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Photoelectron spectroscopic studies of some model molecules for poly(3-alkylthiophene)
Synthetic Metals, 41-43 (1991) 1373-1376
1.373
PHOTOELECTRON SPECTROSCOPIC STUDIES OF SOME MODEL MOLECULES FOR POLY(3-ALKYLTHIOPHENE)
M. KEANE**, W. R. SALANECK*, S. SVENSSON**, A.N. DE BRITO**, N. CORREIA**. and S. LUNELL** *Department of Physics, IFM, Link6ping University, S-581 83 Link6ping (Sweden) and **Departments of Physics or Quantum Chemistry, Uppsala University, Box 530, S-752 21 Uppsala (Sweden) ABSTRACT Previously, we have used X-ray photoelectron spectroscopy (XPS) to study, in the gas phase, certain model molecules for poly(3-alkylthiophene), in relation to the thermochromic effect observed in these polymers. Here we present an extension of these studies to include the trimer of polythiophene, which supports further the previously proposed model of soft confol~Tational defects in determining the thermochromic behavior of the poly-alkylthiophenes.
INTRODUCTION The nature of the thermochromic behavior of the poly(3-alkylthiophene)s, especially poly(3hexylthiophene, I ), or P3HT,
I
I
I
S
S
(I)
I
I
I
I
has been the subject of several studies employing both Ultraviolet and X-ray photoelectron
spectroscopy, or UPS and XPS [2-4]. A geometrical model of the therrnochromic behavior was suggested [2,4], in which, upon increasing temperature, rotations of neighboring thiophene rings lead to a decrease in the g-conjugation, which in turn leads to the electronic smactural changes responsible for the color changes. The driving force for the ring rotation has been suggested to be 0379-6779/91/$3.50
© Elsevier Sequoia/Printed in The Netherlands
1374 temperature-dependent interactions among the alkyl side-groups [5]. Recently, through comparison with XPS (also known as ESCA) spectra of thiophene dimer molecules in the gas phase [6,7], we have shown that the subtle "shake-up" satellite features in the core-electron XPS spectra of the solid polyalkylthiophenes are consistent with a reduction in g-conjugation with increasing temperature. In this short report we present an extension of the XPS shake-up spectra to calculations on the thiophene trimer, which, in comparison with the previously published corresponding spectra of hexyl- [2] and butyl- [3] substituted polythiophenes, adds further support to the model of soft confolvnational defect model [2,7]. DISCUSSION In Fig. 1 are shown the XPS shake-up satellite spectra of P3HT [2] and P3BT, where B = butyl [3], in comparison with the shake-up spectra of three molecular species. The details of these spectra, and the significance for the thermochrornic behavior of the polyalkylthiophenes, have been discussed seperately [6,7]. INDO/CI CALCULATED C1 s SHAKE-UP SPECTRA
3-HEXYL]~HIOPHENE fiAS
DIMER
THIOPHENE GAS
'E .0
20X ~ _ _ ~ ~
BITHIOPHENEGAS Z W 13_
,ox
hi < "l-
TRIMER
W
_>
P3HTI÷190C) ,0,
P3HTI-80Cl /
V I!
