Chain-Length- and Concentration-Dependent Isomerization of Bithiophenyl-Based Diaminotriazine Derivatives in Two-Dimensional Polymorphic Self-Assembly§

Two-Dimensional Self-Assembly of BODIPY Derivatives with Different Functional Groups at the Liquid-Solid Interface

The 4,4-difluoro-boradiazaindacene (BODIPY) unit possesses a rigid aromatic backbone, which facilitates the formation of self-assemblies and aggregates. However, most current studies on the self-assemblies of BODIPY derivatives have relied on spectroscopic methods to indirectly gather information about the self-assembled structures. In this study, we presented three BODIPY derivatives (B-3OC12, B-3OC12-2I, and B-DOB-2OC12) that shared the same core but were decorated with different functional groups. The self-assembled structures were revealed using scanning tunneling microscopy (STM) in combination with density functional theory (DFT). The results showed that all the molecules self-assembled into lamellar structures. When modified with three dodecyloxy chains or with the introduction of additional halogen atoms, the B-3OC12 and B-3OC12-2I molecules tended to distribute in a staggered form to build a tetramer or dimer. In contrast, the B-DOB-2OC12 molecule, which contains a dioxaborole group, self-assembled in a head-to-tail manner. These results demonstrated that BODIPY derivatives self-assembled into different structures, depending on their distinct patterns of intermolecular interactions influenced by functional groups.

Identifying Target Molecule and Trace Amount of the Byproduct by Two-Dimensional Self-Assembly with Different Solution Concentrations

Scanning tunneling microscopy (STM) is a powerful way to realize the recognition of self-assembled nanostructures on the atomic scale. In this article, dihexadecyl 6,9-bis((4-(hexadecyloxy)phenyl)ethynyl) phenanthro[9,10-c]thiophene-1,3-dicarboxylate (D-PT) and dihexadecyl 6-bromo-9-((4-(hexadecyloxy) phenyl)ethynyl)phenanthrol[9,10-c]thiophene-1,3-dicarboxylate (S-BrPT) with different substituents were chosen as the target system. D-PT with four side chains as the target molecule and S-BrPT with three side chains and a bromine substituent as the byproduct were mixed in a molar concentration ratio of 20:1. The effect of solution concentration on the molecular self-assembly of the mixture was investigated by STM at the hexadecane/HOPG interface. At high concentrations, only D-PT molecules formed a dimer pattern resulting from the intermolecular van der Waals force and self-adaption. Further diluting the solution, D-PT formed the coexisting dimer and linear structures, in which the linear pattern was formed via solvent coadsorption. At low concentrations, S-BrPT molecules forming N-shaped dimers appeared and filled the linear structure fabricated by D-PT molecules. With further decrease in the concentration, S-BrPT molecules formed N-shaped dimers covering almost half of the surface area, resulting from the C-Br···π and Br···H-C bonds. At very low concentrations, S-BrPT molecules formed N-shaped dimers to arrange the matrix architecture due to the coadsorption of more hexadecane molecules. Density functional theory (DFT) calculations demonstrated that the stronger intermolecular C-Br···π and Br···H-C bonds were significant factors in determining the formation of N-shaped dimers and the stability of this nanostructure. This work enriches the diversity of self-assembled motifs and provides a strategy to characterize different symmetric molecules with trace amounts in a mixed system by STM.

Breaking Radial Dipole Symmetry in Planar Macrocycles Modulates Edge-to-Edge Packing and Disrupts Cofacial Stacking

Dipolar interactions are ever-present in supramolecular architectures, though their impact is typically revealed by making dipoles stronger. While it is also possible to assess the role of dipoles by altering their orientations by using synthetic design, doing so without altering the molecular shape is not straightforward. We have now done this by flipping one triazole unit in a rigid macrocycle, tricarb. The macrocycle is composed of three carbazoles (2 Debye) and three triazoles (5 Debye) defining an array of dipoles aligned radially but organized alternately in and out. These dipoles are believed to dictate edge-to-edge tiling and face-to-face stacking. We modified our synthesis to prepare isosteric macrocycles with the orientation of one triazole dipole rotated 40°. The new dipole orientation guides edge-to-edge contacts to reorder the stability of two surface-bound 2D polymorphs. The impact on dipole-enhanced π stacking, however, was unexpected. Our stacking model identified an unchanged set of short-range (3.4 Å) anti-parallel dipole contacts. Despite this situation, the reduction in self-association was attributed to long-range (~6.4 Å) dipolar repulsions between π-stacked macrocycles. This work highlights our ability to control the build-up and symmetry of macrocyclic skeletons by synthetic design, and the work needed to further our understanding of how dipoles control self-assembly.

Solvent Effects Used for Optimal Simultaneous Analysis of Amino Acids via 19F NMR Spectroscopy

¹⁹F NMR has been extensively used in simultaneous analysis of multicomponent due to its 100% natural isotope abundance, high NMR-sensitivity, and wide-range chemical shifts. The solvent effects are usually observed in NMR spectroscopy and cause large changes in ¹⁹F chemical shifts. Herein, we propose that the simultaneous analysis of a complex mixture can be achieved using solvent effects via ¹⁹F NMR spectroscopy, such as a mixture solution of amino acids (AAs). AAs are not only cell-signaling molecules, but are also considered as biomarkers of some diseases. Hence, the analysis of AAs is important for human health and the diagnosis of diseases. In this work, the key to the success of sensing 19 biogenic AAs is the use of 2-fluorobenzaldehyde (2FBA) as a highly sensitive derivatizing agent and solvent effects to produce distinguishable ¹⁹F NMR signals. As a result, the resolution of ¹⁹F NMR spectroscopy of multiple 2FBA-labeled AAs is obviously higher than other methods based on ¹⁹F NMR. Moreover, 14 and 18 AAs can be satisfactorily differentiated and unambiguously identified in different complicated media supporting the growth of mammalian cells. Furthermore, quantification of the concentration of AAs can be made, and the limit of detection reaches 10 μM. Our work provides new insights into the simultaneous analysis of a multicomponent mixture based on solvent effects by ¹⁹F NMR spectroscopy.

Surface supramolecular assemblies tailored by chemical/physical and synergistic stimuli: a scanning tunneling microscopy study

Supramolecular self-assemblies formed by various non-covalent interactions can produce diverse functional networks on solid surfaces. These networks have recently attracted much interest from both fundamental and application points of view. Unlike covalent organic frameworks (COFs), the properties of the assemblies differ from each other depending on the constituent motifs. These various motifs may find diverse applications such as in crystal engineering, surface modification, and molecular electronics. Significantly, these interactions between/among the molecular tectonics are relatively weak and reversible, which makes them responsive to external stimuli. Moreover, for a liquid-solid-interface environment, the dynamic processes are amenable to in situ observation using scanning tunneling microscopy (STM). In the literature, most review articles focus on supramolecular self-assembly interactions. This review summarizes the recent literature in which stimulation sources, including chemical, physical, and their combined stimuli, cooperatively tailor supramolecular assemblies on surfaces. The appropriate design and synthesis of functional molecules that can be integrated on different surfaces permits the use of nanostructured materials and devices for bottom-up nanotechnology. Finally, we discuss synergic effect on materials science.