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Reversible Piezofluorochromic Property and Intrinsic Structure Changes of Tetra(4-methoxyphenyl)ethylene under High Pressure Jinxia Wu,† Jia Tang,† Hailong Wang,† Qingkai Qi,‡ Xiaofeng Fang,‡ Yifei Liu,‡ Shuping Xu,† Sean Xiao-An. Zhang,‡ Houyu Zhang,† and Weiqing Xu*,† State Key Laboratory of Supramolecular Structure and Materials, Institute of Theoretical Chemistry and ‡State Key Laboratory of Supramolecular Structure and Materials, College of Chemistry, Jilin University, Changchun 130012, People’s Republic of China

Downloaded by UNIV OF NEW SOUTH WALES on September 7, 2015 | http://pubs.acs.org Publication Date (Web): August 20, 2015 | doi: 10.1021/acs.jpca.5b02362

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ABSTRACT: During the past decade, luminescent mechanochromism has received much attention. Despite the garnered attention, only a few studies have reported the effect of internal molecular structure change on the performance of mechanochromic fluorescence. Here, we chose tetra(4-methoxyphenyl)ethylene (TMOE) as a model molecule to study the correlation between structure and fluorescence property under a hydrostatic pressure produced by a diamond anvil cell (DAC). TMOE is a methoxysubstituted tetraphenylethylene (TPE) derivative and has a nearly centrosymmetric structure and a natural propeller shape. Ultraviolet−visible absorption and fluorescence spectra of TMOE and TPE in solution proved that the presence of methoxy groups in TMOE is responsible for the difference in fluorescence emissions of TMOE and TPE. Under a hydrostatic pressure, the in situ fluorescence spectra of TMOE at different concentrations show that the fluorescence intensity gradually weakens, accompanied by an obvious redshift. The Raman peak intensities decrease gradually, and the peaks disappear eventually with the pressure increasing. These spectral changes are attributed to the changes in the intramolecular conformation, that is, the strengthening of the weak C−H···O hydrogen bonds in TMOE molecules, which is caused by the twisted dihedral angle between the benzene ring and the carbon rigid plane of ethylene. Density functional theory simulation further confirms that the decreased dihedral angle could weaken Raman peak intensity, which is consistent with our experimental results.

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from a screw Zn(II) complex. Its fluorescence color change (from blue green to light blue) under mechanically grinding is also explained by the decrease of the intermolecular π−π stacking interactions under the external pressure, which is confirmed by the X-ray single-crystal analysis.18 However, the fluorescence emission and structural change of piezochromic materials under the hydrostatic pressure have been seldom reported. Tetraphenylethylene (TPE) derivatives (ethoxy- and butoxysubstituted TPE) synthesized by Dong and Tang et al. presented different morphologies in solid state that were found to show various luminous colors under different external stimuli.19 TPE derivatives are often used as models for the aggregation-induced emission (AIE) or aggregation-induced emission enhancement (AIEE) studies.20,21 It was reported that many TPE derivatives exhibit piezofluorochromic property.22,23 The compounds possessing both the piezofluorochromic property and AIE or AIEE characteristic are called piezofluorochromic aggregation-induced emission (PAIE) or piezofluorochromic aggregation-induced emission enhancement (PAIEE) materials.24 However, to our knowledge, PAIE/ PAIEE compounds are still in the preliminary stage of studies, and PAIE compounds are rarely found.25 Recently, Zhang et al.

INTRODUCTION Stimuli-response molecules for fluorescence changes are widely used to study the basic theory of the relationship between chemical structure and physical properties. They are also a current hot topic because of their extensive practical application in the domains of chem-/biosensing and analyzing.1 On the basis of existing stimuli-responsive molecular structure and corresponding properties, many stimuli-responsive mechanisms have been established and classified in reports, such as piezochromism,2−4 vapochromism,5−7 thermochromism,8−10 acid-dependent luminescence,11,12 and photochromism.13 Piezochromic luminescent materials exhibit emission shifts due to the intramolecular conformational changes or intermolecular interactions under pressure,14 and thus they have gained great attention because of the great application potential in the areas of rewritable media,15 display, and information storage.16 For most of the reported piezochromic materials, the piezochromic phenomena are mainly explained by the transformation from crystal phase to amorphous phase under grinding, which is supported by their spectral and powder X-ray diffraction (PXRD) data. For example, the X-ray structure analysis of 4,4,-(acenaphtho[1,2-b]quinoxaline-8,11-diyl)bis(N,N-diphenylaniline), a donor−acceptor−donor-type molecule, shows that its transformation from crystal phase to amorphous phase via mechanical grinding causes a luminescent color change from green to orange.17 Another example comes © 2015 American Chemical Society