~GAS
/ ili \ "~'.i
,
10 0 RELATIVE BINDING ENERGY leVI
_.1 W I
%|
15
10
5
0
RELATIVE BINDING ENERGY (eV)
Figure I 0.eft): .C(ls) shake-up spectra of selected polythiophenes and model molecules [2,3]. Figure 2 (Right): INDO/CI Calculated C(ls) shake-up spectra of the thiophene dimer and trimer [7]. It is important, however, to go beyond the dimer molecules in the comparison of the shake-up specn'a with those of the polymers in the solid state. The vapor pressure of the n'imcr molecule did not permit gas phase X P S measurements of the shake-up spectra. But, fortunately, the INDO/CI simulations of the monomer and dimer shake-up spectra reproduce well the experimental results
1375 [6,7], at least at shake-up energies to within about 10 to 15 eV of the main line, and can be used to study the Irimer. The energy range limitation comes from the use of a lin~ted number of
configurations (- 4000) in the CI simulations, in order to limit the use of computer space and time. The results of an INDO/CI level calculation of shake-up spectra of a thiophene timer and trimer are shown in Fig. 2. Of primary interest is the relative energy of the first major shake-up peak in comparison with the main C(ls) line [6], and how this relative energy varies with the length of the polymer chain (oligomer size). In Fig. 1, the weak cross-hatched satellite peak near 3 eV in the lowest curve represents the long-chain limit to the shake-up spectrum. This peak, however, contains simultaneous contributions from electron energy-loss scattering of the primary C(1 s) electrons (so-called extrinsic effects), as well as the (intrinsic) shake-up contribution. In the longchain limit, the combination peak should correspond approximately in energy to the maximum in the optical absorption spectrum, which occurs near 3.2 eV [1]. Note, that both optical absorption and electron energy loss scattering correspond (mainly) to single-electron transitions in the neutral molecule, while shake-up corresponds to many-electron (but often apparent single-electron-like) transitions in the molecular ion [8]. For large unsaturated molecules, inelastic scattering peaks and shake-up peaks fall within the same energy range [9]. In spectra obtained in the gas phase at sufficiently low vapor pressure (as the case presented here), the electron energy-loss events are eliminated. The INDO/CI calculations produce the first major shake-up peak at slightly too low energy, relative to the main line, although thereafter, the structure are well reproduced [7]. One can correct for the slight under estimation between the calculations and the data by comparing the data on the thiophene monomer and dimer, where both experimental and theoretical spectra are available [7]. The calculated trimer spectra can thus be shifted by the same amount. Following this proceedure, an estimate can be obtained of the relative energy at which the first major shake-up satellite peak will occur in real XPS spectra of the trimer in the gas phase. The energy of the first major shake-up peak is plotted versus the size (number of non-hydrogen atoms) of the thiophene oligomer in Fig. 3. >
o 6 >(.9 n- 4
t.IJ Z I.H n D < I CO
",~
SHAKE-UP ENERGY vs OLIGOMER SIZE 5 "HEAVY ATOMS" PER MONOMER UNIT
-,,
"'"'B
..........
ITL_ -
I!
2 0 0
5
10 15 20 NUMBER OF HEAVY ATOMS (n)
,,co,,
Figure 3: The variation of the energy of the first shake-up peak with oligomer size. Note that n = the number of non-hydrogen atoms, i.e., carbon and sulfur. The energy of the first major shake-up transition appears to saturate between n = 10 (dimers) and n = 15 (trimers), to a value consistent with the weak satellite peak near 3 eV in Fig. 1. This is consistent with a saturation observed for both shake-up shifts and valence band structure in, e.g.,
1376
the alkane series [10]. Thus the "valley" in intensity between the main C(ls) line and the first shake-up peak will not contain any shake-up peaks at any value of "n", consistent with the shake-up spectra for the polymers in Fig. 1. This is an essential point in verifying the soft conformational defect model of the thermochromic behavior of the polyalkylthiophenes, as will be discussed elsewhere [7]. This arguement can be used in reverse also. As a function of increasing temperature, the first shake-up peak in the C(ls) XPS spectra of the polymers moves away from the main peak, toward higher relative energies. This trend is what occurs as one looks from the trimer molecule toward the monomer molecule of polythiophene, i.e., toward a more localized n-electronic system. In addition, in a study of the C(1 s) shake-up spectra of the fused-ring aromatic molecules, Riga et al have shown that for more spatially localized 7t-systems (i.e., benzene as compared to anthracene), (i) the the first major peak, as well as