Photophysical and Thermodynamic Properties of Ag<sub>29</sub>(BDT)<sub>12</sub>(TPP)<sub><i>x</i></sub> (<i>x</i> = 0–4) Clusters in Secondary Ligand Binding–Dissociation Equilibria Unraveled by Photoluminescence Analysis
Yoshiki Niihori, N. Takahashi, Masaaki Mitsui
Abstract
Despite the huge effort that has been devoted to the atomically precise synthesis and structural and physicochemical property characterization of ligand-protected noble metal clusters, quantitative knowledge of thermodynamic equilibrium in solutions of these clusters to date remains lacking. In this article, we report the determination of equilibrium constants for reversible binding–dissociation reactions of a secondary ligand, triphenylphosphine (TPP), in Ag29(BDT)12(TPP)x (x = 0–4) clusters (BDT: 1,3-benzenedithiolate) achieved through photoluminescence (PL) analysis. Detailed analyses of TPP-concentration dependencies of PL decay times and PL quantum yields of the clusters under equilibrium allowed us to extract the PL lifetimes (τx) and PL quantum yields (Φx) of each composition (x = 0–4). Using these PL parameters, we reveal in this study how the relative abundances of each composition vary with the TPP concentration. From the simulation of these dependencies, we obtained the stability and equilibrium constants for each binding–dissociation reaction step in the sequential equilibrium. More importantly, the parent clusters, Ag29(BDT)12(TPP)4, in the solution underwent sequential loss of TPP ligands and became the binding–dissociation equilibria due to modest magnitudes of the equilibrium constant (K = 102–104 M–1). Hence, our PL analysis approach is highly effective in determining the photophysical and thermodynamic parameters of inseparable clusters in dynamic equilibrium.