Electronic structure of polyacetylene systems containing disilanylene

Synthetic Metals, 62 (1994) 113-116

113

Electronic structure of polyacetylene systems containing disilanylene Yoichi Yamaguchi Osaka R&D Laboratories, Sumitomo Electric Industries, Ltd., 1-1-3 Shimaya, Konohana-ku, Osaka 554 (Japan)

(Received June 28, 1993; in revised form August 23, 1993; accepted August 25, 1993)

Abstract

The electronic structures in the optimized geometries of polyacetylenesystemscontaining disilanylene (-SiH2-SiH2-) are theoretically studied based on the one-dimensional tight-binding self-consistent field-crystal orbital (SCF-CO) method. Results show that the delocalization of silicon o-electrons is possible along the Si-Si bond direction of the disilanylene and that the ~r-tr* bandgap from the disilanylene pseudo-chain is comparable to the ~r-lr* bandgap from the carbon chain. It is assumed that, if they are doped with a p-type dopant, the injected positive charge will remain in the disilanylene sites, with the disappearance of bond alternation in the carbon chain skeleton.

1. Introduction

Recently, much attention has been focused on polysilanes because of their intriguing semiconducting properties with doping [1], photoluminescence [2] and photoconduction [3]. The effect of a-electron delocalization and polarization in polysilanes plays an important role in these properties, but until now little experimental or theoretical work has been done beyond the silicon o-conjugation frameworks. We are interested in the rr-conjugated polyenes with Si-Si tr-bonds, as illustrated in Fig. 1. The electronic structure of polymers A and C in Fig. 1 might be influenced by the interaction between the -n-conjugated carbon backbone and the it-conjugated silicon pseudochain (Si-Si-..Si). Although these polymers had not been synthesized, polyene structure containing disilanylene was observed by spectroscopic analysis of the WC16 catalytically polymerized films of (HC=CSiMe2)2 [4]. Also, the electrical conductivity (tr) was 10 -9 S/cm for this pristine film and 10 -3 S/cm for iodinedoped film [4]. Therefore, it is worthwhile to examine the electronic properties of these polyenes by quantum chemical calculation. In this article, we study the electronic structures of three kinds of all-trans polyacetylene containing disilanylene (-SiH2-SiH2-), namely, polymers A-C, as illustrated in Fig. 1. For comparative studies, all-trans polyacetylene (PA) was also studied. The electronic structures of doped oligomers were examined instead of these polymers.

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2. Method of calculation

All of the calculations were performed on the basis of the one-dimensional tight-binding self-consistent field-crystal orbital (SCF-CO) method at the complete neglect of differential overlap, version 2 (CNDO/2) level of the approximation [5], using the geometries that we obtained from modified neglect of diatomic overlap, parametric method 3 (MNDO PM3) solid-state calculations [6]. Details of this CNDO/2 method have been given in a previous paper [7]. Twenty-one representative wave vectors were chosen, with regular intervals (-rr/10a, where a is the unit vector of the translational symmetry, being parallel to the polymer chain axis) in the first Brillouin zone. The band levels were drawn in the reduced zone with O~
3. Results and discussion

The unit cells for the polymers are illustrated in Fig. 1. The results of the optimized geometries of the skeletons of all the polymers obtained by the MNDO PM3 method are also shown in Fig. 1. The bond alternant structures are found along the carbon chain in all the polymers. In comparison with PA, the C-C and C = C bond lengths are slightly larger in polymers A and C. On the other hand, the geometrical structure of polymer

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B is similar to that of PA. This indicates, therefore, that the bond alternant structures in the carbon chain part are not significantly affected by the disilanylene unit in these polymers, especially polymer B. The Si~-Si2 bond lengths of polymers A and C are shorter than those of polymer B because of the repulsion between hydrogens connecting neighboring silicons (Si--.Si). The bond angles /_SiSiC are nearly 90° because the distances in polymer B (C~-.-C3), for example, are about 2.47 /~ which are comparable to the Si~-Si2 bond lengths (2.405 /~,). Therefore, the disilanylene (-SiH2-SiH2-) unit fits the all-trans polyacetylene structure. Since the distances in polymers A and C (Si. • •Si) are about 2.64/~, which are comparable to the Si~-Si2 bond lengths (2.33-2.35/k), a-conjugation

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along the silicon pseudo-chain (Si-Si.-.Si) might be possible in addition to the ~--conjugation of the carbon chain skeleton. The comparative band structures for polymers A-C are shown in Fig. 2 along with those of PA. The electronic properties derived from the band calculations are listed in Table 1. Since the chain skeletons of these polymers are planar, the crystal orbitals (COs) are classified into o and ~-types. The lowest unoccupied COs (LUCOs) in all the polymers show anti-bonding carbon 2p~-bands, as in PA. On the other hand, the highest occupied COs (HOCOs) in polymers A and C show bonding Si 3po orbitals along the silicon pseudochain (Si-Si. • •Si) direction. In polymers A and C, the o-o* bandgap and the HOCO or bandwidth for the silicon pseudo-chain part are comparable to the ~'-~r* bandgap and the HOCO ~- bandwidth for the carbon chain part, respectively. In Table 1, large cr-bond order of Si- • • Si indicates that o-conjugation along the silicon pseudo-chain (Si-Si. • • Si) is formed in polymers A and C. If the neighboring disilanylene units are connected,

