Structural Tuning and Piezoluminescence ... - ACS Publications

Jan 11, 2017 - High Pressure Collaborative Access Team (HPCAT), Geophysical ... Physics, Institute of Fluid Physics, China Academy of Engineering Phys...
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Structural Tuning and Piezoluminescence Phenomenon in Trithiocyanuric Acid Qian Li,†,‡,§,∥ Shourui Li,∥,⊥ Kai Wang,† Yuanyuan Zhou,† Zewei Quan,§ Yue Meng,‡ Yanming Ma,† and Bo Zou*,† †

State Key Laboratory of Superhard Materials, Jilin University, Changchun 130012, China High Pressure Collaborative Access Team (HPCAT), Geophysical Laboratory, CIW, Argonne, Illinois 60439, United States § Department of Chemistry, South University of Science and Technology of China, Shenzhen, Guangdong 518055, China ⊥ National Key Laboratory of Shock Wave and Detonation Physics, Institute of Fluid Physics, China Academy of Engineering Physics, Mianyang 621900, China ‡

S Supporting Information *

ABSTRACT: As an essential thermodynamic variable, pressure has a powerful ability to accurately control molecular structures and properties by modulating the noncovalent interactions therein. Based on this point, we utilized pressure to tune the structure and properties of trithiocyanuric acid (C3H3N3S3, TTCA), gaining deeper insight into its structural nature and structure−property relationships. During compression, layered TTCA undergoes molecular distortion and relative slippage between interlayers, as well as anisotropic and stepwise shrinkage of intralayer six-molecule rings. Importantly, these structural variations have a great influence on the luminescence properties of TTCA. Piezoluminescence is observed above ∼4 GPa, acompanied by the calculated shifting of valence-band top. In experiments, detailed stepwise compression of the intralayer structure was captured directly. The observations combine the microscopic structure and macroscopic properties together and are beneficial for the further design of luminescence materials, as well as pressure sensors and pressure switches.



INTRODUCTION Noncovalent interactions (hydrogen bonding, π-stacking, van der Waals force, etc.) play central roles in most biological and chemical processes.1−4 Compared with covalent interactions, relatively weak and versatile noncovalent interactions are modified more easily by physical perturbations.5,6 On this point, great efforts have been made to control noncovalent interactions, so as to generate targeted structures with unique functions. Gaining deeper insight into structure−property relationships and finding an effective method for precisely controlling architectures are two of the central issues in both materials and chemical science.7−9 The goal is not only to expand material applications but also to shed light on the development direction of material synthesis and design.10,11 Molecular crystals are the ultimate exmples of noncovalent interactions.12 Their structural nature makes them typical prototypes for high-pressure research. Pressure can be used to precisely control the molecular structure by tuning the direction, strength, and cooperativity of the noncovalent interactions therein.13−18 Thus, new molecular materials with some unique structures and properties can be generated at high pressure. For instance, through the rearrangement of the hydrogen-bonded networks within sulfamic acid layers, © 2017 American Chemical Society

pressure successfully introduces relative slippage between adjacent molecular chains and an isosymmetric phase transition.19 Furthermore, high-pressure treatment promotes the crinkling of hydrogen-bonded arrays and the approach of adjacent layers in the energetic material acetamidinium nitrate, fabricating a new high-density and high-energy structure.20 So far, although pressure has proven to be an effective means of adjusting molecular structures, there is still an open question about how microscopic structural adjustments influence macroscopic properties at high pressure. at the same time, a detailed exploration of the mechanism by which pressureinduced structural and property changes are generated is still needed. Solving these problems is of pivotal importance for the application and understanding of molecular crystals, as well as the further study of chemical and materials science. Among molecular crystals, layered architectures always exhibit intriguing structural patterns and high-pressure properties.21,22 Layered structures normally have an interlayer direction with weak noncovalent interactions, so that an Received: November 13, 2016 Revised: January 9, 2017 Published: January 11, 2017 1870

DOI: 10.1021/acs.jpcc.6b11435 J. Phys. Chem. C 2017, 121, 1870−1875

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Figure 1. Ambient TTCA structure: (a) unit cell, (b) single molecular layer with hydrogen-bonded network, (c) two adjacent layers (marked by different colors) viewed perpendicular to the plane of the layers.

application of high pressure is a promising method for functionalizing molecular crystals. Through the tuning of designed molecular structures and the interactions therein, pressure is capable of introducing new luminescence properties into nonluminous architectures. This piezoluminescence phenomenon is significant for the further design of fluorescent materials, as well as pressure sensors and switches.

