Relaxation Dynamics of Electronically Coupled Au₂₀(SC₈H₉)₁₅-n-glyme-Au₂₀(SC₈H₉)₁₅ Monolayer-Protected Cluster Dimers

The electronic relaxation dynamics of photoexcited Au₂₀(SC₈H₉)₁₅-n-glyme and Au₂₀(SC₈H₉)₁₅-n-glyme-Au₂₀(SC₈H₉)₁₅ (n-= di-, tri-, and tetra-) dimers, where glyme refers to n-ethylene glycol dimethyl ether bridging molecules, were studied using pump-probe femtosecond time-resolved transient absorption spectroscopy (fsTAS). The utilization of n-glyme molecular bridging linkers provided a method to prepare Au₂₀(SC₈H₉)₁₅-n-glyme-Au₂₀(SC₈H₉)₁₅ dimers with control over intercluster spatial separation. A dimer-specific electronic absorption resonance was observed at 2.6 eV. Analysis of fsTAS differential spectra for dimer species revealed a pump-probe waiting time-dependent blue shift of the low energy excited-state absorption (ESA) feature, suggesting electronic relaxation into a dimer-specific excited-state. Single probe-energy differential signal amplitude analysis of the ESA feature yielded a distance-dependent growth component for the electronic relaxation dynamics with time constants of 130 ± 20, 60 ± 8, and 36 ± 6 ps for the diglyme, triglyme, and tetraglyme-bridged dimers, respectively. The increase in relaxation time was attributed to intercluster distance-dependent stabilization of dimer-specific electronic excited states. These results suggest a state-specific mechanism for intercluster electronic relaxation.