Despite the accumulation of abundant knowledge through structure-property correlation studies, research on organic conductors is still in the basic research stage, and there is a gap between basic research and device research. This is because organic conducting single crystals are considered to have poor solution processability and are unsuitable for large-scale synthesis. As a next-generation material that could bridge this gap, there is growing interest in charge-transfer complexes formed by electron-rich donor molecules and electron-deficient acceptor molecules. Charge-transfer complexes are classified into “mixed-stack type”, where donors and acceptors are mixed-stacked, and “separated-layer type”, where donor layer and acceptor sheet are alternately stacked. In separated-layer-type complexes, complexes exhibiting high conductivity, including metallic states, have been identified. However, it has been generally accepted that mixed-stack-type charge-transfer complexes, which are relatively easier to obtain, exhibit little electrical conductivity. This low conductivity was caused by the charge transfer quantity $\delta$, which indicates the amount of electrons transferred from the donor to the acceptor, is in the neutral region (0 – 0.4) or the ionic region ($\delta >0.75$) (Fig. 1b), resulting in few effective carriers involved in charge transport. While there had been expectations that synthesizing charge-transfer complexes in the neutral-ionic boundary region could enhance electrical conductivity, such complexes remained elusive for decades.

Our research group has recently developed an oligomer model of doped poly(3,4-ethylenedioxythiophene) (PEDOT) as an electron-rich donor molecule [1–3]. We have found that the shortest dimer (2O, Fig. 1a) [1] and its oxygen/sulfur atom substituted derivatives (2S, Fig. 1a)

possess an ideal electronic structure for constructing neutral-ionic boundary region complexes with electron-deficient fluorine-substituted tetracyanoquinodimethane derivatives (F4 and F2, Fig. 1a). To realize such a boundary region, it is expected that a small energy difference between the donor's highest occupied molecular orbital (HOMO) and the acceptor's lowest unoccupied molecular orbital (LUMO) is essential, and the combination of 2O/2S donors and F4/F2 acceptors well satisfies these conditions. Furthermore, the symmetry of the molecular orbital structure after charge transfer is also well matched, and it is expected that a highly conductive carrier conduction pathway with strong hybridization between the two orbitals can be realized.

By focusing on the molecular orbitals of the donor and acceptor in the design, we successfully enhanced the conductivity of the mixed-stack charge transfer complexes and achieved the highest room-temperature conductivity (${\sigma }_{\mathrm{R}\mathrm{T}}$) in a one-dimensional single crystal as shown in the dotted surrounding circle in Fig. 1b: ${\sigma }_{\mathrm{R}\mathrm{T}}$ [2S-F4, $0.10$ Scm, $\delta =0.69(2)$] $>$ ${\sigma }_{\mathrm{R}\mathrm{T}}$ [2O-F2, $1.4\times {10}^{-2}$ Scm, $\delta =0.71(4)$] $>$ ${\sigma }_{\mathrm{R}\mathrm{T}}$ [2S-F2, $6.9\times {10}^{-3}$ Scm, $\delta =0.46(3)$] $>$ ${\sigma }_{\mathrm{R}\mathrm{T}}$ [2O-F4, $4.9\times {10}^{-4}$ Scm, $\delta =0.79(2)$]. These mixed-stack charge-transfer complexes are scalable for large-scale synthesis, exhibit high solubility in organic solvents, and remain stable for extended periods without decomposition in solution, thereby demonstrating significant potential as a coating-type conductive material. It holds great promise as next-generation organic conducting materials.

References

• [1] R. Kameyama et al., Chemistry - A Euro. J. 27, 6696 (2021).
• [2] K. Onozuka et al., J. Am. Chem. Soc. 145 (28), 15152 (2023).
• [3] T. Fujino et al., Faraday Discuss. 250, 348 (2024).
• [4] T. Fujino et al., Nat. Commun. 15, 3028 (2024).

Authors

• T. Fujino, R. Kameyama, K. Onozuka, K. Matsuo, S. Dekura, T. Miyamoto, Z. Guo^a, H. Okamoto^a, T. Nakamura^b, K. Yoshimi, S. Kitou^a, T. Arima^a,c, H. Sato^d, K. Yamamoto^e, A. Takahashi^f, H. Sawa^g, Y. Nakamura^h, and  H. Mori
• ^aDepartment of Advanced Materials Science, The University of Tokyo
• ^bInstitute for Molecular Science
• ^cRIKEN Center for Emergent Matter Science (CEMS)
• ^dRigaku Corporation
• ^eOkayama University of Science
• ^fNagoya Institute of Technology
• ^gNagoya University
• ^hJapan Synchrotron Radiation Research Institute (JASRI), SPring-8