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Electronic structure of lithium phthalocyanine studied by ultraviolet photoemission spectroscopy
Chemical Physics 253 (2000) 125±131
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Electronic structure of lithium phthalocyanine studied by ultraviolet photoemission spectroscopy Takafumi Kimura a, Michinori Sumimoto a, Shigeyoshi Sakaki b, Hitoshi Fujimoto c,*, Yukinobu Hashimoto c, Susumu Matsuzaki c b
a Graduate School of Science and Technology, Kumamoto University, Kurokami 2-39-1, Kumamoto 860-8555, Japan Department of Applied Chemistry and Biochemistry, Faculty of Engineering, Kumamoto University, Kurokami 2-39-1, Kumamoto 860-8555, Japan c Department of Chemistry, Faculty of Science, Kumamoto University, Kurokami 2-39-1, Kumamoto 860-8555, Japan
Received 14 September 1999
Abstract The electronic structure of the stable organic radical compound, lithium phthalocyanine (LiPc), was investigated using ultraviolet photoemission spectroscopy (UPS). The observed UPS spectra were compared with those of metal free phthalocyanine (H2 Pc) and zinc phthalocyanine (ZnPc). The spectral features of LiPc and ZnPc were analyzed with the aid of ab initio MO (molecular orbital) and density functional theory (DFT) methods. These three compounds showed similar spectral features, and the binding energy of each spectral structure had almost the same value. In addition to this, the ionization threshold of LiPc in the solid state was found to be nearly equal to those of the other compounds: the values are 4.95 eV for LiPc, 5.12 eV for H2 Pc, and 5.03 eV for ZnPc. These observations show that these phthalocyanines have similar electronic structures in the top region of the valence band and that the radical electron exists in the molecular orbital of the phthalocyanine ring, which is the highest occupied molecular orbital (HOMO) of H2 Pc and ZnPc. In contrast to this, the work function of LiPc was about 0.2 eV higher than those of other compounds, and its value was found to be 4.43 eV. This means that the unoccupied states of LiPc should dier from the closed shell compounds such as H2 Pc and ZnPc. Ó 2000 Elsevier Science B.V. All rights reserved. Keywords: Lithium phthalocyanine; Radical; Ultraviolet photoemission spectroscopy; Electronic structure
1. Introduction Phthalocyanine compounds with the formula MPc (Pc phthalocyaninato anion C32 H16 N2± 8 , M H2 or divalent metals) have been widely used as organic dye stu because of their intense absorption of light in the visible and ultraviolet regions, their excellent stability to chemical or thermal treatments, and their relatively low cost *
Corresponding author.
[1±4]. Pcs have a macrocyclic ring with a large p-conjugated aromatic system consisting of four benzoisoindole units. The most stable structure of MPc has been shown to be square planar and to be classi®ed into a D4h point group [5±7]. In the case of M H2 , the molecular symmetry is reduced to D2h due to two hydrogen atoms in the middle of the Pc ring. The radical species of MPc have also been synthesized with monovalent lithium or trivalent lanthanoid, and it has been shown that these radical compounds are stable even in air [8]. The schematic structure of lithium phthalocyanine
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T. Kimura et al. / Chemical Physics 253 (2000) 125±131
Fig. 1. Structure of LiPc.
(LiPc) is shown in Fig. 1 with an electron pair, an unpaired electron and a lithium ion in an inner ring of the Pc macrocycle. The real structure of LiPc is expected to be also square planar, and an unpaired electron would be delocalized on the whole p-electron system. It is well known that the solid state of MPc shows polymorphism. Especially, thin ®lms of MPc (M H2 , Cu, Ni, Co, Zn, etc.) deposited on a glass substrate have at least two polymorphs of a metastable a and a stable b forms. The a form can be prepared by vacuum-depositing on the glass substrate kept at room temperature, whereas the b form can be obtained by heat treatment of the a form or exposure of the a form to certain organic solvents [9,10]. The intermolecular distance of MPc should be dierent for these crystal systems, which alters the strength of the intermolecular interaction. It has been actually reported that the optical and electric properties depend on the crystal systems. A great dierence was especially observed in the ultraviolet and visible (UV±VIS) absorption spectra for each crystal system of LiPc [11] and H2 Pc. Other MPcs such as zinc phthalocyanine (ZnPc) also show a spectral change depending on the crystal structure similar to H2 Pc [9]. The origin of these dierences can be considered to be due to the electronic structures of these compounds, which would dier from each other among these crystal systems.
