Comparison of Shearing Force and Hydrostatic Pressure on Molecular

Jan 30, 2015 - Oil and Gas Wastewater is a Cheap Fix for Road Dust but Comes at a Toxic Cost. Wastewater from oil and gas wells that is spread on unpa...
9 downloads 0 Views 2MB Size
Download PDF
Article pubs.acs.org/JPCA

Comparison of Shearing Force and Hydrostatic Pressure on Molecular Structures of Triphenylamine by Fluorescence and Raman Spectroscopies Jinxia Wu, Hailong Wang, Shuping Xu, and Weiqing Xu* State Key Laboratory of Supramolecular Structure and Materials, Institute of Theoretical Chemistry, Jilin University, Changchun 130012, People’s Republic of China S Supporting Information *

ABSTRACT: Luminescent mechanochromism (e.g., shearing force and hydrostatic pressure) has been intensively studied in recent years. However, there are few reported studies on the difference of the molecular configuration changes induced by these stresses. In this study, we chose triphenylamine, C18H6N (TPA), as a model molecule to study different molecular configuration changes under shearing force and hydrostatic pressure. Triphenylamine is an organic optoelectric functional molecule with a propeller-shaped configuration, a large conjugate structure, and a single molecular fluorescence material. Fluorescence and Raman spectra of TPA were recorded in situ under different pressures (0−1.9 GPa) produced by the mechanical grinding or using a diamond anvil cell (DAC). Our results show that the crystal phase of TPA transformed to the amorphous phase by grinding, whereas no obvious phase transition was observed under hydrostatic pressure up to 1.9 GPa, indicating the stability of TPA. Hydrostatic pressure by DAC induces molecular conformation changes, and the pressure-induced emission enhancement phenomenon of TPA is observed. By analyzing the Raman spectra at high pressure, we suggest that the molecular conformation changes under pressure are caused by the twisted dihedral angle between the benzene and the nitrogen atom, which is different from the phase transformation induced by the shearing force of grinding.

■

INTRODUCTION Tuning the emission property of organic luminescent molecules by changing environmental factors has recently attracted considerable attention as a way to improve the performance of organic optoelectronic devices. The stimuli response for fluorescence changes includes many mechanisms, such as pizochromism,1−3 vapochromism,4,5 thermochromism,6,7 aciddependent luminescence,8,9 and photochromism.10 Piezochromic luminescent materials have recently been frequently studied because they exhibit shifts in emission wavelengths resulting from the intramolecular conformational changes or intermolecular interactions under pressure, rather than perturbations produced by changes in chemical reactions.11 These organic compounds may be applied widely in display, information storage,12 and chemical or biological sensing.13 It is generally recognized that many organic emission materials change their fluorescence color due to the transformation from the crystal phase to the amorphous phase upon grinding.14−16 However, under hydrostatic pressure, the molecule may generate a new phase or a new crystal structure, causing the fluorescence color change.17 To compare the effects of different mechanical stimuli (e.g., shearing force and hydrostatic pressure) on the molecular configurations, we employed fluorescence and Raman spectroscopies to address this issue in this study. © XXXX American Chemical Society

Triphenylamine (TPA) is a typical nonplanar molecule, and it is an important molecule in organic light-emitting materials studies.18,19 Figure 1 shows the structure of a TPA molecule from two view angles and the electrostatic potential around the molecular surface (a movie of a TPA molecule is also supplied in the Supporting Information). It has a larger conjugate

Figure 1. Calculated electrostatic potential on the molecular surface of T. The electrostatic potential scale ranges from −2.160 (red) to 2.160 au (blue). Received: November 13, 2014 Revised: January 30, 2015

A

DOI: 10.1021/jp511380a J. Phys. Chem. A XXXX, XXX, XXX−XXX

Article

The Journal of Physical Chemistry A

spectra data were treated with Baseline using Origin 8.5 software. The high-pressure fluorescence spectra under hydrostatic conditions were measured on a fluorescence microscope (IX71, Olympus, 50× , NA = 0.5) equipped with a spectrometer (Jobin Yvon iHR320).30 The light source was a mercury lamp with an excitation wavelength of 365 nm. The high-pressure Raman spectra under hydrostatic conditions were performed at a Horiba Jobin Yvon JY-T64000 Raman spectrometer. The argon ion laser with a 514.5 nm line was used to excite samples. The output power was 20 mW.

