- Email: [email protected]
Realization of molecular interconnection for molecular electronics: Theoretical aspects
Computational Materials Science 36 (2006) 130–134 www.elsevier.com/locate/commatsci
Realization of molecular interconnection for molecular electronics: Theoretical aspects R.V. Belosludov a
a,b,*
, A.A. Farajian a, Y. Kikuchi a, H. Mizuseki a, Y. Kawazoe
a
Institute for Materials Research, Tohoku University, 2-1-1 Katahira, Aoba-ku, Sendai 980-8577, Japan b Nikolaev Institute of Inorganic Chemistry SB RAS, Novosibirsk 630090, Russia Received 14 July 2004; accepted 22 November 2004
Abstract The structural and electronic properties of conducting polymer covered with cyclodextrin molecules (CDs) have been investigated using quantum mechanical simulations. Thus, the results of calculations showed that the structure of polyaniline, in the cases of b-CDs and molecular nanotube of cross-linking a-CDs has near-planar geometry, with the electronic configuration of the optimized structure being practically same as the one in the planar conformation. It has also been found that in these cases, there are no charge transfer between polymer fragment and frameworks of CDs. The doping effect on the geometric and electronic properties of polyaniline encapsulated by CDs has been also investigated. It has been shown that the doped polymer chain can be stabilized inside the molecular nanotube of cross-linking a-CDs. These results support the realization of molecular electronic device based on this complex. Ó 2005 Elsevier B.V. All rights reserved. PACS: 31.15.A; 07.05.T Keywords: Molecular nanotube; Polyaniline; Cyclodextrins; Molecular interconnection; Quantum mechanical simulations; Molecular electronics
1. Introduction Despite a remarkable miniaturization trend in the semiconductor industry, in the next 10–15 years, conventional Si-based microelectronics is likely face fundamental limitations when feature lengths shrink below 100 nm [1]. Therefore, fundamentally new approaches for realization of electronic parts are required. In 1974, the possibility of an organic molecule functioning as a molecular rectifier was first theoretically demonstrated by Aviram and Ratner [2] and later this was confirmed experimentally [3]. After that and specifically
* Corresponding author. Address: Institute for Materials Research, Tohoku University, 2-1-1 Katahira, Aoba-ku, Sendai 980-8577, Japan. Tel.: +81 22 215 2182; fax: +81 22 215 2188. E-mail address: [email protected] (R.V. Belosludov).
0927-0256/$ - see front matter Ó 2005 Elsevier B.V. All rights reserved. doi:10.1016/j.commatsci.2004.11.012
in the last several years, there have been many scientific efforts and significant advances in the realization of electronic devices (such as the wire, diode or transistor) integrated on the molecular scale [4,5]. Since the size of these molecular devices is a few nanometers, in parallel with the progress of more effective fabrication technologies, theoretical study of promising molecular structures is also one of the key factors for designing new electronic devices with the desired physical characteristics. For application to molecular electronics, the wire is a very important component because it can be used as an interconnection between a metal electrode and other functional molecules, such as a molecular diode or transistor, to create complex molecular circuits. It is also important that the conducting part of a molecular wire should have metallic characteristics. For these reasons, conducting polymers are very attractive materials
R.V. Belosludov et al. / Computational Materials Science 36 (2006) 130–134
among the different candidates for molecular wires. Their electrical conductivity can be controlled over the full range from insulator to metal by chemical or electrochemical doping [6–8]. They can be synthesized with highly controlled lengths and can be integrated in complex circuits by chemically bonding with other functional molecules without changing their electronic properties. In order to prevent the possible interaction between different molecular wires it would be better if a single polymer chain were to be encapsulated into a bulky insulated structure and hence forming a molecular enamel wire. According to the ‘‘molecular enamel wire’’ concept [4], the insulators are placed around a conducting center. It has been also suggested that the ‘‘molecular enamel wire’’ would be one of the key concepts for realizing a high performance molecular supercomputer. The possible approach for the realization of this concept is the formation of an inclusion complex between the conducting polymer and cyclic cyclodextrin (CD) molecules as shown in Fig. 1. The cavity size of CD can be regulated by the number of D-glucose units in each CD molecule (6, 7, and 8 for a-, b-, and c-CD, respectively) and a molecular tube can be created by cross-linking adjacent a-CD units using a hydroxypropylene bridge [9]. Recently, atomic force microscopy (AFM) and scanning tunneling microscopy (STM) observations indicated the formation of an inclusion complex in which the polymer is fully covered by b-CD molecules [10] and also a molecular nanotube of cross-linking a-CD molecules [11]. Moreover, theoretical studies also indicated that b-CD molecules can be used as an insulated molecular structure for stabilization of the isolated near-planar configuration of polymers, with the electronic configuration of the optimized structure being almost the same as that in the planar conformation [12–14]. Here, we report the structural and electronic properties of various CD-polyaniline inclusion complexes. The aim of this study is to demonstrate the possibility of the formation of single molecular enamel wires, using quantum mechanical simulations.
