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High pressure study of acetophenone azine
Solid State Communications 149 (2009) 301–306
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Solid State Communications journal homepage: www.elsevier.com/locate/ssc
High pressure study of acetophenone azine X.D. Tang, Z.J. Ding ∗ , Z.M. Zhang Hefei National Laboratory for Physical Sciences at Microscale and Department of Physics, University of Science and Technology of China, Hefei, 230026, Anhui, People’s Republic of China
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info
Article history: Received 4 July 2008 Accepted 29 November 2008 by R. Merlin Available online 6 December 2008 PACS: 62.50.+p 61.50.Ks 68.35.Rh Keywords: A. Acetophenone azine C. Phase transformation E. High pressure E. Raman
a b s t r a c t High pressure Raman spectra of acetophenone azine (APA) have been measured up to 17.7 GPa with a diamond anvil cell. Two crystalline-to-crystalline phase transformations are found at pressures about 3.6 and 5.8 GPa. A disappearance of external modes and the C–H vibration at pressures higher than 8.7 GPa suggests that the sample undergoes a phase transition to amorphous or orientationally disordered (plastic) state, and the amorphization was completed at about 12.1 GPa. The disordered state is unstable and, then, a polymerization transformation reaction occurs with a further pressure increase. After the pressure has been released, the polymerization state can remain at the ambient condition, indicating that the virgin crystalline state is not recovered. The results show that the phenomenon underlying the pressure induced phase transition of APA may involve profound changes in the coordination environments of the symmetric aromatic azine. © 2008 Elsevier Ltd. All rights reserved.
1. Introduction A common feature of the molecular structure of symmetric aromatic azines is the presence of two strong conjugated carbon–nitrogen double bonds and some of these compounds have already been studied by vibrational spectroscopy [1,2]. It is known that many physical and chemical properties of organic compounds can be greatly altered by high pressure in a way to undergo structural phase transitions or chemical reactions. Pressure conditions can also accelerate the polymerization of unsaturated molecules. Thus, organic polymer chemistry at high pressures [3], including applications to electric conducting polymers and high strength materials [4,5], is of considerable interest. Chemical reactions at high pressure have been observed for many types of unsaturated chemical bonds including those in alkenes, alkynes, nitriles, carbonyls and carbon sulfides. Indeed all unsaturated molecules are expected to be unstable in high pressure conditions with respect to associative, cross-linking reactions which form denser, more saturated species [6]. Azines, with the crisscross addition [7] which is very important in that closely related azaderivatives react in a Diels-Alder fashion [8], have attracted many researchers to study its properties in stereochemistry, stereoelectronics and crystal packing [9,10].
The properties of a material depend not only on its chemical formula but also on its molecular conformation and supramolecular arrangement [11]. High pressure Raman scattering is an efficient and relatively easy technique to probe structural changes, phase transitions and inter-chain interactions in organic molecular solids. Particularly, it plays an important role in the study of the mechanisms of pressure induced crystalline-to-crystalline and crystalline-to-amorphous phase transitions. High pressure fluorescence spectra [12,13] is a tool to study the changes of molecular orbital energy level; it may also be used to indicate evidence for the mechanism of pressure-induced phase transitions, characterize the state of molecular electronic energy level and validate the related theory of electrons and/or molecules interactions. In this work, we have studied Raman spectra and fluorescence spectra of acetophenone azine (APA) to investigate phase transition and vibrational property under high pressure. The observed Raman frequencies are discussed in relation to carbon–nitrogen double bonds. Raman measurements reveal that APA crystals undergo two crystalline-to-crystalline phase transitions at about 3.6 and 5.8 GPa. Pressure-induced crystalline-to-amorphous phase transition starts at approximately 8.7 GPa, being completed by 12.1 GPa and irreversible above 17.7 GPa. It is suggested that APA may transform to a supra-molecular structure or be polymerized under high pressure. 2. Experiment
∗
Corresponding author. E-mail address: [email protected] (Z.J. Ding).
0038-1098/$ – see front matter © 2008 Elsevier Ltd. All rights reserved. doi:10.1016/j.ssc.2008.11.034
APA crystals were synthesized from acetophenone and hydrated hydrazine according to Ref. [14]. Crystals of APA were further purified from a solution of APA in a mixture of chloroform and
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X.D. Tang et al. / Solid State Communications 149 (2009) 301–306
3. Results and discussion 3.1. High pressure Raman spectra
Fig. 1. The molecular structure of APA.
ethyl alcohol by slow evaporation of the solvent at room temperature. Samples were loaded into a diamond anvil cell (DAC) for high pressure Raman spectra measurement. A hardened stainless-steel gasket was pre-indented by the diamonds to an initial thickness of about 25 µm and then drilled to produce a 0.18 mm diameter cavity as the sample chamber by electrical spark erosion (MH20M). Small ruby chips were placed in the sample chamber with the samples in order to allow in situ pressure measurement using the standard ruby fluorescence technique. Pressure-transmitting media was not used because APA is soluble in most organic solvents. Pressure acting on the sample was determined by the wavelength shift of the ruby R1 fluorescence line [15]. By checking the line widths and the separation of the R1 and R2 lines, we have confirmed that a quasi-hydrostatic condition was satisfied in the pressure ranges studied. Raman spectra and fluorescence spectra were recorded by an integrated laser Raman system (LABRAM HR, Jobin Yvon) with a confocal microscope, stigmatic spectrometer and a multichannel air cooled CCD detector with a typical resolution of 1 cm−1 in the measured frequency region. An argon ion laser operating at a line of 514.5 nm and at powers up to 10 mW was used as the exciting source. All spectra were measured in the backscattering geometry for the laser beam focused on the area of 5 × 5 µm at the room temperature. The accuracy of the frequency calibration was ±1 cm−1 established by the peak position of the silicon thin film Raman line (520 cm−1 ) as an internal standard. The Raman spectra were collected in the region of 100–4000 cm−1 . All spectra data were recorded at a small pressure step with a time of 30–40 min for establishing the equilibrium after a pressure increment. Wave number reproducibility is estimated to be ±0.5 cm−1 and the pressure estimation accuracy is ±0.2 GPa. No evidence of significant pressure gradients in the sample was found, as indicated by the width of ruby fluorescent peaks. However appreciable hysteresis effects were observed for spectra obtained after reducing the pressure.
