Phosphorescence spectral shift of singlet oxygen, O-2((1) Delta(g)) was studied in the presence of a nitroxide radical quencher. The O-2-nitroxide complex causes a blue-shifts in both molecular solvents and in ionic liquids. The amount of shift is found to be nearly the same for both solvents, about 50 cm(-1). This shift was attributed to quartet-doublet energy separation of O-2((3)Sigma(g))-nitroxide pair by exchange interaction of O-2 and nitroxide radical. (C) 2018 Elsevier B.V. All rights reserved.