the center-of-gravity of the shake-up spectrum moves away from the main line, and (ii) the overall intensity of the shake-up spectrum increases relative to the main line [11 ]. The growth, with increasing temperature, of significant shake-up intensity at higher relative binding energies in the C(ls) spectra of Fig. l, is precisely the motivation for the soft confopvnational defect model of the thermochromism in the polyalkylthiophenes put forth previously [2]. ACKNOWLEDGEMENTS This work has been supported by the Neste Corporation, Finland, the Swedish Board for Technical Development (STU), and the Swedish Natural Sciences Research Council (NFR). REFERENCES 1 0 . Ingan~is, W. R. Salaneck, J. -E. Osterholm, and J. Laakso, Svnth. Met.. 22 (1988) 395. 2 W.R. Salaneck, O. Ingan~is, B. Th6mans, J. O. Nilsson, B. Sj6gren, J. -E. Osterholm, J. -L. Br&las, and S. Svensson, J. Chem. Phvs. 89 (1988) 4613. 3 W.R. Salaneck, O. Ingan~is, J. O. Nilsson, J. -E. Osterholm, B. Th6mans, and J. -L. Br6das, Svnth. Met. 28 (1989) C451. 4 B. Th6mans, W. R. Salaneck, and J. -L. Br6das, Synth. Met. 28 (1989) C359. 5 G. Zerbi, B. Chierichetti and O. Inganas, to be published. 6 W.R. Salaneck, R. La77~'oni, N. Sato, M. LOgdlund, B. Sj~Sgren,M. P. Keane, S. Svensson, A. Naves de Brito, N. Correia, and S. Lunell, in J. -L. Br&las and R. R. Chance (eds.), Conjugated Polymeric Material~: Opportunities in Electronics. Optoelectronics, and Molecular Electronics, NATO ASI Series E, Vol. 182, Kluwer Academic, Dordrecht, 1990, p-101. 7 M.P. Keane, S. Svensson, A. Naves de Brito, N. Correia, and S. LuneU, B. Sj6gren, O. Inganas, and W. R. Salaneck, J. Chem. Phys., in press. 8 H. -J. Freund and R. W. Bigeiow, Phvsica Scri~ta T17 (1987) 50. 9 D. Nordfors, A. Nilsson, N. Mh-tensson, S. Svensson, U. Gelius, and S. Lunell, J. Chem. Phvs. 88 (1988) 2630. 10 J.J. Pil'eaux, S. Svensson, E. Basilier, P. Malmqvist, U. Gelius, R. Caudano, and K. Siegbahn, Phys. Rev. A14 (1976) 2133. 11 J. Riga, J. J. Pireaux, R. Caudano, and J. J. Verbist, Physica Scripta 16 (1977) 346.
1.373
PHOTOELECTRON SPECTROSCOPIC STUDIES OF SOME MODEL MOLECULES FOR POLY(3-ALKYLTHIOPHENE)
M. KEANE**, W. R. SALANECK*, S. SVENSSON**, A.N. DE BRITO**, N. CORREIA**. and S. LUNELL** *Department of Physics, IFM, Link6ping University, S-581 83 Link6ping (Sweden) and **Departments of Physics or Quantum Chemistry, Uppsala University, Box 530, S-752 21 Uppsala (Sweden) ABSTRACT Previously, we have used X-ray photoelectron spectroscopy (XPS) to study, in the gas phase, certain model molecules for poly(3-alkylthiophene), in relation to the thermochromic effect observed in these polymers. Here we present an extension of these studies to include the trimer of polythiophene, which supports further the previously proposed model of soft confol~Tational defects in determining the thermochromic behavior of the poly-alkylthiophenes.
INTRODUCTION The nature of the thermochromic behavior of the poly(3-alkylthiophene)s, especially poly(3hexylthiophene, I ), or P3HT,
I
I
I
S
S
(I)
I
I
I
I
has been the subject of several studies employing both Ultraviolet and X-ray photoelectron
spectroscopy, or UPS and XPS [2-4]. A geometrical model of the therrnochromic behavior was suggested [2,4], in which, upon increasing temperature, rotations of neighboring thiophene rings lead to a decrease in the g-conjugation, which in turn leads to the electronic smactural changes responsible for the color changes. The driving force for the ring rotation has been suggested to be 0379-6779/91/$3.50
© Elsevier Sequoia/Printed in The Netherlands
1374 temperature-dependent interactions among the alkyl side-groups [5]. Recently, through comparison with XPS (also known as ESCA) spectra of thiophene dimer molecules in the gas phase [6,7], we have shown that the subtle "shake-up" satellite features in the core-electron XPS spectra of the solid polyalkylthiophenes are consistent with a reduction in g-conjugation with increasing temperature. In this short report we present an extension of the XPS shake-up spectra to calculations on the thiophene trimer, which, in comparison with the previously published corresponding spectra of hexyl- [2] and butyl- [3] substituted polythiophenes, adds further support to the model of soft confolvnational defect model [2,7]. DISCUSSION In Fig. 1 are shown the XPS shake-up satellite spectra of P3HT [2] and P3BT, where B = butyl [3], in comparison with the shake-up spectra of three molecular species. The details of these spectra, and the significance for the thermochrornic behavior of the polyalkylthiophenes, have been discussed seperately [6,7]. INDO/CI CALCULATED C1 s SHAKE-UP SPECTRA
3-HEXYL]~HIOPHENE fiAS
DIMER
THIOPHENE GAS
'E .0
20X ~ _ _ ~ ~
BITHIOPHENEGAS Z W 13_
,ox
hi < "l-
TRIMER
W
_>
P3HTI÷190C) ,0,
P3HTI-80Cl /
V I!