115

TABLE 1. Calculated results of each polymer~ Polymer

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C

PA

Bandgap cr-o'* Bandgap ~'-¢r * H O o-bandwidth (Si) L U o-bandwidth (Si) H O ~- bandwidth (C) L U ~- bandwidth (C) Ionization potential Electron affinity o-Bond order b Sil-Si2 Si2-. • SiV Atomic net charge C1 C2 C3 C4 Si 1 Si2

8.664 8.195 4.140 0.066 5.032 3.593 7.939 0.258 0.652 0.260 -0.121 + 0.026 -0.124 + 0.036 + 0.387 + 0.389

11.636 8.129 0.323 0.084 3.674 2.830 8.241 0.112 0.656 0.013 -0.092 + 0.001 -0.083

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for example, by oxygen atoms, a larger Si a-conjugation is possible in polymers A and C [8]. On the other hand, polymer B has a large o-o* bandgap and a small HOCO ~r bandwidth because a Si pseudo-chain is not formed, due to small o--bond order of Si-.-Si. The carbon atoms of polymers A-C attached to silicon atoms are negatively charged, due to o charge transfer from the silicon atoms. This is especially so in polymer C, the carbon atoms of which are all negatively charged. Therefore, it is of interest to examine the changes in

the atomic net charges and the geometries when polymers A and C are doped with p-type dopant in HOCO, namely, Si 3po-bonding band. The MNDO PM3 level calculation has been performed to the neutral and cationic ( + 1) oligomers B', C' and PA' containing 12 ethenylene ( - C = C - ) units instead of polymers B, C and PA, respectively. The unrestricted Hartree-Fock (UHF) method is employed in order to examine the electronic properties of these cationic oligomers. The results of the optimized geometries of the skeletons of all the oligomers indicate that the carbon bond alternant structures almost totally disappear in the cationic state and that the disilanylene structures containing Si-C bonds show little changes in the neutral and cationic state. The highest occupied molecular orbital (HOMO) in oligomers B' and C' shows bonding C 2p~- orbitals and bonding Si 3po orbitals as the HOCO in polymers B and C, respectively. Changes in the atomic net charges of each atom site for the neutral and cationic states of oligomers C' and PA' are shown in Figs. 3 and 4. The proportions of the injected positive charge for each atom are shown in Table 2. In Fig. 3, the injected positive charge in oligomers C' and PA' is in the center of the carbon chain part. However, there is little change in the net charges on carbon sites in oligomer C' in the neutral

116 and cationic state. On the other hand, in Fig. 4, there are noticeable changes in the silicon net charge and the two central silicons Sia2 and Si13 form the boundary between the two alternant charge distributions along each silicon pseudo-chain (Si-Si-.. Si). Table 2 shows that, contrary to oligomer PA', a large amount of the injected positive charge in oligomers B' and C' is in the silicon sites. Therefore, in oligomer C', the injected positive charge is in the bonding Si 3ptr orbital, which will become a o--conduction hole along the silicon pseudo-chain (Si-Si - • • Si). In oligomer B', the injected positive charge on the silicon sites will be localized, while it will be delocalized on the carbon sites, as in the oligomer PA'. In the cationic oligomer C', it is interesting to note the relationship between the distribution of an injected positive charge on the silicon sites and the disappearance of the bond alternation in the carbon chain skeleton. Considering the electronic structures of the present polymers B and C, it is assumed that these polymers will have similar behavioral patterns to oligomers B' and C', respectively.

4. Conclusions The electronic structures of polyacetylene systems containing disilanylene (-SiH2-SiH2-) are studied theoretically using the one-dimensional tight-binding SCFCO method under CNDO/2 approximation. It has been found that the delocalization of silicon g-electrons is possible along the Si-Si bond direction of the disilan-

ylene and that the or-o* bandgap from the disilanylene pseudo-chain is comparable to the 7r-er* bandgap from the carbon chain. If they are doped with a p-type dopant, it is assumed that the injected positive charge will be existent in the disilanylene sites, with the disappearance of the bond alternation in the carbon chain skeleton.

Acknowledgements The author would like to thank Professor T. Yamabe of Kyoto University for his discussions. Part of this work was performed by Sumitomo Electric Industries under the management of the Japan High Polymer Center as part of the Industrial Science and Technology Frontier Program supported by the New Energy and Industrial Technology Development Organization.

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