In addition, the b-HOMO to b-SOMO and the a-SOMO to a-LUMO transitions could not be confirmed for NO2/OMe (Tables S2 and S3), which suggests that the electronic transition between the optimized D1 state and the D0 state is a forbidden transition for NO2/OMe. To clarify the difference between fluorescent and non-fluorescent DAAN radicals, we further analyzed the results of the DFT and TD-DFT calculations. In general, the luminescence quantum yield of radical species (𝜙) is expressed using the rate constant: 33 a R² = 0.958