Fig. 1: One example for the biradical systems and the spin coupling mechanism are shown.

(a) TYY radical, (b) DPA spin coupler, and (c) combined double-radical triplet system. In (d), we have shown the coherent and incoherent processes. The coherent processes include spin‒spin interactions among radicals and triplet and optical driving fields (V), while the incoherent processes include intersystem crossing and decays of the triplet and singlet excited states. The decay rates kST, kTG, and kEG are also illustrated.