Using charged porphyrins, researchers created stacked ion and radical pairs revealing fascinating new properties

π-Stacked ion and radical pairs are extremely useful for the creation of new electronic materials. Yet, the study of these has been limited due to multiple difficulties in their creation and characterization. A new study by researchers from Japan uses charged porphyrins to synthesize and study π-electronic ion pairs. Their findings can lead to the creation of novel materials and new types of batteries and memory devices.

Ions are created when an atom or molecule either loses or gains electrons, thus gaining a charge. When two oppositely charged ions are combined, it can lead to the creation of an ion pair. The influence of different ion pairs on the physical properties of the material they are present in has been widely studied as it can lead to the creation of new functional electronic materials.

Of particular interest is the study of π-electronic ion pairs for their ability to employ intermolecular interactions to produce dimension-controlled assemblies. These are useful for creating new electronic materials. These interactions (known as “ⁱπ−ⁱπ interactions”) cause the stacking of ions into π-stacked ion pairs (π-sips). Controlling the electronic state of these π-sips can create π-stacked radical pairs (π-srps). Both of these have attractive properties for the creation of electronic materials, but have not been studied much owing to the difficulties in preparation and structure determination.

To solve this problem, a team of researchers led by Professor Hiromitsu Maeda of 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 from and more advantageous than porphyrin derivatives with an electronically neutral core and peripheral charged substituents. We found that porphyrins with metal ions can be used for the creation of π-sips and π-srps and thus help us study their structure and properties.”

The researchers first used the theory of hard and soft acids and bases to perform ion-pair metathesis and synthesize porphyrin ion pairs at a yield ranging from 59% to 85%. These were then characterized using spectrometry and single-crystal X-ray analyses. The researchers found that the porphyrin ions formed solid-state assemblies and π-sips in solutions. Moreover, 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 electronic states of the π-sips and thus to the creation of π-srps. The creation of π-srps occurred through electron transfer due to solvent polarity and photoexcitation. The researchers also observed temperature-dependent spin distribution changes in π-srps. “The spin−spin interactions are influenced by the stacking structure of hetero π-electronic systems, which are characteristic of π-srps derived from π-sips,” comments Prof. Maeda.

“Investigating the properties of ion pairs as discrete chemical species is crucial for the development of functional materials. These new materials will have multiple different applications in fields like nanomagnetism, catalytic reactions, and ferroelectrics, thus creating new types of batteries and memory devices,” he concludes.

Title of original paper: π Stacked Ion Pairs: Tightly Associated Charged Porphyrins in Ordered Arrangement Enabling Radical-Pair Formation
Journal: Journal of The American Chemical Society
DOI: https://doi.org/10.1021/jacs.2c09589
Latest Article Publishing Date: November 15 2022
Method of Research: Experimental Study
Subject of Research: Not Applicable
COI Statement: The authors declare no competing financial interest

About Ritsumeikan University, Japan

Ritsumeikan University is one of the most prestigious private universities in Japan. Its main campus is in Kyoto, where inspiring settings await researchers. With an unwavering objective to generate social symbiotic values and emergent talents, it aims to emerge as a next-generation research university. It will enhance researcher potential by providing support best suited to the needs of young and leading researchers, according to their career stage. Ritsumeikan University also endeavors to build a global research network as a “knowledge node” and disseminate achievements internationally, thereby contributing to the resolution of social/humanistic issues through interdisciplinary research and social implementation.

Website: http://en.ritsumei.ac.jp/

About Professor Hiromitsu Maeda from Ritsumeikan University, Japan

Prof. Hiromitsu Maeda is a professor at the Department of Applied Chemistry, College of Life Sciences, Ritsumeikan University. He completed his PhD from Kyoto University in 2004. Professor Maeda’s research interests include topics like physical organic chemistry, supramolecular chemistry, and materials science on π-electronic systems. Prof. Maeda has received several prizes, including ChemComm Emerging Investigator Lectureship (2012) and Fellow of the RSC (2015), and has over 185 publications.

Funding information

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 (2017−22 and 2022−27).