surface, and a single TPA molecule presents a tetrahedral configuration. Owing to not having the π−π intermolecular interaction in stable stages, TPA can effectively avoid the fluorescence quenching.20 Three benzene rings are linked to the central nitrogen atom via the C−N single bond, allowing the benzene rings to rotate freely. This weakens the fluorescence character of TPA. However, if the benzene is modified with substitution groups, the fluorescence of TPA derivatives will be obviously enhanced. Furthermore, different substitution groups can also lead to different fluorescence colors.21,22 What’s more, TPA is frequently used as an electron donor group because its nonplanar structure can avoid the intermolecular interaction and molecular packing and the center nitrogen atom has a lone electron pair.18 Therefore, these TPA derivatives form donor−acceptor structure, exhibiting the typical characteristics of intramolecular charge transfer (ICT)23 and can be used as organic photoelectric functional materials.24 Because of their propeller-shaped structure, TPA-based derivatives are expected to change their fluorescence emission in response to external stimuli.21 In this work, we studied the molecular structure changes of TPA after grinding and hydrostatic pressure using fluorescence and Raman spectroscopies. We found that the fluorescence emission of TPA showed a slight shift in wavelength due to both mechanical grinding and hydrostatic pressure. However, the intensity decreased obviously upon grinding but increased gradually with the increase of pressure upon hydrostatic pressure, demonstrating pressure-induced emission enhancement (PIEE) behavior. In addition, the Raman spectra after mechanical grinding and under hydrostatic pressure were compared.

■

RESULTS AND DISCUSSION 1. Crystal Structures of TPA. We grew a TPA single crystal using the method of evaporating solvents and carried out the single-crystal XRD detection. Single-crystal XRD was performed in order to get insight into the molecular arrangements in the solid state. The crystal data and structural refinement of TPA are shown in Table. 1. The crystal structure Table 1. Crystal Data and Structure Refinement for Compound TPA Compound TPA empirical formula formula mass temperature [K] system space group a [Å] b [Å] c [Å] α [deg] β [deg] γ [deg] volume [Å3] Z parameters/restraints reflections collected/unique R1 [I > 2σ] wR2 (all data) goodness of fit

■

EXPERIMENTAL SECTION TPA was purchased from Chongqing Aikeda Chemical Reagent. Toluene, n-hexane, CH2Cl2, CHCl3, and tetrahydrofuran (THF) were purchased from Beijing Chemical Reagent. All of the above chemicals were of analytical grade and used as received without further purification. The crystal of TPA was grown from the n-hexane/dichloromethane solution. Ultraviolet−visible (UV−vis) spectra of TPA solutions were recorded on a Shimadzu UV-3600 spectrophotometer for solutions contained in a 1.0 cm light path quartz cuvette. Fluorescence spectra of TPA (before and after grinding) at atmospheric pressure were obtained on a Shimadzu 5301PC fluorescence spectrophotometer. Powder X-ray diffraction (XRD) patterns were measured with a Japan Rigaku SmartLab (3) X-ray diffractomer with Cu−Ka radiation (λ = 1.5418 Å, 40 kV, 30 mA) at 25 °C (scan range: 4−50°). Single-crystal X-ray was recorded on a Japan Science R-AXIS RAPID X-ray singlecrystal diffractometer. Hydrostatic pressure was obtained using a diamond anvil cell (DAC) (Diacell Lever DAC-Mini A65000) with 1.0 mm culet diamonds, where the pressure is applied by a screw mechanism. A 0.24 mm thick T301 stainless steel gasket was drilled to make a hole with a diameter of 0.3 mm as the sample chamber. TPA was placed in the sample chamber, and a small ruby chip was introduced into the hole for in situ pressure measurement. Then, a methanol and ethanol mixture (4:1, v:v)25,26 was added as a pressure-transmitting medium (PTM). The presence of a PTM provides a guarantee for obtaining the hydrostatic pressure according to Pascal’s principle. By monitoring the widths and separation of ruby R1 and R2 fluorescence shifts, we can measure the value of the pressure.27−29 Also, all Raman