Fig. 1. Schematic diagram of molecular circuit: (a) inclusion complex formation of CDs and conducting polymer and (b) polymer chain into molecular nanotube of cross-linking a-CDs.
131
2. Theoretical methods Since the treated inclusion complexes consist of a huge number of atoms, the combined quantum mechanics and molecular mechanics calculations has been applied for optimization of the polymer structure into CD molecular nanotubes. The two-layered ‘‘own Nlayered integrated molecular orbital and molecular mechanics’’ (ONIOM) method [15] has been applied. In this hybrid method, the structure of the polymer fragment is treated quantum mechanically while the remainder of the system (CD molecules) is treated by a molecular mechanics force field. By this method, the structure of the selected cluster model has been optimized. In order to examine the possible charge transfer as well as the interaction between polymer fragments and CDs, the single-point energy calculation has been performed for optimal configuration of this inclusion complex. These calculations have been performed using the Gaussian 98 set of programs [16].
3. Results and discussion 3.1. Structure of polymer chain in CDs It is important to know what configuration of polyaniline (PANI) fragment is formed in CDs because the source of conductivity for a conjugated conducting polymer is a set of p-type molecular orbitals that lie above and below the plane of the molecule when it is in a planar or near-planar conformation. The long polymer fragment (nine monomer units) has been optimized in free space using both full optimization in order to find the lowest energy structure and partial optimization while maintaining the planar configuration of the PANI fragment. Fig. 2a and d shows the respective optimized structures. In the case of full optimization, the imaginary frequencies are absent, which means that it is a local minimum. In this configuration, the adjacent benzene rings have a dihedral angle of 90°. This would reduce the extent of p-orbital overlap between adjacent rings, break up the electron channels, and decrease the conductivity of the molecular wire. The planar structure is higher in energy by 38.65 kcal/mol at HF/6-31G* level compared to the most stable structure. Moreover, the imaginary frequencies are found for the planar configuration and correspond to the combination of out-of-plane (bent) vibrations of benzene rings. Therefore it is necessary to apply external forces for stabilization of the planar configuration. The geometry of the PANI has been optimized in b-CDs and the cross-linking a-CDs and these structures are shown in Fig. 2b and c, respectively. The structure of the PANI in b-CDs lies higher in energy by 7.10 kcal/ mol as compared to the most stable configuration of
132
R.V. Belosludov et al. / Computational Materials Science 36 (2006) 130–134
planar configuration of the polymer chain is formed in a molecular nanotube of cross-linking a-CDs due to weak interactions such as Coulomb and van der Waals interactions between the host framework of the CDs and the PANI. 3.2. Electronic structure of polymer chain in CDs
Fig. 2. Structural analysis of PANI fragments: (a) the most stable configuration in free space; (b) in b-CDs; (c) in molecular nanotube of cross-linking a-CDs and (d) the planar configuration in free space.
