Efficient quasi-stationary charge transfer from quantum dots to acceptors physically-adsorbed in the ligand monolayer

Alkanoate-coated CdSe/CdS core/shell quantum dots (QDs) with near-unity photoluminescence (PL) quantum yield and mono-exponential PL decay dynamics are applied for studying quasi-stationary charge transfer from photo-excited QDs to quinone derivatives physically-adsorbed within the ligand monolayer of a QD. Though PL quenching efficiency due to electron transfer can be up to > 80%, transient PL and transient absorption spectra reveal that the charge transfer rate ranges from single-digit nanoseconds to sub-nanoseconds, which is ∼ 3 orders of magnitude slower than that of static charge transfer and ∼ 2 orders of magnitude faster than that of collisional charge transfer. The physically-adsorbed acceptors can slowly (500–1,000 min dependent on the size of the quinone derivatives) desorb from the ligand monolayer after removal of the free acceptors. Contrary to collisional charge transfer, the efficiency of quasi-stationary charge transfer increases as the ligand length increases by providing additional adsorption compartments in the elongated hydrocarbon chain region. Because ligand monolayer commonly exists for a typical colloidal nanocrystal, the quasi-stationary charge transfer uncovered here would likely play an important role when colloidal nanocrystals are involved in photocatalysis, photovoltaic devices, and other applications related to photo-excitation.