Semiconductor nanocrystals interfaced with functional ligands are widely studied as optoelectronic materials for photon upconversion, downconversion or light-harvesting applications. To ensure energy or charge transfer across the inorganic-organic interface, ligand concentrations at the nanocrystal surface are maximized. However, this can lead to ligand-ligand interactions and the formation of molecular aggregates that can alter important energetic parameters, such as donor-acceptor coupling or energetic driving force, that characterize transfer at the interface. Indeed, we show that ligand-ligand interactions impact the nanocrystal-to-ligand electron transfer rate in lead (II) sulfide (PbS) nanocrystals containing perylenediimide (PDI) ligands. PDI ligands were synthesized with a linker, consisting of an ethyl, pentyl or benzyl group, that joins the PDI chromophore with a carboxylate that binds to the PbS surface. PbS nanocrystals with natively bound oleate underwent a ligand exchange with increasing concentration of each unique PDI ligand. The mean number of bound ligands follows a Langmuir adsorption model. Ground-state absorption spectra exhibit a redistribution in the vibronic progression of PDI indicating ligand aggregation with increasing PDI concentration. Following excitation of the PbS nanocrystal, the system undergoes electron transfer as evidenced by a PDI ground state bleach and PDI radical anion excited-state absorption in transient absorption measurements. We find the intrinsic electron transfer rate for a single PbS-to-PDI unit decreases as the mean number of bound ligands and the distribution of aggregates increases. Intrinsic electron transfer rates also decreased with increasing linker length of the PDI ligands. The formation of aggregates at the nanocrystal surface shows that transfer across the inorganic-organic interface can be fine tuned by collective ligand-ligand and nanocrystal-ligand interactions.

Ligand aggregation on PbS nanocrystal surfaces impacts nanocrystal-to-molecule electron transfer rates

ACS Spring 2022

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