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Raman and photoluminescence spectroscopy study of benzoic acid at high pressures
Journal of Physics and Chemistry of Solids 66 (2005) 895–901 www.elsevier.com/locate/jpcs
Raman and photoluminescence spectroscopy study of benzoic acid at high pressures Z.P. Wang, X.D. Tang, Z.J. Ding* 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 Received 11 May 2004; revised 28 October 2004; accepted 31 October 2004
Abstract Benzoic acid (C6H5COOH, BA) has been studied by high pressure Raman and fluorescence spectroscopy up to about 13.40 GPa using a diamond anvil cell at room temperature. The changes of lattice modes are interpreted as the crystal structure transformation. Three possible phase transitions, with the pressure increasing up to about 0.55, 3.67 and 11.10 GPa, are, respectively, elucidated as crystalline-to-crystalline, crystalline-to-amorphous transitions. A new material formed when the pressure is up to above 11.10 GPa remains stable after the pressure is released. q 2005 Elsevier Ltd. All rights reserved. Keywords: D. Phase transitions
1. Introduction Organic molecular crystals differ from other classes of solids in being made of discrete molecules. Although intramolecular forces are strong, intermolecular forces (hydrogen bonds or van der Waals bonds) are generally weak and short-range in their effect. The combination of strong and weak forces in molecular crystals is in marked contrast to the dominance of strong long-range Coulomb forces in simple ionic crystals such as sodium chloride, and introduces diversity to the properties of molecular crystals. Thus some properties of molecular crystals (e.g. molecular dimensions and vibrational frequencies) are essentially those of the free molecules due to the dominance of strong intra-molecular forces, while others (e.g. charge transport and energy transfer) are strongly influenced by intermolecular interactions. The structures of molecular crystals, which also affect many of the physical and chemical properties of these materials, are influenced by both intramolecular and intermolecular forces. Intra-molecular * Corresponding author. Tel.: C86 0551 360 6842; fax: C86 0551 3606948 801. E-mail address: [email protected] (Z.J. Ding). 0022-3697/$ - see front matter q 2005 Elsevier Ltd. All rights reserved. doi:10.1016/j.jpcs.2004.10.013
forces determine molecular shapes, which in turn play an important role in determining the most effective ways of packing the molecules together in the crystal. If the intermolecular forces are particularly strong and/or dependent on the relative orientation of adjacent molecules, they may modify the crystal structure deduced from simple considerations of molecular packing. An understanding of the origin and magnitude of the intermolecular forces, and their dependence on molecular properties and intermolecular separation and orientation, is therefore essential for studying many physical properties of molecular crystals [1]. On the other hand, pressure is an important physical parameter to induce the dramatic changes in physical properties of materials. The essential effect of pressure is to reduce the intermolecular distances, leading to modifications in lattice constants of crystalline solids as well as to the changes in atomic positions within a crystallographic cell. In general, reductions of lattice spacing and force constants between atoms induce modifications in band structures of crystals and thus in electronic properties [2]. Therefore, high pressure technique is also a powerful tool for the research for the molecular solids not only for producing entirely new phases but also for providing
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a better understanding of the physical properties at atmospheric pressure. In this view, high pressure Raman spectroscopy with a diamond anvil cell (DAC) is an efficient and relatively easy technique to probe order–disorder structural changes, phase transitions and inter-chain interactions in organic molecular solids, especially hydrogen-bonded molecular crystals. The hydrogen bond, which is one of the softest links in molecular crystal, typically exhibits a drastic change in bond strength associated with structure phase transitions under ultra-high pressures [3–5]. It also plays an important role in the study of the mechanisms of pressure-induced crystalline-to-crystalline, crystalline-to-amorphous transitions. High pressure fluorescence spectra is a useful tool to study the changes of molecular orbital energy level with the intensity variation; it can also provide evidence for the mechanism of pressure-induced phase transitions and characterize the state of molecular electronic energy level and validate the related theory of electrons and/or molecules interactions. In this work, we aimed at investigation of Raman and fluorescence spectroscopy of Benzoic acid (C6H5COOH, hereafter referred to BA) single crystals from ambient pressure up to about 11 GPa at room temperature. BA is known to be associated to form a nearly planar, centrosymmetric dimmer containing two intermolecular hydrogen bonds, with the molecule and dimmer structure shown by Fig. 1. There are three types of force fields for the BA crystal: a covalent bond force field in the monomer, a hydrogen bond force field in the carboxylic ring and a van der Waals force field in the crystal. At ambient pressure it crystallizes with a monoclinic structure, prismatic class of crystals with space group P21/c and has four monomer molecules (two dimmers) per unit cell [6]. The cyclic BA dimmer has a planar structure and belongs to the C2h point group. The total number of normal vibrations is 84 including 29 Ag (planar)C28 Bu (planar)C14 Au (out-of-plane)C13 Bg (out-of-plane). The Ag and Bg vibrations are Raman active, while Bu and Au ones are IR active. It also has 21 external vibrations, out of these twelve are Raman active (six Ag and six Bg) while the remaining nine are IR allowed (five Au and four Bu) and are expected to be observed in the Raman and IR experiments [7–11]. Horsewill et al. [12] have studied the dynamics of hydrogen atoms in the hydrogen bonds of BA dimmers as a functions of hydrostatic pressure up to 0.40 GPa by measuring the proton spin-lattice relaxation time with nuclear magnetic resonance technique. The experiment has indicated that there are two phase transitions below 0.40 GPa, one of which may be correlated with the reduction in asymmetry of
Fig. 1. The molecular and dimmer structure of benzoic acid.
