Atomic theory says singlet and triplet energies for the same orbital differ because of exchange interaction. However, molecular orbital (MO) theory describes the energy between the HOMO (highest occupied MO) and LUMO (lowest unoccupied MO) as the same regardless of whether triplets or singlets are being considered. Why is this the case?

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    $\begingroup$ Where did you get that "MO theory describes the energy between HOMO and LUMO as the same regardless of whether triplets or singlets are being considered"? $\endgroup$
    – Andrew
    Oct 15, 2021 at 16:30
  • $\begingroup$ when reading papers on TADF molecules, where spin state is relevant, only a set of HOMO and LUMO are usually presented. I get how HOMO is the same but not why they only show one LUMO $\endgroup$
    – siete
    Oct 18, 2021 at 11:18
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    $\begingroup$ I think maybe you're confusing MO diagrams with MO theory. The theory predicts different energies that are often not captured in the simplistic diagrams. They have to be simplistic because otherwise you'd have to redraw them every time you changed the electron configuration. That's why we typically use a diagram of system states rather than MOs for something like TADF. For example upload.wikimedia.org/wikipedia/commons/thumb/9/9e/… Note that the lines here are states of the system (S or T) not individual MOs $\endgroup$
    – Andrew
    Oct 18, 2021 at 11:58
  • $\begingroup$ That makes sense. In that case, when papers depict the simplistic LUMO and HOMO distributions of particular molecules, what transition is that meant to represent? $\endgroup$
    – siete
    Oct 22, 2021 at 10:03
  • $\begingroup$ It would be helpful if you showed a specific diagram that is confusing you $\endgroup$
    – Andrew
    Oct 22, 2021 at 12:34


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