Fig. 1 shows the molecular structure of acetophenone azine (APA). The APA molecule has a trans configuration involving two C=N bonds and two benzene rings. The lattice of APA single crystal is monoclinic with four molecules in the unit cell. The APA single crystal belongs to the space group p21/n, with a factor group isomorphic to the C2h point group, a = 1.48744(8) nm, b = 0.75556(4) nm, c = 1.17097(4) nm, β = 97.7575(20)◦ and V = 1.30397(12) nm3 . Molecules are grouped in a zigzag way in space with the planes of the benzene rings in alternate layers (nearest) being inclined to one another [14]. The color of the sample is yellow in ambient conditions. The Raman and infrared spectra of APA crystal in ambient conditions are shown in Fig. 2. According to Ref. [1,16], the N–N and symmetric C=N vibrations can be assigned to the Raman bands at 1039 and 1560 cm−1 , respectively. The N–N vibration can only be observed in the Raman spectra and are forbidden in the infrared spectra. The band in the region of 2900–3100 cm−1 belongs to CH vibrations. In the present work Raman spectra of the APA were recorded at pressures up to 17.7 GPa. Fig. 3 shows several Raman spectra at high pressures. The details of these Raman spectra in the region of 100–3180 cm−1 at ambient pressure and as a function of increasing pressure up to 8.7 GPa are shown in Fig. 4. It is well established that the intramolecular modes are in the high-frequency region of the Raman spectra of molecular crystals while the external phonon modes, representing essentially the translations and the rotations of rigid molecules, are in the lowfrequency region. For organic molecules bounded by weak van der Waals interactions the external modes and internal modes are well separated from each other and located in different regions of wave number. In Fig. 4(a) the external modes occupy up to 230 cm−1 , however, the Raman intensity was unfortunately quite weak to be observed below 100 cm−1 due to the influence of the filter used to screen the Rayleigh scattering line of the exciting laser beam (514 nm). At high pressures all the external modes move more quickly than the internal modes towards higher wave numbers. This is because during compression the decrease of the inter-molecular distance is more than that of the intra-molecular one, resulting in an increase in the effective force constants more than the latter. In the lattice modes region (see Fig. 4(a)), with pressure increasing from 1 bar to 3.2 GPa the intensity of the Raman
Fig. 2. (a) Infrared and (b) Raman spectra of APA crystal at ambient condition.
X.D. Tang et al. / Solid State Communications 149 (2009) 301–306
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Fig. 3. Raman spectra of APA crystal under high pressures in the ranges of 80–1250 cm−1 .
bands in the region of 130–180 cm−1 increased gradually. No perceptible new mode was observed in this compressing process. At pressures of about 3.6 GPa and 5.8 GPa new Raman modes at 132 cm−1 and 147 cm−1 appeared, respectively. All the modes in the lattice region were broadened and weakened, and vanished at pressures above about 8.7 GPa. Another two new modes at 515 and 708 cm−1 become remarkable at pressures above 6 GPa (see Fig. 4(b)). In fact, there was a slight trace of the mode 515 cm−1 appearing at pressures of about 5.8 GPa. Taking into account the rising of the fluorescence background which began to raise at pressures of about 5.8 GPa the 708 cm−1 Raman bands may actually appear at even lower pressures. The above two bands are even stronger with pressures increasing up to about 10 GPa and then gradually become weaker and vanish until pressures are up to 17.7 GPa (see Fig. 3, shown by arrows). The splitting of the CH stretching (originally at 3066 cm−1 ) was also observed, but, the CH3 symmetric stretching (originally at 2921 cm−1 ) had no significant change except shifting to higher wave numbers (see Fig. 4(e)). Attention must be paid to the C=N symmetric stretching (originally at 1560 cm−1 ). The C=N symmetric and asymmetric stretching of the o-hydroxy acetophenone azine in the IR spectra were reported to be located at 1605 cm−1 and in the region of 1500–1600 cm−1 , respectively [16]. Around 3.6 GPa, the 1560 cm−1 Raman line was split into two lines (see Fig. 4(d)). The shift rates of the wave number with pressure for the chain modes (N–N and C=N) are considered to be approximately the same. Thus, by comparing the shift rates for the ring modes and for the chain mode (N–N, 1039 cm−1 ), it is reasonable to consider that the line of the higher wave number within the 1560 band is due to C=C stretching of the ring and the lower wave number is C=N symmetric stretching. Since the APA molecules are nearly planar and essentially parallel with the ab-plane, the compressibility along the c-axis is larger than in any other directions under low pressure. That is why the splitting of the 1560 cm−1 band cannot be observed at lower pressures, below 3.6 GPa. The shift rate for the C=C stretching of the ring is quicker than that of C=N mode with pressure increasing over 3.6 GPa. It means that the force on the a-axis direction is lower than on b- and c-axes above 3.6 GPa. The evolution of the N–N mode (originally at 1039 cm−1 ), that is important in the azine bridge, is shown in Fig. 4(c). With increasing pressure, the intensity of this mode first increased and then decreased. The intensity of the C=N symmetric mode evolved in a similar way, too.