the potential. Till now, fluorescence has not been observed in aromatic carbonyl compounds such as BA [13]. The nonfluorescent nature of BA has been ascribed to efficient intersystem crossing (S1/T1) [14]. However, Baba and Kitamura had reported fluorescence for dimeric BA at 77 K in a mixture of isopentane and methylcyclohexane (6:1 by volume) [15]. The main purpose of the present study was to find evidence for any solid state phase transitions or molecular distortions under high pressures, to obtain Raman spectra in both the internal and lattice mode regions, to acquire fluorescence spectra in visible region for crystalline BA to determine if the non-fluorescent nature of BA would change under high pressures, to estimate changes in the principal force constants, i.e., intra-molecular and intermolecular forces with increasing pressure, and to determine the system stability under the conditions of high pressure. The study of existence or dissociation of hydrogen bonds at high pressures is also a part our present work.
2. Experiment BA crystals were synthesized by the oxidation of benzaldehyde and recrystallized with ethanol and distilled water. Samples were loaded into a DAC for high-pressure measurement. The stainless steel gasket was pre-indented by the diamonds to an initial thickness of about 25 mm and then drilled to produce a 0.18 mm diameter cavity as the sample chamber with electronic park erosion (MH20M). A few grains of ruby powder were included for in situ measurements of the sample pressure using the standard ruby fluorescent technique. Pressure-transmitting media was not used because BA is soluble in most organic solvents. Pressure acting on the sample was determined by the wavelength shift of the ruby R1 fluorescence line, and we have confirmed from the line widths and the separation of the R1 and R2 lines that the hydrostatic or quasihydrostatic condition is satisfied in the pressure range under consideration. Raman and fluorescent 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 cmK1 in the measured frequency region. An argon ion laser operating at the line of 514.5 nm and at powers up to 10 mW was used as the exciting source. All the spectra were measured in the backscattering geometry for the laser beam focused on the area of 5!5 mm2 at room temperature. The accuracy of the frequency calibration was G1 cmK1 established by the peak position of the silicon thin film Raman line (520 cmK1) as an internal standard. The Raman spectra were collected in the range of 30– 4000 cmK1, but Raman line at 1322 and 1372 cmK1 of BA sample were occulted by the presence of the strong diamond Raman line (1332 cmK1) of the first-order diamond phonon
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mode. Fluorescent spectra were collected in the wavelength range of 520–800 nm. All the spectral data were recorded at a small pressure step within time duration of 30–40 min for establishing the equilibrium after a pressure increment. Wavenumber reproducibility is estimated to be G0.5 cmK1 and the pressure estimation accuracy is G0.2 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.
3. Results and discussion 3.1. High pressure Raman spectra The Raman spectra of the BA crystal at ambient pressure and at 13.40 GPa in the range of 30–1800 cmK1 are shown in Fig. 2. The spectral features at ambient pressure and room temperature agree with those reported in [7]. Details of Raman spectra as a function of increasing pressure are shown in Figs. 3–5. The pressure dependence of the observed frequencies is plotted in Fig. 6. At 11.10 GPa the spectra shows the disappearance of all the lattice modes and the internal modes (997.5, 1024.5 and 1628.9 cmK1) remain visible but in lower intensity; other internal modes become broadened and the intensity is decreased with the increasing pressure. When the pressure was reduced from 13.40 GPa to the ambient pressure, the Raman signals were not detected and the intensity of the background fluorescence became stronger. We can see from Fig. 3 that, with the increasing pressure, a new Raman line (117 cmK1) appears in the lattice mode region at about 0.55 GPa, lattice modes changes greatly at about 3.67–4.53 GPa. In Fig. 4 two internal modes for CaO stretching at 1629 cmK1 and C–C torsion in phenyl ring at 1599 cmK1 finally coupled at 1.57 GPa. At 11.10 GPa,
Fig. 2. Raman spectra of BA crystal in the region of 30–1800 cmK1 at ambient pressure and 13.40 GPa.
Fig. 3. Raman spectra of BA crystal in the lattice mode region under various pressures up to 11.10 GPa.
