A quantitative description of the binding equilibria of para-substituted aniline ligands and CdSe quantum dots

This paper describes the use of ¹H NMR spectroscopy to measure the equilibrium constants for the solution-phase binding of two para-substituted aniline molecules (R-An), p-methoxyaniline (Me0-An) and p-bromoaniline (Br-An), to colloidal 4.1 nm CdSe quantum dots (QDs). Changes in the chemical shifts of the aromatic protons located ortho to the amine group on R-An were used to construct a binding isotherm for each R-An/QD system. These isotherms fit to a Langmuir function to yield K_a, the equilibrium constant for binding of the R-An ligands to the QDs; K_a almost equal to 150 M^-1 and DeltaG_ads almost equal to -19 kJ/mol for both R = MeO and R = Br. ³¹P NMR indicates that the native octylphosphonate ligands, which, by inductively coupled plasma atomic emission spectroscopy, cover 90% of the QD surface, are not displaced upon binding of R-An. The MeO-An ligand quenches the photoluminescence of the QDs at much lower concentrations than does Br-An; the observation, therefore, that K_a,_K-MeO-An almost equal to K_a,_Br-An shows that this difference in quenching efficiencies is due solely to differences in the nature of the electronic interactions of the bound R-An with the excitonic state of the QD.