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NEXAFS spectra of metallotetraphenylporphyrins with adsorbed nitrogen monoxide
Journal of Electron Spectroscopy and Related Phenomena 114–116 (2001) 849–854 www.elsevier.nl / locate / elspec
NEXAFS spectra of metallotetraphenylporphyrins with adsorbed nitrogen monoxide a, b b b,c T. Okajima *, Y. Yamamoto , Y. Ouchi , K. Seki a
Experimental Facilities Division, Japan Synchrotron Radiation Research Institute, 1 -1 -1, Kouto, Mikazuki-cho, Sayo-gun, Hyogo 679 -5198, Japan b Department of Chemistry, Graduate School of Science, Nagoya University, Furo-cho, Chikusa-ku, Nagoya 464 -8602, Japan c Research Center for Materials Science, Nagoya University, Furo-cho, Chikusa-ku, Nagoya 464 -8602, Japan Received 8 August 2000; received in revised form 7 September 2000; accepted 9 September 2000
Abstract Near edge X-ray absorption fine structure (NEXAFS) spectra of nitrogen K-edge region were obtained using total electron yield detection for four metallotetraphenylporphyrins (M–TPPs) (M5Cr, Co, Zn and Ru) before and after nitrogen monoxide (NO) adsorption. The observed spectra of all M–TPPs were drastically changed by NO adsorption. The difference spectra showed new intense peaks appearing around 404 eV for all M–TPPs. Furthermore, small peaks below 400 eV corresponding to p * transitions were observed for ZnTPP and CrTPP. Comparison of these results and oxidation potentials for M–TPPs revealed that NO is adsorbed only at ligand for ZnTPP, at only central metal for CoTPP and RuTPP, and at both ligand and the central metal for CrTPP. 2001 Elsevier Science B.V. All rights reserved. Keywords: NEXAFS spectroscopy; Porphyrin; Metallotetraphenylporphyrins; Nitrogen monoxide; Electronic structure; Adsorption
1. Introduction The electronic structure of metallotetraphenylporphyrins (M–TPPs) is of particular interest due to many possible applications, such as catalysts and electronic devices (e.g. gas sensors, electroluminescent displays, and field effect transistors) [1,2]. M– TPPs have a porphyrin macrocycle with a large p -conjugated aromatic system. In order to attain good chemical and physical properties, it is necessary to elucidate their electronic structure. The p electrons in the highest occupied molecular orbital *Corresponding author. Tel.: 181-791-58-2750; fax: 181-791-582752. E-mail address: [email protected] (T. Okajima).
(HOMO) and lowest unoccupied molecular orbital (LUMO) play an important role for the chemical and physical properties. The details of these energy levels depend on the metal in M–TPP. On the other hand, M–TPPs can be used for the contact decomposition materials of nitrogen monoxide (NO) [3,4]. NO is a free radical containing an electron in the antibonding p (p *) orbital. NO can be reduced and oxidized easily as expected from the oxidation-reduction potentials [5]. The studies of the electronic structure of M–TPPs with adsorbed NO molecule and the state of adsorption of NO will give useful information about the contact decomposition of NO. NEXAFS spectroscopy, also known as X-ray absorption near edge structure (XANES), is a recently developed branch of X-ray spectroscopy. It is
0368-2048 / 01 / $ – see front matter 2001 Elsevier Science B.V. All rights reserved. PII: S0368-2048( 00 )00268-1
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T. Okajima et al. / Journal of Electron Spectroscopy and Related Phenomena 114 – 116 (2001) 849 – 854
useful for the study of unoccupied electronic states and molecular orientation in organic thin films [6]. As for porphyrins, N K-edge NEXAFS study of 5,10,15,20-tetraphenylporphyrin zinc (ZnTPP) [7], free-base 5,10,15,20-tetraphenylporphyrin (H 2 TPP) [7], 5,10,15,20-tetraphenylporphyrin vanadyl (VOTPP) [8], and 2,3,7,8,12,13,17,18-nickeloctaethylporphyrin (NiOEP) [8] were reported. In this work, we applied NEXAFS spectroscopy to four typical porphyrins, ZnTPP, 5,10,15,20-tetraphenylporphyrin cobalt (CoTPP), 5,10,15,20-tetraphenylporphyrin chromium chloride (CrTPPCl), and 5,10,15,20-tetraphenylporphyrin ruthenium dipyridine (RuTPP(py) 2 ). These porphyrins are shown in Fig. 1 together with H 2 TPP. The purpose in this
study is to study the unoccupied electronic structure of M–TPPs with adsorbed NO molecule and to discuss the state of adsorption of NO from N K-edge NEXAFS spectroscopy. Since the HOMO and LUMO contain contribution from the N atoms in porphyrin macrocycle, N K-edge NEXAFS spectroscopy is suitable for this study.
