Realization of Self-Terminating Underpotential Electropolymerization by Strong Monomer-Electrode Interaction

In this report, we show that electropolymerization of 3,4-ethylenedioxythiophene (EDOT) can take place at 0.4 V, a voltage far lower than the conventionally employed voltage of 1.0 V, in a self-terminating manner, when gold is employed as a working electrode. DFT calculations revealed that the highest occupied molecular orbital (HOMO) of EDOT is elevated by approximately 1 eV when it directly interacts with the gold surface compared to that without such direct interaction. In other words, monomers become highly reactive for oxidative polymerization when they are directly adsorbed onto gold. Once the polymer forms and covers the electrode surface, the monomer can no longer interact with the electrode surface, thus the polymerization is terminated. Such selfterminating polymerization behavior was supported by in situ electrochemical surface-enhanced Raman spectroscopy (EC-SERS) and the current profile during the voltage scan. This is in sharp contrast to electropolymerization under ordinary high-voltage conditions, where the polymer film continuously grows. This study may provide solutions to control the polymerization on the solid surface, which is considered to be very important for adhesive, coatings, and solid electrolyte interphase (SEI) formation. It is also expected to lead to new synthetic methods via electropolymerization for monolayer polymers, which have recently attracted attention for their potential to exhibit various functions.