Electronic energy transfer in a single donor–acceptor pair with triplet–triplet absorption

Förster resonance energy transfer (FRET) is examined in a single donor $(D)$–acceptor $(A)$ pair whose molecules have both singlet and triplet states. Two cases are considered: (i) the excitation of the $D$–$A$ pair occurs with the participation of a singlet band of the donor, and (ii) the excitation also involves the participation of an additional triplet–triplet absorption. The triplet state of the donor is shown to have no effect on the expression for the FRET efficiency, but it generates off intervals in the fluorescence track of both the donor and the acceptor. The triplet state of the acceptor strongly affects the expression for the FRET efficiency, making the complete energy transfer impossible and generating off intervals in the acceptor fluorescence track. It is shown that, if the light can excite not only the singlet transition in the donor, but also the triplet–triplet absorption band, then a weak luminescence appears in off intervals, which is orders of magnitude weaker than that in on intervals.