Gas-Phase Anion Photoelectron Spectroscopy of Alkanethiolate-Protected PtAu 12 Superatoms: Charging Energy in Vacuum vs Solution.

The charging behavior of molecular Au clusters protected by alkanethiolate (SC _n H _2n+1 =SCn) is, under electrochemical conditions, significantly affected by the penetration of solvents and electrolytes into the SCn layer. In this study, we estimated the charging energy E _C (n) associated with [PtAu ₂₄ (SCn) ₁₈ ] ^- +e ^- →[PtAu ₂₄ (SCn) ₁₈ ] ^2- (n=4, 8, 12, and 16) in vacuum using mass-selected gas-phase anion photoelectron spectroscopy of [PtAu ₂₄ (SCn) ₁₈ ] ^z (z=-1 and -2). The E _C (n) values of PtAu ₂₄ (SCn) ₁₈ in vacuum are significantly larger than those in solution and decrease with n in contrast to the behavior reported for Au ₂₅ (SCn) ₁₈ in solution. The effective relative permittivity (ϵ _m *) of the SCn layer in vacuum is estimated to be 2.3-2.0 based on the double-concentric-capacitor model. Much smaller ϵ _m * values in vacuum than those in solution are explained by the absence of solvent/electrolyte penetration into the monolayer. The gradual decrease of ϵ _m * with n is ascribed to the appearance of an exposed surface region due to the bundle formation of long alkyl chains.
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