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Ultraviolet photoelectron spectroscopy of poly(p-phenylene sulfide) (PPS)
Volume 130, number 3
CHEMICAL PHYSICS LETTERS
3 October 1986
ULTRAVIOLET PHOTOELECTRON SPECTROSCOPY OF POLY (p_PHENYLENE SULFIDE) (PPS)
Satoshi ASADA ‘, Kazuhiko SEKl and Hiroo INOKUCHI Instrtute for Molecular Science, Myodaoi, Okazaki 444, Japan
Received 28 April 1986; in final form 7 July 1986
Ultraviolet photoelectron spectra were measured for films of poly(pphenylene sulfide) (PPS) prepared by vacuum evaporation. The threshold ionization potential was determined to be 6.0 k 0.1 eV. The peaks in the photoelectron spectra are assigned by comparison with theoretical calculations, and the II:bandwidths of PPS and related compounds are discussed.
1. Introduction
The physico-chemical properties of many conducting polymers have recently been studied with much interest. Polymers such as polyacetylene [ 11, poly (p-phenylene ) (PPP ) [ 2 1, poly (pphenylene sulfide) (PPS) [ 31 and poly(p-phenylene vinylene) [4] doped with strong acceptors exhibit electrical conductivities greater than 1 S/cm. However, hardly any of them are melt- or solution-processible, a requirement which is important for practical applications. In this aspect, PPS is unique in being meltand solution-processible. The valence electronic structure of PPS was investigated by Riga et al. [5] by X-ray photoelectron spectroscopy (XPS), and a detailed assignment was given by Bredas et al. [ 6 ] using a calculation by the valence effective Hamiltonian (VEH) method. The electronic structures of PPS oligomers were also calculated by Duke et al. [ 71 by the CNDO/S3 method. As for the experimental study of oligomer, UV photoelectron spectroscopy (UPS) of gaseous diphenyl sulfide was reported by Colonna et al. [ 8 1. In spite of these efforts, a detailed experimental study is still lacking for the uppermost It-valence levels of PPS, which are important in understanding its electric conductivity. Examination of this part by XPS [ 51 was hindered by the limited energy resoluPermanent address: TOYOTA Motor Corporation, Toyota 47 1, Japan.
0 009-2614/86/$ 03.50 0 Elsevier Science Publishers B.V. (North-Holland Physics Publishing Division)
tion and by the small ionization cross section for the n-levels derived from the C,, orbitals. UPS is suitable for such a study because of its higher sensitivity for the &,-derived K orbitals and its generally higher resolution than XPS. In the present paper, we report a UPS study of PPS. By virtue of its sufficient resolution, fine structures in the uppermost tr bands, which were not resolved by XPS, were clearly observed. They are discussed in comparison with theoretical calculations and data for related compounds.
2. Experimental
Commercially obtained PPS powder from Scientific Polymer Co. Ltd. was used without further purification. A crystalline thin film of PPP for UPS measurements was prepared on a Cu substrate by vacuum evaporation [ 91, in an ultra-high-vacuum (UHV) chamber with a base pressure of 3 x 10W7Pa. The temperatures of the source and the substrate were 400 and 200’ C, respectively,Deposition of PPS was confirmed by measuring the IR spectrum of a film evaporated on a KBr plate. The film thickness was 30-50 nm, which was monitored with a quartz oscillator. This thickness was adequate for avoiding both sample charging and electron emission from the Cu substrate through the film. The photoelectron spectra were measured in situ after cooling the substrate to room temperature. 155
.
CHEMICAL PHYSICS LETTERS
Volume 130. number 3
The light sources were rare gas resonance lines (ArI, 11.83 and 11.62 eV; NeI, 16.85 and 16.67 eV; and HeI, 2 1.22 eV) . A spherical retarding-field-type analyzer was used for electron-energy analysis. The UPS spectra were obtained by current differentiation by superposition of a 0.2 V (peak to peak) ac modulation (4 Hz) onto the retarding potential and lock-in detection, after amplification of the signal by a Cary3 1 vibrating-reed electrometer [ lo]. The experimental error for the observed energy value was estimated to be < 20.1 eV.