the same PANI fragment in free space. In the case of the cross-linking a-CD molecular nanotube, this energy difference is found to be a 20.91 kcal/mol. Moreover, the ˚ ) is very length of polymer chain in this case (45.50 A close to the length of planar conformation of polymer ˚ ) as shown in Fig. 2. These results indicate chain (46.92 A that the configurations of PANI in the cross-linking aCD molecular nanotube are closer to the planar structure of PANI in free space than the configuration of PANI in b-CDs. It has also been found in the both cases that there is no charge transfer between polymer fragment and CDs, and hence the interaction between these molecules has a non-covalent character. Thus, the near-
In order to understand the electron transport through polymer chain in CDs, we have analyzed the spatial extent of the frontier orbital, which provides a strategy by which the transport properties of these systems can be understood. Analysis of the molecular orbital energy diagrams (Fig. 3) for the configurations of PANI in the CDs host framework shows that the lowest unoccupied (LUMO and LUMO+1) orbitals (as well as LUMO+2) are located on the polymer fragment and their contours are similar to those in the case of the planar configuration of PANI in free space. Moreover, in these cases, there is no overlap of electron density between CDs and the PANI fragment. This reveals that the inclusion complex based on the crosslinking a-CD nanotube and b-CD host frameworks can be used as molecular enamel wire. This result may be important from a practical point of view because the electron will transport only through the polymer chain and there is no current leakage across the CD molecules. 3.3. Doping of polymer chain in CDs In order to realize the concept of molecular enamel wire, it is also necessary to understand the stability and the electronic properties of the conducting polymers
Fig. 3. Contour of the lowest unoccupied orbitals: (a) LUMO of PANI-b-CDs; (b) LUMO of PANI-cross-linking a-CDs; (c) LUMO+1 of PANI-bCDs and (d) LUMO+1 of PANIs-cross-linking a-CDs.
R.V. Belosludov et al. / Computational Materials Science 36 (2006) 130–134
133
Fig. 4. Contour of the selected molecular orbitals of ES of PANI fragment in molecular nanotube of cross-linking a-CDs: (a) SOMO; (b) SOMO+1; (c) LUMO+2 and (d) LUMO+3.
in the metallic state when they are encapsulated within molecular nanotubes. For this purpose, the PANI fragment in the metallic (emeraldine salt) state has been optimized inside the cross-linking a-CD molecular nanotube. It is well known through experiment that protonation by the acid–base chemistry leads to an internal redox reaction and the conversion from semiconductor (the emeraldine base, EB) to metal (the emeraldine salt, ES) [8]. The five monomers in free space have been optimized using both full optimization, in order to find the lowest energy structure, and partial optimization while maintaining the planar configuration of PANI with ES. It is found that the lowest energy structure of ES with five monomer units has a total spin of S = 1 which indicates the existence of two unpaired spins. The HOMO–LUMO energy difference is significantly reduced as compared with the same energy difference for EB which has the same number of benzene rings. This indicates the transition of PANI from semiconducting to metallic state. It is also found that by using the cross-linking CD molecular nanotubes one can stabilize the near-planar configuration of the metallic form of PANI. Analysis of molecular orbital energy diagrams for the configuration of EB in molecular nanotube shows that the single occupied molecular (SOMO, SOMO+1) as well the lowest unoccupied (LUMO+2 and LUMO+3) orbitals (Fig. 4) are located on the polymer fragment and their contours are similar to those in the case of the planar configuration of ES. These orbitals are located on polymer chain and hence the CDs can be used as insulator between different single molecular wires. Therefore the present theoretical results provide support for the ‘‘molecular enamel wire’’ concept [4] and hence indicate a high possibility of realizing this concept in molecular electronics.