Fig. 4. Raman spectra of APA crystal at various pressures up to 8.74 GPa in the wave number region of: (a) 80–300 cm−1 ; (b) 400–750 cm−1 ; (c) 900–1150 cm−1 ; (d) 1420–1650 cm−1 , (e) 2900–3200 cm−1 .
3.2. Pressure induced phase transition Fig. 5 shows the Raman intensity ratios for bands between 1039 cm−1 (N–N vibration) and 1025 cm−1 (CH(ring), in plane bend), and 1560 cm−1 (ring) and ν (C=N) as a function of pressure.
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to determine the phase transformation pressure accurately in this work due to the rapid increase of the fluorescence background at such high pressures. 3.3. High pressure fluorescence spectra
Fig. 5. Pressure dependence of Raman intensity ratio between the Raman bands: (a) 1560 cm−1 (ring) and ν1560 (C = N); (b) 1039 and 1025 cm−1 .
The inflexions of the tendencies are found at 3.6 and 5.8 GPa. This behavior is quite suggestive to imply that the change of the signal intensity ratio should be simply associated with modification of the molecular configuration, while maintaining the crystal with the same monoclinic symmetry as that at the ambient pressure condition. Another possibility is due to the pressure induced phase transitions at about 3.6, 5.8 and 8.7 GPa. The similar behavior of Raman band intensities were also observed for the pressure induced phase transition of L-alanine crystal [17]. It is worth mentioning that, in this work, the behavior was found only in such two bands that their Raman frequencies were close to each other. Fig. 6 plots the pressure dependence of wave number for Raman bands. The figure thus shows two possible crystallineto-crystalline phase transitions at about 3.6 and 5.8 GPa by the abrupt slope change and termination of lines. The line slope for CH stretching modes (in the region of 2900–3100 cm−1 ) has an abrupt change at 5.8 GPa. At pressures higher than 8.7 GPa it is suggested that the sample undergoes a crystalline-to-amorphous phase transition. The fact that external modes and CH stretching lines were weakened and broadened with increasing pressure, and vanished around 8.7 GPa suggests that the long-range order of the periodicity of the crystal has broken down. Additional evidence for this phase transition is that the 1560 cm−1 (ring) Raman line disappeared at this pressure. To clear these processes, the width of the 1560 cm−1 (including ring C=C and C=N symmetric vibration) peak was analyzed which has significantly changed with increasing pressure. Fig. 7 shows the inverse of the width of this peak at high pressure 1ωp with respect to that at ambient pressure 1ω0 . A slow change in 1ω0 /1ωp of 1560 cm−1 line (ring) goes up to about 4 GPa, followed by a rapid decrease to about 6 GPa. The rapid decrease in 1ω0 /1ωp of the ν (C=N) occurs from 4 GPa to about 6 GPa, and then the change becomes slow up to 9 GPa, followed by another rapid decrease up to 12 GPa. The increase of peak width above 12 GPa means that the force within APA molecules decreases. The decrease of the force does not result from external condition changes but from an internal mechanism, e.g. chemical reaction induced negative volume effect. For comparison, the one of 1494 cm−1 (ring 19a) mode is also shown in Fig. 7. It follows from Figs. 6 and 7 that APA starts to amorphize around 8.7 GPa, at which all of the lattice modes and some of the intramolecular modes have disappeared, and that the amorphization is completed at pressure about 12 GPa. Pressure induced amorphization has also been observed in other organic molecular crystals such as benzophenone [18] at 11 GPa and hexamethylenetetramine [19] at 15 GPa. It was, however, hard
The variation of the fluorescence spectrum as a function of pressure is shown in Fig. 8. The sharp and strong line at 552 nm is the diamond Raman line, and the strong peaks around 694.3 nm are ruby peaks. The intensity of the fluorescence spectra from 5.8 to 17.7 GPa was normalized by the intensity of the diamond Raman line. Another spectrum was measured in ambient conditions a week later without the diamond anvil being covered. An increase of the intensity of fluorescence occurs with pressures from 5.8 to 12.1 GPa with the position of the spectrum maximum shifted to shorter wave lengths, followed by a more rapid increase to about 15.6 GPa. Above this pressure, the fluorescence intensity becomes lower. As the pressure is released to atmospheric pressure the character of extensive fluorescence remains, moreover, the peak feature and fluorescence intensity still existed one week later after the pressure had been released. We now consider the origin of the fluorescence of APA. At ambient pressure there was no observable fluorescence from the sample excited by a 514 nm laser under the same experimental conditions as that under pressure. This feature is quite analogous to benzal compounds [18,20]. It is well known that aromatic carbonyl compounds, including benzaldehyde, generally show strong phosphorescence but no fluorescence [21,22]. An NMR spectroscopic study has shown that the azine bridge acts as a conjugation stopper [23], this explains why no fluorescence of the APA monomer can be observed at ambient pressure. Under high pressure, the distances between molecules are shortened and the obits of π -conjugated between two adjacent molecules overlap to form the dimer which results in the occurrence of fluorescence. Taking notice of the difference of the changes in the fluorescence intensity around 12 GPa, the increasing rate before 12 GPa is slower than that after 12 GPa. It indicates that the mechanism for pressure induced fluorescence of APA may change around 12 GPa. Compared with benzophenone [18], its pressure induced dimerization states were unstable when the pressure was released from 11 GPa at which the benzophenone crystal was just at the threshold of an amorphization state. As for pressure induced dimer of APA, the rapid increase of the intensity above 12 GPa implies that a new interaction has occurred. The molecules under such a high pressure condition are unstable; with changes in the environment, either by the irradiation of laser or by increasing pressure, they can be transformed to a supra-molecular structure or be polymerized. The recovered sample after release from high pressure is denser in color than the original one. This fact shows that permanent residual stresses were in the sample after releasing the pressure and the final compound was in a new conjugated state other than that in the original one of APA molecules. Fig. 8 shows that the peak feature and fluorescence intensity still existed one week later after pressure had been released, illustrating that the virgin structure was not recovered and, therefore, the final compound with strong fluorescence is a new structure and more stable in ambient conditions. The small mass of the sample (10−6 g) and the weakness of the Raman signal did not allow us to precisely determine what chemical process occurs above 12 GPa. However, the dark-yellow color of the recovered compounds suggests a higher conjugate polymer. These results are deduced by comparison with the white color for ethylene, the orange colors obtained with carotene (conjugate polyene of 25 carbons), and the black color for the higher conjugate polyene as polyacetylene. The same results were obtained for high pressure induced chemical transformation
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Fig. 6. Pressure dependence of Raman bands of APA crystal.
