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:V ²⁺ yields the QD - ligand adsorption constant, K_a (4.4 × 10⁴ 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^+• 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 ∼ 1.7 × 10¹⁰ 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.