Diazadioxa[8]circulene – a platform for stable antiaromatic radicals with strong NIR absorption

Abstract

Stable organic radicals exhibiting absorption capabilities in the visible and near-infrared regions (NIR) are appealing candidates for a wide range of applications, including photovoltaics, photothermal applications, anticounterfeiting and information encryption/decryption. Hetero[8]circulenes represent a promising π-extended platform for stable delocalized radicals with rich possibilities for chemical tuning. In this paper, we present the stable cation radical of diazadioxa[8]circulene with reduced symmetry showing a sharp and strong peak (ε ∼ 2 × 10⁴ M⁻¹ cm⁻¹) in the NIR at 939 nm. The diazadioxa[8]circulene cation radical can be obtained via chemical doping using Magic Blue, or via electrochemical doping. It remains stable for several days without any significant change in the absorption spectra. Furthermore, according to magnetic criteria of aromaticity, it exhibits antiaromatic behaviour. Advanced theoretical calculations predict NIR emission for the radical cation through an anti-Kasha D₄ → D₀ transition, which is experimentally not detectable due to non-radiative deactivation pathways. Our RI-CC2 theoretical calculations also corroborate the blue emission of neutral circulene from the bright lowest singlet excited state S₁, which was previously unexplained due to TD-DFT artefact predicting the dark S₁ state. These results open the opportunities for the practical application of diazadioxa[8]circulene cation radicals in photovoltaics, sensors and photothermal imaging.