The ultraviolet photoemission spectroscopy (UPS) is a powerful method to investigate the electronic structure in the top region of the valence band. The values such as the photoionization threshold and the work function, which characterize the electronic structure of the valence band, can be accurately obtained. These values should be aected by the intermolecular interaction and the existence of the unpaired electron. In this article, UPS has been applied for the purpose of investigating the electronic structures of LiPc. The eects of the molecular symmetry and the unpaired electron were carefully examined by comparing the UPS spectra to those of H2 Pc and ZnPc. The spectral features of LiPc and ZnPc were analyzed with the aid of theoretical calculations. 2. Experimental LiPc was prepared according to the method described by Petit et al. [12]. ZnPc and H2 Pc were purchased from Tokyo Kasei Co. Ltd. and puri®ed by vacuum sublimation just before use. Purities of these compounds were checked by infrared and UV±VIS spectroscopies. Thin ®lms for UPS measurements were prepared by vapor deposition on a copper substrate kept at room temperature under vacuum. The deposition rate and the ®lm thickness were monitored with the use of a quartz thickness monitor, and were controlled at 1 nm/ min and 40 nm, respectively. Characterization of the ®lms, such as the purity and the polymorph, was performed by vibrational and electronic absorption spectroscopies [9±11]. The UPS system contained a Minuteman 302VM monochromator with a 20 cm diameter Rowland circle. A Hinteregger-type hydrogen discharge lamp was attached to the monochromator providing ultraviolet light from 6 to 12 eV. The helium resonance line (He I, 21.2 eV) was also used for the excitation light of UPS measurements. Electron energy analysis was carried out using a spherical retarding-®eld-type analyzer coated with gold on its inner side [13,14]. The photocurrent (I ) was monitored using an ADVANTEST TR8652 digital electrometer by varying the retarding potential (VR ) controlled by a microcomputer, and
T. Kimura et al. / Chemical Physics 253 (2000) 125±131
the data were stored in the computer. The energy distribution curves were obtained by dierentiating the I±VR data as shown in Fig. 2. The work function of the electron-energy analyzer (/a ) was determined to be 4.21 eV by measuring the ionization threshold of HOPG (highly oriented pyrolytic graphite) cleaved in vacuum. The electric contact was achieved by using silver paste. The ionization threshold energy observed for HOPG was found to be 4.71 eV, which is in good agreement with the reported value of 4:7 0:1 eV [15]. The value of /a was checked before and after the measurements of the samples, and no obvious dierence was observed. The geometry of ZnPc was optimized at the AM1 [16,17], PM3 [18±20] and ab initio MO methods. In the HF calculation, two kinds of basis sets, STO-3G and 3-21G [21,22] were used. The geometry of LiPc was optimized at the ROHF
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level. In this calculation, the basis sets of 3-21G [22] were used. The G A U S S I A N 9 4 [23] program was used for these calculations. All geometries were fully optimized and energy minima located without any symmetry restrictions. 3. Results and discussion A typical photoemission spectrum obtained by hydrogen discharge is shown along the VR scale in Fig. 2. The saturation (VS ) and the stopping (V0 ) voltages can be determined by linearly extrapolating as shown in this ®gure. The absence of tailing in the region around VS and the small values of VS armed that the samples were free from electrostatic charging. The ionization energy (Is ) relative to the vacuum level is given by using the photon energy (hm) [24], Is hm ÿ e VR ÿ VS ;
1
where the subscript s of Is emphasizes that the ionization energy belongs to the solid state, and e is the elementary electric charge. In particular, the ionization threshold (Isth ) is obtained by Isth hm ÿ e V0 ÿ VS :
Fig. 2. Photocurrent curve (upper) and photoemission spectrum (lower) for an LiPc ®lm obtained at hm 7.75 eV. The spectrum is shown along the retarding potential (VR ) scale. The positions denoted by VS and V0 are the saturation and the stopping voltages, respectively.
2
Fig. 3 depicts the UPS spectra measured for the deposited ®lm of LiPc on the Is scale. These spectra were obtained by hydrogen discharge except for the one at hm 21.2 eV, which was measured with He I. In these spectra, several structures depicted by A±C are observed, as shown in the ®gure. In order to precisely and accurately determine their ionization energies, the positions of V0 and these spectral features are plotted as the function of hm in Fig. 4. Using the above equations and the least-squares method, Isth and Is of the spectral features are obtained and the values are given in Table 1. The value of the work function of the samples, /s , can be estimated from the values of /a and VS of the sample as [15,24] /s /a eVS :
3
Using the value of /a 4.21 eV and the average of the data shown in Fig. 3 for VS 0.22 eV, /s for
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Fig. 4. Graphical determination of the ionization potentials for the LiPc thin ®lm. The positions of V0 , VS and spectral features are indicated by , D and +, respectively.
Fig. 3. The UPS spectra of the LiPc thin ®lm and simulated spectrum on the Is scale. The simulated spectrum was calculated by convoluting delta functions located at each orbital energy, which was evaluated by the DFT calculation, with a Gaussian function. The spectral features are denoted by the characters A±C.
