Mixed monolayers formed from methyldisulfide-functionalized metal bis(terpyridine) complexes

Multicomponent self-assembled monolayers (SAMs) often suffer from segregation of like components into nanoscale domains. One strategy to form perfectly mixed SAMs is to use a modular series of molecules with similar structures and dimensions, ensuring comparable formation kinetics as well as surface and intermolecular interactions. To help evaluate this concept, we prepared a new family of methyldisulfide-functionalized metal bis(terpyridine) complexes, [M(tpySSMe)2](PF6)2 (M = Fe, Co, Zn), and characterized their single- and multi-component SAMs on gold surfaces. We find these complexes are air-stable in solution for >7 d, in stark contrast to their thiol-functionalized analogues, [M(tpySH)2](PF6)2 (MSH, M = Fe, Co), which decompose in <1 d. Though CoSH has been previously reported, we explicitly detail its synthesis and characterization here for the first time. Solution and surface voltammetry experiments show these complexes can indeed form SAMs with compositions representative of their assembly solutions, a prerequisite for perfect mixing at the molecular scale. However, we find MSS and CoSS SAMs generally exhibit remarkably low saturation surface coverages. This observation agrees with earlier SAM studies involving different polypyridyl-based metal complexes, and is typically attributed to repulsive Coulomb interactions between charged complexes on the surface. Despite their apparent loose packing, we show that purportedly vacant surface sites in MSS SAMs are not readily accessible for coordination by uncharged thiols in solution. X-ray photoelectron spectroscopy studies of CoSS SAMs further reveal clear S 2p (primarily of thiolate character) and N 1s features, but show no F 1s signal attributable to PF6− counterions (either due to their absence or the low surface coverage of MSS). This study raises important new questions regarding the assembly mechanism and nanoscale structure of these and other, structurally-related, charged SAMs, also serving to highlight key challenges in the design and characterization of emerging multi-component monolayer assemblies.