Metal Nanoparticles Functionalized with Molecular and Supramolecular Switches (original) (raw)

2009, Journal of The American Chemical Society

The advent of mechanically interlocked molecules (MIMs), such as catenanes and rotaxanes, 1 has contributed significantly to the development of tunable molecular architectures on scales from the molecular 2 to the nanoscopic. 3 Highly programmable and efficient syntheses of bistable catenanes and rotaxanes 1 employing templatedirected protocols 4 enable the introduction of bistability, 5 a prerequisite for the operation of molecular switches and machines 6 having potential uses in molecular electronic devices (MEDs), 7 molecular switch tunnel junctions (MSTJs), 8 mechanized mesoporous silica nanoparticles (MMSNPs) 9 for drug delivery, and nanoelectromechanical systems (NEMS). 10 Despite these achievements, the challenge that remains for these bistable molecules is their incorporation into integrated nanosystems. The first step in this direction is the conjugation of the MIMs with nanoscopic building blocks, such as metal nanoparticles (MNPs), which display a range of novel optical, 11 electronic, 12 catalytic, 13 and mechanical 14 properties. Traditional methods 15 of MNP functionalization are based on the use of thiolate ligands in a reducing environment; such conditions are not compatible with a wide range of functional groups 16 in molecules including bistable donor-acceptor catenanes and rotaxanes. 5 Herein, we describe an alternative strategy for the preparation of functionalized MNPs with a variety of cores (Au, Pt, Pd) using weakly protected MNP "precursors" and dithiolanes terminated in either redox-active MIM components, such as tetrathiafulvalene (TTF), or actual MIMs, such as a bistable [2]catenane. Characterization of these nanostructures by transmission electron microscopy (TEM), -potential measurements, and cyclic voltammetry (CV) confirms that the "switches" retain their activity when attached to the NPs. At the same time, the oxidation potentials of the adsorbed switches depend on and can be modulated (up to a factor of 2) by their surface fraction on the MNP surfaces. These experimental results suggest the presence of cooperative effects within the MIM monolayer and can be rationalized in terms of electrostatic arguments based on the Poisson-Boltzmann equation describing electric potentials around the switchable particles.