of tribromobenzene (TBB) [5]. The possible mechanism for the chemical reaction process above 12 GPa is suggested as the opening of the unsaturated bonds of the monomers. The polymerization of unsaturated molecules can be explained by a chain propagation in which the hybridization of the atoms involves covalent bond changes during the reaction, e.g., sp2 to sp3 or sp to sp2 in polymerization [5]. Though the APA molecules are quite stable in ambient conditions, however, under high pressure the intermolecular interaction is comparable with the intramolecular one; the molecules can be unstable. Thus, two possible chemical reaction paths are suggested as follow: (1) Opening of the C=N bond. APA has a resonance form for which the C=N double bonds can be opened. The N atom exists in the molecule with sp2 hybridization form [23]. With increasing pressure the energy needed to break the bond decreases [24] and the broken C=N bond will connect to the other resonance one to form a polymer molecule. (2) Opening of the C=C of the ring. Comparative examples are: dimethyl phenyl silanol (DMPS) under high pressure [25], where the opening of the C=C bonds of the benzene ring was obtained at 15 GPa, and tribromobenzene (TBB), where the opening of the C=C bonds of the benzene ring was obtained at 26 GPa. This difference in pressure of the opening of aromatic ring is due to the departure of the symmetry for π -bonds. Therefore, it is reasonable to consider that the C=C bonds of APA were broken at above 12.1 GPa.
Fig. 7. Pressure dependence of the inverse of half width. Solid triangle: 1560 cm−1 (ring), solid circle: ν1560 (C=N) and solid square: 1494 cm−1 (ring, 19a), respectively.
4. Conclusion The high pressure behavior of APA crystals has been investigated up to 17.7 GPa with Raman spectroscopy and fluorescence spectra measurements. The ambient monoclinic structure is found to undergo two possible crystalline-to-crystalline phase transitions at pressures about 3.6 and 5.8 GPa. The disappearance of the external modes and vanishing of some internal modes suggest that amorphization occurs with onset pressure of 8.7 GPa and final pressure of 12.1 GPa. Above 12.1 GPa, an irreversible polymerization reaction may occur. Upon releasing the pressure, the new state remains stable as evident from its fluorescence spectra.
Fig. 8. The fluorescence spectral evolution of the APA with pressure raised from 5.8 to 17.7 GPa and released to ambient pressure for one week.
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Acknowledgements This work was supported by the National Natural Science Foundation of China (Grant Nos. 10874160, 10574121), Chinese Education Ministry and Chinese Academy of Sciences. References [1] [2] [3] [4] [5] [6] [7] [8] [9] [10]
M.M.A. Aly, Spectrochim. Acta. A 55 (1999) 1711. E.C. Lim, R. Li, Y.H. Li, J. Chem. Phys. 50 (1969) 4925. A.M. Katz, D. Schiferl, R.L. Mills, J. Phys. Chem. 88 (1984) 3176. R.B. Heiman, J. Kleiman, N.M. Salansky, Carbon 22 (1984) 147. F. Cansell, D. Fabre, J.P. Petitet, J.P. Itie, A. Fontaine, J. Phys. Chem. 99 (1995) 13109. C.S. Yoo, M.F. Nicol, J. Phys. Chem. 90 (1986) 6726. R. Grashey, in: A. Padwa (Ed.), Azomethine Imines In 1 3-Dipolar Cycloaddition Chemistry, vol 1, John Wiley & Sons, Inc., New York, 1984, pp. 733–814. L. Ghosez, J. Am. Chem. Soc. 97 (1975) 4409. R. Glaser, G.S. Chen, C.L. Barnes, J. Org. Chem. 58 (1993) 4336. G.S. Chen, J.K. Wilbur, C.L. Barnes, R. Glaser, J. Chem. Soc. Perkin Trans. 2 (1995) 2311.
