How does a little difference in structure determine whether molecules have thermally activated delayed fluorescence characteristic or not?

The results indicate that the HOMO and LUMO distributions of 1 and 2 are similar and the radiative decay rate (kr) of 1 is even greater than that of 2. However, 2 has smaller singlet-triplet energy splitting (ΔEST) and larger spin-orbital coupling (SOC) strength than 1. Besides, 2 has a smaller energy gap between S1/T1 minimum energy crossing point (MECP) and S1 than 1, indicating that it is easy for 2 to realize reverse intersystem crossing (RISC) from T1 to S1. Therefore, the parameters including ΔEST, SOC and MECP between S1 and T1 are fairly important factors determining the TADF characteristic of 2.238