~GAS
/ ili \ "~'.i
,
10 0 RELATIVE BINDING ENERGY leVI
_.1 W I
%|
15
10
5
0
RELATIVE BINDING ENERGY (eV)
Figure I 0.eft): .C(ls) shake-up spectra of selected polythiophenes and model molecules [2,3]. Figure 2 (Right): INDO/CI Calculated C(ls) shake-up spectra of the thiophene dimer and trimer [7]. It is important, however, to go beyond the dimer molecules in the comparison of the shake-up specn'a with those of the polymers in the solid state. The vapor pressure of the n'imcr molecule did not permit gas phase X P S measurements of the shake-up spectra. But, fortunately, the INDO/CI simulations of the monomer and dimer shake-up spectra reproduce well the experimental results
1375 [6,7], at least at shake-up energies to within about 10 to 15 eV of the main line, and can be used to study the Irimer. The energy range limitation comes from the use of a lin~ted number of
configurations (- 4000) in the CI simulations, in order to limit the use of computer space and time. The results of an INDO/CI level calculation of shake-up spectra of a thiophene timer and trimer are shown in Fig. 2. Of primary interest is the relative energy of the first major shake-up peak in comparison with the main C(ls) line [6], and how this relative energy varies with the length of the polymer chain (oligomer size). In Fig. 1, the weak cross-hatched satellite peak near 3 eV in the lowest curve represents the long-chain limit to the shake-up spectrum. This peak, however, contains simultaneous contributions from electron energy-loss scattering of the primary C(1 s) electrons (so-called extrinsic effects), as well as the (intrinsic) shake-up contribution. In the longchain limit, the combination peak should correspond approximately in energy to the maximum in the optical absorption spectrum, which occurs near 3.2 eV [1]. Note, that both optical absorption and electron energy loss scattering correspond (mainly) to single-electron transitions in the neutral molecule, while shake-up corresponds to many-electron (but often apparent single-electron-like) transitions in the molecular ion [8]. For large unsaturated molecules, inelastic scattering peaks and shake-up peaks fall within the same energy range [9]. In spectra obtained in the gas phase at sufficiently low vapor pressure (as the case presented here), the electron energy-loss events are eliminated. The INDO/CI calculations produce the first major shake-up peak at slightly too low energy, relative to the main line, although thereafter, the structure are well reproduced [7]. One can correct for the slight under estimation between the calculations and the data by comparing the data on the thiophene monomer and dimer, where both experimental and theoretical spectra are available [7]. The calculated trimer spectra can thus be shifted by the same amount. Following this proceedure, an estimate can be obtained of the relative energy at which the first major shake-up satellite peak will occur in real XPS spectra of the trimer in the gas phase. The energy of the first major shake-up peak is plotted versus the size (number of non-hydrogen atoms) of the thiophene oligomer in Fig. 3. >
o 6 >(.9 n- 4
t.IJ Z I.H n D < I CO
",~
SHAKE-UP ENERGY vs OLIGOMER SIZE 5 "HEAVY ATOMS" PER MONOMER UNIT
-,,
"'"'B
..........
ITL_ -
I!
