C–H···π Interaction Induced Formation of Microtubes with Enhanced

Article pubs.acs.org/crystal

C−H···π Interaction Induced Formation of Microtubes with Enhanced Emission Cuiping Zhao,†,# Zhongliang Wang,‡,# Yinlong Yang,† Chao Feng,‡ Wei Li,† Yanan Li,‡ Yuping Zhang,† Feng Bao,† Yuliang Xing,† Xiujuan Zhang,*,† and Xiaohong Zhang*,‡ †

Functional Nano & Soft Materials Laboratory (FUNSOM) and Jiangsu Key Laboratory for Carbon-Based Functional Materials & Devices, Soochow University, Suzhou, Jiangsu 215123, China ‡ Nano-organic Photoelectronic Laboratory and Laboratory of Organic Optoelectronic Functional Materials and Molecular Engineering, Technical Institute of Physics and Chemistry, Chinese Academy of Sciences, Beijing 100101 S Supporting Information *

ABSTRACT: High luminescent bis(salicylaldehyde)o-phenylenediimine(salophen) microtubes with rectangular cross sections were successfully synthesized by a self-assembly method. Accompanied by the formation of microtubes, a remarkable enhanced emission was observed. Crystal structure analysis and theoretical studies were both investigated in detail. It was found that a conformation change induced by multiple C−H···π interactions between adjacent molecules was responsible for the formation of microtubes. The edge-to-face C−H···π interactions also resulted in molecular structural rigidification, which made salophen a stronger emitter in microtubes.

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INTRODUCTION In recent years, π-conjugated materials based organic nanostructures have aroused a surge of interest, due to their outstanding optical and electrical properties. Applications in light emitting diodes,1 optical waveguides and lasers,2,3 and field-effect transistors4−6 have been successfully demonstrated. Different from their inorganic counterparts, noncovalent intermolecular face-to-face π−π interactions (or π−π stacking) usually play the key role in crystal engineering and corresponding properties tuning.7 Meanwhile, it is also evidenced that face-to-face π−π interactions would result in a cofacial configurations, which would increase the amount of π−π orbital overlap and thus be detrimental to luminescence in the nanostructures.8,9 So far, there has been little evidence that edge-to-face π−π interaction termed as C−H···π interaction can lead to the formation of organic nanostructures. Although a single C−H···π interaction is kinetically labile and the enthalpy is only about 1.6−2.5 kcal/mol,10−13 could multiple C−H···π interactions be applied to engineer nanoscale molecular assembly as well as affect their luminescence behavior? Herein, in this paper we describe the formation of bis(salicylaldehyde) o-phenylenediimine (salophen) microtubes mediated by C− H···π interactions. Accompanied by the formation of microtubes, a remarkable enhanced emission is observed. Crystal structure analysis and theoretical studies are systematically investigated. © 2012 American Chemical Society

EXPERIMENTAL SECTION

Materials. 1,2-Phenylenediamine was obtained from Alfa Aesar (China) and used without further treatment. Ethanol (A. R.) and 1,4dioxane (A. R.) were purchased from Beijing Chemical Reagent Corp., China. High purity water was generated with a Milli-Q apparatus (Millipore) and was filtered by an inorganic membrane with a pore size of 0.02 μm (Whatman International, Ltd.) just before use. Bis(salicylaldehyde)o-phenylenediimine (Salophen) was synthesized using standard procedures involving the reaction of salicylaldehyde with 1,2-phenylenediamine (2:1 molar ratio) in ethanol.14 The yellow needle products were purified by recrystallization from ethanol (yield: 80−90%). The product was then confirmed by MS and NMR (1HNMR: δ 6.6−7.4 (m, 12 H, phenyl), 8.6 (s, 2 H, NCH), 13.2 (s, 2H, OH)). Characterization. The morphologies and sizes of salophen nanoaggregates and microtubes were measured by a field-emission scanning electron microscope (FESEM, Hitachi S-4300) operated at an accelerating voltage of 5 kV. The transmission electron microscopy (TEM) study was performed with a Philips CM200FEG microscope operated at an accelerating voltage of 200 kV. Powder X-ray diffraction (PXRD) patterns were investigated using an X-ray diffractometer (Mac Science M18AHF, Japan) equipped with Cu K-alpha radiation (λ = 1.54050 Å), employing a scanning rate of 0.1 deg/s in the 2θ range from 5° to 60°. The UV−visible absorption spectra were recorded using a Hitachi U-3010 spectrophotometer. Photoluminescenc (PL) spectra were measured at an excitation wavelength of 332 nm by a Hitachi F-4500 spectrometer. Received: September 15, 2011 Revised: December 25, 2011 Published: January 18, 2012 1227

