QM/MM study of photoinduced reduction of a tetrahedral ag₂₀ ⁺ cluster by a ag atom

A hybrid quantum mechanics/classical mechanics (QM/MM) approach was developed to investigate photoinduced electron transfer (PIET) from a neutral Ag atom to an ionized tetrahedral Ag₂₀⁺ cluster, both of which are solvated in water. In this approach, PIET was modeled as a coherent quantum process involving both vertical excitation and electron injection by our recently developed constrained real-time time-dependent density functional theory (C-RT-TDDFT) (J. Phys. Chem. C 2011, 115, 18810), whereas the aqueous solvation structure for the (Ag-Ag₂₀)⁺ complex was determined by the empirical flexible simple point charge (SPC/Fw) force field (J. Chem. Phys. 2006, 124, 024503). An electrostatic embedding scheme was chosen to accurately represent the mutual polarization between the QM subsystem (the (Ag-Ag₂₀)⁺ complex) and the MM subsystem (water molecules) in a self-adaptive manner that turns out to be critical to the relative stability of the electron transfer diabatic states in addition to their electronic coupling strengths in both ground and excited states. It was found that photoinduced electron transfer through an indirect coherent route, which is mediated by a short-lived virtual excited state, can be substantially faster than the sequential two-step process, which is typically limited by the light absorption efficiency. Moreover, the unusually wide plateau of near-unity quantum yields that we found near the plasmon-like resonance wavelength of the (Ag-Ag₂₀)⁺ complex implies the possibility of designing exceptionally efficient plasmon-enhanced photocatalytic systems with an easily tunable range of activation wavelength by varying their plasmonic architectures.