external force can efficiently decrease the intralayer distances and introduce relative slippage between layers.23,24 The decreased distance and slippage between molecular layers lead directly to the destruction of types of interactions, as well as the construction of the new crystal and electronic structures. Based on these considerations, we chose one canonical example of a perfectly layered structure, trithiocyanuric acid (C3H3N3S3, TTCA), as the model to explore the effects of pressure on its interactions, structure, and properties. Moreover, we performed both high-pressure experiments and calculations to gain deeper insight into its structure−property relationship and ensure the influence of structural tuning on variations in its properties. TTCA exhibits a perfectly regular structure that is applied extensively in synthetic chemistry. The two-dimensional hydrogen-bonding networks and rich donor atoms of TTCA make it a strong candidate for the synthesis of supramolecular cocrystals.25 Under ambient conditions (Figure 1a), TTCA adopts triclinic P1̅ symmetry with Z = 2 in one unit cell.26 As shown in Figure 1b,c, TTCA molecules are arranged in perfect layers and connected by extensive N−H···S hydrogen-bonded networks. Within one sheet, six TTCA molecules are connected through 12 hydrogen bonds, forming one six-molecule ring. Owing to the large offset of the rings, no π-stacking interactions are observed between layers. However, between the two corresponding TTCA molecules in adjacent layers, one S atom of one molecule is located perfectly above the center of another ring at a close distance and with the opposite charge and potential (see Figures S1 and S2 and the detailed structural parameters in the Supporting Information). The unique geometrical locations and electronic structures of the S atoms and adjacent TTCA rings indicate the attractive nature between them.27 Such interlayer attractive interactions and the higher mass of S atoms (compared with C, H, and N atoms) are both beneficial for slowing the slippage between the sheets, allowing the structural evolution to be captured in detail. In this work, high-pressure synchrotron X-ray diffraction (XRD) was used to monitor changes in symmetry. In addition, we performed high-pressure Fourier transform infrared (FTIR) and Raman scattering experiments to investigate the local structure and interaction behaviors of TTCA. The in situ photoluminescence (PL) properties were also explored under similar thermodynamic conditions. More information on the structural and property tuning was obtained with assistance of theoretical calculations. This study thus combined the adjustments in microscopic structure and the changes in macroscopic properties together. Such an approach is expected both to allow deeper insight into the nature of the noncovalent interactions and to offer some new strategies for fabricating novel structures with targeted properties in crystal engineering. Meanwhile, the new pressure-induced PL emission of TTCA confirms that the



EXPERIMENTAL SECTION The TTCA sample used in this work was obtained commercially (Aldrich, 95%). A diamond anvil cell (DAC) with 0.4-mm diamond culets was used to generate high pressure. For each experiment, a powdered sample was loaded into a 0.15-mm-diameter aperture that had been drilled in a preindented T301 steel gasket, together with two ruby balls for pressure calibration.28 All of the experiments were performed at room temperature. High-pressure Raman spectra were obtained using a Renishaw inVia Raman microscope in the backscattering geometry, with 514.5- and 830-nm semiconductor diode lasers as excitation sources. The acquisition time of each spectrum was 30 s. High-pressure FTIR absorption experiments were performed with a Bruker Vertex80 V FTIR spectrometer in transmission mode. A custom-made microscope was used to focus the IR beam onto the sample. In addition, the IR spectra were collected by a nitrogen-cooled broadband mercury cadmium telluride (MCT) detector with a resolution of 4 cm−1. In situ PL measurements were performed on a QuantaMaster 40 spectrometer (Photon Technology Inc.) in reflection mode. The excitation source was the 405-nm line of a violet diode laser with a spot size of 20 μm, and the emission spectra were recorded by a monochromator equipped with a photomultiplier. In situ angle-dispersive X-ray diffraction (ADXRD) measurements were performed using the High Pressure Collaborative Access Team’s (HPCAT’s) 16 μBD beamline facilities at the Advanced Photon Source (APS) at Argonne National Laboratory. The diffraction data were collected using the monochromatic 0.6199-Å X-ray beam with a typical beam size ∼10 × 10 μm2. In addition, the Bragg diffraction rings were recorded using a Mar345 CCD detector with an average acquisition time of 60 s. The two-dimensional XRD images were analyzed using FIT2D software, yielding one-dimensional intensity versus 2θ patterns.29 High-pressure lattice constants were obtained using the Pawley refinement method with the Reflex module in the commercial program Materials Studio 5.5, applying ultrafine convergence. A pseudo-Voigt peak-shape function and the Berar−Baldinozzi correction were used to fit the peak profiles. Parameters including the full width at half-maximum (fwhm; U, 1871

