Unsubstituted π-electronic systems with expanded π-planes are highly desirable for improving charge-carrier transport in organic semiconductors. However, their poor solubility and high crystallinity pose major challenges in processing and assembly, despite their favourable electronic properties. The strategic arrangement of these molecular structures is crucial for achieving high-performance organic semiconductive materials.
In a significant breakthrough, a research team led by Professor Hiromitsu Maeda from Ritsumeikan University, including Associate Professor Yohei Haketa from Ritsumeikan University, Professor Shu Seki from Kyoto University, and Professor Go Watanabe from Kitasato University, has synthesized a novel organic electronic system incorporating gold (AuIII) and benzoporphyrin molecules, enabling enhanced solubility and conductivity. The findings of the study were published online in the Journal Chemical Science on February 19, 2025.
π-Electronic systems are molecular structures with delocalized π-electrons, arising from the overlapping of π-orbitals in conjugated systems. These systems allow efficient charge transport with electronic interactions and are most commonly applied in organic semiconductors. However, their application is hindered owing to their low solubility. The researchers used a novel technique involving ion pairing of the π-electronic cation-based system, which improves interactions for solubility and reduces the electrostatic repulsion while stacking into structures.
"Low solubility of expanded π-electronic systems is often a challenge in fabricating assembled structures for organic electronic materials. In our study, we have introduced a new approach to enhance the solubility of expanded π-electronic cations by combining them with appropriate bulky counteranions." says lead author, Prof. Maeda.
Charge segregated systems are where positively and negatively charged π-electronic molecules form distinct molecular stacking arrangements. This allows for efficient charge transfer and conductivity. To build these charge-segregated systems, the researchers first synthesized a benzoporphyrin AuIII complex, which serves as an expanded π-electronic cation. The expansion of the π-system increases dispersion forces (weak intermolecular forces arising due to a change in electron distribution), which helps overcome electrostatic repulsion between identically charged molecules. Further, the researchers paired these expanded π-electronic cations with bulky counterions, forming soluble ion pairs.
"We introduced four different bulky counteranions, including PF₆⁻, FABA⁻, BArF⁻, and PCCp⁻, evaluating each ion pair for their structural properties and conductivity," reports Dr. Yohei Haketa.
On the basis of the stacking of the benzoporphyrin AuIII complex, the ion pairs are assembled in two different polymorphic states: single-crystal and less-crystalline (LeC) states. The single-crystal states were formed in controlled crystallization conditions and exhibited a high-ordered stacking with a rigid crystalline structure. Alternatively, the LeC states, which were formed via recrystallization in particular solvents, exhibited a less ordered arrangement of the ion pairs. The structural properties were confirmed through advanced techniques, including X-ray diffraction and solid-state NMR measurements, along with molecular dynamics simulations.
"We observed that although the pseudo polymorphs exhibited different structural stacking, both types of structures exhibited electrical conductivity with tunable conductive properties, allowing their use in a broad range of applications," explains Prof. Maeda.
The findings of the study were remarkable. The combination of the planar expanded π-electronic cation and bulky anions resulted in the formation of soluble ion pairs, which in turn led to the ordered arrangement of charged π-electronic systems. The formed ion pairs can therefore be used for a solution-processed fabrication of conductive materials, enabling the development of novel electronic materials and devices.
The study therefore paves the way for solution-processed conductive materials, which could potentially lead to next-generation organic semiconductors. Furthermore, the researchers will focus on refining molecular designs to optimize charge transport properties and explore applications in electronic circuits, sensors, and energy storage technologies.
Discussing the significance of their findings, Prof. Maeda remarked, "Our study demonstrates new aspects of molecular assemblies and their functionalities through molecular design and synthesis, which are essential for the future applications of π-electronic materials."
Building on previous findings, their research pushes the limits of molecular assembly and electronic materials, shaping the next generation of electronic technologies.