3. Results and discussion Fig. 1 shows the photoelectron spectra of PPS for the three photon energies ( ArI, NeI, and HeI). The solid-state binding energy (ionization potential) Z, relative to the vacuum level is deduced as [ 10,111 Z,=hv-e(
I/,-R,)
,
n
00 s-t, UPS (solid)
ArI
Ne I
He1 I
I
I
20
15
10
Binding
Energy
I
L
5&Jc=O ( eV 1
Fig. 1. UV photoelectron spectra of poly(p-phenylene sulfide) (PPS) by ArI (11.7 eV), NeI (16.8 eV) and He1 (21.2 eV) resonance lines.
156
3 October 1986
where hv represents the photon energy (mean values are used for the ArI, NeI doublets), VR is the retarding potential, e is the elementary charge, and V, (saturation voltage) is the value of Vn at the left-hand cutoff of the spectrum, which is -0.3 eV in the present measurements. Since V, corresponds to the contact-potential difference between the sample and the collector [ 111, the work function of PPS is estimated to be 4.5 eV from this value of V, and the observed work function of the Au collector (4.8 eV). Four peaks (A: 6.9 eV, B: 8.3 eV, C: 11.2 eV, and D: 13.9 eV) are observed in the photoelectron spectra. The threshold energy for photoemission from the solid Zihis 6.0 f 0.1 eV. This value is determined by use of the high-energy components of the ArI and NeI doublets as the incident photon energies, as an exception to the abovestated use of the mean values. Under the assumption that PPS is an intrinsic semiconductor, the bandgap is estimated to be 3.0 eV, i.e. twice the difference between the If” and the work function, 4.5 eV. This value agrees roughly with the absorption edge of PPS at 3.4 eV [ 21. The He1 UV photoelectron spectrum is compared in fig. 2 with the XPS [ 51 and the density of states (DOS) calculated by the VEH method [ 61. One should note in this comparison that these spectra have been observed for solid samples, while the DOS is calculated for an isolated chain. Nevertheless, it is known that there is a good one-to-one correspondence between the spectrum of a molecular solid and that of the constituent molecule in the gas phase, if a proper shift in the energy scale is allowed for [ 121. Thus the solid-state spectra can be compared with the theoretical calculation. This shift is due to the electric stabilization of the ionized molecule by the electronic polarization of the surrounding molecules. Hence it is called polarization energy [ 131. In the present case, the calculated DOS is adjusted to align the peak B with that of the UV photoelectron spectrum. Since the reported binding energy of the XPS is relative to the Fermi level, its energy scale is also adjusted by use of the abovementioned work function, 4.5 eV. Though correspondence among figs. 2a, 2b, and 2c is generally good, there are some differences. Firstly, peaks A and B, which correspond well to the calculated DOS, are clearly resolved in the UPS, while the XPS shows only a broad peak. They originate from
Volume 130, number 3
Binding Energy
CHEMICAL PHYSICS LETTERS
5Eva,=0 (eV i
Fig. 2. (a) X-ray photoelectron spectrum of PPS [ 51, (b) He1 UV photoelectron spectrum of PPS, and (c) calculated density of states of PPS by the VEH method [ 61.
the uppermost n levels, as we will discuss them later in more detail. Secondly, the binding energies of deeper peaks C and D are slightly different between UPS and XPS. Such a difference can be explained by comparison with the DOS based on the VEH calculation [ 61. The XPS and UPS peaks correspond to different peaks in the calculated DOS as labelled in fig. 2, and the XPS shows shoulders at the energies of C and D. According to the VEH calculation, peak C has a major contribution from the S3$and &,,, orbitals and peak D has CZs,CZp,and H,, characters,while C’ and D’ are formed by overlapping bands of various characters [ 141. A theoretical simulation of the XPS including the effect of photoionization cross section [ 6 ] shows that peaks C and D are weakened relative to those of C’ and D’. Thus the difference in the peak positions for the two experiments can be ascribed to the photon-energy dependence of photoionization cross sections.
