Organic radicals are open-shell molecules with unpaired electrons, and their single electron-occupied frontier orbitals are extremely susceptible to electron transfer and release, making them suitable in energy transfer and storage applications such as conducting materials and rechargeable batteries. The non-zero magnetic moments and distinctive photophysical features of free radicals, particularly their complex spin configurations in the ground and excited states, have also attracted considerable attention in the fields of molecular spintronics, optoelectronics, and quantum information technology. Despite the intriguing prospects for the application of radical materials, the most of radicals are unstable at ambient conditions due to their high reactivity and short lifetimes, and thus their materials-related research for practical applications is currently limited. Developing and manufacturing stable organic radicals, while also extending their operational capability in complex and severe conditions, continue to be difficult challenges.
Porphyrins are a family of conjugated macrocyclic compounds composed of pyrroles connected by carbon atoms and possessing a unique π-conjugated electronic structure capable of delocalizing spins and stabilizing radicals. Porphyrins, which include heme and chlorophyll, are commonly found in nature and are referred to as the pigments of life. The most of porphyrins have aromatic [4n+2] π-conjugated electron structures. In recent years, Professor Shen Zhen's group at Nanjing University's School of Chemistry and Chemical Engineering has been investigating the radical properties of corrole, a ring-contracted porphyrin analogue. The structure of corrole is similar to the main skeleton of vitamin B12, in that it contains one less carbon in the meso position than that of the typical porphyrin. This small structural difference leads to corrole's unique coordination ability, as when coordinated with a certain metal, it readily generates radical based on the [4n+1] electronic structure of corrole ligand. Prof. Shen Zhen's previous work examined the structures and properties of copper and silver corrole complexes and modulated the interaction between the central metal spin and the ligand radical spin via ligand modification, surface adsorption, and chemical oxidation. On corrole, Shen’s group were able to convert the electronic structures of a closed-shell singlet to an open-shell singlet, an open-shell singlet to a stable triplet, and a radical ligand to an antiaromatic ligand. By exploring their applicability in spintronics, these works highlighted the remarkable stability and adjustability of corrole complexes as spin carriers. (Nat. Commun. 2015, 6 (1),7547；Angew. Chem., Int. Ed. 2021, 60 (21), 11702−11706；Chem. Commun., 2021,57, 383-386.)
The present study developed a very stable neutral nickel corrole radical by a facile synthetic strategy using acid-catalyzed condensation and retro-Diels-Alder reaction. The radical is a special a1u type, with most of its spin density (>90%) distributed on the α-carbon atom of the inner pyrrole ring, which is protected by the comprehensive blocking of the peripheral β-fused benzene ring and the meso-phenyl rings, thus greatly enhancing the stability of the molecule. This radical can be converted reversibly to aromatic and anti-aromatic ionic states and exhibits exceptional photothermal stability and photothermal conversion efficiency (Figure 1). This study provides an in-depth insight into the relationship between the corrole radical's special hybrid semi-aromatic-semi-anti-aromatic frontier molecular orbitals and their corresponding photophysical properties through detailed theoretical analysis and spectroscopic measurements, thus further establishing a new idea and platform for the design and application of radical functional materials.
The work was published online as Highly Stable Neutral Corrole Radical: Amphoteric Aromatic-Antiaromatic Switching and Efficient Photothermal Conversion in Journal of the American Chemical Society (DOI: 10.1021/jacs.1c11716). The first author is Ph.D. student Hu Gao, with Dr. Fan Wu and Prof. Zhen Shen as co-corresponding authors. This work was supported by the National Natural Science Foundation of China (NSFC) (Grants 21771102, 22071103, 22001119, and 21911540069).
Figure 1. Aromatic/antiaromatic reversible conversion and photothermal effect of nickel corrole radical.