SERS of the Positive Charge Bearing Pyridinium Ring Terminated Self-Assembled Monolayers: Structure and Bonding Spectral Markers

Abstract

Bifunctional self-assembled monolayers (SAMs) are widely used for construction of surfaces with desirable properties. To predict and control the function of SAMs, molecular level understanding of monolayer architecture is required. In this work, structure and bonding of positive charge bearing SAM of N-(6-mercapto)hexylpyridinium (MHP) on Au and Ag electrodes was probed by SERS, first-principles calculations, isotopic substitution, and reductive desorption voltammetry. Based on analysis of immersion time- and temperature-dependent SERS spectra as well as DFT calculations, the marker bands for MHP structure, bonding, and orientation have been established. The presence of a band around 1083 cm–1 was found to be a reliable SERS marker for the all-trans conformation of hydrocarbon chain. Soft C–H stretching mode near 2832 cm–1 was recognized as a marker for the probing of direct interaction of alkyl chain with the metal. Based on solvent-dependent Raman spectra the pyridinium ring ═C–H band (ν2) was proposed ...