4. Conclusions The structure of PANI fragments in various inclusion complexes based on CD molecules was optimized using the combined quantum mechanics and molecular mechanics method. The results of calculations showed that the structures of the PANI in molecular nanotubes of cross-linking a-CDs have near-planar geometry with the electronic configuration of the optimized structure being almost the same as that in the planar conformation. Moreover, the single chain of ES PANI (metallic form) can be covered with the insulator CD molecular nanotube. The theoretical results, which are in agreement with experimental data [10,11] may also advance the application of such inclusion complexes in molecular circuits with more complex functionality. Acknowledgements The authors would like to express their sincere thanks to the staff of the Center for Computational Materials Science of the Institute for Materials Research, Tohoku University for their continuous support of the SR8000 supercomputing facilities. This study was performed through the Special Coordination Funds for Promoting Science and Technology from the Ministry of Education, Culture, Sports, Science and Technology of the Japanese Government. References [1] C. Joachim, J.K. Gimzewski, A. Aviram, Electronics using hybrid-molecular and mono-molecular devices, Nature 408 (2000) 541–548. [2] A. Aviram, M.A. Ratner, Molecular rectifiers, Chem. Phys. Lett. 29 (1974) 277–283.
134
R.V. Belosludov et al. / Computational Materials Science 36 (2006) 130–134
[3] M. Fujihira, N. Ohishi, T. Osa, Photocell using covalently-bonded dyes on semiconductor surface, Nature 268 (1977) 226–228. [4] Y. Wada, M. Tsukada, M. Fujihira, K. Matsushige, T. Ogawa, M. Haga, S. Tanaka, Prospects and problems of single molecule information devices, Jpn. J. Appl. Phys. 39 (2000) 3835–3849, and references therein. [5] J.C. Ellenbogen, J.C. Love, Architectures for molecular electronic computers: 1. Logic structures and an adder designed from molecular electronic diodes, Proc. IEEE 88 (2000) 386–426, and references therein. [6] H. Shirakawa, The discovery of polyacetylene film: the dawning of an era of conducting polymers (nobel lecture), Angew. Chem. Int. Ed. 40 (2001) 2574–2580. [7] A.G. MacDiarmid, ‘‘Synthetic metals’’: a novel role for organic polymers (nobel lecture), Angew. Chem. Int. Ed. 40 (2001) 2581– 2590. [8] A.J. Heeger, Semiconducting and metallic polymers: the fourth generation of polymeric materials (nobel lecture), Angew. Chem. Int. Ed. 40 (2000) 2591–2611. [9] A. Harada, J. Li, M. Kamachi, Double-stranded inclusion complexes of cyclodextrin threaded on poly(ethylene glycol), Nature 364 (1994) 516–518. [10] K. Yoshida, T. Shimomura, K. Ito, R. Hayakawa, Inclusion complex formation of cyclodextrin and polyaniline, Langmuir 15 (1999) 910–913.
[11] T. Shimomura, T. Akai, T. Abe, K. Ito, Atomic force microscopy observation of insulated molecular wire formed by conducting polymer and molecular nanotube, J. Chem. Phys. 116 (2002) 1753–1755. [12] R.V. Belosludov, H. Mizuseki, K. Ichinoseki, Y. Kawazoe, Theoretical study of inclusion complex of polyaniline covered by cyclodextrins for molecular device, Jpn. J. Appl. Phys. 41 (2002) 2739–2741. [13] R.V. Belosludov, H. Sato, A.A. Farajian, H. Mizuseki, Y. Kawazoe, Theoretical study of insulated wires based on polymer chains encapsulated in molecular nanotube, Thin Solid Films 438– 439 (2003) 80–84. [14] R.V. Belosludov, H. Sato, A.A. Farajian, H. Mizuseki, Y. Kawazoe, Theoretical study of molecular enamel wire based on polythiophene-cyclodextrin inclusion complexes, Mol. Cryst. Liquid Cryst. 406 (2003) 205. [15] S. Humbel, S. Sieber, K. Morokuma, The IMOMO method: integration of different levels of molecular orbital approximations for geometry optimization of large systems: test for n-butane conformation and S(N)2 reaction: RCl + Cl , J. Chem. Phys. 105 (1996) 1959–1967. [16] M.J. Frisch et al., Gaussian 98, Revision A.1; Gaussian, Inc., Pittsburg, PA, 1998.
