We investigate1 the electronically excited states of two recently synthesized organic fluores- cent photoredox catalysts of the dihydrophenazine family.2,3 The mixed charge transfer and local excitation behaviour of dark and bright transitions is unveiled by multi-configuration pair- density functional theory (MC-PDFT)4 based on a restricted active space configuration interac- tion (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 con- ventional complete active space SCF (CASSCF) calculations. These results were achieved by the inclusion of many valence orbitals in the active space and their optimisation within a cost- effective RASSCF 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.
(1) Presti, D., Truhlar, D. G. and Gagliardi, L., Intramolecular Charge Transfer and Local Excitation in Organic Fluorescent Photoredox Catalysts Explained by RASCI-PDFT. J. Phys. Chem. C (2018).
(2) Theriot, J.C., Lim,C.-H., Yang,H., Ryan,M.D., Musgrave,C.B. and Miyake,G.M. Organo-catalyzed atom transfer radical polymerization driven by visible light. Science 352, 1082-1086 (2016).
(3) Lim, C.-H., Ryan, M.D., McCarthy, B.G., Theriot, J.C.,Sartor, S.M.,Damrauer,N.H., Musgrave, C.B. and Miyake, G. M. Intramolecular Charge Transfer and Ion Pairing in N,N-Diaryl Dihydrophenazine Photoredox Catalysts for Efficient Organocatalyzed Atom Transfer Radical Polymerization. J. Am. Chem. Soc. 139, 348-355 (2017).
(4) Li Manni, G., Carlson, R. K., Luo, S., Ma, D., Olsen, J., Truhlar, D. G. and Gagliardi, L. Multicon- figuration Pair-Density Functional Theory. J. Chem. Theory Comput. 10, 3669-3680 (2014).