Direct observation of the potential dependent protonation of 4-(Dimethylamino)pyridine on gold electrodes

The pyridine derivative, 4-(dimethylamino)pyridine (DMAP), is important in a surprisingly wide variety of applications spanning chemical, biological and technological domains. Empirically, the protonation state of DMAP and how this protonation state is changed with adsorption on a solid surface, is known to play a crucial role in many of these applications. However, the direct spectroscopic evidence for bias dependent DMAP (de)protonation at solid/aqueous interfaces is still very limited. Here, we employed surface specific vibrational sum frequency (VSF) spectroscopy to study the (de)protonation process of DMAP(H$^{+}$) on a polycrystalline gold electrode at neutral pH. Because VSF is interface specific by its symmetry selection rules, its application allows us to distinguish adsorbed molecules from those close to, but not at, the interface and identify their speciation and orientation with respect to the surface normal. We find that the potential-induced (de)protonation of flat-lying DMAPH$^{+}$ molecules occurs at relatively low potentials but that both species adsorb horizontally on the surface. A horizontal to vertical phase transition occurs only at more positive biases. The physical picture these results suggest confirms and extends that inferred previously from electrochemical measurements and offers a general approach for characterizing interface-induced acid/base chemistry and its relation to interfacial molecular structure.