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Design principle for increasing charge mobility of π-conjugated polymers using regularly localized molecular orbitals.

Terao J, Wadahama A, Matono A, Tada T, Watanabe S, Seki S, Fujihara T, Tsuji Y - Nat Commun (2013)

Bottom Line: The zigzag wires exhibited higher intramolecular charge mobility than the corresponding linear wires.When the length of the linear region of the zigzag wires was increased to 10 phenylene-ethynylene units, the intramolecular charge mobility increased to 8.5 cm(2) V(-1) s(-1).Theoretical analysis confirmed that this design principle is suitable for obtaining ideal charge mobilities in π-conjugated polymer chains and that it provides the most effective pathways for inter-site hopping processes.

View Article: PubMed Central - PubMed

Affiliation: Department of Energy and Hydrocarbon Chemistry, Graduate School of Engineering, Kyoto University, Nishikyo-ku, Kyoto 615-8510, Japan. terao@scl.kyoto-u.ac.jp

ABSTRACT
The feasibility of using π-conjugated polymers as next-generation electronic materials is extensively studied; however, their charge mobilities are lower than those of inorganic materials. Here we demonstrate a new design principle for increasing the intramolecular charge mobility of π-conjugated polymers by covering the π-conjugated chain with macrocycles and regularly localizing π-molecular orbitals to realize an ideal orbital alignment for charge hopping. Based on theoretical predictions, insulated wires containing meta-junctioned poly(phenylene-ethynylene) as the backbone units were designed and synthesized. The zigzag wires exhibited higher intramolecular charge mobility than the corresponding linear wires. When the length of the linear region of the zigzag wires was increased to 10 phenylene-ethynylene units, the intramolecular charge mobility increased to 8.5 cm(2) V(-1) s(-1). Theoretical analysis confirmed that this design principle is suitable for obtaining ideal charge mobilities in π-conjugated polymer chains and that it provides the most effective pathways for inter-site hopping processes.

No MeSH data available.


Schematics of the orbital distribution and orbital localization in the para- and meta-junctioned wires.(a) Schematic of a wire of widely distributed localized molecular orbitals and its energy diagram for the para-system. The blue elliptical regions indicate the non-zero amplitudes in each orbital, En, where En is a localized orbital in which the orbital amplitude is localized within n units. The position (r) in the energy diagram corresponds to the atomic/fragment position along the one-dimensional molecular wire. The horizontal bars (blue) in the energy diagram indicate the areas in which the orbital amplitudes have non-zero values. The dotted arrows are charge hopping processes, and the grey dotted arrows are the processes with large energy changes that are not included in the meta-junctioned system. (b) Schematic of a wire of narrowly distributed localized molecular orbitals and its energy diagram in the meta-junctioned system. The red elliptical regions indicate the non-zero amplitudes in each orbital, En. The horizontal bars (red) in the diagram indicate the areas with non-zero orbital amplitudes. The solid red arrows are fast hopping processes because of the orbital overlap. (c) The para-junctioned π-conjugated fragment. (d) The meta-junctioned π-conjugated fragment. (e,f) The list of localized orbitals in the para- and meta-fragments by the rotations at positions 1 (1′), 2 (2′) and 3 (3′) shown in Fig. 1c and the counts of each orbital present. The numbers in the parentheses are the orbital energies in the unit of hopping integral calculated using the simple Hückel model. The cyan circles indicate the non-zero orbital amplitudes. The orbitals with a prime or double prime, E′n and E′′n, indicate that the localization area is the same to that of the orbital without a prime, En. However, their position of localization is different from En. For example, the orbital amplitude of E′1 by Rotation 1 and 2 shown in Fig. 1e is localized at the second benzene fragment from the right-hand edge. The orbital overlap at the meta-joint position in the meta-system can be easily confirmed (for example, E3 and E4 in Rotation 1).
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f1: Schematics of the orbital distribution and orbital localization in the para- and meta-junctioned wires.(a) Schematic of a wire of widely distributed localized molecular orbitals and its energy diagram for the para-system. The blue elliptical regions indicate the non-zero amplitudes in each orbital, En, where En is a localized orbital in which the orbital amplitude is localized within n units. The position (r) in the energy diagram corresponds to the atomic/fragment position along the one-dimensional molecular wire. The horizontal bars (blue) in the energy diagram indicate the areas in which the orbital amplitudes have non-zero values. The dotted arrows are charge hopping processes, and the grey dotted arrows are the processes with large energy changes that are not included in the meta-junctioned system. (b) Schematic of a wire of narrowly distributed localized molecular orbitals and its energy diagram in the meta-junctioned system. The red elliptical regions indicate the non-zero amplitudes in each orbital, En. The horizontal bars (red) in the diagram indicate the areas with non-zero orbital amplitudes. The solid red arrows are fast hopping processes because of the orbital overlap. (c) The para-junctioned π-conjugated fragment. (d) The meta-junctioned π-conjugated fragment. (e,f) The list of localized orbitals in the para- and meta-fragments by the rotations at positions 1 (1′), 2 (2′) and 3 (3′) shown in Fig. 1c and the counts of each orbital present. The numbers in the parentheses are the orbital energies in the unit of hopping integral calculated using the simple Hückel model. The cyan circles indicate the non-zero orbital amplitudes. The orbitals with a prime or double prime, E′n and E′′n, indicate that the localization area is the same to that of the orbital without a prime, En. However, their position of localization is different from En. For example, the orbital amplitude of E′1 by Rotation 1 and 2 shown in Fig. 1e is localized at the second benzene fragment from the right-hand edge. The orbital overlap at the meta-joint position in the meta-system can be easily confirmed (for example, E3 and E4 in Rotation 1).

