| Summary: | We used quantum chemistry methods at the levels of mixed-reference spin-flip time-dependent density functional theory and multireference perturbation theory to study diboron- and diaza-doped anthracenes and phenanthrenes. This class of structures recently surged as potential singlet fission chromophores. We studied electronic structures of their excited states and clarified the reasons why they satisfy or fail to satisfy the energy criteria for singlet fission chromophores. Many studied structures have their S₠states not dominated by HOMO → LUMO excitation, so they cannot be described using the conventional two site model. This is attributed to frontier orbital energy shifts induced by the doping and different charge-transfer energies in different one-electron singlet excitations or, in other words, different polarizations of hole and/or particle orbitals in their S₠and T₠states. There is a mirror relation between the orbital energy shifts induced by diboron- and diaza-dopings, which together with alternant hydrocarbon pairings of occupied and unoccupied orbitals, leads to more mirror relations between the excited states of the two types of doped structures. © 2020 American Chemical Society. Received: July 28, 2020; Revised: September 7, 2020; Published: September 9, 2020. We thank York University for the start-up grant (T.Z.: 481333) and the Natural Sciences and Engineering Research Council (NSERC) of Canada (T.Z.: RGPIN-2016-06276) for financial support. We thank the Neese group at the University of Bonn for their continuous development of the ORCA program package. We thank the Gordon group at the Iowa State University for their continuous development of the GAMESS-US program package. This research was enabled in part by support provided by WestGrid (www.westgrid.ca), SciNet (www.scinethpc.ca), and Compute Canada (www.computecanada.ca). The authors declare no competing financial interest. Supplemental Material - jp0c06915_si_001.pdf
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