C18H15N 245.31 293(2) K monoclinic Cc 15.678(3) 15.823(3) 22.276(5) 90 91.18(3) 90 5524.7(19) 16 685/2 26001/11975 0.0429 0.1183 0.974

and the molecular packing of one unit are shown in Figure 2 (a movie showing its crystal structure is provided in the Supporting Information). We can see that TPA is a highly symmetrical molecule, and there are large dihedral angles between each benzene ring and the center nitrogen atom. The twisted conformation of TPA reduces intermolecular interactions. The crystal structure of TPA displays that a TPA molecule is centrosymmetric and has a tetrahedral shape. We believe that no π−π stacking or any type of H- or J-aggregation intermolecular interactions exist in TPA crystals, indicating that the emission from a TPA crystal is due to the behavior of TPA molecules. The UV−vis spectra of different concentrations of TPA (1.0 × 10−5−1.0 × 10−3 M) in different solvents (i.e., toluene, CH2Cl2, CHCl3, THF) at standard temperature (see Figure S1, Supporting Information) also show that the TPA is a single molecular luminescence material and that no intermolecular interaction exists.31 2. Grinding-Induced Phase Transformation of TPA. As it is well-known that the weak intermolecular interactions can lead to defects in the crystal, the crystal structure has low lattice B

DOI: 10.1021/jp511380a J. Phys. Chem. A XXXX, XXX, XXX−XXX

Article

The Journal of Physical Chemistry A

Figure 2. Stacks of crystal structures along the crystallographic a, b, and c axes directions, respectively (left to right).

Figure 3. (A) Fluorescence spectra of TPA before and after grinding. (B) Raman spectra of TPA before and after grinding. The excitation wavelength was 365 nm for fluorescence measurements and 514.5 nm for Raman spectra.

energy and is easily destroyed by external pressure.32 On the basis of the nonplanar structure of TPA, we speculated that the crystal structure of TPA will be easily damaged under external pressure, which may result in the luminescent properties of TPA changing. Therefore, we analyzed the fluorescence and Raman spectra of the solid TPA under pressure. After grinding with a pestle, TPA displays no obvious luminous color changes to the naked eye. Figure 3 is the fluorescence and Raman spectra of TPA before and after grinding. The emission peak of ground TPA shows a wavelength blue shift (from 443 to 435 nm) accompanying the intensities that obviously weaken. This suggests that TPA has better molecular coplanarity and conjugation in the crystal state.33 TPA molecules accumulate close with a well-ordered state in the crystal state, leading to the limitation of the free vibrations of the phenyl ring. The nonradiation transition is weak, and the fluorescence intensity is relatively strong. However, the TPA crystal structure is destroyed by the action of grinding, transforming the crystal phase transform into an amorphous phase, damaging the molecular close packing accordingly, and putting the molecule in a poorly organized state.34,35 The limits on the benzene ring are released, enhancing the nonradiation transition; therefore, the fluorescence intensity is significantly reduced. The Raman spectra of TPA before and after grinding are almost the same (Figure 3B), which indicates that neither large conformational changes nor obvious chemical environmental changes have occurred.36 The shearing force of grinding leads only to an increase in the disordered phase.37

Our hypothesis has been proven by the powder XRD measurements of TPA before and after grinding. Figure 4

Figure 4. Powder XRD curves of TPA under different conditions: (a) pristine, (b) grinding for 1 min, (c) the ground powder after annealing at 150 °C for 1 min, and (d) the annealed sample after grinding.

shows that TPA forms different molecular packing modes before and after grinding and heating. We find that the powder XRD pattern of the pristine TPA is strong and sharp, suggesting a highly ordered crystal structure. In contrast, the powder XRD pattern of ground TPA shows only weak diffraction peaks, while some diffraction peaks have disappeared completely. The powder XRD patterns show that the decrease in the fluorescence intensity of TPA is directly caused by the C

