Orbital hybridization of donor and acceptor to enhance the conductivity of mixed-stack complexes.

Mixed-stack complexes which comprise columns of alternating donors and acceptors are organic conductors with typically poor electrical conductivity because they are either in a neutral or highly ionic state. This indicates that conductive carriers are insufficient or are mainly localized. In this study, mixed-stack complexes that uniquely exist at the neutral-ionic boundary were synthesized by combining donors (bis(3,4-ethylenedichalcogenothiophene)) and acceptors (fluorinated tetracyanoquinodimethanes) with similar energy levels and orbital symmetry between the highest occupied molecular orbital of the donor and the lowest unoccupied molecular orbital of the acceptor. Surprisingly, the orbitals were highly hybridized in the single-crystal complexes, enhancing the room-temperature conductivity (10-4-0.1 S cm-1) of mixed-stack complexes. Specifically, the maximum conductivity was the highest reported for single-crystal mixed-stack complexes under ambient pressures. The unique electronic structures at the neutral-ionic boundary exhibited structural perturbations between their electron-itinerant and localized states, causing abrupt temperature-dependent changes in their electrical, optical, dielectric, and magnetic properties.

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.

Metallic State of a Mixed-Sequence Oligomer Salt That Models Doped PEDOT Family.

Modern organic conductors are typically low-molecular-weight or polymer-based materials. Low-molecular-weight materials can be characterized using crystallographic information, allowing structure-conductivity relationships to be established and conduction mechanisms to be understood. However, controlling their conductive properties through molecular structural modulation is often challenging because of their relatively narrow conjugate areas. In contrast, polymer-based materials have highly π-conjugated structures with wide molecular-weight distributions, and their structural inhomogeneity makes characterizing their structures difficult. Thus, we focused on the less-explored intermediate, i.e., single-molecular-weight oligomers that model doped poly(3,4-ethylenedioxythiophene) (PEDOT). The dimer and trimer models provided clear structures; however, the short oligomers led to much lower conductivities (<10-3 S cm-1) than that of doped PEDOT. Herein, we elongated the oligomer to a tetramer through geometrical tuning based on a mixed sequence. The "P-S-S-P" sequence (S: 3,4-ethylenedithiothiophene; P: 3,4-(2',2'-dimethypropylenedioxy)thiophene) with twisted S-S enhanced the solubility and chemical stability. The subsequent oxidation process planarized the oligomer and expanded the conjugate area. Interestingly, the sequence involving sterically bulky outer P units allowed the doped oligomer to form a pitched π-stack in the single-crystal form. This enabled the inclusion of excess counter anions, which modulated the band filling. The combined effects of conjugate area expansion and band-filling modulation significantly increased the room-temperature conductivity to 36 S cm-1. This is the highest value reported for a single-crystalline oligomer conductor. Furthermore, a metallic state was observed above room temperature in a single-crystalline oligoEDOT for the first time. This unique mixed-sequence strategy for oligomer-based conductors enabled the precise control of conductive properties.

Conjugation length effect on the conducting behavior of single-crystalline oligo(3,4-ethylenedioxythiophene) (nEDOT) radical cation salts.

The conjugation length is a unique structural factor for oligomer-based π-conjugated conductors as it modulates their electronic structures. Herein, we demonstrated the conjugation length effects on conductivity by comparing a dimer and trimer of single-crystalline oligo(3,4-ethylenedioxythiophene) radical cation salts. The dimer showed a uniform-stacked columnar structure, while the trimer showed stacked columns of the π-dimerized donor and weaker intracolumnar interactions. Nevertheless, the trimer exhibited higher conductivity, suggesting a considerable decrease in the on-site Coulomb repulsion energy of the conjugation-expanded system.

The Simplest Model for Doped Poly(3,4-ethylenedioxythiophene) (PEDOT): Single-crystalline EDOT Dimer Radical Cation Salts.

Invited for the cover of this issue is the group of Tomoko Fujino and Hatsumi Mori at the University of Tokyo. The image depicts the structural information of doped PEDOT uncovered by the single-crystalline EDOT dimer model. Read the full text of the article at .10.1002/chem.202005333.

The Simplest Model for Doped Poly(3,4-ethylenedioxythiophene) (PEDOT): Single-crystalline EDOT Dimer Radical Cation Salts.

