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Electronic structure of polyacetylene with bonded oxygen
Synthetic Metals, 51 (1992) 175-178
175
Electronic structure of polyacetylene with bonded oxygen Francisco C. Lavarda a'b, Douglas S. Galv~o a and Bernardo Laks a aInstituto de Fisica, UNICAMP, 13081 Campinas, SP (Brazil) bDepartamento de Fisica, UNESP.Bauru, 17033 Bauru, SP (Brazil)
Abstract In this work we have studied the electronic structure of finite polyacetylene chains with structural oxygen-bonding models following data from ~3C-NMR experiments. We have used a combination of Austin Method One and Hydrogenic Atoms in Molecules version 3 methods to perform geometric and spectroscopic calculations. Our results show that the electronically-active states are generally unaffected by the incorporation of oxygen.
Introduction The discovery that doped polyacetylene (PA) films could increase their conductivity by many orders of magnitude stimulated much theoretical and experimental work [1]. Unfortunately, the technological use of PA is limited by the rapid degradation of its electronic and mechanical properties when exposed to atmosphere [2]. Oxygen has been suggested as the principal agent responsible for this degradation. Experimental data have shown that the conductivity of a pristine PA sample first increases, and then falls to a value below its initial value when exposed to an oxygen atmosphere [2]. This fact seems to indicate that the degradation is both electronic and chemical. Many mechanisms for oxygen-polymer interactions and/or reactions have been proposed [2, 3] to explain PA film degradation. Data from '3C NMR experiments suggested that the oxygen-polymer bond should be in the form of a terminal carboxyl group or, at least, there should be a double bond between the carbon and oxygen atoms [4]. The purpose of this work is to investigate the electronic structure of finite PA chains with structural oxygen bonding models following the suggestion of 13C NMR data [4]. To do this, we have chosen a set of representative molecules: - - ( I ) C22H24: dimerized PA chain without defect: - - ( I I ) C21H2~: dimerized PA chain with a soliton-like defect in its center; - - ( I I I ) C21H220: a structure similar to (II) where the hydrogen atom of the central carbon atom is substituted by an oxygen atom connected by a double bond;
0379-6779/92/$5.00
~c 1992 Elsevier Sequoia. All rights reserved
176 - - ( I V ) C22H2302: a structure similar to (I) where the oxygen atoms are bonded like a carboxyl group at the end of the chain; - - ( V ) C21H2202: a structure similar to (II) where the oxygen atoms are bonded like a carboxyl group at the end of the chain. As the t r a n s isomer is thermodynamically more stable, and since both heat treatment and doping induce t r a n s isomerization [1], here we have studied only the t r a n s isomers of molecules I - V .
Methodology The same procedure has been adopted for all the molecules studied in this work. First, the geometries of the molecules are optimized using the well-known Austin Method One (AM1) [5]. Although AM1 gives very good results for geometries and heats of formation, its spectroscopic results are very poor. It is therefore strongly recommended that the AM1 method should be used in conjunction with a second method specifically designed to perform spectroscopic calculations. For this reason, we have chosen the Hydrogen Atoms in Molecules (HAM/3) method [6]. This is a functional density method which is well suited to treat electronic correlation and self-repulsion. In spite of good results presented for ionization energies, the HAM/3 method had been criticized for the empirical assumptions made in its development. However, the theoretical formulation has been improved and the method is now successfully used to perform photoelectron energy calculations.
Results and discussion The results obtained are presented and discussed below. First, we give the AM1 results and then the HAM/3 results. In order to obtain a reasonable idea of the geometrical transformations induced by oxygen, the most significant result is that of the bondlength alternation pattern (Ar). The plot of Ar versus the carbon number for molecule I can be seen in Fig. l(a). The chain is dimerized and the bond-lengths for the single and double bonds are 1.444 and 1.347 A: the Ar value is 0.097 A with distortions at the chain ends. Molecule II shows the typical pattern of a soliton defect [7]. Molecules IV and V are, respectively, molecules I and II with added oxygen in the form of carboxyl groups at the ends of the chains. We can see that molecule I (II) goes from closed (open) to open (closed) electronic shells. The general Ar pattern is almost unchanged within the class of molecules (open/closed electronic shells), as will be seen from Fig. 1. Molecule III (Fig. l(c)) presents different behavior with a more abrupt inversion of the dimerization pattern than in II. This molecule is impor-
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Fig. 1. The Ar pattern for molecules I (a); II (b); III (c); IV (d); V (e). Fig. 2. The HAM/3 ~ electronic spectra for molecules I - V . The highest fully occupied molecular orbital was set at 0.0 eV for all molecules. The ( - . - ) bar assigned a levels. Omitted from this diagram is a further ~ level, 1.86 eV (1.94 eV), below the final one in the IV (V) spectra.