a broad band appears and the O–H stretching modes at 3062 and 3072 cmK1 disappear in Fig. 5. These facts elucidate that the lattice constants would be altered first by applying pressure; the van der Waals forces field in the crystal would then be changed and finally destroyed. Because the pressure would change the distance between O and H and strengthen the hydrogen bond force field in the carboxylic ring, higher pressure would also alter the covalent bond in the molecules. The disappearance of O–H stretching modes at 11.10 GPa indicates that the hydrogen bonds has been destroyed and new material is formed at such a high pressure. The assignments of all the Raman lines including internal and external modes are straightforward and listed in Table 1. The conventional Wilson notation [16] is used to sign the vibrations of phenyl ring. The initial values of slop dn/dP of the lines obtained from Fig. 6 are also given in Table 1. The line frequencies in these spectra will be cited hereafter by their values at ambient pressure. One can see from Table 1 that the changes of the frequency with pressure are very rapid for the lattice modes. The vibrations of hydrogen-bond in the carbonxylic ring can be described by the relative motion of two monomers. In terms of crystal structure, these vibrations are, in fact, assigned to lattice vibrations. Consequently, each dimmer possesses three Raman active and three IR active modes characteristic of hydrogen-bond vibrations [7]. Only one frequency of hydrogen bond vibration (91.3 cmK1) was observed in lattice mode region and is shown in Fig. 3, and other related hydrogen bond vibrations between aO/H– are given in Fig. 5. The slight
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Fig. 4. Raman spectra of BA in the internal mode region (500–1800 cmK1) under various pressures up to 11.10 GPa.
pressure dependence of the frequencies of hydrogen bond vibrations implies that the aO/H–O hydrogen-bond system still exists up to at least 11.10 GPa. However, its bonding is very weak. The Raman frequencies shift to higher wave numbers with increasing pressure. In external mode region, the upper-lying modes are observed to be more pressure sensitive compared with the lower frequency ones, and the reverse is the case in the internal mode region. 3.2. High pressure fluorescence spectra BA molecule has carboxylic functional group and may be regarded as a hydroxyl derivative of benzaldehyde. It is well known that aromatic carbonyl compounds, including benzaldehyde, generally show strong phosphorescence but no fluorescence [14,17]. This emission property is related to the fact that the lowest excited singlet state in these compounds is of (n, p*) type [14,18]. The fact that the BA monomer with the lowest excited singlet of (p, p*) character does not fluoresce is explained by assuming that the 3(n, p*) state is situated below the 1Lb state; the efficient spin-orbit coupling between 1(p, p*) and 3(n, p*) states must lead to a high rate of the intersystem crossing between 1 Lb and 3(n, p*) state, the monomer will then become nonfluorescent but strongly phosphorescent [18]. High-pressure fluorescence spectra as a function of increasing and decreasing pressure are shown in Figs. 7 and 8, respectively. The sharp and strong line at 552 nm is the diamond Raman line, and the band at 667 and 692.8 and 694.3 nm are ruby peaks. The spectral feature of BA is the quite smooth broad band extending over the visible light wavelength range. With the increased pressure, the nature of BA crystal is changed: the separation between the molecules is shortened and the interaction among the molecules becomes stronger; which induces the change in the outer electron orbits and
cause increase in the energy of bonding orbit. Hydrogen bond force plays an important role for stabilizing the dimmer of BA. The spin–orbit interaction between 1(p, p*) singlet and triplet states is vanishingly small; the rate of the direct intersystem crossing is low enough as compared to that of the fluorescence transition which results prominent occurrence of fluorescence in the dimmer. The intensity of fluorescence increases gradually with the increasing pressure and attains the maximum at about 13.40 GPa. Meanwhile, the O–H stretching mode at 3062 and 3072 cmK1 of Raman spectra disappears. It is observed that the fluorescence intensity decreases as the pressure is further increased above 13.40 GPa. This can be qualitatively understood as follows: The p orbits abut against each other in molecules, and an overlapping of p bonding electron densities of nearest neighbour molecules increases rapidly at high pressures. The interactions could
Fig. 5. Raman spectra of BA in the internal mode region (2100– 3300 cmK1) under various pressures up to 11.10 GPa.
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Fig. 6. The pressure dependence of Raman lines in the lattice mode and internal mode region of BA.
lower the excited state of the molecules. According to the energy gap law [19], the smaller the energy gap between the ground state and the excited state, the easier the internal conversion, and the lower the luminescent efficiency. The relative energy gap between the excited singlet state and triplet state varies with pressure. In Fig. 8, the spectral features vary with the decreasing pressure. First, the intensity of the fluorescence increases, and then becomes weak showing two relatively strong peaks at about 555 and 607 nm. According to the fluorescence spectra of benzil (C6H5COCOC6H5) crystal [20] and n–p or p–p* emission spectra [21], combined with the fact that the O–H stretching mode in Raman spectra of Fig. 5 disappears at about 11.10 GPa, we have inferred that the BA crystal does not recover but a new material, benzoic acid anhydride
(C6H5CO–O–COC6H5), forms and remains stable at ambient pressure. 3.3. Pressure induced phase transition The present Raman spectra study shows three possible phase transitions with increasing pressure up to about 0.55, 3.67 and 11.10 GPa. The spectral features of the phase transformation are summarized as follows. In the low frequency region (see Fig. 3), all external lines get weakened and broadened gradually with increasing pressure. A new Raman line (119 cmK1) appears at about 0.55 GPa, indicating a phase transition. This new phase may be due to the change in crystal structure or alteration in the symmetry of crystal. With the pressure arriving at about
Table 1 Frequencies ni, dni/dP and assignments of the Raman lines of BA crystal at ambient pressure ni (cmK1)
dni/dP (cmK1 GPaK1)
Assignment
ni (cmK1)
76 91.3 117.5
25 18.80 37.20
External motion O/H stretching Stretching
417.8 1439.9 1177.2
6.82 6.43
191.8
13.74
Torsion
1128.8 1287.9 1599.2
7.50 12.18 4.96
C–C torsion
9.05 10.68
CaO stretching Hydrogen bond vibrations Second-order diamond phonon mode O-H stretching (s) O–H stretching (as)
613.5
CaO out of plane deformation
792.6 806.8
5.54 1.94
1152.6
1.48
997.6 1024.5
2.61 4.36
C–H out of plane deformation
dni/dp (cmK1 GPaK1)
1628.9 2462.5 2661.3
Phenyl ring breathing CCC in plane deformation
3062.5 3072.3
Assignment C–C–C torsion C–O stretching CCH in plane deformation
900
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amorphous state does not remain stable on releasing pressure from 13.40 GPa, as shown by Figs. 7 and 8. A chemical transformation takes place during releasing the pressure. We inferred that benzoic acid anhydride ((C6H5CO)2O) which contain a big p conjugated structure is formed. Similar pressure-induced amorphization has also been observed in other organic molecular crystals, such as pentacene [22,23], tetra-cyanoethylene [24,25] and furan [26].
