The ligand shell structure of the aspicule Ag44(4-MBA)30 (MBA: mercaptobenzoic acid, in the thiolate form) was modified in a precise, site-specific manner. Laser irradiation at 633 nm of a monolayer assembly of plasmonic Ag nanoparticles (NPs) covered with Ag44(4-MBA)30 clusters leads to decarboxylation of 4-MBA ligands forming thiophenolate (TP) ligands. While the molecular identity and integrity of aspicules post laser irradiation were confirmed by ESI MS, time-dependent SERS spectra and computational studies suggest that the phenomenon of decarboxylation is limited to the 4-MBA ligands facing the NP surface. This creates modified Ag44 clusters, with 4-MBA ligands on one side and TP ligands on the other, giving them a two-faced (Janus) ligand structure. The ligand distribution of such clusters gets equilibrated in solution. We show that such selective transformation can be used to create molecular patterns. Janus clusters may be important in chemistry at biphasic interfaces. © 2016 American Chemical Society.