Quantifying the Ligand-Induced Triplet Energy Transfer Barrier in aQuantum Dot-Based Upconversion System br

During photon upconversion, quantum dots (QDs) transfer energy to moleculesin solution through a long ligand shell. This insulating ligand shell imparts colloidal stability atthe expense of efficient photosensitization. For thefirst time, we quantify the barrier thesealiphatic ligands pose for triplet energy transfer in solution. Using transient absorptionspectroscopy, we experimentally measure a small damping coefficient of 0.027 A-1for a ligandexceeding 10 carbons in length. The dynamic nature of ligands in solution lowers the barrier tocharge or energy transfer compared to organic thinfilms. In addition, we show that surfaceligands shorter than 8 carbons in length allow direct energy transfer from the QD, bypassing theneed for a transmitter ligand to mediate energy transfer, leading to a 6.9% upconversionquantum yield compared with 0.01% for ligands with 18 carbons. This experimentally derivedinsight will enable the design of efficient QD-based photosensitizers for catalysis and energyconversion.