As expected, we found that the various electron donating and withdrawing substituents have a significant effect on the ∆EHL energy difference between the singlet and quintet minima (c.f. Table ). Strong electron donating substituents can substantially decrease the energy of the quintet state, whereas electron withdrawing groups destabilize the quintet state. Substitution in the 4' position by EW/ED groups thus provides an opportunity to tune the ∆EHL energy gap.