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Computational Method. The geometric structures of salophen molecule were optimized by employing the Hartree−Fock theory and an economic basis set,15 and the energies of the minimum energy configurations were then calculated by using density functional theory (DFT) and standard 6-31G* basis set. All DFT calculations were performed using the B3LYP exchange-correlation functional which combined the Becke three-parameter exchange functional with the gradient-corrected correlation functional of Lee, Yang, and Par.16,17 All the calculations were conducted by the Gaussian03 package. The precise energy calculation for those structures was performed in the framework of DFT with the generalized gradient approximation (GGA) by the LYP functional for exchange-correlation energy, as implemented in the SIESTA code.18 It should be noted that the double-ζ polarized (DZP) numerical atomic orbital basis set is adopted to provide sufficient accuracy, and the core electrons are represented by norm-conserving pseudopotentials. Periodic boundary condition was employed for both crystal and molecule models, using the same lattice parameters as discussed above. The cutoff of the grid integration of charge density was set to 150 Ry, and the Brillouin zone integration was performed on a Monkhorst−Pack (MP) 8 × 8 × 8 kpoint mesh.

Figure 2. XRD patterns of (a) salophen microtubes and (b) the starting powders.

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RESULTS AND DISCUSSION Salophen microtubes was prepared by a simple solution method. In a typical preparation procedure, initial salophen powder was dissolved in dioxane to a concentration of 3 × 10−3 M and 200 μL of solution was injected into 5 mL of continuously stirred high purity water drop by drop at room temperature (25 °C). Then the solution was left undisturbed for about 4 h for stabilization. Figure 1 shows the SEM images

Figure 3. SEM images of salophen sub-microtubes at different aging time. (a) 5 min, (b) 10 min, (c) 30 min, (d) 60 min, (e) 120 min, and (f) 4 h.

Figure 1. (a) Typical SEM image of salophen microtubes; inset is the enlarged SEM image of a single tube; (b) TEM image of a single salophen microtube; inset is the corresponding selected-area electron diffraction pattern.

fused together (Figure 3c). Increasing the aging time to 1 h, half-tubular structures came to emerge among these particles. With ripening proceeding, the number of particles greatly decreased (Figure 3d), and these half-tubular structures slowly sewed together into single rectangular tubes with smooth surfaces and open ends (Figure 3e), indicating that growth proceeded by oriented attachment of particles. If the aging time was extended to 4 h, particles finally disappeared, leaving behind all rectangular microtubes with very high morphological purity (Figure 3f). The schematic illustration of the formation process was proposed in Figure 3g. Strong directional molecular interactions are supposed to be the main driving forces for the particles to self-organize and join together along a certain direction to form microtubes, in a tendency to reduce the high surface energy of the whole system. This process is very similar to the “oriented attachment” mechanism proposed by Banfield et al. in the formation of anatase (TiO2) nanorods.20 We also investigated the time-dependent photoluminescence (PL) properties as shown in Figure 4a. It was found that there was a dramatic change of fluorescence intensity during the formation of microtubes. The dioxane solution and the initial nanoparticle suspensions both had two very weak emission peaks centered at 545 and 435 nm. With increasing the aging

