Exploring Collective Substituent Effects: Dependence of the Lifetime of Charged States of Au25(SCnH2n+1)18 Nanoclusters on the Length of the Thiolate Ligands

Abstract

AbstractElectrochemical methods have been often successfully employed to study substituent effects, a field pioneered by Petr Zuman. Here we used cyclic voltammetry to explore a new dimension of the effect of substituents on the lifetime of electrogenerated species. 18 thiolate ligands were collectively changed in molecular Au25(SCnH2n+1)18 nanoclusters by changing the number of carbon atoms forming the alkanethiolate linear chains: 2, 3, 4, 5, 6, 8, 10, and 12. Whereas on the voltammetric timescale the −1, 0, and +1 charge state of Au25 clusters are stable, formation of the −2 and +2 states causes fragmentation of the cluster. Voltammetric analysis indicates that the former entails reactions involving ligands in the Au12(SCnH2n+1)18 protecting shell, whereas the latter is accompanied by a slower but also more substantial fragmentation affecting the integrity of the central Au13 core. Whereas the lifetime of the dianion is ca. 3 ms and virtually unaffected by the length of the thiolate substituent, the lifetime of the dication increases from 0.11 to 5 s as the alkanethiolate chains are made longer. This difference in behavior is explained by the different effect exerted on the two types of fragmentation by the structure of the protecting monolayer.