Photon upconversion based on sensitized triplet–triplet annihilation in bi-component systems is a multistep process that involves a triplet–triplet energy transfer (ET) from a donor to an acceptor moiety. This is aimed at sensitizing the population of annihilating optically dark triplets that generates the high energy photoluminescence. A large resonance between the involved triplets is usually recommended because it increases the energy gain between absorbed and emitted upconverted photons. However, it also enables efficient back-ET from acceptor to donor triplets, with potential detrimental consequences on the system performance. Here, we analyze a model system, where the energy difference between donor and acceptor triplets is lower than kBT at room temperature by means of time resolved and steady state photoluminescence spectroscopy, and develop a kinetic model, which describes the iterative loop that transfers the triplet exciton between the donor and acceptor molecules. In such a way, we obtained the guidelines for the optimization of the system composition required to overcome the back-ET effect and maximize the upconversion quantum yield. [ABSTRACT FROM AUTHOR]
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