Spin-spin interactions between π-electronic edge-localized spins and molecular oxygen in defective carbon nano-onions

π-Electronic edge-localized spin magnetism in defective carbon nano-onions (DCNOs) derived from detonation nanodiamonds was investigated in terms of molecular oxygen adsorption and microwave (MW) irradiation effect. At low MW power (PMW ⩽ 200 μW) electron paramagnetic resonance (EPR) spectra of air-contained DCNO samples show progressive broadening of the g = 2.0014(2) edge-spin signal on temperature decrease. Below 140 K, the edge-spin signal becomes unobservable and the only weak narrow g = 2.0022(1) signal ascribed to sp3-like defects is observed. Upon higher power (PMW > 20 mW) MW-irradiation at T < 60 K, that weak signal demonstrates unusual enhancement in peak/integral intensities and line broadening accompanied with upward deviations from the conventional saturation behavior. On abrupt reduction of PMW to low (PMW < 1 mW) levels, a new strong narrow signal with the same g = 2.0022(1) emerges. Below 60 K the intensity of the strong signal shows non-Curie-Weiss behavior. This EPR signal is attributed to new magnetic entities originating from antiferromagnetically coupled dimers of edge-spin and negatively charged O2− molecule. The O2− molecules, formed by charge transfer from π-electronic reservoir of underlaid nano-graphene sheet, appear in the vicinity of nano-graphene edges in the process of desorption/adsorption of oxygen molecules caused by strong MW-irradiation and subsequent abrupt reduction of PMW.