Frequently, the limited choice of these groups leads to compounds that show too delocalized frontier molecular orbitals (FMO), leading to larger ΔEST and inefficient RISC. Meanwhile, the presence of higher-energy triplet states has been suggested to assist in the harvesting of triplet exciton in the device. One possible mechanism requires that the higher-energy triplet states Tn (n > 1) lie essentially degenerate with S1 yet be much higher in energy than T1, which renders RISC competitive with IC. The nature of the excited states involved in this mechanism usually show mixed locally-excited (LE) and charge transfer (CT) character (so called HLCT) that consequently leads to enhanced spin-orbit coupling between S1 and these higher-lying triplet states. The nature of the HLCT state provides a balance between fast radiative transition rates manifested in high ΦPL and sufficiently small energy gaps between S1 and Tn (ΔES1Tn), facilitating RISC. Compounds emitting via either a traditional TADF mechanism, involving RISC between T1 and S1 mediated by a small ΔEST, or emitting via a mechanism involving RISC from a Tn state to S1 (n>1) have been widely explored as emitters for OLEDs. For TADF emitters based on a twisted D-A structure, the S1 and T1 states are usually of CT character, and according to El Sayed's rules, direct RISC from T1 to S1 of states with the same orbital symmetry is not allowed, which is reflected in the very low spin-orbit coupling (SOC) between these two states. Previous research results have shown that intermediate triplet states between S1 and T1 can assist the RISC process, and as the orbital type of Tn usually differs from that of the S1 state, strong SOC exists. Thus, the RISC process is facilitated by spin-vibronic coupling between T1 and Tn and enhanced SOC between Tn and S1 when the excited states involved in RISC are energetically closely aligned. Our previous work on the pyrazine-based TADF emitter DTCz-Pz also showed that the presence of an intermediate T2 state of HLCT character that is of different orbital type to S1 (CT character) can provide an indirect route for RISC to occur despite the relatively large ΔEST of 0.27 eV in 2,8bis(diphenyl-phosphoryl)-dibenzo[b,d]thiophene (PPT). 12 Makhseed et al. recently reported a TADF emitter 4CzPyz where four carbazoles were connected to a pyrazine core, and the multiple carbazoles can present slightly different electronic coupling with the central acceptor, thus providing a dense population of excited states. 24 4CzPyz exhibited narrowed ΔEST of 0.23 eV and shorter delayed lifetime (τd) of 144 µs in 10 wt% bis [2-(diphenylphosphino)phenyl]ether oxide (DPEPO) film compared to corresponding values of 0.27 eV and 5.5 ms for DTCz-Pz in 7% PPT film. 12,24 However, the electroluminescence behavior of 4CzPyz was not explored and the increased molecular conjugation caused significantly red-shifted emission as 4CzPyz showed sky-blue emission with λPL of 502 nm, while for DTCz-Pz which only contains two tert-butylcarbazoles were connected to pyrazine, the emission is deep blue with λPL of 460 nm. 12,24