Unravelling quantum dot-molecule interactions for π-conjugated ligands: insights into binding and anchoring group effects

Quantum dots (QDs) coated with π-conjugated ligands display triplet energy transfer (TET), which opens the path for photon upconversion via QD photosensitization. Herein we study the effect of the ligand binding and its orientation on the triplet energy transfer efficiency through analysing the quenching of the QD photoluminescence. Comparing anthracene ligands with different anchoring groups, we find that replacing carboxylate with thiol or dithiol groups enhances quenching rates by factors of 3 and 4.5, respectively. To obtain this quantitative information, we devise a modified Stern-Volmer model taking into account the Poisson distribution of the ligand binding on the QDs. To this end, we show that bound anthracene-based ligands exhibit distinct spectral changes in their absorption spectra, including a ligand-dependent bathochromic shift with a modified vibronic progression and broadened spectral width. These changes, related to the deprotonation of the anchoring groups upon binding and the confined environment on the QD surface, enable the distinction of the crossover from bound to free ligands upon ligand addition. This allows us to incorporate accurate ligand binding stoichiometry to extract reliable quenching rates. Consistent with DFT calculations, the improved quenching for the thiolated anthracenes is ascribed to the parallel orientation of the π-system relative to the QD surface enabling larger orbital overlap that leads to faster TET rates via the Dexter mechanism. This work contributes to the design principles for efficient QD-organic hybrid systems towards improved triplet energy transfer.