4. Conclusion
Fig. 7. The fluorescence spectra excited by Argon laser in the visible region with increasing pressure. The strong sharp line at 552 nm is the Raman line of diamond and slightly weak sharp lines at two sides are also the Raman lines. The ruby fluorescence peaks are the line at 692.80 and 694.28 nm. The arrow represents pressure increasing from bottom to top.
0.96 GPa, this Raman line splits into two lines (120 and 132 cmK1). At about 1.57 GPa only one Raman line (124 cmK1) remains, but it vanishes at about 3.67 GPa. All lattice (external) modes disappear at about 11.10 GPa. Additional evidence for three transformations is also found in the internal mode region (see Figs. 4–6). At 0.55 GPa, where several peaks (originally at 1128, 1152, 1177, 1187, 1599 and 1629 cmK1) originating from vibrations of the phenyl ring, Ph–C and carboxylic group still persist, but with increased width and lowered intensity up to 11.10 GPa. Hence, it is quite clear that the BA sample became amorphous when compressed to about 13.4 GPa. This
A high pressure Raman spectroscopy study shows that BA crystal undergoes three possible phase transitions with increasing pressure. Two phase transitions at 0.55 and 3.67 GPa may be interpreted as changes in crystal structure or crystal symmetry. The other phase transition occurs at 11.10 GPa, where all the external modes vanish, indicating a transition from crystalline to amorphous state and formation of a new material, i.e. benzoic acid anhydride. Pressure induces fluorescence in BA with increasing pressure, in contrast to non-fluorescent feature of BA at atmospheric pressure. As the high pressure alters the electronic state structure of BA molecules and changes the singlet and triple states, the intersystem crossing is reduced and the efficiency of fluorescent quantum yield is enhanced. The new material benzoic acid anhydride has thus the similar fluorescence spectra like benzil. However, additional measurements of X-ray diffraction and IR spectra will be necessary to confirm this argument.
Acknowledgements The authors are grateful to Professors Jian Zuo and Cunyi Xun for help on experiments and useful discussions. This work was supported by the National Natural Science Foundation of China (Grant No.10025420 and 90206009).
References
Fig. 8. The fluorescence spectra excited by Argon laser in the visible region with decreasing pressure. The strong sharp line at 552 nm is the Raman line of diamond and slightly weak sharp lines at two sides are also the Raman lines. The ruby fluorescence peaks are the line at 692.80 and 694.28 nm. The upper curve labled ‘1 bar’ is for the sample releasing from high pressure, and the lower ‘1 bar’ for the virgin sample.
[1] M. Pope, E.S. Charles, Electronic Processes in Organic Crystals and Polymers, second ed., Oxford University Press, New York, 1999. p. 1. [2] J.D. Wright, Molecular Crystals, Cambridge University Press, New York, 1987. p. 11. [3] K.R. Hirsch, W.B. Holzapfel, J. Chem. Phys. 84 (1986) 2771. [4] M. Gauthier, Ph. Pruzan, J.C. Chervin, J.M. Besson, Phys. Rev. B37 (1988) 2102. [5] H. Shimizu, K. Nagata, S. Sasaki, J. Chem. Phys. 89 (1988) 2743. [6] G.A. Sim, J.M. Robertson, T.H. Goodwin, Acta Crystallogr. 8 (1955) 157. [7] G. Klausberger, K. Furic´, L. Colombo, J. Raman Spectrosc. 6 (1977) 277. [8] S.G. Stepanian, L.D. Reva, E.D. Radchenko, G.G. Sheina, Vibr. Spectrosc. 11 (1996) 123. [9] H.T. Flakus, M. Chelmecki, Spectrochim. Acta A58 (2002) 179.
