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Influence of nitrogen substitution on the electronic band structure of poly(peri-naphthalene)
ELSEVIER
Synthetic
Metals
69 (1995) 705-706
Influence of nitrogen substitution on the electronic band structure of poly(peri-naphthalene) P.M. Viruela, R. Viruela, and E. Orti Dept. Quimica
Fisica, Univ. Valencia,
E-46100 Burjassot
(Valencia),
Spain
Abstract The electronic valence band structure of PPDAN, a chemical modification of poly@ri-naphthalene) where the central two carbon atoms of each naphthalene unit cell are substituted by nitrogen atoms, is theoretically investigated using the nonempirical VEH method. VEH calculations predict that PPDAN is a semiconductor with a small bandgap of 0.64 eV when a planar DZh structure is assumed for the unit cell. The bandgap increases to 1.3 - 1.6 eV when the more stable CZv and CZh structures are used.
1. INTRODUCTION N-N = 1.450 A
Ladderlike polymers such as poly(peri-naphthalene) (PPN) are investigated as good candidates for intrinsic conductors [ l31. These polymers are formed from fused aromatic rings and can be visualized as structural intermediates between 1D polyacetylene and 2D graphite. They partially suppress the bond length alternancy present in 1D systems by incorporating all K bonds into aromatic rings. This bond length altemancy is one of the main factors determining the splitting between the occupied and unoccupied electronic levels. Theoretical HF calculations predict an average bond length altemancy of 0.048 8, for the peripheral carbon chains of PPN [4,S]. This alternancy is significantly smaller than that experimentally reported for polyacetylene (0.08 A) [6]. A bandgap of only OS6 eV is thus calculated for PPN [4] compared with the 1.4 eV obtained for polyacetylene [7]. Tanaka et al. [8,9] have suggested the possibility of totally removing this bandgap by controlling the number of r~ electrons in the polymer skeleton through the introduction of heteroatoms in the polymer backbone. In this paper, we present a theoretical investigation of the electronic properties of the poly(peri-diazanaphthalene) sketched in Fig. 1, hereafter called PPDAN. This polymer represents a structural modification of PPN where the two carbons atoms forming the central bond of the naphthalene unit cell are substituted by nitrogen atoms. It therefore has 12 7~electrons per unit cell, two more that PPN, and is claimed to show metallic properties [9].
2. RESULTS
AND
DISCUSSION
Figure 1 displays the geometric parameters used to define the unit cell of PPDAN. Bond lengths and bond angles are taken from the central part of the geometry of the dimer optimized at the ab initio 6-3lG* level assuming a planar Dzh symmetry. The unit cell shows a highly localized structure. The Cl-C2 bonds (1.331 A) correspond to formal double
0379~6779/95/$09.50 0 1995 Elsevier SSDI 0379-6779(94)02624-8
Science
S.A. All rights reserved
N-Cl = 1.402 A ci-cz = 1.331 A c2-c2 = I ,473 A
ci--c= I,408 A C-H = I ,067 A N-NCl=
118.4”
N-Cl-C2 = 122.1” Cl-cz-c2 Cl-CP-H=
= 119.5 0 121.0’
Figure 1. D2h unit cell used for VEH band structure calculations on PPDAN. bonds (1.339 A for ethylene) [lo]. The C2-C2 bonds (1.473 A) have a defined single bond character similar to that found in nonaromatic 1,3-cyclohexadiene (1.468 A) [ll]. The bond length alternancy between Cl-C2 and C2-C2 bonds in PPDAN is therefore of 0.14 A very much larger than in PPN (0.02 A) [4]. The peri bonds between adjacent unit cells are found to be significantly longer for PPDAN (1.488 A) than for PPN (1.460 A). The peripheral carbon chains are then predicted to be less delocalized in PPDAN than in PPN. The central N-N bond has a length of 1.450 8, similar to that observed for hydrazine (1.451 A) [ 121 and therefore corresponds to a formal N-N single bond. The adjacent N-Cl bonds (1.402 A) are however shorter than in saturated hydrazines (1.47- 1.48 A) suggesting some conjugation between the n lone pairs of the nitrogen atoms and the x system of the peripheral carbon chains. The repulsion between the nitrogen lone pairs determines that the planar Dzh structure actually does not correspond to a minimum. Nonplanar Czv and C2, structures, for which the central two nitrogens are pyramidalized in the same and in opposite directions, respectively, were found to be 1.8 and 5.2 kcal/mol lower in energy than the planar Dzh structure for the monomer of PPDAN. It is to note that these structures preserve the planarity of the carbon backbone.
