The mechanisms of endothermic triplet energy transfer in photochemical systems

Short-range triplet energy transfer (TET) is a photophysical phenomenon that is central to important photochemical and photophysical processes. For instance, Dexter-type energy transfer facilitates the extraction of dark triplet excitons in electroluminescent systems and sensitizes metastable triplet states that drive photochemical reactions or undergo triplet–triplet annihilation to achieve upconversion. The rate of TET is expected to be vanishingly small when the energy gap between the donor and the acceptor is several times larger than the magnitude of thermal energy. However, recent studies have shown that TET in such “endothermic” cases can be surprisingly efficient. In this review, we present a detailed account of experimental and theoretical work that sheds light on the mechanisms that influence the rate of “uphill” TET. We show that well-understood factors that are often not considered, such as molecular flexibility and low-frequency vibrations, may play a major role in facilitating endothermic TET. We further provide insights regarding how the prevalence of endothermic TET in certain systems can be understood from a kinetic, rather than a thermodynamic, perspective. This review will be relevant for scientists who seek to exploit endothermic TET to design and use novel donor–acceptor systems.