Mentions: In π-conjugated molecules, orbital delocalization over the planar region is quite effective for obtaining high charge mobility, because the delocalized orbitals provide the ideal pathways for coherent charge transport. However, the delocalization is easily disturbed by the thermally activated structural fluctuation/deformation that occurs in most long π-conjugated molecules, particularly polymers, but not carbon nanotubes or graphene derivatives. Thus, at ambient temperature, the charge transport in π-conjugated polymers can no longer be described by the electronic band picture because the structural deformation results in the localization of the π-orbitals and in turn charge hopping processes become dominant. Because the orbital energies of the localized π-orbitals depend on the area of localization, uncontrolled structural deformations will result in widely distributed orbitals (that is, wide density of states (DOS)) over a given energy range (Fig. 1a), which is not suitable for efficient charge hopping. Therefore, a narrow distribution of π-orbital levels (that is, narrow DOS) is the key condition for the efficient charge hopping in π-conjugated polymers (Fig. 1b). To realize this situation, we adopted oligo(phenylene–ethynylene) (OPE) as test oligomers (Fig. 1c), and introduced meta-junctions (Fig. 1d) to regularly break the orbital delocalization at the joint positions18, leading to regularly localized orbitals (see Supplementary Notes 1 and 2 for the orbital localizations by the meta-junctions). It can be seen that the narrow DOS can be realized in the meta-system by observing the π-orbitals at ambient temperature, where the benzene fragments can rotate almost freely (that is, a structural deformation). Considering the perpendicularly rotated benzenes at position 1 (1′), 2 (2′) or 3 (3′) and at those combinations in the seven benzene unit shown in Fig. 1c, six and four levels of localized π-orbitals are obtained in the para- and meta-junctioned systems, respectively, and the energetically higher orbitals, E5 and E6, do not appear in the meta-system (Fig. 1e). Note that orbitals EM3 and EM5 in the symmetric meta-fragments seem to be different to the four orbitals E1–E4, but the orbitals EM3 and EM5 appear within the energy range from E1 to E4 despite the wider localization area, and in fact EM3 and EM5 show almost the same energy levels of E2 and E3, respectively. Figure 1a shows examples of energy level alignments in a 70-benzene unit based on the orbital counts. It is clear that the meta-system shows a narrower DOS than the para-system. In addition, in the para-system, a large energy change is required for one of the hopping processes (dotted circle in Fig. 1a), corresponding to the case of Rotation 1, and this process becomes the rate-determining step. Such a slow step is not expected in the meta-system because Rotation 1 leads to the localized orbitals of E1 and partially overlapped orbitals E3 and E4 (dotted circle in Fig. 1b), and the rate-determining step is consequently the hopping process from E3 to E1 or E4 to E1, the energy difference of which is smaller than the rate-determining step in the para-system (E6 to E1). Therefore, the regularly localized molecular orbitals introduced by the meta-junctions are expected to be effective for high charge mobility.


