Synthesis and Enhanced Linear and Nonlinear Optical Properties of Chromophore–Au Metal Cluster Oligomers

Using nanoclusters as building blocks for supracrystals can offer unique optical and electronic properties of metal nanoclusters for designing new materials. The advantage of building nanocluster crystals through molecular linkers is that the linker can be used as a functional group for the nanoclusters (e.g., dye–nanocluster systems). In this study, we employed the ligand exchange reaction to synthesize chromophore–Au₂₅ nanocluster oligomers and investigated their linear and nonlinear optical properties. The chromophore–Au₂₅ nanocluster oligomers product mixture was separated into four bands by polyacrylamide gel electrophoresis and characterized by matrix-assisted laser desorption ionization mass spectrometry and scanning transmission electron microscopy imaging. The linear optical properties of the systems were investigated by steady state UV–vis absorption and fluorescence spectroscopy. The chromophore–Au₂₅ nanocluster oligomers showed increased oscillator strength and transition dipole moment compared to single Au₂₅ nanoclusters. Energy transfer from the chromophore 4,4′-thiodibenzenethiol (TBT) to the metal cluster was observed in the chromophore–Au₂₅ nanocluster dimer system. The excited state and fluorescence dynamics were investigated by transient absorption spectroscopy, time-resolved fluorescence upconversion, and time-correlated single photon counting. The chromophore–Au₂₅ nanocluster oligomers have a long-lived surface state due to the contribution of energy transfer by two nanocluster cores. The two-photon absorption cross sections of the chromophore–Au₂₅ nanocluster oligomers showed an increasing enhancement trend with increasing oligomer length. An enhancement factor of up to 68 times was found compared to single Au₂₅ nanoclusters. Finally, we performed a structure–property correlation analysis to explain the observed optical properties of these systems.