Computational Study of the Influence of the Binding Geometries of Organic Ligands on the Photoluminescence Quantum Yield of CdSe Clusters

This article describes density-functional-theory- (DFT-) based calculations of the rate constants for radiative (k_R) and nonradiative (k_NR) decay from the lowest singlet excited state (S₁) to the ground state (S₀) of a Cd₁₆Se₁₃ cluster ligated with various molecules in various binding geometries. The value of k_R is suppressed by ligands whose localized orbitals, as a result of their binding geometry, become the cluster's frontier orbitals and, thereby, decrease the overlap of the electron densities of the HOMO and LUMO necessary for efficient dipole coupling. Thiolate ligands in a monodentate geometry and dithioate ligands in a bridging geometry decrease k_R in this manner. The value of k_NR is also sensitive to the binding geometries of the ligands: binding geometries that are less rigid yield a greater change in nuclear coordinates between the S₁ and S₀ electronic states, which, in turn, increases the rate of nonradiative decay by maximizing the vibronic coupling between the band-edge exciton and the vibrational modes of the ligands. This work suggests that the photoluminescence quantum yield of CdSe nanoparticles can be maximized by ensuring that the bridging binding mode is the dominant binding mode of the ligands; bridging modes decrease the nonradiative decay rate and eliminate mid-band-gap trap states in all cases studied, except for dithioate ligands. (Chemical Equation Presented).