DOI: 10.1021/jp511380a J. Phys. Chem. A XXXX, XXX, XXX−XXX

Article

The Journal of Physical Chemistry A phase transition from the crystalline to amorphous state via grinding.13,14 The ground sample is in an unstable state, and it will rapidly crystallize under 150 °C heating for 1 min, as shown by the narrow and intense peaks of TPA obtained after annealing. The annealed crystalline TPA will transform to an amorphous form after grinding again, as can be seen from the disappearance of peaks in the powder XRD pattern (Figure 4d). The result of powder XRD shows that grinding destroyed the crystal structure. In the amorphous phase, the molecules are disordered. Therefore, the intramolecular vibrational rotations are more flexible, which significantly weakens the fluorescence intensity. 3. Hydrostatic Pressure-Induced Molecular Configuration Changes of TPA. The pressure produced by grinding is nonhydrostatic; however, the pressure caused using the DAC is semihydrostatic.38 Compared with grinding, DAC can produce higher pressures. Because the grinding pressure is limited and uncontrollable, we used the DAC to further explore the effect of high pressure on the structure of TPA via fluorescence and Raman spectroscopies of TPA under different pressures. The pressure-induced fluorescence spectra of TPA are revealed in Figure 5. A plot of the fluorescence peak

Figure 6. Raman spectra of TPA at different pressure values via DAC. The excitation wavelength was 514 nm.

stretches, 1173 cm−1 is attributed to the C−N stretches with associated large C−C stretches, and 1600 cm−1 is attributed to the quadrant stretch of phenyl rings with associated C−N stretches.39 The Raman peak at 1600 cm−1 appears to red shift gradually with the increase of pressure accompanied by the peak intensity gradually weakening, which means that TPA becomes close-packing under pressure.40,41 In addition, the intensities of all Raman peaks gradually weaken with increasing pressure. This further illustrates that molecular vibrations are limited by the close packing induced by pressure, which causes the losses of Raman vibration activity and the intensity.42,43 In the close-packing process, the dihedral angle between the benzene and the nitrogen atom displays an obvious change, resulting in the stretching vibration of benzene becoming significantly restricted. Compared with the Raman peaks at 1100 and 1173 cm−1, the Raman peak at 1600 cm−1 shows a significant red shift and decreased intensity. At 1.9 GPa, the maximum pressure in the experiment, TPA almost completely loses Raman activity compared with that under atmospheric pressure. When the hydrostatic pressure returns to atmospheric, most Raman peaks recover in intensity. No Raman peaks disappeared, and no new Raman peaks were observed, indicating that the change of molecular conformation is not a phase change. Under external pressure, TPA might form a new metastable structure. This new metastable structure of TPA still has a very strong spatial structure and is far from a planar configuration. Although the distance between molecules is reduced, there is not a π−π stacking existing between two TPA molecules due to the structure of TPA under our maximum pressure still being spatial, but the intermolecular interaction is enhanced. Therefore, with the increase of pressure, the fluorescence spectra of TPA show a weak red shift (Figure 5, inset). At this pressure, the intramolecular vibration/rotation is still dominant for the fluorescence emission rather than the intermolecular interaction. Figure 5 shows that when the pressure returns to atmospheric, the fluorescence spectrum is close to that before pressurization, indicating that the changes of TPA caused by external pressure are reversible15 and it is not a chemical transformation. Compared with grinding, DAC-produced hydrostatic pressure turns TPA into a metastable phase with

Figure 5. Fluorescence spectra of TPA under different pressure values via DAC. The excitation wavelength was 365 nm. (Inset) Pressure versus peak wavelength.

wavelength versus pressure is inset in Figure 5. When the pressure is less than 0.1 GPa, the emission wavelength shows a small blue shift, which is in accordance with the spectral change under grinding (see Figure 3A). As the external pressure increases, the fluorescence intensity of TPA gradually increases, accompanied by a slight red shift of several nanometers. When the pressure reaches 1.9 GPa, the maximum pressure of our experiment, the fluorescence intensity doubles, compared with that at atmospheric pressure. We speculate that TPA may have become a twisted structure under this pressure. The dihedral angles between the benzene rings and the central nitrogen atom may decrease under external pressure. The TPA molecule tends to flatten so that the steric hindrance between the phenyl rings decreases with pressure, which makes the molecular structure close-packing. The rotation of the benzene ring is restricted, nonradiation transition weakens, and the fluorescence intensity increases. In order to further explore the internal molecular changes caused by pressure, we conducted high-pressure Raman experiments. The Raman spectra of TPA at different pressure values are shown in Figure 6. The Raman peak at 997 cm−1 is attributed to the C−C stretches with associated small C−N D

