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.