[11] O. Franco, I. Orgzall, W. Regenstein, B. Schulz, J. Phys. Condens. Matter 18 (2006) 1459. [12] J. Zeman, M. Zigone, G.L.J.A. Rikken, G. Martinez, Thin Solid Films 276 (1996) 47. [13] L.T. Wang, S.H. Wang, X.H. Zhao, J.R. Sun, J. Alloys. Compounds 225 (1995) 174. [14] G.S. Chen, M. Anthamatten, C.L. Barnes, R. Glaser, J. Org. Chem. 59 (1994) 4336. [15] E.B. Wilson, Phys. Rev. 45 (1934) 706. [16] B.A. El-Sayed, M.M. Abo Aly, A.A.A. Emara, S.M.E. Khalil, Vib. Spectrosc. 30 (2002) 93. [17] A.M.R. Teixeira, P.T.C. Freire, A.J.D. Moreno, J.M. Sasaki, A.P. Ayala, J.M. Filho, F.E.A. Melo, Solid State Commun. 116 (2000) 405. [18] D.L. Zhang, G.X. Lan, S.F. Hu, H.F. Wang, J.M. Zheng, J. Phys. Chem. Solids 56 (1995) 27. [19] R. Rao, T. Sakuntala, S.K. Deb, A.P. Roy, V. Vijayakumar, B.K. Godwal, S.K. Sikka, Chem. Phys. Lett. 313 (1999) 749. [20] S.K. Deb, M.A. Rekha, A.P. Roy, V. Vijayakumar, S. Meenakshi, B.K. Godwal, Phys. Rev. B. 47 (1993) 11491. [21] S.K. Lower, M.A. El-Sayed, Chem. Rev. 66 (1966) 199. [22] T. Takemura, H. Baba, Bull. Chem. Soc. Jpn. 42 (1969) 2756. [23] M. Lewis, R. Glaser, J. Org. Chem. 67 (2002) 1441. [24] H. Luo, S. Desgreniers, Y.K. Vohra, A.L. Ruoff, Phys. Rev. Lett. 67 (1991) 2998. [25] F. Cansell, J.P. Petitet, D. Fabre, J. Appl. Phys. 65 (1989) 3280.
Contents lists available at ScienceDirect
Solid State Communications journal homepage: www.elsevier.com/locate/ssc
High pressure study of acetophenone azine X.D. Tang, Z.J. Ding ∗ , Z.M. Zhang Hefei National Laboratory for Physical Sciences at Microscale and Department of Physics, University of Science and Technology of China, Hefei, 230026, Anhui, People’s Republic of China
article
info
Article history: Received 4 July 2008 Accepted 29 November 2008 by R. Merlin Available online 6 December 2008 PACS: 62.50.+p 61.50.Ks 68.35.Rh Keywords: A. Acetophenone azine C. Phase transformation E. High pressure E. Raman
a b s t r a c t High pressure Raman spectra of acetophenone azine (APA) have been measured up to 17.7 GPa with a diamond anvil cell. Two crystalline-to-crystalline phase transformations are found at pressures about 3.6 and 5.8 GPa. A disappearance of external modes and the C–H vibration at pressures higher than 8.7 GPa suggests that the sample undergoes a phase transition to amorphous or orientationally disordered (plastic) state, and the amorphization was completed at about 12.1 GPa. The disordered state is unstable and, then, a polymerization transformation reaction occurs with a further pressure increase. After the pressure has been released, the polymerization state can remain at the ambient condition, indicating that the virgin crystalline state is not recovered. The results show that the phenomenon underlying the pressure induced phase transition of APA may involve profound changes in the coordination environments of the symmetric aromatic azine. © 2008 Elsevier Ltd. All rights reserved.
1. Introduction A common feature of the molecular structure of symmetric aromatic azines is the presence of two strong conjugated carbon–nitrogen double bonds and some of these compounds have already been studied by vibrational spectroscopy [1,2]. It is known that many physical and chemical properties of organic compounds can be greatly altered by high pressure in a way to undergo structural phase transitions or chemical reactions. Pressure conditions can also accelerate the polymerization of unsaturated molecules. Thus, organic polymer chemistry at high pressures [3], including applications to electric conducting polymers and high strength materials [4,5], is of considerable interest. Chemical reactions at high pressure have been observed for many types of unsaturated chemical bonds including those in alkenes, alkynes, nitriles, carbonyls and carbon sulfides. Indeed all unsaturated molecules are expected to be unstable in high pressure conditions with respect to associative, cross-linking reactions which form denser, more saturated species [6]. Azines, with the crisscross addition [7] which is very important in that closely related azaderivatives react in a Diels-Alder fashion [8], have attracted many researchers to study its properties in stereochemistry, stereoelectronics and crystal packing [9,10].
The properties of a material depend not only on its chemical formula but also on its molecular conformation and supramolecular arrangement [11]. High pressure Raman scattering is an efficient and relatively easy technique to probe structural changes, phase transitions and inter-chain interactions in organic molecular solids. Particularly, it plays an important role in the study of the mechanisms of pressure induced crystalline-to-crystalline and crystalline-to-amorphous phase transitions. High pressure fluorescence spectra [12,13] is a tool to study the changes of molecular orbital energy level; it may also be used to indicate evidence for the mechanism of pressure-induced phase transitions, characterize the state of molecular electronic energy level and validate the related theory of electrons and/or molecules interactions. In this work, we have studied Raman spectra and fluorescence spectra of acetophenone azine (APA) to investigate phase transition and vibrational property under high pressure. The observed Raman frequencies are discussed in relation to carbon–nitrogen double bonds. Raman measurements reveal that APA crystals undergo two crystalline-to-crystalline phase transitions at about 3.6 and 5.8 GPa. Pressure-induced crystalline-to-amorphous phase transition starts at approximately 8.7 GPa, being completed by 12.1 GPa and irreversible above 17.7 GPa. It is suggested that APA may transform to a supra-molecular structure or be polymerized under high pressure. 2. Experiment
∗
Corresponding author. E-mail address: [email protected] (Z.J. Ding).
