Nanographene-Oxy Radical Stabilized by a Carbaporphyrin Framework

Stable organic radicals are garnering increased attention due to the unusual properties of open-shell electronic systems. However, there are limitations associated with their preparation. For instance, hexa-peri-hexabenzocoronene (HBC), a representative nanographene, has armchair edges and shows good stability, reflecting the presence of Clar sextet rings. This renders HBC-based organic radicals challenging to prepare. Here, we report HBC-based organic radicals that are stabilized within a recently reported carbaporphyrin-like dipyrromethene-fused nanographene hybrid (HBCP). Using a straightforward process involving acetoxylation followed by hydrolysis, an inner-hydroxy-substituted HBCP, termed HBCP-OH, is obtained. Oxidation of HBCP-OH with MnO₂ gives the HBC-oxy radical HBCP-O. This radical is highly stable and is tolerant to air, moisture, and silica gel column chromatography. As prepared, HBCP-O displays organic π radical character, as confirmed by electron paramagnetic resonance (EPR) spectroscopy, cyclic voltammetry, femtosecond transient absorption measurements, and density functional theory (DFT) calculations. Furthermore, this HBC-oxy radical can act as a ligand to coordinate Cu(III) and Pd(II) as representative metal cations. The resulting Cu(III) (HBCP-OCu) and Pd(II) (HBCP-OPd) complexes show distinct electronic features. While the former is a closed-shell nonradical complex, the latter exists as an open-shell organic radical. The present study demonstrates a new way to access stable nanographene-based organic radicals that can be further functionalized through metal complexation.