Molecular-Level Insights into N–N π-Bond Rotation in the pH-Induced Interfacial Isomerization of 5-Octadecyloxy-2-(2-pyridylazo)phenol Monolayer Investigated by Sum Frequency Generation Vibrational Spectroscopy

Abstract

In-situ and real-time characterization of molecular structure of pH stimuli-responsive assembling systems at interfaces is critical to understand the nature of interfacial driving force and weak molecular interaction behind such reactions and provide important clues to control them in a desired manner. In this study, sum frequency generation vibrational spectroscopy (SFG-VS) has been applied, supplemented by surface pressure (π)–area (A) isotherm measurements, and Brewster angle microscopy images, to investigate the interfacial tautomerism and isomerization reactions occurring in 5-octadecyloxy-2-(2-pyridylazo)phenol (PARC18) monolayer at air/buffer solution interface in situ. The isomerization mechanism was examined by measuring interfacial structure of PARC18 molecule at various subphase pH. Time-dependent change of the SFG intensity of the characteristic band was kinetically measured after spreading PARC18 chloroform solution onto different subphase pH buffer solutions. It was found that hydrazone form prevails on the air/water interface in acidic and neutral conditions while azo form dominates at subphase pH ≥ 11.6. The hydrazone form adopts a planar geometry at pH = 4.5 and 7.0, whereas the azo form adopts a nonplanar cis or cis-like conformation. It was indicated that the trans–cis isomerization processes follow a rotation mechanism. The deprotonation rate constant was deduced to be 0.20–0.42 M–1 s–1 at pH = 10.3–12.6. This is the first reported application of SFG-VS to elucidate the isomerization mechanism and deduce the deprotonation rate constant of azoaromatic compounds at interface. Resulting from this study will aid in a better understanding of the interfacial pH-controlled assembly processes.