Gas-Phase Anion Photoelectron Spectroscopy of Alkanethiolate-Protected PtAu12 Superatoms: Charging Energy in Vacuum vs Solution

The charging behavior of molecular Au clusters protected by alkanethiolate (SCnH2n+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 [PtAu24(SCn)(18)](-)+e(-)->[PtAu24(SCn)(18)](2-) (n=4, 8, 12, and 16) in vacuum using mass-selected gas-phase anion photoelectron spectroscopy of [PtAu24(SCn)(18)](z) (z=-1 and -2). The E-C(n) values of PtAu24(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 (& varepsilon;(m)*) of the SCn layer in vacuum is estimated to be 2.3-2.0 based on the double-concentric-capacitor model. Much smaller & varepsilon;(m)* values in vacuum than those in solution are explained by the absence of solvent/electrolyte penetration into the monolayer. The gradual decrease of & varepsilon;(m)* with n is ascribed to the appearance of an exposed surface region due to the bundle formation of long alkyl chains.