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

Charging behavior of molecular Au clusters protected by alkanethiolate (SCnH2n+1 = SCn) is, under electrochemical conditions, significantly affected by the penetration of solvent and electrolyte into the SCn layer. In this study, we estimated the charging energy EC(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 EC(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 Au25(SCn)18 in solution. The effective relative permittivity (εm*) of the SCn layer in vacuum is estimated to be 2.3-2.0 based on the double-concentric-capacitor model. Much smaller εm* values in vacuum than those in solution are explained by the absence of solvent/electrolyte penetration into the monolayer. The gradual decrease of εm* with n is ascribed to the appearance of an exposed surface region due to the bundle formation of long alkyl chains.

Electron Affinities of Ligated Icosahedral M13 Superatoms Revisited by Gas-Phase Anion Photoelectron Spectroscopy.

The electron binding energies of the ligand-protected gold/silver-based cluster anions, [Au25(SR)18]-, [XAg24(SR')18]2- (X = Ag+, Au+, Pd0, or Pt0), and [PdAu24(C≡CR″)18]2- having icosahedral M13 superatomic cores, were reexamined by gas-phase photoelectron spectroscopy (PES) on a significantly intensified mass-selected ion beam. Laser fluence-dependent PE spectra and pump-probe PES revealed that the previous PE spectra were contaminated by PE signals due to the two-photon electron detachment via long-lived photoexcited states. Although the adiabatic electron affinities (AEAs) of the corresponding oxidized forms were found to be 1-2 eV larger than those previously reported, the effects of doping and ligation were not qualitatively affected. (1) The AEA of the Ag13 superatom (∼4 eV) was not appreciably affected by doping a Au atom at the center but was reduced by ∼2 eV by doping Pd or Pt, and (2) the AEA of PdAu12 protected by Au2(C≡CR″)3 units was much larger than that of PdAg12 protected by Ag2(SR')3 units.

Electron Binding in a Superatom with a Repulsive Coulomb Barrier: The Case of [Ag44(SC6H3F2)30]4- in the Gas Phase.

The electron binding mechanism in [Ag44(SC6H3F2)30]4- (SC6H3F2 = 3,4-difluorobenzenethiolate) tetra-anion was studied by photoelectron spectroscopy (PES), collision-induced dissociation mass spectrometry (CID-MS), and density functional theory (DFT) computations. PES showed that [Ag44(SC6H3F2)30]4- is energetically metastable with respect to electron autodetachment {[Ag44(SC6H3F2)30]3- + e-} and features a repulsive Coulomb barrier (RCB) with a height of 2.7 eV. However, CID-MS revealed that [Ag44(SC6H3F2)30]4- does not release an electron upon collisional excitation but undergoes dissociation. DFT computations performed on the known structure of [Ag44(SC6H3F2)30]4- confirmed the negative adiabatic electron affinity of [Ag44(SC6H3F2)30]3- and interpreted the experimental PE spectrum by taking into account tunneling electron photodetachment through the RCB.