Dr. Shinokubo created unconventional types of porphyrins―such as heteroatom-substituted porphyrins and antiaromatic norcorroles―using a synthetic method that employs metal ions as templates. For heteroatom-substituted porphyrins, he demonstrated that introducing heteroatoms or transition metals directly into the cyclic π-conjugated system allows control over optical and electronic properties as well as aromaticity. He also succeeded in the efficient synthesis of stable norcorroles, which exhibit pronounced antiaromaticity. He clarified their unique characteristics that differ from those of typical aromatic compounds, including their excellent redox properties, three-dimensional aromaticity in closely stacked arrangements, and bonding interactions between stacked molecules.

1. Exploration of heteroatom-substituted porphyrins
Dr. Shinokubo successfully introduced heteroatoms and transition metals into porphyrin frameworks. Furthermore, by carefully examining the structures and properties of the synthesized unconventional porphyrins, he successfully realized the functions of heteroatom-substituted porphyrins.

By introducing various heteroatoms, he created unconventional porphyrins incorporating nitrogen, sulfur, silicon, phosphorus, and boron. These porphyrin analogues exhibit optical and electrochemical properties that reflect the characteristics of the introduced elements. He further expanded the introduced elements to transition metals. He synthesized platinacorrole, a porphyrin analogue with a transition metal within its π-conjugated framework. This achievement provided valuable insights into metallaaromaticity by clearly identifying the number of d-electrons involved in the cyclic π-conjugation.

He also developed skeletal editing of the porphyrin framework, enabling the substitution of carbon atoms in the porphyrin skeleton with heteroatoms such as nitrogen, oxygen, and sulfur.

2. Creation of Antiaromatic Norcorroles and Elucidation of Their Functions
Dr. Shinokubo developed a metal-template synthetic method that allows the efficient preparation of stable antiaromatic norcorroles. He elucidated the fundamental properties and reactivities of norcorrole and also demonstrated the prospective features of norcorrole for applications in molecular electronics.

In terms of reactivity, he demonstrated that antiaromatic norcorroles possess both high HOMO and low LUMO levels, which grant them sufficient reactivity in electrophilic and nucleophilic reactions, enabling the highly regioselective introduction of various functionalities. He also found that peripheral π-extension of norcorroles modulates the electronic structure, giving rise to open-shell character and near-infrared photothermal conversion properties.

Furthermore, he focused on the stable redox properties of norcorroles and demonstrated their applicability as an electrode-active material for secondary batteries. He also demonstrated that norcorroles exhibit higher single-molecule conductivity than the corresponding porphyrin analogues.

3. Verification of Aromaticity in Closely Stacked Antiaromatic Compounds
It has been theoretically predicted that antiaromatic compounds should exhibit three-dimensional aromaticity when they are stacked closely. However, as the synthesis of stacked antiaromatic compounds was challenging, this phenomenon had not been experimentally observed. Dr. Shinokubo synthesized stacked antiaromatic dimers using norcorrole and verified the emergence of aromaticity for the first time. By controlling the stacking orientation of two norcorroles, he demonstrated that aromaticity and antiaromaticity can be dynamically interconverted, providing a novel perspective on the classical concept of aromaticity.

He discovered that norcorrole dimers connected by linkers adopt closely stacked structures, exhibiting a distinct aromatic nature. Furthermore, he found that a norcorrole with electron-withdrawing substituents forms an exceptionally close face-to-face stacking arrangement with a distance of 2.97 Å without any linkers. Molecular orbital calculations revealed that the close stacking originates from bonding orbital interactions between two norcorroles. He also succeeded in visualizing the bonding electrons present between the norcorroles. This clarified that the three-dimensional aromaticity of stacked antiaromatic compounds arises from intermolecular molecular orbital interactions. He further demonstrated that stacking alters the electronic state and drastically changes photophysical properties, including nonlinear optical characteristics. In addition, he successfully constructed supramolecular assemblies based on the attractive interactions between norcorroles.

In summary, Dr. Shinokubo has developed synthetic methods that enable elemental substitution of porphyrins as well as the efficient synthesis of stable antiaromatic norcorroles. He explored the properties and functions of these unconventional porphyrins. In particular, his achievements with norcorroles have significantly contributed to the development of the chemistry of antiaromatic compounds, which had long remained unexplored due to their inherent instability. These achievements are recognized as worthy of the prestigious Chemical Society of Japan Award.