Received: March 10, 2015 Revised: August 11, 2015 Published: August 11, 2015 9218

DOI: 10.1021/acs.jpca.5b02362 J. Phys. Chem. A 2015, 119, 9218−9224

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The Journal of Physical Chemistry A

Figure 1. UV−vis absorption spectra of TMOE and TPE dissolved in THF with different concentrations. (insets) The molecular structures of TMOE and TPE and the photographs of the color changes via TFA and TEA adding.

with a spectrometer (Jobin Yvon iHR320) and a camera.32 The light source is a mercury lamp with an excitation wavelength of 365 nm. The high-pressure Raman spectra of TMOE under hydrostatic conditions were recorded on a Horiba Jobin Yvon JY-T64000 confocal Raman spectrometer. The argon ion laser with 514.5 nm line was used to excite the samples. The output power was 20 mW. Geometries of the complexes were optimized via Becke’s LYP (B3LYP)33,34 exchange-correlation functional with 631G(d, p) basis set based on density functional theory (DFT). All the calculations were performed with the Gaussian 09 package.35

reported a PAIE molecule, tetra(4-methoxyphenyl)ethylene (TMOE), a methoxy-substituted TPE derivative. They attributed the mechanofluorochromism of TMOE to the phase transition from crystalline to amorphous state upon grinding according to the disappearance of its XRD peaks.26 Since the grinding pressure is limited and uncontrollable, we intented to perform an in-depth high-pressure experiment based on a diamond anvil cell (DAC) to explore the underlying mechanism of the piezochromism of TMOE. In this study, TMOE was chosen as a model molecule to investigate its intrinsic structure changes for mechanofluorochromism based on its PAIEE property.26 We compared the fluorescence emission of TMOE with that of TPE in tetrahydrofuran (THF) and under high pressure. The molecular structural changes of TMOE under the hydrostatic pressure were explored via fluorescence and Raman spectroscopies. Moreover, the theoretical simulation of Raman spectra of TMOE was performed to assign the spectral changes. The detailed spectral analyses were presented.

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RESULTS AND DISCUSSION 1. Fluorescence Emission Spectra of TMOE and TPE in Solution. Insets in Figure 1 show the molecular structures of TMOE and TPE. In Liu and Zhang’s study,26 the emission of TMOE crystal was thought to be a single molecular behavior based on the analysis of the intermolecular interactions in TMOE crystal, in which no π−π stacking interaction or any type of H or J-aggregation exists due to its centrosymmetric and propeller shape. Compared with TPE, TMOE has four substituted methoxy groups on the para positions of the phenyl groups. The existence of the methoxy groups in the TMOE molecule may affect the fluorescence property of TMOE under high pressure. To corroborate our speculation, UV−vis absorption and fluorescence spectra of TMOE and TPE in THF were measured. Figure 1 shows the UV−vis absorption spectra of TMOE and TPE with different concentrations (1.0 × 10−5, 2.5 × 10−5, 5.0 × 10−5, and 1.0 × 10−4 M) in THF at room temperature. It was observed that the absorption spectra of TMOE show three absorption peaks centered at ∼230, 260, and 330 nm, respectively. The absorption peak at 230 nm is attributed to the n−σ* electron transition of the oxygen atom in methoxy group, whereas the bands at 260 and 330 nm are attributed to the π−π* local electron transition of the conjugate system.36 In the absorption spectra of TPE, there are only two π−π* local electron transition absorption peaks centered at ∼240 and 310 nm, respectively. We speculated that the lone pair of electrons in the oxygen atom of TMOE may be responsible for the difference in the fluorescence emissions of TMOE and TPE. TFA is a strong electron acceptor due to the trifluoromethyl group. TFA can combine with TMOE to form a D−A