0038-1098/$ – see front matter © 2008 Elsevier Ltd. All rights reserved. doi:10.1016/j.ssc.2008.11.034
APA crystals were synthesized from acetophenone and hydrated hydrazine according to Ref. [14]. Crystals of APA were further purified from a solution of APA in a mixture of chloroform and
302
X.D. Tang et al. / Solid State Communications 149 (2009) 301–306
3. Results and discussion 3.1. High pressure Raman spectra
Fig. 1. The molecular structure of APA.
ethyl alcohol by slow evaporation of the solvent at room temperature. Samples were loaded into a diamond anvil cell (DAC) for high pressure Raman spectra measurement. A hardened stainless-steel gasket was pre-indented by the diamonds to an initial thickness of about 25 µm and then drilled to produce a 0.18 mm diameter cavity as the sample chamber by electrical spark erosion (MH20M). Small ruby chips were placed in the sample chamber with the samples in order to allow in situ pressure measurement using the standard ruby fluorescence technique. Pressure-transmitting media was not used because APA is soluble in most organic solvents. Pressure acting on the sample was determined by the wavelength shift of the ruby R1 fluorescence line [15]. By checking the line widths and the separation of the R1 and R2 lines, we have confirmed that a quasi-hydrostatic condition was satisfied in the pressure ranges studied. Raman spectra and fluorescence spectra were recorded by an integrated laser Raman system (LABRAM HR, Jobin Yvon) with a confocal microscope, stigmatic spectrometer and a multichannel air cooled CCD detector with a typical resolution of 1 cm−1 in the measured frequency region. An argon ion laser operating at a line of 514.5 nm and at powers up to 10 mW was used as the exciting source. All spectra were measured in the backscattering geometry for the laser beam focused on the area of 5 × 5 µm at the room temperature. The accuracy of the frequency calibration was ±1 cm−1 established by the peak position of the silicon thin film Raman line (520 cm−1 ) as an internal standard. The Raman spectra were collected in the region of 100–4000 cm−1 . All spectra data were recorded at a small pressure step with a time of 30–40 min for establishing the equilibrium after a pressure increment. Wave number reproducibility is estimated to be ±0.5 cm−1 and the pressure estimation accuracy is ±0.2 GPa. No evidence of significant pressure gradients in the sample was found, as indicated by the width of ruby fluorescent peaks. However appreciable hysteresis effects were observed for spectra obtained after reducing the pressure.
Fig. 1 shows the molecular structure of acetophenone azine (APA). The APA molecule has a trans configuration involving two C=N bonds and two benzene rings. The lattice of APA single crystal is monoclinic with four molecules in the unit cell. The APA single crystal belongs to the space group p21/n, with a factor group isomorphic to the C2h point group, a = 1.48744(8) nm, b = 0.75556(4) nm, c = 1.17097(4) nm, β = 97.7575(20)◦ and V = 1.30397(12) nm3 . Molecules are grouped in a zigzag way in space with the planes of the benzene rings in alternate layers (nearest) being inclined to one another [14]. The color of the sample is yellow in ambient conditions. The Raman and infrared spectra of APA crystal in ambient conditions are shown in Fig. 2. According to Ref. [1,16], the N–N and symmetric C=N vibrations can be assigned to the Raman bands at 1039 and 1560 cm−1 , respectively. The N–N vibration can only be observed in the Raman spectra and are forbidden in the infrared spectra. The band in the region of 2900–3100 cm−1 belongs to CH vibrations. In the present work Raman spectra of the APA were recorded at pressures up to 17.7 GPa. Fig. 3 shows several Raman spectra at high pressures. The details of these Raman spectra in the region of 100–3180 cm−1 at ambient pressure and as a function of increasing pressure up to 8.7 GPa are shown in Fig. 4. It is well established that the intramolecular modes are in the high-frequency region of the Raman spectra of molecular crystals while the external phonon modes, representing essentially the translations and the rotations of rigid molecules, are in the lowfrequency region. For organic molecules bounded by weak van der Waals interactions the external modes and internal modes are well separated from each other and located in different regions of wave number. In Fig. 4(a) the external modes occupy up to 230 cm−1 , however, the Raman intensity was unfortunately quite weak to be observed below 100 cm−1 due to the influence of the filter used to screen the Rayleigh scattering line of the exciting laser beam (514 nm). At high pressures all the external modes move more quickly than the internal modes towards higher wave numbers. This is because during compression the decrease of the inter-molecular distance is more than that of the intra-molecular one, resulting in an increase in the effective force constants more than the latter. In the lattice modes region (see Fig. 4(a)), with pressure increasing from 1 bar to 3.2 GPa the intensity of the Raman
Fig. 2. (a) Infrared and (b) Raman spectra of APA crystal at ambient condition.
