There has been a long-standing controversy in organic chemistry regarding the relationship between aromaticity and planarity of the representative aromatic compound, benzene. Synthesis of an [n]paracyclophane with the smallest possible methylene bridge has been proposed to be the best model to address this open controversy. Dr. Tobe successfully synthesized the smallest isolable congener, [6]paracyclophane, and its anthracene homologues by developing an ingenious synthetic method; he clarified  structures and unconventional properties of these molecules, which are due to strain imposed on the aromatic ring. Moreover, he identified the next smallest [n]paracyclophane, [5]paracyclophane, and experimentally proved that its aromaticity was sustained, thereby elucidating the aromaticity-planarity relationship.

2. Generation of Highly Reactive Cyclic Polyynes Based on [2+2] Cycloreversion

Cyclocarbons are all-carbon, π-conjugated monocyclic molecules; they have attracted interest because of not only their unique structures and aromaticity, but also their utility in exploring the mechanism of fullerene formation from small carbon fragments. Dr. Tobe synthesized stable cyclocarbon precursor molecules with [4.3.2]propellane units as masked triple bonds. He generated cyclocarbon ions through [2+2] cycloreversion of the precursors by means of laser irradiation under high-vacuum conditions and clarified the structures of cyclocarbons through ultraviolet photoelectron spectroscopy measurements of cyclocarbon ions. Utilizing the same protocol, he identified the most severely deformed cyclic triyne in an argon matrix at a cryogenic temperature. Moreover, he demonstrated the formation of fullerene ions under vacuum from their respective three-dimensional polyyne ions through multiple-step cyclization, thereby providing experimental evidence for a mechanism of fullerene formation.

3. Synthesis and Unconventional Physical Properties of Nonalternant Diradicaloid Aromatic Hydrocarbons

Diradicaloid hydrocarbons have been examined intensively owing to their low band-gap semiconductor and nonlinear optical properties. Dr. Tobe focused on nonalternant diradicaloid hydrocarbons, expecting them to manifest unconventional physical properties due to their unique electronic configurations. For example, although a larger π-conjugated organic molecule typically has a smaller electronic transition energy of its first excitation band compared to that of a smaller homologue, Dr. Tobe found through absorption spectra that this trend was reversed in nonalternant diradicaloid hydrocarbons with moderate diradical character, such as indeno[2,1-b]fluorene and its larger homologue fluoreno[2,3-b]fluorene, both of which he synthesized. He also synthesized what is potentially the first tetraradicaloid hydrocarbon, tertacyclopentatetraphenylene, which consists of anti-aromatic external 20-π and internal 8-π circuits. These studies are unique and outstanding among those in the field owing to the novel structures and unconventional physical properties they have revealed.

4. Construction of Porous Two-Dimensional Molecular Networks by Self-Assembly of Cyclic Aryl Acetylenes at Liquid-Solid Interfaces

Construction of two-dimensional nanostructures by means of molecular self-assembly on surfaces has attracted tremendous current interest from the perspective of bottom-up organic electronics and tailored chemical reaction fields. Of these architectures, porous molecular networks are among the most interesting because they can form nanostructures consisting of multiple components. Dr. Tobe made ingenious use of the shapes and chemical properties of triangular aryl acetylenes, exploiting the power of organic synthesis and surface technology; using scanning tunneling microscopy, he found that the triangular molecules self-assembled at liquid-solid interfaces via van der Waals interaction between the alkyl chains attached to the triangle core, forming porous molecular networks of honeycomb-like structures. He also proposed a semi-quantitative thermodynamic model to predict structural patterns of the molecular assemblies. In addition, he successfully demonstrated size- and shape-selective co-adsorption of guest molecules to the porous networks and proved that the adsorption was reversibly induced by external stimuli, thus achieving potentially useful functions via two-dimensional supramolecular chemistry.

In summary, Dr. Tobe has synthesized a number of central π-conjugated organic molecules with exotic structures and electronic characteristics; he has clarified their structures and physical properties to make great contributions to physical organic chemistry. In addition, he has investigated the formation and control of porous two-dimensional molecular networks by self-assembly on surfaces, thereby advancing a frontier of organic chemistry. His accomplishments have broadly impacted fundamental as well as materials chemistry and are highly evaluated worldwide. Dr. Tobe therefore deserves The Chemical Society of Japan Award for these achievements.