DOI: 10.1021/acs.jpcc.6b11435 J. Phys. Chem. C 2017, 121, 1870−1875

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The Journal of Physical Chemistry C V, W), profile parameters (NA, NB), asymmetry (P1−P4), background coefficients, lattice (a, b, c, α, β, γ), and lattice strain (A, B, C) were refined. Theoretical calculations of the charge and electrostatic potential distributions, as well as the simulated IR and Raman vibrations, were performed using the CASTEP package in Materials Studio 5.5 with the density-functional-theory-based pseudopotential plane-wave method. During the calculations, the local density approximation (LDA) exchange-correlation, Broyden−Fletcher−Goldfarb−Shanno (BFGS) algorithm, and norm-conserving pseudopotentials were employed. The convergence tolerance was set as a maximum force of 0.03 eV/Å and a maximum stress of 0.05 GPa. In addition, the plane-wave cutoff energy was set to 700 eV. The self-consistent field (SCF) tolerance was set to 1 × 10−6 eV/atom. Finally, 3 × 2 × 2 kpoints were used for the electronic Brillouin zone integration. The underlying ab initio structural relaxations and electronic structure calculations were performed using density functional theory within the Perdew−Burke−Ernzerhof (PBE) parametrization of the generalized gradient approximation (GGA) as implemented in the Vienna ab initio simulation package (VASP) code.30,31 The all-electron projector-augmented-wave (PAW) method was employed with the potentials, where 2s22p2, 2s22p3, 3s23p4, and 1s1 were treated as valence electrons for C, N, S, and H atoms, respectively. The energy cutoff of 1000 eV and appropriate Monkhorst−Pack k-meshes of 0.03 Å−1 were used to ensure that all of the enthalpy calculations were well-converged to better than 1 meV/atom.32



RESULTS AND DISCUSSION ADXRD patterns offer straightforward information on structural evolution. As shown in Figures S3−S7, the evolution of the diffraction patterns of TTCA was continuous, suggesting structural stabilization below ∼10 GPa. At the same time, it is apparent that the (1 2̅ 0) diffraction peak shifted at the highest rate during compression. The largest compressibility between TTCA layers results from the weakest interactions between them. Although no phase transition was observed, the question remains: How does pressure affect the local crystalline structure (such as inter- and intralayer structures) and electronic structure of TTCA. The high-pressure contraction of unilaminar TTCA molecules can be deduced from the evolution of the N−H stretching vibrations in the IR spectra (Figure 2), which present the behavior of hydrogen-bonded networks. Three stretching modes (ν1, ν2, and ν3) were detected at ambient conditions (Figure 2a), as expected from the calculated results shown in Figure 2b. The three modes mainly originate from the donors’ stretching vibrations along the opposite sides of the intralayer six-molecule rings (Figure 2d). With increasing pressure, all of the ν(NH) vibrations exhibited red shifts, owing to the sustained strengthening of the N−H···S hydrogen bonds.33,34 However, as shown in Figure 2c, the pressure-dependent peak positions of ν3 and ν2 displayed obvious discontinuities at 3.4 GPa. Their pressure coefficients changed dramatically from −13.37 to −1.38 cm−1/GPa and from −1.28 to −3.35 cm−1/ GPa, respectively. The pressure coefficients of stretching modes are commonly used to estimate the compressibility of chemical bonds at high pressure. Below 3.4 GPa, the much faster shift of ν3 indicates pressure mainly caused the A and F rings to approach each other, as well as the C and D rings. With further compression, the ν3 vibrations were restrained. However, ν2 displayed a much larger pressure dependence than before,

Figure 2. (a) Selected high-pressure FTIR spectra of N−H stretching modes. (b) Calculated FTIR N−H stretching vibrations under ambient conditions. (c) Corresponding wavenumber as a function of pressure. (d) Detailed graphical representation of three N−H stretching vibrations. The vibration amplitudes are indicated by the lengths of the arrows.

