To gain greater insight into the nature of the excited states, the natural transition orbitals (NTOs) were analyzed at the ground-state optimized geometries (Figure ). Both the T1 hole and particle are localized on the donor for AZB-Ph-TRZ, indicating localized excitonic (LE) character for this state. For the T2 state, the hole and particle are situated on the AZB and triazine-phenylene bridge, respectively, clearly indicating CT character. For AZB-TRZ the orbital nature of T1 and T2 is reversed. Both molecules possess S1 states with the expected CT character that is emblematic of TADF emitters. Spin-orbit coupling (SOC) values were also calculated based on the optimized T1 geometries. Due to the difference in orbital type between the triplet states and S1, the SOC is considerably larger for T1 (with <S1|ĤSOC|T1> = 0.72 cm -1 ) than for T2 (with <S1|ĤSOC|T2> = 0.01 cm -1 ) in AZB-Ph-TRZ, reflecting a strong adherence to El-Sayed's rule for direct RISC between T1 and S1. On the other hand, AZB-TRZ behaves in a complementary manner, with <S1|ĤSOC|T2> = 0.27 cm -1 , which is larger compared to <S1|ĤSOC|T1> = 0.0 cm -1 . For AZB-Ph-TRZ, T1 and T2 lie just below the S1 state, with the energy differences between S1-T1, S1-T2 and T2-T1 being 0.10, 0.01 and 0.09 eV, respectively. These energy gaps are sufficiently small to support efficient RISC through the spin-vibronic coupling between T2 and T1 states to the S1 state. In the case of AZB-TRZ, T2 is slightly higher in energy than S1 and the energy differences between S1-T1, S1-T2 and T2-T1 are 0.02, -0.09 and 0.11 eV, respectively. A similar spin-vibronic mechanism is therefore also expected in this compound to support efficient RISC. Cyclic (CV) and differential pulse (DPV) voltammetry in dichloromethane were conducted to ascertain experimentally the HOMO and LUMO levels (Figure ). At a scan rate of 100 mV s -1 , both compounds showed reversible reduction waves while the oxidation waves were found to be irreversible. The oxidation potentials, obtained from the maxima of the DPVs are 1.46 V and 1.51 V while the reduction potentials are at -1.65 V and -1.55 V for AZB-Ph-TRZ and AZB-TRZ, respectively. The HOMO levels were thus estimated to be -5.79 eV and -5.85 eV and LUMO levels were 2.69 eV and 2.79 eV, respectively, for AZB-Ph-TRZ and AZB-TRZ. The estimated redox gaps of 3.10 eV and 3.06 eV reflect the predicted trends in the calculated HOMO-LUMO gaps of 3.68 and 3.59 eV for AZB-Ph-TRZ and AZB-TRZ, respectively. The reported oxidation potentials of Cz-Ph-TRZ (also known as Cz-TRZ in the literature), DMAC-TRZ, and azasiline-Ph-TRZ (also known as DTPDDA in the literature) are 1.33 V, 0.98 V and 1.14 V, respectively, which are all significantly cathodically shifted with respect to the two AZB-containing compounds and clearly evidences the weak nature of the azaborine donor and its poor conjugation with the rest of the compound. The reduction potential of Cz-Ph-TRZ and DMAC-TRZ at -1.55 V and -1. azasiline-Ph-TRZ 34,e 1.14 -1.63 -5.58 -2.72 2.86 a Obtained from DPV peaks and referenced with respect to SCE (Fc/Fc + = 0.46 V for DCM). b EHOMO/LUMO = -(E ox/red (vs Fc/Fc + ) + 4.8) eV. c ∆EHL = |EHOMO-ELUMO|. 39 d obtained from the CV peaks maxima and referenced with respect to SCE (Fc/Fc + = 0.46 V for DCM, used for oxidation scan; and 0.45 V in DMF, used for the reduction scan), Cz-Ph-TRZ is known as Cz-TRZ in the literature e Onset potentials referenced with respect to SCE (Fc/Fc + = 0.36 V for CHCl3), azasiline-Ph-TRZ is known as DTPDDA. Progressing to an evaluation of the optical properties of AZB-Ph-TRZ and AZB-TRZ, the absorption and emission spectra in toluene are shown in Figure . Absorption maxima at 389 nm (ε = 22 × 10 3 M -1 cm -1 ) and 382 nm (ε = 16 × 10 3 M -1 cm -1 ) for AZB-Ph-TRZ and AZB-TRZ, respectively, are assigned to bands associated with !→!* transitions on the AZB fragment, as they match the absorption spectrum of the AZB donor (Figure ). CT absorption bands are practically absent due to the strong electronic decoupling of donor and acceptor moieties in the ground state, although there is a noticeable low-energy foot in the absorption spectrum of AZB-TRZ (Figure ), which we suggest arises from the relatively larger electronic coupling in this compound due to the reduced distance between the donor and acceptor moieties, a picture that aligns with the DFT study. In toluene AZB-Ph-TRZ shows a lPL at 400 nm along with a weak shoulder band around 417 nm, indicating emission from an LE state that matches that of the donor (Figure ). AZB-TRZ instead exhibits a broad and comparatively red-shifted CT emission at lPL of 494 nm. In toluene, Cz-Ph-TRZ, DMAC-TRZ, and azasiline-Ph-TRZ emit at 422 nm, 498 nm, and ~460 nm, respectively, all of which are red-shifted compared to the lPL of AZB-Ph-TRZ. The emission spectra of AZB-Ph-TRZ and AZB-TRZ both show the typical bathochromic shift with increasing solvent polarity (positive solvatochromism), indicating that the S1 state is CT in nature in sufficiently polar media (Figure ). Comparing the emission onset wavelengths of the structurally analogous series in more polar THF solvent (which elicits only CT emission), we identify that Cz-Ph-TRZ has the shortest wavelength (highest energy) onset at 400 nm, followed by AZB-Ph-TRZ (425 nm) and then azasilane-Ph-TRZ (450 nm). This comparison of emission onsets largely reveals the order of donor strengths, although the distinct donor steric environment and consequently lower CT character of Cz-Ph-TRZ makes it less sensitive to host polarity, and blue-shifts its emission more than might be expected based on the strength of the donor from calculations alone (Figure ). Similar comparisons of emission onsets between azasilane-Ph-TRZ and DMAC-TRZ place the donor strength of DMAC as yet stronger than azasilane. The FPL in degassed toluene solution of AZB-Ph-TRZ and AZB-TRZ are 48% and 13%, respectively, which decreased to 40% and 6% upon exposure to oxygen. The PL of AZB-TRZ toluene solution decays with biexponential kinetics, with a prompt lifetime, tp, of 28 ns (60%) and a delayed lifetime, td, of 117 ns (40%). Upon exposure to oxygen, the tp shortened to 13 ns (62%) while the td decreased to 52 ns (38%). In contrast to AZB-TRZ, the decay profile of AZB-Ph-TRZ exhibited monoexponential kinetics with a lifetime of 4 ns in degassed toluene solution, which did not measurably change when exposed to oxygen at 3.5 ns (Figure ).