P.M. viruela et al. / Synthetic Metals 69 (1995) 705-706
706
the valence band of PPDAN and is separated by a small gap of 0.64 eV, similar to that found for PPN (0.56 eV), from the conduction band. The use of the nonplanar Czv and Czh unit cells leads to band structures similar to that shown in Fig. 2, the main difference being that the bandgap opens to 1.27 and 1.60 eV, respectively. These gaps are of the same order than that found for polyacetylene (1.40 eV) [7] since both for the CZv and the CZh cell the peripheral polyacetylene chains preserve their planarity. In conclusion, PPDAN is predicted to be a semiconductor with a small bandgap. The magnitude of this bandgap depends on the planarity of the polymer and will be determined by the packing forces in the solid state. Compared to PPN (4.08 eV) and polyacetylene (4.7 eV), PPDAN is calculated to have a very low ionization potential (2.67 eV). It could therefore be expected that PPDAN leads to highly stable p-doped conducting materials. -18
I 0
w k--1
rrla
Figure 2. VEH band structures of PPDAN (solid lines) and PPN (broken lines). Only x bands are displayed. Labels used to denote the z bands of PPDAN are enclosed in circles. Figure 2 shows the electronic valence band structure calculated for PPDAN using the nonempirical VEH method [ 133 and the planar Dz~ unit cell depicted in Fig. 1, together with that previously reported for PPN [4]. Looking at the atomic orbital patterns at the center (k=O) of the Brillouin zone, a one-to-one correspondence is found between the n bands of PPDAN and those of PPN. The x1, x3, and x4 bands of PPDAN respectively correlates with the n,, x4, and n3 bands of PPN. These bands are stabilized for PPDAN due to the more electronegative character of the nitrogen atoms which largely contribute to them. Bands n2 and x5 show the same topology for PPDAN and PPN. They have no contribution from the central atoms and are lower in energy for PPDAN due to the shortening of the Cl-C2 bonds. The highest occupied or vaIence.band (VB) in PPDAN corresponds to the x6 band. This band has the same topology than the ns band of PPN which is stabilized by the large contributions from nitrogen atoms. It therefore has no relation with the lowest unoccupied or conduction band (CB) of PPN (band ng) which for PPDAN corresponds to the ~7 band and is calculated higher in energy. These energetic rearrangements determine that for PPDAN the conduction band does not cross any other unoccupied band while for PPN it crosses the ~7 band. This result invalidates the argument used by Tanaka et al. [9]. They stated,that as long as the planarity of the polymer skeleton and the mirrdr plane runni,ng along the chain axis were retained a finite density of states would be obtained at the Fermi level of PPDAN. They suggested that the crossing observed for PPN between the ng (CB) and 7r7 bands was preserved for PPDAN. The existence of two extra n electrons per unit cell in PPDAN therefore transforms ng in the valence band and metallic properties would be obtained since unoccupied levels would be immediately above the occupied levels. Our VEH calculations discard these expectations due to the stabilization of the ns band of PPN which is converted in
The research reported in this communication was supported by the DGYCIT project No. PB91-0935. We thank the Servei d’Inform8tica de la Universitat de ValBncia for the use of their computing facilities. REFERENCES 1.
