in the Supporting Information). By contrast, kf is twice as large in PyFlCz than in PyPhCz, and this correlates with the higher oscillator strength determined for PyFlCz compared to PyPhCz (cf. Tables and). We hypothesize that the second component τ2 originates from triplet-to-singlet conversion producing a delayed contribution to fluorescence. When considering the available electronic states in PyFlCz and in PyPhCz as determined by TD-DFT, we observe that the energy gap between S1 and T1 state is more than 1 eV. Also, the energy gap between T1 and the next excited triplet state T2 are large, which can prevent vibrational coupling and at the same time, this leads to better energetic alignment between the S1 and higher excited triplet states. Considering that the molecular orbitals of S1 and T1 are typically very similar, one can conclude that the molecular orbitals of the hot triplet states (> T1) will be more dissimilar to the S1 orbital. According to the El-Sayed rule, the rate for (reverse) intersystem crossing from these hot triplet states to the S1 can be very large. Such triplet-to-singlet conversion is termed a hot exciton mechanism. To ensure a high RISC rate, the energy difference between the S1 and the corresponding hot triplet states (ΔEST) needs to be small. states in PyPhCz that seem directly aligned with the S1 (T6 and T7). This energy landscape around the S1 state can explain the much-extended delayed component τ2