Diffusion Assisted Bimolecular Electron Injection to CdS Quantum Dots: Existence of Different Regimes in Time Dependent Sink Term of Collins-Kimball Model

Excited state lifetime and steady state fluorescence of a series of CdS quantum dots (QDs) with different sizes in toluene were quenched by electron donor molecule N-methyl aniline (NMA). Static quenching Collins-Kimball (SQCK) diffusion model enabled convincing fittings to the steady-state and time-resolved data using nearly a same set of parameters, only after considering the presence of inherent quencher sites statistically distributed over the quantum dot surface. Electron injection rate shows strong chemical driving force dependency. QD with largest dimension (similar to 5.4 nm) used in this study exhibits a slightly higher chemical driving force (-Delta G(0) = 0.80 eV) of electron transfer as compared to that (-Delta G(0) = 0.79 eV) obtained for the smallest size QD (similar to 3.8 nm). However, such a small change in driving force causes nearly similar to 3 times acceleration of the ET rate coefficient (k(0) = 8.30 X 10(9) M-1 s(-1)) within the larger size QD as compared to that (k(0) = 2.74 X 10(9) M-1 s(-1)) observed in smaller size QD. The time evolution of the sink term obtained from the Collins-Kimball fitting of ET kinetics shows different regimes of the kinetics (static and nonstationary).