3 October 1986
In the following we will discuss the uppermost z bands. According to the reported calculations [ 6-8 1, bonding of the benzene rings with the sulfur atoms affects the doubly degenerate highest-occupied rt orbitals of benzene in different ways. One of these orbitals is not influenced, since it has no electron density on the carbon atom connected to sulfur. The other, which has a high electron density on the bonding carbon atom, interacts strongly with the sulfur 3p orbital to form split levels. As the polymer-chain length increases to infinity, the former non-bonding orbital forms a flat band at the energy of the HOMO of benzene and exhibits a high density of states. On the other hand, the interacting (benzene+sulfur) orbitals form wide antibonding n bands above and below the flat band, respectively. They make high densities of states at their top and bottom edges at wave vector k=O. Such features are clearly seen in the calculated DOS (fig. 2c) in the region of 7-10 eV. We note that the energy difference A between the highest antibonding orbital and the non-bonding orbital corresponds to the bandwidth of the antibonding band. It gives a measure of the strength of the interaction among the units forming a polymer chain, or the degree of the hole delocalization in a chain. Thus peaks A and B in the UV photoelectron spectrum (fig. 2b) can easily be ascribed to the upper edge of the antibonding band and the non-bonding band, respectively, with A= 1.4 eV. The lower edge of the bonding band is merged with the deeper-lying bands to form peak C in the observed spectrum. This fairly large value of A arises from the good efficiency of the SJp levels in transferring the interaction among the benzene rings [6] even in the largely twisted structureofPPP [15]. There has been some controversy about the possibility of observing extended ICstates by photoelectron spectroscopy [ 161. The present clear observation of such a state (A) adds another example to the recent observations of similar features in polypyrole [ 171 and poly( 3-methylthiophene) [ 181. The present results on the uppermost n band are compared in table 1 with those for related compounds. First we note that the value of A for PPS is slightly larger than that for diphenylsulfide [ 8 1, which corresponds to a higher hole delocalization in the longer chain. 1.57
3 October 1986
CHEMICAL PHYSICS LETTERS
Volume 130, number 3
The data for benzene, poly(pxylylene) (PPX) and poly(pphenylene) (PPP) are included in table 1 for comparison with other polymers including benzene rings in the principal chain. The discussion about the formation of the non-bonding, bonding, and antibonding bands is also applicable to PPP and PPX by changing the bonding unit from an S atom to a direct bond and a -CH2CH2- group, respectively. The binding energy of non-bonding band is shown to be almost constant at the HOMO energy of benzene, as expected from the model described above by assuming a common polarization energy. PPX shows almost the same spectra as those for benzene, which indicates an almost flat n band with AZ 0 [ 191. On the other hand, PPP shows a wider rt band (A= 2.0 eV) than PPS (A= 1.4 eV) [ 201. Thus we conclude that the interaction between the highest x orbitals on the neighboring benzene rings in PPP is slightly reduced by inserting an S atom, and it is completely removed by inserting a -CH2CH2- group. A similar ordering of PPP > PPS > poly( phenylene oxide) in the delocalization of ICelectrons has also been deduced by electron energy loss spectroscopy (EELS) [ 211. However, EELS is concerned with the intrachain delocalization of the K-+X* electronic excitation (exciton), and the results of the present UPS study give more direct and quantitative experimental
information about the degree of hole delocalization. Finally, we discuss the ionization threshold of PPS and its related eompounds. The value of PPS (6.0 k 0.1 eV) is 1.4 eV smaller than that of gaseous diphenyl su!tide (7.4 + 0.1 eV). A part of this difference can be ascribed to the abovementioned difference in A, but a major contribution should originate from the polarization energy. (The value of 1.4 eV is comparable with the polarization energies for ordinary organic solids ranging 1.2-l .8 eV [ 221.) As expected from the above discussion, the thresholds of other polymers in table 1 are primarily determined by the rc bandwidth A. In accordance with the discussion of A, the threshold energy of PPS is slightly higher than that of PPP [20] but is about 1.5 eV smaller than those of benzene [ 231 and PPX [ 191. Since the ionization threshold gives a measure of the ease for acceptor doping, we see that the x-electron delocalization is effective not only for intrachain hole transport but also for carrier generation. Thus the present result that delocalization in PPS is slightly smaller than in PPP is qualitatively consistent with the reported smaller electric conductivity of AsF5dopedPPS(3Scm-‘)thanthatofPPP(500Scm-’) [ 21, although the effects of interchain transport and possible structural change under the doping process [ 21 must also be taken into account.
