Room Temperature Phosphorescence Once the T 1 states get populated, a possible relaxation mechanism is phosphorescence in both systems. Similar as fluorescence, the ultralong phosphorescence in IPA is more efficient compared to the TPA, with the computed rates of 9.92 s -1 and 0.5 s -1 , respectively. In IPA, according to our results, phosphorescence occurs at 2.91 eV with a lifetime of ∼100 ms. The experimental phosphorescence energies and lifetimes are in relatively good agreement with our results. RTP is observed at 506 nm (2.45 eV) with a lifetime of 290 ms. Our computations predict an ultralong phosphorescence at 2.53 eV for TPA, in good agreement with the experimental weak phosphorescence signal at 2.43 eV (511 nm). Reverse Intersystem Crossing To examine the deactivation pathways from the triplets populated from the singlet states through intersystem crossing, we optimised the geometries of the T 1 and T 2 states in both systems, and the T 3 state in TPA, and computed the reverse ISC rates from the minima of the triplet states. Because these systems experimentally exhibit DF, there must be mechanisms that efficiently populate S 1 from the triplet manifold.