Electron transfer as a probe of the permeability of organic monolayers on the surfaces of colloidal PbS quantum dots

Transient absorption measurements on both the picosecond and microsecond time scales reveal that the efficiencies with which a series of alkyl-substituted p-benzoquinone (s-BQ) molecules participate in static and collisional photoinduced electron transfer (PET) with colloidal PbS quantum dots (QDs) in dichloromethane solution depend on both the size and shape of the s-BQ molecule. The efficiencies of both static and collisional PET are limited by the presence of the oleate ligand shell on the surface of the QDs and decrease with an increase in molecular volume, V_Q, of the s-BQ, in general; however, the substitution patterns on the BQ ring that facilitate static PET are not the same patterns that facilitate collisional PET. A model for the dependence of the collisional quenching efficiency on V_Q allows quantitative characterization of both the permeability and average thickness of the oleate ligand shell of the QDs in a dichloromethane solution.