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EXPERIMENTAL SECTION TPE and TMOE were synthesized by Zhang and his coworkers.26 The structures of TPE and TMOE are shown in Figure 1. Trifluoroacetic acid (TFA), triethylamine (TEA), and THF were purphased from Beijing Chemical Reagent. All the above chemicals were of analytical grade and were used as received without further purification. Ultraviolet−visible (UV−vis) spectra of TMOE and TPE in THF in a 1.0 cm light path quartz cuvette were recorded on a Shimadzu UV-3600 spectrophotometer. Hydrostatic pressure was supplied by a DAC (Diacell Lever DAC−Mini A65000) with 1.0 mm diamond culet. A T301 stainless steel gasket with a thickness of 0.24 mm was drilled to make a hole having a diameter of 0.3 mm to be used as the sample chamber. A small ruby chip was introduced into the hole for in situ pressure calibration. Then, TMOE was placed in the sample chamber. A methanol/ethanol mixture (4:1, v/v) was added as a pressure transmitting medium (PTM).27,28 PTM can ensure hydrostatic pressure according to Pascal principle. By monitoring the widths and separation of R1 and R2 fluorescence shifts of ruby, the values of the pressures can be obtained.29−31 The hydrostatic pressure fluorescence spectra of TMOE and those of TPE were measured on a self-built fluorescence microscope (IX71, Olympus, 50× , numerical aperture = 0.5) equipped 9219


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Figure 2. Raman spectra of TMOE (A) and TPE (B) before and after grinding. Excitation wavelength was 532 nm. (inset) Photographs of partial TMOE and TPE before and after grinding.

Figure 3. Fluorescence spectra of TMOE (A) and TPE (B) under different pressures. The numbers indicate the pressure in units of gigapascals. Excitation wavelength was 365 nm. (inset) Photographs of partial TMOE and TPE crystals before and after hydrostatic pressure was performed, taken by a camera fixed on the fluorescence microscope.

configuration and may affect the fluorescence property of TMOE. On the contrary, TEA is able to neutralize TFA. Thus, the added TEA could weaken the TFA’s influence on TMOE in the THF. To further examine the function of methoxy group on the fluorescence of TMOE, an experiment of TFA to interact with TMOE was performed. When TFA (several drops) was added into the TMOE solution (1.0 × 10−4 M, 2.0 mL), the blue solution became almost colorless. Then, the TFA in TMOE solution was neutralized with TEA (several drops), and the solution with a deep blue color was restored. We suspected that the dynamic and reversible C−O···H hydrogen bond generated between the TFA and TMOE molecules would decrease the excited-state energy of TMOE molecules, quenching the fluorescence and changing the color of the TMOE solution. When TEA was added to neutralize TFA in the solution, the hydrogen bond would be destroyed. Hence, the fluorescence intensity of TMOE was restored. In contrast, the additions of TFA and TEA have no effect on the fluorescence emission of the TPE solution (inset in Figure 1). Compared with that of TMOE, the free rotation of the benzene ring of TPE has stronger activity due to the absence of

methoxy groups. Therefore, TPE solution is almost nonfluorescent. From the comparisons of UV−vis absorption spectra of TMOE and TPE and the test of the TFA/TEA addition, we can deduce that the chemical structural change of TMOE (the presence of methoxy groups) may bring the difference in fluorescence emission, especially under external stimuli. 2. Raman Spectra of TMOE and TPE under Grinding. As a TPE derivative, TMOE shows a remarkable change in its fluorescence spectra compared with TPE. Upon grinding, a color change of TMOE from blue to cyan occurred, which suggests a piezochromic property (see inset in Figure 2A). However, the fluorescence emission of TPE did not change through a grinding process (Figure 2B inset).26 Zhang and his co-workers’ investigation of the piezochromic mechanism of TMOE by XRD indicates that the mechano-fluorochromism was caused directly by crystalline-to-amorphous phase transition, while TPE maintained the same color upon grinding because of no phase transition.26 To identify the different structure changes between TMOE and TPE after grinding and to understand the piezochromic property of TMOE, the Raman 9220


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Figure 4. Raman spectra of TMOE at different pressure values. The numbers indicate the pressure in units of gigapascals. The excitation wavelength was 514 nm.

