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.