suggesting a more obvious compression along the two sides AB and ED above 3.4 GPa. In addition, because no discontinuity was observed for ν1, it can be inferred that the compressibility of the remaining two sides, BC and EF, remained stable all the way to ∼10 GPa. The six-molecule rings of TTCA were thus compressed in steps and anisotropically, which was also observed in the following Raman experiments. Raman spectra are extensively used to obtain local structural information. Our assignments of the Raman modes of TTCA were based on the calculated results and an early related study.35 When the 830-nm laser was used as the exciatation source, most of the Raman peaks exhibited continuous normal blue shifts until ∼10 GPa, except for the modes shown in Figure 3. At 3.2 GPa, new bending modes of δ(CS) and δ(NH) appeared, indicating the beginning of the conformational distortion of the planar TTCA molecules. During further compression, the molecules became further distorted, as evidenced by the emergence of some additional δ(CS) and δ(NH) modes at 6.7 and 7.3 GPa, respectively.23 The Raman peaks marked by the asterisks in Figure 3a shifted from the lower-frequency region. In addition, the out-of-plane ring vibration modes, δ(ring), exhibited continuous red shifts with increasing pressure (Figure 3b), which is ascribed to the enhanced attractive interactions between the molecular layers during their gradual approach.16 It is worth noting that the 1872

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Figure 4. (a) TTCA structure under ambient conditions. (b) Calculated structure at 10 GPa. Bottom: Three adjacent layers (marked by different colors) viewed perpendicular to the plane of the layers.

caused the inevitable approach and relative slippage of adjacent layers. Along with the slippage, the corresponding S atoms could no longer simply be located above the ring centers. The stacked configuration of the TTCA rings was destroyed, causing the weakening of the attractive interactions between the S atoms and the corresponding rings. One can see that pressure could be used to successfully tune both the interactions and the relative arragements between the TTCA molecules, transferring the construction into a puckered structure with a new stacking arrangement. On the basis of the combined TTCA experimental and calculated results, we infer the pressure-tuning mechanism of TTCA as follows: External force leads to the contraction of both the interlayer and intralayer constructions. For the interlayer structure, the interlayer space is compressed rapidly by high pressure owing to the relatively weak interactions. Below ∼3.2 GPa, to balance the increased Gibbs free energy, the TTCA layers undergo a gradual slippage to remain stable. In addition, during this slippage, within the one-layer sixmolecule rings, the slippage directions of the A and F rings, as well as the C and D rings, are exactly opposite (Figures 2 and 4), leading to the weakening of the interactions betwen them. Therefore, sides AF and CD of the one-layer six-molecule rings are more easily compressed up to ∼3.2 GPa. With further compression, layer slippage alone can no longer handle the increased energy, so the molecular conformation begins to be distorted at ∼3.2 GPa. Along with the slippage, the S atoms deviate dramatically from the ring centers, giving rise to much weakened attractive interactions between the S atoms and the rings (Figure 4b). This directly results in the departure of the S atoms from the molecular plane, forming distorted TTCA molecules and puckered layer structures. At the same time, this distortion also influences the compression of the intralayer structures. Specifically, within one six-molecule ring, sides AB and ED becomes more easily compressed than original sides AF and CD. In addition to the molecular distortion, both the structural nature (the irregular hexagonal geometry of the sixmolecule rings) and interaction properties (directionality of the hydrogen bonds) contribute to the anisotropic contraction of the intralayer structure. Moreover, for the neighboring TTCA

Figure 3. (a,b) Selected Raman modes of TTCA. (c) Peak positions of N−H bending modes of δ1 at high pressure. The inset is the detailed graphical representation of δ1. The vibration amplitudes are indicated by the lengths of the arrows.

pressure dependence of the Raman shift of one N−H bending mode δ1 also showed an obvious discontinuity at 3.2 GPa (Figure 3c). The calculations showed that δ1 mainly involved the N−H bending vibrations of sides AF and CD in the sixmolecule rings. Therefore, combining the results of both Raman and IR experiments, it can be inferred that the sixmolecule rings within one layer undergo a stepwise and anisotropic compression at high pressure. Below ∼3.2 GPa, pressure mainly caused the shrinkage of sides AF and CD. However, with further compression, sides AB and ED were compressed more dramatically. Only sides BC and EF exhibited continuous contractions all the way to ∼10 GPa. This unique phenomenon might result from the structural nature of TTCA, as the six-molecule rings are not regular hexagons. In addition, the directionality of the hydrogen bonds also contributed to the anisotropic compression. For layered molecular materials, it is rarely possible to directly capture the detailed stepwise contraction of intralayer structures in high-pressure experiments. To gain deeper insight into the structural tuning, theoretical calculations were also performed (Figure 4a,b). Within one sheet, the TTCA molecules were distorted at high pressure, leading to wrinkling of the layers. This was found to be coincident with the complex behaviors of δ(CS) and δ(NH) in the Raman experiments. However, the hydrogen-bonded networks were not rearranged at high pressure, as evidenced by the unchanged profile of the NH stretching modes in the IR spectra. For interlayer structure, it is evident that pressure 1873

