Researchers from Ritsumeikan University reveal how counterions can control molecular shape, electron transfer, assembly, and pressure-responsive propertiesn

Electronic materials can change their behavior when molecules interact with surrounding ions, but controlling these effects remains challenging. Now, researchers from Ritsumeikan University, Japan, have developed π-electronic cations whose molecular shape and electronic properties are controlled by counteranions. The ion-dependent changes modulate photoinduced electron transfer, pressure-responsive photophysical behavior, and solid-state assembly, offering design principles for smart materials, molecular switches, sensors, and tunable charge-transport technologies. These findings could accelerate development of next-generation responsive electronic devices.

Orthogonally arranged π-electronic systems that combine electron-donating and -accepting units display distinctive electronic and photophysical behavior. Fine-tuning their electronic structure offers a route for controlling photoinduced electron transfer. Building on this idea, complexing boron with 1,3-diketones and 9-oxidophenalenone may result in electron-deficient cationic π-electronic systems.

A research team led by Professor Hiromitsu Maeda, along with Professor Yohei Haketa and Professor Yoichi Kobayashi from Ritsumeikan University, Japan, and Professor Gaku Fukuhara from Kyushu University, Japan, extended the approach of introducing a range of π-electronic diol units at the boron center and was able to successfully incorporate a phenalenyl unit into the framework. Their findings were published in the journal Chemical Science on August 17, 2026.

“By introducing a phenalenyl unit into our previously studied anion-responsive molecular framework, we were able to create a cationic π-electronic system with two orthogonally arranged components,” says Prof. Maeda. “We expected that this arrangement would allow counteranions to influence molecular conformation, electronic states, and subsequent photophysical behavior.”

The researchers first prepared chloride ion pairs and then exchanged chloride for BF₄^-, PF₆^-, B(C₆F₅)₄^-, and pentacyanocyclopentadienide. The identity of the counteranion strongly influenced the shape of the anion-binding unit. Chloride binding induced inversion of two pyrrole rings, whereas larger counteranions favored an unbound conformation. Nuclear magnetic resonance and UV/visible spectroscopy confirmed these changes, while calculations showed distinct electronic distributions between the electron-rich anion-binding region and the electron-deficient phenalenyl unit.

These structural differences also influenced electron transfer after photoexcitation. Transient absorption measurements showed electron transfer from the dipyrrolyldiketone unit to the phenalenyl unit, generating a reduced phenalenyl species. The process depended on the counteranion: for 3b⁺-B(C₆F₅)₄^-, electron transfer occurred with a time constant of 200 fs, whereas the corresponding chloride complex reacted faster than the 150-fs instrumental response. Counteranion binding therefore provided a molecular means of modulating ultrafast electron-transfer behavior.

The molecules also responded reversibly to hydrostatic pressure up to 280 MPa. Increasing pressure caused gradual red shifts in their absorption spectra, but the magnitude of the response depended on the counteranion. For example, 3b⁺-B(C₆F₅)₄^- showed a slope of −0.714 cm^-1 MPa^-1, compared with −0.616 cm^-1 MPa^-1 for 3b⁺-Cl^-. The smaller response of chloride complexes was attributed to structural rigidification caused by chloride binding.

“Counteranions are often viewed simply as charge-balancing partners, but our results show that they can actively control molecular behavior,” says Prof. Maeda. “This ability to regulate electron transfer and pressure-responsive photophysical properties could help establish new design strategies for stimulus-responsive electronic and photophysical materials.”

In the solid state, single-crystal X-ray analysis revealed ion-pairing assemblies in which the orthogonal molecules formed one-dimensional arrays through double ⁱπ–ⁱπ interactions. Favorable electrostatic and dispersion interactions stabilized these structures, showing that counteranion-dependent conformations can influence both molecular properties and crystal packing.

Overall, the study demonstrates that counteranion selection can control the conformation, electronic states, electron transfer, pressure response, and solid-state assembly of π-electronic cations. This strategy could support the development of smart materials such as pressure sensors, molecular switches, and tunable charge-transport systems, contributing to smaller, lighter, and more energy-efficient electronic and photonic technologies.

Title of original paper: Anion-controlled ion pairing and assembly of π-electronic cations with orthogonal π-systems
Journal: Chemical Science
DOI: 10.1039/d6sc05424b

About Ritsumeikan University, Japan

Ritsumeikan University is one of the most prestigious private universities in Japan. With an unwavering objective to generate social symbiotic values and emergent talents, it aims to emerge as a next-generation research-intensive university. It will enhance researcher potential by providing support best suited to the needs of young and leading researchers, according to their career stage. Ritsumeikan University also endeavors to build a global research network as a “knowledge node” and disseminate achievements internationally, thereby contributing to the resolution of social/humanistic issues through interdisciplinary research and social implementation.

Website: http://en.ritsumei.ac.jp/
Ritsumeikan University Research Report: https://www.ritsumei.ac.jp/research/radiant/eng/

About Professor Hiromitsu Maeda from Ritsumeikan University, Japan

Prof. Hiromitsu Maeda is a Professor in the Department of Applied Chemistry, College of Life Sciences, Ritsumeikan University, Japan and a Fellow of the Ritsumeikan Advanced Research Academy (RARA). He earned his Ph.D. from Kyoto University in 2004, following a three-month research stay at the University of Texas at Austin in 2001. He joined Ritsumeikan University's College of Science and Engineering in 2004 and transferred to the College of Pharmaceutical Sciences in 2008, becoming a Professor in 2014. He moved to the College of Life Sciences in 2016 and has held several additional research appointments in Japan and abroad.

Funding information

This work was supported by JSPS KAKENHI Grant Numbers JP18H01968, JP22H02067, and JP23K23335 for Scientific Research (B); JP24K08389 for Scientific Research (C); JP23K17951 for Challenging Research (Exploratory); and JP20H05863 for Transformative Research Areas (A) “Condensed Conjugation,” the Cooperative Research Program of “Network Joint Research Center for Materials and Devices (MEXT),” and the Ritsumeikan Global Innovation Research Organization (R-GIRO) project (2017–22 and 2022–27). Theoretical calculations were partially performed using the Research Center for Computational Science, Okazaki, Japan (Projects: 23-IMS-C069, 24-IMS-C067, 25-IMS-C069, and 26-IMS-C067). Synchrotron-radiation analysis was performed at BL40XU (2023A1240, 2023B1390) and BL02B1 (2023A1645) of SPring-8 with the approval of the Japan Synchrotron Radiation Research Institute (JASRI).