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.