2 0 0
5
10 15 20 NUMBER OF HEAVY ATOMS (n)
,,co,,
Figure 3: The variation of the energy of the first shake-up peak with oligomer size. Note that n = the number of non-hydrogen atoms, i.e., carbon and sulfur. The energy of the first major shake-up transition appears to saturate between n = 10 (dimers) and n = 15 (trimers), to a value consistent with the weak satellite peak near 3 eV in Fig. 1. This is consistent with a saturation observed for both shake-up shifts and valence band structure in, e.g.,
1376
the alkane series [10]. Thus the "valley" in intensity between the main C(ls) line and the first shake-up peak will not contain any shake-up peaks at any value of "n", consistent with the shake-up spectra for the polymers in Fig. 1. This is an essential point in verifying the soft conformational defect model of the thermochromic behavior of the polyalkylthiophenes, as will be discussed elsewhere [7]. This arguement can be used in reverse also. As a function of increasing temperature, the first shake-up peak in the C(ls) XPS spectra of the polymers moves away from the main peak, toward higher relative energies. This trend is what occurs as one looks from the trimer molecule toward the monomer molecule of polythiophene, i.e., toward a more localized n-electronic system. In addition, in a study of the C(1 s) shake-up spectra of the fused-ring aromatic molecules, Riga et al have shown that for more spatially localized 7t-systems (i.e., benzene as compared to anthracene), (i) the the first major peak, as well as the center-of-gravity of the shake-up spectrum moves away from the main line, and (ii) the overall intensity of the shake-up spectrum increases relative to the main line [11 ]. The growth, with increasing temperature, of significant shake-up intensity at higher relative binding energies in the C(ls) spectra of Fig. l, is precisely the motivation for the soft confopvnational defect model of the thermochromism in the polyalkylthiophenes put forth previously [2]. ACKNOWLEDGEMENTS This work has been supported by the Neste Corporation, Finland, the Swedish Board for Technical Development (STU), and the Swedish Natural Sciences Research Council (NFR). REFERENCES 1 0 . Ingan~is, W. R. Salaneck, J. -E. Osterholm, and J. Laakso, Svnth. Met.. 22 (1988) 395. 2 W.R. Salaneck, O. Ingan~is, B. Th6mans, J. O. Nilsson, B. Sj6gren, J. -E. Osterholm, J. -L. Br&las, and S. Svensson, J. Chem. Phvs. 89 (1988) 4613. 3 W.R. Salaneck, O. Ingan~is, J. O. Nilsson, J. -E. Osterholm, B. Th6mans, and J. -L. Br6das, Svnth. Met. 28 (1989) C451. 4 B. Th6mans, W. R. Salaneck, and J. -L. Br6das, Synth. Met. 28 (1989) C359. 5 G. Zerbi, B. Chierichetti and O. Inganas, to be published. 6 W.R. Salaneck, R. La77~'oni, N. Sato, M. LOgdlund, B. Sj~Sgren,M. P. Keane, S. Svensson, A. Naves de Brito, N. Correia, and S. Lunell, in J. -L. Br&las and R. R. Chance (eds.), Conjugated Polymeric Material~: Opportunities in Electronics. Optoelectronics, and Molecular Electronics, NATO ASI Series E, Vol. 182, Kluwer Academic, Dordrecht, 1990, p-101. 7 M.P. Keane, S. Svensson, A. Naves de Brito, N. Correia, and S. LuneU, B. Sj6gren, O. Inganas, and W. R. Salaneck, J. Chem. Phys., in press. 8 H. -J. Freund and R. W. Bigeiow, Phvsica Scri~ta T17 (1987) 50. 9 D. Nordfors, A. Nilsson, N. Mh-tensson, S. Svensson, U. Gelius, and S. Lunell, J. Chem. Phvs. 88 (1988) 2630. 10 J.J. Pil'eaux, S. Svensson, E. Basilier, P. Malmqvist, U. Gelius, R. Caudano, and K. Siegbahn, Phys. Rev. A14 (1976) 2133. 11 J. Riga, J. J. Pireaux, R. Caudano, and J. J. Verbist, Physica Scripta 16 (1977) 346.