Density functional theory-based rapid and accurate estimation of reduction potentials of acridinium derivatives in ground and excited state

We devised a quantum chemical simulation protocol that can rapidly and accurately predict ground and excited state reduction potentials (E⁰ and E*, respectively) of acridinium photoredox catalysts (PCs). To bypass time-consuming excited state geometry optimizations, we used ground state equilibrium geometries to compute the electronic energy of excited states, which provides reasonable E⁰ and E* estimates. The contribution of Hartree-Fock exchange (HFX) in density functionals was systematically varied to estimate E⁰ and E*. In addition to reproducing experimental results, physically sensible models, such as correct descriptions of exciton behavior, are highly necessary. Based on the exciton correlation values, the appropriate amount of HFX in the B3LYP functional was determined to be 30 % to yield physically sensible bound electron-hole pairs for small organic molecules. We also investigated the impact of basis sets on the predictability of E⁰ and E*. Geometry optimizations with B3LYP-D2(HFX 20 %)/6-31G and single point energy refinement with (TD-)B3LYP-D2(HFX 30%)/6-311++G(d,p) yielded the best results. The transferability of the suggested protocol was confirmed with a test set consisting of eight acridinium derivatives. This study can provide reasonably accurate results with relatively small amounts of computational resources, and it is therefore expected to greatly contribute to the development of new acridinium PCs.