1. Open-shell singlet diradicaloids and polyradicaloids
Traditionally, most reported neutral π-conjugated molecules show a closed-shell electronic structure in the ground state. However, our group and others have demonstrated that certain type of polycyclic hydrocarbons could show open-shell singlet diradical or even polyradical character in the ground state and exhibited unique electronic, optical and magnetic properties. We have developed various synthetic methods to prepare soluble and stable diradical-like compounds (diradicaloids) and clearly disclosed the fundamental structure-physical properties relationships. The studies revealed how the intermediate π-bonding fundamentally affected the electronic properties of π-conjugated molecules. The obtained diradicaloids and polyradicaloids have found promising material applications for such as electronics, photonics, spintronics, magnets and quantum information processing.
2. Aromaticity in 2D open-shell macrocycles and 3D fully conjugated molecular cages
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Aromaticity is one of the vital concepts in Organic Chemistry to understand the electronic properties of cyclic π-conjugated molecules. Over a century, different aromaticity rules have been developed. However, these rules are limited to two-dimensional (2D) monocyclic π-conjugated systems with closed-shell ground state. The fundamental rules for the open-shell systems and three-dimensional (3D) π-conjugated molecules remain poorly understood. We recently conducted systematic research on the global aromaticity in 2D macrocyclic polyradicaloids and 3D fully π-conjugated molecular cages. The studies revealed unusual aromaticity in the open-shell systems where both Hückel’s rule and Baird’s rule are applicable depending on the spin state. In addition, we have experimentally demonstrated the first 3D global aromaticity and disclosed close relationships between 3D global aromaticity and 2D Hückel/Baird aromaticity. These findings started a new chapter of the concept of “Aromaticity” in Organic Chemistry.
3. Molecular nanocarbons with controlled edge structures and topologies
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Carbon nanostructures play a key role in materials science and their electronic properties are strongly dependent on their edge structures and topologies. We have conducted bottom-up syntheses of various nanocarbons with controlled electronic structures and physical properties by both solution-phase chemistry and on-surface synthesis. We also developed efficient synthesis of a few novel carbon nanostructures that have been sought by chemists for a long time. Examples include tube-like carbon nanobelts, Archimedean cages, twisted carbon nanobelts with a figure-eight or triply twisted Möbius shape, and new synthetic molecular carbon allotropes (e.g., graphyne spoked wheel). These new topological molecular carbons are not only structurally appealing, but also useful materials with chiroptical properties and controlled charge/spin transporting behaviour in two- or three dimensions.
