Shell Thickness and Heterogeneity Dependence of Triplet Energy Transfer between Core-Shell Quantum Dots and Adsorbed Molecules.

Quantum dot (QD)-sensitized triplet energy transfer (TET) has found promising applications in photon upconversion and photocatalysis. However, the underlying mechanism of TET in the QD-acceptor complex remains unclear despite the well-developed TET theory for the molecular donor-acceptor systems. Herein, the coupling strength of TET from CdSe/CdS core-shell QDs to 9-anthracene carboxylic acid (ACA) was studied by measuring the TET rate as a function of shell thickness with time-resolved photoluminescence. The change of TET-coupling strength with increasing shell thickness was further compared to those of electron and hole transfers from QDs so that we could test whether QD-sensitized TET is mediated by the charge transfer virtual state and can be considered as simultaneous electron and hole transfers as in molecular donor-acceptor systems. The measured coupling strength of TET from the CdSe/CdS QD decreases exponentially with the CdS shell thickness : ||() = ||(0)e, with an exponential decay factor β of 0.19 Å, which is smaller than the sum of the measured decay factors for electron transfer to methyl viologen (0.18 Å) and hole transfer to phenothiazine (0.29 Å) from the same QD. This inconsistency is explained by the broadening of QD shell thicknesses in the distance dependence study, which significantly modifies the TET-coupling strength and driving force, resulting in a shallower distance dependence of the TET rate constants. This study sheds light on the fundamental mechanisms of QD-sensitized TET reactions.