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Fig. 3. Raman spectra of APA crystal under high pressures in the ranges of 80–1250 cm−1 .
bands in the region of 130–180 cm−1 increased gradually. No perceptible new mode was observed in this compressing process. At pressures of about 3.6 GPa and 5.8 GPa new Raman modes at 132 cm−1 and 147 cm−1 appeared, respectively. All the modes in the lattice region were broadened and weakened, and vanished at pressures above about 8.7 GPa. Another two new modes at 515 and 708 cm−1 become remarkable at pressures above 6 GPa (see Fig. 4(b)). In fact, there was a slight trace of the mode 515 cm−1 appearing at pressures of about 5.8 GPa. Taking into account the rising of the fluorescence background which began to raise at pressures of about 5.8 GPa the 708 cm−1 Raman bands may actually appear at even lower pressures. The above two bands are even stronger with pressures increasing up to about 10 GPa and then gradually become weaker and vanish until pressures are up to 17.7 GPa (see Fig. 3, shown by arrows). The splitting of the CH stretching (originally at 3066 cm−1 ) was also observed, but, the CH3 symmetric stretching (originally at 2921 cm−1 ) had no significant change except shifting to higher wave numbers (see Fig. 4(e)). Attention must be paid to the C=N symmetric stretching (originally at 1560 cm−1 ). The C=N symmetric and asymmetric stretching of the o-hydroxy acetophenone azine in the IR spectra were reported to be located at 1605 cm−1 and in the region of 1500–1600 cm−1 , respectively [16]. Around 3.6 GPa, the 1560 cm−1 Raman line was split into two lines (see Fig. 4(d)). The shift rates of the wave number with pressure for the chain modes (N–N and C=N) are considered to be approximately the same. Thus, by comparing the shift rates for the ring modes and for the chain mode (N–N, 1039 cm−1 ), it is reasonable to consider that the line of the higher wave number within the 1560 band is due to C=C stretching of the ring and the lower wave number is C=N symmetric stretching. Since the APA molecules are nearly planar and essentially parallel with the ab-plane, the compressibility along the c-axis is larger than in any other directions under low pressure. That is why the splitting of the 1560 cm−1 band cannot be observed at lower pressures, below 3.6 GPa. The shift rate for the C=C stretching of the ring is quicker than that of C=N mode with pressure increasing over 3.6 GPa. It means that the force on the a-axis direction is lower than on b- and c-axes above 3.6 GPa. The evolution of the N–N mode (originally at 1039 cm−1 ), that is important in the azine bridge, is shown in Fig. 4(c). With increasing pressure, the intensity of this mode first increased and then decreased. The intensity of the C=N symmetric mode evolved in a similar way, too.
Fig. 4. Raman spectra of APA crystal at various pressures up to 8.74 GPa in the wave number region of: (a) 80–300 cm−1 ; (b) 400–750 cm−1 ; (c) 900–1150 cm−1 ; (d) 1420–1650 cm−1 , (e) 2900–3200 cm−1 .
3.2. Pressure induced phase transition Fig. 5 shows the Raman intensity ratios for bands between 1039 cm−1 (N–N vibration) and 1025 cm−1 (CH(ring), in plane bend), and 1560 cm−1 (ring) and ν (C=N) as a function of pressure.
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to determine the phase transformation pressure accurately in this work due to the rapid increase of the fluorescence background at such high pressures. 3.3. High pressure fluorescence spectra
Fig. 5. Pressure dependence of Raman intensity ratio between the Raman bands: (a) 1560 cm−1 (ring) and ν1560 (C = N); (b) 1039 and 1025 cm−1 .
The inflexions of the tendencies are found at 3.6 and 5.8 GPa. This behavior is quite suggestive to imply that the change of the signal intensity ratio should be simply associated with modification of the molecular configuration, while maintaining the crystal with the same monoclinic symmetry as that at the ambient pressure condition. Another possibility is due to the pressure induced phase transitions at about 3.6, 5.8 and 8.7 GPa. The similar behavior of Raman band intensities were also observed for the pressure induced phase transition of L-alanine crystal [17]. It is worth mentioning that, in this work, the behavior was found only in such two bands that their Raman frequencies were close to each other. Fig. 6 plots the pressure dependence of wave number for Raman bands. The figure thus shows two possible crystallineto-crystalline phase transitions at about 3.6 and 5.8 GPa by the abrupt slope change and termination of lines. The line slope for CH stretching modes (in the region of 2900–3100 cm−1 ) has an abrupt change at 5.8 GPa. At pressures higher than 8.7 GPa it is suggested that the sample undergoes a crystalline-to-amorphous phase transition. The fact that external modes and CH stretching lines were weakened and broadened with increasing pressure, and vanished around 8.7 GPa suggests that the long-range order of the periodicity of the crystal has broken down. Additional evidence for this phase transition is that the 1560 cm−1 (ring) Raman line disappeared at this pressure. To clear these processes, the width of the 1560 cm−1 (including ring C=C and C=N symmetric vibration) peak was analyzed which has significantly changed with increasing pressure. Fig. 7 shows the inverse of the width of this peak at high pressure 1ωp with respect to that at ambient pressure 1ω0 . A slow change in 1ω0 /1ωp of 1560 cm−1 line (ring) goes up to about 4 GPa, followed by a rapid decrease to about 6 GPa. The rapid decrease in 1ω0 /1ωp of the ν (C=N) occurs from 4 GPa to about 6 GPa, and then the change becomes slow up to 9 GPa, followed by another rapid decrease up to 12 GPa. The increase of peak width above 12 GPa means that the force within APA molecules decreases. The decrease of the force does not result from external condition changes but from an internal mechanism, e.g. chemical reaction induced negative volume effect. For comparison, the one of 1494 cm−1 (ring 19a) mode is also shown in Fig. 7. It follows from Figs. 6 and 7 that APA starts to amorphize around 8.7 GPa, at which all of the lattice modes and some of the intramolecular modes have disappeared, and that the amorphization is completed at pressure about 12 GPa. Pressure induced amorphization has also been observed in other organic molecular crystals such as benzophenone [18] at 11 GPa and hexamethylenetetramine [19] at 15 GPa. It was, however, hard