Realization of molecular interconnection for molecular electronics: Theoretical aspects R.V. Belosludov a
a,b,*
, A.A. Farajian a, Y. Kikuchi a, H. Mizuseki a, Y. Kawazoe
a
Institute for Materials Research, Tohoku University, 2-1-1 Katahira, Aoba-ku, Sendai 980-8577, Japan b Nikolaev Institute of Inorganic Chemistry SB RAS, Novosibirsk 630090, Russia Received 14 July 2004; accepted 22 November 2004
Abstract The structural and electronic properties of conducting polymer covered with cyclodextrin molecules (CDs) have been investigated using quantum mechanical simulations. Thus, the results of calculations showed that the structure of polyaniline, in the cases of b-CDs and molecular nanotube of cross-linking a-CDs has near-planar geometry, with the electronic configuration of the optimized structure being practically same as the one in the planar conformation. It has also been found that in these cases, there are no charge transfer between polymer fragment and frameworks of CDs. The doping effect on the geometric and electronic properties of polyaniline encapsulated by CDs has been also investigated. It has been shown that the doped polymer chain can be stabilized inside the molecular nanotube of cross-linking a-CDs. These results support the realization of molecular electronic device based on this complex. Ó 2005 Elsevier B.V. All rights reserved. PACS: 31.15.A; 07.05.T Keywords: Molecular nanotube; Polyaniline; Cyclodextrins; Molecular interconnection; Quantum mechanical simulations; Molecular electronics
1. Introduction Despite a remarkable miniaturization trend in the semiconductor industry, in the next 10–15 years, conventional Si-based microelectronics is likely face fundamental limitations when feature lengths shrink below 100 nm [1]. Therefore, fundamentally new approaches for realization of electronic parts are required. In 1974, the possibility of an organic molecule functioning as a molecular rectifier was first theoretically demonstrated by Aviram and Ratner [2] and later this was confirmed experimentally [3]. After that and specifically
* Corresponding author. Address: Institute for Materials Research, Tohoku University, 2-1-1 Katahira, Aoba-ku, Sendai 980-8577, Japan. Tel.: +81 22 215 2182; fax: +81 22 215 2188. E-mail address: [email protected] (R.V. Belosludov).
0927-0256/$ - see front matter Ó 2005 Elsevier B.V. All rights reserved. doi:10.1016/j.commatsci.2004.11.012
in the last several years, there have been many scientific efforts and significant advances in the realization of electronic devices (such as the wire, diode or transistor) integrated on the molecular scale [4,5]. Since the size of these molecular devices is a few nanometers, in parallel with the progress of more effective fabrication technologies, theoretical study of promising molecular structures is also one of the key factors for designing new electronic devices with the desired physical characteristics. For application to molecular electronics, the wire is a very important component because it can be used as an interconnection between a metal electrode and other functional molecules, such as a molecular diode or transistor, to create complex molecular circuits. It is also important that the conducting part of a molecular wire should have metallic characteristics. For these reasons, conducting polymers are very attractive materials
R.V. Belosludov et al. / Computational Materials Science 36 (2006) 130–134
among the different candidates for molecular wires. Their electrical conductivity can be controlled over the full range from insulator to metal by chemical or electrochemical doping [6–8]. They can be synthesized with highly controlled lengths and can be integrated in complex circuits by chemically bonding with other functional molecules without changing their electronic properties. In order to prevent the possible interaction between different molecular wires it would be better if a single polymer chain were to be encapsulated into a bulky insulated structure and hence forming a molecular enamel wire. According to the ‘‘molecular enamel wire’’ concept [4], the insulators are placed around a conducting center. It has been also suggested that the ‘‘molecular enamel wire’’ would be one of the key concepts for realizing a high performance molecular supercomputer. The possible approach for the realization of this concept is the formation of an inclusion complex between the conducting polymer and cyclic cyclodextrin (CD) molecules as shown in Fig. 1. The cavity size of CD can be regulated by the number of D-glucose units in each CD molecule (6, 7, and 8 for a-, b-, and c-CD, respectively) and a molecular tube can be created by cross-linking adjacent a-CD units using a hydroxypropylene bridge [9]. Recently, atomic force microscopy (AFM) and scanning tunneling microscopy (STM) observations indicated the formation of an inclusion complex in which the polymer is fully covered by b-CD molecules [10] and also a molecular nanotube of cross-linking a-CD molecules [11]. Moreover, theoretical studies also indicated that b-CD molecules can be used as an insulated molecular structure for stabilization of the isolated near-planar configuration of polymers, with the electronic configuration of the optimized structure being almost the same as that in the planar conformation [12–14]. Here, we report the structural and electronic properties of various CD-polyaniline inclusion complexes. The aim of this study is to demonstrate the possibility of the formation of single molecular enamel wires, using quantum mechanical simulations.