Table 1 Ionization energies of PPS and related compounds relative to the vacuum level (in eV) Compound a’
Experiment threshold
Calculation (VEH method) [ 61 peak
bandwidth b,
method
ref.
peak
bandwidth b,
A
PPS
6.0
highest occupied state
main
4.9
8.3
1.4
q.4
7.88
9.20
1.32
UPS (solid) XPS (solid) UPS (gas)
7.1 6.9 5.65 *’
8.2 8.24 6.1 d’
8.2 8.24 8.1 d’
0 0 2.0 d’
UPS (solid) UPS (solid) UPS (solid)
2.6 Cl diphenyl sulfide benzene PPX PPP
A
this work
[23] [191
highest occupied state
main
7.1
8.2
1.2
-
-
-
7.9
9.5
1.8
151 [8]
1201
a’ PPS, poly(pphenylene sulfide): PPX, poly(pxylylene); PPP, poly(pphenylene). ” The energy difference between the highest antibonding orbital and the non-bonding orbital originating from the uppermost levels of benzene. See text. c’ The value relative to the Fermi level. See text. ” Values estimated from extrapolation of the data of oligomers.
158
Volume 130. number 3
CHEMICAL PHYSICS LETTERS
Acknowledgement
We are grateful to Dr. Takeshi Kawakami and Mr. Hidetoshi Oosawa of TOYOTA Motor Corporation for valuable discussion, and to Dr. J.L. Bredas of FacultCs Universitaires Notre-Dame de la Paix for informing the details of his VEH calculation. This work was carried out during the stay of SA at the Institute for Molecular Science (1985- 1986 ). It was supported in part by a Grant-in-Aid for Scientific Research (No. 60 104006 ) from the Ministry of Education, Science and Culture of Japan.
References [ I] Y.W. Park, M.A. Druy, C.K. Chiang, A.G. MacDiarmid,
[2] [3] (41 [5]
[6]
A.J. Heeger, H. Shirakawa and S. Ikeda, J. Polym. Sci. Polym. Letters Ed. 17 (1979) 195. L.W. Shacklette, R.R. Chance, H. Eckhardt, J.E. Frdmmer and R.H. Baughman, J. Chem. Phys. 75 (1981) 1919. J.F. Robolt, T.C. Clarke, K.K. Kanazawa, J.R. Reynolds and G.B. Street, J. Chem. Sot. Chem. Commun. (1980) 347. G.E. Wnek, J.C.W. Chien, F.F. Karasz and L.P. Lillya, Polymer20 (1979) 1441. J. Riga, J.P. Boutique, J.J. Pireaux and J.J. Verbist, Proceedings of the ACS International Symposium on PhysicoChemical Aspects of Polymer Surfaces, New York (198 I ) p. 45. J.L. Brbdas, R.R. Chance, R. Silbey, G. Nicolas and Ph. Durand, J. Chem. Phys. 77 (1982) 371.