DOI: 10.1021/jp511380a J. Phys. Chem. A XXXX, XXX, XXX−XXX

Article

The Journal of Physical Chemistry A a close-packing structure. Accordingly, the fluorescence intensity of TPA is significantly enhanced. The hydrostatic pressure-induced Raman results of TPA are consistent with the fluorescence data. This further proves that our supposition that the close packing of TPA resulting from external pressure is the main reason for the observed fluorescence enhancement. We also found that TPA has the aggregation-induced emission enhancement (AIEE) property (Figure S2, Supporting Information), and the enhanced fluorescent intensity in the aggregation state can be attributed to the restricted intramolecular rotation (RIR), resulting in the energy loss caused by the nonradiation transition reduced.44 This further proves that the intramolecular free rotation of benzene is the main reason for the fluorescence intensity of TPA.

and Technology of China No. 2011YQ03012408, and Innovation Program of the State Key Laboratory of Supramolecular Structure and Materials. We highly appreciate the unknown reviewer for his/her careful revision.

■

(1) Bi, H.; Chen, D.; Li, D.; Yuan, Y.; Xia, D.; Zhang, Z.; Zhang, H.; Wang, Y. A green emissive amorphous fac-Alq3 solid generated by grinding crystalline blue fac-Alq3 powder. Chem. Commun. 2011, 47 (14), 4135−4137. (2) Zhang, Z.; Yao, D.; Zhou, T.; Zhang, H.; Wang, Y. Reversible piezo- and photochromic behaviors accompanied by emission color switching of two anthracene-containing organic molecules. Chem. Commun. 2011, 47 (27), 7782−7784. (3) Zhang, X.; Chi, Z.; Zhang, J.; Li, H.; Xu, B.; Li, X.; Liu, S.; Zhang, Y.; Xu, J. Piezofluorochromic properties and mechanism of an aggregation-induced emission enhancement compound containing nhexyl-phenothiazine and anthracene moieties. J. Phys.Chem. B 2011, 115 (23), 7606−7611. (4) Takahashi, E.; Takaya, H.; Naota, T. Dynamic vapochromic behaviors of organic crystals based on the open−close motions of Sshaped donor−acceptor folding units. Chem.Eur. J. 2010, 16 (16), 4793−4802. (5) Hudson, Z. M.; Sun, C.; Harris, K. J.; Lucier, B. E.; Schurko, R. W.; Wang, S. Probing the structural origins of vapochromism of a triarylboron-functionalized platinum (II) acetylide by optical and multinuclear solid-state NMR spectroscopy. Inorg. Chem. 2011, 50 (8), 3447−3457. (6) Feng, J.; Tian, K.; Hu, D.; Wang, S.; Li, S.; Zeng, Y.; Li, Y.; Yang, G. A triarylboron-based fluorescent thermometer: Sensitive over a wide temperature range. Angew. Chem., Int. Ed. 2011, 50 (35), 8072− 8076. (7) Zhao, Y.; Gao, H.; Fan, Y.; Zhou, T.; Su, Z.; Liu, Y.; Wang, Y. Thermally induced reversible phase transformations accompanied by emission switching between different colors of two aromatic-amine compounds. Adv. Mater. 2009, 21 (31), 3165−3169. (8) Zhou, T. L.; Jia, T.; Zhao, S. S.; Guo, J. H.; Zhang, H. Y.; Wang, Y. Acid-stimuli-luminescence and carbonyl-proton interaction dependent emission properties of 2,6-biphenyl-4-pyrone crystals. Cryst. Growth Des. 2012, 12 (1), 179−184. (9) Sato, T.; Higuchi, M. A vapoluminescent Eu-based metallosupramolecular polymer. Chem. Commun. 2012, 48 (41), 4947−4949. (10) Browne, W. R.; Pollard, M. M.; de Lange, B.; Meetsma, A.; Feringa, B. L. Reversible three-state switching of luminescence: A new twist to electro- and photochromic behavior. J. Am. Chem. Soc. 2006, 128 (38), 12412−12413. (11) Wang, R.; Li, S.; Wang, K.; Duan, D.; Tang, L.; Cui, T.; Liu, B.; Cui, Q.; Liu, J.; Zou, B.; Zou, G. Pressure-induced phase transition in hydrogen-bonded supramolecular structure: Guanidinium nitrate. J. Phys. Chem. B 2010, 114 (20), 6765−6769. (12) Mizobe, Y.; Ito, H.; Hisaki, I.; Miyata, M.; Hasegawa, Y.; Tohnai, N. A novel strategy for fluorescence enhancement in the solid-state: Affording rigidity to fluorophores packing. Chem. Commun. 2006, 20, 2126−2128. (13) Toal, S. J.; Jones, K. A.; Magde, D.; Trogler, W. C. Luminescent silole nanoparticles as chemoselective sensors for Cr (VI). J. Am. Chem. Soc. 2005, 127 (33), 11661−11665. (14) Dou, C.; Han, L.; Zhao, S.; Zhang, H.; Wang, Y. Multi-stimuliresponsive fluorescence switching of a donor−acceptor π-conjugated compound. J. Phys. Chem. Lett. 2011, 2 (6), 666−670. (15) Ni, J.; Zhang, X.; Qiu, N.; Wu, Y. H.; Zhang, L. Y.; Zhang, J.; Chen, Z. N. Mechanochromic luminescence switch of platinum (II) complexes with 5-trimethylsilylethynyl-2,2′-bipyridine. Inorg. Chem. 2011, 50 (18), 9090−9096. (16) Nagura, K.; Saito, S.; Yusa, H.; Yamawaki, H.; Fujihisa, H.; Sato, H.; Shimoikeda, Y.; Yamaguchi, S. Distinct responses to mechanical grinding and hydrostatic pressure in luminescent chromism of

