Dithieno[3,4-b:3',4'-d]arsole: A Novel Class of Hetero[5]radialenes

[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.