Dithieno[3,4‐<i>b</i>:3',4'‐<i>d</i>]arsole: A Novel Class of Hetero[5]radialenes

Abstract

[5]Radialene is known as an unstable cyclic hydrocarbon with a cross‐conjugated system. Incorporation of heteroatoms into [5]radialene skeleton is an effective strategy for stabilization. Herein we synthesized dithieno[3,4‐ b :3',4'‐ d ]arsole ( 1 ) as a novel class of hetero[5]radialenes since trivalent arsenic atom is much more stable than phosphorus one, which was used for hetero[5]radialene but unstable in air. The characteristic nature of the arsa[5]radialene was experimentally and computationally studied by comparing with the isomer, dithieno[3,2‐ b :2',3'‐ d ]arsole ( 2 ). Structural analysis by X‐ray crystallography and computational evaluation of aromaticity revealed the radialene character of 1 . Interestingly, 1 showed phosphorescence though only fluorescence was observed for 2 . Time‐dependent density functional theory (TD‐DFT) calculations implied that intersystem crossing could readily occur upon excitation for 1 . Furthermore, it was computationally elucidated that dimerization and/or oligomerization via Diels‐Alder reaction, which convert [5]radialene, were circumvented to offer stability for 1 .