36−38 The experiment is realized by adjusting the ratio of THF/water from 0% to 100% (volume/ volume percentages) to monitor the changes of luminescence, while maintaining the concentration of solutions at 1 × 10−5 mol L−1. As shown in Figure 4, the emission intensity of 1 in THF/water system starts to increase when the water constituent exists. After water fractions exceed 30%, the intensity shows a moderate increase until reaching 100%. The enhancement of intensity is nearly 43-fold than the initial one. For 2, with increasing of water fractions ( f w), only a small increase in emission intensity is observed when up to ∼80 vol % water was added to the THF solution but it started to swiftly increase afterward. When the volume fraction of water in the THF/water mixture continued to increase, notable emission enhancement happened compared to pure THF, which is almost 10-fold higher than that of the THF solvent when reaching out to 100%. On the basis of the discussion and analysist about our SOF system above, we assume that the restriction of intramolecular vibrations (RIR) mechanism may be responsible for the AIE effect of 1 and 2.39 In solution, the phenyl or pyridyl rotors of 1 and 2 can dynamically rotate via the single-bond axes, which serves as a nonradiative pathway for the excitons to decay. In aggregates, such rotations are mainly suppressed due to the physical constraint. The radiationless pathway is thus nearly blocked, while the radiative channel is opened. To support the assumption we made, an external control experiment that has been conducted. Since decreasing temperature, especially cryogenic cooling, can also retard the intramolecular rotations and fortify the RIR effect,40 the temperature effects on the light emissions were investigated for 1 and 2. As shown in Figure S13, when the temperature decreases to 77 K, the intensity of the emission band is sharply

increased for both 1 and 2. That may be because reducing temperature impedes the intramolecular rotations; thus it activates the radiative transitions and boosts the emission intensity. Influence of Conformation Induced via C−H···π and π−π Interactions on AIE. The molecular packing modes in the crystalline state are related to intermolecular interactions that typically govern the properties of the materials.41 As we mentioned above, it is easy to observe that 1 and 2 both show the AIE phenomena, and the structure common to 1 and 2 is the head-to-tail intrinsic O−H···N hydrogen bonded 1D chain, which may be used to interpret the restriction of rotation for 1 and 2 and enhance emission intensity in the aggregates. However, the AIE activity of 2, especially the enhancive degree of intensity, is obviously weaker than that in 1. Naturally, the corresponding variation of AIE activity induced by traditional intermolecular hydrogen-bond interactions can be ruled out. Thus, the emission property of the crystal is mainly dependent on the two individual molecules; however, to the carboxyl part, the benzene rings are both nearly perpendicular to each other, so molecular conformation of bipy plays a dominant role in determining the emissions of 1 and 2. For 1, the dihedral angle between two pyridine rings is 33.48°, which is realized via intermolecular C−H···π interactions that exist between the adjacent bipy and H2bpdc molecules. These noncovalent interactions help to hold the molecules together, stabilize the crystal packing, and fix the molecular conformations. Namely, the mutiple intermolecular C−H···π interaction results in the nonplanarity of bipy and H2bpdc molecules and synergistically constrains the molecules in the crystal lattices and suppresses the rotations of their benzene and phenyl rings in the solid aggregate state. By contrast, the coplanarity of two pyridine rings in 2 is much better (dihedral angle is 0.29°), which creates a positive opportunity for formation of π−π stacking. Furthermore, although the radiative transition channels by RIR process are opened in the aggregate state, the quenching effect due to the π−π stacking interactions between the benzene and pyridine rings is also activated. Hence, the solid of 2 exhibits a lower luminescent enhancement than that of 1, leading to a weaker AIE activity. Clearly, the hydrogen bonds have beneficial effects for the AIE phenomena, and the elongation in the electronic conjugation has antagonistic effects on the emissions in aggregate states. Nevertheless, whatever the conformation bipy molecule is, the intramolecular rotation is ubiquitous theoretically, but why does it show a different contribution to AIE activity in 1 and 2? To answer this question, it is necessary to use the simple model elaborated.42 Figure 5 illustrates the correlation among the geometrical planarity, conformational flexibility, intramolecular motion, and light emission behavior. The bipy molecule is simplified as comprised of two pyridine units A and B through the linkage of a rotatable C−C single bond. In the diagram, θr and Ψr denote the extent of intramolecular motion and the dihedral angle between units A and B, respectively. In other words, θr specifies the structural flexibility or rigidity of a molecule, while Ψr serves as an indicator for its conformational planarity. When bipy crystallized in 2, pyridine units A and B are aligned in almost parallel fashion (Ψr ≈ 0°). Such a molecule (A-B) enjoys a maximal electronic conjugation and a minimal potential energy. As shown in Figure 5, the C−C linkage is endowed with some pseudodouble-bond character by the extended cross-chromophore π-electronic delocalization. The F



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may efficiently regulate the force stimuli-response ability, leading to a result that 1 shows better steady performances in materials applications. In addition, 1 displays blue-white light in the solid state and superior AIE activity which can be recognized by the naked eye. Encouraged by that, we consider inducing the 1 into polymer matrixes to further verify and explore the valuable and potential usefulness in practical application. Therefore, we carried out a solid-state “dilution” experiment and prepared a series of composite films of poly(methyl methacrylate) (PMMA) doped with 1. Upon setting the 365 nm UV-light as the excitation, the emissive peak positions of the 1−PMMA films, in which concentrations start from 0.4% with a step of 0.4% (w/w) increment, remain roughly identical with 1. With an increase of the content of 1, the intensity and lifetimes of the 1−PMMA films increase moderately. When the content of 1 reaches at 2.0%, the intensity of 1−PMMA is comparable with 1 in the solid (Figure 6). Hence, taking the transparency and economics