the deposited ®lm of LiPc is estimated to be 4.43 eV. The measurements were repeated several times using dierent ®lms; however, no obvious dierence was observed for the values of Is , Isth and /s . The ab initio MO calculations were performed to analyze the UPS spectra. Using the geometry of LiPc optimized by the ROHF level with the 3-21G basis set, the orbital energy was evaluated by the UHF and the density functional theory (DFT) calculations. For comparison with the observed UPS spectra, a trial simulated spectrum was calculated, as shown in Fig. 3, by convoluting delta functions located at each orbital energy with a Gaussian function without correction of crosssection eects. In order to investigate the eects of the molecular symmetry and the unpaired electron on the electronic structure, similar measurements were
performed for the deposited ®lms of H2 Pc and ZnPc. Figs. 5 and 6 compare the UPS spectra of three Pcs taken at hm 7.75 and 10.33 eV, respectively, and the values of Is , Isth and /s for H2 Pc and ZnPc are also given in Table 1. As seen from Figs. 5 and 6, the UPS spectral pattern of these three compounds resemble one another. This means that the top region of the valence band consists of the p-molecular orbitals of the Pc ring, and there is no contribution from the inner hydrogen or metal atoms. The theoretical calculations also show that the molecular orbital with an unpaired electron has a p character. A careful examination of the UPS spectra shown in Fig. 5 for three Pcs reveals several differences. Firstly, the peak A of LiPc is depressed in its intensity as compared with H2 Pc and ZnPc. For the closed shell system, such as H2 Pc and ZnPc, the highest occupied molecular orbital (HOMO) Table 1 Energy values of spectral features
a b
Sample
Isth
Is (A)
Is (B)
Is (C)
/s
Eg a
LiPc H2 Pc ZnPc
4.95 5.12 5.03
5.43 5.59 5.62
7.52 8.10 7.77
8.76 ±b ±b
4.43 4.26 4.19
1.04 1.72 1.68
Estimated values from twice the dierence between Isth and /s . This structure could not be clearly observed.
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Fig. 5. Comparison of the UPS spectra obtained for the deposited ®lms of LiPc, ZnPc and H2 Pc at hm 7.75 eV on the Is scale.
contains two electrons and the ionization of the electron from this orbital corresponds to peak A of the observed UPS spectra. On the contrary, in LiPc, the singly occupied molecular orbital (SOMO) corresponds to this peak. The theoretical calculation shows that both orbitals have the same p character, and it would be expected that the cross-sections of ionization from these orbitals are nearly equal. Therefore, the origin of the dierence in intensity of peak A is thought to be the number of the occupied electrons in the orbital related to the peak. Secondly, peak A was observed at a slightly dierent position between LiPc and H2 Pc or ZnPc: the binding energy for LiPc is lower than that of the others (see the values of Is listed in Table 1). The work function of LiPc is also higher by about 0.2 eV than those of other compounds. LiPc tends to show the smallest value of Isth among the three compounds studied; however, the dierence in the values is not signi®cant and the values of Isth are
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Fig. 6. Comparison of the UPS spectra obtained for the deposited ®lms of LiPc, ZnPc and H2 Pc at hm 10.33 eV on the Is scale.
within 5:0 0:1 eV for the three Pcs. The band gap was estimated from twice the dierence between the values of Isth and /s assuming that these compounds are intrinsic semiconductors, and the obtained values are listed in the last column of Table 1. These estimated values of the band gap should be identical to the position of the absorption onset. Fig. 7 shows the absorption spectra of the deposited ®lms on glass substrates for LiPc, H2 Pc and ZnPc. These spectra are similar to the reported absorption spectra [9,11]. The values of the band gap estimated by UPS spectroscopy show quite a good coincidence with the absorption onset of about 0.6 eV for the deposited ®lm of LiPc and 1.6 eV for H2 Pc and ZnPc. Two reasons correspond to the origin of the above-mentioned variations between LiPc and other Pcs: the dierence in the crystal system and the existence of an unpaired electron. H2 Pc and ZnPc are crystallized on a copper substrate kept at room temperature by vacuum sublimation into the
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T. Kimura et al. / Chemical Physics 253 (2000) 125±131
Fig. 7. Absorption spectra of the deposited ®lms on glass substrates for LiPc, ZnPc and H2 Pc.
a form, which has not been well characterized as yet and of which two crystal systems, tetragonal and monoclinic, have been reported until now [25,26]. In the case of LiPc, it also shows a similar polymorphic change depending on the substrate temperature: the dominant phase is a tetragonal system (P4/mcc) for the substrate temperature below 150°C, whereas a monoclinic phase (C2/c) dominates at the substrate temperatures between 150°C and 200°C [27]. All samples used in this study were prepared by vacuum sublimation on the copper substrate kept at room temperature, and the infrared and UV±VIS absorption spectra showed that the ®lms of H2 Pc and ZnPc consist of the a form and that the space group of the LiPc ®lm is P4/mcc. The intermolecular distance of the Pc ring should be dierent for these crystal systems, which alters the strength of the intermolecular interaction. Both forms of H2 Pc and ZnPc seem to have the same intermolecular interaction because they all show similar UV±VIS spectra, whereas LiPc shows quite dierent spectra depending on the crystal system. There is some possibility that the a form of H2 Pc and ZnPc has a dierent crystal system from P4/mcc of LiPc and a
