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Novel functional tripodal monomolecular films

Subject Area Physical Chemistry of Molecules, Liquids and Interfaces, Biophysical Chemistry
Physical Chemistry of Solids and Surfaces, Material Characterisation
Term from 2019 to 2024
Project identifier Deutsche Forschungsgemeinschaft (DFG) - Project number 437355715
 
Final Report Year 2024

Final Report Abstract

When employing self-assembled monolayers (SAMs) for tuning surface and interface properties, organic molecules that enable strong binding to the substrate, large-area structural uniformity, precise alignment of functional groups, and control of their density are highly desirable. To achieve these goals, tripod systems bearing multiple bonding sites have been developed as an alternative to conventional monodentate systems. Bonding of all three sites has, however, hardly been achieved, with the consequence that structural uniformity and orientational order in tripodal SAMs are usually quite poor. Facing this challenge, we have established a new type of tripodal monomolecular films based on the triptycene unit within the given project. A large variety of the respective SAM-forming molecules, decorated with different anchoring and functional tail groups in the tripodal and bridgehead configurations, was custom-designed, synthesized (by our partners), and used for the SAM assembly. Homogeneous, tripodal-anchored SAMs could be successfully prepared on application-relevant, coinage metal (Au and Ag) and oxide (indium tin oxide) substrates. On Ag(111), these SAM exhibited distinct polymorphism, with a particular interesting chiral arrangement of the molecules in the honeycomb network in one of the structural phases. The decoration of the triptycene framework with nitrile tail groups allowed the monitoring of the charge transfer (CT) dynamics in these systems, with the particular emphasis on the relative weights of alternative CT pathways. This issue was further extended by design and characterization of a series of monopodal SAMs, allowing to address matrix effects in CT across monomolecular assemblies. In addition, nitrile-decorated triptycene SAMs provided a showcase for the work function tuning by tripodal SAMs – an issue important in context of organic electronics and photovoltaics. Introduction of ethynyl tail groups allowed to create tripodal monomolecular template suitable for click reaction with functional azide-decorated substituents. The on-surface fabrication of tailor-designed, functional tripodal SAMs become thus possible, without timeintensive synthesis of individual precursors in each particular case. Introduction of ferrocene tail group resulted in rectification behavior of the respective molecular assembly at the exceptionally low bias, with two distinctly different conductance states observed at specific bias sweeping modes in two-terminal molecular junctions. Introduction of fullerene tail groups provided important insights in the mechanisms of molecular assembly. Generally, applicability and broad versatility of triptycene-based, tripodal SAMs was demonstrated, opening new ways for further research and applications.

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