where 𝑘 ! is the radiative rate constant and 𝑘 "! is the non-radiative rate constant. Based on equation (3), it is necessary for the radicals to emit fluorescence when the 𝑘 "! value is small and the 𝑘 ! value is large. Therefore, we discuss 𝑘 "! in terms of the molecular structure in the ground state and in the lowest excited state, as well as 𝑘 ! in terms of the transition dipole moment.