■

SUMMARY AND CONCLUSIONS We studied the difference between shearing forces from grinding and hydrostatic pressure on the molecular structures of TPA crystals using fluorescence and Raman spectroscopies. Upon grinding, the TPA crystal changes reversibly to a disordered phase. However, in the case of using the DAC, to reach high hydrostatic pressures, the tetrahedral configuration of the molecule can be distorted from the change of the dihedral angle between the benzene rings and the nitrogen atom, and TPA molecules start to be close-packed, resulting in enhanced fluorescent emission and weakened Raman intensity with a red shift. Even under such a high pressure, there is no phase change, indicating the good stability of TPA. This study will be a reference in the future designs of TPA devivatives. The change of dihedral angle between the benzene rings and the nitrogen atom can also be achieved by decorating the benzene with different substitute groups. For the organic light-emitting materials composed of TPA derivatives, the particular substituents can be selected according to the luminous colors that we are looking forward to, for instance, phenanthrene, anthracene, quinazoline, acridine, and so forth.23 These TPA-based derivatives will be a new sort in PIEE family and own potential in the fields of display information storage and chemical or biological sensing. Moreover, these TPA derivatives also have a large space structure as the TPA, and they are good model molecules for the studies of structurerelated spectral properties.

■

ASSOCIATED CONTENT

S Supporting Information *

Two movies displaying a TPA molecule structure and a TPA crystal structure, UV−vis spectra of different concentrations of TPA in different solvents, and the fluorescence change of TPA in pure THF and THF/water mixtures are provided. This material is available free of charge via the Internet at http:// pubs.acs.org.

■

REFERENCES

AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Tel: +86-431-85168505. Notes

The authors declare no competing financial interest.