2. Experimental section ZnTPP, CoTPP and CrTPPCl were commercially obtained from Tokyo Chemical Industry Co. Ltd. RuTPP(py) 2 was also commercially obtained from Nard Institute Ltd. Their thin films were prepared by vacuum deposition onto Si (100) wafer at room temperature under vacuum of 10 23 Pa range. The film thickness was about 50 nm as measured by a quartz oscillator. The adsorption of NO on M–TPPs was performed in a glass dedicator filled with helium gas with 5% NO at room temperature. The reaction time was about 1 h. N K-edge NEXAFS spectra were measured at the soft X-ray beamline 11A of Photon Factory at Institute of Materials Structure Science. Synchrotron radiation from the storage ring was monochromatized by a Grasshopper monochromator with a grating of 2400 lines / mm [9,10]. The spectra were obtained in the total electron yield mode using a channeltron with a base pressure of 10 26 Pa. X-ray incidence angle from sample surface was 558 (so-called magic angle). At this incidence angle, possible effects of preferred molecular orientation can be removed for samples without azimuthal order, as expected for the present case [6,11]. The photon energy was calibrated using 7,7,8,8-tetracyanoquinodimethane (TCNQ) in standard material, which was calibrated using the vapor spectra of Ar [12], N 2 [13], O 2 [14], and SF 6 [15]. The main peak energy of TCNQ at the N K-edge region was taken to be at 399.7 eV.
3. Results and discussions
Fig. 1. Structural chemical formulas of porphyrins: CoTPP (a), CrTPPCl (b), ZnTPP (c), RuTPP(py) 2 (d), and H 2 TPP (e).
Fig. 2 shows the N K-edge NEXAFS spectra of (a) CrTPPCl, (b) CoTPP, (c) ZnTPP, and (d) RuTPP(py) 2 thin films. Regardless of the difference
T. Okajima et al. / Journal of Electron Spectroscopy and Related Phenomena 114 – 116 (2001) 849 – 854
Fig. 2. N K-edge NEXAFS spectra of CrTPPCl (a), CoTPP (b), ZnTPP (c), and RuTPP(py) 2 (d).
