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
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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 (<i>A</i>) 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<sup>–1</sup> s<sup>–1</sup> 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