Niederhausen, J.; Zhang, Y.; Cheenicode Kabeer, F.; Garmshausen, Y.; Schmidt, B.; Li, Y.; Braun, K.; Hecht, S.; Tkatchenko, A.; Koch, N.; Hla, S.: Subtle Fluorination of Conjugated Molecules Enables Stable Nanoscale Assemblies on Metal Surfaces. The Journal of Physical Chemistry C 122 (2018), p. 18902-18911
10.1021/acs.jpcc.8b03398
Open Access Version
Abstract:
We elucidate the structure of assemblies formed by three conjugated molecules with very similar chemical structure, differing only by two fluorine atoms, on an Ag(111) surface to gain insight into the intricate interplay of attractive and repulsive interactions that govern the self-assembly of molecules on a metal surface. With scanning tunneling microscopy we observe substantially different self-assembled structures for sub-monolayers of parasexiphenyl (6P) and its two partially fluorinated derivatives ortho-2F-6P and meta-2F-6P (see Fig. 1), which we can fully rationalize only with state-of-the-art density functional theory modeling. By deliberate discriminating all involved interactions, that is, electrostatic due to permanent charges, polarization and dispersion, as well as hydrogen bonding, we can provide new insights for advanced self-assembly strategies. We demonstrate that fluorination at selected positions of conjugated molecules provides for sufficiently strong, yet nonrigid, H•••F bonding capability to circumvent dominating repulsive interactions at very low molecular surface coverage. This enables the realization and steering of individual and stable nanoscale molecular assemblies as well as their study, without the need to approach monolayer coverage, which could substantially alter the obtained structure. Furthermore, the insight provided here helps to understand how fluorine substitution in conjugated molecules and polymers contributes to thin film and bulk structures, which, in turn will enable realizing organic electronic materials with superior optical and charge-transport properties for electronic and optoelectronic applications.