Z.P. Wang et al. / Journal of Physics and Chemistry of Solids 66 (2005) 895–901 [10] S. Hayashi, M. Oobatake, R. Nakamura, K. Machida, J. Chem. Phys. 94 (1991) 4446. [11] H.R. Zesmann, Z. Mielke, Chem. Phys. Lett. 186 (1991) 501. [12] A.J. Horsewill, P.J. McDonald, D. Vijayaraghavan, J. Chem. Phys. 100 (1994) 1889. [13] M. Kasha, Radiat. Res. Suppl. 2 (1960) 243. [14] S.K. Lower, M.A. El-Sayed, Chem. Rev. 66 (1966) 199. [15] H. Baba, M. Kitamura, J. Mol. Spectrosc. 41 (1972) 302. [16] E.B. Wilson, Phys. Rev. 45 (1934) 706. [17] T. Takemura, H. Baba, Bull. Chem. Soc. Jpn 42 (1969) 2756. [18] M.A. El-Sayed, J. Chem. Phys. 38 (1963) 2843.
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R. Englman, J. Jortner, Mol. Phys. 18 (1970) 145. S.C. Ganguly, N.K. Ghoudhury, J. Chem. Phys. 21 (1952) 554. C. Redit, J. Chem. Phys. 23 (1953) 1906. H.G. Drickamer, Science 156 (1967) 1183. R.B. Aust, W.H. Bentley, H.G. Drickamer, J. Chem. Phys. 41 (1964) 1856. [24] S.L. Chaplot, A. Mierzejewski, G.S. Pawley, Mol. Phys. 56 (1985) 115. [25] S.L. Chaplot, R. Mukhopadhyay, Phys. Rev. B33 (1986) 5099. [26] M. Ceppatelli, M. Santoro, R. Bini, V. Schettino, J. Chem. Phys. 118 (2003) 1499.
Raman and photoluminescence spectroscopy study of benzoic acid at high pressures Z.P. Wang, X.D. Tang, Z.J. Ding* 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 Received 11 May 2004; revised 28 October 2004; accepted 31 October 2004
Abstract Benzoic acid (C6H5COOH, BA) has been studied by high pressure Raman and fluorescence spectroscopy up to about 13.40 GPa using a diamond anvil cell at room temperature. The changes of lattice modes are interpreted as the crystal structure transformation. Three possible phase transitions, with the pressure increasing up to about 0.55, 3.67 and 11.10 GPa, are, respectively, elucidated as crystalline-to-crystalline, crystalline-to-amorphous transitions. A new material formed when the pressure is up to above 11.10 GPa remains stable after the pressure is released. q 2005 Elsevier Ltd. All rights reserved. Keywords: D. Phase transitions
1. Introduction Organic molecular crystals differ from other classes of solids in being made of discrete molecules. Although intramolecular forces are strong, intermolecular forces (hydrogen bonds or van der Waals bonds) are generally weak and short-range in their effect. The combination of strong and weak forces in molecular crystals is in marked contrast to the dominance of strong long-range Coulomb forces in simple ionic crystals such as sodium chloride, and introduces diversity to the properties of molecular crystals. Thus some properties of molecular crystals (e.g. molecular dimensions and vibrational frequencies) are essentially those of the free molecules due to the dominance of strong intra-molecular forces, while others (e.g. charge transport and energy transfer) are strongly influenced by intermolecular interactions. The structures of molecular crystals, which also affect many of the physical and chemical properties of these materials, are influenced by both intramolecular and intermolecular forces. Intra-molecular * Corresponding author. Tel.: C86 0551 360 6842; fax: C86 0551 3606948 801. E-mail address: [email protected] (Z.J. Ding). 0022-3697/$ - see front matter q 2005 Elsevier Ltd. All rights reserved. doi:10.1016/j.jpcs.2004.10.013
forces determine molecular shapes, which in turn play an important role in determining the most effective ways of packing the molecules together in the crystal. If the intermolecular forces are particularly strong and/or dependent on the relative orientation of adjacent molecules, they may modify the crystal structure deduced from simple considerations of molecular packing. An understanding of the origin and magnitude of the intermolecular forces, and their dependence on molecular properties and intermolecular separation and orientation, is therefore essential for studying many physical properties of molecular crystals [1]. On the other hand, pressure is an important physical parameter to induce the dramatic changes in physical properties of materials. The essential effect of pressure is to reduce the intermolecular distances, leading to modifications in lattice constants of crystalline solids as well as to the changes in atomic positions within a crystallographic cell. In general, reductions of lattice spacing and force constants between atoms induce modifications in band structures of crystals and thus in electronic properties [2]. Therefore, high pressure technique is also a powerful tool for the research for the molecular solids not only for producing entirely new phases but also for providing
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Z.P. Wang et al. / Journal of Physics and Chemistry of Solids 66 (2005) 895–901
a better understanding of the physical properties at atmospheric pressure. In this view, high pressure Raman spectroscopy with a diamond anvil cell (DAC) is an efficient and relatively easy technique to probe order–disorder structural changes, phase transitions and inter-chain interactions in organic molecular solids, especially hydrogen-bonded molecular crystals. The hydrogen bond, which is one of the softest links in molecular crystal, typically exhibits a drastic change in bond strength associated with structure phase transitions under ultra-high pressures [3–5]. It also plays an important role in the study of the mechanisms of pressure-induced crystalline-to-crystalline, crystalline-to-amorphous transitions. High pressure fluorescence spectra is a useful tool to study the changes of molecular orbital energy level with the intensity variation; it can also provide evidence for the mechanism of pressure-induced phase transitions and characterize the state of molecular electronic energy level and validate the related theory of electrons and/or molecules interactions. In this work, we aimed at investigation of Raman and fluorescence spectroscopy of Benzoic acid (C6H5COOH, hereafter referred to BA) single crystals from ambient pressure up to about 