t a n t b e c a u s e it simulates some C H - C O copolymers t h a t have shown similar PA c o n d u c t i v i t y values [8]. Figure 2 shows a c o m p a r a t i v e diagram of the HAM/3 results for the energy levels, w h e r e the highest fully occupied m o l e c u l a r orbital energies of the five molecules are set to the same energy reference value of 0.0 eV. For molecule I, the energy 'gap' b e t w e e n u n o c c u p i e d and occupied states is 1.45 eV, in good agreement with optical absorption d a t a where the optical gap onset is at 1.4 and the peaks at 1.6-1.8 eV [9]. Molecules III, IV, and V show spectra similar to I and II. The exceptions are some a levels associated with oxygen atoms. As far as the geometry is concerned, the electronic spectra show the same two types of classes of results and the electronic s t r u c t u r e s are very similar within each class (open/closed electronic shells). An analysis of the eigenfunctions of the electronically-active levels (the highest fully occupied, the semi-occupied, and the lowest fully u n o c c u p i e d m o l e c u l a r orbitals) show t h a t the presence of the oxygen atoms affects these states in a r a t h e r w e a k fashion.
S u m m a r y and conclusions In this w o r k we have carried out semi-empirical calculations on a series of PA molecules with, and without, b o n d e d oxygen. The results
178
have shown that the energy positions of the electronic-active states are little modified by the presence of oxygen either as a central atom or as a carboxyl terminal group. In addition, the spatial distribution pattern of the wave functions associated with these states is also little modified. Based on these results, and on the experimental fact that the doped C H - C O copolymers have shown the same high conductivity values as doped PA, we conclude that the conductivity degradation has its origins dominated by a chemical process such as the generation of oxides and sp 3 defects. We must emphasize that our results are related to isolated chains and the conductivity process in the real samples is a macroscopic phenomenon dominated by charge-transfer mechanisms along the chains. The presence of carboxylic groups strongly affects their polarizability and, consequently, the macroscopic conductivity. It is not clear to us what is the real contribution of this mechanism to the PA degradation. Further studies in these directions are under way at our laboratory.
Acknowledgements We acknowledge Professor Y. Takahata for kindly making available the HAM/3 program. The authors also thank the Brazilian agencies CAPES, CNPq, and FAPESP for their financial support.
References 1 T. A. Skotheim (ed.), Handbook of Conducting Polymers, Marcel Dekker, New York, 1986. 2 J. M. Pochan, The oxidative stability of dopable polyenes, in T. A. Skotheim (ed.), Handbook of Conducting Polymers, Marcel Dekker, New York, 1986, Ch. 39. 3 X.-Z. Yang and J. C. W. Chien, J. Polym. Sci., Polym. Chem. Ed., 23 (1985) 859. 4 M. Helmle, J. D. Becker and M. Mehring, 13C NMR investigation of an oxygen defect in trans-polyacetylene, in H. Kuzmany, M. Mehring and S. Roth (eds.), Electronic Properties of Polymers and Related Compounds, Springer, Berlin, 1985, p. 275. 5 M. J. S. Dewar, E. G. Zoebisch, E. F. Healy and J. J. P. Stewart, J. Am. Chem. Soc., 107 (1975) 3902. 6 E. Lindholm and L. ~sbrink, Molecular Orbitals and their Energies, Studied by the Semiempirical HAM Method, Spinger, Berlin, 1985. 7 W. P. Su, J. R. Schrieffer and A. J. Heeger, Phys. Rev. Lett., 42 (1979) 1698. 8 J. C. W. Chien, G. N. Babu and J. A. Hirsch, Nature (London), 314 (1985) 723. 9 T. C. Chung, F. Moraes, J. D. Flood and A. J. Heeger, Phys. Rev. B, 29 (1984) 2341.