M. Kertesz, Y.S. Lee and J.J.P. Stewart, Int. .I. Quantum Chem., 25 (1989) 305; Y.-D. Gao and H. Hosoya, Theor. and J.L. C&m. Acta, 81 (1991) 105; J.M. Toussaint Bredas, Synth. Met., 46 (1992) 325; C.-M. Liegener, A.K. Bakshi and J. Ladik, Chem Phys. L&t., 199 (1992) 62; C. Hu. J. Chem. Phys., 98 (1993) 7621. 2. M. Baumgarten, S. Karabunarliev, K.-H. Koch, K. Miillen and N. Tyutyulkov, Synth. Met., 47 (1992) 21; N. Tyutyulkov, F. Dietz, K. Miillen and M. Baumgarten, Chem. Phys., 163 (1992) 5.5. S. Karabunarliev, K. Miillen and M. 3. N. Tyutyulkov, Baumgarten, Synth. Met., 53 (1993) 205. 4. R. Viruela-Martin, P.M. Viruela-Martin and E. Orti, J. Chem. Phys., 97 (1992) 8470. S. Karaburnaliev, M. Baumgarten, K. Mullen and N. 5. Tyutyulkov, Chem. Phys., 179 (1994) 421. C.R. Fincher, C.E. Chen, A.J. Heeger, A.G. McDiarmid 6. and J.B. Hastings, Phys .?ev. Lett., 48 (1982) 100. 7. J.L. BrCdas, R.R. Chance, R. Sibey, G. Nicolas and Ph. Durand, J. Chem. Phys., 75 (1981) 255. K. Tanaka, K. Ueda, T. Koike, M. Ando and T. Yamabe, 8. Phys. Rev. B, 32 (1985) 4279; K. Tanaka, M. Murashima and T. Yamabe, Synth. Met., 24 (1988) 371. K. Tanaka, S. Yamanaka, K. Ueda, S. Takeda and T. 9. Yamabe, Synth. Met., 20 (1987) 333. 10 B.P. Stoicheff and J.M. Dowling, Can. J. Phys., 37 (1959) 703. 11 H. Oberhaumer and S.H. Bauer, J. Am. Chem. Sot., 91 (1969) 10. I. Ichishima, T. Shimanouchi and S. 12. A. Yamaguchi, Mizushima, Spectrochim. Actu, 16 (1960) 1471. 13. J.M. Andre, J.L. BrBdas, J. Delhalle, D.J. Vanderveken, D.P. Vercauteren and J.G. Fripiat, in E. Clementi (ed.), MOTECC-90 (Modern Techniques in Computational Chemistry), ESCOM Science Publishers, Leiden 1990, Ch. 15, p. 745.
Synthetic
Metals
69 (1995) 705-706
Influence of nitrogen substitution on the electronic band structure of poly(peri-naphthalene) P.M. Viruela, R. Viruela, and E. Orti Dept. Quimica
Fisica, Univ. Valencia,
E-46100 Burjassot
(Valencia),
Spain
Abstract The electronic valence band structure of PPDAN, a chemical modification of poly@ri-naphthalene) where the central two carbon atoms of each naphthalene unit cell are substituted by nitrogen atoms, is theoretically investigated using the nonempirical VEH method. VEH calculations predict that PPDAN is a semiconductor with a small bandgap of 0.64 eV when a planar DZh structure is assumed for the unit cell. The bandgap increases to 1.3 - 1.6 eV when the more stable CZv and CZh structures are used.
1. INTRODUCTION N-N = 1.450 A
Ladderlike polymers such as poly(peri-naphthalene) (PPN) are investigated as good candidates for intrinsic conductors [ l31. These polymers are formed from fused aromatic rings and can be visualized as structural intermediates between 1D polyacetylene and 2D graphite. They partially suppress the bond length alternancy present in 1D systems by incorporating all K bonds into aromatic rings. This bond length altemancy is one of the main factors determining the splitting between the occupied and unoccupied electronic levels. Theoretical HF calculations predict an average bond length altemancy of 0.048 8, for the peripheral carbon chains of PPN [4,S]. This alternancy is significantly smaller than that experimentally reported for polyacetylene (0.08 A) [6]. A bandgap of only OS6 eV is thus calculated for PPN [4] compared with the 1.4 eV obtained for polyacetylene [7]. Tanaka et al. [8,9] have suggested the possibility of totally removing this bandgap by controlling the number of r~ electrons in the polymer skeleton through the introduction of heteroatoms in the polymer backbone. In this paper, we present a theoretical investigation of the electronic properties of the poly(peri-diazanaphthalene) sketched in Fig. 1, hereafter called PPDAN. This polymer represents a structural modification of PPN where the two carbons atoms forming the central bond of the naphthalene unit cell are substituted by nitrogen atoms. It therefore has 12 7~electrons per unit cell, two more that PPN, and is claimed to show metallic properties [9].