11 GPa at room temperature. BA is known to be associated to form a nearly planar, centrosymmetric dimmer containing two intermolecular hydrogen bonds, with the molecule and dimmer structure shown by Fig. 1. There are three types of force fields for the BA crystal: a covalent bond force field in the monomer, a hydrogen bond force field in the carboxylic ring and a van der Waals force field in the crystal. At ambient pressure it crystallizes with a monoclinic structure, prismatic class of crystals with space group P21/c and has four monomer molecules (two dimmers) per unit cell [6]. The cyclic BA dimmer has a planar structure and belongs to the C2h point group. The total number of normal vibrations is 84 including 29 Ag (planar)C28 Bu (planar)C14 Au (out-of-plane)C13 Bg (out-of-plane). The Ag and Bg vibrations are Raman active, while Bu and Au ones are IR active. It also has 21 external vibrations, out of these twelve are Raman active (six Ag and six Bg) while the remaining nine are IR allowed (five Au and four Bu) and are expected to be observed in the Raman and IR experiments [7–11]. Horsewill et al. [12] have studied the dynamics of hydrogen atoms in the hydrogen bonds of BA dimmers as a functions of hydrostatic pressure up to 0.40 GPa by measuring the proton spin-lattice relaxation time with nuclear magnetic resonance technique. The experiment has indicated that there are two phase transitions below 0.40 GPa, one of which may be correlated with the reduction in asymmetry of
Fig. 1. The molecular and dimmer structure of benzoic acid.
the potential. Till now, fluorescence has not been observed in aromatic carbonyl compounds such as BA [13]. The nonfluorescent nature of BA has been ascribed to efficient intersystem crossing (S1/T1) [14]. However, Baba and Kitamura had reported fluorescence for dimeric BA at 77 K in a mixture of isopentane and methylcyclohexane (6:1 by volume) [15]. The main purpose of the present study was to find evidence for any solid state phase transitions or molecular distortions under high pressures, to obtain Raman spectra in both the internal and lattice mode regions, to acquire fluorescence spectra in visible region for crystalline BA to determine if the non-fluorescent nature of BA would change under high pressures, to estimate changes in the principal force constants, i.e., intra-molecular and intermolecular forces with increasing pressure, and to determine the system stability under the conditions of high pressure. The study of existence or dissociation of hydrogen bonds at high pressures is also a part our present work.
2. Experiment BA crystals were synthesized by the oxidation of benzaldehyde and recrystallized with ethanol and distilled water. Samples were loaded into a DAC for high-pressure measurement. The stainless steel gasket was pre-indented by the diamonds to an initial thickness of about 25 mm and then drilled to produce a 0.18 mm diameter cavity as the sample chamber with electronic park erosion (MH20M). A few grains of ruby powder were included for in situ measurements of the sample pressure using the standard ruby fluorescent technique. Pressure-transmitting media was not used because BA is soluble in most organic solvents. Pressure acting on the sample was determined by the wavelength shift of the ruby R1 fluorescence line, and we have confirmed from the line widths and the separation of the R1 and R2 lines that the hydrostatic or quasihydrostatic condition is satisfied in the pressure range under consideration. Raman and fluorescent 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 cmK1 in the measured frequency region. An argon ion laser operating at the line of 514.5 nm and at powers up to 10 mW was used as the exciting source. All the spectra were measured in the backscattering geometry for the laser beam focused on the area of 5!5 mm2 at room temperature. The accuracy of the frequency calibration was G1 cmK1 established by the peak position of the silicon thin film Raman line (520 cmK1) as an internal standard. The Raman spectra were collected in the range of 30– 4000 cmK1, but Raman line at 1322 and 1372 cmK1 of BA sample were occulted by the presence of the strong diamond Raman line (1332 cmK1) of the first-order diamond phonon
Z.P. Wang et al. / Journal of Physics and Chemistry of Solids 66 (2005) 895–901
897
mode. Fluorescent spectra were collected in the wavelength range of 520–800 nm. All the spectral data were recorded at a small pressure step within time duration of 30–40 min for establishing the equilibrium after a pressure increment. Wavenumber reproducibility is estimated to be G0.5 cmK1 and the pressure estimation accuracy is G0.2 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.
3. Results and discussion 3.1. High pressure Raman spectra The Raman spectra of the BA crystal at ambient pressure and at 13.40 GPa in the range of 30–1800 cmK1 are shown in Fig. 2. The spectral features at ambient pressure and room temperature agree with those reported in [7]. Details of Raman spectra as a function of increasing pressure are shown in Figs. 3–5. The pressure dependence of the observed frequencies is plotted in Fig. 6. At 11.10 GPa the spectra shows the disappearance of all the lattice modes and the internal modes (997.5, 1024.5 and 1628.9 cmK1) remain visible but in lower intensity; other internal modes become broadened and the intensity is decreased with the increasing pressure. When the pressure was reduced from 13.40 GPa to the ambient pressure, the Raman signals were not detected and the intensity of the background fluorescence became stronger. We can see from Fig. 3 that, with the increasing pressure, a new Raman line (117 cmK1) appears in the lattice mode region at about 0.55 GPa, lattice modes changes greatly at about 3.67–4.53 GPa. In Fig. 4 two internal modes for CaO stretching at 1629 cmK1 and C–C torsion in phenyl ring at 1599 cmK1 finally coupled at 1.57 GPa. At 11.10 GPa,
Fig. 2. Raman spectra of BA crystal in the region of 30–1800 cmK1 at ambient pressure and 13.40 GPa.