of the as-prepared samples, which are mainly composed of a large number of microtubes with smooth surface, uniform diameter, and distinctive rectangular cross section. The average length and outside diameter of these tubes are ca. 10−20 μm and 500 nm, respectively. Inset of Figure 1a shows a single microtube, with the wall thickness of about 100 nm. The microstructures of the salophen microtubes were also studied by TEM; the sharp spots in the selected area electron diffraction (SAED) pattern clearly demonstrate that the microtube is single-crystalline. X-ray diffraction (XRD) patterns were recorded to further investigate the crystal structure of the microtubes and the starting materials as shown in Figure 2. Both XRD patterns can be readily indexed by a monoclinic space group P21 with the unit cell dimensions a = 6.064 Å, b = 16.306 Å, c = 16.541 Å, α = β = 90°, and γ = 91.5° (CCDC Refcode: OPHSAL10).19 To investigate the formation mechanism of the rectangular microtubes, samples with different ripening durations were studied. As shown in Figure 3a, disordered rod-like nanoparticles with diameters of 80−120 nm and lengths of 400−600 nm were first observed 5 min right after stirring. The rod-like particles began to aggregate (Figure 3b) and then gradually 1228

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Figure 4. Time-dependent (a) fluorescence and (b) UV−visible spectra of salophen aggregates in water.

time, emission at 545 nm decreased obviously, while the emission at 435 nm experienced a red shift to 451 nm, accompanied with the intensity gradually enhanced and finally reaching the maximum of about 100 times higher than original. To gain the insight into the cause of the unique PL behavior, the UV absorption spectra were measured as well. Figure 4b shows the UV−vis spectra acquired every 5 min in the formation process. It can be seen that a new peak at 335 nm was emerging and gradually become predominant, while the absorption band at 410 nm, associated with n → π* transition of keto tautomer decreased correspondingly. In addition, the absorption band associated with π → π* transitions centered at 257 nm also increased with the ripening proceeding. The isoabsorptive point appeared at 266 nm, indicating a conversion from the keto to the enol form existed during the formation process of salophen microtubes. This is also consistent with the PL results, where the emission of the enol form increased gradually with the crystallization proceeding, while the emission of initial nanoparticles, which mainly consisted of the keto forms, dropped off accordingly. When the salophen molecules meet with protic solvent (water), hydrogen bondings between the CO group in salophen and hydrogen atom in water would be easily formed, which is in favor of formation of keto tautomers. Thus, as proposed by Herzfeld,21 the amorphous particles formed at the early stage should be mostly composed of keto tautomers. However, according to the absorption spectra, in the following crystal growth process, most of keto tautomers gradually transformed into enol tautomers. Accompanied with the changes, obvious emission enhancement was observed as well. What is the driving force for this transformation? Theory calculation and crystal structure analysis were further investigated to explore the conformation and energy difference of salophen molecules in the isolated state and in the microtubes. We define the dihedral angle between C1−C2−N1−C3 as the distortion angle and calculated the energy of different configurations of isolated salophen molecules, with the C2−N1 δ bond rotating from 0° to 180°. Two lowest energy configurations appeared with the distortion angle of about 35° and 140°, as shown in Figure 5. The two configurations were then further optimized by using the B3LYP/6-31G* method. It was found that they were almost isoenergetic, with an energy difference of 2.01 kJ/mol, and the dihedral angle C1−C2−N1−C3 being 38.1° and 142.5°, respectively. We also studied the molecular configuration in the crystals. Figure 5c shows the crystal structure viewed along the a axis; it is found that R2 is almost coplanar to the middle phenyl ring R1 with

Figure 5. (a) Chemical structure of salophen molecule (red, blue, green, and white spheres represent oxygen, nitrogen, carbon, and hydrogen atoms, respectively); (b) calculated energy of salophen molecule at different distortion angles and the structures of two optimized configurations with lowest energies. (c) Projection of salophen structures viewed along the [100] direction; (d) perspective view of the aromatic C−H···π interaction in the crystal, which are denoted by dotted lines.