3 October 1986
[ 71 C.B. Duke, A. Paton and W.R. Salaneck, Mol. Cryst. Liquid Cryst. 83 (1982) 177. [ 81 F.P. Colonna, G. Distefano, V. Galasso, G.C. Pappalardo and G. Scarlata, J. Chem. Sot. Faraday Trans. II 73 (1977) 822. [9] K. Misoh, S. Tanaka, S. Miyata and H. Sasabe, J. Chem. Sot. Japan 32 (I 983) 763 (in Japanese). [IO] T. Hlrooka, K. Tanaka, K. Kuchitsu, M. Fujihira, H. Inokuchi and Y. Harada, Chem. Phys. Letters I8 (1973) 390. [I I ] Y. Harada and H. Inokuchi, Bull. Chem. Sot. Japan 39 (1966) 1443. [ 121 K. Seki, H. lnokuchi and Y. Harada, Chem. Phys. Letters 20 (1973) 197. [I31 L.E.Lyons, J.Chem.Soc. (1957) 5001. [ 141 J.L. Brtdas, private communication. [ 151 B.J. Tabor, E.P. Magrb and J. Boon, European Polym. J. 7 (1971) 1127. [ 161 W.R. Salaneck, CRC Crit. Rev. Solid State Materials Sci. I2 (1985) 267. [ 171 P. Rfluger, U.M. Gubler and G.B. Street, Solid State Commun.49(1984)911. [ 181 Y. Jugnet, G. Tourillon and T.M. Due, Phys. Rev. Letters 56 (1986) 1862. [ 191 Y. Takai. T. Mizutani, M. Ieda, K. Seki and H. Inokuchi, Polymer Photochem. 2 (I 982) 33. [20] K. Seki, U. Karlsson, R. Engelhardt, E.E. Koch and W. Schmidt, Chem. Phys. 9 I (1984) 459. [2l] G. Crecelius, J. Fink, J.J. Ristko, M. Stamm, H.-J. Freund and H. Gonska, Phys. Rev. 828 (1983) 1802. [22] N. Sato, K. Seki and H. Inokuchi, J. Chem. Sot. Faraday Trans. II 77 (1981) 1621. [23] K. Seki, unpublished.
159
CHEMICAL PHYSICS LETTERS
3 October 1986
ULTRAVIOLET PHOTOELECTRON SPECTROSCOPY OF POLY (p_PHENYLENE SULFIDE) (PPS)
Satoshi ASADA ‘, Kazuhiko SEKl and Hiroo INOKUCHI Instrtute for Molecular Science, Myodaoi, Okazaki 444, Japan
Received 28 April 1986; in final form 7 July 1986
Ultraviolet photoelectron spectra were measured for films of poly(pphenylene sulfide) (PPS) prepared by vacuum evaporation. The threshold ionization potential was determined to be 6.0 k 0.1 eV. The peaks in the photoelectron spectra are assigned by comparison with theoretical calculations, and the II:bandwidths of PPS and related compounds are discussed.
1. Introduction
The physico-chemical properties of many conducting polymers have recently been studied with much interest. Polymers such as polyacetylene [ 11, poly (p-phenylene ) (PPP ) [ 2 1, poly (pphenylene sulfide) (PPS) [ 31 and poly(p-phenylene vinylene) [4] doped with strong acceptors exhibit electrical conductivities greater than 1 S/cm. However, hardly any of them are melt- or solution-processible, a requirement which is important for practical applications. In this aspect, PPS is unique in being meltand solution-processible. The valence electronic structure of PPS was investigated by Riga et al. [5] by X-ray photoelectron spectroscopy (XPS), and a detailed assignment was given by Bredas et al. [ 6 ] using a calculation by the valence effective Hamiltonian (VEH) method. The electronic structures of PPS oligomers were also calculated by Duke et al. [ 71 by the CNDO/S3 method. As for the experimental study of oligomer, UV photoelectron spectroscopy (UPS) of gaseous diphenyl sulfide was reported by Colonna et al. [ 8 1. In spite of these efforts, a detailed experimental study is still lacking for the uppermost It-valence levels of PPS, which are important in understanding its electric conductivity. Examination of this part by XPS [ 51 was hindered by the limited energy resoluPermanent address: TOYOTA Motor Corporation, Toyota 47 1, Japan.
