Molecules with high symmetry and structural aesthetics have attracted attention of many organic chemists due to their synthetic challenges and their potential for chiroptical properties. Our group has been engaged in the synthesis and functional exploration of highly symmetric π-conjugated compounds incorporating axial (binaphthyl) and planar chirality ([2.2]paracyclophane) as chiral elements. X-ray crystallography has revealed the structures, and their chiroptical properties such as circular dichroism (CD) and circular polarized luminescence (CPL) have been investigated. We have also developed chiral-induced additives based on π-extended planar chiral [2.2]paracyclophane. A significant amplification of CPL was observed with only 3 wt% doping to the achiral π-conjugated luminescent polymer. In this review, we summarize the synthesis and properties of the chiral compounds which we have developed so far. We sincerely believe that this paper will be a source of ideas not only for researchers in structural organic chemistry and materials organic chemistry who work with π-conjugated compounds, but also for synthetic organic chemists who work with chiral compounds, especially those who are involved in asymmetric synthesis.

Macrocyclic compounds exhibit unique properties due to their large ring-shaped structures, which have captivated chemists in recent decades. These molecules exert specific functions in a multidisciplinary basis, e.g. chemistry, materials science, and biology. Crown ethers, calix [n] arenes, and porphyrinoids, which contain several ether/phenol/pyrrole subunits, represent milestones in these molecular categories with unparalleled functional diversity. In contrast, macrocycles exploiting quinoline as a key unitary element to construct specific macrocyclic architecture have received limited attention and been barely explored. In this report, we summarize the recent advances in oligo-quinoline macrocycles TriQuinoline (TQ), oxa-TriQuinoline (o-TQ), TEtraQuinoline (TEQ), and indole/quinoline hybrid macrocycle In2Q2. Particular emphasis is placed on the synthesis, structure, and application of these molecules in organometallic and supramolecular chemistry, which may guide new molecular designs in the heterocycle-based macrocycle family.

Biomacromolecules, such as DNA and proteins, form characteristic secondary structures, represented by α-helix and β-sheets, which are further assembled into tertiary and quaternary structures through specific intra- and intermolecular interactions. Such higher-order structures play a key role in the expression of the sophisticated biological functions. While advances in polymerization technologies have made it possible to precisely control the primary structures of synthetic polymers, the design and construction of higher-order structures, even secondary structures, remains a major challenge in polymer science. Recently, the author's group has developed a novel synthetic methodology called “defect-free ladderization” and combined it with “chiral molecular design” to realize the tailored construction of diverse secondary structures to meet specific functional requirements. In this article, we present our recent results with emphasis on the efficient synthesis of defect-free ladder polymers with specific secondary structures via alkyne benzannulations and the development of functional materials based on the rational design of artificial secondary structures. We anticipate that these results will provide new directions for advancing the field of organic synthesis and polymer science.

The author has proposed and developed FLAP (Flexible and Aromatic Photofunctional Systems), a unique hybrid molecular system with flexible and rigid moieties, that can induce a dynamic motion to express meaningful macroscopic events. The essence of the molecular design is to change the electronic structure as well as the conformation of the molecule by hybridizing flexible and rigid parts based on deep consideration of molecular orbitals. Specifically, we have proposed a molecular design with a flexible 8π ring structure at the center and fused rigid π-conjugated skeletons on both sides. The structure of the flexible moiety of FLAP is not necessarily limited to an eight-membered ring, and countless structures are possible for FLAP by combining flexible and rigid moieties. Although it is interesting to study to explore functional FLAPs from the chemical space, this paper only focuses on the FLAP skeleton with cyclooctatetraene (COT) at the center as a typical 8-membered ring of the 8π system. Synthetic trials and errors and development history of FLAPs are described.

Synthesis of ever-longest carbo- or hetero-helicenes is a challenging task in synthetic organic chemistry, but it often involves handling problems such as insolubility in common organic solvents and oxidative degradation under aerobic conditions. We designed benzo-annulated aza[n]helicenes ([n]AH), which benefit from both the suppressed elevation of HOMO energies and high solubility due to hydrogen-bonding with solvent molecules at the pyrrole NH sites. This strategy enabled the synthesis of six single aza[n]helicenes (n=9, 11, 13, 15, 17, 19) and double aza[9]helicene (D[9]AH) via intramolecular oxidative fusion reactions. The solubilities were further tuned by N-alkylation reaction. Aza[9]helicenes with terminal substituents were also synthesized to investigate the substituent effects. The structures of all the synthesized aza[n]helicenes were determined by X-ray diffraction analysis, which revealed triple-layered structures of [17]AH and [19]AH as well as chiral (P,P) and (M,M) structures of D[9]AH-Et as a racemic mixture. Their electrochemical and (chir)optical properties have been comprehensively studied by cyclic voltammetry, UV/Vis absorption, emission, CD, and CPL spectra. These systematic studies provide an insight on the structure-property relationships of azahelicenes.