behaviors may be attributed to the molecular structure twist. The dihedral angle between the benzene ring and the ethylene core transformed under external pressure and the steric hindrance between the phenyl rings decreased. High pressure led to the dense packing of TMOE and TPE molecules due to the reduction in dihedral angle and distance of molecules, so the fluorescence intensities of TMOE and TPE were quenched by the intermolecular π−π stacking interactions between the planar molecules. However, the strength of the weak C−H···O hydrogen bonds in TMOE molecules was gradually enhanced with the increase of pressure, leading to an obvious red shift of the fluorescence emission.38 Note that the change of the C− H···O hydrogen bonds under pressure is reversible. In contrast, there is no C−H···O interaction among TPE molecules so that the fluorescence spectra display some red shift, but the red shift was too weak to be observed; the intermolecular π−π stacking interaction still partially existed even though the external pressure had been relieved. 4. Pressure-Dependent Raman Spectra of TMOE. To further verify the molecular internal changes caused by pressure, the high-pressure Raman spectra of TMOE were obtained with the help of a DAC. The Raman spectra of TMOE at different pressures are shown in Figure 4. The Raman peak at 1133 cm−1 is attributed to the stretching vibration of the single C−C bond formed by the carbon atom of the rigid planar ethylene core and the carbon atom of the benzene ring. The peak at ∼1600 cm−1 is attributed to the stretching vibrations of the CC and benzene ring. With the increase of pressure, the Raman peak at ∼1600 cm−1 slightly red shifts, accompanied by the peak intensity gradually weakening. This indicates that TMOE molecules started to closely pack under the external pressure.39,40 The molecular conformation tended to planarity, and the CC bond length became shorter gradually. This will be confirmed by the DFT simulation results that we will discuss later. In addition, all the Raman peak intensities gradually weakened. This further illustrates that molecular vibrations are limited by the close packing induced by pressure, which causes the losses of Raman vibration activity and intensity.41,42 When the pressure reached

spectra were acquired upon grinding. Figure 2 shows the Raman spectra of TMOE and TPE before and after grinding. It can be found that the Raman spectra of TMOE and TPE are almost the same except that the strength decreases to a half after grinding. Little difference in Raman spectra of TMOE and TPE indicates that no large conformational changes occurred after grinding.37 Therefore, the destruction of the molecular interaction between molecules might answer for above data. There are many flexible intermolecular interactions, such as C− H···O hydrogen bonds and C−H···π covalent bonds from methoxy groups among TMOE molecules. In contrast, only C−H···π interactions exist among TPE molecules. So, we expect that the crystal phase transition of TMOE from crystal to the amorphous phase is allowed. However, the crystal structure of TPE is too fragile under shearing force to be detected by its color change.26 3. Pressure-Dependent Fluorescence Spectra of TMOE and TPE. Considering that the grinding pressure is limited and uncontrollable, we further used DAC to explore the effect of high pressure on the structure of TMOE via the fluorescence and Raman spectroscopies. Figure 3 displays the pressure-dependent fluorescence spectra of TMOE and TPE. We can see that with the external pressure increasing, the fluorescence emission band of TMOE gradually red shifts, and the fluorescence intensity weakens as well. Changes in fluorescence color (from 480 to 540 nm) caused by external pressure from atmospheric level to 2.6 GPa, which is the maximum pressure in the experiment, are much larger than those obtained by grinding.26 The fluorescence band at 480 nm gradually decreases with the increase of pressure, while a new fluorescence band emerges gradually at 540 nm. When the pressure returned to atmospheric level, the fluorescence intensity almost recovered to that before pressing. However, with the external pressure increasing, the fluorescence intensity of TPE gradually decreased. Also, almost no emission band shifted, and no new fluorescence peak emerged (Figure 3B). When the pressure returned to atmospheric pressure, the fluorescence intensity of TPE did not recover to the original level. We speculated that the hydrostatic pressure-related 9221


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Chart 1. Simulation of the Structure of TMOE at Different Dihedral Angles

a

(a) The structure at ambient, when the dihedral shown in cyan-colored atoms angle is 47.577°. (b−f) The structures when the dihedral angles are 57.577°, 67.577°, 77.577°, respectively.

