Study of Diffusion-Assisted Bimolecular Electron Transfer Reactions: CdSe/ZnS Core–Shell Quantum Dot Acts as an Efficient Electron Donor and Acceptor

Excited-state lifetimes and steady-state emission of two different size CdSe/ZnS core–shell quantum dots (QDs) in toluene were quenched by an electron donor molecule N-methyl aniline (NMA) and an electron acceptor molecule 2,4-dinitrotoluene (DNT) in two separate sets of experiments. Static quenching Collins-Kimball (SQCK) diffusion model enabled a conclusive fitting only to the electron transfer (ET) kinetics of QD-NMA pairs. However, for QD-DNT pairs, a clear break down of SQCK model was observed. Interestingly, when we considered a QD-to-DNT static complex formation, we observed even a classic Stern–Volmer (SV) fitting equation can provide an adequate fitting to the ET kinetics. ET kinetics we studied here are strongly controlled by the chemical driving forces (ΔG). For example, electron injection rates (by NMA) to the two QDs with core dimensions ∼3.4 nm (QD560) and ∼2.5 nm (QD480) were found to be similar (∼1.50 × 10⁹–1.60 × 10⁹ M^–1 S^–1), which is nicely correlated with their nearly same values of the chemical driving force (−ΔG ∼ 0.18–0.19 eV) associated with their ET reactions. Conversely, electron donating rates (to DNT) of the same two QDs are found to be ∼7.0 × 10⁹ M^–1 S^–1 (QD480) and ∼3.7 × 10⁹ M^–1 S^–1 (QD560), respectively, for QD480 and QD560, which is again congruent to their chemical free energy changes (−ΔG_QD480‑DNT ∼ 1.18 eV and −ΔG_QD560‑DNT ∼ 0.44 eV). A nonadiabatic sink term of ET kinetics from QD-NMA pair shows distinct regimes associated with the ET reaction (i.e., static, nonstationary, and stationary).