As mentioned before, we previously showed that the identity of the acceptor monomer has the highest correlation to a stable triplet ground-state . However, the absolute classification of monomers between acceptor and donor is vague because these are relative terms. These are usually described by their HOMO levels, as a donor monomer will have a high HOMO level, while an acceptor will have a small one. However, an absolute scale is hard to derive, as a monomer with a low HOMO level compared to its oligomer counterpart will behave as the acceptor (Figure ). A higher difference between the HOMO levels would entail a stronger donor-acceptor pair and vice versa.