Simultaneous Determination of the Adsorption Constant and the Photoinduced Electron Transfer Rate for a Cds Quantum Dot-Viologen Complex

Transient absorption (TA) spectroscopy of solution-phase mixtures of colloidal CdS quantum dots (QDs) with acid-derivatized viologen molecules, N-[1-heptyl],N'[3-carboxypropyl]-4,4'-bipyridinium dihexafluorophosphate (V2+), indicates electron transfer occurs from the conduction band of the QD to the LUMO of V2+ after photoexcitation of a band-edge exciton in the QD. Analysis of the magnitude of the ground state bleach of the QD as a function of the molar ratio QD:V2+ yields the QD-ligand adsorption constant, K-a (4.4 x 10(4) M-1) for V2+ ligands adsorbed in geometries conducive to electron transfer. The value of K-a, together with the measured rates of (i) formation of the V+center dot electron transfer product and (ii) recovery of the ground state bleach of the QD, enables determination of the intrinsic rate constant for charge separation, k(CS,int) similar to 1.7 x 10(10) s(-1), the rate for a single QD-V2+ donor-acceptor pair. This analysis confirms previous reports that the number of ligands adsorbed to each QD is well-described by a Poisson distribution. This is the first report where the QD-ligand charge transfer and binding equilibria are quantitatively investigated simultaneously with a single technique.