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 <i>N</i>-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 (Δ<i>G</i>). 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 × 10<sup>9</sup>–1.60 × 10<sup>9</sup> M<sup>–1</sup> S<sup>–1</sup>), which is nicely correlated with their nearly same values of the chemical driving force (−Δ<i>G</i> ∼ 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 × 10<sup>9</sup> M<sup>–1</sup> S<sup>–1</sup> (QD480) and ∼3.7 × 10<sup>9</sup> M<sup>–1</sup> S<sup>–1</sup> (QD560), respectively, for QD480 and QD560, which is again congruent to their chemical free energy changes (−Δ<i>G</i><sub>QD480‑DNT</sub> ∼ 1.18 eV and −Δ<i>G</i><sub>QD560‑DNT</sub> ∼ 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).