Local Conformational Switching of Supramolecular Networks at the Solid/Liquid Interface

FERNANDO P. COMETTO; KLAUS KERN; MAGALÍ A. LINGENFELDER

Villa de Merlo, San Luis

Congreso; 100ª Reunión Nacional de la Asociación Física Argentina; 2015

Self-assembled monolayers (SAMs) on surfaces constitute the basis for molecular nanodevices. The tuning of charge transport properties is crucial for molecular electronics and is critically determined by conformation dependent molecule-surface and molecule-molecule interactions. Ultimately, most applications require the possibility of controlling molecular conformation in a predictable way. Thus, different external stimuli such as light and temperature have been explored to gain control over the morphology of supramolecular layers. Due to the strong intermolecular interactions that need to be reconfigured to change the intermolecular connectivity landscape, it has been difficult to obtain reversible conformational switching in SAMs by means of electric fields. Accordingly, the systems studied so far included either metal-organic complexes with high intrinsic dipolar moments that can flip aligning to an external field or ionized species being adsorbed/desorbed or restructured from the solution in response to a charged interface. Specially challenging is the possibility to find robust and reproducible systems that can keep the same response and reversibility after numerous switching cycles.In this work, we use the electric field in a scanning tunneling microscope to manipulate the transition between open and close packed 2D supramolecular networks of neutral molecules in nonpolar media. We found that while the magnitude of the applied field is not decisive, it is the sign of the polarization that needs to be maintained to select one particular polymorph. Moreover, the switching is independent of the solvent used and fully reversible. We propose that the orientation of the surface dipole determined by the electric field might favor different conformational-depended charge transfer mechanisms of the adsorbates to the surface, inducing open (closed) structures for negative (positive) potentials. Our results show new avenues for the use of local fields to select the polymorphic outcome of supramolecular assemblies at the solid/liquid interface. The effect has potential to control locally the capture and release of analytes in host-guest systems and the 2D morphology in multicomponent layers.The presented tunable morphology could be used in functional molecular architectures in which nanoporous could act as a target site for catalysis, controllable drug release, host-guest chemistry, etc. and this capability could be switch on/off in-situ. Moreover we are currently using this effect to investigate tunable rearrangements in mixed phases with planar and non-planar adsorbates that respond distinctly to the orientation of the surface dipole, allowing local control of the partial interfacial concentration. In a more general context, we expect this work to draw attention into the ?non-innocent? role of the sample polarity in determining the morphological outcome of supramolecular adlayers, even for neutral species in nonpolar media.