in the central metal, the general appearance of the spectra is similar. There are three peaks A, B, and C at low photon energy, and two peaks D, and E at higher energy. These two groups correspond to the excitations to p * and s * orbitals, respectively [7]. The peak A observed in the spectrum of RuTPP(py) 2 contains contribution from the strong p * resonance
851
of pyridine (py) at |400 eV [16]. These features were assigned in detail by Narioka et al. using polarization dependence of NEXAFS spectra and comparison with CNDO / S2 molecular orbital calculation with a limited configuration interaction for free base porphyrin and beryllium porphyrin [7]. In M– TPPs, all four nitrogen atoms in porphyrin macrocycle are equivalent. The energies and assignments of the features are listed in Table 1. Fig. 3 shows the NEXAFS spectra of M–TPPs without and with adsorbed NO. All spectra are changed by NO adsorption. In the case of ZnTPP and CrTPPCl, three new peaks a, b and c are seen. On the other hand, only two new peaks a and c are seen for RuTPP(py) 2 and CoTPP. In all spectra, intense two peaks A and B assigned to p * transitions at low photon energy are broadened. These results show that NO molecule is adsorbed on all M–TPPs used in this study. On the other hand, in the study of fourier transfer infrared (FT–IR) spectroscopy for H 2 TPP and M–TPPs (M5Cr, Co, Zn, Ru, and Ag), it was reported that NO was adsorbed only on CoTPP [17]. The origin of this difference is not clear at present. NEXAFS spectrum of reaction species by NO adsorption is obtained by subtracting appropriately scaled spectrum before NO adsorption from the spectrum after NO adsorption. The scaling coefficient was determined to make the intensity of the peak A equal before and after NO adsorption. The obtained difference spectra for the M–TPPs are also shown in Fig. 3 together with the NEXAFS spectra before and after NO adsorption. These spectra are similar to each other. Weak peaks corresponding to peaks A and B are observed below 400 eV for all M–TPPs, and these peaks are shifted to the
Table 1 Peak energies and assignments of N K-edge NEXAFS spectra for four metallotetraphenylporphyrins (M–TPPs) (M5Cr, Co, Zn, and Ru) before nitrogen monoxide adsorption Feature
A B C D E
Energy / eV
Assignment
CrTPPCl
CoTPP
ZnTPP
RuTPP(py) 2
398.5 401.3 404.5 406.7 416
398.7 401.4 404.5 406.7 416
398.4 401.3 404.0 406.5 416
398.8 401.4 404.5 406.8 416
p * (e g ) p * (b2 u , e g ) p * (a2 u ) s* s*
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Fig. 3. N K-edge NEXAFS spectra of M–TPPs before (a), after NO adsorption (b), and the difference spectra (c).
low photon energy side by NO adsorption. In the case of ZnTPP, the peaks below 400 eV in the differential shape are most intense indicating large shift. Those of CrTPPCl shows medium intensity,
and they are weak for CoTPP and RuTPP(py) 2 . Above 400 eV two intense peaks a and c are observed at 403.7 eV and 414 eV for CoTPP and RuTPP(py) 2 , while ZnTPP and CrTPPCl show three
T. Okajima et al. / Journal of Electron Spectroscopy and Related Phenomena 114 – 116 (2001) 849 – 854
intense peaks a, b and c at 403.7 eV, 405.2 eV and 414 eV, respectively. Considering their peak energies, we may suppose that peaks a and b are p * transitions and peak c is s * transition. NEXAFS spectrum of NO molecule at N K-edge shows an intense peak attributed to N 1s→p * transition at 399.7 eV [18]. The energy of the lowest peak a observed in this study at 403.7 eV is much different from that of NO molecule. Tsuji et al. and Miyamoto et al. reported that NO is adsorbed on CoTPP in the state of NO anion, NO – , using FT–IR spectroscopy [3,17]. Thus we can attribute the peak a at 403.7 eV to NO – . Similarity of the difference spectra suggests the peaks a and b in other M–TPP can be also attributed to NO – . Oxidation potentials for M–TPPs showed that the first oxidation occurs at the central metal for CoTPP, RuTPP(py) 2 , and CrTPPCl, whereas ligand oxidation occurs first for ZnTPP [19–21]. Subsequent oxidations also occur at the ligand in all cases [19–21]. From the difference of these oxidation potentials, it is expected that NO – is formed by the electron transfer from the ligand for ZnTPP and that NO – is adsorbed to the ligand. In the case of CrTPPCl, the first and the second oxidation potentials are close (0.2 eV difference), and the similarity of the difference spectrum with that of ZnTPP suggests electron transfer may occur from the ligand. Note that the differential peaks A and B below 400 eV in the difference spectra corresponding to p * transition for ZnTPP and CrTPPCl are more significant than those for CoTPP and RuTPP(py) 2 . This indicats that p * orbitals of ZnTPP and CrTPPCl are actually affected by the adsorption of NO at the ligand and thus supports the above arguments. On the other hand, the differential peaks A and B below 400 eV for CoTPP and RuTPP(py) 2 are less significant. This suggests that NO is adsorbed predominantly on central metal for CoTPP and RuTPP(py) 2 . The difference from the case of ZnTPP and CrTPPCl may be attributed to the large difference between first oxidation potential and second oxidation potential (0.67 eV and 1.05 eV for CoTPP and RuTPP(py) 2 , respectively), which hinders the electron transfer from the ligand [19,21]. The result that two intense peaks a and b are observed in the spectra of ZnTPP and CrTPPCl may indicate that there are two difference adsorption states of NO – for ligand.
