Probing a Reactive Alkyne Center Aligned via a Triphenylmethane Tripod on Au(111) for Electric Field-Induced...

Electric field-induced chemistry at the solid/vacuum interface has distinct advantages over enzyme- and electrocatalysis because chemical reactivity is solely governed by external electrical fields (EEFs), while other factors such as solvent and ion effects are ruled out. Intense EEFs on the order of 10⁹ V m^-1 are generated between the metal tip of a scanning tunneling microscope (STM) and a single-crystalline metal surface. Reactive centers can be aligned along the direction of the EEF, that is, orthogonal to the metal surface, for experiencing the maximum field. Here, a strategy is introduced to achieve this: a terminal alkyne as a reactive center is connected to the sp³-hybridized core of a triphenylmethane-based rigid molecular tripod chemisorbed on Au(111), while its upright configuration is probed experimentally by low-temperature (LT) ultrahigh-vacuum (UHV) single-molecule STM and STM-based tip-enhanced Raman scattering (TERS) in conjunction with density functional theory (DFT). The orientation of the alkyne moiety relative to the surface normal is inferred from STM height profiles and particularly the Raman intensity of the dominant C≡C stretching peak. The upright configuration and stability of the alkyne reactive center in the presence of intense EEFs paves the way for future E-field-induced chemistry monitored in situ by TERS.