Ultraviolet Photodissociation of Selected Gold Clusters: Ultraefficient Unstapling and Ligand Stripping of Au₂₅(pMBA)₁₈ and Au₃₆(pMBA)₂₄

Abstract

We report the first results of ultraviolet photodissociation (UVPD) mass spectrometry of trapped monolayer-protected cluster (MPC) ions generated by electrospray ionization. Gold clusters Au(pMBA) and Au(pMBA) (pMBA = para-mercaptobenzoic acid) were analyzed in both the positive and negative modes. Whereas activation methods including collisional- and electron-based methods produced relatively few fragment ions, even a single ultraviolet pulse (at λ = 193 nm) caused extensive fragmentation of the positively charged clusters. Upon photoactivation using a low number of laser pulses, the staple motifs of both clusters were cleaved and stripped of the protecting ligand portions without removal of any contained gold atoms. This striking process involved Au-S and C-S bond cleavages via a pathway made possible by 6.4 eV photon absorption. Monomer evaporation (neutral gold atom loss) occurred upon exposure to multiple pulses, resulting in a size series of bare gold-cluster ions. All tandem mass spectrometric methods produced the singly charged ring tetramer ion, [Au(pMBA) + Na], for each cluster.

Isomeric Effect of Mercaptobenzoic Acids on the Synthesis, Stability, and Optical Properties of Au25(MBA)18 Nanoclusters

“…For p -MBA, Au 36 (MBA) 24 is also formed and can be isolated during the purification process. 25 Clearly, an isomeric effect of mercaptobenzoic acids on the preparation of Au 25 (MBA) 18 NCs is observed. This may be explained qualitatively based on steric hindrance of the carboxylic group (bulkiness of the carboxylic groups with respect to other benzene rings in the ligand shell, see Scheme 1) and its ion-pairing with tributylamine.…”

“…CID on gold NCs in mass spectrometers was reported in previous works for different charge states, in both positive and negative modes, and with different ligands. 25,37−40 Usually, whatever the ligand, CID experiments lead to dissociation of the protecting ligand staples and formation of ions with higher- m / z ratios. Of note, the nature (protonation, deprotonation, counterion adducts, etc) as well as the number of charges affect the fragmentation pattern; however, the obtained fragments are not sufficient for structural characterization.…”

“…These fragmentation patterns are very similar to those observed by Brodbelt, Whetten, and co-workers on negatively charged Au 25 ( p -MBA) 18 by conventional CID and higher-energy collisional dissociation. 25 Of note, the electron affinity of Au 25 (SR) 18 was reported to be as low as 2.2 eV, 41 and thus the electron detachment might compete with the CID. For [Au 25 (MBA) 18 ] 3– and [Au 25 (MBA) 18 ] 4– precursor ions, only fragmentation channels are observed, while for [Au 25 (MBA) 18 ] 5– precursor ions (for the ortho isomer), electron detachment from collisionally heated clusters competes with fragmentation.…”

Activation of atom-precise clusters for catalysis

“…For example Black et al have recently investigated the use of high energy UV light to efficiently strip ligands from Au 25 (SR) 18 and Au 36 (SR) 24 clusters. 118 Ultraviolet photodissociation mass spectrometry measurements showed that single high energy ultraviolet pulses (l ¼ 193 nm) could cause extensive stripping of ligands off the staple motifs without removing any of the underlying Au atoms, while ve or more pulses resulted in the formation of Au 4 (SR) 4 tetramers and Au atom loss from the clusters. While this essentially is a pure mass spectrometry report, the use of high energy ultraviolet light pulses to remove ligands would be a novel concept towards activating clusters on supports.…”

The emerging interface of mass spectrometry with materials

“…While both CID and SID produces a lesser number of fragments, extensive fragmentation can be observed by ultraviolet photodissociation (UVPD) as observed in the case of Au 25 (pMBA) 18 and Au 36 (pMBA) 24 NCs 93 . UVPD on these clusters using λ = 193 nm involved high-energy fragmentation pathways and cleavage of Au-S and C-S bonds.…”