853
4. Conclusions In this work, we studied four kinds of metallotetraphenylporphyrins (M–TPPs) (M5Cr, Co, Zn, and Ru) before and after nitrogen monoxide (NO) adsorption by near edge X-ray absorption fine structure (NEXAFS) spectroscopy. N K-edge NEXAFS spectra obtained from M–TPPs were similar to each other. At NO adsorption, NEXAFS spectra were drastically changed. The difference spectra showed new intense peaks appearing around 404 eV for all M–TPPs. Following previous study, we ascribe this peak to NO adsorbed in the state of NO anion, NO – . On the other hand, small differential peaks below 400 eV corresponding to p * transitions were observed in the difference spectra for ZnTPP and CrTPPCl and were little observed for other M–TPPs. From the comparison of these results and oxidation potentials for M–TPPs, it seems that NO is only adsorbed at ligand for ZnTPP, only at central metal for CoTPP and RuTPP(py) 2 , and at both ligand and central metal for CrTPPCl. Furthermore, there are two difference adsorption states of NO – for adsorption to ligand.
Acknowledgements One of the authors (T.O.) thanks to Mr. Makoto Miyamoto at Mitsubishi Electric Corporation for useful discussions and sample preparation. This work has been performed under the approval of Photon Factory Advisory Committee (proposals 95G372 and 95G374). This work had supported in part by Grantsin-Aid for Scientific Research from the Ministry of Education, Science, Sports and Culture of Japan (07 NP0301 and 07CE2004).
References [1] F.J. Kampas, K. Yamashita, J. Fajer, Nature 284 (1980) 40. [2] Y. Hanazato, T. Kamiya, N. Ohta, M. Miyamoto, S. Isoda, in: A.D. Jamico, C. Di Natale (Eds.), Proceedings of Fifth International Meeting on Chemical Sensors, Sensors and Actuators B24–25, 1995, p. 1148. [3] K. Tsuji, H. Fujitsu, K. Takeshita, I. Mochida, J. Mol. Catal. 9 (1980) 389. [4] F. Steinbach, H.-J. Joswig, J. Catal. 55 (1978) 272.
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[5] A.R. Butler, D.L.H. Williams, Chem. Soc. Rev. 22 (1993) 233. ¨ [6] J. Stohr, NEXAFS Spectroscopy, Springer-Verlag, New York, 1992. [7] S. Narioka, H. Ishii, Y. Ouchi, T. Yokoyama, T. Ohta, K. Seki, J. Phys. Chem. 99 (1995) 1332. [8] S. Mitra-Kirtley, O.C. Mullins, J. van Elp, S. George, J.J. Chen, S.P. Cramer, J. Am. Chem. Soc. 115 (1993) 252. [9] F.C. Brown, R.Z. Bachrach, N. Lien, Nucl. Inst. Methods 152 (1978) 76. [10] M. Yanagihara, H. Maezawa, T. Sasaki, Y. Ikuchi, KEK Rep. 84-17 (1984) 1–19. ¨ [11] J. Stohr, D.A. Outka, Phys. Rev. B 36 (1987) 7891. [12] G.C. King, M. Tronc, F.H. Read, R.C. Bradfort, J. Phys. B 10 (1977) 2479.
