By functionalizing surfaces with a single, often highly ordered, layer of physisorbed or chemisorbed molecules, it becomes possible to modify their properties at the nanoscale using organic chemistry. Such self-assembled monolayers (SAMs) have been widely applied over the past 40 years: to protect reactive metal surfaces; probe fundamental electrochemical, charge transport and thermodynamic questions; alter metal work functions; or anchor proteins or catalysts. However, conventional approaches produce SAMs that exhibit severe limitations in terms of stability and/or function. Researchers around the world are exploring different strategies to address these issues, including the introduction of new surface-linker groups or the development of patterning techniques based on supramolecular interactions (hydrogen/halogen bonding, 𝜋-interactions).
Figure 1. A schematic of SAM formation. Molecules with appropriate structures and surface-binding groups can self-organize into periodic structures on metal surfaces.
OUR INITIAL RESEARCH DIRECTION(S):
Tuning surface-based chemical (catalytic) reactions. The functionality of chemical groups embedded in SAMs - such as chemical reactivity or photoswitching yields/rates - is often found to be severely attenuated compared to their behavior in solution. In many cases, this can be attributed to intermolecular steric hinderance from adjacent components in the surface-bound layer. Several groups have pursued different, typically redox-inactive, “platform”-based approaches to control the spacing and enhance the activity of functional elements in a SAM. We are working to develop new families of modular, redox-active platform molecules that are capable of forming well-mixed SAMs on metal surfaces. Among other possible advantages, electroactive components provide invaluable additional handles for SAM characterization using surface voltammetry or electrochemical scanning tunneling microscopy (see “other useful references” (1), below). One goal of our research is to leverage such platform-based SAMs to explore new strategies for tuning the reactivity, selectivity, and stability of surface-bound homogenous catalysts by controlling their local nano-environment (Fig. 2).
Figure 2. A schematic model of a multicomponent well-mixed SAM, with each cube representing a different surface-bound molecule. If the purple sites are functionalized with a homogeneous catalyst, their nano-environment can be modulated by functional groups appended to green/yellow sites.
Key techniques (* = project dependent):
Solution and surface voltammetry
X-ray photoelectron spectroscopy
Electrochemical quartz crystal microbalance*
Scanning tunnelling microscopy*
Air-free chemistry (glovebox, Schlenk line)*
Single-crystal X-ray diffraction*
Other useful references:
(1) N.J. Tao, "Probing Potential-Tuned Resonant Tunneling through Redox Molecules with Scanning Tunneling Microscopy" Phys. Rev. Lett., 1996, 76, 4066–4069. (2) J.C. Love, L.A. Estroff, J.K. Kriebel, R.G. Nuzzo, G.M. Whitesides, "Self-Assembled Monolayers of Thiolates on Metals as a Form of Nanotechnology" Chem. Rev., 2005, 105, 1103–1170. (3) B. Baisch, D. Raffa, U. Jung, O.M. Magnussen, C. Nicolas, J. Lacour, J. Kubitschke, R. Herges, "Mounting freestanding molecular functions onto surfaces: The platform approach" J. Am. Chem. Soc., 2009, 131, 442–443. (4) M. Valášek, M. Mayor, "Spatial and Lateral Control of Functionality by Rigid Molecular Platforms" Chem. Eur. J., 2017, 23, 13538–13548.