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.

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.

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 self-assembly of 2,7-bis­(decyloxy)-9-fluorenone on highly oriented pyrolytic graphite is investigated at the solid/gas interface by scanning tunneling microscopy, which allows us to determine the effect of its molecular structure on the formation of monolayer morphology. By varying the solution concentration in dichloromethane, seven types of supramolecular assemblies can be obtained. The concentration-dependent structural polymorphism is discussed in terms of thermodynamics. In particular, these different patterns are identified to be bound with weak intermolecular C­(sp2)–H···OC hydrogen bonds. As discerned by its position in the fluorenone moiety, the C­(sp2)–H group with larger chemical shift value and stronger acidity displays a higher priority when involving in the formation of a C­(sp2)–H···O hydrogen bond. Owing to the high density of C­(sp2)–H donors, various hydrogen-bonding configurations arise, further leading to the occurrence of structural polymorphism. Besides, intermolecular van der Waals interactions as well as the dipole–dipole interactions act as the complementary roles to stabilize the whole monolayer. The underlying mechanism is further confirmed by the density functional theory calculations.

Self-assembled behaviors of three fluorenone derivatives substituted by different halogen atoms, 2-(pentadecyloxy)-6-bromo-fluorenone (Br-FC15), 2-(pentadecyloxy)-6-chloro-fluorenone (Cl-FC15), and 2-(pentadecyloxy)-6-fluoro-fluorenone (F-FC15), were investigated at the 1-phenyloctane/highly oriented pyrolytic graphite interface by scanning tunneling microscopy combined with density functional theory calculations in comparison with the self-assembly of 2-pentadecyloxy-fluorenone (H-FC15). It is found that different charge distributions on halogen substituents lead to a subtle change of the molecular packing nanostructures. By varying the solution concentrations, X-FC15 (X = Br, Cl, H) can self-assemble into polymorphic nanostructures, whereas only one pattern can be observed for the F-FC15 adlayer due to the stronger continuous C–H···F bonds. The intermolecular C–H···OC hydrogen bonds are the main driving forces for all the self-assembled patterns. Particularly, the halogen-based hydrogen bonds and the type-I X···X (X = Br, Cl) bonds act as the collaborative forces to stabilize the alternate adlayers. Furthermore, the halogen bonds (C–Br···OC and C–Cl···OC) make the crucial contribution to the distinct lamellar and the dumbbell-like patterns. The investigation suggests that the engineering of organic nanoarchitectures can be effectively tailored by the introduction of different halogen atoms.

We design a bifunctional molecule (5-bromo-2-hexadecyloxy-benzoic acid, 5-BHBA) with a bromine atom and a carboxyl group and its two-dimensional self-assembly is experimentally and theoretically investigated by using scanning tunneling microscopy (STM) and density functional theory (DFT) calculations. The supramolecular self-organization of 5-BHBA in two different solvents (1-octanoic acid and n-hexadecane) at the liquid-solid interface at different solution concentrations is obviously different due to the cooperative and competitive intermolecular halogen and hydrogen bonds. Three kinds of nanoarchitectures composed of dimers, trimers and tetramers are formed at the 1-octanoic acid/graphite interface based on -COOHHOOC-, triangular C[double bond, length as m-dash]OBrH-C, -BrO(H), BrBr, and OH interactions. Furthermore, by using n-hexadecane as the solvent, two kinds of self-assembled linear patterns can be observed due to the coadsorption, in which the dimers are formed by intermolecular -COOHHOOC- hydrogen bonds. The molecule-solvent and solvent-solvent van der Waals force and intermolecular hydrogen bonds dominate the formation of coadsorbed patterns. We propose that the cooperative and competitive halogen and hydrogen bonds are related to the polarity of the solvent and the type of molecule-solvent interaction. The intermolecular binding energy of different dimers and their stability are supported by theoretical calculations. The result provides a new and innovative insight to induce the 2D self-assembled nanostructures by halogen and hydrogen bonds at the liquid-solid interface.