[13] R.N. Sodhi, C.E. Brion, J. Electron Spectrosc. Relat. Phenom. 34 (1984) 363. [14] A.P. Hitchcock, C.E. Brion, J. Electron Spectrosc. Relat. Phenom. 18 (1980) 1. [15] A.P. Hitchcock, C.E. Brion, Chem. Phys. 33 (1978) 55. ¨ [16] J.A. Horsley, J. Stohr, A.P. Hitchcock, D.C. Newbury, A.L. Johnson, F. Sette, J. Chem. Phys. 83 (1985) 6099. [17] M. Miyamoto, Y. Hanazato, Nippon Kagaku Kaishi (1998) 338 (in Japanese). [18] N. Saito, I. Suzuki, Phys. Rev. A43 (1991) 3662. [19] A. Wolberg, J. Manassen, J. Am. Chem. Soc. 92 (1970) 2982. [20] G.M. Brown, F.R. Hopf, J.A. Ferguson, T.J. Meyer, D.G. Whitten, J. Am. Chem. Soc. 95 (1973) 5939. [21] C.M. Newton, D.G. Davis, J. Magn. Reson. 20 (1975) 446.
NEXAFS spectra of metallotetraphenylporphyrins with adsorbed nitrogen monoxide a, b b b,c T. Okajima *, Y. Yamamoto , Y. Ouchi , K. Seki a
Experimental Facilities Division, Japan Synchrotron Radiation Research Institute, 1 -1 -1, Kouto, Mikazuki-cho, Sayo-gun, Hyogo 679 -5198, Japan b Department of Chemistry, Graduate School of Science, Nagoya University, Furo-cho, Chikusa-ku, Nagoya 464 -8602, Japan c Research Center for Materials Science, Nagoya University, Furo-cho, Chikusa-ku, Nagoya 464 -8602, Japan Received 8 August 2000; received in revised form 7 September 2000; accepted 9 September 2000
Abstract Near edge X-ray absorption fine structure (NEXAFS) spectra of nitrogen K-edge region were obtained using total electron yield detection for four metallotetraphenylporphyrins (M–TPPs) (M5Cr, Co, Zn and Ru) before and after nitrogen monoxide (NO) adsorption. The observed spectra of all M–TPPs were drastically changed by NO adsorption. The difference spectra showed new intense peaks appearing around 404 eV for all M–TPPs. Furthermore, small peaks below 400 eV corresponding to p * transitions were observed for ZnTPP and CrTPP. Comparison of these results and oxidation potentials for M–TPPs revealed that NO is adsorbed only at ligand for ZnTPP, at only central metal for CoTPP and RuTPP, and at both ligand and the central metal for CrTPP. 2001 Elsevier Science B.V. All rights reserved. Keywords: NEXAFS spectroscopy; Porphyrin; Metallotetraphenylporphyrins; Nitrogen monoxide; Electronic structure; Adsorption
1. Introduction The electronic structure of metallotetraphenylporphyrins (M–TPPs) is of particular interest due to many possible applications, such as catalysts and electronic devices (e.g. gas sensors, electroluminescent displays, and field effect transistors) [1,2]. M– TPPs have a porphyrin macrocycle with a large p -conjugated aromatic system. In order to attain good chemical and physical properties, it is necessary to elucidate their electronic structure. The p electrons in the highest occupied molecular orbital *Corresponding author. Tel.: 181-791-58-2750; fax: 181-791-582752. E-mail address: [email protected] (T. Okajima).