Figure 5. Simulated Raman spectra of TMOE at different dihedral angles in two different spectral ranges.

to 1.5 GPa, except the CC stretching, there was almost no Raman signal. When the pressure reaches 1.7 GPa the Raman activity was seriously weakened, and all Raman peaks disappeared. When the hydrostatic pressure returned to atmospheric level, all the Raman peaks recovered, indicating that the changes in molecular conformation are reversible. The fact all Raman peaks recovered and no new band generated once the pressure relieved that no phase change occurred in TMOE crystals under such a high pressure. 5. DFT Simulation of TMOE. To confirm pressure-induced torsion around the single C−C bond formed by the carbon atom of the rigid planar ethylene core connected with the carbon atom of the benzene ring then tuning the efficiency of TMOE molecule conjugation related to the structure change caused by external pressure, DFT simulation was conducted. We changed the dihedral angle between the CC center and phenyl ring and simulated the Raman spectra of TMOE molecules at different angles. The molecular structures of TMOE at different dihedral angles are shown in Chart 1. With the increase of one of the dihedral angles between the benzene

rings and the rigid planar ethylene core, molecular twist decreased with the shortening of the bond length of CC. When the dihedral angle is more than 89.577°, molecular structure will reverse its direction of twisting (data are not shown). Figure 5 exhibits the simulated Raman spectra of TMOE at different dihedral angles. As shown in Figure 5, under the atmospheric pressure, the crystal of TMOE displays a dihedral angle of 47.577°.26 Several vibration bands can be identified from its Raman spectrum (top curve), such as the stretching vibration of the single C−C bond formed by the carbon atom of the rigid planar ethylene core connected with the carbon atom of the benzene ring (1158 cm−1), the rocking vibration of C−C bond and the stretching vibration of benzene ring (1356 cm−1), the stretching vibrations of CC and benzene ring (1600 cm−1), etc. With the gradual increase of the dihedral angle between the benzene ring and the carbon atom of planar ethylene core from 47.577 to 77.577° and the gradual decrease in the adjacent dihedral angle, the intensities of all the Raman peaks of TMOE gradually weaken, accompanied by red shifts. 9222




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Especially the Raman peak at ∼1600 cm−1 shows a significant red shift and a decreased intensity compared with the others. When the dihedral angle is over 77.577°, the Raman spectra are nearly unchanged, which can be explained by the almost planar molecular structure and the closest molecular packing. This is consistent with our experimental results in which the Raman spectra are nearly unchanged, and all the Raman peaks do not appear any more when the pressure is over 1.5 GPa. We also found that the Raman band intensities recover when the dihedral angle is larger than 89.577°, indicating a reverse twisting direction as stated above. These changes in the simulated Raman spectra are consistent with the results of our high-pressure Raman experiment. From Chart 1, it can also be found that the dihedral angle between the planar CC core and phenyl ring gradually increases, while the adjacent dihedral angle gradually decreases. These changes make the TMOE molecule tend to closely pack. The intermolecular π−π stacking interaction among the molecules leads to the decrease of fluorescence intensity. This further proves that the piezochromism of TMOE comes from the distortion of the torsion angle between the benzene ring and the planar ethylene core, thus leading to the enhanced intermolecular interactions caused by the close packing as well as the C−H···O hydrogen bonds interactions under external pressure.

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SUMMARY AND CONCLUSIONS In this work, we studied the piezochromic characteristic of TMOE using fluorescence and Raman spectroscopies under the hydrostatic pressure via a DAC. Under the hydrostatic pressure, the geometry of TMOE starts to distort through the change of the dihedral angle between the benzene ring and the planar ethylene core. As a result, the molecules become close-packed. And the π−π stacking interaction among TMOE molecules is enhanced, resulting in the quenching of its fluorescence. In addition, the C−H···O hydrogen bonds in TMOE molecules are strengthened with increasing pressure, responsible for the obvious red shift of the fluorescence band. Raman peaks under the hydrostatic pressure and the theoretical calculation both prove that the distortion of the torsion angle leads to enhanced intermolecular interactions and facilitates C−H···O hydrogen bond interactions. This study provides experimental evidence for the intramolecular interaction responsible for PAIE. And the mechanism can be extended to other PAIE material systems that possess weak enough intermolecular interaction. The TMOE study will be helpful for learning the structure− property correlations of other TPE derivatives, especially for the ones having the PAIE characteristic.

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AUTHOR INFORMATION

Corresponding Author

*Phone: +86-431-85159383. E-mail: [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was supported by the National Natural Science Foundation of China (NSFC) Grant Nos. 91441105 and 21373096 and National Instrumentation Program of the Ministry of Science and Technology of China No. 2011YQ03012408. 9223


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