NSF-DFG MISSION: In Operando Nonlinear Optical Spectroscopy at Solid/Liquid Interfaces: Driving Surface Wettability with Light and Electric Field Gradients

• Applicants: Professor Dr. Björn Braunschweig; Professor Dr. Bart Jan Ravoo
• Subject Area: Physical Chemistry of Molecules, Liquids and Interfaces, Biophysical Chemistry ; Organic Molecular Chemistry - Synthesis and Characterisation
• Term: since 2024
• Project identifier: Deutsche Forschungsgemeinschaft (DFG) - Project number 542225359

The ability to control the molecular orientation of functional groups and their charge states on solid surfaces holds the promise of access to surface properties that can be dynamically switched on or off using external stimuli such as light or applied potentials. As a means of example, consider the wetting of a bathroom mirror surface under normal versus high relative humidity: as the mirror fogs up, its reflectivity drastically changes, and as it clears up it changes back to the starting condition. Indeed, the wetting of solid surfaces by liquids is known from everyday experience and is surprisingly easy to observe. Yet, changing surface properties in general with spatially and temporally directed external stimuli (light, applied potential) in a controlled and reversible way is a formidable challenge. This proposal will test the hypothesis that the advancement of switchable surfaces can be accelerated by quantifying, under operando conditions, the "on" and "off" states of a molecular switch in terms of the following fundamental molecular properties: a) Functional group orientation distributions; b) Switching kinetics and mechanism (collective vs. defect driven) c) Total surface potentials and fields; and d) Hydrogen-bond network strength. Using operando nonlinear optical spectroscopy the plan is to obtain these 3 missing pieces of molecular scale information as a function of the following stimuli: a) light intensity and wavelength; b) applied electrode potential; c) ionic strength and relative humidity, while tailoring the following structural and molecular properties of the switchable nanolayers: a) surface switch density and surface patterning; b) molecular identity of the terminal group. Testing the central hypothesis is important because it would represent a significant advance towards a fundamental understanding of these fascinating interfacial systems. The results from in the proposed collaboration will 1) improve the understanding of surface wetting beyond macroscopic descriptors like contact angles, 2) eventually enable a better control and prediction of the wettability of switchable surfaces, and 3) generally contribute to getting the field closer to controlled droplet motion. Molecular/macroscopic insights into the function of molecular switches would be a powerful tool to eventually direct the flow of liquids locally on surfaces of, say, microfluidic systems or microreactors where the reaction path is controlled by the way the liquid wets the surface, or to turn on heterogeneous catalytic reactions with spatio-temporal control.

• DFG Programme: Research Grants
• International Connection: USA
• Cooperation Partner: Professor Dr. Franz M. Geiger