Methyl Viologen Groups Bound to Pseudo-Two-Dimensional Cadmium Selenide Nanocrystals as X-Type Ligands Are More Effective Quenchers Than When Bound as L-Type Ligands

Abstract

Oleylamine-ligated wurtzite CdSe quantum platelets (QPs) and quantum belts (QBs) are exchanged with carboxylate-terminated, methyl-viologen-containing MV2+(CH2)nCO2– ligands (where n = 2, 4, or 6), resulting in the progressive quenching of the QP and QB photoluminescence (PL). Spectroscopic and compositional analyses indicate that the initial ligand exchange responsible for PL quenching occurs on the nonpolar, thin, long-edge nanocrystal facets. The X-type MV2+(CH2)nCO2– ligands are shown to be more-effective PL quenchers than the corresponding L-type MV2+(CH2)nNH2 ligands studied previously [ Meyer, H. M.; et al.J. Phys. Chem. C2023, 127, 18506–18517.]. Transient-absorption spectroscopy measurements reveal electron transfer to the ligands occurs for fully quenched QPs, but not for the fully quenched QBs, establishing that electron transfer occurs selectively at the polar, thin, short-edge facets of the nanocrystals. The electron transfer occurs in <50 fs for the X-type ligation and within 100 fs for the L-type ligation. This difference is ascribed to the greater preference for the binding of X-type over L-type ligands on polar facets, and this results in a significant contribution of electron transfer to the overall PL quenching.