Although doped poly(3,4-ethylenedioxythiophene) (PEDOT) is extensively used in electronic devices, their molecular-weight distributions and inadequately defined structures have hindered the elucidation of their underlying conduction mechanism. In this study, we introduce the simplest discrete oligomer models: EDOT dimer radical cation salts. Single-crystal structural analyses revealed their one-dimensional (1D) columnar structures, in which the donors were uniformly stacked. Band calculations identified 1D metallic band structures with a strong intracolumnar orbital interaction (band width W≈1 eV), implying the origin of the high conductivity of doped PEDOT. Interestingly, the salts exhibited semiconducting behavior reminiscent of genuine Mott states as a result of electron-electron repulsion (U) dominant over W. This study realized basic models with tunable W and U to understand the conduction mechanism of doped PEDOT through structural modification in oligomers, including the conjugation length.

Effect of Alkyl Chain Length on Charge Transport Property of Anthracene-Based Organic Semiconductors.

Anthracene, a simple planar building block for organic semiconductors, shows strong intermolecular interactions and exhibits strong blue fluorescence. Thus, its derivatives have a great potential to integrate considerable charge carrier mobility and strong emission within a molecule. Here, we systematically studied the influence of alkyl chain length on the crystal structures, thermal properties, photophysical characteristics, electrochemical behaviors, and mobilities for a series of 2,6-di(4-alkyl-phenyl)anthracenes (Cn-Ph-Ants, where n represents the alkyl chain length). Among them, Cn-Ph-Ants (n = 0, 1, 2, and 3) display similar layered herringbone (LHB) packing motifs, which facilitate two-dimensional charge transport and thereby enables high-performance organic field-effect transistors (OFETs). All Cn-Ph-Ants exhibit similar work functions and show strong blue fluorescence with photoluminescence quantum yields (PLQY) of approximately 40% in toluene. In addition, the absolute powder PLQYs of C0-, C2-, C3-, C4-, and C6-Ph-Ants are 24.6, 8.2, 5.7, 10.9, and 8.6%, respectively. Note that the alkyl chain length shows a significant effect on the charge mobilities of Cn-Ph-Ants. Our newly synthesized C1-, C3-, and C4-Ph-Ants show hole mobilities of up to 2.40, 1.34, and 1.00 cm2 V-1 s-1, respectively, with mobilities of 3.40, 1.57, and 0.82 cm2 V-1 s-1 for C0-, C2-, and C6-Ph-Ants, indicating an increasing tendency of mobility with shorter alkyl chain length. This feature is related to the microstructures of the thin films, which reveal the enhanced film order, crystallinity, and grain size with a decrease in the alkyl chain length. Moreover, we theoretically analyze the intermolecular transfer integrals of HOMOs, which increase at T-shaped contacts as the alkyl chain length decreases, which improves the intermolecular charge transport properties, leading to the increases in mobility. Interestingly, the anisotropy of the transfer integral tends to decrease upon substitution with longer alkyl chains, suggesting that alkyl chain adjustments may facilitate isotropic charge transport property in 2,6-alkylated anthracenes.

Combined Iron/Hydroxytriazole Dual Catalytic System for Site Selective Oxidation Adjacent to Azaheterocycles.

This report details a new method for site-selective methylene oxidation adjacent to azaheterocycles. A dual catalysis approach, utilizing both an iron Lewis acid and an organic hydroxylamine catalyst, proved highly effective. We demonstrate that this method provides complementary selectivity to other known catalytic approaches and represents an improvement over current heterocycle-selective reactions that rely on stoichiometric activation.

Reductive Cross-Coupling of Conjugated Arylalkenes and Aryl Bromides with Hydrosilanes by Cooperative Palladium/Copper Catalysis.

A method for the reductive cross-coupling of conjugated arylalkenes and aryl bromides with hydrosilanes by cooperative palladium/copper catalysis was developed, thus resulting in the highly regioselective formation of various 1,1-diarylalkanes, including a biologically active molecule. Under the applied reaction conditions, high levels of functional-group tolerance were observed, and the reductive cross-coupling of internal alkynes with aryl bromides afforded trisubstituted alkenes.