Fig. 3. Raman spectra of BA crystal in the lattice mode region under various pressures up to 11.10 GPa.
a broad band appears and the O–H stretching modes at 3062 and 3072 cmK1 disappear in Fig. 5. These facts elucidate that the lattice constants would be altered first by applying pressure; the van der Waals forces field in the crystal would then be changed and finally destroyed. Because the pressure would change the distance between O and H and strengthen the hydrogen bond force field in the carboxylic ring, higher pressure would also alter the covalent bond in the molecules. The disappearance of O–H stretching modes at 11.10 GPa indicates that the hydrogen bonds has been destroyed and new material is formed at such a high pressure. The assignments of all the Raman lines including internal and external modes are straightforward and listed in Table 1. The conventional Wilson notation [16] is used to sign the vibrations of phenyl ring. The initial values of slop dn/dP of the lines obtained from Fig. 6 are also given in Table 1. The line frequencies in these spectra will be cited hereafter by their values at ambient pressure. One can see from Table 1 that the changes of the frequency with pressure are very rapid for the lattice modes. The vibrations of hydrogen-bond in the carbonxylic ring can be described by the relative motion of two monomers. In terms of crystal structure, these vibrations are, in fact, assigned to lattice vibrations. Consequently, each dimmer possesses three Raman active and three IR active modes characteristic of hydrogen-bond vibrations [7]. Only one frequency of hydrogen bond vibration (91.3 cmK1) was observed in lattice mode region and is shown in Fig. 3, and other related hydrogen bond vibrations between aO/H– are given in Fig. 5. The slight
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Z.P. Wang et al. / Journal of Physics and Chemistry of Solids 66 (2005) 895–901
Fig. 4. Raman spectra of BA in the internal mode region (500–1800 cmK1) under various pressures up to 11.10 GPa.
pressure dependence of the frequencies of hydrogen bond vibrations implies that the aO/H–O hydrogen-bond system still exists up to at least 11.10 GPa. However, its bonding is very weak. The Raman frequencies shift to higher wave numbers with increasing pressure. In external mode region, the upper-lying modes are observed to be more pressure sensitive compared with the lower frequency ones, and the reverse is the case in the internal mode region. 3.2. High pressure fluorescence spectra BA molecule has carboxylic functional group and may be regarded as a hydroxyl derivative of benzaldehyde. It is well known that aromatic carbonyl compounds, including benzaldehyde, generally show strong phosphorescence but no fluorescence [14,17]. This emission property is related to the fact that the lowest excited singlet state in these compounds is of (n, p*) type [14,18]. The fact that the BA monomer with the lowest excited singlet of (p, p*) character does not fluoresce is explained by assuming that the 3(n, p*) state is situated below the 1Lb state; the efficient spin-orbit coupling between 1(p, p*) and 3(n, p*) states must lead to a high rate of the intersystem crossing between 1 Lb and 3(n, p*) state, the monomer will then become nonfluorescent but strongly phosphorescent [18]. High-pressure fluorescence spectra as a function of increasing and decreasing pressure are shown in Figs. 7 and 8, respectively. The sharp and strong line at 552 nm is the diamond Raman line, and the band at 667 and 692.8 and 694.3 nm are ruby peaks. The spectral feature of BA is the quite smooth broad band extending over the visible light wavelength range. With the increased pressure, the nature of BA crystal is changed: the separation between the molecules is shortened and the interaction among the molecules becomes stronger; which induces the change in the outer electron orbits and
cause increase in the energy of bonding orbit. Hydrogen bond force plays an important role for stabilizing the dimmer of BA. The spin–orbit interaction between 1(p, p*) singlet and triplet states is vanishingly small; the rate of the direct intersystem crossing is low enough as compared to that of the fluorescence transition which results prominent occurrence of fluorescence in the dimmer. The intensity of fluorescence increases gradually with the increasing pressure and attains the maximum at about 13.40 GPa. Meanwhile, the O–H stretching mode at 3062 and 3072 cmK1 of Raman spectra disappears. It is observed that the fluorescence intensity decreases as the pressure is further increased above 13.40 GPa. This can be qualitatively understood as follows: The p orbits abut against each other in molecules, and an overlapping of p bonding electron densities of nearest neighbour molecules increases rapidly at high pressures. The interactions could
Fig. 5. Raman spectra of BA in the internal mode region (2100– 3300 cmK1) under various pressures up to 11.10 GPa.
