Triplet energy migration pathways from PbS quantum dots to surface-anchored polyacenes controlled by charge transfer

Sensitization of molecular triplets using PbS quantum dots (QDs), followed by efficient triplet fusion, has been developed as a novel route to near-infrared-to-visible photon upconversion. Fundamentally, however, the mechanisms of triplet energy transfer (TET) from PbS QDs to surface-anchored polyacence acceptors remain highly debated. Here we study and side-by-side compare the kinetic pathways of TET from photoexcited PbS QDs to surface-anchored tetracene and pentacene derivatives using broad-band transient absorption spectroscopy spanning multiple decades of timescales. We find that the TET pathways are dictated by charge-transfer energetics at the QD/molecule interface. Charge transfer from QDs to tetracene was strongly endothermic, and hence spectroscopy showed one-step transformation from QD excited states to tetracene triplets in 302 ns. In contrast, hole transfer from QDs to pentacene was thermodynamically favoured and was confirmed by the formation of pentacene cation radicals in 13 ps, which subsequently evolved into pentacene triplets through a 101 ns electron transfer process. These results not only are consistent with a recently-established framework of charge-transfer-mediated TET, but also provide a route to manipulate triplet sensitization using lead-salt QDs for efficient upconversion of near-infrared photons.