When X has a dark S1, all dimer's oscillator strength is higher than monomer X but still lower than that of C22. This is because the involvement of C22's bright LE state increases the dimer's oscillator strength compared to X, but the participation of CT states limits the dimer's oscillator strength below that of C22. The dark-low subgroup exhibits the smallest oscillator strength as the dimer exciton localized on the dark LE state of monomer X, while that of the dark-high group is very close to C22 because the dimer S1 is dominated by the bright LE state of C22.