Researchers develop highly conductive assemblies of gold complexes using ion-pairing

π-Electronic systems with delocalized electrons facilitate charge transport and molecular stacking. Ordered arrangements of π-electronic systems are crucial for charge transport but often result in lower solubility. Researchers from Ritsumeikan University have developed a new strategy for assembling such conductive materials using a highly planar benzoporphyrin gold (Au^III) complex. This breakthrough in π-electronic systems offers enhanced solubility and charge segregation, paving the way for innovative electronic applications.

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 (Au^III) 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 Au^III 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 Au^III 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.

Title of original paper: Electrically conductive charge-segregated pseudo-polymorphs comprising highly planar expanded π-electronic cations
Journal: Chemical Science
DOI: https://doi.org/10.1039/d4sc07576e

About Professor Hiromitsu Maeda from Ritsumeikan University, Japan

Dr. Hiromitsu Maeda is a professor at the Department of Applied Chemistry, College of Life Sciences, Ritsumeikan University and a Ritsumeikan Advanced Research Academy (RARA) Fellow. He completed his PhD from Kyoto University in 2004. Professor Maeda’s research interests include physical organic chemistry, supramolecular chemistry, and materials science on π-electronic systems. Prof. Maeda has received several prizes, including the ChemComm Emerging Investigator Lectureship (2012) and Fellow of the Royal Society of Chemistry (2015), and has over 200 publications.

Funding information

This work was supported by JSPS KAKENHI Grant Numbers JP18H01968, JP22H02067 and JP23K23335 for Scientific Research (B), JP19K05444 and JP24K08389 for Scientific Research (C), JP20J22745 for JSPS Fellows, JP23K17951 for Challenging Research (Exploratory) and JP20H05863 for Transformative Research Areas (A) “Condensed Conjugation”, JST SPRING, Grant Number JPMJSP2101 and Ritsumeikan Global Innovation Research Organization (R-GIRO) project (2017–22 and 2022–27).