(HOMO) and lowest unoccupied molecular orbital (LUMO) play an important role for the chemical and physical properties. The details of these energy levels depend on the metal in M–TPP. On the other hand, M–TPPs can be used for the contact decomposition materials of nitrogen monoxide (NO) [3,4]. NO is a free radical containing an electron in the antibonding p (p *) orbital. NO can be reduced and oxidized easily as expected from the oxidation-reduction potentials [5]. The studies of the electronic structure of M–TPPs with adsorbed NO molecule and the state of adsorption of NO will give useful information about the contact decomposition of NO. NEXAFS spectroscopy, also known as X-ray absorption near edge structure (XANES), is a recently developed branch of X-ray spectroscopy. It is
0368-2048 / 01 / $ – see front matter 2001 Elsevier Science B.V. All rights reserved. PII: S0368-2048( 00 )00268-1
850
T. Okajima et al. / Journal of Electron Spectroscopy and Related Phenomena 114 – 116 (2001) 849 – 854
useful for the study of unoccupied electronic states and molecular orientation in organic thin films [6]. As for porphyrins, N K-edge NEXAFS study of 5,10,15,20-tetraphenylporphyrin zinc (ZnTPP) [7], free-base 5,10,15,20-tetraphenylporphyrin (H 2 TPP) [7], 5,10,15,20-tetraphenylporphyrin vanadyl (VOTPP) [8], and 2,3,7,8,12,13,17,18-nickeloctaethylporphyrin (NiOEP) [8] were reported. In this work, we applied NEXAFS spectroscopy to four typical porphyrins, ZnTPP, 5,10,15,20-tetraphenylporphyrin cobalt (CoTPP), 5,10,15,20-tetraphenylporphyrin chromium chloride (CrTPPCl), and 5,10,15,20-tetraphenylporphyrin ruthenium dipyridine (RuTPP(py) 2 ). These porphyrins are shown in Fig. 1 together with H 2 TPP. The purpose in this
study is to study the unoccupied electronic structure of M–TPPs with adsorbed NO molecule and to discuss the state of adsorption of NO from N K-edge NEXAFS spectroscopy. Since the HOMO and LUMO contain contribution from the N atoms in porphyrin macrocycle, N K-edge NEXAFS spectroscopy is suitable for this study.
2. Experimental section ZnTPP, CoTPP and CrTPPCl were commercially obtained from Tokyo Chemical Industry Co. Ltd. RuTPP(py) 2 was also commercially obtained from Nard Institute Ltd. Their thin films were prepared by vacuum deposition onto Si (100) wafer at room temperature under vacuum of 10 23 Pa range. The film thickness was about 50 nm as measured by a quartz oscillator. The adsorption of NO on M–TPPs was performed in a glass dedicator filled with helium gas with 5% NO at room temperature. The reaction time was about 1 h. N K-edge NEXAFS spectra were measured at the soft X-ray beamline 11A of Photon Factory at Institute of Materials Structure Science. Synchrotron radiation from the storage ring was monochromatized by a Grasshopper monochromator with a grating of 2400 lines / mm [9,10]. The spectra were obtained in the total electron yield mode using a channeltron with a base pressure of 10 26 Pa. X-ray incidence angle from sample surface was 558 (so-called magic angle). At this incidence angle, possible effects of preferred molecular orientation can be removed for samples without azimuthal order, as expected for the present case [6,11]. The photon energy was calibrated using 7,7,8,8-tetracyanoquinodimethane (TCNQ) in standard material, which was calibrated using the vapor spectra of Ar [12], N 2 [13], O 2 [14], and SF 6 [15]. The main peak energy of TCNQ at the N K-edge region was taken to be at 399.7 eV.
3. Results and discussions
Fig. 1. Structural chemical formulas of porphyrins: CoTPP (a), CrTPPCl (b), ZnTPP (c), RuTPP(py) 2 (d), and H 2 TPP (e).