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further design and synthesis of pressure sensors/switches, as well as luminescence materials.

layers, both the stacking method and interactions are modulated by high pressure. It is indisputable that pressure provides an effective means of tuning the molecular structure of TTCA. Yet, we still need to answer the question of how the microscopic structure influences the macroscopic properties. In our Raman experiments, when the 514.5-nm laser was used as the excitation source (Figure S8), the fluorescence background of TTCA increased dramatically above ∼3.7 GPa and gradually lost its intensity above ∼8.0 GPa. Similarly, in high-pressure PL measurements, although the detection of the fluorescence signal was influenced by the diamond emission signal, we also clearly observed a new emmission peak in the pressure range 5.4−9.3 GPa (Figure 5). The piezoluminescence appeared at



CONCLUSIONS In summary, high pressure has been used successfully to bring about a new molecular arrangement and piezoluminescence phenomenon of TTCA. Within one layer, accompanied by the distortion of the molecules, the anisotropic and stepwise compression is directly captured within the six-molecule rings. Meanwhile, between the adjacent layers, the TTCA molecular sheets exhibit relative slippage, tuning both the stacking method and the interactions between molecules. Furthermore, the calculated results show that the shifting of valence-band top greatly contributes to the piezoluminescence phenomenon of TTCA. High-pressure investigations on TTCA are beneficial for providing a deeper insight into the structure−property relationships of molecular crystals, as well as offering new strategies for fabricating functionalized architectures.



ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.jpcc.6b11435. Calculated structural and band-gap information, highpressure lattice constants, and ADXRD and PL data (PDF)



AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. ORCID

Bo Zou: 0000-0002-3215-1255 Author Contributions ∥

Q.L. and S.L. contributed equally to this work.

Notes

The authors declare no competing financial interest.



Figure 5. Selected high-pressure photoluminescence spectra of TTCA.

ACKNOWLEDGMENTS This work was supported by the National Science Foundation of China (NSFC) (Nos. 91227202, 21673100, 11604141), the Shenzhen Fundamental Research Programs (No. JCYJ20160530190717385), the Changbai Mountain Scholars Program (No. 2013007), and the Program for Innovative Research Team (in Science and Technology) in University of Jilin Province. High-pressure experiments were performed at HPCAT’s beamline facility (Sector 16) of the Advanced Photon Source at Argonne National Laboratory. HPCAT is supported by the Carnegie Institution of Washington, the Carnegie/DOE Alliance Center, UNLV, and LLNL through funding from DOE-BES, DOE-NNSA, and NSF. APS is supported by DOE-BES (Grant DE-AC02-06CH11357). The support provided by the China Scholarship Council (CSC) during a visit of Q.L. to HPCAT is also acknowledged.

∼3 GPa, which was just the pressure point at which the TTCA moleculars started to be distorted. This indicates that the molecular distortion and stacking destruction have a strong impact on the electronic structure of TTCA, presenting the new PL properties at high pressure.36 Molecular conformations have a significant influence on band gaps, which directly determine the PL properties of molecular crystals.37 Based on this point, calculations of the TTCA band structure were performed to explain the pressure-induced fluorencence. As illustrated in Figures S9 and S10, the band gap of TTCA decreased continuously during the whole compression. When the pressure was increased to 3 GPa, the band gap changed from near F−Γ type to near Γ−Γ type, showing the variation trend from indirect to direct. Such shifting of the valence-band top from the F point to the Γ point is beneficial for the electron transition, as evidenced by the appearance and enhancement of the new emission peak in this pressure range.38 Then, at 10 GPa, the valence-band top moved in the opposite direction, back toward the F point, which is in agreement with the gradual disappearance of the emission peak above ∼7 GPa. Our calculations show that the shifting of the valence-band top from the F point to the Γ point greatly contributes to the piezoluminescence phenomenon of TTCA. These pressureinduced new PL properties of TTCA might be beneficial for the



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