2. RESULTS
AND
DISCUSSION
Figure 1 displays the geometric parameters used to define the unit cell of PPDAN. Bond lengths and bond angles are taken from the central part of the geometry of the dimer optimized at the ab initio 6-3lG* level assuming a planar Dzh symmetry. The unit cell shows a highly localized structure. The Cl-C2 bonds (1.331 A) correspond to formal double
0379~6779/95/$09.50 0 1995 Elsevier SSDI 0379-6779(94)02624-8
Science
S.A. All rights reserved
N-Cl = 1.402 A ci-cz = 1.331 A c2-c2 = I ,473 A
ci--c= I,408 A C-H = I ,067 A N-NCl=
118.4”
N-Cl-C2 = 122.1” Cl-cz-c2 Cl-CP-H=
= 119.5 0 121.0’
Figure 1. D2h unit cell used for VEH band structure calculations on PPDAN. bonds (1.339 A for ethylene) [lo]. The C2-C2 bonds (1.473 A) have a defined single bond character similar to that found in nonaromatic 1,3-cyclohexadiene (1.468 A) [ll]. The bond length alternancy between Cl-C2 and C2-C2 bonds in PPDAN is therefore of 0.14 A very much larger than in PPN (0.02 A) [4]. The peri bonds between adjacent unit cells are found to be significantly longer for PPDAN (1.488 A) than for PPN (1.460 A). The peripheral carbon chains are then predicted to be less delocalized in PPDAN than in PPN. The central N-N bond has a length of 1.450 8, similar to that observed for hydrazine (1.451 A) [ 121 and therefore corresponds to a formal N-N single bond. The adjacent N-Cl bonds (1.402 A) are however shorter than in saturated hydrazines (1.47- 1.48 A) suggesting some conjugation between the n lone pairs of the nitrogen atoms and the x system of the peripheral carbon chains. The repulsion between the nitrogen lone pairs determines that the planar Dzh structure actually does not correspond to a minimum. Nonplanar Czv and C2, structures, for which the central two nitrogens are pyramidalized in the same and in opposite directions, respectively, were found to be 1.8 and 5.2 kcal/mol lower in energy than the planar Dzh structure for the monomer of PPDAN. It is to note that these structures preserve the planarity of the carbon backbone.
P.M. viruela et al. / Synthetic Metals 69 (1995) 705-706
706
the valence band of PPDAN and is separated by a small gap of 0.64 eV, similar to that found for PPN (0.56 eV), from the conduction band. The use of the nonplanar Czv and Czh unit cells leads to band structures similar to that shown in Fig. 2, the main difference being that the bandgap opens to 1.27 and 1.60 eV, respectively. These gaps are of the same order than that found for polyacetylene (1.40 eV) [7] since both for the CZv and the CZh cell the peripheral polyacetylene chains preserve their planarity. In conclusion, PPDAN is predicted to be a semiconductor with a small bandgap. The magnitude of this bandgap depends on the planarity of the polymer and will be determined by the packing forces in the solid state. Compared to PPN (4.08 eV) and polyacetylene (4.7 eV), PPDAN is calculated to have a very low ionization potential (2.67 eV). It could therefore be expected that PPDAN leads to highly stable p-doped conducting materials. -18
I 0
w k--1
rrla
