Intramolecular Charge Transfer and Local Excitation in Organic Fluorescent Photoredox Catalysts Explained by RASCI-PDFT

We investigate the electronically excited states of two recently synthesized organic fluorescent photoredox catalysts of the dihydrophenazine family. The mixed charge transfer and local excitation behavior of dark and bright transitions is unveiled by multiconfiguration pair-density functional theory (MC-PDFT) based on a restricted active space configuration interaction (RASCI) wave function (RASCI-PDFT). The RASCI-PDFT calculations give an accurate description of the experimental optical absorption spectra with active spaces too large for conventional complete active space self-consistent-field calculations. These results were achieved by the inclusion of many valence orbitals in the active space and their optimization within a cost-effective restricted active space self-consistent field framework without a RAS2 subspace, followed by calculations at the RASCI level including orbitals in RAS2. This novel strategy can be extended to systems that need a large number of orbitals in the active space.