Fig. 1. Schematic diagram of molecular circuit: (a) inclusion complex formation of CDs and conducting polymer and (b) polymer chain into molecular nanotube of cross-linking a-CDs.
131
2. Theoretical methods Since the treated inclusion complexes consist of a huge number of atoms, the combined quantum mechanics and molecular mechanics calculations has been applied for optimization of the polymer structure into CD molecular nanotubes. The two-layered ‘‘own Nlayered integrated molecular orbital and molecular mechanics’’ (ONIOM) method [15] has been applied. In this hybrid method, the structure of the polymer fragment is treated quantum mechanically while the remainder of the system (CD molecules) is treated by a molecular mechanics force field. By this method, the structure of the selected cluster model has been optimized. In order to examine the possible charge transfer as well as the interaction between polymer fragments and CDs, the single-point energy calculation has been performed for optimal configuration of this inclusion complex. These calculations have been performed using the Gaussian 98 set of programs [16].
3. Results and discussion 3.1. Structure of polymer chain in CDs It is important to know what configuration of polyaniline (PANI) fragment is formed in CDs because the source of conductivity for a conjugated conducting polymer is a set of p-type molecular orbitals that lie above and below the plane of the molecule when it is in a planar or near-planar conformation. The long polymer fragment (nine monomer units) has been optimized in free space using both full optimization in order to find the lowest energy structure and partial optimization while maintaining the planar configuration of the PANI fragment. Fig. 2a and d shows the respective optimized structures. In the case of full optimization, the imaginary frequencies are absent, which means that it is a local minimum. In this configuration, the adjacent benzene rings have a dihedral angle of 90°. This would reduce the extent of p-orbital overlap between adjacent rings, break up the electron channels, and decrease the conductivity of the molecular wire. The planar structure is higher in energy by 38.65 kcal/mol at HF/6-31G* level compared to the most stable structure. Moreover, the imaginary frequencies are found for the planar configuration and correspond to the combination of out-of-plane (bent) vibrations of benzene rings. Therefore it is necessary to apply external forces for stabilization of the planar configuration. The geometry of the PANI has been optimized in b-CDs and the cross-linking a-CDs and these structures are shown in Fig. 2b and c, respectively. The structure of the PANI in b-CDs lies higher in energy by 7.10 kcal/ mol as compared to the most stable configuration of
132
R.V. Belosludov et al. / Computational Materials Science 36 (2006) 130–134
planar configuration of the polymer chain is formed in a molecular nanotube of cross-linking a-CDs due to weak interactions such as Coulomb and van der Waals interactions between the host framework of the CDs and the PANI. 3.2. Electronic structure of polymer chain in CDs
Fig. 2. Structural analysis of PANI fragments: (a) the most stable configuration in free space; (b) in b-CDs; (c) in molecular nanotube of cross-linking a-CDs and (d) the planar configuration in free space.
