MECP STATE1 for excited singlet state (S1 or S2) to the first triplet state (T1)

I have a query about the calculation of the MECP from an excited singlet state (S1 or S2) to the first triplet state (T1). If we assume that a molecule with a singlet ground state has multiple calculated excited states, and that the oscillator strength is highest in the second excited state (S2) and lowest in the first (S1).
Should I use the strongest oscillator to calculate the MECP from S2 to T1 or the lowest excited state from S1 to T1? In other words, is the MECP STATE1 determined by oscillator strength or the excited state energy?

I don’t know that this is a question that can be answered in general as it depends on the details of the molecule and the experiment (i.e., there are known exceptions to what I am about to say). With those caveats, the typical case is covered by Kasha’s rule, which states that emission occurs from the lowest excited state of a given spin symmetry, and the underlying physics behind that rule is that internal conversion is fast relative to emission or intersystem crossing. On that basis, one might imagine that the excitation from S0 primarily populates the state with the largest oscillator strength (S2 in your case), but that S2 → S1 internal conversion is fast so that intersystem crossing happens from S1 → T1.

Dear John,

Your thoughtful reply is much appreciated.

I looked for a relevant article to your response and came upon the one below.

According to Fig. 3 of this article, I was wondering if it makes sense for MECP S2/S1 to have less energy (2.83 eV) than the optimized minimum S2 state (3.40 eV).

If this is a correct trend, why should the optimization be stopped in S2 and not continue until MECP S2/S1 (more stable and without energy barrier)?

Do you consider this to be logical, or do you think the authors, or the Q-CHEM program are at fault?

Thank you

There’s nothing inherently wrong with having a S2 local minimum at 3.40 eV and then a S2/S1 MECP at somewhat lower energy. Presumably there is something of a barrier in between those points, because the minimum needs to have positive curvature in all directions. You could consider contacting Shirin Faraji, who is a corresponding author on that paper and also a Q-Chem developer. I presume it was her group that did the calculations in this paper.