TRZ-DI (∆EST: 0.023 eV), designed by Kwon et al., incorporates a D3-symmetric triazatruxene donor. Compared with Cz in Cz-TRZ, the extended conjugation within triazatruxene effectively localizes the HOMO only on the donor group, leading to a very small ∆EST of 0.02 eV. A small dihedral angle (24.5 o ) between the plane of triazatruxene and the TRZ acceptor, along with the rigid molecular structure, contribute to a FPL of 87% (25 wt% in TCTA: Bepp2 exciplex host). A green OLED using TRZ-DI [λEL: 526 nm; CIE = (0.31, 0.57)]