Nonequilibrium Adsorption
and Reorientation Dynamics
of Molecules at Electrode/Electrolyte Interfaces Probed via Real-Time
Second Harmonic Generation
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Abstract
Nonequilibrium adsorption and subsequent reorientation
of organic
molecules at electrode/electrolyte interfaces are important steps
in electrochemical reactions and other interfacial processes, yet
real-time quantitative characterization and monitoring of these processes,
particularly for the reorientation step, are still challenging experimentally.
Herein, we investigated the nonequilibrium adsorption process of 4-(4-(diethylamino)styryl)-<i>N</i>-methyl-pyridinium iodide (D289) molecules from acetonitrile
solution onto a polycrystalline platinum electrode surface using real-time
second harmonic generation (SHG) in combination with the potential
step method. The time-dependent SHG curves exhibit two distinct regimes,
which were interpreted with a two-step adsorption model consisting
of a fast adsorption and a slow reorientation step for D289 on the
surface. D289 was assumed to initially adsorb in an orientation perpendicular
to the surface and then reorient to a parallel orientation. We derived
a quantitative mathematical expression containing a biexponential
function to fit the temporal SHG curves and obtain the rate constants
for the two steps. The rate constants for fast adsorption and the
slower reorientation processes show similar potential-dependent behavior:
the rate decreases with an increase in the negative potential. We
further proposed a molecular mechanism involving the displacement
of D289 and CH<sub>3</sub>CN molecules adsorbed on the electrode interface
to explain this potential-dependent behavior. On the basis of such
analysis, we obtained a detailed picture of the adsorption of D289
molecules on the Pt electrode/CH<sub>3</sub>CN electrolyte, which
consists of three consecutive steps: diffusion, adsorption, and reorientation.
The results of this study may shed light on adsorption mechanisms
at the electrode/electrolyte interface as well as at biological and
other functional material interfaces