Spontaneous Intercluster Electron Transfer X2-+X0 → 2 X- (X=PtAu24(SCnH2n+1)18) in Solution: Promotion by Long Alkyl Chains

In this work, we systematically investigated the ligand effects on spontaneous electron transfer (ET) between alkanethiolate-protected metal clusters in solution. The donor and acceptor clusters used were [PtAu24(SCnH2n+1)18]2- (8e(Cn)) and [PtAu24(SCmH2m+1)18]0 (6e(Cm)) (n, m = 2-16), which have icosahedral Pt@Au12 cores with eight and six valence electrons, respectively. The ET rate constant (kET) from 8e(Cn) to 6e(Cm) in benzene exhibited a novel turnover behavior as a function of the total chain length n + m: the kET decreased with n + m in the range of 4-12, whereas it monotonically increased with n + m in the range of 12-32. Electrospray ionization mass spectrometry of the mixture of 8e(Cn) and 6e(Cm) detected the dimer complex 8e(Cn)center dot 6e(Cm), the relative population of which increased with n + m. The activation energy (Ea), determined based on the Arrhenius plots for n = m, monotonically decreased with n (>= 6). Based on these results, we proposed that the promotion of ET by longer alkanethiolates was ascribed to two effects on the key intermediate 8e(Cn)center dot 6e(Cm): (1) elongation of the lifetime and (2) the contraction of the distance between 8e(Cn) and 6e(Cm) due to the stronger van der Waals interaction between the longer alkyl chains. Such alkyl-chain-promoted ET is specific to ultrasmall clusters in solution because a nonuniform ligand layer could be formed due to the large curvature of the cluster core.

Automated summary

In this study, we investigated the effects of ligands on spontaneous electron transfer between alkanethiolate-protected metal clusters in solution. The rate of electron transfer showed a turnover behavior depending on the total chain length of the ligands. The promotion of electron transfer by longer alkanethiolates was attributed to the elongation of lifetime and the contraction of distance between the clusters due to stronger van der Waals interaction. This alkyl-chain-promoted electron transfer is specific to ultrasmall clusters in solution due to nonuniform ligand layer formation.