Assembling Supramolecular Rotors on Surfaces Under Ambient Conditions

Abstract

The formation of supramolecular rotor-type systems on surfaces under ambient conditions is discussed. We present in detail the assembly of a rotor on a gold surface from solution, where the surface acts as a stator, a pyridyl containing thiol acts as the axle, and a C4 symmetrical porphyrin bearing “paddle-like” arms is the intended rotator. The effective non-covalent attachment of the rotator to the axle is achieved instantly in solution. However, when the axle forms part of a self-assembled monolayer (SAM), the complex formation is practically negated if the SAM’s organization is not appropriate. Thus, the SAM formed on gold by the pyridyl thiol subject of this study does not bind the rotator part of the system—a zinc(II)porphyrin bearing four biphenyl moieties. This negative allosteric surface effect can be overturned by combining the pyridyl thiol with dodecanethiol: the resulting mixed SAM contains pyridyl groups which are oriented quasi-perpendicular to the surface and are therefore available for binding. The difference in the surface organization is achieved using only 1 part in 10 of the alkanethiol (which forms small ordered domains alone). This dramatic effect may find use in other areas where SAMs are used as template layers because the capacity for the surface-anchored ligand to bind solution-borne compounds is affected by its orientation. Exposure of this mixed monolayer to a solution of the zinc(II)porphyrin results in attachment of this rotator component to the surface, which was imaged by scanning tunnelling microscopy (STM). Alternatively, formation of the axle–rotator complex in solution followed by chemisorption of this supramolecular object to the metal surface leads (in places) to a dense layer of the rotators. The presence of the porphyrin was confirmed by mass spectrometry. The results show how the bottom-up route employed can influence the arrangement of ligating moieties in a monolayer, provide a protocol for the preparation of sparse and dense layers of rotors on surfaces, and thereby help plot the road map to the bottom-up creation of surface-based molecular machines based on interconnected rotors.