Efficient non-radiative FRET from the triplet state of the B-fiber donor to the singlet states of the RhB acceptor (TS-FRET) was confirmed by the decay profiles thereby eliminating the possibility of simple energy transfer (emission-reabsorption process), since for this situation the donor lifetime was not expected to change. On the other hand, no afterglow emission in Bfiber/RhB was observed upon excitation of the RhB acceptor at 500 nm (Supplementary Fig. ). Taken the results together, it is evident that the long-lived triplet excitons of the donor Bfiber were the only source of the delayed population of the singlet state of the acceptor RhB via an efficient TS-FRET mechanism, resulting in persistent delayed fluorescence.