Solvent Dependent Excited State Behaviors of Luminescent Gold(I)–Silver(I) Cluster with Hypercoordinated Carbon

Abstract

Polynuclear Au­(I) complexes continues to attract considerable attention because of their bright emissions in the visible wavelength, which hold promise in applications in luminescence, fluorescence sensing, and bioimaging. Despite various spectroscopic investigations on their steady state properties, detailed understanding of the origin of their emissions and excited state relaxations is still lacking. Here, we report femtosecond time-resolved transient absorption experiments combined with quantum chemical calculations on a brightly emissive [Au₆Ag₂(C)­(dppy)₆]­(BF₄)₄ cluster in different solvents. Global analysis on the transient absorption spectra based on a sequential model gives three spectral components: (1) excited state absorption (ESA) of ¹MLCT_Au state (τ = 1–3 ps); (2) ESA of ³MLCT_Au state (τ = 11–40 ps), and (3) ESA of ³MLCT_Ag state (long-lived). By variation of the solvent’s polarity and hydrogen bonding ability, the relative population of the triplet MLCT states and the emission properties can be modulated. Especially in methanol, an additional site specific O–H···π bond is formed between methanol molecules and aromatic rings of ligands, which enhances the ultrafast nonradiative decay from the hydrogen bond stabilized ³MLCT_Au state and reduces the population of the emissive ³MLCT_Ag state. The results presented here about the excited state dynamics of luminescent gold­(I)–silver­(I) cluster allow a deeper insight into the origin of their emissions by monitoring the population of the emissive ³MLCT_Ag state and dark ³MLCT_Au state in different environments