only 2.5° titled in the molecule (the enol tautomer). The coplanarization of the groups R2 and R1 will enlarge the πconjugated system and thus activate the radiation process, making the salophen molecules more luminescent,22−24 while the coplanarization is also in favor of forming a slipped cofacial π−π stacking between adjacent molecules, which may induce certain luminescence quenching. In addition, R3 is significantly distorted from R1, with a twisted angle up to 53°, which is obviously different than that optimized in the isolated state. But at this point, most of the phenyl rings take on the edge-to-face mode, which favors the formation of C−H···π interactions between molecules (Figure 5d). C−H···π interaction is a weak attractive force occurring between a soft acid (C−H group from the edge of π system) and a soft base (the face of π system). Multiple C−H···π interactions would not only lock the twisted conformation and rigidify the structure, making salophen stronger emitter, but also would act as the driving force to induce the directional crystal growth to form one-directional microtubes. Thus the combination effect of planarization between R1 and R2 and twisted conformations between R1 and R3 should be responsible for the enhanced emission in the 1229

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Crystal structure analysis indicates that salophen molecules take on more twisted conformations in the crystalline state compared to those in the isolated state. The multiple aromatic C−H···π interactions induce and stabilize the twisted conformation, which endow microtubes with enhanced emission. Theory calculations indicate that conformation change of salophen molecules also causes large negative ΔG, which encourages self-assembly of salophen molecules into microtubes.

microcrystals. The multiple C−H···π interactions is the major driving force for the conformation change and the formation of microtubes. The intensity of intermolecular interactions between salophen molecules was also investigated by calculating the binding energy Eb, which is defined by Eb = Et,cell/4 − Et,mol, where Et,cell is the total energy of the unit cell of salophen crystal, Et, mol is the total energy of an isolated salophen molecule, and 4 denotes the unit cell consist of four salophen molecules. The atomic coordinates of the cell and the lattice parameters were adopted from CCDC, while the optimal geometric structure of the isolated salophen molecule was obtained from the above optimization calculations. The total optimized binding energy was calculated to be 262.3 kJ/mol, which was the driving force for the assembly of microtubes. This means the large negative Gibbs energy ΔG (binding energy) mainly comes from the multiple C−H···π interactions along with some π−π interactions. On the basis of experimental observations, crystal structure analysis and calculation results, we proposed the growth process of salophen microtubes, as illustrated in Figure 6. When

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ASSOCIATED CONTENT

S Supporting Information *

Figure S1: MS of the as-synthesized salophen. Figure S2 IR spectra of the samples collected at the ripening time of 5 min and 8 h. This material is available free of charge via the Internet at http://pubs.acs.org.

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AUTHOR INFORMATION

Corresponding Author

*(Xiujuan Zhang) Fax: +86-512-65882846; tel:+86-51265880955; e-mail:[email protected]. (Xiaohong Zhang) Tel: +86-10-82543510; e-mail: [email protected]. Author Contributions #

These authors contributed equally to this work.

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ACKNOWLEDGMENTS This work was supported by Major Research Plan of the National Natural Science Foundation of China (Nos. 91027021, 91027041), National Natural Science Foundation of China (Nos. 50903059, 51173124, 51172151, 50825304), National Basic Research Program of China (973 Program, Grant Nos. 2010CB934500, 2011CB808400), and the Specialized Research Fund for the Doctoral Program of Higher Education of China (Grant No. 20093201120020). We also thank Natural Science Foundation of Jiangsu Province (No. BK2010003) and a Project Funded by the Priority Academic Program Development of Jiangsu Higher Education Institutions.

Figure 6. Schematic energy and conformation diagram of salophen molecules during the self-assembly process.

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salophen dioxane solution were dropped into water, aggregation of salophen molecules would occur due to the low solubility in the mixed solvent, during which competitive equilibrium existed between the keto tautomer and the enol tautomer. Particles composed of keto tautomers would be much more easily formed at the beginning, because the keto tautomer has a lower energy barrier Ek. However, in the mixed solution, water would offer protons and then facilitate the isomerization transform from keto form to enol form. More importantly, during the next Ostwald ripening process, strong intermolecular interactions (multiple C−H···π interactions) will strengthen the molecular conformation change and lock the molecules in enol forms in crystals, as the spectra indicated. Accordingly, particles composed of keto tautomers will gradually evolve into particles constituted with enol tautomers and then eventually grow into microtubes under directional intermolecular interactions as the driving force.

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CONCLUSIONS In summary, single-crystal salophen microtubes with rectangular cross sections have been prepared. During the formation of microtubes, a remarkable fluorescence increase is observed. 1230

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