Satoru Hiroto

Recently, numerous heterohelicene derivatives have been synthesized; however, creating unsymmetrical heterohelicenes, particularly those with embedded nitrogen atoms, remains a challenge. In this study, we successfully synthesized unsymmetric azahelicenes through oxidative cyclodehydrogenation of N‐bridged π‐conjugated molecules. Due to the high regioselectivity at the 1‐positions of amino‐substituted anthracenes during oxidative couplings, we were able to obtain the corresponding azahelicenes by oxidizing N‐bridged phenanthrene, anthracene and pyrenes. The UV/vis absorption spectra of these products exhibited a bathochromic shift as π‐conjugation increased, while the fluorescence quantum yields correspondingly decreased. DFT calculations revealed that the configuration of the allowed HOMO–1 to LUMO transition in the lowest energy S₀–S₁ transition enhanced the quantum yield of anthracene‐based azahelicenes. These findings offer valuable insights for designing emissive helicenes with unsymmetrical structures.

Panchromatic dyes have been highly useful in the realm of optical devices. Here, we report that panchromatic dyes with heterohelicenes have been successfully synthesized using a donor‐acceptor strategy. Our synthesis resulted in the creation of π‐extended aza[5]helicene oligomers with butadiyne linkages, which displayed bathochromically shifted absorption and emission spectra. The solvent‐dependent optical measurements revealed the intramolecular charge transfer characteristic of these molecules, and theoretical calculations described the biased molecular orbitals on the azahelicene units that generated the charge‐transfer characteristic. Encouraged by these results, we also prepared donor‐acceptor‐donor dyads using azahelicenes and dimide derivatives, resulting in panchromatic absorbing characteristics covering the range from 250 nm to 800 nm. Theoretical calculations showed the presence of mixed charge‐transfer transitions and localized transitions on the azahelicene units, which led to a broad light‐absorbing property covering the near IR region. Additionally, we conducted measurements of circular dichroism and circularly polarized luminescence for the obtained products. The g‐values were reduced by oligomerization, indicating that the lowest energy transitions were allowed in nature.

Polar crystals have attracted interest for the applications to polar materials with piezo‐ and pyroelectricity, and second harmonic generation. Despite their potential utility for flexible polar materials, a strategy for ordering polar helicenes has remained elusive. Here, we demonstrate the creation of polar crystals with unsymmetrically substituted aza[5]helicenes tuned by substituents. The unsymmetric aza[5]helicenes have been prepared through regioselective mono‐protiodesilylations. We disclose that triisopropylsilyl‐substituted derivatives show 1D chain columnar packings. In particular, enantiopure crystals showed spontaneous polarization. Optical and single‐crystal X‐ray diffraction experiments with other derivatives, as well as theoretical calculations, revealed that the presence of triisopropylsilyl or electron‐withdrawing aryl substituents is essential for forming the 1D chain columnar structure. Hirshfeld surface analyses further showed that CH‐π interactions between 1D chain columns regulate the polar assembly. Finally, we determined the polarizability of the nitro derivative by ab initio calculation to be 4.53 μC/cm². This value corroborates the first example of a spontaneously polar crystal of helicenes. We believe that this study will contribute to the development of polar materials from organic molecules.

Three-dimensional π-conjugated molecules have received extensive attention in many chemical research areas due to their interesting structure and unique characteristics. In particular, twisted π-conjugated systems have been focused on for potential applications to solution processible organic materials. To date, numerous twisted organic molecules such as twistacenes have been synthesized. Under these situations, we found that oxidation of aromatic amines facilitates construction of twisted π-conjugated molecules. For example, oxidation of beta-amino substituted porphyrins provided three-dimensional porphyrins in excellent yield. Sequential use of amination and oxidation toward 5,10,15,20-tetramesitylporphyrin afforded highly twisted porphyrin oligomers. For this reaction, structures of the dimers proved dependent on the meso-substituents. This time, we investigated the substituent effect on the dimerization reactions. Introduction of more bulky substituents at meso-positions on oxidation process provided the corresponding twisted porphyrin dimers with larger twisted angles. In the case of p-tolyl and other aryl substituents without ortho substituents afforded porphyrin dimers with double spirocyclic linkages. The electrochemical analysis revealed the presence of electronic communications between two porphyrin units for the obtained dimers even through the spirocyclic linkages. The central metal also affects the conformations of the dimers. The central Ni(II) metal can be easily removed by treatment of sulfonic acid. Insertion of Zn(II) and Cu(II) metals reduced the twisted angle of the corresponding dimers mainly due to the planar features of the porphyrin units. The presence of magnetic interactions between two porphyrin units were revealed by temperature dependent magnetic susceptibility measurements on SQUID with Cu(II) analogues. In this presentation, we will disclose the structural characterizations of these dimers and would like to show the relationship between electrochemical properties and the conformations of porphyrin dimers.

Depth of a conjugated system! The chemistry of π‐conjugated compounds is discussed by Guest Editors Satoru Hiroto and Jishan Wu in their Editorial for this special issue.

Three-dimensional π-conjugated molecules have received extensive attention in many chemical research areas due to their interesting structure and unique characteristics. Among these situations, we found that oxidation of aromatic amines was useful for construction of distorted π-conjugated molecules under laboratory conditions. For example, oxidation of beta-amino substituted porphyrins provided three-dimensional porphyrins in excellent yield. In particular, sequential use of amination and oxidation toward 5,10,15,20-tetramesitylporphyrin effectively afforded highly twisted porphyrin oligomers. These oligomers exhibited high solubility in common organic solvent, which is promising for application to solution-processible molecular wire. In addition, we further investigated the substituent effect on the oxidation of beta-aminoporphyrins. We disclosed that dimerization products were varied dependent on meso-aryl substituents. In this presentation, we will show the electrochemical properties of these porphyrin oligomers in order to investigated the effect of distorted conformation on π-electronic network between porphyrin units. We also found oxidation of N-aryl beta-amino substituted porphyrins produced dehydropyrazine-bridged porphyrin dimers. Recently, we have succeeded in the synthesis of π-extended dihydrophenazine through oxidation of amino-substituted anthracenes. This molecule exhibited conformational change upon electrochemical and chemical redox conditions. Spectral analysis and theoretical calculation revealed that the aromatic and non-aromatic switch of the central dihydropyrazine moiety by two electron oxidation play important role for this electrochemical responsive behaviors. This time, we also prepared π-extended dihydrophenazine based on porphyrins. The structure was clearly elucidated by X-ray diffraction analysis, showing saddle-shape conformation. The electrochemical analysis exhibited irreversible characteristic upon one-electron oxidation and reduction as observed for anthracene derivatives. In this presentation, we will show the synthetic details and electrochemical behaviors of π-extended dihydrophenazines and related porphyrin arrays.