■

ACKNOWLEDGMENTS This work was supported by the National Natural Science Foundation of China NSFC Grant 21373096 and 91441105, National Instrumentation Program of the Ministry of Science E

DOI: 10.1021/jp511380a J. Phys. Chem. A XXXX, XXX, XXX−XXX

Article

The Journal of Physical Chemistry A tetrathiazolylthiophene. J. Am. Chem. Soc. 2013, 135 (28), 10322− 10325. (17) Dong, Y.; Xu, B.; Zhang, J.; Tan, X.; Wang, L.; Chen, J.; Lv, H.; Wen, S.; Li, B.; Ye, L.; Zou, B.; Tian, W. Piezochromic luminescence based on the molecular aggregation of 9,10-bis((E)-2-(pyrid-2yl)vinyl)anthracene. Angew. Chem., Int. Ed. 2012, 51 (43), 10782− 10785. (18) Lee, H. J.; Sohn, J.; Hwang, J.; Park, S. Y.; Choi, H.; Cha, M. Triphenylamine-cored bifunctional organic molecules for two-photon absorption and photorefraction. Chem. Mater. 2004, 16 (3), 456−465. (19) Ge, Z.; Hayakawa, T.; Ando, S.; Ueda, M.; Akiike, T.; Miyamoto, H.; Kajita, T.; Kakimoto, M. A. Spin-coated highly efficient phosphorescent organic light-emitting diodes based on bipolar triphenylamine-benzimidazole derivatives. Adv. Funct. Mater. 2008, 18 (4), 584−590. (20) Mishra, A.; Fischer, M. K.; Bäuerle, P. Metal-free organic dyes for dye-sensitized solar cells: From structure:property relationships to design rules. Angew. Chem., Int. Ed. 2009, 48 (14), 2474−2499. (21) Xue, P.; Chen, P.; Jia, J.; Xu, Q.; Sun, J.; Yao, B.; Zhang, Z.; Lu, R. A triphenylamine-based benzoxazole derivative as a high-contrast piezofluorochromic material induced by protonation. Chem. Commun. 2014, 50 (20), 2569−2571. (22) Guo, Z.-H.; Jin, Z.-X.; Wang, J.-Y.; Pei, J. A donor−acceptor− donor conjugated molecule: Twist intramolecular charge transfer and piezochromic luminescent properties. Chem. Commun. 2014, 50 (46), 6088−6090. (23) Li, W.; Pan, Y.; Yao, L.; Liu, H.; Zhang, S.; Wang, C.; Shen, F.; Lu, P.; Yang, B.; Ma, Y. A hybridized local and charge-transfer excited state for highly efficient fluorescent OLEDs: Molecular design, spectral character, and full exciton utilization. Adv. Opt. Mater. 2014, 2 (9), 892−901. (24) Kamada, K.; Ohta, K.; Kubo, T.; Shimizu, A.; Morita, Y.; Nakasuji, K.; Kishi, R.; Ohta, S.; Furukawa, S.; Takahashi, H.; Nakano, M. Strong two-photon absorption of singlet diradical hydrocarbons. Angew. Chem., Int. Ed. 2007, 46 (19), 3544−3546. (25) Piermarini, G.; Forman, R.; Block, S. Viscosity measurements in the diamond anvil pressure cell. Rev. Sci. Instrum. 1978, 49 (8), 1061− 1066. (26) Besson, J.; Pinceaux, J. Uniform stress conditions in the diamond anvil cell at 200 kilobars. Rev. Sci. Instrum. 1979, 50 (5), 541−543. (27) Piermarini, G.; Block, S. Ultrahigh pressure diamond-anvil cell and several semiconductor phase transition pressures in relation to the fixed point pressure scale. Rev. Sci. Instrum. 1975, 46 (8), 973−979. (28) Eggert, J. H.; Goettel, K. A.; Silvera, I. F. Ruby at high pressure. I. Optical line shifts to 156 GPa. Phys. Rev. B 1989, 40 (8), 5724. (29) Chervin, J.; Canny, B.; Mancinelli, M. Ruby-spheres as pressure gauge for optically transparent high pressure cells. Int. J. High Pressure Res. 2001, 21 (6), 305−314. (30) Li, H.; Wang, H.; Huang, D.; Liang, L.; Gu, Y.; Liang, C.; Xu, S.; Xu, W. Note: Raman microspectroscopy integrated with fluorescence and dark field imaging. Rev. Sci. Instrum. 2014, 85 (5), 056109. (31) Chen, F.; Zhang, J.; Wan, X. Design and synthesis of piezochromic materials based on push−pull chromophores: A mechanistic perspective. Chem.Eur. J. 2012, 18 (15), 4558−4567. (32) Liu, W.; Wang, Y. L.; Sun, M. X.; Zhang, D. T.; Zheng, M.; Yang, W. J. Alkoxy-position effects on piezofluorochromism and aggregation-induced emission of 9, 10-bis(alkoxystyryl) anthracenes. Chem. Commun. 2013, 49 (54), 6042−6044. (33) Yang, S.; Kertesz, M. Bond length alternation and energy band gap of polyyne. J. Phys. Chem. A 2006, 110 (31), 9771−9774. (34) Zhang, G. Q.; Lu, J. W.; Fraser, C. L. Mechanochromic luminescence quenching: Force-enhanced singlet-to-triplet intersystem crossing for iodide-substituted difluoroboron-dibenzoylmethane-dodecane in the solid state. Inorg. Chem. 2010, 49 (23), 10747−10749. (35) Wang, K.; Huang, S.; Zhang, Y.; Zhao, S. S.; Zhang, H. Y.; Wang, Y. Multicolor fluorescence and electroluminescence of an ICTtype organic solid tuned by modulating the accepting nature of the central core. Chem. Sci. 2013, 4 (8), 3288−3293.