Fig. 2 shows the N K-edge NEXAFS spectra of (a) CrTPPCl, (b) CoTPP, (c) ZnTPP, and (d) RuTPP(py) 2 thin films. Regardless of the difference
T. Okajima et al. / Journal of Electron Spectroscopy and Related Phenomena 114 – 116 (2001) 849 – 854
Fig. 2. N K-edge NEXAFS spectra of CrTPPCl (a), CoTPP (b), ZnTPP (c), and RuTPP(py) 2 (d).
in the central metal, the general appearance of the spectra is similar. There are three peaks A, B, and C at low photon energy, and two peaks D, and E at higher energy. These two groups correspond to the excitations to p * and s * orbitals, respectively [7]. The peak A observed in the spectrum of RuTPP(py) 2 contains contribution from the strong p * resonance
851
of pyridine (py) at |400 eV [16]. These features were assigned in detail by Narioka et al. using polarization dependence of NEXAFS spectra and comparison with CNDO / S2 molecular orbital calculation with a limited configuration interaction for free base porphyrin and beryllium porphyrin [7]. In M– TPPs, all four nitrogen atoms in porphyrin macrocycle are equivalent. The energies and assignments of the features are listed in Table 1. Fig. 3 shows the NEXAFS spectra of M–TPPs without and with adsorbed NO. All spectra are changed by NO adsorption. In the case of ZnTPP and CrTPPCl, three new peaks a, b and c are seen. On the other hand, only two new peaks a and c are seen for RuTPP(py) 2 and CoTPP. In all spectra, intense two peaks A and B assigned to p * transitions at low photon energy are broadened. These results show that NO molecule is adsorbed on all M–TPPs used in this study. On the other hand, in the study of fourier transfer infrared (FT–IR) spectroscopy for H 2 TPP and M–TPPs (M5Cr, Co, Zn, Ru, and Ag), it was reported that NO was adsorbed only on CoTPP [17]. The origin of this difference is not clear at present. NEXAFS spectrum of reaction species by NO adsorption is obtained by subtracting appropriately scaled spectrum before NO adsorption from the spectrum after NO adsorption. The scaling coefficient was determined to make the intensity of the peak A equal before and after NO adsorption. The obtained difference spectra for the M–TPPs are also shown in Fig. 3 together with the NEXAFS spectra before and after NO adsorption. These spectra are similar to each other. Weak peaks corresponding to peaks A and B are observed below 400 eV for all M–TPPs, and these peaks are shifted to the
Table 1 Peak energies and assignments of N K-edge NEXAFS spectra for four metallotetraphenylporphyrins (M–TPPs) (M5Cr, Co, Zn, and Ru) before nitrogen monoxide adsorption Feature
A B C D E
Energy / eV
Assignment
CrTPPCl
CoTPP
ZnTPP
RuTPP(py) 2
398.5 401.3 404.5 406.7 416
398.7 401.4 404.5 406.7 416
398.4 401.3 404.0 406.5 416
398.8 401.4 404.5 406.8 416
p * (e g ) p * (b2 u , e g ) p * (a2 u ) s* s*
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Fig. 3. N K-edge NEXAFS spectra of M–TPPs before (a), after NO adsorption (b), and the difference spectra (c).
