The self-assembled behaviors of two fluorenone derivatives, 2,7-bis((11-hydroxyundecyl)oxy)-9-fluorenone (BHUF) and 2,7-bis((10-carboxydecyl)oxy)-9-fluorenone (BCDF), were investigated at the liquid–solid interface by scanning tunneling microscopy. Two solvents, 1-octanoic acid and 1-phenyloctane, were employed in consideration of their distinct polarity and solubility. It is observed that the BHUF molecules self-assemble into seven different polymorphs upon adsorption, while only two different polymorphs are observed in the BCDF monolayer. The theoretical calculation is performed to reveal the underlying mechanism. As compared to that of CO···HO hydrogen bonds, the enhanced binding energy of intermolecular CO···HOOC hydrogen bonds in the BCDF monolayer would dominate the intermolecular van der Waals (vdWs) interactions and the molecule–solvent interactions, thereby resulting in a limitation of expression of structural polymorphism. In addition, the concentration-dependent polymorphism as well as the relative phase transition is discussed in terms of the stability and packing density of different polymorphs. Furthermore, the different self-assembled behaviors of BHUF molecules in these two solvents at lower concentrations are associated with the different energy gain upon solvent coadsorption. The investigation provides a simple and alternative strategy to construct the structural polymorphs by utilizing multiple hydrogen bonds at the liquid–solid interface.
Herein, the photophysical properties of two π-conjugated thienophenanthrene derivatives (6,9- and 5,10-DBTD) are reported. Their self-assembled monolayers in aliphatic hydrocarbon solvents under different concentrations were investigated by scanning tunneling microscopy on a graphite surface. The STM results revealed that the self-assembled structures of the two geometrical isomers exhibited absolutely different behaviors. At the aliphatic solvent/graphite interface, 6,9-DBTD produced almost a single stable coassembled linear structure, except for that with n-tridecane as the solvent. However, the self-assembly of 5,10-DBTD showed structural diversity, and it presented a gradient variety through increasing the chain length of the aliphatic solvents as well as the solution concentration. All ordered self-assembled adlayers critically depend on not only the interchain van der Waals (vdW) interactions, but also on multiple intermolecular interactions, including BrO[double bond, length as m-dash]C and BrS hetero-halogen bonds, homo-BrBr interactions, and HBr and HO hydrogen bonds. We proposed that the cooperation and competition of the intermolecular interactions involving a Br atom and interchain vdW forces induce this structural variety. Density functional theory calculations support to unravel the different elementary structural units based on halogen bonds and hydrogen bonds and were useful tools to dissect and explain the formation mechanism.
Two-dimensional supramolecular assemblies of a series of 2,7-bis(10-n-alkoxycarbonyl-decyloxy)-9-fluorenone derivatives (BAF-Cn, n = 1, 3-6) consisting of polar fluorenone moieties and ester alkoxy chains were investigated by scanning tunneling microscopy on highly oriented pyrolytic graphite surfaces. The chain-length effect was observed in the self-assembly of BAF-Cn. Self-assembly of BAF-C1 was composed of a linear I pattern, where the side chains adopted a fully interdigitated arrangement. As the length of side chains increased, the coexistence of a linear I pattern and a cyclic pattern for the self-assembly of BAF-C3 was observed. Upon increasing the length of the alkoxy chain even further (n = 4-6), another linear II structure was observed in the BAF-Cn monolayer, in which the side chains in adjacent rows were arranged in a tail-to-tail configuration. It is reasonable to conclude that not only the van der Waals forces but also the dipole-dipole interactions from both the fluorenone cores and the ester alkoxy chains play critical roles in the self-assemblies of BAF-Cn. Our work provides detailed insights into the effect of intermolecular dipole-dipole and van der Waals interactions on the monolayer morphology of fluorenone derivatives.
The organization of molecular motors in supramolecular assemblies to allow the amplification and transmission of motion and collective action is an important step toward future responsive systems. Metal-coordination-driven directional self-assembly into supramolecular metallacycles provides a powerful strategy to position several motor units in larger structures with well-defined geometries. Herein, we present a pyridyl-modified molecular motor ligand (MPY) which upon coordination with geometrically distinct di-Pt(II) acceptors assembles into discrete metallacycles of different sizes and shapes. This coordination leads to a red-shift of the absorption bands of molecular motors, making these motorized metallacycles responsive to visible light. Photochemical and thermal isomerization experiments demonstrated that the light-driven rotation of the motors in the metallacycles is similar to that in free MPY in solution. CD studies show that the helicity inversions associated with each isomerization step in the rotary cycle are preserved. To explore collective motion, the trimeric motor-containing metallacycle was aggregated with heparin through multiple electrostatic interactions, to construct a multi-component hierarchical system. SEM, TEM, and DLS measurements revealed that the photo- and thermal-responsive molecular motor units enabled selective manipulation of the secondary supramolecular aggregation process without dissociating the primary metallacycle structures. These visible-light-responsive metallacycles, with intrinsic multiple rotary motors, offer prospects for cooperative operations, dynamic hierarchical self-assembled systems, and adaptive materials.
Halogen bonding with high specificity and directionality in the geometry has proven to be an important type of noncovalent interaction to fabricate and control 2D molecular architectures on surfaces. Herein, we first report how the orientation of the ester substituent for thienophenanthrene derivatives (5,10-DBTD and 5,10-DITD) affects positive charge distribution of halogens by density functional theory, thus determining the formation of an intermolecular halogen bond and different self-assembled patterns by scanning tunneling microscopy. The system presented here mainly includes heterohalogen X···O═C and X···S halogen bonds, H···Br and H···O hydrogen bonds, and I···I interaction, where the directionality and strength of such weak bonds determine the molecular arrangement by varying the halogen substituent. This study provides a detailed understanding of the role of ester orientation, concentration, and solvent effects on the formation of halogen bonds and proves relevant for identification of multiple halogen bonding in supramolecular chemistry.
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