0 009-2614/86/$ 03.50 0 Elsevier Science Publishers B.V. (North-Holland Physics Publishing Division)
tion and by the small ionization cross section for the n-levels derived from the C,, orbitals. UPS is suitable for such a study because of its higher sensitivity for the &,-derived K orbitals and its generally higher resolution than XPS. In the present paper, we report a UPS study of PPS. By virtue of its sufficient resolution, fine structures in the uppermost tr bands, which were not resolved by XPS, were clearly observed. They are discussed in comparison with theoretical calculations and data for related compounds.
2. Experimental
Commercially obtained PPS powder from Scientific Polymer Co. Ltd. was used without further purification. A crystalline thin film of PPP for UPS measurements was prepared on a Cu substrate by vacuum evaporation [ 91, in an ultra-high-vacuum (UHV) chamber with a base pressure of 3 x 10W7Pa. The temperatures of the source and the substrate were 400 and 200’ C, respectively,Deposition of PPS was confirmed by measuring the IR spectrum of a film evaporated on a KBr plate. The film thickness was 30-50 nm, which was monitored with a quartz oscillator. This thickness was adequate for avoiding both sample charging and electron emission from the Cu substrate through the film. The photoelectron spectra were measured in situ after cooling the substrate to room temperature. 155
.
CHEMICAL PHYSICS LETTERS
Volume 130. number 3
The light sources were rare gas resonance lines (ArI, 11.83 and 11.62 eV; NeI, 16.85 and 16.67 eV; and HeI, 2 1.22 eV) . A spherical retarding-field-type analyzer was used for electron-energy analysis. The UPS spectra were obtained by current differentiation by superposition of a 0.2 V (peak to peak) ac modulation (4 Hz) onto the retarding potential and lock-in detection, after amplification of the signal by a Cary3 1 vibrating-reed electrometer [ lo]. The experimental error for the observed energy value was estimated to be < 20.1 eV.
3. Results and discussion Fig. 1 shows the photoelectron spectra of PPS for the three photon energies ( ArI, NeI, and HeI). The solid-state binding energy (ionization potential) Z, relative to the vacuum level is deduced as [ 10,111 Z,=hv-e(
I/,-R,)
,
n
00 s-t, UPS (solid)
ArI
Ne I
He1 I
I
I
20
15
10
Binding
Energy
I
L
5&Jc=O ( eV 1
Fig. 1. UV photoelectron spectra of poly(p-phenylene sulfide) (PPS) by ArI (11.7 eV), NeI (16.8 eV) and He1 (21.2 eV) resonance lines.
156
3 October 1986
where hv represents the photon energy (mean values are used for the ArI, NeI doublets), VR is the retarding potential, e is the elementary charge, and V, (saturation voltage) is the value of Vn at the left-hand cutoff of the spectrum, which is -0.3 eV in the present measurements. Since V, corresponds to the contact-potential difference between the sample and the collector [ 111, the work function of PPS is estimated to be 4.5 eV from this value of V, and the observed work function of the Au collector (4.8 eV). Four peaks (A: 6.9 eV, B: 8.3 eV, C: 11.2 eV, and D: 13.9 eV) are observed in the photoelectron spectra. The threshold energy for photoemission from the solid Zihis 6.0 f 0.1 eV. This value is determined by use of the high-energy components of the ArI and NeI doublets as the incident photon energies, as an exception to the abovestated use of the mean values. Under the assumption that PPS is an intrinsic semiconductor, the bandgap is estimated to be 3.0 eV, i.e. twice the difference between the If” and the work function, 4.5 eV. This value agrees roughly with the absorption edge of PPS at 3.4 eV [ 21. The He1 UV photoelectron spectrum is compared in fig. 2 with the XPS [ 51 and the density of states (DOS) calculated by the VEH method [ 61. One should note in this comparison that these spectra have been observed for solid samples, while the DOS is calculated for an isolated chain. Nevertheless, it is known that there is a good one-to-one correspondence between the spectrum of a molecular solid and that of the constituent molecule in the gas phase, if a proper shift in the energy scale is allowed for [ 121. Thus the solid-state spectra can be compared with the theoretical calculation. This shift is due to the electric stabilization of the ionized molecule by the electronic polarization of the surrounding molecules. Hence it is called polarization energy [ 131. In the present case, the calculated DOS is adjusted to align the peak B with that of the UV photoelectron spectrum. Since the reported binding energy of the XPS is relative to the Fermi level, its energy scale is also adjusted by use of the abovementioned work function, 4.5 eV. Though correspondence among figs. 2a, 2b, and 2c is generally good, there are some differences. Firstly, peaks A and B, which correspond well to the calculated DOS, are clearly resolved in the UPS, while the XPS shows only a broad peak. They originate from
Volume 130, number 3
Binding Energy
CHEMICAL PHYSICS LETTERS
5Eva,=0 (eV i
Fig. 2. (a) X-ray photoelectron spectrum of PPS [ 51, (b) He1 UV photoelectron spectrum of PPS, and (c) calculated density of states of PPS by the VEH method [ 61.
