We report that post-synthetic functionalization (often referred to as “top-down” synthesis) of CdSe quantum dots (QDs) with a series of imidazolylidene N-heterocyclic carbene (NHC) ligands results in a bathochromic shift of the ground state absorption and emission spectra of the QDs. This bathochromic shift is indicative of the expansion of the wavefunction of the exciton of the QD beyond the physical radius of the QD and into the ligand shell. Such behavior is known as exciton delocalization, and is of interest for QD-mediated photocatalysis, since exciton delocalization enables efficient extraction of one or multiple equivalents of charge carriers from the QD to a molecular redox partner. Exciton delocalization has been previously demonstrated for chalcogenide-based QDs with ligands that contain thiolate binding groups, like thiophenol and phenyldithiocarbamate. Thiolate-based exciton delocalizing ligands, however, suffer from several disadvantages, including i) instability due to disulfide formation upon oxidation, ii) quenching of the emission of the QD unless excess molecular cadmium is added to solution, and iii) in the case of phenyldithiocarbamates in particular, the spontaneous degradation of the ligand when unbound to a QD surface into multiple degradation products. This work is the first example of an exciton-delocalizing ligand that uses a carbene binding group and demonstrates that NHC ligands do not fully quench the QD emission nor degrade in solution. We perform post-synthetic ligand exchange with three 1,3-methyl-4,5-disubsituted imidazolylidene NHC derivatives and use a combination of ground state optical spectroscopies and nuclear magnetic resonance spectroscopy to characterize the change in the apparent radius of the QD as a function of bound NHC ligands. We discuss the effect of σ and π interactions between the carbene carbon and the QD valence band, which influences the magnitude of the apparent radius of the QD per bound NHC ligand.

N-heterocyclic carbenes as exciton-delocalizing ligands for CdSe quantum dots

Macromolecular Chemistry: The Second Century

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ACS Spring 2020 National Meeting & Expo

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