Design principle for increasing charge mobility of π-conjugated polymers using regularly localized molecular orbitals.

Terao J, Wadahama A, Matono A, Tada T, Watanabe S, Seki S, Fujihara T, Tsuji Y - Nat Commun (2013)

Schematics of the orbital distribution and orbital localization in the para- and meta-junctioned wires.(a) Schematic of a wire of widely distributed localized molecular orbitals and its energy diagram for the para-system. The blue elliptical regions indicate the non-zero amplitudes in each orbital, En, where En is a localized orbital in which the orbital amplitude is localized within n units. The position (r) in the energy diagram corresponds to the atomic/fragment position along the one-dimensional molecular wire. The horizontal bars (blue) in the energy diagram indicate the areas in which the orbital amplitudes have non-zero values. The dotted arrows are charge hopping processes, and the grey dotted arrows are the processes with large energy changes that are not included in the meta-junctioned system. (b) Schematic of a wire of narrowly distributed localized molecular orbitals and its energy diagram in the meta-junctioned system. The red elliptical regions indicate the non-zero amplitudes in each orbital, En. The horizontal bars (red) in the diagram indicate the areas with non-zero orbital amplitudes. The solid red arrows are fast hopping processes because of the orbital overlap. (c) The para-junctioned π-conjugated fragment. (d) The meta-junctioned π-conjugated fragment. (e,f) The list of localized orbitals in the para- and meta-fragments by the rotations at positions 1 (1′), 2 (2′) and 3 (3′) shown in Fig. 1c and the counts of each orbital present. The numbers in the parentheses are the orbital energies in the unit of hopping integral calculated using the simple Hückel model. The cyan circles indicate the non-zero orbital amplitudes. The orbitals with a prime or double prime, E′n and E′′n, indicate that the localization area is the same to that of the orbital without a prime, En. However, their position of localization is different from En. For example, the orbital amplitude of E′1 by Rotation 1 and 2 shown in Fig. 1e is localized at the second benzene fragment from the right-hand edge. The orbital overlap at the meta-joint position in the meta-system can be easily confirmed (for example, E3 and E4 in Rotation 1).
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Related In: Results  -  Collection