noncovalent forces, such as dipole-dipole interaction, [3] electrostatic interaction, [4] van der Waals interaction, [5] hydrogen bonds, [6] and π-π stacking. [7] Through physical adsorption, achiral molecules can self-assemble into chiral configurations on the achiral substrate due to the reduced freedom and constraint of the substrate lattice. [2d,8] Because of the rotation characteristics, porous networks show great potential in the process of chiral recognition and have received wide attentions. [9] However, nonporous networks also possess possibility of forming chiral polymorphs as long as the molecules are properly designed. Thus obtaining chirality via tiny structure modification will be a new attempt on fabricating chiral structures and will provide a new method on designing advanced functional materials in the field of surface science.Fluorenone derivatives have been reported to be used as liquid crystals. [10] Moreover, they were also good candidates for studying the self-assemblies at the liquid/highly oriented pyrolytic graphite (HOPG) interface, which have been widely explored in our group. We have previously reported that structural diversity was common to be observed and could be induced via changing the length of the alkyl chain, the concentration, and the solvent. For the bis-substituted fluorenone derivatives, molecules were arranged into a variety of nanopatterns which were regularly ordered but not chiral. [3,11] For the monosubstituted fluorenone derivatives, molecules adopted both chiral and achiral configurations and structural transition could be regulated between them. [12] Therefore, fabrication of chiral nanostructures using unsymmetrically substituted fluorenone derivatives seems to be a challenging but meaningful target. Fortunately, we realized this goal by efficient modification of the side chains.When the π-conjugated fluorenone core was substituted at the equal positions but substituted by unequal alkyl chains, chiral nanostructures were observed in its self-assembled monolayer on HOPG. The molecule we use in this study is 2-decyloxy-7-pentadecyloxy-9-fluorenone (DPF), as shown in Figure 1a. The first typical feature induced by the two different alkyl chains (C 10 H 21 and C 15 H 31 ) is that the long chains and the short chains are orderly interdigitated. As a result of this interdigitation, DPF molecules are usually notThe self-assemblies of 2-decyloxy-7-pentadecyloxy-9-fluorenone (DPF) are characterized by scanning tunneling microscopy at the liquid/solid interface. Achiral Dimer and chiral S-like structures are observed in 1-octanoic acid. The solvent molecule takes part in the self-assembly via forming COOH⋯OC and COOH⋯COOH hydrogen bonds. When 1-phenyloctane and n-tetradecane are used as the solvents, DPF self-assembles into chiral Z-like structure, which are classified into Types I-IV according to the packing styles. For the purpose of exploring the effect of solvent and the competition between these three structures, the self-assemblies of DPF in mixed solvents and ...

The supramolecular self-assembly of a push-pull dye is investigated using scanning tunneling microscopy (STM) at the liquid-solid interface. The molecule has an indandione head, a bithiophene backbone and a triphenylamine-bithiophene moiety functionalized with two carboxylic acid groups as a tail. The STM images show that the molecules adopt an "L" shape on the surface and form chiral Baravelle spiral triangular trimers at low solution concentrations. The assembly of these triangular chiral trimers on the graphite surface results in the formation of two types of chiral Kagomé nanoarchitectures. The Kagomé-α structure is composed of only one trimer enantiomer, whereas the Kagomé-β structure results from the arrangement of two trimer enantiomers in a 1:1 ratio. These Kagomé lattices are stabilized by intermolecular O-H•••O hydrogen bonds between carboxylic acid groups. These observations reveal that the complex structure of the push-pull dye molecule leads to the formation of sophisticated two-dimensional chiral Kagomé nanoarchitectures. The subsequent deposition of coronene molecules leads to the disappearance of the Kagomé-β structure, whereas the Kagomé-α structure acts as the host template to trap the coronene molecules.