Fig. 1: Schematic depiction of the exothermic Förster transfer mechanism studied in this work.

Transfer to emitters with lower emission energy is enhanced by the facts that: (a) the lowest-energy states for emission and absorption, which are predominantly of triplet character (T_I−III states), are in resonance due to the Stokes shifts of the donor and acceptor molecules, and (b) the excitation can be transferred to energetically higher-lying excited acceptor states. When these states have strong singlet character (S_n) the transfer can be strongly enhanced.