dierent strength of intermolecular interaction from LiPc. Therefore, it seems that the dierences in the region of peak A originate from the variation of the intermolecular interaction due to the dierent crystal systems. However, the good agreement of the observed UPS spectrum with the calculated one shows that the observed UPS spectra of the LiPc ®lm corresponds to the electronic structure of the free LiPc molecule; therefore, it should not be assumed that the observed dierence is due to the variation of the intermolecular interaction. The most plausible origin of the dierences observed in the region of peak A is the eect of the unpaired electron. Our theoretical calculations show that the orbital energies of HOMO for ZnPc and SOMO for LiPc are nearly equal, but the lowest unoccupied molecular orbital (LUMO) of LiPc is much lower in energy than the LUMO of ZnPc. Actually, the observed UPS spectra of LiPc and ZnPc show a similarity in the region of peak A except for the intensity; therefore, the dierences between LiPc and other Pcs in the values of /s and the band gap could be due to the unoccupied states. Finally, there are some spectral changes due to the central elements of the Pc ring in the regions of peaks B and C: the position of peak B shifts to the low-energy side in the order of H2 Pc, ZnPc and LiPc, and peak C is clearly observed in LiPc but could not be detected in the other Pcs. The calculated UPS spectra for both LiPc and ZnPc can simulate the observed ones, and the calculations show that the central elements of the Pc rings contribute to the orbitals in these region. Therefore, it might be tentatively assigned that the variation in the regions of peaks B and C is due to the contributions of the central elements in the Pc rings to the molecular orbitals. The detailed investigation of the calculated results is being carried out now, and will be published soon elsewhere. 4. Conclusion We measured the UPS spectra of the deposited ®lms for LiPc (P4/mcc), H2 Pc (a form) and ZnPc (a form), and the spectral features of LiPc were analyzed with the aid of the theoretical calcula-
T. Kimura et al. / Chemical Physics 253 (2000) 125±131
tions. The UPS spectral patterns of these three compounds resemble one another. This means that the top region of the valence band consists of the p-molecular orbitals of the Pc ring. At the same time, there are several dierences. The lowest binding-energy peak of LiPc is depressed in its intensity as compared with H2 Pc and ZnPc. This could be explained by the number of electrons in the HOMO: the SOMO corresponds to the peak in the case of LiPc, whereas the HOMO of H2 Pc and ZnPc contains two electrons. The position of this peak of LiPc is slightly lower than the others and the work function of LiPc is higher, about 0.2 eV than those of the others. We consider two reasons: the dierence in the crystal system and the existence of the unpaired electron. The observed UPS spectrum of LiPc agrees well with the simulated one from the calculated orbital energy of the free LiPc molecule. Thus, we consider the most plausible origin of the dierences in the peak position and the work function should be attributed to the unpaired electron. Moreover, the peak located at around 8 eV is observed at dierent positions to each other. It might be tentatively assigned that the shift is caused by the contributions of the central elements in Pc rings to the molecular orbitals in this region. Acknowledgements The authors would like to thank Mr. Toshiaki Noda for preparing the vacuum sublimation system. The calculations were carried out by using the SP2 computer at Computer Center of Institute for Molecular Science (Okazaki, Japan). This work was partly supported by Saneyoshi Scholarship Foundation through Grant 09-12 and Grant-inAid for Scienti®c Research on Priority Areas ``Molecular Physical Chemistry'' (11166253) from the Ministry of Education, Culture, Sports and Science. References [1] C.C. Lezno, A.B.P. Lever, in: Phthalocyanines, Properties and Applications, VCH, Weinheim, vol. 1, 1989.
