Single-crystalline oligomer-based conductors modeling the doped poly(3,4-ethylenedioxythiophene) family

Conductive polymers with highly conjugated systems, such as the doped poly(3,4-ethylenedioxythiophene) (PEDOT) family, are commonly used in organic electronics. However, their structural inhomogeneity with various chain lengths makes it difficult to control their conductivities and structural details. On the other hand, low-molecular-weight materials have well-defined structures but relatively narrow conjugate areas with a limited range of Coulomb repulsion between carriers (Ueff), which hamper the flexible control of conductivities. To bridge this gap, we developed oligomer-based conductors, which are intermediate materials between polymers and low-molecular-weight materials. Using a library of single-crystal charge-transfer salts of oligo(3,4-ethylenedioxythiophene) (oligoEDOT) analogs that model the doped PEDOT family, we have investigated the structure-determining factors affecting their conductivities, such as counter anion variations, lengths of oligomer donor, and band fillings. Through the screening study, we developed oligoEDOT analogs with tunable room temperature conductivities by several orders of magnitude, including a metallic state above room temperature. In this study, we consistently evaluated the electronic structural insights by first-principles calculations and revealed that Ueff is the dominant factor that determines the relationship between the structures and conductivities. The unique features of oligoEDOT conductor systems with widely variable Ueff can differentiate these systems from strongly electron-correlated systems. Oligomer-based charge-transfer complexes display a diversified carrier-to-carrier Coulomb repulsion (Ueff) and intermolecular interactions (W), enabling flexible tuning of electrical conductivities.