Z.P. Wang et al. / Journal of Physics and Chemistry of Solids 66 (2005) 895–901
899
Fig. 6. The pressure dependence of Raman lines in the lattice mode and internal mode region of BA.
lower the excited state of the molecules. According to the energy gap law [19], the smaller the energy gap between the ground state and the excited state, the easier the internal conversion, and the lower the luminescent efficiency. The relative energy gap between the excited singlet state and triplet state varies with pressure. In Fig. 8, the spectral features vary with the decreasing pressure. First, the intensity of the fluorescence increases, and then becomes weak showing two relatively strong peaks at about 555 and 607 nm. According to the fluorescence spectra of benzil (C6H5COCOC6H5) crystal [20] and n–p or p–p* emission spectra [21], combined with the fact that the O–H stretching mode in Raman spectra of Fig. 5 disappears at about 11.10 GPa, we have inferred that the BA crystal does not recover but a new material, benzoic acid anhydride
(C6H5CO–O–COC6H5), forms and remains stable at ambient pressure. 3.3. Pressure induced phase transition The present Raman spectra study shows three possible phase transitions with increasing pressure up to about 0.55, 3.67 and 11.10 GPa. The spectral features of the phase transformation are summarized as follows. In the low frequency region (see Fig. 3), all external lines get weakened and broadened gradually with increasing pressure. A new Raman line (119 cmK1) appears at about 0.55 GPa, indicating a phase transition. This new phase may be due to the change in crystal structure or alteration in the symmetry of crystal. With the pressure arriving at about
Table 1 Frequencies ni, dni/dP and assignments of the Raman lines of BA crystal at ambient pressure ni (cmK1)
dni/dP (cmK1 GPaK1)
Assignment
ni (cmK1)
76 91.3 117.5
25 18.80 37.20
External motion O/H stretching Stretching
417.8 1439.9 1177.2
6.82 6.43
191.8
13.74
Torsion
1128.8 1287.9 1599.2
7.50 12.18 4.96
C–C torsion
9.05 10.68
CaO stretching Hydrogen bond vibrations Second-order diamond phonon mode O-H stretching (s) O–H stretching (as)
613.5
CaO out of plane deformation
792.6 806.8
5.54 1.94
1152.6
1.48
997.6 1024.5
2.61 4.36
C–H out of plane deformation
dni/dp (cmK1 GPaK1)
1628.9 2462.5 2661.3
Phenyl ring breathing CCC in plane deformation
3062.5 3072.3
Assignment C–C–C torsion C–O stretching CCH in plane deformation
900
Z.P. Wang et al. / Journal of Physics and Chemistry of Solids 66 (2005) 895–901
amorphous state does not remain stable on releasing pressure from 13.40 GPa, as shown by Figs. 7 and 8. A chemical transformation takes place during releasing the pressure. We inferred that benzoic acid anhydride ((C6H5CO)2O) which contain a big p conjugated structure is formed. Similar pressure-induced amorphization has also been observed in other organic molecular crystals, such as pentacene [22,23], tetra-cyanoethylene [24,25] and furan [26].
4. Conclusion
Fig. 7. The fluorescence spectra excited by Argon laser in the visible region with increasing pressure. The strong sharp line at 552 nm is the Raman line of diamond and slightly weak sharp lines at two sides are also the Raman lines. The ruby fluorescence peaks are the line at 692.80 and 694.28 nm. The arrow represents pressure increasing from bottom to top.
0.96 GPa, this Raman line splits into two lines (120 and 132 cmK1). At about 1.57 GPa only one Raman line (124 cmK1) remains, but it vanishes at about 3.67 GPa. All lattice (external) modes disappear at about 11.10 GPa. Additional evidence for three transformations is also found in the internal mode region (see Figs. 4–6). At 0.55 GPa, where several peaks (originally at 1128, 1152, 1177, 1187, 1599 and 1629 cmK1) originating from vibrations of the phenyl ring, Ph–C and carboxylic group still persist, but with increased width and lowered intensity up to 11.10 GPa. Hence, it is quite clear that the BA sample became amorphous when compressed to about 13.4 GPa. This
A high pressure Raman spectroscopy study shows that BA crystal undergoes three possible phase transitions with increasing pressure. Two phase transitions at 0.55 and 3.67 GPa may be interpreted as changes in crystal structure or crystal symmetry. The other phase transition occurs at 11.10 GPa, where all the external modes vanish, indicating a transition from crystalline to amorphous state and formation of a new material, i.e. benzoic acid anhydride. Pressure induces fluorescence in BA with increasing pressure, in contrast to non-fluorescent feature of BA at atmospheric pressure. As the high pressure alters the electronic state structure of BA molecules and changes the singlet and triple states, the intersystem crossing is reduced and the efficiency of fluorescent quantum yield is enhanced. The new material benzoic acid anhydride has thus the similar fluorescence spectra like benzil. However, additional measurements of X-ray diffraction and IR spectra will be necessary to confirm this argument.
Acknowledgements The authors are grateful to Professors Jian Zuo and Cunyi Xun for help on experiments and useful discussions. This work was supported by the National Natural Science Foundation of China (Grant No.10025420 and 90206009).
References
Fig. 8. The fluorescence spectra excited by Argon laser in the visible region with decreasing pressure. The strong sharp line at 552 nm is the Raman line of diamond and slightly weak sharp lines at two sides are also the Raman lines. The ruby fluorescence peaks are the line at 692.80 and 694.28 nm. The upper curve labled ‘1 bar’ is for the sample releasing from high pressure, and the lower ‘1 bar’ for the virgin sample.
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