Helically twisted conductive nanocarbon materials hold great potential for optoelectronic and electromagnetic molecular devices operating on the nanometer scale. With recent advances in synthetic organic chemistry, there has been increasing interest in the synthesis and characterization of compounds with large π-conjugated systems, which were previously challenging to synthesize. Our research focuses on the synthesis of homogeneously π-extended helicene derivatives and the investigation of their electronic structures. In 2018, we successfully synthesized and performed single-crystal structure analysis of a π-extended [7]helicene (2, C₄₈H₂₄). In 2022, we synthesized π-extended [5]helicene (1, C₃₄H₁₆(CH₃)₂) and π-extended [9]helicene (3, C₆₂H₃₀). Through our study of their electronic structures, we discovered that the unique electronic and photophysical properties of π-extended helicenes originate from a polyene-like electronic state within the helical framework along the helical inner rim. In this article, we present our latest findings on the synthesis of homogeneously π-extended helicene derivatives and their distinct physical properties, which arise from their molecular structures.

In the field of physical organic chemistry, the important thing is how to design and synthesize beautiful molecules with a unique structure or construct molecules with desired functions. During molecular design, the author determines himself to focus on the following points: (1) highly distorted molecules and (2) molecules that can undergo redox interconversion. This is because he believes that imparting strain to a molecule not only gives it a structure with unusual geometrical parameters but also allows it to exhibit functions that normal molecules do not have. However, the more strain a molecule has, the more difficult it is to synthesize the molecule. To solve this issue, the author adopts the key strategy of designing a redox-active molecule, so that the final reaction yielding the desired product could be conducted by the redox reaction. Based on such a design concept, the author's group has developed highly strained redox-active molecules with unique geometries and functions. Herein, the author focuses on overcrowded ethylenes (OCEs), which exhibit nonplanar geometries due to the bulky substituents on the alkene moiety. For example, well-designed OCEs can adopt multiple conformations/configurations, such as folded and twisted forms, and their frontier orbital levels can change significantly based on easily interconvertible molecular structures, allowing for dynamic changes in physical properties. In this paper, the author describes his molecular design concept and successful examples of stimuli-responsive systems with metastable structures and their properties.

Cyclic molecular nanocarbons represented by carbon nanorings and nanobelts are recent emerging molecules in π-conjugated chemistry and materials science. Since the synthesis of cycloparaphenylene (CPP), which is the simplest nanoring, has been achieved in 2008, creation of diverse types of nanorings and their unique properties have been reported in this decade. In contrast to nanorings, the molecules called nanobelts have structures where aromatic rings are fused to be tubular shape. A number of chemists have attempted the synthesis of nanobelts from long time ago even before that of CPP; however, it has never been achieved until 2017. The breakthrough synthesis of (6,6)carbon nanobelt has led huge progress of nanobelt chemistry in recent years. On the other hand, synthesis of nanobelts often requires multiple reaction steps resulting in low yields of target products. These drawbacks have prevented to produce large amount of nanobelts and derivatives with diverse belt sizes. In 2020, we have developed a new nanobelt, methylene-bridged [6]cycloparaphenylene ([6]MCPP), where neighboring phenylene units in [6]CPP are respectively connected by a methylene moiety. The synthesis of [6]MCPP needs only three steps from readily available pillar [6]arene, which has enabled its rapid commercialization. [6]MCPP has unique properties such as the smallest diameter and small HOMO-LUMO gap, encouraging us to further study MCPPs. This article describes the chemistry and applications of methylene-bridged nanobelts including other belt sizes of MCPPs, methylene-bridged cyclonaphthylene (MCN) and a water-soluble [6]MCPP derivative.

Carbon and hydrogen are fundamental components of organic molecules. Various functional π-systems have been created via the structural modification of conventional π-conjugated hydrocarbons. Novel π-conjugated hydrocarbon skeletons with attractive electronic structures are a source of novel functional materials. Notably, π-conjugated hydrocarbons with a narrow HOMO-LUMO gap are attractive research targets. Hydrocarbons with antiaromaticity or diradical character often exhibit narrow HOMO-LUMO gaps. However, these molecules are inherently reactive, thus requiring kinetic stabilization by sterically hindered peripheral substituents. This article describes the synthesis and properties of as-indaceno[3,2,1,8,7,6-ghijklm]terrylene, indeno[1,2,3,4-pqra]perylene, and its π-extended derivatives. These π-conjugated hydrocarbons are sufficiently stable under ambient conditions despite their narrow HOMO-LUMO gaps.

Fluorine is a unique element, characterized by the highest electronegativity, the second smallest atomic radius after hydrogen, and low polarizability. When incorporated into organic molecules, fluorine significantly modifies their structures and properties. In this work, the author and coworkers successfully introduced multiple fluorine atoms into a cubic-shaped molecule called cubane, by employing their “PERFECT method”, a liquid-phase fluorination technique using fluorine gas. The first part focuses on synthesis of perfluorocubane, which has been expected to accept an electron inside its cubic skeleton. They successfully isolated the target molecule, and demonstrated the electron acceptance in the cube. The second part is about synthesis and structural analysis of hexafluorodihalocubanes. Based on the X-ray diffraction analysis of crystal and X-ray absorption spectroscopy in solution, the C-X bond (X=Cl, Br, I) in the molecules was found to be exceptionally short. Comparing to halotrinitromethanes, which is the representative series of compounds with short C-X bond, reasons for the bond-shortening will be comprehensively discussed.