Figure 5. Schematic illustration of the relationship among conformational planarity, torsional motion, and luminescence intensity, where θr(′) is rotation amplitude, ψr(′) is dihedral angle between units A(′) and B(′), respectively.

molecular conformation is thus stiffened, and its resistance against the intramolecular rotations is strengthened. Although the pyridine units may still swing to a small extent (θr), the quenching effect induced via the low-frequency motion is insufficient enough that the 2 shows the stronger emission in the solution state. In contrast to 2, the two pyridine units A′ and B′ in 1 are twisted out of plane due to the involved intermolecular C− H···π restrained effect. In this case, Ψr′ is 33.48°. The overlap between the π-electron clouds of the A′ and B′ plates becomes smaller, and the cross-chromophore π-electronic conjugation becomes weaker. The C−C linkage between A′ and B′ units thus possesses little restraint to the intramolecular rotations. Because the θr′ value can be varied in a much wider range (theoretically |0|°−|90|°), the rotational amplitude becomes larger. Much energy is therefore dissipated by the large twisting motions, resulting in a weaker emission of 1 in the solution state. According to such a pictorial model, luminescence behavior of a molecule in a solution can be predicted from Ψr (planarity) and θr (rotatability), where the structural and conformational stiffness plays a decisive role. Stimuli Responses Properties to Mechanical Force. From a potential application and workability view, luminescent materials which show more high-level stability will be considered preferentially. Mechanical force is one of the most common physical external stimuli that is used to evaluate the stability of materials showing AIE activity.43−46 For clarity, the as-synthesized samples and ground samples are hereafter abbreviated as the letters “A” and “G”, respectively (e.g., the sample 1A is the as-synthesized 1 through the hydrothermal method; 1G is the ground sample of 1A). After grinding with a pestle and mortar, emission position of 1G is roughly consistent with that of 1A, while 2G is found to display a spectral blue-shift of 32 nm (Figure S14), probably attributing to a reduction in the overlapping area of two π-stacked degree after grinding.47−50 Apparently, the external force, caused by grinding treatment, is insufficient for the rearrangement of 1 in the solid state. These results, from another perspective, also illustrate that 1 in the aggregated state pack more tightly compared to 2, owing to the multi-intermolecular C−H···π hydrogen bond and triple helical structures. Properties of PMMA Polymer Doped with 1. Apparently, the conformation induced via C−H···π interactions

Figure 6. Emission spectra of PMMA polymer doped with 1 in 0.4− 2.0% at 298 K under excitation at 365 nm and the pictures of 1− PMMA under UV irradiation.

of 1−PMMA into account, the proper doping concentration is 2.0%. Additionally, as shown in Table S4, the lifetimes of 1− PMMA films increase with the enhancement of the content of 1, and τ reaches 8.43 μs at 2.0%. More importantly, after doping with PMMA, TG analysis of 1−PMMA film shows a slight increase of 34 °C for the Tonset in comparison with the pure PMMA (Figure S15), and the major weight-loss occurs over a 278−423 °C temperature interval, which suggests that the thermal stability of the 1−PMMA film is essentially improved by doped. The results suggest that the hybrid materials can serve as ideal candidates in the pursuit of applications in farm plastic−film with optical transfer function and also can be used as a new type of organic luminescent glass.51

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CONCLUSION In summary, 3D supramolecular organic frameworks (1 and 2) driven by C−H···π and π−π interactions, which feature the novel 41 square hydrogen bonding triple helix and 1D wavelike chain respectively, have been constructed only based on simple aromatic molecules under hydrothermal conditions. Upon ultraviolet light as the excitation, 1 and 2 show stronger bluewhite and green-white emission in the solid state than those in THF. The AIE activity has been confirmed, and the RIR mechanism is responsible for this. Notably, 1 displays more G



Article

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striking enhanced AIE compared with 2. It indicates that twisted hydrogen-bonding helical structure could be used as an available strategy for exploring the AIE-active system. Furthermore, 1 shows poorer stimuli-responses but greater stability to mechanical force than that of 2. Hence, 1 was doped with PMMA film, and the results illustrate that the comparable intensity with solid and enhanced thermostability can be realized at a relatively lower concentrate. The present study perhaps discloses the effect of molecular conformation and arrangement on self-assembly and luminescent properties. Follow-on studies on this platform will address self-assembly driven via diverse noncovalent interactions and are currently underway in our laboratory.

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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.cgd.6b00791. Structural information for compounds 1 and 2, IR spectra, TGA curves, PXRD patterns, and selected bond lengths and angles for compounds 2 (PDF) Accession Codes

CCDC 1480850 contains the supplementary crystallographic data for this paper. These data can be obtained free of charge via www.ccdc.cam.ac.uk/data_request/cif, or by emailing data_ [email protected], or by contacting The Cambridge Crystallographic Data Centre, 12 Union Road, Cambridge CB2 1EZ, UK; fax: +44 1223 336033.

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AUTHOR INFORMATION

Corresponding Authors

*(R.F.) Fax: +86-451-86413710. E-mail: [email protected]. *(J.F.) E-mail: [email protected]. *(Y.Y.) E-mail: [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was supported by National Natural Science Foundation of China (Grant 21371040, 21571042 and 21171044), the National Key Basic Research Program of China (973 Program, No. 2013CB632900).

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