175
Electronic structure of polyacetylene with bonded oxygen Francisco C. Lavarda a'b, Douglas S. Galv~o a and Bernardo Laks a aInstituto de Fisica, UNICAMP, 13081 Campinas, SP (Brazil) bDepartamento de Fisica, UNESP.Bauru, 17033 Bauru, SP (Brazil)
Abstract In this work we have studied the electronic structure of finite polyacetylene chains with structural oxygen-bonding models following data from ~3C-NMR experiments. We have used a combination of Austin Method One and Hydrogenic Atoms in Molecules version 3 methods to perform geometric and spectroscopic calculations. Our results show that the electronically-active states are generally unaffected by the incorporation of oxygen.
Introduction The discovery that doped polyacetylene (PA) films could increase their conductivity by many orders of magnitude stimulated much theoretical and experimental work [1]. Unfortunately, the technological use of PA is limited by the rapid degradation of its electronic and mechanical properties when exposed to atmosphere [2]. Oxygen has been suggested as the principal agent responsible for this degradation. Experimental data have shown that the conductivity of a pristine PA sample first increases, and then falls to a value below its initial value when exposed to an oxygen atmosphere [2]. This fact seems to indicate that the degradation is both electronic and chemical. Many mechanisms for oxygen-polymer interactions and/or reactions have been proposed [2, 3] to explain PA film degradation. Data from '3C NMR experiments suggested that the oxygen-polymer bond should be in the form of a terminal carboxyl group or, at least, there should be a double bond between the carbon and oxygen atoms [4]. The purpose of this work is to investigate the electronic structure of finite PA chains with structural oxygen bonding models following the suggestion of 13C NMR data [4]. To do this, we have chosen a set of representative molecules: - - ( I ) C22H24: dimerized PA chain without defect: - - ( I I ) C21H2~: dimerized PA chain with a soliton-like defect in its center; - - ( I I I ) C21H220: a structure similar to (II) where the hydrogen atom of the central carbon atom is substituted by an oxygen atom connected by a double bond;
0379-6779/92/$5.00
~c 1992 Elsevier Sequoia. All rights reserved
176 - - ( I V ) C22H2302: a structure similar to (I) where the oxygen atoms are bonded like a carboxyl group at the end of the chain; - - ( V ) C21H2202: a structure similar to (II) where the oxygen atoms are bonded like a carboxyl group at the end of the chain. As the t r a n s isomer is thermodynamically more stable, and since both heat treatment and doping induce t r a n s isomerization [1], here we have studied only the t r a n s isomers of molecules I - V .
Methodology The same procedure has been adopted for all the molecules studied in this work. First, the geometries of the molecules are optimized using the well-known Austin Method One (AM1) [5]. Although AM1 gives very good results for geometries and heats of formation, its spectroscopic results are very poor. It is therefore strongly recommended that the AM1 method should be used in conjunction with a second method specifically designed to perform spectroscopic calculations. For this reason, we have chosen the Hydrogen Atoms in Molecules (HAM/3) method [6]. This is a functional density method which is well suited to treat electronic correlation and self-repulsion. In spite of good results presented for ionization energies, the HAM/3 method had been criticized for the empirical assumptions made in its development. However, the theoretical formulation has been improved and the method is now successfully used to perform photoelectron energy calculations.
Results and discussion The results obtained are presented and discussed below. First, we give the AM1 results and then the HAM/3 results. In order to obtain a reasonable idea of the geometrical transformations induced by oxygen, the most significant result is that of the bondlength alternation pattern (Ar). The plot of Ar versus the carbon number for molecule I can be seen in Fig. l(a). The chain is dimerized and the bond-lengths for the single and double bonds are 1.444 and 1.347 A: the Ar value is 0.097 A with distortions at the chain ends. Molecule II shows the typical pattern of a soliton defect [7]. Molecules IV and V are, respectively, molecules I and II with added oxygen in the form of carboxyl groups at the ends of the chains. We can see that molecule I (II) goes from closed (open) to open (closed) electronic shells. The general Ar pattern is almost unchanged within the class of molecules (open/closed electronic shells), as will be seen from Fig. 1. Molecule III (Fig. l(c)) presents different behavior with a more abrupt inversion of the dimerization pattern than in II. This molecule is impor-
177
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Fig. 1. The Ar pattern for molecules I (a); II (b); III (c); IV (d); V (e). Fig. 2. The HAM/3 ~ electronic spectra for molecules I - V . The highest fully occupied molecular orbital was set at 0.0 eV for all molecules. The ( - . - ) bar assigned a levels. Omitted from this diagram is a further ~ level, 1.86 eV (1.94 eV), below the final one in the IV (V) spectra.
