Changing counterions gives molecular materials new electronic behaviors
Orthogonally arranged π-electronic systems that combine electron-donating and electron-accepting units display distinctive electronic and photophysical behavior. Fine-tuning their electronic structure offers a way to control photoinduced electron transfer. Building on this idea, complexing boron with 1,3-diketones and 9-oxidophenalenone may produce electron-deficient cationic π-electronic systems.
Researchers at Ritsumeikan University in Japan, led by Hiromitsu Maeda, have developed a new method to alter the electronic properties of molecular materials by changing their counterions. By incorporating a phenalenyl unit into a previously studied anion-responsive molecular framework, the team was able to create a cationic π-electronic system with two orthogonally arranged components.
The identity of the counteranion strongly influenced the shape of the anion-binding unit, affecting molecular conformation, electronic states, and subsequent photophysical behavior. The researchers found that electron transfer from the dipyrrolyldiketone unit to the phenalenyl unit could be modulated by the counteranion, with time constants ranging from 150 fs for chloride complexes to 200 fs for BF₄⁻ complexes.
Additionally, the molecules responded reversibly to hydrostatic pressure up to 280 MPa, with the magnitude of the response depending on the counteranion. This research demonstrates that counterions can actively control molecular behavior, offering a new way to regulate electron transfer and pressure-responsive photophysical properties in molecular materials.
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