Evidence for a Through-Space Pathway for Electron Transfer from Quantum Dots to Carboxylate-Functionalized Viologens

Ultrafast transient absorption measurements reveal that the rate constant for photoinduced electron transfer (PET) from colloidal CdS quantum dots (QDs) to alkylcarboxylate-functionalized viologens is independent of the number of methylene groups in the alkyl chain (<i>n</i>). The rate constant for PET is (1.2 ± 0.3) × 10¹⁰ s^–1 for <i>n</i> = 1, 2, and 3, and for <i>n</i> = 0 (methylviologen). The insensitivity of the electron transfer rate constant to the length of the functional groups on the viologen suggests that a “through-space” pathway, where the electron bypasses the alkylcarboxylate and tunnels instead through only the orbitals of the QD and of the bipyridinium core, is the dominant PET pathway

Organic Surfactant-Controlled Composition of the Surfaces of CdSe Quantum Dots

The ratio of Cd to Se (Cd/Se) within colloidal CdSe quantum dots (QDs) synthesized with 90% trioctylphosphine oxide (TOPO) as the coordinating solvent increases from 1.2:1 for QDs with radius <i>R</i> ≥ 3.3 nm to 6.5:1 for <i>R</i> = 1.9 nm, as measured by inductively coupled plasma atomic emission spectroscopy (ICP-AES). The highest value of Cd/Se reported previously for CdSe QDs was 1.8:1. The dependence of Cd/Se on <i>R</i> fits a geometric model that describes the QDs as CdSe cores with Cd/Se = 1:1 encapsulated by a shell of Cd−organic complexes. Use of 99% TOPO as the coordinating solvent produces QDs with Cd/Se ≈ 1:1 for all values of <i>R</i>, and use of 99% TOPO “doped” with <i>n</i>-octylphosphonic acid (OPA), an impurity in 90% TOPO, produces QDs with values of Cd/Se up to 1.5:1. These results imply that Cd enrichment of the QDs is driven by tight-binding Cd²⁺−alkylphosphonate complexes that stabilize the interface between the polar CdSe core and the organic medium