Ions are formed when an atom or molecule loses or gains electrons, thereby gaining a charge. When two oppositely charged ions combine, this can lead to the formation of an ion pair. The influence of different ion pairs on the physical properties of the material in which they are present has been extensively studied as it can lead to the creation of new functional electronic materials.
Of particular interest is the study of π-electron ion pairs for their ability to exploit intermolecular interactions to produce dimensionally controlled assemblies. These are useful for creating new electronic materials. These interactions (known as “Iπ−Iπ-Interactions) cause ions to stack into π-stacked ion pairs (π-sips). Controlling the electronic state of these π-sips can generate π-stacked radical pairs (π-srps). Both have attractive properties for the manufacture of electronic materials, but have not been studied much due to the difficulties in fabrication and structure determination.
To solve this problem, a research team led by Professor Hiromitsu Maeda from Ritsumeikan University, Japan synthesized and studied π-sips and π-srps using charged porphyrins. This study was subsequently published in the Journal of the American Chemical Society. According to Prof. Maeda: “The charged porphyrins investigated in this study are different and more advantageous than porphyrin derivatives with an electronically neutral core and peripherally charged substitutes. We found that porphyrins with metal ions can be used to prepare π-sips and π-srps, thus helping us to study their structure and properties.”
Researchers first used the theory of hard and soft acids and bases to perform ion-pair metathesis and synthesize porphyrin ion pairs in 59% to 85% yield. These were then characterized by spectrometry and single-crystal X-ray analysis. The researchers found that the porphyrin ions formed solid-state arrays and π-sips in solution. In addition, the introduction of electron-donating and electron-withdrawing groups activated the anions and cations, respectively. This activation of cations and anions leads to the ability to control the electronic states of the π-sips and hence the formation of π-srps. The formation of π-srps occurred through electron transfer due to solvent polarity and photoexcitation. The researchers also observed temperature-dependent changes in the spin distribution in π-srps. “The spin-spin interactions are influenced by the stacking structure of hetero-π-electronic systems characteristic of π-sips-derived π-srps,” comments Prof. Maeda.
“The study of the properties of ion pairs as discrete chemical species is crucial for the development of functional materials. These new materials will have several different applications in areas such as nanomagnetism, catalytic reactions and ferroelectrics, creating new types of batteries and storage devices,” he concludes.
About Ritsumeikan University, Japan
Ritsumeikan University is one of the most renowned private universities in Japan. Its main campus is in Kyoto, where inspiring environments await researchers. With an unwavering goal of creating social symbiotic values and emerging talent, it aims to establish itself as the next-generation research university. It will increase researcher potential by providing support that best suits the needs of young and leading researchers according to their career stage. Ritsumeikan University also strives to build a global research network as a “knowledge hub” and disseminate achievements internationally, thus contributing to the solution of social/humanistic problems through interdisciplinary research and societal implementation.
About Professor Hiromitsu Maeda from Ritsumeikan University, Japan
Teacher. Hiromitsu Maeda is a professor in the Department of Applied Chemistry, College of Life Sciences, Ritsumeikan University. He received his PhD from Kyoto University in 2004. Professor Maeda’s research interests include topics such as physical organic chemistry, supramolecular chemistry and materials science on π-electronic systems. Teacher. Maeda has received multiple awards including ChemComm Emerging Investigator Lectureship (2012) and Fellow of the RSC (2015) and has published over 185 papers.
This work was supported by JSPS KAKENHI Grant Numbers JP18H01968 and JP22H02067 for Scientific Research (B), JP20J22745 for JSPS Fellows and JP20H05863 for Transformative Research Areas (A) “Condensed Conjugation” and the Ritsumeikan Global Innovation Research Organization (R-GIRO) project supported (2017-22 and 2022-27).
Journal of the American Chemical Society
subject of research
Stacking π-ion pairs: Closely associated charged porphyrins in an ordered array allow for the formation of radical pairs
Article publication date
November 15, 2022
The authors declare no competing financial interests.
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