Caffeine is the most commonly used drug and has a wide range of real world applications. Besides its effect on the human central nervous system, it is a corrosion inhibitor for copper[1] and can be used to improve the performance and thermal stability of perovskite solar cells [2].
As a crystal, caffeine molecules show a polymorphic behavior with an α- and β-phase. Furthermore, different orientations were found in surface induced phases [3].
A reason for this is the asymmetric and achiral character of the caffeine molecules which leads to two on-surface chiralities. These chiral alignments have an impact on the on-surface formation.
In order to investigate these effects, the structural formation of caffeine monolayers on a Au(111)-surface is analyzed by scanning tunneling microscopy (STM), low energy electron diffraction (LEED), x-ray photoelectron spectroscopy (XPS), and density functional theory (DFT) calculations. The monolayers are prepared by molecular beam epitaxy (MBE) and are studied at various temperatures.
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left: STM images of two domains of caffeine molecules on a Au(111) surface. right: DFT simulation of the investigated monolayer film. M. G. H. Schulte et al., PRB 101, 245414 (2020). |
[1] F. S. Souza et al., Mater. Sci. Eng. C 32 2436–2444 (2012)
[2] R. Wang et al., Joule 3 1-14 (2019)
[3] C. Röthel et al., CrystEngComm 19 2936-2945 (2017)
Theobromine molecules and related compounds such as caffeine and theophylline are of importance for pharmacology, toxicology, and biochemistry. In this field it is fundamental to study the emergence of chirality. Chirality can emerge due to surface adsorption. Therefore, it is necessary to study the chiral adsorption of theobromine on the surface. We studied theobromine monolayers on Au(111) and on few layer graphene on 6H-SiC(0001). The self-assembly of theobromine is characterized utilizing STM and LEED.
We found that the adsorbate unit cell is rectangular and consists of four molecules. Theobromine shows geometric similar unit cells on both substrates. On Au(111), the molecules are prochiral, with two enantiomers in a zig-zag structure, which is explained by glide reflections.
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left: STM image of theobromine on Au(111) showing a zig-zag arrangement. right: Structural model showing the orientations of the theobromine molecules with respect to the Au(111) surface. Ismail Baltaci et al., The Journal of Physical Chemistry C Article ASAP DOI: 10.1021/acs.jpcc.0c06138 . |
