Polarization Effect and Electric Potential Changes in the Stimuli-Responsive Molecular Monolayers Under an External Electric Field

Under the exerted external electric field (E-ex), significant polarization effects are demonstrated in the switching process of positively charged oligopeptide chains I on the gold surface by using the polarizable force-field-based molecular dynamics (MD) simulations. The changes of electrostatic environment during the conformational switching are described by calling density functional theory (DFT) calculations of atomic charges or fragment-centered dipole moments. The charge-variable polarizable force field (Q-POL) model shows a good agreement with the experimental observations of both the open-circuit state (OC) without E-ex and extended/bent conformation (ON/OFF) with upward/downward E-ex, while the nonpolarizable force field (Non-POL) with the fixed atomic partial charges fails to reproduce experiments for the ON state. The charged oligopeptide chains (with each residue furnished with a positive charge) challenge the application of the coarse-graining polarizable force field based on the fragment dipoles (D-frag-POL). Through tracing the dynamics of charge centers of both counterions and oligopeptide chains in the switching process, a qualitative picture is depicted for understanding the polarization phenomena in the dielectric monolayer in OC/ON/OFF states. With the applied electric field, the rearrangement of ions leads to the induced internal E-field, E-in, in the direction opposite to E-ex. The values of total electric field, E-total, and the interfacial potential Phi(x) were evaluated. The relationship between the applied electric potential and the interfacial potential is built for the switching OC/ON/OFF states. The analysis of the induced internal E-field and interfacial potential may shed light on the further modification of theoretical models to better understand the electrical-induced switching mechanism.