the uppermost n levels, as we will discuss them later in more detail. Secondly, the binding energies of deeper peaks C and D are slightly different between UPS and XPS. Such a difference can be explained by comparison with the DOS based on the VEH calculation [ 61. The XPS and UPS peaks correspond to different peaks in the calculated DOS as labelled in fig. 2, and the XPS shows shoulders at the energies of C and D. According to the VEH calculation, peak C has a major contribution from the S3$and &,,, orbitals and peak D has CZs,CZp,and H,, characters,while C’ and D’ are formed by overlapping bands of various characters [ 141. A theoretical simulation of the XPS including the effect of photoionization cross section [ 6 ] shows that peaks C and D are weakened relative to those of C’ and D’. Thus the difference in the peak positions for the two experiments can be ascribed to the photon-energy dependence of photoionization cross sections.
3 October 1986
In the following we will discuss the uppermost z bands. According to the reported calculations [ 6-8 1, bonding of the benzene rings with the sulfur atoms affects the doubly degenerate highest-occupied rt orbitals of benzene in different ways. One of these orbitals is not influenced, since it has no electron density on the carbon atom connected to sulfur. The other, which has a high electron density on the bonding carbon atom, interacts strongly with the sulfur 3p orbital to form split levels. As the polymer-chain length increases to infinity, the former non-bonding orbital forms a flat band at the energy of the HOMO of benzene and exhibits a high density of states. On the other hand, the interacting (benzene+sulfur) orbitals form wide antibonding n bands above and below the flat band, respectively. They make high densities of states at their top and bottom edges at wave vector k=O. Such features are clearly seen in the calculated DOS (fig. 2c) in the region of 7-10 eV. We note that the energy difference A between the highest antibonding orbital and the non-bonding orbital corresponds to the bandwidth of the antibonding band. It gives a measure of the strength of the interaction among the units forming a polymer chain, or the degree of the hole delocalization in a chain. Thus peaks A and B in the UV photoelectron spectrum (fig. 2b) can easily be ascribed to the upper edge of the antibonding band and the non-bonding band, respectively, with A= 1.4 eV. The lower edge of the bonding band is merged with the deeper-lying bands to form peak C in the observed spectrum. This fairly large value of A arises from the good efficiency of the SJp levels in transferring the interaction among the benzene rings [6] even in the largely twisted structureofPPP [15]. There has been some controversy about the possibility of observing extended ICstates by photoelectron spectroscopy [ 161. The present clear observation of such a state (A) adds another example to the recent observations of similar features in polypyrole [ 171 and poly( 3-methylthiophene) [ 181. The present results on the uppermost n band are compared in table 1 with those for related compounds. First we note that the value of A for PPS is slightly larger than that for diphenylsulfide [ 8 1, which corresponds to a higher hole delocalization in the longer chain. 1.57
3 October 1986
CHEMICAL PHYSICS LETTERS
Volume 130, number 3
