Synthesis of Cationic Azatriphenylene Derivatives by Anodic Pyridination and Their Optoelectronic Properties
Authors: Ohno Yushi , Ando Shogo , Furusho Daisuke , Sato Kosuke , Hifumi Ryoyu , Tomita Ikuyoshi , Inagi Shinsuke 
Published: December 2024

Polycyclic aromatic hydrocarbons (PAHs) have attracted attentions due to their unique properties derived from their planar structure with wide π-conjugation. Heteroatom-doping of PAHs is a powerful method to control their optoelectronic properties and packing structures. Among heteroatom-doped PAHs, cationic nitrogen (N
+)-doped PAHs have stable electrochemical properties, fluorescent properties, and peculiar aggregation behavior that can lead their application in organic photocatalysts, fluorescence molecules, and bioimaging[1]. However, the approaches to synthesizing N
+-doped PAHs are still limited,[2] thus a facile synthesis is required to explore further molecular design.

In this study, we demonstrate the facile synthesis of cationic azatriphenylene derivatives by electrochemical pyridination[3]. This protocol is a quite simple and efficient method without transition metal catalysts.

First, the electrochemical oxidation of 1a was carried out under constant-current conditions in a H-type divided cell equipped with a carbon felt anode, and a Pt plate cathode under the reported standard conditions[4]. Under the optimized conditions, the 6-endo-trig cyclization successfully proceeded to yield product 2a in 60% isolated yield (Figure 1a). The structure of 2a was characterized by NMR, HR-MS, elemental analysis, and X-ray analysis. Furthermore, this cyclization was tolerant to various aryl substitutions such as dimethoxyphenyl, phenyl, and naphthyl groups, but 1f provided spirocyclic product 2f’ selectively via 5-exo-tet cyclization. These differences in reactivity and regioselectivity depending on the position of functional groups can be explained by DFT simulation.

In UV-vis absorption and fluorescence measurements, it was observed that the obtained cationic azatriphenylene derivatives exhibited good emission properties attributed to the donor-acceptor structure (Figure 1b). The cyclic voltammograms of obtained products showed irreversible reduction peak at lower potential. This result indicates that the reduced species of 2a is unstable to undergo side reactions such as dimerization.

In single crystal X-ray diffraction analysis, the cationic azatriphenylene derivatives formed relatively high planar structures and columnar packing patterns with cation species like charge-segregated assemblies (Figure 1c)[5]. Although counter-anions are expected to be important components to adjust layer distance between cationic π-surfaces, we could not observe remarkable differences among three anions (BF4
−, PF6
−, ClO4
−). This result is due to the rigid structures of the triphenylene unit, indicating that a more bulky counter anion is required to change the crystal structure.

In conclusion, we successfully achieved the facile synthesis of cationic azatriphenylene derivatives for the first time by intramolecular anodic pyridination. This method is very useful for introducing a nitrogen cation into PAHs to perturb the π-electron system.

-----Reference--------------------------------------------------------------------------------------------------------

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Figure 1