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.
By replacing the electron-pairing counterions encircling a molecular framework, researchers discovered fresh electronic and photonic properties in π-electronic materials. The team headed by Professor Hiromitsu Maeda of Ritsumeikan University introduced a phenalenyl unit into a previously studied anion-responsive system, creating a cationic π-electronic structure.
The identity of the counterion significantly influenced the molecular conformation, electronic states, and photophysical behavior. Chloride ions caused two pyrrole rings to invert, while larger counterions favored an unbound configuration. The electron-rich anion-binding region and electron-deficient phenalenyl unit exhibited distinct electronic distributions, affecting electron transfer after photoexcitation.
Transient absorption measurements revealed electron transfer from the dipyrrolyldiketone unit to the phenalenyl unit, with electron transfer rates varying depending on the counterion. Pressure-induced spectral shifts also depended on the counteranion, with smaller responses observed for chloride complexes due to structural rigidification.
Counteranion selection can control conformation, electronic states, electron transfer, pressure response, and solid-state assembly in π-electronic cations, paving the way for smart materials like pressure sensors, molecular switches, and tunable charge-transport systems.
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