The molecular structures of the DAAN derivatives in the optimized D0 state and the optimized D1 state were obtained from TD-DFT calculations in order to clarify the characteristics of the radiative and non-radiative processes. Specifically, after the structural optimization, the lengths of all the carbon-carbon bonds in the diphenylmethane moiety and the dihedral angle between the two aromatic rings of each DAAN derivative were calculated (Figure ). In the cases of the non-symmetric DAAN derivatives R 1 /R 2 , the carbon atoms were numbered with the ipso-position of R 1 as C1. The results indicate that the bond lengths between the central carbon atom and the aryl moiety (C6-C7-C8) and between the carbon atoms at the orthoand meta-positions (C2-C4, C3-C5, C9-C11, C10-C12) of the optimized D1 state are shorter than those in the optimized D0 state, except in the case of NO2/OMe (Table ). These results thus also indicate an increased quinoid character for the aryl groups in the optimized D1 state, except in the case of NO2/OMe. Moreover, the dihedral angle between the aromatic rings decreases from the optimized D0 state to the optimized D1 state (except in the case of NO2/OMe), confirming the improved planarity of the molecules (Table ). The decrease in the dihedral angle should promote stronger p-conjugation between the two aromatic rings (Figure ), and the enhanced p-conjugation should suppress the thermal fluctuation and vibration of molecules, which would lead to non-radiative decay. These findings provide some evidence that the 𝑘 "! of is significantly higher than that of the other DAAN derivatives. The transition dipole moment between the optimized D1 and D0 states (𝜇 #$ ) is related to 𝑘 ! , and molecules with a large 𝜇 #$ value show high luminescence efficiency. The 𝜇 #$ value was calculated based on the results of the TD-DFT calculations, and a positive correlation was observed between the experimental fluorescence quantum yields and 𝜇 #$ (Table , Figure ). These results suggest that it is possible to use TD-DFT calculations in order to estimate whether a radical species can exhibit fluorescence. In addition, NO2/OMe showed extremely low values compared to the other DAAN derivatives (Table ). This result suggests that NO2/OMe is a non-luminescent radical.