Molecular self-assembly is currently one of the most commonly used methods for surface fabrication of nanostructures. The next frontier in fabricating molecular nanodevices is the ability to dynamically control the structure-function properties of nanostructures on demand, and the reversible formation of hydrogen bonds is a prevalent mechanism for controlling molecular assembly in biological systems. And two-dimensional supramolecular networks based on non-covalent interactions (e.g., H-bonding) build well-organized and potentially switchable engineered nanostructures. However, achieving predictable reversibility in artificial two-dimensional (2D) materials remains a major challenge.
In recent times, Prof. Fernando P. Cometto and his team from the Laboratory of Molecular Nanoscience and IPHYS at the Max-Planck-Ecole Polytechnique Fédérale de Lausanne (EPFL) have been able to use scanning tunneling microscopy to trigger switching of hydrogen-bonded 2D networks using an external electric field (EEF) at the solid/liquid interface. The switching effect of the external electric field was analyzed by systematically varying the molecule-to-molecule interactions (i.e., hydrogen bond strengths) as well as the molecule-to-substrate interactions with the aid of density functional theory and molecular dynamics simulations. By tuning the hydrogen bonding capacity of the building blocks as well as the substrate properties and charges, the switching properties were induced or frozen and the final polymorphic output of the 2D network was controlled. The aim is to provide insights into the design and control of reversible molecular assemblies in 2D materials, with promising applications in a wide range of fields such as sensors and electronics.
The results show that the switching effect is not intrinsic but related to the overall energetic perspective of the adsorbent/substrate system under different experimental conditions. This experiment discusses the importance of intermolecular H-bonding interactions for the switching mechanism by replacing -COOH groups (BTB to C3-Ald) with -CHO groups. This functional group modification weakens the intermolecular H-bonds and prevents the expression of CHO-based porous patterns, thereby inhibiting the switch from stable close-packed structures to potentially unstable porous networks. The team also found that the formation of polarizable species correlates with the dynamics of carboxylate rotation and proton exchange, a dynamic situation influenced by an applied electric field that promotes the switching of the molecule to the most stable structure based on surface polarity.
The reaction kinetics starts with several H exchange processes between the dimer-COOH groups and the rotation of the carboxyl groups occurring
In summary this work provides insights into the design and control of reversible molecular assemblies in two-dimensional materials, exploiting their potential applications in a wide range of fields including sensors and electronics.
