This article has been reviewed according to Science X's editorial process and policies. Editors have highlighted the following attributes while ensuring the content's credibility: 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. A research team led by Hiromitsu Maeda, a professor at Ritsumeikan University in Japan, along with Ritsumeikan professors Yohei Haketa and Yoichi Kobayashi and Kyushu University professor Gaku Fukuhara, extended the approach by introducing a range of π-electronic diol units at the boron center. They incorporated a phenalenyl unit into the framework.
Their findings were published in Chemical Science. "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," Maeda says. "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 caused two pyrrole rings to invert, 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.
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