The decay profiles of the q-eq. band in toluene shown in Figures and clearly shows a double exponential decay with lifetimes of 16 ± 0.1ns and 544 ± 12 ns, respectively. Because the band shape is independent of the delay time as shown in Figure , the shorter and longer lifetime decays are assigned to the lifetimes of prompt fluorescence and TADF, respectively. In THF, the lifetime of prompt fluorescence becomes much shorter, 0.97 ± 0.01 ns and no TADF is observed. This result indicates that the nonradiative decay from S1 to S0 is accelerated in a higher permittivity solvent in qeq. This is presumably because the solvation by a higher permittivity solvent heavily stabilized the CT excited S1 state and the smaller energy gap between S1 and S0 (Figure ) causes efficient nonradiative decay, which is known as the energy gap law . This nonradiative acceleration mechanism also explains the experimental fact that the intensity of steady-state emission decreases with increasing the permittivity of solvent as shown in Figure .