Scientists Can Now Tune Molecular Behavior by Swapping a Single Ion

Researchers at Ritsumeikan University in Japan have found that changing the ions surrounding certain molecules can alter their shape, electron transfer, response to pressure and how they assemble in solid form, potentially opening new approaches to sensors, molecular switches and electronic materials.

The researchers developed positively charged molecules whose shape and electronic properties could be controlled by changing the negatively charged ions paired with them. The changes affected how the molecules responded to light and pressure and how they arranged themselves in crystals, according to findings published in the journal Chemical Science on Aug. 17.

Molecules containing electron-donating and electron-accepting components can display distinctive electronic and light-responsive properties. Researchers have been exploring ways to adjust those properties to control the movement of electrons after exposure to light.

A team led by Professor Hiromitsu Maeda, with Professors Yohei Haketa and Yoichi Kobayashi of Ritsumeikan University and Professor Gaku Fukuhara of Kyushu University, built on earlier work by introducing different electronic components around a boron center and incorporating a phenalenyl unit into the molecular structure.

“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,” Prof. Maeda told Impact Newswire. “We expected that this arrangement would allow counteranions to influence molecular conformation, electronic states, and subsequent photophysical behavior.”

The researchers initially prepared molecules paired with chloride ions and then replaced the chloride with BF₄⁻, PF₆⁻, B(C₆F₅)₄⁻ and pentacyanocyclopentadienide.

The choice of ion changed the shape of the molecular structure. Chloride caused two pyrrole rings to flip, while larger ions favored a configuration in which the binding site remained open. Nuclear magnetic resonance and UV/visible spectroscopy confirmed the changes, while calculations showed different electron distributions within the molecule.

The structural changes also affected how quickly electrons moved after the molecules were exposed to light. Measurements showed electrons moving from one part of the molecule to the phenalenyl unit, producing a reduced phenalenyl species.

The speed of this process varied with the ion. For 3b⁺-B(C₆F₅)₄⁻, electron transfer occurred with a time constant of 200 femtoseconds, while the corresponding chloride complex reacted faster than the 150-femtosecond response limit of the instrument.

The findings indicate that changing the surrounding ions can provide a way to control extremely fast electron-transfer processes at the molecular level.

The molecules also responded reversibly to hydrostatic pressure of up to 280 megapascals. As pressure increased, their absorption spectra gradually shifted toward longer wavelengths, with the extent of the shift depending on the ion.

For example, 3b⁺-B(C₆F₅)₄⁻ showed a response of −0.714 cm⁻¹ MPa⁻¹, compared with −0.616 cm⁻¹ MPa⁻¹ for 3b⁺-Cl⁻. The researchers attributed the smaller response of the chloride complex to a more rigid structure 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.”

Tests of the materials in solid form also showed that the molecules could assemble into one-dimensional structures. Single-crystal X-ray analysis found that the molecules formed arrays through interactions between their electronic systems, with electrostatic and dispersion forces helping to stabilize the structures.

The researchers said the results demonstrate that selecting the ion paired with a molecule can affect its shape, electronic properties, electron-transfer rate, response to pressure and arrangement in the solid state.

Such control could be useful in developing pressure sensors, molecular switches and materials in which the movement of electric charge can be adjusted, the researchers said.

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