Figure 2. VEH band structures of PPDAN (solid lines) and PPN (broken lines). Only x bands are displayed. Labels used to denote the z bands of PPDAN are enclosed in circles. Figure 2 shows the electronic valence band structure calculated for PPDAN using the nonempirical VEH method [ 133 and the planar Dz~ unit cell depicted in Fig. 1, together with that previously reported for PPN [4]. Looking at the atomic orbital patterns at the center (k=O) of the Brillouin zone, a one-to-one correspondence is found between the n bands of PPDAN and those of PPN. The x1, x3, and x4 bands of PPDAN respectively correlates with the n,, x4, and n3 bands of PPN. These bands are stabilized for PPDAN due to the more electronegative character of the nitrogen atoms which largely contribute to them. Bands n2 and x5 show the same topology for PPDAN and PPN. They have no contribution from the central atoms and are lower in energy for PPDAN due to the shortening of the Cl-C2 bonds. The highest occupied or vaIence.band (VB) in PPDAN corresponds to the x6 band. This band has the same topology than the ns band of PPN which is stabilized by the large contributions from nitrogen atoms. It therefore has no relation with the lowest unoccupied or conduction band (CB) of PPN (band ng) which for PPDAN corresponds to the ~7 band and is calculated higher in energy. These energetic rearrangements determine that for PPDAN the conduction band does not cross any other unoccupied band while for PPN it crosses the ~7 band. This result invalidates the argument used by Tanaka et al. [9]. They stated,that as long as the planarity of the polymer skeleton and the mirrdr plane runni,ng along the chain axis were retained a finite density of states would be obtained at the Fermi level of PPDAN. They suggested that the crossing observed for PPN between the ng (CB) and 7r7 bands was preserved for PPDAN. The existence of two extra n electrons per unit cell in PPDAN therefore transforms ng in the valence band and metallic properties would be obtained since unoccupied levels would be immediately above the occupied levels. Our VEH calculations discard these expectations due to the stabilization of the ns band of PPN which is converted in
The research reported in this communication was supported by the DGYCIT project No. PB91-0935. We thank the Servei d’Inform8tica de la Universitat de ValBncia for the use of their computing facilities. REFERENCES 1.
M. Kertesz, Y.S. Lee and J.J.P. Stewart, Int. .I. Quantum Chem., 25 (1989) 305; Y.-D. Gao and H. Hosoya, Theor. and J.L. C&m. Acta, 81 (1991) 105; J.M. Toussaint Bredas, Synth. Met., 46 (1992) 325; C.-M. Liegener, A.K. Bakshi and J. Ladik, Chem Phys. L&t., 199 (1992) 62; C. Hu. J. Chem. Phys., 98 (1993) 7621. 2. M. Baumgarten, S. Karabunarliev, K.-H. Koch, K. Miillen and N. Tyutyulkov, Synth. Met., 47 (1992) 21; N. Tyutyulkov, F. Dietz, K. Miillen and M. Baumgarten, Chem. Phys., 163 (1992) 5.5. S. Karabunarliev, K. Miillen and M. 3. N. Tyutyulkov, Baumgarten, Synth. Met., 53 (1993) 205. 4. R. Viruela-Martin, P.M. Viruela-Martin and E. Orti, J. Chem. Phys., 97 (1992) 8470. S. Karaburnaliev, M. Baumgarten, K. Mullen and N. 5. Tyutyulkov, Chem. Phys., 179 (1994) 421. C.R. Fincher, C.E. Chen, A.J. Heeger, A.G. McDiarmid 6. and J.B. Hastings, Phys .?ev. Lett., 48 (1982) 100. 7. J.L. BrCdas, R.R. Chance, R. Sibey, G. Nicolas and Ph. Durand, J. Chem. Phys., 75 (1981) 255. K. Tanaka, K. Ueda, T. Koike, M. Ando and T. Yamabe, 8. Phys. Rev. B, 32 (1985) 4279; K. Tanaka, M. Murashima and T. Yamabe, Synth. Met., 24 (1988) 371. K. Tanaka, S. Yamanaka, K. Ueda, S. Takeda and T. 9. Yamabe, Synth. Met., 20 (1987) 333. 10 B.P. Stoicheff and J.M. Dowling, Can. J. Phys., 37 (1959) 703. 11 H. Oberhaumer and S.H. Bauer, J. Am. Chem. Sot., 91 (1969) 10. I. Ichishima, T. Shimanouchi and S. 12. A. Yamaguchi, Mizushima, Spectrochim. Actu, 16 (1960) 1471. 13. J.M. Andre, J.L. BrBdas, J. Delhalle, D.J. Vanderveken, D.P. Vercauteren and J.G. Fripiat, in E. Clementi (ed.), MOTECC-90 (Modern Techniques in Computational Chemistry), ESCOM Science Publishers, Leiden 1990, Ch. 15, p. 745.