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[2] C.C. Lezno, A.B.P. Lever, in: Phthalocyanines, Properties and Applications, VCH, Weinheim, vol. 2, 1993. [3] C.C. Lezno, A.B.P. Lever, in: Phthalocyanines, Properties and Applications, VCH, Weinheim, vol. 3, 1994. [4] C.C. Lezno, A.B.P. Lever, in: Phthalocyanines, Properties and Applications, VCH, Weinheim, vol. 4, 1996. [5] W.R. Scheidt, W. Dow, J. Am. Chem. Soc. 99 (1977) 1101. [6] J.F. Kirner, W. Dow, W.R. Scheidt, Inorg. Chem. 15 (1976) 1687. [7] T. Kobayashi, F. Kurokawa, N. Uyeda, E. Suito, Spectrochim. Acta 26A (1970) 1305. [8] P. Turek, P. Petit, J.J. Andre, J. Simon, R. Even, B. Boudjema, G. Guillaud, M. Maitrot, J. Am. Chem. Soc. 109 (1987) 5119. [9] E.A. Lucia, F.D. Verderame, J. Chem. Phys. 48 (1968) 2674. [10] J.H. Sharp, M. Abkowitz, J. Phys. Chem. 77 (1973) 477. [11] H. Homborg, C.L. Teske, Z. Anorg. Allg. Chem. 527 (1985) 45. [12] M.A. Petit, T. Thami, R. Even, J. Chem. Soc. Chem. Commun. (1989) 1059. [13] A.A. Zagrubskii, F.I. Vilesov Fiz. Tverd. Tela 13 (1971) 2300 [Sov. Phys. Solid State 13 (1972) 1927]. [14] T. Hirooka, K. Tanaka, M. Fujihira, H. Inokuchi, Y. Harada, K. Kuchitsu, Chem. Phys. Lett. 18 (1973) 930. [15] L. Apker, E. Taft, J. Dickey, Phys. Rev. 74 (1948) 1462. [16] M.J.S. Dewar, K.M. Merz Jr., Organometallics 7 (1983) 522. [17] M.J.S. Dewar, E.G. Zoebisch, E.F. Healy, J.J.P. Stewart, J. Am. Chem. Soc. 107 (1985) 3902. [18] J.J.P. Stewart, J. Comput. Chem. 10 (1989) 209. [19] J.J.P. Stewart, J. Comput. Chem. 10 (1989) 221. [20] J.J.P. Stewart, J. Comput. Chem. 12 (1991) 320. [21] K.D. Dobbs, W.J. Hehre, J. Comp. Chem. 8 (1987) 861. [22] J.S. Binkley, J.A. Pople, W.J. Hehre, J. Am. Chem. Soc. 102 (1980) 939. [23] M.J. Frisch, G.W. Trucks, H.B. Schlegel, P.M.W. Gill, B.G. Johnson, M.A. Robb, J.R. Cheeseman, T.A. Keith, G.A. Petersson, J.A. Montgomery, K. Raghavachari, M.A. Al-Laham, V.G. Zakrzewski, J.V. Ortiz, J.B. Foresman, J. Cioslowski, B.B. Stefanov, A. Nakayakkara, M. Challacombe, C.Y. Peng, P.Y. Ayala, W. Chen, M.W. Wong, J.L. Andres, E.S. Rreplogle, R. Gomperts, R.L. Martin, D.J. Fox, J. Baker, J.P. Stewart, M. HeadGordon, C. Gonzalez, J.A. Pople, G A U S S I A N 9 4 , Gaussian, Pittsburgh, PA, 1995. [24] Y. Harada, H. Inokuchi, Bull. Chem. Soc. Jpn. 39 (1966) 1443. [25] M.T. Robinson, G.E. Klein, J. Am. Chem. Soc. 74 (1952) 6294. [26] M. Ashida, N. Uyeda, E. Suito, Bull. Chem. Soc. Jpn. 39 (1966) 2616. [27] M. Brinkmann, J.C. Wittmann, C. Chaumont, J.J. Andre, Thin Solid Films 292 (1997) 192.
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Electronic structure of lithium phthalocyanine studied by ultraviolet photoemission spectroscopy Takafumi Kimura a, Michinori Sumimoto a, Shigeyoshi Sakaki b, Hitoshi Fujimoto c,*, Yukinobu Hashimoto c, Susumu Matsuzaki c b
a Graduate School of Science and Technology, Kumamoto University, Kurokami 2-39-1, Kumamoto 860-8555, Japan Department of Applied Chemistry and Biochemistry, Faculty of Engineering, Kumamoto University, Kurokami 2-39-1, Kumamoto 860-8555, Japan c Department of Chemistry, Faculty of Science, Kumamoto University, Kurokami 2-39-1, Kumamoto 860-8555, Japan
Received 14 September 1999
Abstract The electronic structure of the stable organic radical compound, lithium phthalocyanine (LiPc), was investigated using ultraviolet photoemission spectroscopy (UPS). The observed UPS spectra were compared with those of metal free phthalocyanine (H2 Pc) and zinc phthalocyanine (ZnPc). The spectral features of LiPc and ZnPc were analyzed with the aid of ab initio MO (molecular orbital) and density functional theory (DFT) methods. These three compounds showed similar spectral features, and the binding energy of each spectral structure had almost the same value. In addition to this, the ionization threshold of LiPc in the solid state was found to be nearly equal to those of the other compounds: the values are 4.95 eV for LiPc, 5.12 eV for H2 Pc, and 5.03 eV for ZnPc. These observations show that these phthalocyanines have similar electronic structures in the top region of the valence band and that the radical electron exists in the molecular orbital of the phthalocyanine ring, which is the highest occupied molecular orbital (HOMO) of H2 Pc and ZnPc. In contrast to this, the work function of LiPc was about 0.2 eV higher than those of other compounds, and its value was found to be 4.43 eV. This means that the unoccupied states of LiPc should dier from the closed shell compounds such as H2 Pc and ZnPc. Ó 2000 Elsevier Science B.V. All rights reserved. Keywords: Lithium phthalocyanine; Radical; Ultraviolet photoemission spectroscopy; Electronic structure
1. Introduction Phthalocyanine compounds with the formula MPc (Pc phthalocyaninato anion C32 H16 N2± 8 , M H2 or divalent metals) have been widely used as organic dye stu because of their intense absorption of light in the visible and ultraviolet regions, their excellent stability to chemical or thermal treatments, and their relatively low cost *
Corresponding author.
[1±4]. Pcs have a macrocyclic ring with a large p-conjugated aromatic system consisting of four benzoisoindole units. The most stable structure of MPc has been shown to be square planar and to be classi®ed into a D4h point group [5±7]. In the case of M H2 , the molecular symmetry is reduced to D2h due to two hydrogen atoms in the middle of the Pc ring. The radical species of MPc have also been synthesized with monovalent lithium or trivalent lanthanoid, and it has been shown that these radical compounds are stable even in air [8]. The schematic structure of lithium phthalocyanine
0301-0104/00/$ - see front matter Ó 2000 Elsevier Science B.V. All rights reserved. PII: S 0 3 0 1 - 0 1 0 4 ( 9 9 ) 0 0 3 8 1 - X
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T. Kimura et al. / Chemical Physics 253 (2000) 125±131
Fig. 1. Structure of LiPc.
