Moreover, a strong linear correlation emerged when the vibrational frequency of the maximum fluorescence wavelength, obtained from the solid-state fluorescence spectra (Figure ), was plotted as a function of the vertical transition energy from the optimized D1 state to the D0 state, which was obtained from time-dependent (TD) DFT calculations (Figure ). To investigate the dependence of the obtained results on the functional, we performed the same calculations using different functionals (UwB97X-D/6-311+G(d,p) and UCAM-B3LYP/6-311+G(d,p)). Although slight systematic errors were observed, as in the case of the UM06-2X/6-311+G(d,p) level, a strong linear dependence was obtained (Figure ). These results thus indicate that the fluorescence wavelength of the DAAN radicals can be estimated theoretically with high accuracy, and that it can be expected to be possible to design radicals with a target fluorescence wavelength using TD-DFT calculations.