Magic electret clusters of 4-fluorostyrene on metal surfaces

We report a combined experimental and theoretical study of the adsorption and assembly of a simple dipolar molecule, 4-fluorostyrene, on both Cu and Au surfaces. Self-assembly occurs in the form of small highly polar electrets with discrete ("magic") sizes that depend on the surface metal. Charge transfer between the molecule and surface results in a ∼90° reorientation of the electric dipole moment as compared to the gas-phase molecule and a doubling of its magnitude. The magic size can be understood in terms of a balance between attractive interactions in the form of both directional C-H⋯F hydrogen bonding and van der Waals interactions, as well as repulsive forces from Columbic interaction between the charged molecules. While this work illustrates the importance of interfacial charge transfer in molecular dipole engineering at surfaces, it offers unique chiral systems that are highly regular and dipolar with which to study and understand charge- and spin-transfer across metal-organic interfaces.