According to the Arrhenius plot (Figure ), the activation barrier for intra-TTA-UC in mPhTc2 is around 0.011 ± 0.003 eV. Turning to mAdTc2, the lifetime of (S1S0)rel increases from 0.7 ns at 300 K to 3.0 ns at 80 K. Temperature dependence is not noted for the lifetime of (S1S0)(T1T1) CT , while that of (T1T1) becomes longer as the temperature is reduced. As a matter of fact, intra-TTA-UC from (T1T1) is shut down completely below 160 K, as a biexponential fitting turned out to be sufficient for the TCSPC data. For mAdTc2, the activation barrier by means of Arrhenius analysis is around 0.03 eV. Higher fluorescence quantum yields of mAdTc2 at lower temperatures relate to a radiative decay even before (T1T1) is formed.