The absorption and emission spectra of the three compounds are shown in Figure . The absorption spectra of MeTPA-BQ and tBuTPA-BQ consist of bands peaking at 302 and 445 nm, along with a broad shoulder at around 535 nm. Not surprisingly, the molar absorptivity (e) values for these bands are very similar for the two compounds given their similar structures. A similar absorption spectral profile was noted for TPPA-BQ; however, the high-energy band at 312 nm is slightly red-shifted while the low energy band and associated shoulder at 405 and 460 nm are blue- shifted compared to the former two compounds. These low-energy bands in all three compounds are due to the charge transfer transitions from the donor fragments to the benzo[g]quinoxaline-5,10-dione acceptor moiety, while the short wavelength bands can be ascribed to the donorcentered absorption bands assigned based on comparison with the literature. The red-shifted CT bands in MeTPA-BQ and tBuTPA-BQ are due to a shorter phenylene spacer that facilitates increased conjugation as illustrated by the DFT-predicted HOMO and LUMO overlap (Figure ). The steady-state PL and phosphorescence spectra were measured in 2-MeTHF glass at 77 K and S1 and T1 energies were determined from their respective onsets (Figure ). The shape of the steady-state and phosphorescence spectra are broad and structureless, illustrating that both states at cryogenic temperatures are CT in nature. The S1 and T1 energies are 2.16 eV and 2.15 eV for MeTPA-BQ, 2.15 eV and 2.14 eV for tBuTPA-BQ, and 2.32 eV and 2.22 eV for TPPA-BQ. The corresponding ΔEST are 0.01 eV, 0.01 eV, and 0.10 eV. The emissions of MeTPA-BQ, tBuTPA-BQ, and TPPA-BQ in degassed toluene solution decay mono-exponentially, with τPL of 5.4, 5.8 and 3.9 ns, there is no delayed emission observed (Figure ). Next, we investigated the photophysical properties in 4,4′-bis(N-carbazolyl)-1,1′-biphenyl (CBP) host as this host matrix has a sufficiently high triplet energy (T1 = 2.6 eV) to prevent the backward energy transfer from the T1 states of the dopant emitters. An optimized doing concentration of 2 wt% was identified based on an assessment of the ΦPL as a function of dopant concentration from 2-10 wt% in CBP (Table ). The MeTPA-BQ, tBuTPA-BQ, and TPPA-BQ emit at λPL of 670, 650, and 625 nm in 2 wt% doped films in CBP, emission that is blue-shifted compared to their respective spectra in toluene solution at λPL of 680, 680 and 645 nm (Figure ). The corresponding film ΦPL are 32, 30 and 28% under air and these values increased to 42, 41, and 39% under N2. Compounds MeTPA-BQ, tBuTPA-BQ, and TPPA-BQ all showed increasing delayed emission with increasing temperature, indicative that these compounds are TADF emitters (Figure ). The emissions of MeTPA-BQ, tBuTPA-BQ, and TPPA-BQ decay with multiexponential kinetics, with average prompt lifetimes (τavg, p) of 8.0, 9.9 and 12.2 ns, and average delayed lifetimes (τavg,d) of 40.1, 144, and 42.8 µs, in 2 wt% doped films in CBP, respectively (Figures , Table ). Using these average lifetimes, the kISC and kRISC were calculated 23 to be 7.1 × 10 7 and 5.8 × 10 4 s -1 for MeTPA-BQ, 5.8 × 10 7 and 1.6 × 10 4 s -1 for tBuTPA-BQ, and 4.8 × 10 7 and 5.6 × 10 4 s -1 for TPPA-BQ, respectively, indicating that all three emitters have nearly the same RISC rate constants. These kRISC values are relatively slower compared to the reported carbonyl-containing quinoxaline acceptor-based TADF emitter (2,3-bis(4-(10H-phenoxazin-10-yl)phenyl)quinoxalin-6yl)(phenyl)methanone, DPXZ-PQM (kRISC = 2.05 ×10 5 s -1 ). This is due to its higher ΦPL of 88% and shorter τd of 3.8 µs due to the smaller ∆EST of 0.02 eV in 5 wt% doped in DCzDPy (5,5ʹbis(carbazol-9-yl)-3,3ʹ-bipyridine) films.