(LiPc) is shown in Fig. 1 with an electron pair, an unpaired electron and a lithium ion in an inner ring of the Pc macrocycle. The real structure of LiPc is expected to be also square planar, and an unpaired electron would be delocalized on the whole p-electron system. It is well known that the solid state of MPc shows polymorphism. Especially, thin ®lms of MPc (M H2 , Cu, Ni, Co, Zn, etc.) deposited on a glass substrate have at least two polymorphs of a metastable a and a stable b forms. The a form can be prepared by vacuum-depositing on the glass substrate kept at room temperature, whereas the b form can be obtained by heat treatment of the a form or exposure of the a form to certain organic solvents [9,10]. The intermolecular distance of MPc should be dierent for these crystal systems, which alters the strength of the intermolecular interaction. It has been actually reported that the optical and electric properties depend on the crystal systems. A great dierence was especially observed in the ultraviolet and visible (UV±VIS) absorption spectra for each crystal system of LiPc [11] and H2 Pc. Other MPcs such as zinc phthalocyanine (ZnPc) also show a spectral change depending on the crystal structure similar to H2 Pc [9]. The origin of these dierences can be considered to be due to the electronic structures of these compounds, which would dier from each other among these crystal systems.
The ultraviolet photoemission spectroscopy (UPS) is a powerful method to investigate the electronic structure in the top region of the valence band. The values such as the photoionization threshold and the work function, which characterize the electronic structure of the valence band, can be accurately obtained. These values should be aected by the intermolecular interaction and the existence of the unpaired electron. In this article, UPS has been applied for the purpose of investigating the electronic structures of LiPc. The eects of the molecular symmetry and the unpaired electron were carefully examined by comparing the UPS spectra to those of H2 Pc and ZnPc. The spectral features of LiPc and ZnPc were analyzed with the aid of theoretical calculations. 2. Experimental LiPc was prepared according to the method described by Petit et al. [12]. ZnPc and H2 Pc were purchased from Tokyo Kasei Co. Ltd. and puri®ed by vacuum sublimation just before use. Purities of these compounds were checked by infrared and UV±VIS spectroscopies. Thin ®lms for UPS measurements were prepared by vapor deposition on a copper substrate kept at room temperature under vacuum. The deposition rate and the ®lm thickness were monitored with the use of a quartz thickness monitor, and were controlled at 1 nm/ min and 40 nm, respectively. Characterization of the ®lms, such as the purity and the polymorph, was performed by vibrational and electronic absorption spectroscopies [9±11]. The UPS system contained a Minuteman 302VM monochromator with a 20 cm diameter Rowland circle. A Hinteregger-type hydrogen discharge lamp was attached to the monochromator providing ultraviolet light from 6 to 12 eV. The helium resonance line (He I, 21.2 eV) was also used for the excitation light of UPS measurements. Electron energy analysis was carried out using a spherical retarding-®eld-type analyzer coated with gold on its inner side [13,14]. The photocurrent (I ) was monitored using an ADVANTEST TR8652 digital electrometer by varying the retarding potential (VR ) controlled by a microcomputer, and
T. Kimura et al. / Chemical Physics 253 (2000) 125±131
the data were stored in the computer. The energy distribution curves were obtained by dierentiating the I±VR data as shown in Fig. 2. The work function of the electron-energy analyzer (/a ) was determined to be 4.21 eV by measuring the ionization threshold of HOPG (highly oriented pyrolytic graphite) cleaved in vacuum. The electric contact was achieved by using silver paste. The ionization threshold energy observed for HOPG was found to be 4.71 eV, which is in good agreement with the reported value of 4:7 0:1 eV [15]. The value of /a was checked before and after the measurements of the samples, and no obvious dierence was observed. The geometry of ZnPc was optimized at the AM1 [16,17], PM3 [18±20] and ab initio MO methods. In the HF calculation, two kinds of basis sets, STO-3G and 3-21G [21,22] were used. The geometry of LiPc was optimized at the ROHF
127
level. In this calculation, the basis sets of 3-21G [22] were used. The G A U S S I A N 9 4 [23] program was used for these calculations. All geometries were fully optimized and energy minima located without any symmetry restrictions. 3. Results and discussion A typical photoemission spectrum obtained by hydrogen discharge is shown along the VR scale in Fig. 2. The saturation (VS ) and the stopping (V0 ) voltages can be determined by linearly extrapolating as shown in this ®gure. The absence of tailing in the region around VS and the small values of VS armed that the samples were free from electrostatic charging. The ionization energy (Is ) relative to the vacuum level is given by using the photon energy (hm) [24], Is hm ÿ e VR ÿ VS ;
1
where the subscript s of Is emphasizes that the ionization energy belongs to the solid state, and e is the elementary electric charge. In particular, the ionization threshold (Isth ) is obtained by Isth hm ÿ e V0 ÿ VS :
Fig. 2. Photocurrent curve (upper) and photoemission spectrum (lower) for an LiPc ®lm obtained at hm 7.75 eV. The spectrum is shown along the retarding potential (VR ) scale. The positions denoted by VS and V0 are the saturation and the stopping voltages, respectively.
