Triplet Excimer Regulates Triplet Sensitization Efficiency of Semiconductor Quantum Dots

Semiconductor quantum dots (QDs) sensitize molecular triplets via triplet energy transfer (TET) to surface-anchored ligands. QD-ligand energy alignment and distance have been proven to be key factors for TET efficiency. The effect of ligand-ligand interaction, however, is elusive and hard to characterize. In this work, we show that QD triplet sensitization efficiency is greatly affected by ligand triplet excimer, which can be identified by photon upconversion from a three-component upconversion system comprising CdSe QD sensitizer, anthracene ligand, and perylene emitter. Specifically, two anthracene derivatives, anthracen-9-ylphosphonic acid (AnP) and (10-((triisopropylsilyl)ethynyl)anthracen-9-yl)phosphonic acid (TIPS-AnP), with different steric hindrance are bound to CdSe QDs, and the increase of ligand density results in strong ligand-ligand interaction, i.e., triplet excimer formation evidenced by transient absorption spectroscopy. Such triplet excited state-ground state interaction leads to a decrease of upconversion emission due to the low-lying excimer state that is energetically unfavorable for TET to perylene emitter. The observation that the bulkier TIPS-AnP is more prone to triplet excimer formation is attributed to the higher molecular polarity that induces an aggregated distribution on QD surfaces. This work reveals for the first time that the ligand-ligand interaction is crucial for TET from ligand to substrates, which provides a guide to the structural design of ligands on QDs in terms of QD-ligand energy alignment, distance, and ligand-ligand interaction for efficient triplet sensitization.