(36) Li, S.; Li, Q.; Wang, K.; Zhou, M.; Huang, X.; Liu, J.; Yang, K.; Liu, B.; Cui, T.; Zou, G.; Zou, B. Pressure-induced irreversible phase transition in the energetic material urea nitrate: Combined Raman scattering and X-ray diffraction study. J. Phys. Chem. C 2013, 117 (1), 152−159. (37) Nagura, K.; Saito, S.; Yusa, H.; Yamawaki, H.; Fujihisa, H.; Sato, H.; Shimoikeda, Y.; Yamaguchi, S. Distinct responses to mechanical grinding and hydrostatic pressure in luminescent chromism of tetrathiazolylthiophene. J. Am. Chem. Soc. 2013, 135 (28), 10322− 10325. (38) Speziale, S.; Zha, C. S.; Duffy, T. S.; Hemley, R. J.; Mao, H. k. Quasi-hydrostatic compression of magnesium oxide to 52 GPa: Implications for the pressure−volume−temperature equation of state. J. Geophys. Res. 2001, 106 (B1), 515−528. (39) Littleford, R.; Tackley, D.; Cherryman, J.; Dent, G.; Smith, W. A Raman and DFT study of substituted triphenylamines for use as charge transfer materials in light emitting polymers. J. Mol. Struct. 2004, 692 (1), 81−90. (40) Zhao, X.; Yan, T.; Wang, K.; Yan, Y.; Zou, B.; Yu, J.; Xu, R. NaEu3 (GeO4)2(OH)2: A high-pressure-stable photoluminescent lanthanide germanate. Eur. J. Org. Chem. 2012, 2012 (15), 2527−2532. (41) Boldyreva, E. V. High-pressure diffraction studies of molecular organic solids. A personal view. Acta Crystallogr., Sect. A 2007, 64 (1), 218−231. (42) Tan, X.; Wang, K.; Li, S.; Yuan, H.; Yan, T.; Liu, J.; Yang, K.; Liu, B.; Zou, G.; Zou, B. Exploration of the pyrazinamide polymorphism at high pressure. J. Phys. Chem. B 2012, 116 (49), 14441−14450. (43) Yan, T.; Li, S.; Wang, K.; Tan, X.; Jiang, Z.; Yang, K.; Liu, B.; Zou, G.; Zou, B. Pressure-induced phase transition in N−H···O hydrogen-bonded molecular crystal oxamide. J. Phys. Chem. B 2012, 116 (32), 9796−9802. (44) Jason, C.; Jacky, W.; Herman, H.; Tang, B. Deciphering mechanism of aggregation-induced emission (AIE): Is E−Z isomerisation involved in an AIE process? Chem. Sci. 2012, 3 (2), 493−497.

F

DOI: 10.1021/jp511380a J. Phys. Chem. A XXXX, XXX, XXX−XXX