RESUMEN
Self-assemblies of two fluorenone-based derivatives (FE and FEC) consisting of a central 2,7-diphenyl-9-fluorenone polar moiety but differing in the flexible terminal groups were investigated by scanning tunneling microscopy (STM) at the 1-octanoic acid/HOPG interface under different concentrations and density functional theory calculation (DFT). STM results reveal a concentration-dependent polymorphic self-assembly behavior for FE, but without the presence of co-adsorbed solvents. As the concentration decreases, the dimer, bracket-like, and ribbon-like self-assembled structures were observed. On the contrary, FEC molecules assemble into only a type of oval-shaped morphology by the intermolecular N···H-O hydrogen bonds with the solvent molecules. Combined with DFT calculations, it can be deduced that the intermolecular van der Waals forces, dipole-dipole interactions, and hydrogen bonding are the main driving forces to stabilize the molecular packing of fluorenone-based polycatenars with strong polarity. Our work is of significance at the molecular level to further clarify the intermolecular interactions and conformational effects on the formation of molecular packing structures with liquid crystal property.
RESUMEN
Halogen bonding (XB) is of great importance in fabricating a two-dimensional (2D) self-assembly for its adaptive directionality. However, the XBs involving fluorine (F) have barely been studied due to the absence of an σ-hole on F. Here, 2D self-assemblies of a F-substituted 4,7-bis(5-bromo-4-dodecylthiophen-2-yl)-5,6-difluorobenzo[c][1,2,5]thiadiazole (BTZ-BrF) molecule on graphite were investigated using scanning tunneling microscopy (STM) and density functional theory (DFT) calculations. STM experiments revealed that the 2D patterns of BTZ-BrF had a clear solvent and concentration dependence, showing a frame-like pattern in aliphatic acid and aliphatic hydrocarbon solvents at high concentrations. At lower concentrations, a bamboo-like and a wave-like pattern were observed in aliphatic acid, whereas small frame-like and large ladder-like domains at high solution concentrations in aliphatic hydrocarbon were observed. As the concentration further decreased, two linear patterns were observed. DFT calculations suggested that the hetero-XBs of F···Br, F···S, Br···S, and Br···N, the homo-XBs of type-II Br···Br, and the S···S interactions synergistically directed and stabilized the polymorphic 2D architectures. This understanding of intermolecular XBs during the molecular assembly at the molecular level may shed light on the ongoing efforts of regulating nanostructures of multifunctional organics.
RESUMEN
Fabricating tetraphenylethylene (TPE)-functionalized metal-organic frameworks (MOFs) with aggregation- induced emission on surfaces and understanding the growth mechanism have not yet been pursued. Herein, MOFs constructed via the Ullmann-type reaction of a C2-symmetry TPE derivative (p-BrTBE) on Au(111) and Cu(111) surfaces were thoroughly investigated using scanning tunneling microscopy. On a Au(111) surface, p-BrTBE molecules formed the self-assembled pattern at 298 K. Stepwise annealing led to a progressive evolution process, in which the stepwise debromination reaction led to organometallic intermediates, and surface-stabilized radicals and metal-organic networks were formed. By contrast, the relatively ordered MOFs were obtained by replacing the underlying substrate with the more catalytically active Cu(111) at 298 K. Density functional theory calculations demonstrated that the formation of different networks on Au(111) and Cu(111) was determined by the different conformations of the TBE unit on the different substrates due to the different adsorption energy.
RESUMEN
The rational synthesis of thiophene-based cross-coupled polymers on surfaces has been attracting more attention recently. Here, we report the stepwise activation of 5,5'-(2,3-difluoro-1,4-phenylene)bis(2-bromothiophene) as a precursor to synthesize thiophene-based polymeric ribbons on the Au(111) surface. Scanning tunneling microscopy studies showed that the precursor adopted different conformations in the self-assembled structure, organometallic species, and covalent polymers. On annealing the sample at a relatively low temperature (150 °C), the conversion of the organometallic structure into a covalent product with straight lines was observed, in which the Br adatoms arranged between the neighboring chains. On further annealing the sample at 270 °C, the detached Br adatoms played a key role in promoting the C-H bond activation. The cross-linked polymer was achieved by a combination of Ullmann and dehydrogenative coupling. When the annealing temperature was up to 390 °C, the C-F bond activation was triggered, which led to the formation of polymeric ribbons resulting from the cyclodehydrogenation of the fluorinated polymer. This study further supplements the reaction mechanism of thiophene-based dehalogenative, dehydrogenative and defluorinative coupling, and provides us a rational way for synthesizing cross-linked functional materials.