Quantum chemical calculations performed with the ORCA program package provided insights into the electronic structure of 1a. Scalar relativistic complete active space self-consistent field (CASSCF) with n-electron valence second-order perturbation theory (NEVPT2) calculations were conducted on the DFT-optimized geometry of 1a (see Section 11 of the Supporting Information for computational details) with inclusion of spin-orbit coupling (SOC) via quasi-degenerate perturbation theory (QDPT). The calculated ground state is dominated to 95% by a totally symmetric closed shell singlet state 1 A1. The HOMO is dominated by the contributions from the Bi 6pz orbital (orbital 1b1 in Figure ). According to our calculations, the first electronically excited state arises from an excitation of the HOMO to the LUMO (1a2 in Figure ). This excitation gives rise to a pair of singlet and triplet states of B2 symmetry ( 1,3 B2). This state is best characterized as a charge transfer state since the HOMO is mostly localized at the Bi center while the LUMO is dominated by a 𝜋* orbital of the ligand. The energy of the singlet 1 B2 excited state is calculated to be 15022 cm -1 (1.86 eV, 666 nm) in excellent agreement with the spectral feature with λmax = 630 nm in the UV-Vis spectrum (Figure ). SOC splits the corresponding 3 B2 state into three pure triplet states almost degenerate (splitting of the Ms = 0 magnetic sublevel from Ms = ±1 by only 0.41 cm -1 ). This is likely an underestimate because the direct electron spin-spin coupling will also contribute to the excited state zero-field splitting (ZFS). However, given the charge-transfer nature of the excitation, it is likely that these contributions are also smaller than 1 cm -1 and would not change any of our conclusions. Notably, the calculated 1 B2-3 B2 energy gap is only 0.10 eV, again in close agreement with the experimental value. Such small S-T splitting is amenable to the efficient charge-transfer character of MLCT(B2) state, which maximizes the distance between the charge centers in 1b1 and 1a2 orbitals reducing the exchange interaction of the unpaired electrons. Calculations show that bismuth-centered "ligand-field" excited states occur at energy values too high (starting from 17703 cm -1 , 2.20 eV, see also Table ) to interfere with the 3 B2 state which is therefore free to persist long enough to exploit the chemical potential stored in this charge-separated state. Stern-Volmer quenching studies were conducted using fsTAS to study the rate of bimolecular quenching using 0.55 mM solutions of 1a in MeCN and increasing concentrations of either 4-iodobenzonitrile 2 or arylthianthrenium 3. In both cases, only 3 MLCT lifetime was significantly affected by the presence of the electrophiles. Evaluation of the rate constants derived from global analysis afforded bimolecular quenching constants of 5 × 10 8 and 6 × 10 9 M - 1 cm -1 for 2 and 3, respectively (Figure ). Given that 3 (Ep/2 ≈ -1.6 V vs Fc +/0 ) 61 is a better electron acceptor than 2 (Ep/2 = -2.12 V vs Fc +/0 ), and considering the calculated excited-state oxidation potential of -2.62 V vs Fc +/0 for 1a, we wondered whether the order of magnitude differences in quenching constants originates merely from the driving forces for electron transfer, which probably falls in the Marcus normal region for both substrates (ca. 0.5 V and 1 V overpotential for substrates 2 and 3, respectively). Unfortunately, no differential signals arising from the products of the bimolecular reactions were detected, indicating that products react faster than their production rate. Even though it is reasonable to posit an electron-transfer / mesolytic cleavage pathway for the activation of aryl thianthrenium species, aryl iodides can be activated by several different pathways. Spurred by these large differences in quenching constants and in the overall rate of oxidative addition, a study was conducted to gain further insight into the mechanism of oxidative addition into aryl iodides.