Dynamic Covalent Properties of a Novel Indolo[3,2-b]carbazole Diradical

Abstract

This work describes the synthesis and properties of a dicyanomethylene-substituted indolo[3,2-b]carbazole diradical ICz-CN. This quinoidal system dimerises almost completely to (ICz-CN)2, which contains two long C(sp3)−C(sp3) σ-bonds between the dicyanomethylene units. The minor open-shell ICz-CN component in the solid-state mixture was identified by EPR spectroscopy. Cyclic voltammetry and UV–visible spectroelectrochemical data, as well as comparison with reference monomer ICz-Br reveal that the nature of the one-electron oxidation of (ICz-CN)2 at ambient temperature and ICz-CN at elevated temperature is very similar in all these compounds due to the prevailing localization of their HOMO on the ICz backbone. The peculiar cathodic behaviour reflects the co-existence of (ICz-CN)2 and ICz-CN. The involvement of the dicyanomethylene groups stabilizes the close-lying LUMO and LUMO+1 of (ICz-CN)2 and especially ICz-CN compared to ICz-Br, resulting in a distinctive cathodic response at low overpotentials. Differently from neutral ICz-CN, its radical anion and dianion are remarkably stable under ambient conditions. The UV/Vis(–NIR) electronic transitions in parent (ICz-CN)2 and ICz-CN and their different redox forms have been assigned convincingly with the aid of TD-DFT calculations. The σ-bond in neutral (ICz-CN)2 is cleaved in solution and in the solid-state upon soft external stimuli (temperature, pressure), showing a strong chromism from light yellow to blue–green. Notably, in the solid state, the monomeric diradical species is predominantly formed under high hydrostatic pressure (>1 GPa).