the same PANI fragment in free space. In the case of the cross-linking a-CD molecular nanotube, this energy difference is found to be a 20.91 kcal/mol. Moreover, the ˚ ) is very length of polymer chain in this case (45.50 A close to the length of planar conformation of polymer ˚ ) as shown in Fig. 2. These results indicate chain (46.92 A that the configurations of PANI in the cross-linking aCD molecular nanotube are closer to the planar structure of PANI in free space than the configuration of PANI in b-CDs. It has also been found in the both cases that there is no charge transfer between polymer fragment and CDs, and hence the interaction between these molecules has a non-covalent character. Thus, the near-
In order to understand the electron transport through polymer chain in CDs, we have analyzed the spatial extent of the frontier orbital, which provides a strategy by which the transport properties of these systems can be understood. Analysis of the molecular orbital energy diagrams (Fig. 3) for the configurations of PANI in the CDs host framework shows that the lowest unoccupied (LUMO and LUMO+1) orbitals (as well as LUMO+2) are located on the polymer fragment and their contours are similar to those in the case of the planar configuration of PANI in free space. Moreover, in these cases, there is no overlap of electron density between CDs and the PANI fragment. This reveals that the inclusion complex based on the crosslinking a-CD nanotube and b-CD host frameworks can be used as molecular enamel wire. This result may be important from a practical point of view because the electron will transport only through the polymer chain and there is no current leakage across the CD molecules. 3.3. Doping of polymer chain in CDs In order to realize the concept of molecular enamel wire, it is also necessary to understand the stability and the electronic properties of the conducting polymers
Fig. 3. Contour of the lowest unoccupied orbitals: (a) LUMO of PANI-b-CDs; (b) LUMO of PANI-cross-linking a-CDs; (c) LUMO+1 of PANI-bCDs and (d) LUMO+1 of PANIs-cross-linking a-CDs.
R.V. Belosludov et al. / Computational Materials Science 36 (2006) 130–134
133
Fig. 4. Contour of the selected molecular orbitals of ES of PANI fragment in molecular nanotube of cross-linking a-CDs: (a) SOMO; (b) SOMO+1; (c) LUMO+2 and (d) LUMO+3.
in the metallic state when they are encapsulated within molecular nanotubes. For this purpose, the PANI fragment in the metallic (emeraldine salt) state has been optimized inside the cross-linking a-CD molecular nanotube. It is well known through experiment that protonation by the acid–base chemistry leads to an internal redox reaction and the conversion from semiconductor (the emeraldine base, EB) to metal (the emeraldine salt, ES) [8]. The five monomers in free space have been optimized using both full optimization, in order to find the lowest energy structure, and partial optimization while maintaining the planar configuration of PANI with ES. It is found that the lowest energy structure of ES with five monomer units has a total spin of S = 1 which indicates the existence of two unpaired spins. The HOMO–LUMO energy difference is significantly reduced as compared with the same energy difference for EB which has the same number of benzene rings. This indicates the transition of PANI from semiconducting to metallic state. It is also found that by using the cross-linking CD molecular nanotubes one can stabilize the near-planar configuration of the metallic form of PANI. Analysis of molecular orbital energy diagrams for the configuration of EB in molecular nanotube shows that the single occupied molecular (SOMO, SOMO+1) as well the lowest unoccupied (LUMO+2 and LUMO+3) orbitals (Fig. 4) are located on the polymer fragment and their contours are similar to those in the case of the planar configuration of ES. These orbitals are located on polymer chain and hence the CDs can be used as insulator between different single molecular wires. Therefore the present theoretical results provide support for the ‘‘molecular enamel wire’’ concept [4] and hence indicate a high possibility of realizing this concept in molecular electronics.