t a n t b e c a u s e it simulates some C H - C O copolymers t h a t have shown similar PA c o n d u c t i v i t y values [8]. Figure 2 shows a c o m p a r a t i v e diagram of the HAM/3 results for the energy levels, w h e r e the highest fully occupied m o l e c u l a r orbital energies of the five molecules are set to the same energy reference value of 0.0 eV. For molecule I, the energy 'gap' b e t w e e n u n o c c u p i e d and occupied states is 1.45 eV, in good agreement with optical absorption d a t a where the optical gap onset is at 1.4 and the peaks at 1.6-1.8 eV [9]. Molecules III, IV, and V show spectra similar to I and II. The exceptions are some a levels associated with oxygen atoms. As far as the geometry is concerned, the electronic spectra show the same two types of classes of results and the electronic s t r u c t u r e s are very similar within each class (open/closed electronic shells). An analysis of the eigenfunctions of the electronically-active levels (the highest fully occupied, the semi-occupied, and the lowest fully u n o c c u p i e d m o l e c u l a r orbitals) show t h a t the presence of the oxygen atoms affects these states in a r a t h e r w e a k fashion.
S u m m a r y and conclusions In this w o r k we have carried out semi-empirical calculations on a series of PA molecules with, and without, b o n d e d oxygen. The results
178
have shown that the energy positions of the electronic-active states are little modified by the presence of oxygen either as a central atom or as a carboxyl terminal group. In addition, the spatial distribution pattern of the wave functions associated with these states is also little modified. Based on these results, and on the experimental fact that the doped C H - C O copolymers have shown the same high conductivity values as doped PA, we conclude that the conductivity degradation has its origins dominated by a chemical process such as the generation of oxides and sp 3 defects. We must emphasize that our results are related to isolated chains and the conductivity process in the real samples is a macroscopic phenomenon dominated by charge-transfer mechanisms along the chains. The presence of carboxylic groups strongly affects their polarizability and, consequently, the macroscopic conductivity. It is not clear to us what is the real contribution of this mechanism to the PA degradation. Further studies in these directions are under way at our laboratory.
Acknowledgements We acknowledge Professor Y. Takahata for kindly making available the HAM/3 program. The authors also thank the Brazilian agencies CAPES, CNPq, and FAPESP for their financial support.
References 1 T. A. Skotheim (ed.), Handbook of Conducting Polymers, Marcel Dekker, New York, 1986. 2 J. M. Pochan, The oxidative stability of dopable polyenes, in T. A. Skotheim (ed.), Handbook of Conducting Polymers, Marcel Dekker, New York, 1986, Ch. 39. 3 X.-Z. Yang and J. C. W. Chien, J. Polym. Sci., Polym. Chem. Ed., 23 (1985) 859. 4 M. Helmle, J. D. Becker and M. Mehring, 13C NMR investigation of an oxygen defect in trans-polyacetylene, in H. Kuzmany, M. Mehring and S. Roth (eds.), Electronic Properties of Polymers and Related Compounds, Springer, Berlin, 1985, p. 275. 5 M. J. S. Dewar, E. G. Zoebisch, E. F. Healy and J. J. P. Stewart, J. Am. Chem. Soc., 107 (1975) 3902. 6 E. Lindholm and L. ~sbrink, Molecular Orbitals and their Energies, Studied by the Semiempirical HAM Method, Spinger, Berlin, 1985. 7 W. P. Su, J. R. Schrieffer and A. J. Heeger, Phys. Rev. Lett., 42 (1979) 1698. 8 J. C. W. Chien, G. N. Babu and J. A. Hirsch, Nature (London), 314 (1985) 723. 9 T. C. Chung, F. Moraes, J. D. Flood and A. J. Heeger, Phys. Rev. B, 29 (1984) 2341.