Impact of Polarity on the Anisotropic Diffusion of Conjugated Organic Molecules on (10-10) Zinc Oxide Surface

We study the influence of polarity on the binding and diffusion of single conjugated organic molecules on the inorganic (10-10) zinc oxide surface by means of all-atom molecular dynamics simulations at room temperature and above. In particular, we consider the effects of partial fluorination of the para-sexiphenyl (p-6P) molecule with chemical modifications of one head group (p-6PF2) or both (symmetric) head and tail (p-6PF4). Quantum-mechanical and classical simulations both result in consistent and highly distinct dipole moments and densities of the fluorinated molecules, which interestingly lead to a weaker adhesion to the surface than for p-6P. The diffusion for all molecules is found to be normal and Arrhenius-like for long times. Strikingly, close to room temperature the polar molecules diffuse 1-2 orders of magnitudes slower compared to the p-6P reference in the apolar x-direction of the electrostatically heterogeneous surface, while in the polar y-direction they diffuse 1-2 orders of magnitude faster. We demonstrate that this rather unexpected behavior is governed by a subtle electrostatic anisotropic mismatch between the polar molecules and the chemically specific surface, as well as by increased entropic contributions coming from orientational and internal degrees of freedom.