low photon energy side by NO adsorption. In the case of ZnTPP, the peaks below 400 eV in the differential shape are most intense indicating large shift. Those of CrTPPCl shows medium intensity,
and they are weak for CoTPP and RuTPP(py) 2 . Above 400 eV two intense peaks a and c are observed at 403.7 eV and 414 eV for CoTPP and RuTPP(py) 2 , while ZnTPP and CrTPPCl show three
T. Okajima et al. / Journal of Electron Spectroscopy and Related Phenomena 114 – 116 (2001) 849 – 854
intense peaks a, b and c at 403.7 eV, 405.2 eV and 414 eV, respectively. Considering their peak energies, we may suppose that peaks a and b are p * transitions and peak c is s * transition. NEXAFS spectrum of NO molecule at N K-edge shows an intense peak attributed to N 1s→p * transition at 399.7 eV [18]. The energy of the lowest peak a observed in this study at 403.7 eV is much different from that of NO molecule. Tsuji et al. and Miyamoto et al. reported that NO is adsorbed on CoTPP in the state of NO anion, NO – , using FT–IR spectroscopy [3,17]. Thus we can attribute the peak a at 403.7 eV to NO – . Similarity of the difference spectra suggests the peaks a and b in other M–TPP can be also attributed to NO – . Oxidation potentials for M–TPPs showed that the first oxidation occurs at the central metal for CoTPP, RuTPP(py) 2 , and CrTPPCl, whereas ligand oxidation occurs first for ZnTPP [19–21]. Subsequent oxidations also occur at the ligand in all cases [19–21]. From the difference of these oxidation potentials, it is expected that NO – is formed by the electron transfer from the ligand for ZnTPP and that NO – is adsorbed to the ligand. In the case of CrTPPCl, the first and the second oxidation potentials are close (0.2 eV difference), and the similarity of the difference spectrum with that of ZnTPP suggests electron transfer may occur from the ligand. Note that the differential peaks A and B below 400 eV in the difference spectra corresponding to p * transition for ZnTPP and CrTPPCl are more significant than those for CoTPP and RuTPP(py) 2 . This indicats that p * orbitals of ZnTPP and CrTPPCl are actually affected by the adsorption of NO at the ligand and thus supports the above arguments. On the other hand, the differential peaks A and B below 400 eV for CoTPP and RuTPP(py) 2 are less significant. This suggests that NO is adsorbed predominantly on central metal for CoTPP and RuTPP(py) 2 . The difference from the case of ZnTPP and CrTPPCl may be attributed to the large difference between first oxidation potential and second oxidation potential (0.67 eV and 1.05 eV for CoTPP and RuTPP(py) 2 , respectively), which hinders the electron transfer from the ligand [19,21]. The result that two intense peaks a and b are observed in the spectra of ZnTPP and CrTPPCl may indicate that there are two difference adsorption states of NO – for ligand.
853
4. Conclusions In this work, we studied four kinds of metallotetraphenylporphyrins (M–TPPs) (M5Cr, Co, Zn, and Ru) before and after nitrogen monoxide (NO) adsorption by near edge X-ray absorption fine structure (NEXAFS) spectroscopy. N K-edge NEXAFS spectra obtained from M–TPPs were similar to each other. At NO adsorption, NEXAFS spectra were drastically changed. The difference spectra showed new intense peaks appearing around 404 eV for all M–TPPs. Following previous study, we ascribe this peak to NO adsorbed in the state of NO anion, NO – . On the other hand, small differential peaks below 400 eV corresponding to p * transitions were observed in the difference spectra for ZnTPP and CrTPPCl and were little observed for other M–TPPs. From the comparison of these results and oxidation potentials for M–TPPs, it seems that NO is only adsorbed at ligand for ZnTPP, only at central metal for CoTPP and RuTPP(py) 2 , and at both ligand and central metal for CrTPPCl. Furthermore, there are two difference adsorption states of NO – for adsorption to ligand.
Acknowledgements One of the authors (T.O.) thanks to Mr. Makoto Miyamoto at Mitsubishi Electric Corporation for useful discussions and sample preparation. This work has been performed under the approval of Photon Factory Advisory Committee (proposals 95G372 and 95G374). This work had supported in part by Grantsin-Aid for Scientific Research from the Ministry of Education, Science, Sports and Culture of Japan (07 NP0301 and 07CE2004).
References [1] F.J. Kampas, K. Yamashita, J. Fajer, Nature 284 (1980) 40. [2] Y. Hanazato, T. Kamiya, N. Ohta, M. Miyamoto, S. Isoda, in: A.D. Jamico, C. Di Natale (Eds.), Proceedings of Fifth International Meeting on Chemical Sensors, Sensors and Actuators B24–25, 1995, p. 1148. [3] K. Tsuji, H. Fujitsu, K. Takeshita, I. Mochida, J. Mol. Catal. 9 (1980) 389. [4] F. Steinbach, H.-J. Joswig, J. Catal. 55 (1978) 272.
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