2
Fig. 3 depicts the UPS spectra measured for the deposited ®lm of LiPc on the Is scale. These spectra were obtained by hydrogen discharge except for the one at hm 21.2 eV, which was measured with He I. In these spectra, several structures depicted by A±C are observed, as shown in the ®gure. In order to precisely and accurately determine their ionization energies, the positions of V0 and these spectral features are plotted as the function of hm in Fig. 4. Using the above equations and the least-squares method, Isth and Is of the spectral features are obtained and the values are given in Table 1. The value of the work function of the samples, /s , can be estimated from the values of /a and VS of the sample as [15,24] /s /a eVS :
3
Using the value of /a 4.21 eV and the average of the data shown in Fig. 3 for VS 0.22 eV, /s for
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Fig. 4. Graphical determination of the ionization potentials for the LiPc thin ®lm. The positions of V0 , VS and spectral features are indicated by , D and +, respectively.
Fig. 3. The UPS spectra of the LiPc thin ®lm and simulated spectrum on the Is scale. The simulated spectrum was calculated by convoluting delta functions located at each orbital energy, which was evaluated by the DFT calculation, with a Gaussian function. The spectral features are denoted by the characters A±C.
the deposited ®lm of LiPc is estimated to be 4.43 eV. The measurements were repeated several times using dierent ®lms; however, no obvious dierence was observed for the values of Is , Isth and /s . The ab initio MO calculations were performed to analyze the UPS spectra. Using the geometry of LiPc optimized by the ROHF level with the 3-21G basis set, the orbital energy was evaluated by the UHF and the density functional theory (DFT) calculations. For comparison with the observed UPS spectra, a trial simulated spectrum was calculated, as shown in Fig. 3, by convoluting delta functions located at each orbital energy with a Gaussian function without correction of crosssection eects. In order to investigate the eects of the molecular symmetry and the unpaired electron on the electronic structure, similar measurements were
performed for the deposited ®lms of H2 Pc and ZnPc. Figs. 5 and 6 compare the UPS spectra of three Pcs taken at hm 7.75 and 10.33 eV, respectively, and the values of Is , Isth and /s for H2 Pc and ZnPc are also given in Table 1. As seen from Figs. 5 and 6, the UPS spectral pattern of these three compounds resemble one another. This means that the top region of the valence band consists of the p-molecular orbitals of the Pc ring, and there is no contribution from the inner hydrogen or metal atoms. The theoretical calculations also show that the molecular orbital with an unpaired electron has a p character. A careful examination of the UPS spectra shown in Fig. 5 for three Pcs reveals several differences. Firstly, the peak A of LiPc is depressed in its intensity as compared with H2 Pc and ZnPc. For the closed shell system, such as H2 Pc and ZnPc, the highest occupied molecular orbital (HOMO) Table 1 Energy values of spectral features
a b
Sample
Isth
Is (A)
Is (B)
Is (C)
/s
Eg a
LiPc H2 Pc ZnPc
4.95 5.12 5.03
5.43 5.59 5.62
7.52 8.10 7.77
8.76 ±b ±b
4.43 4.26 4.19
1.04 1.72 1.68
Estimated values from twice the dierence between Isth and /s . This structure could not be clearly observed.
T. Kimura et al. / Chemical Physics 253 (2000) 125±131
Fig. 5. Comparison of the UPS spectra obtained for the deposited ®lms of LiPc, ZnPc and H2 Pc at hm 7.75 eV on the Is scale.
contains two electrons and the ionization of the electron from this orbital corresponds to peak A of the observed UPS spectra. On the contrary, in LiPc, the singly occupied molecular orbital (SOMO) corresponds to this peak. The theoretical calculation shows that both orbitals have the same p character, and it would be expected that the cross-sections of ionization from these orbitals are nearly equal. Therefore, the origin of the dierence in intensity of peak A is thought to be the number of the occupied electrons in the orbital related to the peak. Secondly, peak A was observed at a slightly dierent position between LiPc and H2 Pc or ZnPc: the binding energy for LiPc is lower than that of the others (see the values of Is listed in Table 1). The work function of LiPc is also higher by about 0.2 eV than those of other compounds. LiPc tends to show the smallest value of Isth among the three compounds studied; however, the dierence in the values is not signi®cant and the values of Isth are
129
Fig. 6. Comparison of the UPS spectra obtained for the deposited ®lms of LiPc, ZnPc and H2 Pc at hm 10.33 eV on the Is scale.
within 5:0 0:1 eV for the three Pcs. The band gap was estimated from twice the dierence between the values of Isth and /s assuming that these compounds are intrinsic semiconductors, and the obtained values are listed in the last column of Table 1. These estimated values of the band gap should be identical to the position of the absorption onset. Fig. 7 shows the absorption spectra of the deposited ®lms on glass substrates for LiPc, H2 Pc and ZnPc. These spectra are similar to the reported absorption spectra [9,11]. The values of the band gap estimated by UPS spectroscopy show quite a good coincidence with the absorption onset of about 0.6 eV for the deposited ®lm of LiPc and 1.6 eV for H2 Pc and ZnPc. Two reasons correspond to the origin of the above-mentioned variations between LiPc and other Pcs: the dierence in the crystal system and the existence of an unpaired electron. H2 Pc and ZnPc are crystallized on a copper substrate kept at room temperature by vacuum sublimation into the
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Fig. 7. Absorption spectra of the deposited ®lms on glass substrates for LiPc, ZnPc and H2 Pc.