4. Conclusions The structure of PANI fragments in various inclusion complexes based on CD molecules was optimized using the combined quantum mechanics and molecular mechanics method. The results of calculations showed that the structures of the PANI in molecular nanotubes of cross-linking a-CDs have near-planar geometry with the electronic configuration of the optimized structure being almost the same as that in the planar conformation. Moreover, the single chain of ES PANI (metallic form) can be covered with the insulator CD molecular nanotube. The theoretical results, which are in agreement with experimental data [10,11] may also advance the application of such inclusion complexes in molecular circuits with more complex functionality. Acknowledgements The authors would like to express their sincere thanks to the staff of the Center for Computational Materials Science of the Institute for Materials Research, Tohoku University for their continuous support of the SR8000 supercomputing facilities. This study was performed through the Special Coordination Funds for Promoting Science and Technology from the Ministry of Education, Culture, Sports, Science and Technology of the Japanese Government. References [1] C. Joachim, J.K. Gimzewski, A. Aviram, Electronics using hybrid-molecular and mono-molecular devices, Nature 408 (2000) 541–548. [2] A. Aviram, M.A. Ratner, Molecular rectifiers, Chem. Phys. Lett. 29 (1974) 277–283.
134
R.V. Belosludov et al. / Computational Materials Science 36 (2006) 130–134
[3] M. Fujihira, N. Ohishi, T. Osa, Photocell using covalently-bonded dyes on semiconductor surface, Nature 268 (1977) 226–228. [4] Y. Wada, M. Tsukada, M. Fujihira, K. Matsushige, T. Ogawa, M. Haga, S. Tanaka, Prospects and problems of single molecule information devices, Jpn. J. Appl. Phys. 39 (2000) 3835–3849, and references therein. [5] J.C. Ellenbogen, J.C. Love, Architectures for molecular electronic computers: 1. Logic structures and an adder designed from molecular electronic diodes, Proc. IEEE 88 (2000) 386–426, and references therein. [6] H. Shirakawa, The discovery of polyacetylene film: the dawning of an era of conducting polymers (nobel lecture), Angew. Chem. Int. Ed. 40 (2001) 2574–2580. [7] A.G. MacDiarmid, ‘‘Synthetic metals’’: a novel role for organic polymers (nobel lecture), Angew. Chem. Int. Ed. 40 (2001) 2581– 2590. [8] A.J. Heeger, Semiconducting and metallic polymers: the fourth generation of polymeric materials (nobel lecture), Angew. Chem. Int. Ed. 40 (2000) 2591–2611. [9] A. Harada, J. Li, M. Kamachi, Double-stranded inclusion complexes of cyclodextrin threaded on poly(ethylene glycol), Nature 364 (1994) 516–518. [10] K. Yoshida, T. Shimomura, K. Ito, R. Hayakawa, Inclusion complex formation of cyclodextrin and polyaniline, Langmuir 15 (1999) 910–913.
[11] T. Shimomura, T. Akai, T. Abe, K. Ito, Atomic force microscopy observation of insulated molecular wire formed by conducting polymer and molecular nanotube, J. Chem. Phys. 116 (2002) 1753–1755. [12] R.V. Belosludov, H. Mizuseki, K. Ichinoseki, Y. Kawazoe, Theoretical study of inclusion complex of polyaniline covered by cyclodextrins for molecular device, Jpn. J. Appl. Phys. 41 (2002) 2739–2741. [13] R.V. Belosludov, H. Sato, A.A. Farajian, H. Mizuseki, Y. Kawazoe, Theoretical study of insulated wires based on polymer chains encapsulated in molecular nanotube, Thin Solid Films 438– 439 (2003) 80–84. [14] R.V. Belosludov, H. Sato, A.A. Farajian, H. Mizuseki, Y. Kawazoe, Theoretical study of molecular enamel wire based on polythiophene-cyclodextrin inclusion complexes, Mol. Cryst. Liquid Cryst. 406 (2003) 205. [15] S. Humbel, S. Sieber, K. Morokuma, The IMOMO method: integration of different levels of molecular orbital approximations for geometry optimization of large systems: test for n-butane conformation and S(N)2 reaction: RCl + Cl , J. Chem. Phys. 105 (1996) 1959–1967. [16] M.J. Frisch et al., Gaussian 98, Revision A.1; Gaussian, Inc., Pittsburg, PA, 1998.