Mechanistic studies of AFM probe-driven Suzuki and Heck molecular coupling

The initiation and high resolution control of surface confined chemical reactions would be both beneficial for nanofabrication and fundamentally interesting. In this work, mechanistic aspects of spatially confined Suzuki and Heck 'catalytic nanolithography' are investigated by varying the experimental parameters and noting their effects on apparent reactivity and lithographic yield. A variety of molecular couplings can be driven by the activated scanning probe with linewidths ultimately limited by its functional apex. On ramping the speed at which the probe traverses the surface confined reagent, one reaches a limit where the catalytic chemistry becomes limiting and reaction rates can be estimated. Self-assembled monolayer (SAM) confined bromide reagents are observed to be more labile than their iodide analogues in Suzuki couplings. With Heck reactions, coupling rates are observed to be faster with the styrene surface confined than in the 'inverted configuration' with an aryl halide SAM, possibly because the accepted (solution phase) rate-determining step is bypassed in the former configuration. For both Heck and Suzuki coupling reactions, calculations indicate that catalyst turnover numbers per nanoparticle are an order of magnitude or more greater within the confines of the AFM probe-surface junction compared to solution phase reactions. A PVP matrix model is presented to account for these observations and the mechanism of catalytic nanolithography.