The variation of the fluorescence spectrum as a function of pressure is shown in Fig. 8. The sharp and strong line at 552 nm is the diamond Raman line, and the strong peaks around 694.3 nm are ruby peaks. The intensity of the fluorescence spectra from 5.8 to 17.7 GPa was normalized by the intensity of the diamond Raman line. Another spectrum was measured in ambient conditions a week later without the diamond anvil being covered. An increase of the intensity of fluorescence occurs with pressures from 5.8 to 12.1 GPa with the position of the spectrum maximum shifted to shorter wave lengths, followed by a more rapid increase to about 15.6 GPa. Above this pressure, the fluorescence intensity becomes lower. As the pressure is released to atmospheric pressure the character of extensive fluorescence remains, moreover, the peak feature and fluorescence intensity still existed one week later after the pressure had been released. We now consider the origin of the fluorescence of APA. At ambient pressure there was no observable fluorescence from the sample excited by a 514 nm laser under the same experimental conditions as that under pressure. This feature is quite analogous to benzal compounds [18,20]. It is well known that aromatic carbonyl compounds, including benzaldehyde, generally show strong phosphorescence but no fluorescence [21,22]. An NMR spectroscopic study has shown that the azine bridge acts as a conjugation stopper [23], this explains why no fluorescence of the APA monomer can be observed at ambient pressure. Under high pressure, the distances between molecules are shortened and the obits of π -conjugated between two adjacent molecules overlap to form the dimer which results in the occurrence of fluorescence. Taking notice of the difference of the changes in the fluorescence intensity around 12 GPa, the increasing rate before 12 GPa is slower than that after 12 GPa. It indicates that the mechanism for pressure induced fluorescence of APA may change around 12 GPa. Compared with benzophenone [18], its pressure induced dimerization states were unstable when the pressure was released from 11 GPa at which the benzophenone crystal was just at the threshold of an amorphization state. As for pressure induced dimer of APA, the rapid increase of the intensity above 12 GPa implies that a new interaction has occurred. The molecules under such a high pressure condition are unstable; with changes in the environment, either by the irradiation of laser or by increasing pressure, they can be transformed to a supra-molecular structure or be polymerized. The recovered sample after release from high pressure is denser in color than the original one. This fact shows that permanent residual stresses were in the sample after releasing the pressure and the final compound was in a new conjugated state other than that in the original one of APA molecules. Fig. 8 shows that the peak feature and fluorescence intensity still existed one week later after pressure had been released, illustrating that the virgin structure was not recovered and, therefore, the final compound with strong fluorescence is a new structure and more stable in ambient conditions. The small mass of the sample (10−6 g) and the weakness of the Raman signal did not allow us to precisely determine what chemical process occurs above 12 GPa. However, the dark-yellow color of the recovered compounds suggests a higher conjugate polymer. These results are deduced by comparison with the white color for ethylene, the orange colors obtained with carotene (conjugate polyene of 25 carbons), and the black color for the higher conjugate polyene as polyacetylene. The same results were obtained for high pressure induced chemical transformation
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Fig. 6. Pressure dependence of Raman bands of APA crystal.
of tribromobenzene (TBB) [5]. The possible mechanism for the chemical reaction process above 12 GPa is suggested as the opening of the unsaturated bonds of the monomers. The polymerization of unsaturated molecules can be explained by a chain propagation in which the hybridization of the atoms involves covalent bond changes during the reaction, e.g., sp2 to sp3 or sp to sp2 in polymerization [5]. Though the APA molecules are quite stable in ambient conditions, however, under high pressure the intermolecular interaction is comparable with the intramolecular one; the molecules can be unstable. Thus, two possible chemical reaction paths are suggested as follow: (1) Opening of the C=N bond. APA has a resonance form for which the C=N double bonds can be opened. The N atom exists in the molecule with sp2 hybridization form [23]. With increasing pressure the energy needed to break the bond decreases [24] and the broken C=N bond will connect to the other resonance one to form a polymer molecule. (2) Opening of the C=C of the ring. Comparative examples are: dimethyl phenyl silanol (DMPS) under high pressure [25], where the opening of the C=C bonds of the benzene ring was obtained at 15 GPa, and tribromobenzene (TBB), where the opening of the C=C bonds of the benzene ring was obtained at 26 GPa. This difference in pressure of the opening of aromatic ring is due to the departure of the symmetry for π -bonds. Therefore, it is reasonable to consider that the C=C bonds of APA were broken at above 12.1 GPa.
Fig. 7. Pressure dependence of the inverse of half width. Solid triangle: 1560 cm−1 (ring), solid circle: ν1560 (C=N) and solid square: 1494 cm−1 (ring, 19a), respectively.
4. Conclusion The high pressure behavior of APA crystals has been investigated up to 17.7 GPa with Raman spectroscopy and fluorescence spectra measurements. The ambient monoclinic structure is found to undergo two possible crystalline-to-crystalline phase transitions at pressures about 3.6 and 5.8 GPa. The disappearance of the external modes and vanishing of some internal modes suggest that amorphization occurs with onset pressure of 8.7 GPa and final pressure of 12.1 GPa. Above 12.1 GPa, an irreversible polymerization reaction may occur. Upon releasing the pressure, the new state remains stable as evident from its fluorescence spectra.
Fig. 8. The fluorescence spectral evolution of the APA with pressure raised from 5.8 to 17.7 GPa and released to ambient pressure for one week.
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Acknowledgements This work was supported by the National Natural Science Foundation of China (Grant Nos. 10874160, 10574121), Chinese Education Ministry and Chinese Academy of Sciences. References [1] [2] [3] [4] [5] [6] [7] [8] [9] [10]
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