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Metallic State of Sequence-Controlled Oligomer Conductor that Models Doped PEDOT Family

Mori and Ozaki Groups

Organic conducting materials are attracting attention because of their flexibility and solution processability, characteristics not found in inorganic semiconductors and metals which are currently the mainstream materials. Among them, conductive polymers such as doped poly(3,4-ethylenedioxythiophene) (PEDOT) are widely used organic conducting materials because of their easy solution-processability and high conductivity. However, the large molecular-weight distribution of polymers during processing causes less-crystallinity and less-accessible to atomic-level structural information and makes it difficult to elucidate the conduction mechanism and design polymers based on the crystal structure. In this study, we focused on oligomers to challenge these issues. Oligomers have high degrees of freedom in molecular design controlling three parameters (hetero-sequence, chain length, and terminal group). In addition, since oligomers possess single-crystallinity, detailed structural information can be accessible, the conduction mechanism based on crystal structures can be clarified and materials design are possibly proposed.

So far, Mori group has developed 3,4-ethylenedioxythiophene (EDOT, O) oligomeric salts nO-X [1-3]. EDOT dimer salts 2O•X (X = BF4, ClO4, PF6) have a relatively wide calculated bandwidth of around 1 eV [1], but are Mott insulators with 1/2-filled band structure. To deviate from the Mott insulating state and to improve the conductivity, two strategies for reducing the Coulomb repulsion (Ueff) between carriers have been reported. The first is band-filling modulation from the 1/2-filled state [2]. The second strategy is the extension of conjugate length, namely elongation of oligomer length. The trimer salt 3O•PF6(CH2Cl2) has been synthesized for oligomer-length elongation, which leads the reduction of resistivity by about one order of magnitude than that of dimer salt 2O•PF6 [3]. From the above results, the realization of high conductivity and metallic state in oligomeric organic conductors can be achieved by reducing Ueff through conjugate expansion, band-filling modulation, and suppressing stacking dimerization with improving dimensionality of the electronic structure.

Although conjugate-length expansion of oligomers is effective in reducing Ueff, it has been difficult to synthesize long-chain oligomers without introduction of solubility auxiliary groups due to instability against oxidation and solubility issues of oligomers consisting of a single EDOT unit. In this study, we utilized heterogenious units, i.e., 3,4-ethylenedithiothiophene / 3,4-(2',2'-dimethylpropylenedioxy)thiophene (S/P), to synthesize long chain oligomers in solid state. The long-chain oligomers were designed and newly synthesized without the introduction of solubility-supporting groups that inhibit intermolecular interactions in solids. In the neutral 4PS (P-S-S-P), stability and solubility can be imparted by introducing an S-S structure in the center, which is a truncated and twisted structure of the conjugated system. This twisted structure is eliminated by oxidation. By introducing bulky unit P at both ends, dimerization is suppressed by increasing the dimensionality of the electronic structure, and band-filling modulation is achieved by creating a space where excess anions can exist (Fig. 1).

Subsequently, single crystals of 4PS•(PF6)1.2(solvent)m were prepared by constant-current electrochemical oxidation of tetramer donor 4PS. Band calculations show a quasi-one-dimensional band structure (W = 0.41 eV) with band dispersion mainly in the stacking direction, confirming the acquisition of dimensionality. The room temperature resistivity in the stacking direction is 36 S cm-1, which is six orders of magnitude lower than that of 2O•PF6, and metallic conduction behavior is observed in the high temperature region above 280 K (Fig. 1). The IR reflection spectrum reveals a plasma edge, and the first metallic state in a single-crystalline EDOT-based oligomeric organic conductor is observed [4].

This result demonstrates that molecular arrangements and electronic functionalities in the solid state can be controlled by the type and sequence of oligomer units, which is a characteristic molecular design freedom of oligomers, and realizes a new concept in the development of conductor materials. This concept is expected to create a new trend in the development of organic conducting materials.


References
  • [1] R. Kameyama, T. Fujino, S. Dekura, M. Kawamura, T. Ozaki, and H. Mori, Chemistry - A European Journal, 27, 6696 (2021).
  • [2] R. Kameyama, T. Fujino, S. Dekura, S. Imajo, T. Miyamoto, H. Okamoto, and H. Mori, J. Mater. Chem. C, 10, 7543 (2022).
  • [3] R. Kameyama, T. Fujino, S. Dekura, and H. Mori, Phys. Chem. Chem. Phys. 24, 9130(2022).
  • [4] K. Onozuka, T. Fujino, R. Kameyama, S. Dekura, K. Yoshimi, T. Nakamura, T. Miyamoto, T. Yamakawa, H. Okamoto, H. Sato, T. Ozaki, and H. Mori, J. Am. Chem. Soc., 145, 15152 (2023).
Authors
  • K. Onozuka, T. Fujino, R. Kameyama, S. Dekura, K. Yoshimi, T. Nakamuraa, T. Miyamotob, T. Yamakawab, H. Okamotob, H. Satoc, T. Ozaki, and H. Mori
  • aInstitute for Molecular Science
  • bThe University of Tokyo
  • cRigaku Corporation