Diazuleno‐s‐indacene Diradicaloids: Syntheses, Properties, and Local (anti)Aromaticity Shift from Neutral to Dicationic State

Abstract

AbstractNon‐alternant, non‐benzenoid π‐conjugated polycyclic hydrocarbons (PHs) are expected to exhibit very different electronic properties from all‐benzenoid PHs. Reported herein are the syntheses and physical properties of four derivatives of two azulene‐fused s‐indacene isomers, the diazuleno[2,1‐a:2′,1′‐g]‐s‐indacene (DAI‐1) and diazuleno[2,1‐a:1′,2′‐h]‐s‐indacene (DAI‐2). The backbone of both isomers contains 28π electrons and is a 7‐5‐5‐6‐5‐5‐7 fused ring system. X‐ray crystallographic analysis, NMR spectra, and theoretical calculations (ACID, NICS) reveal a structure with two aromatic azulene units fused with a central anti‐aromatic s‐indacene moiety. All compounds exhibit open‐shell diradical character and are magnetically active, but the derivatives of DAI‐2 show larger radical character than the respective ones of DAI‐1. Their dications were isolated in crystalline form and all experimental and theoretical analyses disclose a shift of local (anti)aromaticity along the backbone, with two aromatic tropylium rings at the termini.