The data for benzene, poly(pxylylene) (PPX) and poly(pphenylene) (PPP) are included in table 1 for comparison with other polymers including benzene rings in the principal chain. The discussion about the formation of the non-bonding, bonding, and antibonding bands is also applicable to PPP and PPX by changing the bonding unit from an S atom to a direct bond and a -CH2CH2- group, respectively. The binding energy of non-bonding band is shown to be almost constant at the HOMO energy of benzene, as expected from the model described above by assuming a common polarization energy. PPX shows almost the same spectra as those for benzene, which indicates an almost flat n band with AZ 0 [ 191. On the other hand, PPP shows a wider rt band (A= 2.0 eV) than PPS (A= 1.4 eV) [ 201. Thus we conclude that the interaction between the highest x orbitals on the neighboring benzene rings in PPP is slightly reduced by inserting an S atom, and it is completely removed by inserting a -CH2CH2- group. A similar ordering of PPP > PPS > poly( phenylene oxide) in the delocalization of ICelectrons has also been deduced by electron energy loss spectroscopy (EELS) [ 211. However, EELS is concerned with the intrachain delocalization of the K-+X* electronic excitation (exciton), and the results of the present UPS study give more direct and quantitative experimental
information about the degree of hole delocalization. Finally, we discuss the ionization threshold of PPS and its related eompounds. The value of PPS (6.0 k 0.1 eV) is 1.4 eV smaller than that of gaseous diphenyl su!tide (7.4 + 0.1 eV). A part of this difference can be ascribed to the abovementioned difference in A, but a major contribution should originate from the polarization energy. (The value of 1.4 eV is comparable with the polarization energies for ordinary organic solids ranging 1.2-l .8 eV [ 221.) As expected from the above discussion, the thresholds of other polymers in table 1 are primarily determined by the rc bandwidth A. In accordance with the discussion of A, the threshold energy of PPS is slightly higher than that of PPP [20] but is about 1.5 eV smaller than those of benzene [ 231 and PPX [ 191. Since the ionization threshold gives a measure of the ease for acceptor doping, we see that the x-electron delocalization is effective not only for intrachain hole transport but also for carrier generation. Thus the present result that delocalization in PPS is slightly smaller than in PPP is qualitatively consistent with the reported smaller electric conductivity of AsF5dopedPPS(3Scm-‘)thanthatofPPP(500Scm-’) [ 21, although the effects of interchain transport and possible structural change under the doping process [ 21 must also be taken into account.
Table 1 Ionization energies of PPS and related compounds relative to the vacuum level (in eV) Compound a’
Experiment threshold
Calculation (VEH method) [ 61 peak
bandwidth b,
method
ref.
peak
bandwidth b,
A
PPS
6.0
highest occupied state
main
4.9
8.3
1.4
q.4
7.88
9.20
1.32
UPS (solid) XPS (solid) UPS (gas)
7.1 6.9 5.65 *’
8.2 8.24 6.1 d’
8.2 8.24 8.1 d’
0 0 2.0 d’
UPS (solid) UPS (solid) UPS (solid)
2.6 Cl diphenyl sulfide benzene PPX PPP
A
this work
[23] [191
highest occupied state
main
7.1
8.2
1.2
-
-
-
7.9
9.5
1.8
151 [8]
1201
a’ PPS, poly(pphenylene sulfide): PPX, poly(pxylylene); PPP, poly(pphenylene). ” The energy difference between the highest antibonding orbital and the non-bonding orbital originating from the uppermost levels of benzene. See text. c’ The value relative to the Fermi level. See text. ” Values estimated from extrapolation of the data of oligomers.
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Volume 130. number 3
CHEMICAL PHYSICS LETTERS
Acknowledgement
We are grateful to Dr. Takeshi Kawakami and Mr. Hidetoshi Oosawa of TOYOTA Motor Corporation for valuable discussion, and to Dr. J.L. Bredas of FacultCs Universitaires Notre-Dame de la Paix for informing the details of his VEH calculation. This work was carried out during the stay of SA at the Institute for Molecular Science (1985- 1986 ). It was supported in part by a Grant-in-Aid for Scientific Research (No. 60 104006 ) from the Ministry of Education, Science and Culture of Japan.
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