a form, which has not been well characterized as yet and of which two crystal systems, tetragonal and monoclinic, have been reported until now [25,26]. In the case of LiPc, it also shows a similar polymorphic change depending on the substrate temperature: the dominant phase is a tetragonal system (P4/mcc) for the substrate temperature below 150°C, whereas a monoclinic phase (C2/c) dominates at the substrate temperatures between 150°C and 200°C [27]. All samples used in this study were prepared by vacuum sublimation on the copper substrate kept at room temperature, and the infrared and UV±VIS absorption spectra showed that the ®lms of H2 Pc and ZnPc consist of the a form and that the space group of the LiPc ®lm is P4/mcc. The intermolecular distance of the Pc ring should be dierent for these crystal systems, which alters the strength of the intermolecular interaction. Both forms of H2 Pc and ZnPc seem to have the same intermolecular interaction because they all show similar UV±VIS spectra, whereas LiPc shows quite dierent spectra depending on the crystal system. There is some possibility that the a form of H2 Pc and ZnPc has a dierent crystal system from P4/mcc of LiPc and a
dierent strength of intermolecular interaction from LiPc. Therefore, it seems that the dierences in the region of peak A originate from the variation of the intermolecular interaction due to the dierent crystal systems. However, the good agreement of the observed UPS spectrum with the calculated one shows that the observed UPS spectra of the LiPc ®lm corresponds to the electronic structure of the free LiPc molecule; therefore, it should not be assumed that the observed dierence is due to the variation of the intermolecular interaction. The most plausible origin of the dierences observed in the region of peak A is the eect of the unpaired electron. Our theoretical calculations show that the orbital energies of HOMO for ZnPc and SOMO for LiPc are nearly equal, but the lowest unoccupied molecular orbital (LUMO) of LiPc is much lower in energy than the LUMO of ZnPc. Actually, the observed UPS spectra of LiPc and ZnPc show a similarity in the region of peak A except for the intensity; therefore, the dierences between LiPc and other Pcs in the values of /s and the band gap could be due to the unoccupied states. Finally, there are some spectral changes due to the central elements of the Pc ring in the regions of peaks B and C: the position of peak B shifts to the low-energy side in the order of H2 Pc, ZnPc and LiPc, and peak C is clearly observed in LiPc but could not be detected in the other Pcs. The calculated UPS spectra for both LiPc and ZnPc can simulate the observed ones, and the calculations show that the central elements of the Pc rings contribute to the orbitals in these region. Therefore, it might be tentatively assigned that the variation in the regions of peaks B and C is due to the contributions of the central elements in the Pc rings to the molecular orbitals. The detailed investigation of the calculated results is being carried out now, and will be published soon elsewhere. 4. Conclusion We measured the UPS spectra of the deposited ®lms for LiPc (P4/mcc), H2 Pc (a form) and ZnPc (a form), and the spectral features of LiPc were analyzed with the aid of the theoretical calcula-
T. Kimura et al. / Chemical Physics 253 (2000) 125±131
tions. The UPS spectral patterns of these three compounds resemble one another. This means that the top region of the valence band consists of the p-molecular orbitals of the Pc ring. At the same time, there are several dierences. The lowest binding-energy peak of LiPc is depressed in its intensity as compared with H2 Pc and ZnPc. This could be explained by the number of electrons in the HOMO: the SOMO corresponds to the peak in the case of LiPc, whereas the HOMO of H2 Pc and ZnPc contains two electrons. The position of this peak of LiPc is slightly lower than the others and the work function of LiPc is higher, about 0.2 eV than those of the others. We consider two reasons: the dierence in the crystal system and the existence of the unpaired electron. The observed UPS spectrum of LiPc agrees well with the simulated one from the calculated orbital energy of the free LiPc molecule. Thus, we consider the most plausible origin of the dierences in the peak position and the work function should be attributed to the unpaired electron. Moreover, the peak located at around 8 eV is observed at dierent positions to each other. It might be tentatively assigned that the shift is caused by the contributions of the central elements in Pc rings to the molecular orbitals in this region. Acknowledgements The authors would like to thank Mr. Toshiaki Noda for preparing the vacuum sublimation system. The calculations were carried out by using the SP2 computer at Computer Center of Institute for Molecular Science (Okazaki, Japan). This work was partly supported by Saneyoshi Scholarship Foundation through Grant 09-12 and Grant-inAid for Scienti®c Research on Priority Areas ``Molecular Physical Chemistry'' (11166253) from the Ministry of Education, Culture, Sports and Science. References [1] C.C. Lezno, A.B.P. Lever, in: Phthalocyanines, Properties and Applications, VCH, Weinheim, vol. 1, 1989.
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