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Show All Figures
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f1: Schematics of the orbital distribution and orbital localization in the para- and meta-junctioned wires.(a) Schematic of a wire of widely distributed localized molecular orbitals and its energy diagram for the para-system. The blue elliptical regions indicate the non-zero amplitudes in each orbital, En, where En is a localized orbital in which the orbital amplitude is localized within n units. The position (r) in the energy diagram corresponds to the atomic/fragment position along the one-dimensional molecular wire. The horizontal bars (blue) in the energy diagram indicate the areas in which the orbital amplitudes have non-zero values. The dotted arrows are charge hopping processes, and the grey dotted arrows are the processes with large energy changes that are not included in the meta-junctioned system. (b) Schematic of a wire of narrowly distributed localized molecular orbitals and its energy diagram in the meta-junctioned system. The red elliptical regions indicate the non-zero amplitudes in each orbital, En. The horizontal bars (red) in the diagram indicate the areas with non-zero orbital amplitudes. The solid red arrows are fast hopping processes because of the orbital overlap. (c) The para-junctioned π-conjugated fragment. (d) The meta-junctioned π-conjugated fragment. (e,f) The list of localized orbitals in the para- and meta-fragments by the rotations at positions 1 (1′), 2 (2′) and 3 (3′) shown in Fig. 1c and the counts of each orbital present. The numbers in the parentheses are the orbital energies in the unit of hopping integral calculated using the simple Hückel model. The cyan circles indicate the non-zero orbital amplitudes. The orbitals with a prime or double prime, E′n and E′′n, indicate that the localization area is the same to that of the orbital without a prime, En. However, their position of localization is different from En. For example, the orbital amplitude of E′1 by Rotation 1 and 2 shown in Fig. 1e is localized at the second benzene fragment from the right-hand edge. The orbital overlap at the meta-joint position in the meta-system can be easily confirmed (for example, E3 and E4 in Rotation 1).
Mentions: In π-conjugated molecules, orbital delocalization over the planar region is quite effective for obtaining high charge mobility, because the delocalized orbitals provide the ideal pathways for coherent charge transport. However, the delocalization is easily disturbed by the thermally activated structural fluctuation/deformation that occurs in most long π-conjugated molecules, particularly polymers, but not carbon nanotubes or graphene derivatives. Thus, at ambient temperature, the charge transport in π-conjugated polymers can no longer be described by the electronic band picture because the structural deformation results in the localization of the π-orbitals and in turn charge hopping processes become dominant. Because the orbital energies of the localized π-orbitals depend on the area of localization, uncontrolled structural deformations will result in widely distributed orbitals (that is, wide density of states (DOS)) over a given energy range (Fig. 1a), which is not suitable for efficient charge hopping. Therefore, a narrow distribution of π-orbital levels (that is, narrow DOS) is the key condition for the efficient charge hopping in π-conjugated polymers (Fig. 1b). To realize this situation, we adopted oligo(phenylene–ethynylene) (OPE) as test oligomers (Fig. 1c), and introduced meta-junctions (Fig. 1d) to regularly break the orbital delocalization at the joint positions18, leading to regularly localized orbitals (see Supplementary Notes 1 and 2 for the orbital localizations by the meta-junctions). It can be seen that the narrow DOS can be realized in the meta-system by observing the π-orbitals at ambient temperature, where the benzene fragments can rotate almost freely (that is, a structural deformation). Considering the perpendicularly rotated benzenes at position 1 (1′), 2 (2′) or 3 (3′) and at those combinations in the seven benzene unit shown in Fig. 1c, six and four levels of localized π-orbitals are obtained in the para- and meta-junctioned systems, respectively, and the energetically higher orbitals, E5 and E6, do not appear in the meta-system (Fig. 1e). Note that orbitals EM3 and EM5 in the symmetric meta-fragments seem to be different to the four orbitals E1–E4, but the orbitals EM3 and EM5 appear within the energy range from E1 to E4 despite the wider localization area, and in fact EM3 and EM5 show almost the same energy levels of E2 and E3, respectively. Figure 1a shows examples of energy level alignments in a 70-benzene unit based on the orbital counts. It is clear that the meta-system shows a narrower DOS than the para-system. In addition, in the para-system, a large energy change is required for one of the hopping processes (dotted circle in Fig. 1a), corresponding to the case of Rotation 1, and this process becomes the rate-determining step. Such a slow step is not expected in the meta-system because Rotation 1 leads to the localized orbitals of E1 and partially overlapped orbitals E3 and E4 (dotted circle in Fig. 1b), and the rate-determining step is consequently the hopping process from E3 to E1 or E4 to E1, the energy difference of which is smaller than the rate-determining step in the para-system (E6 to E1). Therefore, the regularly localized molecular orbitals introduced by the meta-junctions are expected to be effective for high charge mobility.

Bottom Line: The zigzag wires exhibited higher intramolecular charge mobility than the corresponding linear wires.When the length of the linear region of the zigzag wires was increased to 10 phenylene-ethynylene units, the intramolecular charge mobility increased to 8.5 cm(2) V(-1) s(-1).Theoretical analysis confirmed that this design principle is suitable for obtaining ideal charge mobilities in π-conjugated polymer chains and that it provides the most effective pathways for inter-site hopping processes.

View Article: PubMed Central - PubMed

Affiliation: Department of Energy and Hydrocarbon Chemistry, Graduate School of Engineering, Kyoto University, Nishikyo-ku, Kyoto 615-8510, Japan. terao@scl.kyoto-u.ac.jp

ABSTRACT
The feasibility of using π-conjugated polymers as next-generation electronic materials is extensively studied; however, their charge mobilities are lower than those of inorganic materials. Here we demonstrate a new design principle for increasing the intramolecular charge mobility of π-conjugated polymers by covering the π-conjugated chain with macrocycles and regularly localizing π-molecular orbitals to realize an ideal orbital alignment for charge hopping. Based on theoretical predictions, insulated wires containing meta-junctioned poly(phenylene-ethynylene) as the backbone units were designed and synthesized. The zigzag wires exhibited higher intramolecular charge mobility than the corresponding linear wires. When the length of the linear region of the zigzag wires was increased to 10 phenylene-ethynylene units, the intramolecular charge mobility increased to 8.5 cm(2) V(-1) s(-1). Theoretical analysis confirmed that this design principle is suitable for obtaining ideal charge mobilities in π-conjugated polymer chains and that it provides the most effective pathways for inter-site hopping processes.

No MeSH data available.