Localization vs Conduction: Anionic Excitations in Alkanethiol Self-Assembled Monolayers

Abstract

Low-energy (6-11 eV) electron injection into Xe-coated self-assembled alkanethiol monolayers (SAMs) is reported. At most energies, the presence of the Xe film has negligible effect on the incident electrons, which penetrate the overlayer and induce significant C--H bond rupture at the terminal methyl sites and the subsurface methylene sites of the organic substrates. However, irradiation at 7.7 +/- 0.2 eV can lead to resonant electronic excitations of the Xe adsorbates to create transient anionic states in the Xe overlayer. Transfer of anionic excitations from the Xe overlayer to the SAM initially prepares excited anionic states at the terminal CH3 groups and leads to highly selective dissociations at the methyl sites, with negligible conduction along the alkane chain which would lead to subsurface C-H bond rupture at the methylene sites. These results demonstrate that the mobility of electronically excited charged states along the alkanethiol chains is significantly less than that of simple excess electrons and that highly site-selective chemical modifications can be induced by low-energy electrons in these highly homogeneous organic films.